F EMORAL ARTICULAR CARTILAGE CHARACTERISTICS AND MECHANICAL KNEE JOINT LOADING IN INDIVIDUALS DURING EARLY PHASES OF RECOVERY FOLLOWING A NT ERIOR CRUCIATE LIGAMENT RECONSTRUCTION By Caroline Michele Lisee A DISSERTATION Submitted to Michi gan State University in partial fulfillment of the requirements for the degree of Kinesiology Doctor of Philosophy 20 20 PUBLIC ABSTRACT FEMORAL ARTICULAR CARTILAGE CHARACTERISTICS AND MECHANICAL KNEE JOINT LOADING IN INDIVIDUALS DURING EARLY PHASES O F RECOVERY FOLLOWING A NT ERIOR CRUCIATE LIGAMENT RECONSTRUCTION By Caroline Michele Lisee Anterior cruciate ligament reconstruction (ACLR) is a ris k factor for the development of accelerated post - traumatic knee osteoarthritis. Pre - radiographic assessment s of early knee joint changes and understanding the impact of movement and activity contributing to worsening knee joint health may help health care providers identify and intervene before detrimental degenerative effects occur. The purpose s of this disser tation compris ed of 3 manuscripts with data collected from 2 studies w ere to: 1) establish the reli ability 2 ultrasound assessment techniques of early knee joint structur e and deformation , 2) assess knee joint structural differences and changes between sur gical and non - surgical knees 4 - and 6 - months post - ACLR in individuals recovering from surgery , and 3) identify if walking movement patterns and amount of activity participation at 4 - months post - ACLR contribute to knee joint structural changes assessed via ultrasound at 6 - months post - ACLR. In the first study, ultrasound images of knee articular cartilage thickness were capture d in the knees of 30 participants without a history of knee injury at rest and after 3,000 steps of walking. The second study was comp leted by 20 participants recovering from ACLR at 4 - an d 6 - months after surgery. At 4 months post - ACLR , participants completed a walking movement pattern assessment and an ultrasound imag ing assessment of knee articular cartilage thickness at rest in both t he surgical and non - surgical knees. Participants were also instructed to wear a physical activity monitor for 7 days to assess average daily steps at this time. At 6 - months post - ACLR, ultrasound images of knee articular cartilage thickness were c aptured at rest for all participants. For the first study , intra - rater and test - retest reliability was excellent for assessing r esting knee articular cartilage thickness in all compartments (ICC 2,k =0.97 - 0.99) . Knee articular cartilage changes after 3,000 s teps of wa lking demonstrated good to excellent intra - rater reliability (ICC 2,k =0.84 - 0.94) , but poor test - retest reliability (ICC 2,k = - 0.36 - 0.46) . For study 2, there were no significant differences or interactions between surgical and non - surgical resting knee articul ar cartilage thickness or between 4 - and 6 - months post - ACLR ( p range =0.22 - 0.92) . Additionally, individuals who with walked with lesser sagittal knee joint forces , but greater steps per day at 4 months post - ACLR had greater knee articular cartilage thicknes s (R 2 = 0.39, p =0.03) . Ultrasound assessment of knee articular cartilage thickness at rest is a reliable measure of knee joint structure that should be used in individuals at risk for knee osteoarthritis, but changes in knee articular cartilage thickness af ter walking are too inconsistent for application over multiple study sessions . Individuals with a history of ACLR do not demonstrate knee articular cartilage structural differences assessed via ultrasound at rest betwee n knees within the first 6 months of recovery. After ACLR , individuals who participate in high amounts of activity before altered knee movement patterns are resolved demonstrated knee articular cartilage thickness associated with cartilage swelling. Future research should determine when ultra sound assessment of knee articular cartilage can be used to identify early knee articular cartilage structural changes after ACLR and if addressing altered walking movement patterns before increasing activity participa t ion promotes long - term knee joint hea lth in individuals after ACLR . ABSTRACT FEMORAL ARTICULAR CARTILAGE CHARACTERISTICS AND MECHANICAL KNEE JOINT LOADING IN INDIVIDUALS DURING EARLY PHASES OF RECOVERY FOLLOWING A NT ERIOR CRUCIATE LIGAMENT RECONSTRUCTION By Caroline Michele Lisee Individuals with a history of anterior cruciate ligament reconstruction ( ACLR ) have a higher risk of developing accelerated knee osteoarthritis compared to individuals without a history of knee injury. It is necessary to establish reliable tools (i.e. ultrasound asse ssment) that assess knee joint health , determine how early these tools can identify poor knee joint health changes in high - risk populations , and examine which modifiable risk factors (i.e. mechanical knee joint loading) contribute to accelerated poor knee joint health development. The purpose s of this dissertati on were to: 1) establish the intra - rater and test - retest reliability of two ultrasound assessment techniques ( resting cartilage and cartilage response to loading assessment s ) of femoral articular car tilage structure and deformatio n , 2) assess resting femoral articular cartilage structural differences between the involved limb and contralateral limb and changes over time from 4 - to 6 - months post - ACLR, and 3) determine the ability of cumulative knee joi nt loading (gait knee biomechanics and volume of lo ading ) at 4 - months post - ACLR to predict resting medial femoral articular cartilage structure at 6 - months post - ACLR. In the first observational study, femoral articular cartilage structure and deformation w ere evaluated via the resting cartilage and cartilage response to loading ultrasound assessment techniques in 30 participants without a history of knee injury . In 2 identical testing sessions, t he resting cartilage and post - loading cartilage images were ca pture after 30 minutes of rest and 3,000 steps of walking , respectively . A total of 20 participants post - ACLR completed the resting cartilage ultrasound assessment in their involved and contralateral limb at 4 - and 6 - months post - ACLR for the second longitu dinal study . At 4 - months post - ACLR, knee gait biomechanics (knee extension moment, knee abduction moment, and vertical ground reaction force) were assessed with motion capture and force plates, and volume of activity (steps/day) were assessed with a hip wo rn accelerometer over 7 days . All ultrasound images were processed using a semi - automated processing technique to divide the total cartilage cross - sectional area into medial, intercondylar, and lateral compartments normalized to compartment length for cart ilage thickness (mm) . Restin g cartilage ultrasound assessment demonstrated excellent test - retest and intra - rater reliability (ICC 2,k = 0.97 - 0.99 ) . Cartilage response to loading ultrasound assessment demonstrated poor test - retest reliability (ICC 2,k = - 0.36 - 0. 46), but good to excellent i ntra - rater reliability (ICC 2,k =0.84 - 0.94 ) . Individuals 4 - to 6 - months post - ACLR did not demonstrate any significant limb main effects ( p range=0.50 - 0.92), time main effects ( p range=0.22 - 0.72), or interactions ( p range=0.24 - 0.49 ) for resting medial, intercondylar, or lateral femoral articular cartilage compartmental thickness. Lesser knee extension moment p =0.02) and greater steps per day (unstandardized p =0.0 4 ) at 4 - months post - ACLR predict greate r medial femoral artic ular cartilage compartmental thickness at 6 - months post - ACLR (R 2 = 0.39, p =0.03) . The r esting cartilage ultrasound assessment is a reliable technique between multiple processing and testing sessions , but the cartilage response to loadi ng ultrasound assessment is not reliable between testing sessions . Femoral articular cartilage structur al differences between limbs or change over time may not be present before 6 months post - ACLR. Individuals with poor biomechanics who take more steps per d ay demonstrate articular cartilage structural changes indicative of articular cartilage swelling within 6 - months post - ACLR . Cumulative m echanical knee joint loading is a multifactorial risk factor of knee joint health during the early phases of recovery af ter ACLR. Copyright by CAROLINE MICHELE LISEE 2020 v A CKNOWLEDGEMENTS First and foremost, I would like to thank Dr. Christopher Kuenze , my research partner in crime . I would not be where I am today without your consistent guidance and support. You go above and beyond the basic expectations of an advisor daily . Because of your support, I have exceeded my own expectations and achieved all my professional goals during the past 4 years . I wish I could give you every mentor award because you deserve it. Thank you for supporting me as a student, an instructor, a researcher and most of all, a human being. I look forward to working with you in the future as I continue my professional research career. I would like to thank Dr. Tracey Covassin, D r. Karin Pfeiffer, and Dr. Jeffrey Kovan for their guidance throughout my comprehensive exams and dissertation process. This project would not have been possible without your de dication and support. I would especially like to thank Dr. Covassin and Dr. Pfe iffer for leading the way for female scientists including myself . I am inspired by your consistent success and unwavering leadership. Thank you for being my role models. I would like to thank Dr. Matthew Harkey for your time teaching me how to assess femo ral articular cartilage with ultrasound. This project would not be possible with your guidance and patience. I would like to thank all members of the Sports Injury Resear ch Laboratory including Abby Bretzin, Kyle Petit, Jen Savage, Tom Birchmeier, Morgan Anderson, Chris Tomczyk, Katie Collins, and Aaron Zynda . I could not imagine maneuvering through these past 4 years without vi you . I believe the culture we have cultivated in the lab is beautiful and rare. I hope to offer you the same support as you continue through the program. I would like to thank our research assistants including Callum Davis, Tess McGuire, Zach , Eric Ballard, Jen Poli n, Henry Burghardt, Moriah Moore, Jason Greib, and John Italiano for all y our help on past and current projects. The lab would not run without your hard work and dedication. Finally, I would like to thank my family , John, Terry, Chris, Michelle, and Kevin . Y our support has been essential to my professional and personal growth. Thank you fo r providing me encouragement in the face of rejection , celebrating my accomplishments, and for teaching me to believe in myself. vii TABLE OF CONTENTS L IST OF TABLES ................................ ................................ ................................ ......................... xi L IST OF FIGURES ................................ ................................ ................................ ..................... xiii CHAPTER 1: INTRODUCTION ................................ ................................ ................................ . 1 S TATEMENT OF THE PROBLEM ................................ ................................ ............................... 1 STATEMENT OF THE PURPOSE ................................ ................................ ................................ 1 R ESEARCH QUESTIONS AND EXPERIMENTAL HYPOTHESES ................................ .......... 5 M ANUSCRIPT 1 RESEARCH QUESTIONS AND HYPOTHESES ................................ ....... 5 M ANUSCRIPT 2 RESEARCH QUESTIONS AND HYPOTHESES ................................ ....... 6 M ANUSCRIPT 3 RESEARCH QUESTIONS AND HYPOTHESES ................................ ....... 7 SIGNIFICANCE OF THE STUDY ................................ ................................ ................................ . 8 CHA PTER 2 : REVIEW OF THE LITERATURE ................................ ................................ ... 10 I NTRODUCTION ................................ ................................ ................................ ......................... 1 0 E PIDEMIOLOGY OF ACL INJURY AND ACLR ................................ ................................ ...... 1 3 A NTERIOR CRUCIATE LIGAMENT (ACL) A NATOMY ................................ .................. 13 P RIMARY ACL I NJURY ................................ ................................ ................................ ........ 1 3 M ECHANISM OF NON - CONTACT ACL INJURY ................................ .............................. 1 5 ACLR E PIDEMIOLOGY ................................ ................................ ................................ ......... 16 ACLR S URGICAL TECHNIQUE ................................ ................................ ........................... 1 6 S ECONDARY AC L I NJURY E PIDEMIOLOGY ................................ ................................ ... 18 R ISK FACTORS FOR SECOND ACL INJURY ................................ ................................ ..... 19 V OLUME OF A CTIVITY IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF KNEE PATHOLOGY ................................ ................................ ................................ ................... 19 PHYSICAL ACTIVITY IN THE GENERAL POPULATION ................................ ................ 2 0 BENEFITS OF PHYSICAL ACTIVITY PARTICIPATION ................................ .................. 2 1 MEASURING PHYSICA L ACTIVITY ................................ ................................ ................... 21 SELF - REPORTED TYPE AND VOLUME OF ACTIVITY THE MARX ACTIVITY SCALE ................................ ................................ ................................ ................................ ...... 22 SELF - REPORTED TYPE OF ACTIVITY T HE TEGNER ACTIVITY SCALE ................ 2 2 SELF - REPORTED INTENSITY AND VOLUME OF ACTIVITY T HE IPAQ - SF ............ 2 3 FREE LIVING VOLUME OF ACTIVITY ACCELEROMETR Y ................................ ....... 2 4 ACTIVITY MEASUREMENT STRENGTHS AND WEAKNESSES ................................ ... 2 6 MEASURING VOLUME AND TYPE OF ACTIVITY AFTER ACLR ................................ . 2 8 VOLUME OF ACTIVITY DIFFERENCES IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF ACLR ................................ ................................ ................................ ................ 29 RELATIONSHIPS BETWEEN VOLUME OF ACTIVITY AND CLINICAL OUTCOMES AFT ER ACLR ................................ ................................ ................................ .......................... 3 0 VOLUME OF ACTIVITY AND KNEE OSTEOARTHRITIS ................................ ............... 3 1 BMI IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF KNEE PATHOLO GY ...... 3 2 BMI IN THE GENER AL POPULATION ................................ ................................ ............... 3 3 viii BMI AFTER ACLR ................................ ................................ ................................ .................. 3 4 BMI AND SELF - R EPORTED K NEE F UNCTION AFTER ACLR ................................ ....... 3 5 G AIT BIOMECHANICS IN INDIVIDUAL S WITH AND WITHOUT A HISTORY OF KNEE PATHOLOGY ................................ ................................ ................................ ............................... 36 M ULTIPLANAR KNEE BIOMECHANICS DURING GAIT AFTE R ACLR ....................... 39 E PIDEMIOLOGY OF KNEE OA ................................ ................................ ................................ . 4 2 A RTICULAR CARTILAGE ANATOMY ................................ ................................ ............... 4 2 A RTICULAR CARTILAGE AND GROWTH ................................ ................................ ........ 4 4 K NEE OA ................................ ................................ ................................ ................................ . 4 5 PTOA ................................ ................................ ................................ ................................ ........ 4 6 MEASURES OF KNEE OA PROGRESSION AND SEV ERITY ................................ ............... 48 R ADIOGRAPHIC KNEE ARTICULAR CARTILAGE ASSESSMENT ............................... 48 W ET BIOMARKERS AND KNEE ARTICULAR CARTILAGE ASSESSMENT ............... 49 MRI - B ASED KNEE ARTICULAR CARTILAGE ASSESSMENT ................................ ....... 51 U LTRASONOGRAPHY AND KNEE ARTICULAR CARTILAGE ASSESSMENT ........... 53 ULTRASONOGRAPHY OF RESTING KN EE ARTICULAR CARTILAGE IN INDIVIDUALS WITH A HISTORY OF ACLR ................................ ................................ ..... 54 K NEE ARTICULAR CARTILAGE HEALTH AND FACTORS OF MECHANICAL LOADING AFTER ACLR ................................ ................................ ................................ ............ 56 V OLUME OF LOADING AND KNEE ARTICULAR CARTILAGE HEALTH AFTER ACLR ................................ ................................ ................................ ................................ ........ 5 6 BMI AND KNEE ARTICULAR CARTILAGE HEALTH ................................ ..................... 5 8 G AIT BIOMECHANICS AND KNEE ARTICULAR CARTILAGE HEALTH AFTER ACLR ................................ ................................ ................................ ................................ ........ 59 C ONCLUSION ................................ ................................ ................................ .............................. 62 CHAPTER 3: KNEE ULTRASOUND ASSESSMENT S OF RESTING CARTILAGE AND CARTILAGE RESPONSE TO LOADING IN HEALTHY PARTICIPANTS: A RELIABILITY STUDY ................................ ................................ ................................ .............. 6 4 A BSTRACT ................................ ................................ ................................ ................................ ... 6 4 INTRODUCTION ................................ ................................ ................................ ......................... 6 6 M ETH ODS ................................ ................................ ................................ ................................ .... 68 P ARTICIPANTS AND SCREENING PROCESS ................................ ................................ ... 68 G AIT SPEED ASSESSMENT ................................ ................................ ................................ 6 9 R ESTING CARTILAGE ULTRASOUND IMAGING ASSESSMENT ................................ . 69 S TANDARDIZED LOADING PROTOCOL ................................ ................................ .......... 72 CARTILAGE RESPONSE TO LOADING ULTRASOUND IMAGING ASSESSMENT .... 72 FOLLOW - UP STUDY VISIT ................................ ................................ ................................ .. 72 IMAGE PROCESS ING ................................ ................................ ................................ ............ 7 3 S TATISTICAL ANALYSIS ................................ ................................ ................................ ..... 7 6 S AMPLE SIZE ESTIMATION ................................ ................................ ................................ 77 R ESULTS ................................ ................................ ................................ ................................ ...... 77 PARTICIPANTS DEMOGRAPHIC, WALKING, AND CARTILAGE CHARACTERISTIC S ................................ ................................ ................................ .............. 77 INTRA - SESSION RELIABILITY RESULTS ................................ ................................ ......... 83 TEST - RETEST RELIABILITY RESULTS ................................ ................................ ............. 83 ix INTRA - RATER RELIABILITY RESULTS ................................ ................................ ............ 8 3 D ISCUSSION ................................ ................................ ................................ ................................ 96 C ONCLUSION ................................ ................................ ................................ ............................ 100 CHAPTE R 4: AVERAGE FEMORAL ARTICULAR CARTILAGE THICKNESS IN INDIVIDUALS AFTER ACLR: A LONGITUDINAL STUDY ................................ ........... 101 ABSTRACT ................................ ................................ ................................ ................................ . 10 1 INTRODUCTION ................................ ................................ ................................ ....................... 10 3 METHODS ................................ ................................ ................................ ................................ .. 10 5 P ARTICIPANT S ................................ ................................ ................................ .................... 10 5 S AMPLE SIZE ESTIMATION ................................ ................................ .............................. 10 6 P ARTICIPANT SCREENING PROCESS ................................ ................................ ............. 10 6 RESTING CA RTILAGE ULTRASOUND IMAGING ASSESSMENT ............................... 10 6 6 MONTH POST - ACLR ASSESSMENT ................................ ................................ .............. 10 7 IMAGE PROCESSING ................................ ................................ ................................ .......... 10 7 STATISTICAL ANALYSIS ................................ ................................ ................................ ... 10 8 RESULTS ................................ ................................ ................................ ................................ .... 10 9 DISCUSSION ................................ ................................ ................................ .............................. 11 6 CONCLUSION ................................ ................................ ................................ ............................ 119 CHAPTER 5: CUMULATIVE KNEE JOINT LOADING AND FEMORAL ARTICULAR CARTILAGE THICKNESS 4 TO 6 MO NTHS POST - ACLR ................................ ............. 120 ABSTRACT ................................ ................................ ................................ ................................ . 12 0 INTRODUCTION ................................ ................................ ................................ ....................... 12 2 METHODS ................................ ................................ ................................ ................................ .. 1 24 P ARTICIPANTS ................................ ................................ ................................ .................... 12 4 G AIT ASSESSMENT 4 MONTHS POST - ACLR ................................ ................................ . 12 5 D AILY STEP COUNT MONITORING 4 MONTHS POST - ACLR ................................ ..... 12 8 U LTRASOUND ASSESSMENT OF FEMORAL ARTICUALR CARTILAGE THICKNESS 6 MONTHS POST - ACLR ................................ ................................ ................................ ...... 1 30 I M AGE PROCESSING ................................ ................................ ................................ .......... 1 3 0 S TATISTICAL ANALYSIS ................................ ................................ ................................ ... 13 2 S AMPLE SIZE ESTIMATION ................................ ................................ .............................. 13 2 RESULTS ................................ ................................ ................................ ................................ .... 13 2 DISCUSSION ................................ ................................ ................................ .............................. 1 39 CONCLUSION ................................ ................................ ................................ ............................ 14 3 CHAPTER 6: SUMMARY AND CONCLUSIONS ................................ ............................... 145 S UMMARY ................................ ................................ ................................ ................................ . 14 5 RELIABILITY OF RESTING CARTILAGE AND CARTILAGE RESPONSE TO LOADING ULTRASOUND ASSESSMENT S O F FEMORAL ARTICULAR CARTILA GE ................................ ................................ ................................ ................................ ................. 14 5 LONGITUDINAL ULTRASOUND ASSESSMENT OF FEMORAL ARTICULAR CARTILAGE THICKNESS 4 TO 6 MONTHS POST - ACLR ................................ .............. 14 6 x ASSOCIATIONS BETWEEN CUMULA TIVE KNEE JOINT LOADING AND MEDIAL FEMORAL ARTICULAR CARTILAGE THICKNESS WITHIN 6 MONTHS PO ST - ACLR ................................ ................................ ................................ ................................ ................. 14 6 L IMITATIONS ................................ ................................ ................................ ............................ 1 47 S TRENGTHS ................................ ................................ ................................ .............................. 1 49 ACCESIBLE ASSESSMENT TOOLS FOR HEALTH CARE PROFESSIONALS ............ 1 49 INTEGRATIVE APPROACH TO UNDERSTANDING KNEE JOINT MECHANICAL LOADING ................................ ................................ ................................ ............................. 15 0 CLINICAL IMPLICATIONS AND FUTURE RESEARCH ................................ ...................... 15 1 C ONCLUSIONS ................................ ................................ ................................ .......................... 15 3 REFERENCES ................................ ................................ ................................ ........................... 1 55 xi LIST OF TABLES Table 1. Summary of gait biomechanics in the involved limb of individuals with a history of ACLR compared to the contralateral limb and the limbs of healthy controls from post - op to greater than 4 years post - surgery ................................ ................................ ............................. 38 Table 2. Relationship between gait biomechanics and measures of knee articular cartilage health after ACLR ................................ ................................ ................................ ............................... 61 Table 3 . Participant and study session characteristics (N=30) ................................ ..................... 79 Table 4 . Participant w alking c haracteristics ( M ean ± SD [ R ange]) ................................ ............. 80 Table 5 . Femor al a rticular c artilage c haracteristics for r ound 1 i mage p rocessing of b oth v isits (Mean ± SD ) ................................ ................................ ................................ ........................... 81 Table 6 . Femoral a rticular c artilage c haracteristics for r ound 2 i mage p rocessing of b oth v isits (Mean ± SD) ................................ ................................ ................................ ........................... 82 Table 7 . Intra - session r eliability (ICC 2 .1 and 95% Confidence Intervals) of i ndividual f emoral a rticular c artilage i mages for a ll c ompartments ................................ ................................ ....... 85 Table 8 . Test - r est r eliability (ICC 2,k and 95% C onfidence I ntervals) , s tandard e rror of m easurement (SEM) and m inimal d etectable c hange (MDC) for r e sting, p ost - l oading, and d eformation f emoral a rticular c artilage o utcomes ................................ ................................ ... 86 Table 9 . Intra - r ater r eliability (ICC 2,k and 95% C onfidence I ntervals) , s tandard e rror of m easurement (SEM) and m inimal d etectable c hange (MDC) for r esting, p ost - l oading, and d e formation f emoral a rticular c artilage o utcomes ................................ ................................ ... 91 Table 10 . Participant and study session characteristics ( Mean ± Standard Deviation ) .............. 1 10 Table 11 . Resting f emoral a rticular c artilage compartmental t hickness (mm) at 4 - and 6 - Months Post - ACLR (Mean ± S D ) ................................ ................................ ................................ ...... 1 11 Table 12 . Resting f emoral a rticular c artilage c ompartmental t hickness (mm) d ifferences b etween l imbs and o ver t ime (Mean ± Standard Deviation) ................................ ............................... 1 12 Table 13. Accelerometer d ata c ollection and a nalysis m ethods ................................ .................. 1 29 Table 1 4 . Participant and stu dy session characteristics (N=19) ................................ ................. 1 34 Table 1 5 . Mechanical l oading o utcomes 4 months p ost - ACLR and m edial f emoral a rticular c artilage compartmental t hickness 6 m onths p ost - ACLR ................................ ..................... 1 35 xii Table 1 6 . Correlation matrix between gait biomechanical outcomes 4 - mon ths post - ACLR and medial femoral articular cartilage compartmental thickness 6 - months post - ACLR ............. 1 36 xiii LIST OF FIGURE S Figure 1 . Loading in Osteoarthritis Development (LOAD) model supporting the mechanical pathways for developing of post - traumatic OA during the initiation phase after ACL R ........ 12 Figure 2 . A.) Patient position for ultrasound assessment with back against wall and knee at 140° of f lexion identified by manual goniometer. Red open circle indicates area where position of posterior aspect of ca lcaneus is recorded and used for all assessments; B. ) Positioning of ultrasound head for image capture. Red circles represent medial and lateral condylar landmarks, the red line indicates where ultrasound head is placed in between condyles ........ 71 Figure 3. A .) Total cross - sectional area of anterior femoral articular cartilage is outlined by the white line and the center point of the articular cartila ge is represented by the red diamond; Figure B. ) Medial (orange), intercondylar (green) and lateral (blue) arti cular cartilage compartments representing the segmented CSA of anterior femoral articular cartilage; central point = red diamond; intercondylar l ength of the middle segment = red line .......................... 72 Figure 4. Average femoral articular cartilage CSA differences betwe en visit 1 and visit 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between visit 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement. Positive ave rage differences indicate that the second visit had greater cartilage CSA compared to the first visit. Negative average differences indicate that the second visit had lesser cartilage CSA compared to the first visit ................................ ...... 87 Figure 5. Average femoral articular cartilage thickness differences between visit 1 and visit 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line r epresents the average mean difference between visit 1 and 2 for each outcome. The dotted lines represent the 95% upper and l ower limits of agreement. Positive average differences indicate that the second visit had greater cartilage thickness compared to th e first visit. Negative average differences indicate that the second visit had lesser cartilage thickness compared to the fi rst visit .......... 89 Figure 6 . Average femoral articular cartilage CSA differences between image processing round 1 and 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between image processing round 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement. Positive average differen ces indicate that the second round had greater cartilage CSA compared to the first round. Negative average differences indicate that the second round had lesser cartilage CSA compared to the first round ................................ ................................ ................................ ...... 92 Figure 7 . Average femoral articular cartilage th ickness differences between image processing round 1 and 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line repr esents the average mean difference between image processing round 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement. Positive average differences indicate that the second round had greater cartilage thickness compared xiv to the first round. Negative average differences indicate that the second round had lesser cartilage thickness com pared to the first round ................................ ................................ ....... 94 Fi gure 8. Medial (A), intercondylar (B), and lateral femoral articular cartilage compartmental thicknes s (C) in the involved and contralateral limbs from 4 to 6 months post ACLR. Green circles and the solid lines represent the contralateral limb and black circles and dotted lines represent the involved limb ................................ ................................ ................................ .... 113 Figure 9. M edial (A & B), intercondylar (C & D), and lateral (E & F) femoral articular cartilage compartmental thickness between limbs at 4 - and 6 - months post - ACLR. The bottom and top of the vertical line represent the minimum and maximum of the cartilage thickness outcome. The bottom and top of th e box indicate the first and third quartile of the cartilage thickness outcomes. The line represents the avera ge cartilage thickness outcome. Green boxes represent the involved limb and gray boxes represent the contralateral limb ........................ 114 Figure 1 0 . A.) Clust ers with 4 retroreflective markers each were placed on the thoracic and lumbar regions. The red circles over t he medial and lateral knee joint line represent stylus placement used to identify the knee joint center. B. ) Clusters with 4 retroreflective marke rs each were place on the outside of the right thigh and shank, and on top of the right foot. Cluster placement was identical on the left leg ................................ ................................ ..... 127 Figure 11 . Relationships between mechanical knee joint loading outcomes during gait at 4 months post - ACLR an d average medial femoral articular cartilage thickness at 6 months post - ACLR. Negative values indicate greater knee extension moment, but positive values indicate greater knee abduction moment and vGRF ................................ .............................. 137 F igur e 12. Association bet ween observed and pred icted involved limb medial femoral articular cartilage thickness ................................ ................................ ................................ .................. 138 1 CHAPTER 1 : INTRODUCTION STATEMENT OF THE PROBLEM Knee osteoarthritis (OA) is a chronic health condition resulting in irreversible tissue damage to the synovial joint especially within the tibiofemoral articular cartilage . 20 K nee OA is a leading cause of disability in daily activities worldwide and individuals with the condition report poor quality of life . 1,2 Traumatic k nee injury , including an terior cruciate ligam ent (ACL) injury , is a leading risk factor for the develop ment of knee post - traumatic osteoarthritis (PT OA ) . 3 Greater than 50% of young , otherwise healthy individuals with ACL injury and/or sub sequent reconstruction ( ACLR ) develop PTOA within 20 years. 3 Unfortunately, there is no cure for knee PT OA so individuals with ACLR often develop PTOA early in life , leading to many years lived with disability. T he refore, t here is a critical need to identify individuals at elevated risk for knee PTOA and to develop interven tions t hat help prevent or delay the development of poor synovial knee joint health , disability, and poor quality of life after ACLR . To address these problems, we must: 1) establish a reliable assessment of early knee joint health changes; 2) determine if this assessment technique can detect early knee joint health changes in ACLR populations; 3) identify which modifiable risk factors contribute t o early , poor knee joint health changes after ACLR. STATEMENT OF THE PURPOSE M echanical knee joint loading is one of three pathway s along with joint metabolism and structure that contribu te to the development of knee PTOA after ACLR . Mechanical knee joint loading refers to the internal and external forces acting on the knee joint during weightbearing activities. Healthy knee a rticular cartilage aids in absorbing these forces , but damaged knee articular cartilage responds poorly to the mechanical forces acti ng on the knee. 4 It is 2 hypothesized that assessing knee articul ar cartilage response to mechanical load ing may provide unique insights into early m icrostructural or compositional changes of the tissue that may precede radiographic evidence of knee PT OA. 5 Ultrasonography is an emerging tool used to assess femoral articular cartilage health. A resting cartilage ultrasound assessment technique that calculates the restin g femoral articular cartilage compartmental thickness is a valid and reliable assessment in individuals with and without a history of knee injury . 6 - 8 Based on resting cartilage technique, the cartila ge response to loading ultrasound assessment technique was developed to evaluate the change in femoral articular cartilage compartmental thickness from resting to post - loading characterized as the response of fem oral articular cartilage after a period of mechanical loading ( referred to as deformation). 7 - 9 Good to excellent intra - rater and test - retest reliability 7 - 9 have been establ ished for the cartilage response to loading ultrasound as sessment techniques applied to healthy populations when completed by an expert rater (> 5 years of experience) , but a gap exists in the literature defining the intra - rater and test - retest reliability of a novice rater (<1 year of experience) . Therefore, the purpose of the first manuscript was to determine the intra - rater and test - retest reliability of the cartilage response to loading ultrasound assessment technique in a novice rater. Novice raters mu st establish simil ar reliability quality to determine if this tool is can also be adopted by new healthcare providers . Knee joint degeneration, especially in the tibiofemoral articular cartilage, is present as early as 6 months post - ACLR. 9 - 12 The earliest changes in knee joint health occur in the medial compartment compared to the intercondylar or lateral compartments due to greater mechanical loading forces acting in this region during weightbearing activities. 13 After ACLR, patients undergo 6 - 12 months of rehabilitation 14 and are progre ssively integrated into activities with increasing intensities of knee joint mechanical loading . A critical period in rehabilitation occurs 3 at 4 and 6 month s post - ACLR when patients are exposed to greater mechanical loading through exposure to more challen ging therapeutic exercises . For example, p atients are integrated into jogging and modified sports activities 4 months post - ACL R , and begin discharge from re habilitation and unrestricted return to physical activity as early as 6 months post - ACLR . 14 Assessment of knee articular cartilage structure to determine knee joint health in individuals with ACLR is imperative during this rehabilitation period so healthcare providers can intervene if necessary . The purpose of the second manuscript was to assess between limb d ifferences and changes over time in femoral articular cartilage structure captured via resting cartilage ultrasound assessment between the involved and contralateral l imb s in patients with a primary, unilateral history of ACLR at 4 - and 6 - months post - surgery. Application of this technique in ACLR populations can help determine if articular cartilage structural changes or limb differences are present while patients remai n un der the care of healthcare providers. Modifiable factors of knee joint mechanical loading include magnitude of loading (characterized by walking biomechanics) and volume of loading (characterized by volume of physical activity ) . Both of these factors contribute to the development of poor knee joint health 4,15 especially in the medial tibiofemoral compartment. 13 These factors of knee joint mechanical loading are often co nsidered separately, but this is an unrealistic representation of daily mechanical loading placed on the knee. Instead, these factors of mechanical loading should be considered concurrently to refl ect the daily cumulative mechanical loading occurring at th e knee. After ACLR, individuals demonstrate a berrant walking biomechanics in knee extension moment, knee abduction moment, and vertical ground reaction forces during the initial phases of rehabilit ation which may persist for years after ACLR. 11,16 Patients also demonstrate lesser volume s of physical activity quantified as steps per day compared to individuals without a 4 history of knee injury within 5 years of surgery . 17 Altered ma gnitude and volume of mechanical knee joint loadi ng are persistently present in individuals with a history of ACLR. However, i t is unclear how these factors are associated with knee joint health while individual s with ACLR are integrated into activities wi th increasing mechanical loading and remain under healthcare supervision. The purpose of the third manuscript was to assess the ability of cumulative mechanical loading ( walking biomechanics and volume of activity ) at 4 - months post - ACLR to predict medial knee articular cartilage compartmental thickness c aptured vi a resting cartilage ultrasound assessment 6 - months post - ACLR . I dentification of modifiable risk factors of mechanical loading during the rehabilitation process after ACLR may en able patient - specific secondary prevention of knee PTOA after ACLR. 5 RESEARCH QUESTIONS AND EXPERIMENTAL HYPOTHESES M ANUSCRIPT 1 RESEARCH QUESTIONS AND HYPOTHESES Primary Purpose: The primary purpose of this study was to assess the intra - rater reliability of a novice assessor using the resting cartilage and cartilage resp onse to loading ultrasound assessment technique s in healthy participants with out a history of knee injury . Secondary Purpose: The secondary purpose of this study was to assess the test - retest reliability of the novice assessor between 2 sessions of both ul trasound assessment technique s in healthy participants with out a history of knee injury. H 1. 1 . The primary hypothesis was that both ultrasound assessment technique s will demonstrate excellent intra - rater reliability for assessing medial, intercondylar, an d lateral femoral articular cartilage compartmental thickness and deformation . H 1. 1 . The secondary hypothesis was that both ultrasound assessment technique s will demonstrate excellent test - retest reliability for assessing medial, intercondylar, and latera l femoral articular cartilage compartmental thickness and deformation. 6 M ANUSCRIPT 2 RESEARCH QUESTIONS AND HYPOTHESES Prima ry Purpose: The purpose of this study was to assess between limb differences (involved limb and contralateral limb) and changes over time (4 - and 6 - months post - ACLR) in resting femoral articular cartilage characteristics (medial , intercondylar, and lateral femoral articular cartilage compartmental thickness ) in individuals recovering after ACLR. H 2.1. The primary hypothesis is that th e involved limb will demonstrate greater resting medial femoral articular cartilage compartmental thickness, but no differences in intercondylar and lateral femoral articular cartilage compartmental thickness compared to the c ontralateral limb at 4 - month s and 6 - month s post - ACLR . H 2.2 . The secondary hypothesis is that the involved limb will demonstrate greater medial femoral articular cartilage compartmental thickness at 6 - months compared to 4 - months post - ACLR . Involved limb intercondylar and lateral femor al articular cartilage compartmental thickness will not be different between 4 - and 6 - months post - ACLR H.2.3. The contralateral limb m edial, intercondylar, and lateral femoral articular cartilage compartmental thickness will not be different between 4 - and 6 - months post - ACLR. 7 M ANUSCRIPT 3 RESEARCH QUESTIONS AND HYPOTHESES Primary Purpose : The purpose of this study was to assess the ability of cumulative mechanical knee joint loading (gait biomechanics and volume of activity) at 4 - months post - ACLR to predi ct involved limb medial femoral articular cartilage compartmental thickness in individuals 6 - months post - ACLR. H 3.1. The primary hypothesis is that greater involved limb peak interna l knee abduction moment, peak internal knee extension moment, and peak v e rtical ground reaction force during the stance phase of walking at 4 - months post - ACLR will be associated with greater involved limb medial femoral articular cartilage compartmental thickness at 6 - months post - ACLR. P eak internal knee abduction moment will d emonstrate the strongest relationship. H 3.2. The secondary hypothesis is that greater involved limb peak internal knee abduction moment and lesser daily steps at 4 - m onths post - ACLR will predict greater involved limb medial femoral articular cartilage com partmental thickness 6 - months post - ACLR in individuals recovering from ACLR. 8 SIGNIFICANCE OF THE STUDY T he proposed studies incorporate emerging ultraso und techniques to assess articular cartilage health which are accessible in orthopedic clinics and act ivity assessments which are accessible through consumer - grade technology . Early identification of tibiofemoral articular cartilage changes is key in secondary prevention efforts, but radiographic and MRI - based assessments are impractical or lack sensitivit y which limits their feasibility in the rehabilitation environment. Ultrasound offers a repeat able assessment approach for longitudinal assessments that with more research has the potential to be integrated into healthcare clinics to identify individuals a t risk for developing knee PT OA. If th ese assessment s are adopted in healthcare clinics, it is important to understand how well individuals with limited experience effectively perform these assessments and the best way to perform this assessment technique . To our knowledge, this is the first study to longitudinally assess femoral articular cartilage structure via ultrasoun d after ACLR. Rehabilitation after ACLR is generally completed within a 6 - to 9 - month period. This study has a longitudinal design to bet ter understand the relationship between factors of loading during rehabilitation and articular cartilage joint health when many individuals are actively engaged with a healthcare provider on a consis tent basis and have not returned to unrestricted activity . T he proposed stud ies also take a multifaceted approach to addressing the early effects of cumulative mechanical loading on knee articular cartilage health after ACLR. Traditionally, walking biomech anics and volume of activity have been assessed individua lly after ACLR, but considering these factors conjointly provides a more comprehensive assessment of contributors to knee mechanical loading following surgery . In the current studies, we u tilize rese arch - grade activity monitoring technology for valid data collection to provide better context about the 9 volume of activity via daily step counts. However, c onsumer - grade activity tracking technology by clinicians and patients through consumer - grade devices such as smart watches or FitBit monitors . The results of t he proposed studies will provide the first step in a line of research to characterize the effects of under - or over - loading behavior on articular cartilage h ealth during critical points of the recovery process following ACLR to slow or mitigate the rapid d evelopment of PTOA commonly observed in this at - risk population. This also captures a period when rehabilitation clinicians can incorporate interventions to improve articular cartilage health. By identifying which load - related factors are associated with p oor knee articular cartilage health, we may be able to develop and implement safe, progressive walking - based protocols during recovery with the goal of limit ing sedentary behaviors and promoting healthy knee articular cartilage. 10 CHAPTER 2: REVIEW OF THE LITERATURE INTRODUCTION Many i ndividuals with a history of anterior cruciate ligament (ACL) injury or reconstruction (ACLR) develop post - traumatic knee osteo arthritis (PTOA) at an accelerated and greater rate compared to individuals without a history of knee injury. The pathogenesis of symptomatic knee post - traumatic osteoarthritis (PTOA) after anterior cruciate ligament reconstruction (ACLR) occurs over an i nitiation phase and subsequent progression phase 18 due to a combination of biological, structural, and mechanical mechanisms. 15 The initiation phase is characterized by early, superficial articular cartilage damage and the pr ogression phase is characterized by long - term, deeper articular cartilage degeneration. 18 Although these mechanisms are symbiotic and should be occur ring conc urrently in the overall development of knee PTOA, the Loading in OsteoArthritis Development (LOAD) model focu ses on the mechanical mechanisms. Mechanical mechanisms 18 refer to the various forces of load applied to knee articular cartilage during movement or activity and can be assessed through biomechanical analysis and wearable technologies that are able to quantify activi ty. Magnitude and volume of loading are 2 mechanical factors considered in the development of knee PTOA. Magnitude of loading considers the magnitude and location of biomechanical forces applied across the articula r cartilage surfaces of the knee joint and volume of loading refers to how often the knee articular cartilage is loaded (or not loaded) during daily activities . After ACLR, individuals experience an abrupt change in movement patterns and extended periods of restricted knee joint loading activity. These alterations in both quality and quan tity of mechanical loading expose knee articular cartilage to abnormal contact forces and cartilage composition which contribute to poor cyclical articular cartilage loading response and 11 disruption. 4,19 Cartilage compositional changes may reflect lesser proteoglycan content, collagen disorganization or greater water content. 20 - 22 The conceptual model (Figure 1 ) that serves as the bas is for this dissertation project focuses on the initiation phase of PTOA and considers a combination of previously proposed mechanical mechanisms to understand how different aspects of loading may impact knee articular cartilage after ACLR. This literature review explores the most recent evidence supporting this model. First, the literature review will explore the e pidemiology of ACL injury, ACLR, and knee OA. The review will also briefly describe the most relevant wet and dry biomarkers used to character ize the progression and severity of knee OA as it relates to knee articular cartilage health. Additionally, the re view will summarize mechanical loading (walking biomechanics and volume of activity) in individuals with and without a history of knee patholo gy. Finally, the review will discuss the relationship between biomarkers of knee articular cartilage health and mo difiable factors of mechanical loading. 12 Figure 1 . Loading in Osteo a rthritis Development (LOAD) model supporting the mechanical pathways fo r developing of post - traumatic OA during the initiation phase after ACLR 13 EPIDEMIOLOGY OF ACL INJURY AND ACLR ANTERIOR CRUCIATE LIGAMENT (ACL) ANATOMY The tibia, femur and patella bones articulate to form the patellofemoral joint (PFJ) and tibiofemoral joi nt (knee). The knee is a synovial, modified hinge joint that moves through flexion, extension, and a minimal d egree of internal and external rotation. Passively, the knee joint is supported by intra - articular ligaments (medial collateral ligament, lateral collateral ligament), extra - articular ligaments (anterior cruciate ligament and posterior cruciate ligament), and a synovial joint capsule. One of the commonly injured extra - articular ligaments in the knee is the anterior cruciate ligament (ACL). The ACL i s divided into posterior - lateral and anterior - medial bundles that originate from the medial portion of the lat eral femoral condyle and inserts on the intercondylar tibial eminence. 23 The primary role of the ACL is to resist anterior translation of the tibia, but the ligament also aids in rotary stability of the tibiofemo ral joint. 24 The ligament primarily consists of type I collagen 23 and includes various mechanoreceptors to aid in knee joint proprioception including Ruffi ni corpusc les, Pacinian corpuscles, golgi tendon organs, and free nerve endings. The meniscus consists of fibrocartilage and is located superiorly to the tibia to aid in compressive force absorption for the tibiofemoral joint. The meniscus is divided into an oval sh aped lateral meniscus and a crescent shaped medial meniscus which also attaches to the medial collateral ligament. The vascularity of the meniscus varies with the outer portion of the meniscus receiving the greatest blood flow and the inner porti on of the meniscus receiving little to no blood flow. 25 PRIMARY ACL INJURY Primary ACL injuries are the result of contact (58.8%) and non - contact mechanisms (37.9%). 26 Contact mechanisms of injury appear to be more prevalent in high school - aged individuals, 26 and non - contact mechanisms are more prevalent in individuals older than 18 14 years. 27 Overall, men susta in more ACL tears compared to females 28 due to higher number of athletic event exposures and greater participation in contact sports, but females have a hig her rate of non - contact ACL tears after controlling for ex posures. 26 Girls have slightly higher ACL injury ra tes compared boys in sex - comparable sports at the high sch ool level. 29 However, boys have a higher incidence of ACL tears after high school between the ages of 19 to 25 years , and girls have a higher preval ence during high school between the ages of 14 to 18 years. 28 In high school and collegiate level sports, the majority of ACL injuries occur during football and rugby in boys 26,27 in comparison to soccer and basketball in girls. 26 Non - modifiable risk factors of primary non - contact ACL injuries include young age (<20 years), female sex especially during the preovulatory phase, participation in sports with high levels of cutting and jumping, and narrow femoral intercondylar notch. 30 Modifiabl e risk factors of primary ACL injuries include dynamic valgus during sport specific movements, stiff landing mechanics at the hip and knee, poor lumbopelvic control, and weakness of hamstrings and hip abductors. 30 Primary injury prevention programs that focus on improv ing lower extremity strength, balance, flexibility and agility and incorporate plyometric exercise 31 may reduce the risk ACL injury by 50% in male and female youth athletes by targeting some of these modifiable risk factors. 32 Other kne e related pathologies may occur concurrently with i solated ACL tears. 28,33,34 meniscus. MCL injuries are reported to occur in 22% of individuals with ACL injuries, but this number may be under - reported. 33 Meniscal pathologies in either the medial or lateral menisci may be as high as 60% in individuals with acute ACL tears. 28 Acute ACL injuries are also consistently associated bone contusions of the tibial plateau and femoral condyles and are reported to occur in 16 to 46% of the pathological population. 34 The highest prevalence of bone 15 c ontusions occur in the lateral portion of the tibial plateau and the lateral femoral condyle. 33 This may result in early disruption of articular cartilage or subchondral bone depending on the severity of the injury . Both meniscal pathologies and bone bruises are potential risk factors for the development of poor articular cartilage health 35 and may be important confounding factor s to consider when assessing articular cartilage health. Meniscal injury involvement will be discussed MECHANISM OF NON - CONTACT ACL INJ URY Non - contact ACL injuries occur most commonly during sport specific movements including landing, cutting or deceleration tasks. 36 Due to the increasing incidence of non - contact ACL injuries, accumulating evidence suggests ACL injuries result from multiplanar injury mechanisms especially dur ing jump landing and cutting tasks. Cadaveric studies ind icate that ACL strain is greatest during 0 - 30° of knee flexion. 37 At 25° of knee flexion, anterior shear loads, abduction moments and internal rotation m oments were applied to cadaveric knees at the same time a s an axial load to simulate multiplanar landing conditions from a jump. 38 Combined forces of anterior shear loads of 268 N, internal rotation moments of 60 Nm or greater, and knee abduction moments of 75 N or great er resulted in incre ased ACL strains from baseline and from forces applied individually. 38 Knee abduction angle explained greater peak ACL strain variance (R 2 =0.45) compared to knee rotation angle (R 2 =0.32). 39 Additionally, cadaveric loading simulations with MCL tears demonstrate greater ACL strain regardless of tibial rot ation position 39 indicating that the MCL plays a role in resis ting multiplanar loads during ACL injuries. These results suggest that anterior translation, knee abduction moments and interna l rotation moments should be considered when assessing mechanisms of non - contact ACL injuries. 16 A meta - analysis assessing video o f ACL injury events during sport also supported these studies and reported that individuals demonstrated on average 16° of knee flexion with a maximum of 30° of knee flexion at the time of injury. 40 A prospe ctive biomechanical analysis of ACL injury in young females athletes reported that young girls with greater peak knee abduction m oments and vertical ground reaction force demonstrate greater risk of primary ACL injury. 41 greater dynamic knee valgu s, hip internal rotation and tibial internal or external rotation may also be a non - contact mechanism of ACL injury in women. 42,43 Expert opinion suggests that the best supported hypothesis of non - contact ACL injur y mechanism is a multiplanar mechanism of ACL injury in the sagittal, frontal and transverse planes of motion. ACLR EPIDEMIOLOGY After ACL injuries, indiv iduals may seek conservative treatment through rehabilitation or surgical intervention to regain knee joint stability. Approximately 130,000 ACL reconstructions (ACLRs) are performed annually in the United States 44 and are most commonly performed on individuals under the age of 20. 44 Since 2002, the incidence of ACLRs has increased especially in individuals 13 to 1 7 years old. 45 In this age group, isolated ACLRs, ACLRs with meniscal repairs, and ACLRs with meniscec tomies have increa se by 37%, 107%, and 63% resepectively. 45 The number of ACLRs performed in the United States has increased in bot h male and female populations. 44 Furthermore, the number of ACLRs has nearly doubled in female populations since 1994. 44 ACLR SURGICAL TECHNIQUE Autograft and allograft tendons are used to surgically reconstruct the ACL after ACL injuries. Approximately, 20 - 100% of surgeri es are completed with an autograft tendon and 0 - 17 80% of surgeries are completed using an allograft tendon . 46 Individuals under the age of 29 years are most commonly treated with autograft tendons , and individuals ol der than 40 years old are more commonly treated with allografts. 46 Hamstring tendon grafts (54%) are the most commonly used autograft tendons for primary ACLRs followed by bone - patellar tendon - bone gra fts (45%) and quadriceps tendon grafts (less than 1%). 47 However, quadriceps tendon autografts are gaining popularity for use in young females since other autograft tendon choices may be too small or disrupt epiphyseal plate gr owth in pre - adolescent females. 48 Individuals undergoing ACLR often have concomitant surgical procedures t o address additional tissue damage. Overall, meniscectomies are the most common concomitant procedure, 45 but meniscal repairs and microf racture surgeries are also consistently performed. Determining the optimal surgical treatment approach for concurrent meniscal pathologies are important to short term 49 and long - term knee health and function after ACLR . 50 Individuals undergoing ACLR and meniscectomy are 3.54 (95% CI = 2.56 - 4.91) times more likely to development knee OA compared to individuals undergoing an isolated ACLR average time to follow - up ranging between 10.7 and 24.5 years pos t - surgery. 50 The meniscus plays a primary role in knee function especially for distributing forces places on the articular cartilage in the knee during weightbearing movement. Even minimal disrupti on in meniscal tissue due to a meniscect omy, may result in altered, high - risk walking mechanics and contact between the tibia and the femur associated with the development of knee OA. 49 Alterations in meniscal tissue r esulting from injury or surgical procedure may shift articulating surface contact increasing the mechanical load during weightbearing to different regions of the articular cartilage and decreasing the mechanical load to other regions. Articular cartilage c ontact and load distribution 18 changes may disrupt the homeostasis of the joint metabolic environment progressing towards tissue degeneration. 18 Bone contusions or bruises may result in bone marrow lesions or chondral damage. Bo ne bruises are identified by greater sig nal intensity changes on MRIs representative of excessive connective tissue growth and increased vascularity. 51 Approximately 80% of patients are diagnosed with bone bruises i n individuals who sustain an ACL injury which disrupts the articular cartilage. 52 Bone bruises occur in 86% of the medial and 87% of the lateral tibial condyles with the majority occurring in the posterior regions. 53 They also occur in 86% of the medial and 94% of the lateral femoral condyles with the majority occurring in the anterior and central regions. 53 Bone bruises are a risk f actor for knee PTOA 54 and greater chondral damage is correlated with greater presence of radiographic tibiofemoral PTOA ( r =0.411) 6 years post - ACLR. 54 Bone bruises may take up to approximately a year to fully heal 55 and due to their association with PTOA should be considered a confounding factor when assessing knee articular cartilage health. SECONDARY ACL INJURY E PIDEMIOLOGY ACLR is generally successful in regaining knee joint stability following ACL injury. However, over 20% of individuals under the age of 25 will sustain a secondary ACL injury including ipsilateral graft tears and contralateral ACL tears within 1 5 years after ACLR. 56 Conversely, a meta - analysis reported that only 15% of individuals sustain secondary ACL tears within the broader population of i ndividuals 10 to 64 years old. 56 Within 2 years of surgery, individuals with a history of ACLR have a 6 tim es greater risk of sustain a secondary ACL injury in the involved or contralateral limb compared to those without a history of knee injury. 57 Approximately 75% of secondary ACL injuries occur as a result on non - contact mechanism. 58 19 RISK FACTORS FOR SECOND ACL INJURY Women with a history of ACLR are 2 times more likely to sustain a contralate ral ACL tear compared to ipsilateral graft re - tears 57 and men with a history of ACLR have a 38% higher risk of sust aining an ipsilateral graft re - tear compared to women with a history of ACLR. 59 A multisite prospective cohort study reported 59 that young, male patients with a hamstring autograft were at increased risk of sustained an involved limb ACLR revision, but y oung, female pati ents were at increased risk of sustaining contralateral limb ACL tears. 59 Overall, patient s with allografts had a significantly higher graft failure rate compared to patients with autograft tears especially from th e ages of 10 to 19 years. 60 Time since surgery was also associated with risk of secondary ACL tears. Individuals returning before 9 months post - surgery were 50% more likely to sustain a secondary ACL tear compared to individuals returning after 9 months. These fa ctors are important to consider when determining graft type and time of return to play after primary ACLR for individuals w ith different demographic backgrounds. VOLUME OF ACTIVITY IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF KNEE PATHOLOGY Quantity of mec hanical knee joint loading can be measured through volume of daily lower extremity weightbearing activity. Knee articular c artilage morphology and deformational behavior in response to mechanical load may change based on the volume of daily weightbearing a ctivity performed, 5,8 especially in populations with a history of knee joint inju ry. Physical activity (PA) which encompasses many types of daily weightbearing activity, has been assessed in healthy and pathological individuals demonstrating protective health benefits to nonco mmunicable diseases including knee OA. Lesser volumes of dai ly activity that are apparent in ACLR populations years after surgery may negatively impact the knee articular 20 cartilage morphology and deformational behavior, but these relationships have not bee n explored. This section of the literature review aims to un derstand the current state of physical activity behavior in general and pathological populations, how to measure activity (i.e. type, intensity, volume), and the relationship between activity and health outcomes. PHYSICAL ACTIVITY IN THE GENERAL POPULATI ON Recently, new national PA guidelines were published by the 2018 Physical Activity Guidelines Advisory Committee. 61 The guidelines state that youth aged 6 to 17 should participate in 60 minutes of moderate to vi gorous PA (MVPA) per day and muscle/bone strengthening exerc ises at least three days per week . 61 Adults should complete either 150 to 300 minutes of moderate intensity activity or 150 minutes of MVPA per week with 2 days of strength training. 61 psychological and physical health, less than 30% of population in the United States meet recommended guidelines for physical activity. 61 Improving PA participation at all ages is impe rative for promoting healthy lifestyles. Longitudinal evidence suggests low to moderate evidence of PA beh avior stability from childhood and adolescence to adulthood indicating that individuals that were more active in their childhood or adolescence remain ed more active into adulthood. 62 In terms of sports participation, individuals who participated in organized sport dur ing childhood were 1.75 times (95% CI = 1.11 - 2.76) more likely to report healthy living habits in adulthood including PA compared to individuals that 6 3 Individuals who participated in organized sport du ring childhood and adolescence also more frequently reported achieving recommended PA guidelines during adulthood compared to individuals who did not participate in organized sport during childhood and a dolescence. 63 This demonstrates the 21 importance of promoting PA and sports participation in children and adolescence to increase PA engagement in adulthood. BENEFITS OF PHYSICAL ACTIVITY PARTICIPATION PA participati mental health 64 - communicable diseases. A prospective study repo rte d that youth and adult populations that participate in greater volume of PA are 10% (95% CI = 83% - 98%) and 22% (95% CI = 70% - 87%) less likely to develop depression compared to youth and adult populations that participate in lesser amounts of PA. 64 Additionally, individuals who are physically inactive have a higher risk of developing coronary artery disease, type 2 diabetes, and certain types of cancer compared to individuals who are active worldwide. 65 Improving PA participation throughout an MEASURING PHYSICAL ACTIVITY Detailed evaluations of activity are best characterized using the FITT principle which considers the fr equency, intensity, time, and type of activity when quantifying PA. 61 Frequency indicates how often activity is performed, intensity describes the rate of energy expended during activity (i.e. metabolic equivalent of a task) and time quantifies the duration of activity tha t is performed. Volume of activity is an outcome des cribing the frequency and duration of activity and can be used to quantify different activity intensities such as MVPA. 61 FITT characterizations of PA are commonly measured via self - reported surveys responses or accelerom etry. Commonly utilized measurements of s elf - reporte d PA utilized in individuals with a history of ACLR include the Marx Activity Scale (Marx S cale , Appendix Marx Scale ) , the Tegner Activity Scale (TAS) 22 and the International Physical Activity Questionnaire Short Form (IPAQ - SF), and more recently acceleromet ry to measure activity in free - living conditions has also been used in this population. SELF - REPORTED TYPE AND VOLUME OF ACTIVITY THE MARX ACTIVITY SCALE The Marx Scale consists of 4 items scored on a 5 - point Likert scale (0 - 5). The questionnaire measure s how often (< 1 time per month to >3 times a week) participants participate in running, cutting, deceleration, and pivoting tasks. The Marx scale was created based on expert opinions from sports medicine health care providers and feedback from patients wi th knee injuries. 66 The test - retest reliability for the Marx scale was excellent (ICC = 0.97). The Marx scale also achieved face a nd content validity as determine d by physicians and allied health care professionals in the rehabilitation profession. 66 Construct validity of the scale was determined through asses sing the relationships to other self - reported activity. 66 The Marx scale demonstrated a moderate relationship with the TAS ( r =0.66), 66 but with MVPA assessed via accelerometry. 67 Divergent validity was established based on an inverse relationship between the Marx scale ( r = - 0.48, p =0 .002) and age according to the hypothesis that individuals will b ecome less active as they age. 66 The Marx scale is a valid and reliable scale for measuring type and volume of activity especially in individua ls with knee injuries. SELF - REPORTED TYPE OF ACTIVITY THE TEGNER ACT IVITY SCALE The TAS (Appendix Tegner Activity Scale) consists of 1 item ranging on an 11 - point Likert scale (0 - 11). This self - reported activity scale was traditionally created to be use d by physicians to measure activity changes early on after ACL surgery to when an individual may be returning to work or later on when returning to some level of recreational activities or sport. 68 A score of zero i ndicates that individual is unable to participate in activity due to knee disability, 23 scores 1 - 5 indicate progressing levels of work intensity participation, and scores 5 - 10 indicate pr ogressing levels of recreational and sport activity participation. The TAS was validated in individuals with a history of ACLR 2, 6, 9, 12 and 24 months post - surgery. 69 In ACLR populations, the TAS demonstrated acceptable test - retest reliability (Intraclass Coefficient = 0.82 95% CI=0 .66 - 0.89) and a minimal d etectable change of 1 on the Likert scale. 69 Based on pre - operative data, the TAS demonstrated acceptable construct validity indicate less than 30% of patients demonstrating floor (8% of pa rticipants reported score of 0) or ceiling effects (3% of participants reported score of 10), weak criterion validity compared to the Short Form - 12 ( =0.2, p <0.05), and acceptable construct validity on all 6 proposed constructs. 69 Between most time points, the TAS demonstrated large effect sizes except at 6 months when the TAS score demonstrated moderate effect sizes for survey responsiveness. 69 The TAS demonstrates acceptab le validity and reliability for measuring type of activity in ACL injured and ACLR populations. However, the TAS is poorly correlated to MVPA assessed via accelerometry in 67 SELF - REPORTED INTENSITY AND VOLUME OF ACTIVITY THE IPAQ - SF The IPAQ - SF (Appendix IPAQ - SF) is a self - reported 9 - item questionnaire assessing the amount of time spent in vigorous, moderate, walking and sitting activities over the course of one week. This questionnaire is a widely used scale especially in epidemiological and longitudinal studies of activity in vario us populations. The IPAQ demonstrates good test - retest reliability with an overall ICC of 0.86 (vigorous activity ICC = 0.86, moderate activity ICC = 0.71, walking activity ICC = 0.89) in adults. 70 However, test - re test reliability was poor to moderate in adolescents ranging in age from 13 to 18 (ICC = 0.10 - 0.62 ). 71 The IPAQ has been validated against various PA measures including accelerometers 72 and doubly labeled water which is 24 considered a more accurate and objective measurement of activity. 73 The IPAQ - SF demonstrated negligible to small relationships with accelerometers when assessing total physical activity and demonstrated moderate relationships when assessing walking. 72 Overall, the IPAQ - SF tends to overesti mate volume when compared to accelerometers. 72 Wh en validated against the doubly labeled water technique, th e IPAQ could only adequately distinguish those who participate in large amounts of activity from physically in active individuals. 72 The IPAQ - SF demonstrates acceptable reliability in adults, but should be used cautiously in adolescents and childr en due to its variable test - retest reliability in this popu lation. 72 Researchers using the questionnaire sho uld also be aware that it tends to over - estimate PA behavior. FREE LIVING VOLUME OF ACTIVITY ACCELEROMETRY Utilizing accelerometers to measure activity is more time consuming and requires technic al expertise by the researcher in selecting parameters chosen to enable appropriate data processing. Accelerometry can be effectively used to measure activity in free - living conditions to overcome the barriers of personal bias in self - report measures. 74 A variety of consumer - and research - grade activity monitors exist, but this portion of the literature review will focus on the research utilizing research - grade Actigraph GT3X and Link GT9X monitors based on their in clusion in the proposed study methodology. The GT3X and Link monitors are triaxial capacitive accelerometers that determine number of counts per minute based on a proprietary algorithm that accounts fo r changes in acceleration along three different axes. 75 Activity counts were traditionally based on accelerometer data obtained from the vertical axis, but more recent Actigraph monitors can assess data bas ed on vector magnitude (VM) which is the square root of the sum of squares of data from each of the three axes. Activity counts take into consideration ample, more intense movements (jogging) are 25 represented by more activity counts per minute when compared to less intense activities (walking). Selection of appropriate a ctivity monitoring methodology and processing data parameters are essential to establis h before data collection. For adults, it is recommended that accelerometer methodology include waist worn monitors sampling at 30, 60 or 90 Hz with a normal filter using an epoch length of 60 seconds for at least 10 hours per day for at least 4 days (1 wee kend day). 74,7 6 Various activity cut - point metrics based on counts per minute (cpm) can be used to assess different intensities of activity and steps completed while wearing the monitors. Two commonly utilized cut - point measurements in adults are the Freedson 1998 77 and Freedson 2011 VM bouts. 78 The Freedson 1998 cut - points utilize counts from the vertical axis to determine sedentary (0 - 100 cpm), light (101 - 1951 cpm), moderate (1952 - 5724 cpm) and vigorous (<572 4 cpm) activity intensities. 77 Freeson 2011 VM cut - points utilize counts from the VM data to determine sedentary (0 - 200 cpm), light (201 - 2690 cpm), moderate (2691 - 6166 cpm), and vi go rous (>6166 cpm) activity intensities. 78 Total activity counts (TACs) can be calculated based on total VM counts or counts from axis 1 to assess total amount of activity. TACs can be helpful in measuring total activity during the wear period while Freedson 1998 or Freed so n 2011 VM cut - points may be used to identify total activity at a specific intensity (i.e. sendentary, light, moderate, and vigorous). Although not perfect, it can help provide context about the intensity of the exercise because more intense activity resu lt s in greater activity counts. However, it is also important to normalize this measure appropriately to wear time or amount of days worn to account for different varying times in activity monitor wear time. Various studies have used different methods to d et ermine the reliability and validity of the Actigraph monitors. When worn for 4 days for at least 10 hours per day, the WGT3X 26 demonstrates acceptable test - retest reliability over 1 to 3 week periods (ICC =0.80 - 0.90) 79 It is recommended that men and women wear the activity monitors for at least 4 days to achieve 80% reliability when assessing MVPA or TACs. 79 However, seasonal variability in activity may affect longitudinal r eliability of activity wear. For example, activity is greatest during the summer months compared to the winter months. 80 As previously stated, doubly labeled water is a recommended validation test for activity ener gy expenditure, and demonstrated weak to moderate correlations with TACs and steps (TACs = 0.33 - 0.44, Steps =0.42). 73 The doubly labeled water technique can be used in free - living situations, in this case over the period of a week, to determine total energy expenditure based on the rate that carbon dioxide leaves an 73 Actigraph monitors tend to underestimate step counts at lower speeds (2.4 - 3.2 km/h) when compared to a manual step count, but demonstrate 83% sensitivity and 89.6% specificit y at identifying MVPA compared to indirect calorimetry. 81 Actigraph monitors demonstrated acceptable reliability when testing time points are one month apart, but season variability must be taken into cons ideration when assessing longitudinal time points farther apart. Validity of the devices va ries based on the validation activity to which the monitor is being compared. ACTIVITY MEASUREMENT STRENGTHS AND WEAKNESSES Self - reported and accelerometry measures of activity each have their own strengths and weaknesses. The TAS, Marx scale, and IPAQ ca n be administered quickly and do not require any technical skills or training to administer. However, the nature of the Marx scale and the IPAQ require individuals t o retrospectively determine their level of PA within the past week or year subjecting the r esults to recall bias. The TAS and Marx scale demonstrate adequate test - retest reliability 66,69 in adults over time which c an be beneficial to understand if individuals are 27 returning their pre - injury level of activity, but accelerometry measures of activity lack reliability longitudinally when utilized in free - living condition s across different seasons. 80 The Marx scale was validated in adults (>18 year old), but demonstrates good test - retest reliability in children and adolescents, but has a large ceiling effect. 82 The TAS was also only validat ed in adults (>18 year old) and demonstrate d lower comprehensibility in children and adolescents. 83 Because the IPAQ is self - reported , it of activity 72 and is not as reliable in individuals under 18 years old. 71 The Physic al Activity Questionnaire for Adolescents (PAQ - A) is a more valid and reliable method for determining physical acti vity level in high school aged adolescents. 84 The Actigraph collects data in free - living situations at all times dur ing the day, but is only worn for short periods of time (1 week) o this fact, accelerometry - based measures physical activity tends to be underestimate actual physical activity part icipation volume. Both the TAS and the Marx scale are catered towards identifying individuals that participate in sport - related activities, w hile the IPAQ and the Actigraph activity monitor assess activity regardless of whether or not its sports related. I ndividuals that participate in recreational activity or achieve the National PA guidelines without participating in sport will report low to mid - range scores on the TAS and Marx scale. For example, the M arx scale is effective for identify ing sport specific activities but may not be good at differentiating individuals that meet PA guidelines with strength and condition ing and moderate to vigorou s physical activity. Individuals who are recreationally active , jog , and strength train three times a week to meet the national PA guidelines would only be recorded as a 3 out of 16 total points on the scale. On the contrary someone who plays soccer or bas ketball would report a 16 out of 16. 85 The IPAQ and 28 the Actigraph cap , even walking , which may be considered light or moderate activity. The limitations of these activity measurements are important to take into consideration when interpreting results of studies that use them. MEASURING VOLUME AND TYPE OF ACTIVITY AFTER ACLR The definition of successful recovery after ACLR varies among individuals based on th eir individualized post - operative goals. Many individuals undergoing ACLR are physical ly active before injury and describe successful recovery as returning to sport - based PA (recreational, non - elite or competitive, elite) after surgery. Approximately 81% o f individuals return to any level of sport - based PA, 65% of individuals return to pre - injury level of sport - based PA, and only 55% of individuals return to competitive lev els of sport - based PA. 86 Return to sport - ba sed activity is often used as a surrogate measure for PA in ACLR populations. Some of the most commonly used tool s to assess self - reported return to activity in individuals with a history of ACLR include the TAS and Marx scale. However, these classificatio ns of activity cannot determine the extent to which an individual is physically active or if they meet national PA guidelines. The TAS only determines the type of activity in which individuals participate and the Marx scale determines the frequency of spor t s pecific activities, but not the intensity or volume. More recent studies 67,87 have included the self - reporte d IPAQ to help better define the quantity and quality of activity in individuals with ACLR since it all ows individuals to report amount of time spend in vigorous, moderate, and walking activity. Due to improvements in research grade technology, acceleromet ry as a means of assessing objective activity in this population has been recently explored. The IPAQ a nd objectively measured PA provide more comprehensive approaches to evaluating activity assessing frequency, intensity and duration of activity. It can a lso be used to determine if individuals are meeting the recommended national 29 PA guidelines; however app lication of these measurement techniques has been limited in the ACLR population to this point. 61 VOLUME OF ACTIVITY DIFFERENCES IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF ACLR Methods of activity assessment in ACLR populations include the TAS, the Marx, the IPAQ and acc elerometry. No differences in TAS or Marx scores were reported between the ACLR and healthy control groups within five years of ACLR. 87 Additionally, step count was weakly correlated to the TAS ( r =0.36 , p =0.04) sco re , but not Marx score ( r =0.16, p =0.27) in individuals after ACLR. However, individuals after ACLR reported decreased Tegner scores compared to healthy age and gender matched controls (ACLR=4, Control=6, p =0.001) but no differences in IPAQ scores (ACLR=156 3, Control=1893 p =not significant ) on average 20 years (range 17 - 28 years) after surgery. 88 It is important to note that correlations between the TAS, Marx, and objectively me asure activity are weak and non - signifi cant ( - 0.03 - 0.31, p range = 0.10 - 0.89 ). 67 and reality of his or her activity engagement. Individuals with and without a history of ACLR tend to overestimate their activity when asked to self - report activit y. These limitations are important to consider when interpreting the results of studies using self - reported activity. In comparison, individuals with a history of ACLR tend to demonstrate poorer accelerometr y measured activity outcomes compared to individu als without a history of knee injury, but no differences in self - reported activity outcomes. Individuals with ACLR demonstrated lesser accelerometry measured m inutes of MVPA per day (ACLR=79.37±23.95 min/day , Control=93.12±23.94 min/day , p <0.02, d = - 0.72, 9 5% CI=[ - 1.21, - 0.22]) , and steps per day (ACLR=8,158±2780 steps/day, Control=9769±980.38 steps/day , p <0.02, d =0.68, 95% CI=[ - 30 1.18, - 0.18] ) compared to age and sex matched healthy controls. 87 Individuals with a history of ACLR were also 2.36 times (95% CI=[1.09 - 5.08]) less likely to meet national physical activity guidelines compared in individuals without a history of knee injury. 89 Other recommendations include meeting 10,000 steps per day. 90 H owever, only 24% of ACLR participants met 10,000 steps per day guidelines compared to 42% healthy controls. 89 Overall, participants with ACLR spent 15 minutes of MVPA less and 1,611 steps less per day compared to h ealthy controls. 87 As stated previously, approximately 70% of the Americans demonstrate poor levels of activity. It is problematic that individuals with a history of ACLR participate in lesser activity on a weekly basis compared to individuals that are representative of the general population. Limited activity participation resulting in low quantities of knee articular cartilage loading may contribute to the mechanica l pathway of poor knee joint health and the prema ture development of knee PTOA after ACLR in addition to the already well described role it plays in the development of chronic disease and the occurrence of premature mortality. RELATIONSHIPS BETWEEN VOLUME OF ACTIVITY AND CLINICAL OUTCOMES AFTER ACLR Less er quadriceps strength 91,92 and poor self - reported knee function based on the Knee Injury and Osteoarthritis Outcome Score (KOOS) 93 are reported in individuals with a history of ACLR and knee OA. Due to the importance of these clinical outcomes in the development of knee OA, relationships between lower extremity strength, self - reported function, and activity levels have been asses sed in individuals after ACLR who are at increased risk of developing knee PTOA. Discrepancies between objective and self - reported measurements of activity may influence these relationships. In thi s population, the Marx scale was weakly correlated with KOO S stiffness and pain, KOOS symptoms, KOOS daily living and KOOS sports and recreation 31 activities (range rho =0.37 - 0.39, range p =0.03 - 0.04). The TAS was moderately correlated with KOOS sports and rec reational activities ( rho =0.42, p =0.02). 67 Objective measures of MVPA were not correlated with self - reported knee function or peak isometric and isokinetic knee extension torque 67 , but individuals with greater isotonic and isokinetic quadriceps limb symmetry strength at 3 months w ere 1.96 times (95% CI = [1.18 - 3.25]) and 1.68 times (95% CI = [1.10 - 2.56]) more likely to report greater than a 6 on the TAS approximately months after ACLR. 94 Activity in high - risk populations may play a minor ro le in poor clinical outcomes commonly measures in individuals with knee OA. Unfortunately, these relationships are inconsistent and longitudinal studies are necessary to better understand the relationship between knee function and activity after ACLR. VOLU ME OF ACTIVITY AND K NEE OSTEOARTHRITS Individuals with knee OA are less physically active compared to individuals without knee OA 95 completing on average 3,000 less steps per day. 96 However, evidence is conflicting regarding how volum e of activity contributes to the progression and development of knee OA. 97 In general, participati ng at recommended levels of MVPA, 17 as assessed by either self - rep orted or measured by accelerometer - based physical activity monitors, 98 does not increase an fect. The effects of physical inactivity and very hig h levels of activity on knee OA development are inconclusive. 17,99,100 The cohorts in these studies 17,98,100 are generally older (>45 years old) than the average age indivi duals who are included in studies evaluating outcomes in individuals after ACL injury. One sy stematic review reported that the prevalence of knee OA is highest in elite and professional runners (13.3%) and lowest in recreational runners (3.5%) with sedenta ry, nonrunners (10.2%) between those groups. 101 The majority of this data was collected from 32 cohort, cross - sectional and case - control studies and a causative relationship cannot be determined. Despite the lower qua lity evidence, this meta - analysis 101 highlights a potential relationship between volume of activity and knee OA development. Specifically, under - or over - loading of knee based on limited or excessive volumes of act ivity may impact the deformational behavior of knee articular cartilage in hig h - risk populations. BMI IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF KNEE PATHOLOGY BMI is a general health outcome that is often used as a measurement of body size which consid 102 The same equation is used for children and adolescents, but BMI values are compared to national percentiles based on age and sex. 102 Greater BMI is positively associated with various noncommunicable disease comorbidities including high blood pressure, high non - HDL cholesterol, diabetes, ischemic heart disease, and stroke. 103 Higher BMI, especially those classified as overweight or obese, is also a ris k factor for the development of clin ical, symptomatic and/or radiographic knee OA. 97 BMI is hypothesized to contribute to the development of symptomati c knee OA through greater mechanical loading, catabolic hormonal and growth factors, and genetics. 104 Greater body mass places gre ater mechanical on the knee joint du ring weight - bearing activities which may exceed knee articular cartilage structural properties over time. 104 Young obese and overw eight adults also demonstrate lesser knee flexion excursion, greater instantaneous vGRF, and are more likely to walk with heel strike transient during walking as compared to individuals with normal weight. 105,106 The differences in gait biomechanics may alter the distribution of forces on the knee articular cartilage to areas that are not accustomed to high loads . 105,106 While BMI is a cl inically applicable quantification of body 33 size, it is a limited measure because it fails to differentiate between fat mass and fat - free mass. For example, individuals with high lean muscle mass may be classified as overweight or obese, b ut have low fat ma ss. This is an important limitation to acknowledge because greater presence of adipose tissue in the body may increase the production of growth factor hormones that may negatively impact articular cartilage health. 104 BMI IN THE GENERAL POPULATION For adults, BMI less than 18.5 kg/m 2 is considered underweight, 18.5 to 24.9 kg/m 2 is considered healthy or normal weight, 25.0 to 29.9 k g/m 2 is considered overweight and greater than 30.0 kg/m 2 is considered obese. 102 For children and adolescents, BMI less than the 5 th percentile is considered underweight, 5 th to 85 th percentile is considere d normal or healthy weight, 85 th to 95 th percentile is considered overweight and greater than the 95 t h percentile is considered obese. 102 In general, BMI is moderately associated with better validated measurements of body fat including dual exergy x - ray absorptiometry. 107 However, individuals with larger muscles mass (i.e. professional athletes) are often categoriz ed as overwei ght or obese based on the BMI classifications despite normal body fat percentages. Average BMIs for young adult men and women in the United States are 27.9 kg/m 2 (20 - 29 years = 26.6 kg/m 2 ) and 28.2 kg/m 2 (20 - 29 years = 26.8 kg/m 2 ) respectively . 108 Approximately, 36% of American young adults are obese. 109 M edian BMI for adolescents is 23.6 kg/m 2 . 110 Approximately 34% of adolescents are greater than the 85 th p ercentile (ei ther overweight or obese) and of those individuals 18% are greater than the 95 th percentile (only obese) . 110 This study suggests that on average young adults between the ages of 20 - 29 years may be overweight. Due to the relationship between higher BMI and knee OA risk, BMI is a potential covariate to consider in individuals at this age. 34 BMI AFTER ACLR BMI h as n ot been adequately tracked longitudinally after ACLR so the casual relationships between ACLR and greater BMI have not been established. In general, adults with a history of ACLR have higher BMI compared to pediatric patients (Adult = 27.2 ± 0.7 kg/m 2 , P edia tric = 24.3 ± 1.1 kg/m 2 , p <0.01). 111 This trend was similar in both male and female patients. 111 One study, tracked BMI at one, three and 6 months post ACLR, but separate d low and high BMI groups a priori . 112 No significant differences in BMI were reported between time points in the low or high BMI group after ACLR indicating that average BMI in the low and high groups were relatively unchanged across time points. 112 A limitation of this study is that it fails to co nsider individual fluctuations in BMI and whether specific groups of individuals are more likely to demonstrate BMI changes after ACLR. 112 On average 20 years after ACLR, individuals ha d higher BMI compared to healthy, age - and gender - matched cont rols. 88 Individuals with a history of ACLR are at elevated risk of developing knee OA and greater BMI may act as a confounding factor that increases th e mechanical load on knee articular cartilage during movement over time. It is hypothesized that BMI may increase the forces of mechanical loading occurring at the knee joint. During a walking task, obese participants demonstrate greater vGRF compared to n ormal weight young adults suggesting greater mechanical loading. 105,106 Others hypothesize that an increase in BMI may alt er metabolic factors involved in the development of knee OA. Obese individuals demonstrate g reater odds of developing hip, knee and hand OA compared to normal weight individuals. 113 Hip and knee joints may experience greater mechanical loading during weightbearing activities regularly, but hand joints do not experience the same frequency of mechanical loading. 113 These f indings 35 suggests other factors known to increase the risk of developing OA such as metabolic factors may play a role in obese individuals. BMI A ND SELF - REPORTED KNEE FUNCTION AFTER ACLR Lower BMI was weakly and negatively associated with higher IKDC sco res ( - 0.08, p =0.04), higher TAS scores ( - 0.08, p <0.05) and younger age ( r =0.23, p <0.05) in individuals ranging from 3.9 to 301.2 months post - surger y. 114 Women with ACLR demonstrated weak associations between BMI and IKDC scores ( r = - 0.13, p =0.009), but no relationships were reported in men. 114 Individuals with patellar tend on autografts demonstrated weak associations between BMI and IKDC scores ( r = - 0.16, p <0.01), but no relationships were reported in individuals with allografts or hamstring tendon autografts. After ACLR, individuals who were underweight or normal had a 1.45 higher odds (95% CI = [1.05,1.99]) of achieving healthy normative IKDC scores compared to individuals who were overweight and obese. 114 While these results are statistically significant, these relationships ar e negative and weak indicating that individuals with higher BMI report poorer knee function. The cross - sectional approach to this study should be considered as a limitation in the interpretation of these results and highlights the need for longitudinal stu dies assessing the relationship between BMI and self - reported knee function after ACLR. One study assessed the longitudinal nature of BMI on knee pain and symptoms 2 - and 6 - years post - surgery. Higher BMI scores at time of surgery were mor e likely to repor t poorer knee function scores 6 years (OR = 0.79, 95% CI = [0.69, 0.91]) post - ACLR. 115 Between 5 to 20 years after surgery, higher BMI at the time the survey was administered is associated with poorer self - reported quality of life and greater reporting of depressive symptom. 116 These results differ in comparison to the results of Pietrosimone et al. that reported weak associations ( r = 0 .8, p = 36 0.04) between BMI and self - rep orted knee function , 114 but the longitudinal study considers the effects of BMI over time. Greater BMI over longer periods of time may progressively impact articular cartilage health and this relationship may not b e apparent in cross - sectional study designs. Physical deficits may also be associated with BMI after ACLR. Individuals who were overweight or obese one month after surgery demonstrated poorer quadriceps strength, single leg hop, and balance testing 6 month s post ACLR compared to individuals who were underweight or normal range BMI. 112 Self - reported knee disability and presence of symptoms are considered in the clinical diagnosis of symptomatic knee OA along with radiographic imaging. Self - reported knee function is persistently poorer compared to individuals without a history of knee in jury 117 and it is necessary to understand which modifiable risk factors such as BMI may relate to long - term perceived knee di sability. GAIT BIOMECHANICS IN INDIVIDUALS WITH AND WITHOUT A HISTORY OF KNEE PATHOLOGY Multiplanar knee kinematic s and kinetic s, and vertical ground reaction forces (vGRFs), differ between individuals with and without a history of ACLR during gait . 118 Knee joint kinematics are defined as angular displacements of the tibiofemoral joint without regards to forces acting on the joint. 119 Knee joint kinetics are defined as internal (i.e. muscular, ligamentous , joint capsule) and external forces (i.e. ground reaction force) acting on the joint during movement. 119 vGRF is defined as the vertical forces of the ground acting on the body. 119 This section will explore gait biomechanical differe nces in individuals with and without a history of ACLR, and how they may be important in the development of knee PTOA. Table 1 summarizes multiplanar gait biomechanical differences between the involved limb of ACLR 37 patients compared to the contralateral li mb and limb of healthy controls which will be discussed in greater detail throughout this section. 38 Table 1 . Summary of gait biomechanics in the involved limb of individuals with a history of ACLR compared to the contralateral limb and the limbs of hea lth y controls from post - op to greater than 4 years post - surgery < 6 months 6 - 12 months 1 - 2 years >4 years Knee flexion angle Greater than healthy control limb Less than contralateral limb Greater than healthy control limb Inconclusive compared to contralat eral limb - - Knee Extension Moment Lesser than healthy control limb Less than contralateral limb Lesser than healthy control limb No differences between involved and contralateral limbs - - Knee adduction angle - - Greater than healthy contr ol limb Greater than contralateral limb Lesser than healthy control limb Lesser than contralateral limb Knee adduction moment Conflictive Evidence of differences between involved contralateral and healthy control limbs Knee internal rotation ang le - - Lesser than contralateral limb Lesser than control limb - Knee internal rotation moment - - Lesser than contralateral limb Lesser than control limb - vGRF Lesser than contralateral limb Greater than contralateral limb - - Abbreviatio ns: vGRF = vertical ground reaction force 39 MULTIPLANAR KNEE BIOMECHANICS DURING GAIT AFTER ACLR Both greater and lesser knee flexion angle and knee extension moments have been associated with poor articular cartilage joint health and radiographic presence of PTOA. 16,120 - 122 Some authors speculate that greater knee extension moment and knee flexion angle during gait are indicative o f greater quadriceps force which increases tibiofemoral contact. 16 Other authors hypothesize that lesser knee flexion angle results in more anterior tibiofemoral joint contact in the medial compartments where articular cartilage is thinner resulting in greater shear and compressive forces. 122 Changes in sagittal plane knee kinematics and kinetics are inconsistent across time in individuals with a history of ACLR 118,123 and may explain why both greater or lesser sagit tal plane knee kinematics and kinetics are associated with knee PTOA. Two recent meta - analyses 118,123 reported differences in sagittal plane knee joint walking gait biomechanics between the involved limb and contr alateral limb of individuals after ACLR and the limbs of healthy controls less than 6 months after surgery. Mean peak knee flexion angles are 23 .65 in the contralateral limb, 23.85 in the healthy control limb, and range between 13.41 to 24.4 in the ACLR limb. 123 I ndi viduals with a history of ACLR have greater knee flexion angles compared to healthy controls, but lesser knee flexion angle compare to their contralateral limb before 6 - months post - ACLR. 118 After 12 months, both st udies reported lesser knee flexion angle in the involved limb of individuals with ACLR compared to the limbs of healthy controls after 12 months 118 for as long as 60 months 123 post - ACLR, but evidence of d ifferences between the involved limb and contralateral limb at this time point is conflicting. 123 Mean peak knee extension m oments are 2.74 Nm/kg in the contralateral limb, 2.73 Nm/kg in the healthy control limb, and range between 1.70 to 2.86 Nm/kg in the A CLR limb. 123 In comparison to healthy controls and the contralateral limb, individuals with ACLR demonstrate lesser knee 40 ext ension moments before 6 months and after a year post - ACLR. 118 After three years post - ACLR, individuals demonstrate no differences compared to their contralateral limb, but lesser knee extension moment compared to h ealthy controls. 118,123 Therefore, both surgery and time since surgery play a role in determining gait patterns among this population. Greater knee adduction moments during gait associated with the progression of k nee OA. 4 Increases in knee adduction moment shift the weightbearing compression forces from the lateral tibial plateau to the medial tibial plateau increasing the mechanical load placed on the medial compartme nt. 15 Slater et al. 123 reported greater knee adduction angles bet ween the involved limb of individuals with ACLR compared to thei r contralateral limb and the limb of healthy controls 11 and 20 months post - ACLR , but smaller knee adduction angles at 48 and 64 months post - ACLR between groups. Hart et al. reported no differ ences in external knee adduction moments between the involved li mb and the contralateral limb of individuals after ACLR and the limbs of healthy controls at any time point. 118 However, Slater et al. 123 suggests that individuals with ACLR report smaller external knee adduction moments compared to the involved limb of ACLR s and limbs of healthy controls at 9, 26 and 34 months post - surgery . Smaller kne e internal rotation angle changes after ACLR alter contact surfaces and shearing forces between the tibia and femur during walking. 4 During this process, articular cartilage sur face areas that have adapted to enduring large repetitive shearing forces are no t exposed to those forces as often and other articular cartilage surface areas are exposed to increased shearing forces. 4 Transverse plane knee kinematics during walking after A CLR were scarcely and inconsistently reported in the literature. 118 Results for transverse plane knee ki nematic and kinetic differences were inconsistent before one year after ACLR. 118 Knee internal - rotation angle a nd transverse plane knee moments were smaller in the involved limb of 41 individuals with ACLR compare to the contralateral limb and healthy control limb more than 2 year s after ACLR. 123 Overloading tissue with shearing forces that is unprepared to resist repetitive shearing due to smaller knee internal rotation angles may result in early degenerative articular cartilage changes. Under - or overloading the knee joint through compressive forces (i.e. vGRF or loading rate) while walking is also associated with poor knee articular cartilage health. 124 In a cross - sectional study of individuals after ACLR, vGRF was greater in the contralateral limb compared to the involved limb after controlling for time since surgery. 125 When considering time since surgery, i ndividuals with worse self - reported k nee function demonstrated lesser peak vGRF less than a year out from surgery, but greater peak vGRF more than 2 years out from surgery. 126 This study demonstrates a shift from underload ing early on post - surgery to overload ing the knee joint in individuals self - reporting greater knee disability once returning to normal activities after ACLR. Obese individuals without a history of knee injury also demonstrate altered loading characteristic s during gait. 106 Obese individuals walked with greater instantaneous loading rates compared to those with normal BMI. 106 Greater BMI may place greater mecha nical load on the knee while walk, but may also cont ribute to altered knee joint kinetics that is detrimental to knee articular cartilage health. Healthy knees respond positively to consistent loading patterns during activities of daily living. This result s in increased thickness in the articular cartilage especially in the posterior lateral compartment and the anterior medial compartment. 4 During the initiation phase of PTOA after ACLR, areas that were once used to a certain degree of shear and compressive forces of loading are now faced with greater or less er mechanical load in different articular cartilage compartments due to a biomechanical shift during various movements. 15 Greater internal joint 42 moments and knee joint angles result in greater shear forces and may shift tibiofemoral joint surface contact to areas that are ill - equipped to handle the same magnitude or repetition of shear forces, while greater vGRFs result in greater compressive forces. The opposite is true for lesser internal moments, knee joint angle s, and vGRFs. For example, greater knee abduction angle after ACLR and lesser vGRF may results in gr eater shear, but lesser compressive forces in the medial tibial and femoral compartment. It is hypothesized that areas of articular cartilage that are not c apable of adapting to new or shifting loading parameters experience superficial fibrillation and col lagen breakdown. 15 The cycle continues to negatively spiral as areas with greater fibrill ation result in increased articular surface friction and shearing causing deeper tissue degeneration. 15 Hence, movement changes after ACLR may contribute to the early development of PTOA. EPIDEMIOLOGY OF KNEE OA OA of various joints affect s approximately 1 in 7 people 127 resulting in an average annual cost of approximately $485 billion in the United States. 128 In addition to economic burden, OA results in significant disability contributing to one - thir d of days of lost work for any reported medical condition 128 and report poorer mental health and limitations in activities of daily living on a weekly basis. 2 Knee OA, specifically, is one of the most debilitating and potent ial, long - term consequences of ACL injury and reconstruction. The primary purpose of this section is to review the anatomy and development of knee joint articular cartilage. The secondary purpose of this section is to review epidemiology of knee OA in the general population and individuals with a history of ACLR. ARTICULAR CARTILAGE A NATOMY Articular cartilage of the knee, also known as hyaline cartilage, is made of fibrous connective tissue located at the distal end of the femur, proximal end of th e tibia, and posterior 43 side of the patella. The primary role of articular cartilage in the knee is transmit and distribute compressive forces and reduce friction at the articular surfaces of the knee. 129 In general, articular cartilage lacks vascul ar and neural innervation. 129 Therefore, articular cartila ge receives its nutrients from joint synovial fluid that is diffused into the cartilage as a re sult of cyclical loading such as walking. 129 Articular car tilage is primarily made of water and consists of an extracellular matrix filled with cells cal led chondrocytes, collagen fibers (mainly type II collagen) and a collection of proteins called proteoglycan (PG) aggrecans. 130 Chondrocytes produce collagen, proteoglycans, and GAGs, but also aid in cartilage resorption. 130 PG aggregans include core proteins, link proteins, 2 types of glycosaminog lycans (GAGs) called chondroitin sulfate and keratin sulfate which bind to hyaluronan chain. 130 Collagen fibers and proteoglycans are hypothesized to resist tensile and compressive forces respectively. 129 Articular cartila ge can be classified into 4 zones that are structured appropriately to optimize function. 131 The superficial zone consists of type II collagen fibers aligned parallel to the surface, some presence of proteoglycans and flat chondrocytes, and the most amount of water compared to the other zones. 129 This zone resists tensile and shearing forces which occur during knee movement as the articulating surfaces move across each other. 129 In the middle zone, collagen is randomly structured, there are more proteoglycans, and sphere - shaped chondrocytes. 129 The deep zone has collagen fibers aligned perpendicular to the surface, column shaped chondrocytes, the greatest amount of proteoglycans, and the smallest amount of water. 129 The calcified zone h as chondrocytes and is separated from the deeper vascular bone by the tide mark. 129 The deeper the zone, the better the articular cartilage is at resisti ng compressive forces. 129 Articular cartilage is the primary tissue that is compromised in individuals with a history of knee OA. 44 The bony endplate separ ates articular cartilage from subchondral bone in the tibia and femur and has neural and vascular innervations unlike the articular cartilage endplate. 132 The bony endplate and subchondral bone play a pivotal role in helping transmit co mpressive forces throughout the articular cartilage. 132 When the endplate and subchondral bone become compromised as a result of bone contusion, bone marrow lesion or natural tissue aging, the tissue may become sclerotic and demonstrate s reduced capabilities in distributing mechanical forces throughout the articular cartilage. 132 Damage to the bony endplate or subchondral after ACLR results in primary damage to the tissue which often heals within a year of injury, but also contributes to altered mechanical loading at the knee both contributing factors to the progression of knee PTOA. 133 ARTI CULAR CARTILAGE AND GROWTH Normal changes in articular cartilage growth occur as individuals age. MRI - based imaging of longitudinal articular growth over one year in adolescents reported a 0.8% increase in boys and a 1.4% increase in girls of the total tib iofemoral compartment. 134 The largest longitudinal change (<2.5% for girls and <1.5% for boys) occurred in the medial femoral condyle. 134 In comparison, adults (mean age = 30 years old) demonstrated minimal decreases (<3%) in knee articular cartilage thickness changes over on e year. 134 The effects of age on knee articular cartilage changes varies depending on maturity. The results of this study 134 are limited because determination of maturity was no t defined and there was significant heterogeneity within the adolescent population. The only maturity - based assessment completed in this study was evaluation of epiphyseal plate openness or closure. Another longitudinal study assessed changes in tibial and patellar articular cartilage thickness changes over 1 - 2 years. 135 This study utilized Tanner staging to determine sexual 45 maturity of the participants aged 9 to 18 years old. 135 Tanner staging was weakly and negatively correlated with articular cartilage changes indicating smaller changes in more sexually mature indiv iduals, with the greatest change occurring in Tanner stage 2 (avera ge age = 10 - 13 years old). 135 Medial and lateral tibial articula r cartilage decreased less than 10% in individuals defined in Tanner stage 4 and 5. This study 135 is also limited because sexual maturity does not account for bone growth changes and skeletal maturity would be the most appropriate maturity - based indicator for this study methodo logy. Longitudinal studies assessing articular cartilage in adolescent patients collectively report minimal increases in articular cartilage thickness changes over 1 - 2 years but lack appropriate identification of skeletal maturity. 135 Adolescents demonstrate slight increases in knee articular cartilage, while presumed s keletally and sexual mature adults demonstrate slight decreases in knee articular cartilage. Studies assessing knee articular cartilages in young participants should ac count for normal growth changes over time. KNEE OA The prevalence of knee OA has increa sed by nearly one - third since 2005 and is one of the leading causes of disability worldwide. 136 The prevalence of knee OA in N orth America i s 5% in females (lower uncertainty interval (UI) = 3.9%, upper UI = 4.6%) and 3.1% in males (lower UI = 2.4%, upper UI = 4.0%) 136 and the peak age of reporting knee OA is 50 years. 1 Individuals with kn ee OA spend on average over $140,000 in health care costs in their lifetime. 137 Worldwide years lived with disability as a result of knee OA has increased by over 60% in t he past decade. 1 Those with knee OA report poorer health - related quality of life, greater disability and greater pain compared to individuals without a history of knee OA. 138 Knee OA is a disease pathology characterized by measurable struct ural articular cartilage damage (radiographic OA) and illness pathology referr ing to patient - reported symptoms (symptomatic OA). 139 46 Structural or compositional damage to the joint can include the bone, cartilage, meniscus or the joint capsule, and is often di agnosed with radiographic or MRI imaging. 139 Patient - reported symptoms most often include knee pain, stiffness, and disability with activities and is diagnosed through patient reported outcomes or clinical presentation. 140 However, not all individuals with radiog raphic presence of knee OA demonstrate symptomatic presentations of knee OA and vice versa. 141 The prevalence of radiographic knee OA is greater than symptomatic knee (37.4% vs. 12.1% in adults older than 60 years old). 142 However, there is some relations hip between radiographic and symptomatic knee OA especially in individuals with more advanced forms of knee OA . 143 Individuals with symp tomatic knee OA report greater disability with activities of daily living like walking and climbing stairs. 142 The Osteoarthritis Research Society International - Food and Drug Administration ( OARSI - FDA ) I nitiation rec ommends that radiographic and symptomatic knee OA shoul d be utilized in research studies to help guide effective prevention strategies, diagnostic tools, and treatment. 139 PTOA Previous knee trauma, including ACL injury, is a significant risk factor of knee O A and this condition is commonly referred to as PTOA. I ndividuals with a history of knee injury are 4.2 times more likely to develop knee radiographic OA compared to individuals without a history of knee injury. 144 PTOA may be classified as radiographic or symptomatic similar to knee OA in the general population Prevalence of radiographic knee PTOA in eithe r the tibiofemoral or patellofemoral joint ranges betwe en 0 - 100% of the involved limb and 2 - 38% of the contralateral limb 10 to 24 years post - ACLR. 145 Prevalence of radiographic and symptomatic tibiofemoral and patellofemoral OA was 35% and 15% respectively 10 to 15 years post - ACLR. Comparisons to the prevalence of knee radiographic PTOA is much higher than the prevalence of symptomatic 47 knee PTOA which i s a similar trend of knee OA prevalence in the general population. 145 Some authors argue the relevance of identifying radiographic knee OA as opposed to symptomatic knee O A because a combination of both will be used in the diagnosis of clinical k nee OA. 145 However, radiographic knee OA will precede symptomatic knee OA and may be beneficial in identifying the earliest stages of poor articular cartilage degeneration . ACLR was once hypothesized to reduce the risk of PTOA after ACL injury, but this hypothesis has been disproven. Current best evidence indicates that surgical reconstruction does n ot protect against increased development of radiographic knee PTOA and that ACLR may actual result in increased harm and risk of PTOA compared to individuals remaining ACL - deficient (Numbers Needed to Harm = 3, 95% CI = [2, 6]; Relative Risk Increase = 44% , 95% CI = [29, 59]. 3 Previous li terature reports that 12% of individuals develop clinically diagnosed knee PTOA within 5 years of ACLR 10 and as many as 50% of individuals develop radiographic PTOA within 20 years of ACLR. 3 PT OA after ACLR is concerning due to the young age that individuals may develop PTOA. Potential risk factors of PTOA in individuals with a history of knee injury include older age, high BMI, chondral injury, and meniscal pathology. 146 Individuals receiving surgery at 35 years old demonstrate 2.44 greater odds (95% CI = [2.1 - 2.8]) of developing clinically - diagnosed PTOA 5 years post - ACLR compared to younger individuals. 10 Meniscectomy, a common developing radiographic PTOA by 3.5 times (95% CI = [2.56 - 4.91]) 10 to 25 years post - ACLR compared to individuals without a meniscec tomy. 50 Due to the early and accelerated development of PTOA in individuals with history of ACLR, health care professionals must determine was modifiable risk factors can be addressed to slow down the progression of the 48 diseases. Further more, collecting reliable information about concomitant procedures such as meniscal procedures is necessary when exploring PTOA as potential confou nders or sensitivity analyses. MEASURES OF KNEE O A PROGRESSION AND SEVERITY Radiographic imaging is used for traditional clinical diagnoses of knee - related OA by identifying osteophytes, cysts, stiffening of subchondral bone, and knee joint space narrowing. 147 While useful for ruling in conditions once they are present, radiographs capture articula r cartilage with irreversible damage during the later stages of the pathology. Other assessment techniques have been explored to assess the metabolic and histological state of the articular c s may help assess knee joint metabolism and inflammation during the early phases of OA development and progression 148 through s ynovial, blood and uri ne biomarkers . Image - and ultrasound, may also have the capabilities of identifying pre - radiographic changes in articular cartilage. Currentl y, wet biomarkers are not used to diagnose knee OA in clinical settings becau se they are expensive, time consuming to process, and require extensive training to analyze. Early identification of individuals with pre - radiographic changes in knee OA, may help health care professionals intervene through injections of targeted biologics or rehabilitation exercise to slow the progression of articular cartilage degeneration in high - risk individuals such as those with a history of knee injury. RADIOGRAPHIC KNEE ARTICULAR CARTILAGE ASSESSMENT The Kellgren and Lawrence scale is used by physic ians to diagnosis the radiographic presence and categorize the severity of knee OA. Classification of knee OA is graded on a scale of 0 to 4 based on presence of joint space narrowing, osteophyte formation, and increasing 49 stiffness of articular cartilage. 147 The Kellgren and Lawrence scale demonstrates variable inter - and intra - rater reliability ranging from moderate to good . 147,149 T he s cale is criticized because it relies heavily on osteophyte formation and joint space size to diagnose OA despite the variable presentatio ns of the condition . 147 T he scale also f ails to evaluate patellofemoral joint articular cartilage health and does not consider the severity of knee OA in relation to patient - reported symptomology disability. 147 This a ssessment technique is effective once tibiofemoral knee OA is present, but fails to consider the earliest signs of articular cartilage structural breakdow n. In order to prevent or delay the development of knee OA, wet and dry biomarkers may be more effecti ve in identifying early signs of arti cular cartilage changes. WET BIOMARKERS AND KNEE ARTICULAR CARTI LA GE ASSESSMENT Biological mechanisms 15 refer to the biochemical environment in the synovial joint that can be me asured through various inflammatory and cartilage protein biomarkers. Biological mechanism of knee PTOA may include pro longed inflammation and altered responses to mechanical stimuli which disrupt cellular metabolism homeostasis resulting articular cartila ge degeneration. 15 Biomarkers can be collected from various sources including synovial fluid within the knee joint, blood, and urine. Synovial, blood and urinary biomarkers including cartilage oligomeric matr ix pr oduct (COMP), collagen type II cleavage product (C2C), C - terminal cross - linked telopeptide of type II collagen (CTX - II) and matrix metalloproteinase 3 (MMP - 3) are elevated when greater collagen degradation occurs in the body, while elevated procollage n II C - propeptide (CPII) is elevated when greater collagen synthesis is occurring. In order to understand homeostasis between collagen breakdown and formation, ratios of C2C to CPII are often assessed. Larger ratios indicate greater collagen degradation to synt hesis. Blood biomarker interleukin 6 (IL - 6) cytokines are associated with pro - inflammatory states within in 50 the body. It is recommended that researchers use synovial biomarkers assessing cartilage degradation (i.e. COMP, C2C, CTX - II, MMP - 3) because th ey ar e the most consistently altered after ACLR. 150 In individuals with a history of ACLR, serum biomarker COMP and IL - 6 decreased from pre - operative to appr oximately one week post - operatively, but no changes were prese nt from 1 to 2 years post - operatively. 150 Similarly, urinary biomarker CTX - II decreased from 4 weeks to 4 mon ths post - operatively, but was greater compared to sex - and age - matched healthy controls up to 4 years aft er ACLR. 151 Urinary biomarker ratio C2C:CPII was not different compared to controls up to 4 years after ACLR, but synovial biomarker ratio was C2C:CPII greater more than a year out. 150 Synovial MMP - 3 has not been assess in ACLR patients, but is greater in ACLD p atients compared to healthy controls for years after surgery. 150 Overall, many biomarkers indicative of cartilage breakdown and pro - inflammatory processes are elevated afte r immediately after surgery and up to 3.5 years post - ACLR providing evidence of a catabolic knee joint environment. 150 Synovial fluid biomarkers are drawn directly fr om the joint of interest for localized assessment and prov ide a more direct measurement of joint metabolism and inflammation. However, these assessments are not commonly performed because they can be very painful and expose the patient to an elevated risk of intracapsular infection. Blood and urine biomarkers may be less invasive, but may identify other metabolic states in the body that could be interacting to elicit changes in protein content. For example, an inflammatory - based blood biomarker may be incre ased in individuals with concurrent injuries and infection s that are not related to knee articular cartilage health. Unfortunately, all of t hese approaches are expensive , require technical expertise to complete and are inconsistent among studies based on a lack of consensus of the best 51 biomarkers that identify po or articular cartilage health . 150 Assessment of dry biomarkers may overcome clinical barriers to identifying early knee articular cartilage degeneration. M RI - BASED KNEE ARTICULAR CARTILAGE ASSESSMENT MRI - based imaging provides a more direct assessment of knee articular cartilage structure as compared to blood and urinary biomarkers, and certain techniques identify early articular cartilage changes compare d to radiographic imaging. Some MRI imaging techniques assess the structure outcomes of articular cartilage including joint space width, cartilage volume, cartilage thickness, cartilage area, cartilage roughness, cartilage homogeneity, and cartilage curv a ture. 152 Joint space width (AUC=0.73) and cartilage roughness (AUC=0.80) demonstrate the best diagnostic capability of all measures to appropriately identify individuals with radiographic knee OA defined in the Kel lgren and Lawrence scale compared to healthy individuals. A composite score combining all MRI structural outcomes and wet biomarker CTX - II outcome had the best diagnostic accuracy (AUC=0.84, p <0.05). 152 MRIs are al so used to identify bone marrow lesions which are identified by increased water signal on an MRI in the tibia or femur. 51 Other MRI imaging techniques utilize and T2 relaxation time of images to assess the energy exchange between water and macromolecules (i.e. proteins) within the articular cartilage 153 and mobility water within the articular cartilage. 154 These measures have been associated with histological changes in articular cartilage as opposed to structural changes. Greater MRI - l e sser proteoglycan and greater water content in articular cartilage. 155,156 Greater T2 relaxation times are sensitive to detecting collagen content, collagen disorganization and water content in articular cartilage. 156 T1 is more se nsitive to early changes in articular cartilage structure compared to T2 52 relaxation. 157 These changes are indicative of early articular cartilage degeneration 158 before radiographic changes such as joint space narrowing and osteophytes can be identified. Multiple studies have evaluated changes over time and differences between ind ividuals with a nd without a history of ACLR of tibial and femoral T1 and T2 relaxati on from 6 months to 2 years after ACLR. According to MRI - based imaging, changes to the medial compartment may begin within the first year after ACLR and as early as three weeks. 157 Significant increases in involved limb femoral T1 and T2 relaxation times have been reported from pre - op to 6 months, 1 year and 2 years post - ACLR indicating lesser proteoglycan content and greater water content within the articular cartilage, 13,16,159 but outc omes remained the same from 6 to 12 months post - ACLR. 159 Significant differences were also reported in medial femoral T1 relaxation times between involved limb o f ACLR and healthy control limb at 1 year post ACLR. 160 Similar changes were reported in the contralateral limb of individuals after ACLR. 13 Individuals with ACLR undergoing concomitant meniscectomy and meniscal repair surgery demonstrate greater involved limb T2 relaxation ti mes compared to individuals who did not undergo meniscal surgery. 161 Fibrocartilage pathologi es are important covarying factors that may affect articular cartilage health and should be considered as confounding fac tors. Based on these overall findings, early proteoglycan and water content changes of involved limb femoral articular cartilage are ev ident after ACLR within the first 2 years of surgery and changes may also occur in the contralateral limb. However, these MRI - based imaging techniques and process are technically demanding and time consuming. While these types of images appear to be the mo st sensitive to changes, they are not clinically feasible for clinicians because image processing is time consuming and r equires extensive technical training. 53 ULTRASONOGRAPHY AND KNEE ARTICULAR CARTILAGE ASSESMENT Diagnostic US is a n accessible tool that may overcome the budget and time barriers associated with MRI assessment of knee joint cartilage health . There are 2 pop ular US assessment techniques utilized to assess femoral articular cartilage outcomes. One technique involves assessing the c ross - sectional area and thickness of resting medial, intercondylar and lateral femoral articular cartilage compartments. 9 US - based assessment resting anterior femoral articular cartilage demonstrates good agreement in medial condyle thickness (ICC=0.719), but poor agreement in lateral condyle (ICC=0.284) and intercondylar thickness (ICC=0.267) compared to macroscopic cadaver femoral articular cartilage full thickness measurements. 6 After eliminating one highly osteoarthritic knee, agreement improved to 0.883, 0.795, 0.732 in the femoral medial, lateral and intercondylar regions, respectively. 6 In general, US assessment tended to underestimate articular cartilage thickness. 6 In individuals without a history of ACLR, this t echnique demon strates strong intra - and intersession reliability between sessions at least seven days apart. 8 In individuals with a history of ACLR, US assessment of resting knee articular cartilage demonstrates st rong intra - (ICC[2,1]=0.98) and inter - session reliability (ICC[2,k]=0.95) and accept able precision. 9 Overall, these studies indicate that resting femoral articular cartilage thickness US assessment is valid and rel iable. The second diagnostic US technique assesses the change in cross - sectiona l area and thickness of the femoral articular cartilage before and after a bout of exercise such as walking. 7,8 This technique is hypot hesized to identify early changes in cartilage composition by understanding the deformational behavior of the cartilage through how it responds to loading during activity. 7,8,157,162 Reliability and validity for th is technique have not been established. 5 p <0.001) between US and MRI assessment of resting knee articular 54 cartilage thickness have previously been reported indicating that simil ar results may be demonstrated in US - based assessments. 163 US - based femoral articular cartilage thickness and cross - sectional area change from pre to post activity in the medial, lateral and intercondylar regions of healthy participants after walking, running, and drop landing indicating femoral articul ar cartilage deformation. 7,8 Medial, lateral and intercondylar femoral articular cartilage deformation is significantly greater after walking, running, and drop - landing tasks compared to a control resting condition in individuals without a history of knee injury. 7,8 Knee articular cartilage thickness deformation may better capture the ability of articular cartilage to withstand mechanical load compared to resting knee articu lar cartilage thickness becau se it represents the response of the tissue. Knee articular cartilage thickness before and after walking assessed via MRI decreased after walking. 164 Van Ginkle et al. assessed MRI - def ined morphological deformation changes before and after 30 minutes of walking in individuals with history of ACLR at time of return sport. 162 Fe moral and tibial deformational changes did not differ between particip ants with ACLR and controls, but participants with ACLR had a slower recovery of knee articular cartilage thickness after running. 162 US - based femoral articular cartilage deformation has not been assessed in indivi duals with a history of ACLR. Deformation outcomes should be used in studies exploring factors of mechanical load after ACLR such as volume of activity or gait biomechanics relat ed to knee PTOA . ULTRASONOGRAPHY OF RESTING KNEE ARTICULAR CARTILAGE IN INDIV IDUALS WITH A HISTORY OF ACLR Harkey et al. 9 explored differences in resting femoral articular cartilage thickness and cross - sectional area between individuals with a history of ACLR and healthy matched controls. 55 T his study identified initial differences in ultrasoun d based femoral articular cartilage size between ACLR patients and individuals without a history of knee injury. Involved limb knee articular cartilage had greater anterior femoral articular cartilage CS A compared to the contralateral limb ( p d =0.46) and a healthy control limb ( p d =0.50). 9 Specifically, the involved limb had greater medial condyle thickness compared to the contralatera l limb and greater medial a nd lateral condyle thickness compared to healthy controls. 9 While knee OA is defined by joint space narrowing and articular cartilage thinning, thickening of the articular cartilage may o ccur during the early phases of OA progression. 165 The authors hypothesize that greater articular cartilage thickness may be the result of swelling, increased water content or hypertrophy of articular cartilage. 9 While this may seem counterintuitive, the beginning sta ges of knee OA have been defined by increases in knee articular cartilage thickness following by articular cartilage thinning and degeneration in later stages. 166 One limitation to this study is that the populati on is heterogenous and individuals with ACLR varied in time since surgery (time since surgery range = 7 to 103 months). 9 Moderate correlations ( r =0.47, p =0.04) were reported between time since surgery and articular c artilage cross - sectional area limb symmetry in individuals with a history of ACLR i ndicate that individuals greater in time since surgery had greater femoral articular cartilage cross - sectional area in the involved limb compared to the contralateral lim b . These findings highlight a need for longitudinal studies utilizing US to assess fem oral articular cartilage size after ACLR to understand when these changes begin to occur and how femoral articular cartilage thickness and cross - sectional area vary over tim e. 56 KNEE ARTICULAR CARTILAGE HEALTH AND FACTORS OF MECHANICAL LOADING AFTER ACLR Researchers have benefitted from advancing technological assessment of knee articular cartilage to explore the progression of knee OA in individuals at high - risk of knee OA such as those with a history of ACLR. Factors which influence mechanical loading hy pothesized to contribute to the development of PTOA include volume of loading during activity, BMI, and gait biomechanics. Volume of loading assessment explains both the fre quency and intensity of loading that occurs at the knee over time. BMI considers th e amount of body mass (including fat and fat free mass) that exerts compressive force placed on the knee during all weight bearing activities. Mechanical loading during gait accounts for the shifts in knee joint contact forces as a result of injury and sur gical reconstruction. Together these factors represent a more complete picture of the loads placed on the knee that may help to understand how mechanical load contributes to the early development of knee PTOA after ACLR. VOLUME OF LOADING AND KNEE ARTICULAR CARTILAGE HEALTH AFTER ACLR Both under - and overloading volumes of activity have been associated with poor knee articular cartilage health. After ACLR, individuals go thro ugh extended periods of immobilization and partial weight bearing in additi on to restricted activity during rehabilitation. They also continue to demonstrate lesser daily PA and a lower daily step count finished rehabilitation and have been c leared for full participation. 87 A nimal and human models indicate that periods of immobilization results in acute periods of articular cartilage thinning . 165,167 Exercise c ombats the deleterious effects of immobilization and may increase articular cartilage thickness 167,168 proteoglycan content. 5 In animal models, articular carti lage thickness is not different compared 57 to controls once loading is reinitiated after periods of immobilization. 165 While these periods may be necessary for tissue healing and cannot be avoided , they may acutely weaken the articular cartilage during normal compres sion. Once patients return to progressively i ntensive weightbearing activities, the weakened articular cartilage may not be adept at handling significant volumes of load quickly potentially requiring a period of acclimatization as weightbearing loads with progressively challenging activities (i.e. wa lking, running, sprinting) are introduced throughout rehabilitation. Therefore, authors suggest that graded activity is best for promoting healthy biochemical and mechanical loading environments when integrating individuals into weight - bearing activities. 169 Graded exposure protocols based on healthy cartilage recovery used to return patients to weight - bearing activities have not been established in individuals after ACLR. New evidence suggests that there may be an optimal volume of load associated with healthy knee articular cartilage. M iddle aged individuals without history of OA longitudinally demonstrates a quadratic relationship between acc elerometry - measured and self - repor ted physical activity and T2 relaxation time of the tibiofemoral joint. 17,19 Th ese stud ies suggest that n both excessive amounts of activity and those e xhibiting disproportionate amounts of sedentary behavior are associated with greater degeneration in knee articular cartilage as opposed to those participating in moderate levels of activity. 17,19 A healthy and con sistent volume of load over time through daily activities may help protect knee articular cartilage from consequential morphological changes. After ACLR, individuals demonstrate lesser volumes of load compared to healthy sex - and aged - ma tched controls, 87 but it is unclear if this lower volume of activity falls outside of the optimal volume of loading that may promote healthy articular cartilage. 58 BMI AND KNEE ARTICULAR CARTI LAGE HEALTH While greater BMI is a risk f actor for knee OA in individuals without a history of knee injury, the predictive relationship between BMI and PTOA after ACLR is unclear. This is especially the case when understanding the relationship between BMI and early articular cartilage degeneratio n. At 6 months post - ACLR, BMI was not associated with blood biomarkers associated with proteoglycan content. 170 Conversely, greater BMI on average 3 years after ACLR was moderately correlated with blood biomarkers indicative of greater collagen breakdown to co llagen synthesis ratio, but the results were not significant. 124 Other studies report overweight and obese individual s at time of surgery are at 2 to 5 times greater odds of displaying MRI - based OA features including cartilage defects, bone marrow lesions and osteophytes in the tibiofemoral and patellofemoral joints 1 to 5 ye ars post ACLR. 12,171 BMI may be related to articular cartilage changes in later phases after ACLR compared to earlier phases because weight changes may occur as a result of an increased accumulation of loading over time. Within five years of ACLR, individuals who gained weigh t as opposed to those who maintained weight demonstrated a weight increase of 8.8 kg. 172 T his indicates that a subgroup of individuals experience significant weight changes after ACLR which may impact articular cartilage health over time. BMI may also indirectly affect to the develop ment of knee PTOA through mechanical loading. Individuals with lower BMI were more likely to participate in higher levels of PA ( i nterquartile range OR: 1.37; 95% CI: [1.04 - 1.82]) based on the Marx scale within 2 years of ACLR 85 indicating that individuals with greater BMI and ACLR may participate in less daily activity. Obesity has also been associated with changes in gait movement patterns in young adults. Obese young adults demonstrate greater instantaneous loadi ng rates, lesser knee flexion excursion, and greater incidence of impulsi ve loading compared to normal weight young 59 adults. 105,106 Impulsive loading and lesser knee flexion angle have been associated with poor arti cular cartilage health. 120,173 Therefore, the complex interaction of greater BMI and different factors of mechanical loading after ACL injury may contribute to t he development of knee PTOA. GAIT BIOMECHANICS AND KN EE ARTICULAR CARTILAGE HEALTH AFTER ACLR As with volume of activity, it is unclear if under - or overloading as expressed through gait kinetics such as knee joint moments and ground reaction forces are as sociated with the development of knee PTOA. Some of t he most commonly explored gait biomechanics include knee extension moment, knee adduction moment, and vertical ground reaction force. Tibial rotation angle, knee flexion angle, and knee abduction angle h ave also been assessed, but to a lesser extent. A cul mination of the relationships between knee gait kinetics and kinematics are reported in Table 2 . Greater internal knee extension moment resulting in greater sagittal plane knee joint shear forces was associated with lesser femoral proteoglycan content with in the first 2 years post - ACLR, 16 but lesser internal knee extension moment resulti ng in lesser shear forces was associated with lesser femoral proteoglycan content and radiograph evidence of OA 3 years post - ACLR. 121,122 Declining knee extension moment could be evi dence of quadriceps weakness over time which is a risk factor for knee OA. 174 Lesser external knee adduction moment resul ted in lesser shear forces 6 months post - ACLR was associated with metabolic changes indicative of articular cartilage degeneration, 175 but greater external knee adduction moment and angles result in greater shear f orces were associated with MRI and ultrasound - defined poor articular cartilage health 6 months to 3 years post - ACLR. 11,176 Minimal external knee adduction moment result increases after ACLR result in shifts of load ing on the knee articular cartilage resulting in greater 60 shear forces on the medial tibial plateau compared to the lateral tibial plateau. 15 Both greater and lesser peak vGR F during gait is associated with poor met abolic and imaging - based articular cartilage health due to greater and lesser compressive forces, respectively. Authors speculate that greater vGRF at initial contact or heel strike result in impulsive loading on th e knee after ACLR. 173 Any changes in gait have the potential to alter articulating joint contact to areas that are thinner and have not adapted to load changes. 177 A more reasonable explanation is that a combination of tri - planar under - and overloading during different phases of the gait cycle collectively contribute to poor quality of knee loading. 61 Table 2. Relationship between gait biomechanics and measures of knee articular cartilage health after AC LR Gait Outcomes Radiographs Wet Biomarkers MRI Imaging Ultrasound Imaging Lesser Peak vGRF (involved or LSI) Greater Peak vGRF (involved) 49.3±27.3 mo. post - ACLR 178 ratio; 37.9±29.27 mo . post - ACLR 124 - 6; 6 mo. post - ACLR 175 lateral femoral - ACLR 11 post - ACLR 121 femoral T2; 1 - 2 years post - ACLR 16 femoral medial condyle; 60±24.8 mo. post - ACLR 176 Lesser Knee Adduction Moment LSI Greater Knee Adduction Moment (involved) Greater Knee Adduction Angle (involved or LSI) - 3; 6 mo. post - ACLR 175 ratio; 6 mo. post - ACLR 175 and medial ti bial - ACLR 11 years post - ACLR 121 femoral medial condyle thickness; 60±24.8 mo. post - ACLR 176 f emoral medial condyle thickness; 60±24.8 mo. post - ACLR 176 Greater knee flexion moment (involved) - 2 years post - ACLR 16 62 Table 2. (cont d ) Lesser k nee flexion moment (involved or LSI) Greater k nee flexion angle (involved) Lesser kne e flexion angle (involved) Greater Knee Flexion Excursion (involved) KL grade >2; 5 years post - ACLR 122 Presence of symptomatic lateral compartment knee OA; 12±7 years post - ACLR 120 KL grade >2; 5 years post - ACLR 122 and femoral post - ACLR 121 T2; 1 - 2 years post - ACLR 16 femoral medial condyle; 60±24.8 mo. post - ACLR 176 femoral medial condyle; 60±24.8 mo. post - ACLR 176 Lesser Knee Internal Rotation Angle (involved) Presence of symptomatic lateral compartment knee OA; 12±7 years post - ACLR 120 Abbreviations: v GRF = vertical ground reaction force, LSI = limb symmetry index, MRI = magnetic resonance imaging, ACLR = anterior cruciate ligament reconstruction, KL = Kellgren - Lawerence Classification of Osteoarthritis, OA = osteoarthritis CONCLUSION PTOA is a common consequence of ACLR and mechanical loading may play a r ole in accelerating the disease process. Two mechanisms of mechanical loading include cumulative load on the joint during daily activities which is decreased after ACLR and altered sur face 63 contact driven through changes in gait movement patterns after ACLR . Both under - and overloading of the knee joint measured through volume of activity and gait knee joint kinetics may contribute to PTOA, but a gap exists in the literature understandin g how these mechanical factors of loading interact to impact poor articu lar cartilage health. Ultrasonography is a pre - radiographic tool to assess resting femoral articular cartilage and femoral articular cartilage response to loading that may overcome the barriers of wet and dry biomarker - based assessments. Early identificati on of articular cartilage changes and effective interventions are necessary to slow the disease progression considering articular cartilage damage is irreversible. 64 CHAPTER 3 : KNEE ULTRASOUND ASSESSMENTS OF RESTING CARTILAGE AND CARTILAGE RESPONSE TO LOAD ING IN HEALTHY PARTICIPANTS: A RELIABILITY STUDY A BSTRACT Reliable and pre - radiographic assessments of knee articular cartilage health are necessary to identify early changes in cartilage health that precede development of knee osteoarthritis . U ltrasono graphy of resting femoral cartilage structure or femoral articular cartilage response to loading provide alternatives to traditional imaging . The purpose of this study was to assess the intra - rater and test - retest reliability of resting cartilage and carti lage response to loading ultrasound assessment s . Thirty p articipants (13 Male/17 Female, age=21.8 years, gait speed=1.3 m/s) sat with knees unloaded for 30 minutes before standard ultrasound assessment was captured bilaterally by a single assessor. Next, p articipants walked 3,000 steps on a treadmill at their habitual gait speed. P ost - loading assessment s w ere captured bilaterally by the same assessor. The same procedures were repeated by the same assess or at least 72 hours later . A single blinded rater segm ented cartilage images using a semi - automated processing technique to calculate femoral articular cartilage compartmental cross - sectional area (CSA) and thickness . Deformation was calculated as the percentage difference between resting and post - loading CSA or thickness. The same blinded rater processed all images 1 month later. Intraclass correlation coefficients were used to determine intra - rater and test - re test reliability for all cartilage outcomes . All cartilage outcomes during resting and post - loading demonstrated excellent test - retest (ICC 2,k = 0 .97 - 0.99, p and intra - rater reliability (ICC 2,k = 0.99, p . All cartilage deformation outcomes demonstrated poor test - retest reliability (ICC 2,k = - 0.36 - 0.46, p - range=0.01 - 0.87) , but good to excellent intra - rater reliability (ICC 2,k =0.84 - 0.94, p . The resting cartilage assessment is reliable when capturing images across multiple visits and between image 65 processing sessions . Caution should be exercised when assessing c artilage deformation over more than one study visit using the cartilage response to lo ading assessment. Femoral cartilage response to loading may not consistently deform over t ime and future research should explore why these differences occur. 66 I NTRODUCTION Knee osteoarthritis ( OA ) is a chronic disease resulting in irreversible damage to t he synovial joint including tibiofemoral articular cartilage . This disease is the 11 th most dominant cause of disability worldwide. 1 Radiographic imaging and clinical presentation are the gold standard for diagnosi ng individuals with knee OA, 147,179 but this approach may lack the sensitivity to detect early synovial joint structural changes that may progress to symptomatic knee OA . Diagnostic ultrasonography, which is availa ble in many orthopedic clinics, is an emerging tool th at may help assess early stages of knee articular cartilage change that may precede radiographic findings indicative of knee OA. 4 Ultrasound assessment may also overcome some of the accessibility and inv asive barriers associated with other assessment types such as blood and synovial wet biomarkers 148 or T1 rho and T2 relaxation time MRI imaging 158 that are effective in ide ntifying early decline in articular cartilage metabolism or composition . Currently, resting cartilage and cartilage response to loading ultrasound assessment technique s have emerged to characterize the structure and loading response of femoral articular ca rtilage . 8,176 Changes in tibiofemoral articular cartilage structure based on conventional MRI imaging across time have been associated with radiographic indications of early development of knee OA 180 and provides the basis for exploring femoral articular cartilage structure using the resting cartilage ultrasound technique. 181 Th is technique involves assessing resting femoral articular cartilage compartmental thickness and cross - sectional area (CSA) in an unloaded condition . Pre vious research indicates the resting cartilage assessment technique has excellent intra - session (ICC = 0.98 - 0.99) and inter - session (ICC = 0.95 - 0.98) reliability for measurement of femoral articular cartilage compartmental thickness and CSA outcomes within a single unblinded, expert assessor . 8,9 Additionally, excellent inter - ra ter reliability (ICC =0.94 - 0.99) has 67 been established between unblinded novice and expert assessor s for imaging segmentation of medial, inter condylar, and lateral femoral articular cartilage compartmental thickness . 182 However, a gap exists in the literature exploring the reliability of the resting cartilage assessment technique in a blinded, novice rater. As this ultrasound technique develops, it is important to understand if th e measure is reliabl e across individuals w ith various levels of training before it can be widely adopted clinically in a variety of healthcare and research settings. Recently a novel ultrasound technique , which assess es the response of femoral articular cartilage compart ment al thickness and CSA to weightbearing loading of walking activity , has been described and applied to individuals without a history of knee injury . This response in articular cartilage size is referred to as deformation . 8 Knee articular cartilage composition may undergo compositional changes such as decreases in proteoglycan content and increases in water content during the beginning stages of knee OA development . 158 Changes in the articular cartilage extracellular matrix may alter its ability to respond to loading (i.e. compression and shear forces) during weightbearing activities. 5 With th e cartilage response to loading ultrasound te chnique, greater deformation following a period of loading indicates poor articular cartilage response to loading 183 and may indicate that cartilage composition has been compromised. 184 Therefore, this technique may provide unique information during the initial stages in knee OA developm ent that may not be apparent on more traditional forms of imaging such as radiographs . I ntra - rater and test - retest reliability of the cartilage response to loading ultrasound technique assessing femoral articular cartilage deformation after activity has no t been reported . T he purpose of was study is to assess intra - rater reliability, test - ret est reliability , and agreement of femoral articular cartilage outcomes of a blinded, novice rater using the resting cartilage and cartilage response to loading ultrasou nd assessment technique s in healthy participants . We 68 hypothesize that the both the resting cartilage and cartilage response to loading ultrasound assessment technique will demonstrate excellent intra - rater and test - retest reliability for assessing femoral articular cartilage compartmental thickness and CSA outcomes . M ETHODS T hi s observational laboratory study was approved by Michigan State U I nstitutional R eview B oard and conducted over 2 identical study sessions at least 72 hours apart. A ll part icipants provide d written informed consent at the begin ning of the first study session. parents or guardians provided written informed consent. P ARTICIPANTS AND SCREENIN G PROCESS A convenience sample of participants w as recruited through flyers, emails, and word of mouth across the faculty and students o n the university campus. After providing consent, participants were screened for inclusion criteria and complete d a gene ral health history form during the first session . Participants were included in this study if they were between 16 and 30 years old and reported no previous history of intra - articular knee injury or surgery. They w ere excluded from the study if they report ed a ny other history of lower extremity orthopedic injury in the past 6 weeks (i.e. ankle sprains, muscle strains, etc.) , rheumatoid arthritis, or any other chronic illnesses that may impede their ability to complete the tasks required of the study. As par t of the screening process, a p articipant s hydration status was assessed by providing a urine sample at both study sessions because hydration status is reported to impair articular cartilage imaging. 154 A n Atago 3730 digital refractometer (ATAGO U.S.A., Inc., Bellevue, WA) was used to measure a p articipant (USG) to det ermine if the participant was 69 adequately hydrated. Participants w ere rescheduled if their USG exceed ed 1.025 which indicates potential dehydration . 185 G AIT SPEED ASSESSMENT Participants perform ed a gait speed asse ssment along a 6 - meter track between 4 T F 100 infrared timing gates (TracTronix, Belton, MO) during the first session only . Participants were asked to walk between the timing gates and along the track at a normal walking speed for 10 trials to determine ave rage habitual gait speed (meters/second ) 8 which was converted to miles per h our and used as the treadmill gait speed in the walking protocol described later in the methods. R ESTING CARTILAGE ULTRASOUND IMAGING AS SESSMENT During both session s , participants sat in a long - sitting (i.e. legs straight on table) position to unload the knee joints for 30 minutes to minimize the effects of loading experienced prior to the assessment. 7 Sitting normalization time was determined based on a study reporting no differences in femoral articular cartilage compartmental thickness 30 minutes post - activity between individuals who walked and controls who did not walk. 8 Ultrasound images of anterior femoral articular cartilage were captured in both knees with a Vivid i Q ultrasound machine and 12 L - RS linear probe (GE Healthcare, Boston, MA). The assessor (C.L.) receiv ed in - depth ultrasound assessment trai ning from an expert assessor with 7 years of experience utilizing the same imaging technique. 7 - 9,183 The proposed imaging technique has been validated using cadaver models 6 and in comparison to MRI imaging of femoral articular cartilage structure . 186 After the 30 minute normalization time , participant s were instructed to sit with their back flat against the wall and bend their domina nt knee to 140° of knee flexion which was identified manually by a goniometer. The position of the posterior aspect of the calcaneus of the flexed knee was recorded 70 using a tape measure affixed to the table to ensure similar knee positioning post - loading a nd between sessions (Figure 2 A ) . To image the resting femoral articular cartilage structure , the ultrasound probe was placed perpendicular to the anterior surface of the femoral condyles and aligned with the most anterior aspects of the medial and lateral femoral condyles, superior to the patella (Figure 2 B) . 8 A transparency grid placed over the monitor display of the image was u sed to record the position of the medial femoral condyle, lateral femoral condyle, and i ntercondylar notch to improve reliability of knee images post exercise and between sessions. 8 A total of three images were colle cted on the dominant limb and the non - dominant legs . 71 Figure 2 . A.) Patient positio n for all phases of ultrasound assessment with back against wall and knee at 140° of flexion identified by manual goniometer. Red open circle indicates area where position of posterior aspect of calcaneus is recorded and used for all assessments; B. ) Posit ioning of ultrasound head for image capture. Red circles represent medial and lateral condylar landmarks, the red line indicates where ultrasound head is placed in between condyles 72 S TANDARDIZED LOADING PRO TOCOL After the resting ultrasound assessment, par ticipants completed 3,000 steps of walking on a treadmill at their average habitual walking speed. This volume of loading was adopted based on reporting that 3,000 steps is an optimal volume of loading to asses s femoral articular cartilage tibiofemoral CSA deformation . 187 The table was located approximately 3.75 meters from the treadmill requiring approximately five additional steps to the step count when leaving and returning to the table. Participants wore Fitbit Charge 2 monitors (Fitbit, Inc., San Francisco, CA) on their dominant wrist to track their step counts in real time with the Fitbit app. The Fitbit Charge 2 was chosen over other activity tracking technology because of its ability to track steps in real t ime via the Fitbit mobile application and Bluetooth syncing capabilities . When participants achieved 2,995 steps, they were instructed to walk five more steps and place their feet on the side rails of the treadmill so the total steps and duration of the ac tivity could be recorded. Participants returned to lab table for post - loading ultrasound imaging assessment. CARTILAGE RESPONSE TO LOADING ULTRASOUND IMAGING ASSESSMENT After engaging in the walking protocol, a total of three images were immediately collec ted on both the dominant and non - dominant legs using the same protocol reported for the ultrasound assessment of the resting femoral articular cartilage structure . F OLLOW - UP STUDY VISIT The second study visit was completed at least 72 hours after the first study visit. Identical methods were used to complete the resting ultrasound imaging assessment, the walking protocol and the post - loading ultrasound imaging assessment. Ave rage habitual gait speed and knee flexion placement based on posterior calcaneus po sition, and transparency g rid with anatomical landmarks from the first study session were used to complete the second study visit. 73 I MAGE PROCESSING Ultrasound i mages were processed with a previous defined semi - automated technique 182 using open source Image J software (National Institute of Health, Bethesda M D ) . All images were deidentified and randomized by an independent study team member to blind the rater for image processing . Total CSA , defined as the space between the oute r borders of the medial and lateral condyles in addition to the superior synovial - c artilage border and the inferior cartilage - bone border (Figure 3 A) , 7 - 9,183 was measured by blinded novice (C.L.) rater . The central point of the femoral articular cartilage was manually identified by the blinded ra ter as the middle of the synovial - cartilage border of the articular cartilage separating the medial and lateral upslopes (Figure 3 A). After identifying the total CSA and the central point of the femoral articular cartilage, the images were processed using a custom MATLAB code (Version 9.2, Mathwork s, Natik, MA) which segment ed the total femoral articular cartilage CSA into medial, intercondylar , and lateral compartments . The intercondylar segment length was defined as the middle 25% of the femoral articular cartilage extending from the manually identified central point (Figure 3 B). The medial compartment length was defined from the medial border of the intercondylar compartment to the outer border of the medial condyle and the la teral compartment length was define d from the lateral border of the intercondylar segment to the outer border . Average m edial , intercondylar and lateral thickness es (mm) normalized to segment length and CSA s (mm 2 ) were calculated for each image of the femoral articular cartilage (Figu re 3 B) . Percentage change 8 between resting femoral articular cartilage and post - loading fem oral articular cartilage w as used to determine femoral articular cartilage deformation of all outcomes (Equation 1) . 74 Greater deformation is indicated by a negative percentage change va lue . The singular rater in this study receive d training in processing the images from the expert rater who has established excellent intra - session and test - retest reliability using traditional manual processing technique . 8,9 The rater for this study and expert rater est ablished excellent inter - rater reliability for processing resting femoral articular cartilage CSA and average thickness outcomes (ICC 2, k = 0.9 94 - 0.99 7 ) using the semi - automated processing technique d escribed in this paper . 182 The same rater (C.L.) completed a second round of blinded image processing approximately 1 month from the first round of image processing in order to establish intra - rater reliability. 75 Figure 3. A.) Total c ross - sectional area of anterior femoral articular cartilage is outl ined by th e white line and the ce nter point of the articular cartilage is represented by the red diamond ; Figure B. ) Medial (orange), intercondylar (green) and lateral (blue) articular cartilage compartments representing the segmented CSA of anterior femoral articul ar cartilage ; central point = red diamond; intercondylar length of the middle segment = red line 76 STATISTICAL ANALYSIS Differences in walking characteristics including steps calculated by the Fitb it and total treadmill walking time were analyzed us ing pai red t - tests if data were normally distributed and Wilcoxon signed rank tests i f data were not normally distributed. R esting, post - loading, and deformation of femoral articular cartilage CSA and thickness for all three compartments were used for the reliabi lity analyses . S eparate intra - class correlation coefficient (ICC 2,1 ) were calculated to determine intra - session reliability of the outcomes between the 3 images captured in each knee during the first round of image processing for both visits. Separate intr a - class correlation coefficient s (ICC 2, k ) were also calculated to determine test - retest reliability and intra - rater reliability of average femoral articular cartilage outcomes from the 3 images capture in each knee between the first and second visit , and t he first and second round of rating by a single rater , respectively . ICC values are classified as poor (ICC<0.49), moderate (ICC=0.5 - 0.74), good (ICC=0.75 - 0.89) and excellent (ICC>0.9). 188 Standard error of measurement (SEM) 8,189 and minimal detectable change based on 90% confidence 8,189 (MDC 90 ) w ere also calculated to determine the precision and clinically relevant change of femoral articular cartilage outcomes for intra - rater r eliability, inter - rater reliability , and test - retest reliability. MDC 90 was used to compare to previous literature assessing standard and novel ultrasound assessments of femoral articular cartilage. 8 Bland Altman plots with 9 5 % limits of agreement were generated to analyze trends in agreement for all outcome measures betwee n testing sessions and processing rounds . 190 The mean difference and the average of each articular cartilage outcome between the testing sessions or 77 processing times were plotted on the Y axis and X ax is, respectively. Systematic t rends of overestimation or underestimation for the articular cartilage outcomes were determined using the Bland Altman plots if the majority of data poin ts are greater than or less than the mean difference, respectively. 190 Trends of overestimation or underestimation based on cart ilage size variation were also assessed. For example, - axis may be evenly - axis, but larger outcomes - axis may trend in the positive di rection. These results would indicate trends of overes timation in participants with large CSA or thickness. SAMPLE SIZE ESTIMATION An a priori sample size estimation was determined using open - source software , RStudio ( Version 1.1.453, RStudio Inc., Boston , MA ) and CRAN Package ICC . We determined a total of 21 participants would be necessary to achieve a dequate power and alpha level 0.05 ) with 2 raters (k=2) . This a priori power analysis is based on recent publications reporting good (0. 83 ) to excellent intra - rater (0.9 9 ) reliability between 2 sessions of the standard ultrasound assessment technique. 8,191 For a conservative estimate, the hypothesized ICC value was set to 0.83 based on the lowest ICC value extracted in the previous study using the standard ultrasound assessment technique , 8 and the null hypothesis ICC was se t to 0. 49 which indicates poor reliability . RESULTS P ARTICIPANT DEMOGRAPHIC, WALKING, AND CARTILAGE CHARACTERISTICS A total of 31 participants enrolled in the study, but only 30 participants were retained in our analysis. The participant removed from the analysis did not return for the second session and did not provide a reason for dropping out. Participant and study session characteristics are 78 reported in Table 3 . Step and walking time data were not normall y di stributed so Wilcoxon 2 - sample rank - sum tests were used to assess differences between testing sessions. The number of steps ( p =0.68) and time in which participants completed the treadmill walking task ( p =0.27) were not different between visit 1 and 2 ( Tabl e 4 ). Femoral articular cartilage outcomes for both visits during round 1 and round 2 of processing are reported in Tables 5 and 6 , respectively. 79 Table 3 . Participant and study session characteristics (N=30) Sex Males = 13, Female = 17 Age (years) 21.8 ± 3.8 [16, 28] a BMI for Adults(kg/m 2 ) 24.8 ± 4.4 [19.0, 33.5] a,c BMI for Adolescents (Percentile) 82 % [71 % , 90 % ] b,d Days Between Testing Sessions 6.4 ± 2.3 [3, 13] a Tegner Activity Level 7 [4, 10] b Gait Speed (m/s) 1.3 ± 0.2 [1.0, 1.1] a a = repor ted as Mean ± Standard Deviation [Minimum, Maximum]; b = reported as Median [Minimum, Maximum], c = based on N=25, d = based on N=5 80 Table 4 . Participant w alking c haracteristics (M ean ± SD [ R ange]) Walking Characteristics Visit 1 Visit 2 p - value Step Count (steps) 3008.6 ± 8.5 [2998, 3032] 3007.8 ± 7.5 [2997, 3034] 0.68 Treadmill Walking Time (min.) 28.4 ± 3.0 [25.1, 42.1] 28.4 ±2.1 [25.5, 36.3] 0.27 a = p <0.05 81 Table 5 . Femoral a rticular c artilage c haracteristics for r ound 1 i mage p rocessing of b oth v isits (Mean ± SD ) Outcome Compartment Visit 1 Visit 2 Resting (mm) Post Loading (mm) Deformation (%) Resting (mm) Post Loading (mm) Deformation (%) C ross - sectional Area Medial 34.63 ± 6.77 34.61 ± 6.83 0.12 ± 5.77 34.52 ± 6.54 34.26 ± 6.23 0.48 ± 5.66 Intercondylar 20.61 ± 4.89 20.77 ± 4.93 0.90 ± 6.32 20.49 ± 4.77 20.38 ± 4.78 - 0.34 ± 6.55 Lateral 35.45 ± 7.44 35.44 ± 7.67 - 0.04 ± 5.22 35.09 ± 7.29 35.23 ± 7.52 0.35 ± 4.91 Average Thickness Medial 2.13 ± 0.40 2.12 ± 0.40 - 0.21 ± 5.35 2.11 ± 0.39 2.09 ± 0.38 - 0.68 ± 4.96 Intercondylar 2.55 ± 0.60 2.57 ± 0.61 0.86 ± 6.30 2.54 ± 0.58 2.52 ± 0.59 - 0.35 ± 6.48 Lateral 2.08 ± 0.36 2.09 ± 0.39 0.20 ± 4.70 2.08 ± 0.36 2.08 ± 0.36 0.44 ± 4.51 82 Table 6 . Femoral a rticular c artilage c ha racteristics for r ound 2 i mage p rocessing of b oth v isits (Mean ± SD) Outcome Compartment Visit 1 Visit 2 Resting (mm) Post - Loading (mm) Deformation (%) Resting (mm) Post - Loading (mm) Deformation (%) C ross - sectional Area Medial 34.75 ± 7.16 34.51 ± 6 .87 - 0.40 ± 5.36 34.67 ± 6.69 34.30 ± 6.46 - 0.84 ± 4.90 Intercondylar 20.34 ± 4.97 20.56 ± 4.93 1.32 ± 6.19 20.17 ± 4.78 20.15 ± 4.77 0.07 ± 5.89 Lateral 35.28 ± 7.26 35.40 ± 7.74 0.18 ± 4.75 35.04 ± 7.35 35.13 ± 7.51 0.31 ± 5.28 Average Thickness Medial 2.12 ± 0.41 2.11 ± 0.40 - 0.50 ± 5.12 2.11 ± 0.39 2.09 ± 0.36 - 0.73 ± 4.77 Intercondylar 2.52 ± 0.61 2.54 ± 0.61 1.24 ± 6.19 2.50 ± 0.59 2.49 ± 0.59 - 0.09 ± 5.97 Lateral 2.08 ± 0.37 2.09 ± 0.39 0.19 ± 4.83 2.07 ± 0.37 2.08 ± 0.37 0.33 ± 4.53 83 INTRA - SESSION RELIABILITY RESULTS I ntra - session reliability for the resting and post - loading outcomes between the 3 images in each knee was excellent during visit 1 and visit 2 (ICC 2,1 range = 0.9 1 - 0.9 7 ) for all outcomes (Table 6 ). I ntra - session reliabi lity for femoral articular cartilage deformation between the 3 images in each knee was poor for visit 1 and visit 2 (ICC 2,1 range = 0.12 - 0.38) for all outcomes (Table 6 ) . TEST - RETEST RELIABILITY RESULTS Test - retest reliability between visit 1 and 2 for ave rage femoral articular cartilage compartmental thickness and CSA during resting and post - loading was excellent (ICC 2,k = 0.97 - 0.99) (Table 7 ). Test - retest reliability was poor for average femoral articular cartilage compartmental deformation (ICC 2,k = - 0.3 6 - 0.4 6) (Table 7 ). Standard error of measurement and minimal detectable change for each cartilage compartment are reported in Table 7 . Based on the Bland Altman plots, there was good agreement between visi ts and systematic trends in error or based on cart ilage size variations between visits were not noted for resting, post - loading, and deformation femoral articular cartilage CSA and thickness ( Figure 4 and 5 ). INTRA - RATER RELIABILITY RESULTS Intra - rater reliability between the 2 rounds of processing for vi sit 1 w as excellent during resting and post - loading (ICC 2,k = 0.99) (Table 9 ). For deformation, intra - rater reliability for all average femoral articular cartilage outcomes ranges from good to excellent (ICC 2,k = 0.84 - 0.94) (Table 9 ). Standard error of measu rement and minimal detectable change between processing rounds for each cartilage compartment are reported in Table 9 . Between image processing rounds 1 and 2, good agreement was noted in the Bland - Altman plots, and systematic trends based on 84 cartilage siz e variation were not noted for resting, post - loading, and deformation femoral articular cartilage CSA and thickness ( Figure 6 and 7 ) . 85 Table 7 . Intra - session r eliability (ICC 2.1 and 95% Confidence Intervals) of i ndividual f emoral a rticular c artilage i mages for a ll c ompartments Outcome Compartment Visit 1 Visit 2 Resting Post - Loadi ng Deformation Resting Post - Loading Deformation Cross - Sectional Area Medial 0.93 a [0.90 , 0.96] 0.93 a [0.90 , 0.96] 0.22 ( p =0.002) [0.07 , 0.39] 0.95 a [0.93 , 0.97] 0.93 a [0.8 9 , 0.95] 0.34 a [0.18 , 0.50] Intercondylar 0.97 a [0.96 , 0.98] 0.96 a [0.94 , 0.98] 0.26 a [0.10 , 0.43] 0.96 a [0.94 , 0.98] 0.96 a [0.93 , 0.97] 0.38 a [0.22 , 0.54] Lateral 0.93 a [0.90 , 0.96] 0.94 a [0.92 , 0.96] 0.13 ( p =0.049) [ - 0.02 , 0.30] 0.94 a [0.91 , 0.96] 0.95 a [0.92 , 0.97] 0.15 ( p =0.03) [ - 0.00 , 0.32] Average Thickness Medial 0.95 a [0.92 , 0.97] 0.95 a [ 0.92 , 0.97 ] 0.30 a [0.14 , 0.46] 0.95 a [0.93 , 0.97] 0.95 a [0.92 , 0.97] 0.22 ( p =0.003) [0.06 , 0.39] Intercondylar 0.97 a [0.96 , 0.98] 0.96 a [0.94 , 0.98] 0.23 (p=0.001) [0.08 , 0.40] 0.96 a [0.94 , 0.98] 0.95 a [0.93 , 0.97] 0.38 a [0.22 , 0.54] Lateral 0.91 a [0.87 , 0.94] 0.94 a [0.90 , 0.96] 0.12 ( p =0.064) [ - 0.03 , 0.29] 0.93 a [0.89 , 0.95] 0.94 a [0.92 , 0.96] 0.18 ( p =0.01) [0.02 , 0.35] a = intra - session reliability p ; Intra - session reliability for all resting and post - loading outcomes were significant ( p <0.05) 86 Table 8 . Test - r est r eliability (ICC 2,k and 95% Confidence Intervals ) , s tandard e rror of m easurement (SEM) and m inimal d etectable c hange (MDC) for r estin g, p ost - l oading, and d eformation f emoral a rticular c artilage o utcomes Outcome Compartment Resting (mm) Post - Loading (mm) Deformation (%) ICC SEM MDC ICC SEM MDC ICC SEM MDC Cross - Sectional Area Medial 0.97 a [0.95 , 0.98] 1.13 2.64 0.96 a [0.93 , 0.9 7] 1.11 2.58 0.27 ( p =0.12) [ - 0.23 , 0.57] 3.71 8.63 Intercondylar 0.99 a [0.98 , 0.99] 0. 48 1.12 0.97 a [0.95 , 0.98] 0. 83 1.93 0.07 ( p =0.39) [ - 0.56, 0.44] 4.47 10.40 Lateral 0.98 a [0.97 , 0.99] 1.03 2.40 0.98 a [0.96 , 0.99] 1.06 2.47 - 0.35 (p =0.87) [ - 1 .30, 0.20] 3.84 8.94 Average Thickness Medial 0.97 a [0.97 , 0.95] 0.07 0. 16 0.97 a [0.95 , 0.98] 0.0 7 0. 15 0.46 ( p =0.01) [0.09, 0.68] 3.05 7.10 Intercondylar 0.99 a [0.98 , 0.99] 0.0 6 0. 14 0.97 a [0.96 , 0.98] 0. 10 0. 24 0.03 ( p =0.45) [ - 0.62, 0.42] 4.49 10. 44 Lateral 0.98 a [0.97 , 0.99] 0.0 5 0. 12 0.98 a [096 , 0.99] 0.0 5 0. 12 - 0.36 (p=0.87) [ - 1.30, 0.20] 3.51 8.16 a = test - retest reliability p <0.001 ; test - retest reliability for all resting and post - loading outcomes were significant ( p <0.05) 87 Figure 4 . Average femoral articular cartilage CSA differences between visit 1 and visit 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between visit 1 and 2 for each outcome. The dotted lines r epresent the 95% upper and lower limits of agreement. Positive average difference s indicate that the second visit had greater cartilage CSA 88 compared to the first visit. Negative average difference s indicate that the second visit had lesser cartil age CSA compared to the first visit 89 Figure 5 . Average femoral articul ar cartilage thickness differences between visit 1 and visit 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between vis it 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement. Positive average difference s indicate that the second visit had greater cartilage thickness 90 compared to the first visit. Negative averag e difference s indicate that the second visit had lesser cart ilage thickness compared to the first visit 91 Table 9 . Intra - r ater r eliability (ICC 2,k and 95% Confidence Intervals) , s tandard e rror of m easurement (SEM) and m inimal d etectable c hange (MDC) for r est ing, p ost - l oading, and d eformation f emoral a rticular c artilage o utcomes Outcome Compartment Resting (mm) Post - Loading (mm) Deformation (%) ICC SEM MDC ICC SEM MDC ICC SEM MDC Cross - Sectional Area Medial 0.99 a [0.98 , 0.99] 0. 69 1.61 0.99 a [0.99, 1.00] 0. 68 1.57 0.87 a [ 0.79, 0.93 ] 1. 89 4.41 Intercondylar 0.99 a [0.99 , 1.00] 0. 49 1.15 0.99 a [0.99 - 1.00] 0. 49 1.15 0.94 a [0.90 - 0.96] 1.49 3.46 Lateral 0.99 a [0.98, 0.99] 0. 73 1.71 0.99 a [0.98 - 0.99] 0. 77 1.78 0.84 a [ 0.73, 0.90 ] 1. 85 4.31 Average Thickness Medial 0.99 a [0.99 , 1.00] 0. 04 0.0 9 0.99 a [0.99, 1.00] 0.0 4 0.09 0.92 a [0.86, 0.95] 1.42 3.31 Intercondylar 0.99 a [0.99, 1.00] 0.0 6 0. 14 0.99 a [0.99 - 1.00] 0.0 6 0. 14 0.94 a [0.90, 0.96] 1.48 3.45 Lateral 0.99 a [0.99, 1.00] 0.0 4 0.0 9 0.99 a [0.99 - 1.00] 0.0 4 0.0 9 0.92 a [0.86, 0.95] 1.29 3.01 a = intra - rater reliability p<0.001 ; intra - rater reliability for all resting, post - loading, and deformation outcomes were significant ( p <0.05) 92 Figure 6 . Average femoral articular cartilage CSA d ifferences between image processing round 1 and 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between image processing round 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement. Positive average difference s indicate that the 93 second round had greater cartilage CSA compared to the first round. Negative average difference s indicate that the second round had lesser cartilage CSA compa red to the first round 94 Figure 7 . Average femoral articular cartilage thickness differences between image proce ssing round 1 and 2 in A.) Resting, B.) Post - Loading, and C.) Deformation. The solid line represents the average mean difference between imag e processing round 1 and 2 for each outcome. The dotted lines represent the 95% upper and lower limits of agreement . Positive average difference s indicate that the 95 second round had greater cartilage thickness compared to the first round. Negative average difference s indicate that the second round had lesser cartilage thickness compared to the first round 96 D ISCUSSION Resting cartilage and cartilage response to loading u ltrasound assessment s of resting , post - load ing , and deformation femor al articular cartilage CSA and thickness h ave been conducted in individuals with 9,192 and without knee pathologies . 8,186,187,193,194 As research pr ogress es , assessment of a rticular cartilage structure and response to loading may provide unique and pre - radiographic assessments to characterize knee joint health , but measurement properties must be established before future application of the technique can be applied more broadl y with confidence . In the current study, e xcellent intra - rater and test - retest reliability w ere demonstrated for resting and post - loading cartilage outcomes . S imilar SEM and MDC thickness outcomes were also obser ved when compared to previous research (Tabl es 5 and 6) . 8 G ood to excellent intra - rater reliability was also established f or cartilage deformation outcomes , but poor test - retest reliability was reported . The results of this study suggest that r esting and pos t - loading ultrasound femoral articular cartilage outcomes are reliable measures with small , acceptable SEM and MDC but differences in deformation outcomes over multiple study sessions should be interpreted with caution as they are l ess reliable. This is the first study to assess post - loading and deformation femoral articular cartilage reliability using a semi - automated processing technique. The results of the current study confirm the previously reported e xcellent intra - rater reliability for resting femor al articular cartilage thickness in all compartments 182 and report the excellent test - retest reliability of resting and post - loading cartilage out comes. CSA and thickness MDCs reported in this study can be applied in pathological populations to determine clinically meaningful longitudinal resting femoral articular cartilage compartmental changes. Restin g femoral articular cartilage compartmental thi ckness MDCs between visits in this study (MDC = 0.12 - 0.16 mm) were similar to Harkey et al. (MDC = 97 0.14 - 0 .18 mm) , 8 but were larger in the intercondylar compartment when compared to previous reports (current study = 2.52 - 2.54 mm; Harkey et al. 8 =2.24 - 2.28 mm). These outcomes may vary because the semi - automated technique calculates femoral articular cartilage compartmental thickness across the compartment by normalizing compart ment CSA to compartment length instead of measur ing thickness at a sing le location . The technique utilized to measure femoral articular cartilage compartmental thickness in this study was developed based on standardized MRI assessments of tibiofemoral arti cular cartilage thickness . 195 T his technique may provide a more thorough assessment of total femoral articular cartilage compartment thickness when compared to thickness estimated at a single point in the articular cartilage . The current study also included 5 adolescent participants younger than 18 year s old . Adolescen ts have greater intercondylar cartilage thickness (average thickness range in healthy teenagers = 2.87 to 3.47 mm) , 196 which may acc ount for small differences in average cartilage thickness size when compared to adult populations (>18 years old) . N ormative resting femoral articular cartilage compartment thickness may vary by age or skeletal maturation stage 134 and should be considered as an important co - va riate in future research assessing resting femoral articular cartilage compartmental CSA or thickness. Articular cartilage d eformatio n was inconsistent between visits , which limits the ability of researchers or clinicians to determine meaningful differences in deformation over time using this technique . In the case of ultrasound assessment, intra - rater reliability assesses the ability to consistently segment total femoral articular carti lage CSA during image processing within a single study visit or session. Test - capture similar areas of the anterior cartilage by aligning the ultrasound probe consistently between 2 separate study visit s or sessions. I n this study, i ntra - rater reliability for cartilage 98 deformation was categorized as good to excellent . However, t est - retest reliability for deformation was categorized as poor . These results suggest that f emoral articular cartilage may not u niformly deform as previously reported. 8,183 Contrary to our expectations, s ome compar tments decreased in size, while others increased in size following the standardized loading protocol . Approximately 47% of limbs consistently deformed across all compartments and only 23% of participants consistently deformed in all compartments in both limbs. I n the medial compartment alone , only 52% of limbs decreased or increased thickness consistently between visits. Based on t hese findings, c aution should be exercised when comparing cartilage deformation outcomes between multiple visits or multiple assessments within the same study visits . Future research should determine i f these inconsistencies are dependent on the cartilage deformation specific to individual study participants or ultrasound assessment methodology (i.e. length of time for knee unloading before assessment or time of day of assessment). When designing trainin g for assessors, more time should be spent on post - loa ding image capturing training , and deformation reliability should be established before integration into data collection. P oor femoral articular cartilage deformation test - retest reliability was reporte d in this study . This finding may have resulted from t he use of a suboptimal loading protocol and limitations in our step tracking approach . Pfeiffer et al. 187 reported that participant s femoral articular cartilage tended to increase in size after walking 1,000, 2,00 0, 4,000 and 5,000 steps on a treadmill . However a fter 3,000 steps , 93% of the participant s demonstrated decreases on total femoral articular cartilage CSA. 187 As a result, participants were required to achieve 3,0 00 steps in the current study to achieve optimal loading for deformation. It is u nclear why this articular cartilage response occurs in healthy individuals, but the authors suggest ed that greater restin g femoral articular cartilage structure is reported as a positive tissue adaptation to adequate and 99 consistent loading. 187 In regards to standardizing the loading protocol , we used wrist - worn Fitbit accelerometers to track step counts because the device provide s real - time data syncing via protocol. Unfortunately, wrist - worn Fitbit s have moderate test - retest reliability (ICC = 0.70 - 0.73) 197 meaning the s tep counts reported in this study may have under - or overestimated the number of actual steps taken by each participant. Therefore, participants may not have achieve d optimal loading conditions resulting in deformation as defined by Pfeiffer e t al. 187 If some participants i n our study accumulated steps greater than or less than 3,000 steps while others accumulated steps closer to 3,000 , this may result in the inconsistent post - loading outcome findings . Future studies should utilize more reliable equipment to ensure that participants are loading the recommended number of steps for optimal deformation such as pedometer with capabilities to display real time feedback (i.e. OneTweak) used in a previous study . 187 As previously stated, t his was the first study to assess post - loading and deformation reliability of femoral articular cartilage outcomes among a sample that included adolescent participants . Knee OA typicall y impacts older individuals, but young individuals with a history of knee injury have an elevated risk of developing early post - traumatic knee osteoarthritis. 3 Early detection of knee OA development though pre - radi ographic assessments are essential in younger populations. While only a few adolescent participants were included in our study , a dolescent or young adult participants in our study may be skeletal immature , which may affect longitudinal assessments . 134 Implementation of gold standard skeletal matur ity assessment s require s increased exposure to radiation through radiographs or longitudinal assessments to determine peak height velocity. 198,199 We were unable to determine a participant s stage of skeletal maturity , but future 100 research should assess longitudinal differences in ultrasound femoral articular cartilage assessments in skeletally immature participants . CONCLUSION Rest ing femoral articular cartilage compartmental CSA or thickness outcomes demonstrate excellent intra - rater and test - retest reliability. The results of this study reinforce th e u tility of semi - automated ultrasound - based measurement of resting femoral articul ar cartilage compartmental CSA or thickness for assessment between multiple study visits . In comparison, femoral articular cartilage deformation outcomes are inconsistent over multiple assessments and should be used with caution . Femoral articular cartilag e may not consistently deform as previously hypothesized. Future research should assess physiological explanations for differences in cartilage response. Val id and reliable step - tracking equipment should be used when assessing cartilage response to loading to ensure optimal loading (3,000 steps) for more consistent cartilage deformation results . 101 CHAPTER 4: AVERAGE RESTING FEMORAL ARTICULAR CARTILAGE THICKNESS IN INDIVIDUALS AFTER ACLR: A LONGITUDINAL STUDY A BSTRACT Individuals with a history of anteri or cruciate ligament reconstruction ( ACLR ) are at elevated risk for accelerated development of post - traumatic knee osteoarthritis. Many biomarkers used for assessing knee joint health in individuals with a history of ACLR are not sensitive to early structu ral changes in the joint or they are impractical for use by health care provide r s . Diagnostic ultrasonography may provide a pre - radiographic assessment of knee joint health, but it is unclear if changes in articular cartilage structure can be detected over time among individuals who have recently undergone ACLR . The purpose of this l ongitudinal study was to compare femoral articular cartilage thickness via ultrasound in the involved and contralateral limb at 4 and 6 months post - ACLR. A total of 20 participa nts recover ing from ACLR ( 10 Male/10 Female, age= 21.1±5.7) completed 2 identical testing sessions at 4 - and 6 - months post - ACLR. A fter 30 minutes of participants unloading their knees , three ultrasound - based femoral articular cartilage images were captured in the involved and contralateral knees at both time point s . Average medial, intercondylar, and lateral femoral articular cartilage compartmental thicknes s es w ere determined using a semi - automated processing technique. Two - way repeated measure ANOVA s were used to compare femoral articular cartilage thickness outc omes between limbs over time. Paired t - test s were used to assess difference between groups if interactions were identified. Individuals with a history of ACLR did not demonstrate statistically signi ficant main effects for limbs ( p - range= 0.50 - 0.92) or time ( p - range= 0.22 - 0.72) , or interactions ( p range = 0.24 - 0.49) for any femoral articular cartilage compartment . Ultrasound asse ssment of femoral articular cartilage 102 thickness may not detect between limb differences or changes over time between 4 to 6 months post - ACLR. 103 I NTRODUCTION Approximately 12% of patients are clinical ly diagnos ed with post - traumatic knee osteoarthritis (PTOA) only 5 years after anterior cruciate ligament reconstruction ( ACLR ) 10 , and 35% of patients experience symptomatic PTOA within 15 years of ACLR . 145 Imaging assessment techniques , such as radiographs and magnetic resonance imaging ( MRI s) of static tibiofemoral articular cartilage are used to assess synovial joint health in individuals with a history of ACLR. 120,122 Traditionally, these imaging techniques capture resting images of the knee providing essential information about the structur e of the synovial joint especially the articular car tilage (i.e. presence of osteophytes , joint space narrowing ) , and are helpful for diagnosing PTOA after degeneration has occurr ed. How ever, traditional imaging assessments may not be able to identify early onset synovial joint changes before permanent tissue damage has occurred . Despite the inability of traditional imaging assessments to detect early articular cartilage changes, other b iomarkers may assess early metabolic, compositional , and structural cartilage changes that may precede the irreversible degenerative tissue damage . Serum, synovial, or urinary biomarkers indicative of cartilage breakdown and pro - inflammatory process es are elevated within the first few weeks or months post - ACLR providing evidence of a catabolic knee joint environment. 150 Furthermore, research grade MRI outcomes ( ) associated with diminished proteoglycan s and increased water content of tibiofemoral articular cartilage are present in the medial compartment of the involved limb of individuals with a history of ACLR compared to their contra lateral limb within the first 6 months to a year after surgery . 13,160 Unfortunately, these tools are impractical and difficult to use to perform longitudinal assessments . Diagnostic ultrasonography is an emerging, pre - radiographic assessment of knee 104 articular cartilage health found in most orthopedic clinics that may overcome some of the barriers associated with assessing the other biomarkers. 148,200 A recent cross - sectiona l study reported differences in involved limb femoral articular cartilage thickness compared to the contralatera l limb in individuals approximately 3 years post - ACLR providing preliminary evidence for the use of this technique. 9 However, it remains unclear how early after ACLR ultrasound assessments can detect between limb differences. Declines in tibiofemoral joint health are present during the initial phases of recovery after surgery 13,150,160 and may be impacted by changes in mechanical loading through weight - bearing activities. 11,124,201 Time points 4 and 6 months post - ACLR mark milestones in the recovery process when health care professionals often recommend distinct changes in patient activity that may increase mechanical loading occurring at the knee. 202 At 4 months post - ACLR, approximately 75% of physical therapists repor t returning patients to jogging and 50% report returning patients to modifi ed sports activity. 14 At 6 months post - ACLR, healthcare professionals often begin to make decisions regarding participation in unrestricted activity and many patients cease rehabilitative care. 14 Changes in activity around these time points may increase mechanical loading occurring at the knee and impact knee articular cartilage health. However, a gap exists in the literature understanding changes in knee articular cartilage structure during thes e periods of increased mechanical loading. There is a critical need to determine if this promis ing ultrasound assessment of femoral articular cartilage thickness can detect changes during this period of increased mechanical loading after ACLR. T he purpose of this longitudinal study is to compare changes in medial intercondylar, and lateral femoral a rticular cartilage compartmental thickness between the involved limb and contralateral limb of individuals recovering from ACLR at 4 - and 6 - months post - surgery . We 105 hypothesize that individuals will demonstrate greater medial femoral articular cartilage com partmental thickness in the involved limb compared to the contralateral limb at both time points post - ACLR. Secondly, w e hypothesize that individuals will demonstrate greater medial femoral articular cartilage compartmental thickness in the involved limb a t 6 months post - ACLR compared to 4 months post - ACLR. Medial, intercondylar, and lateral femoral articular cartilage compartmental thickness will not differ in the contralateral limb between 4 and 6 months post - ACLR. M ETHODS This longitudinal study was co mpleted over 2 testing sessions assessing femoral articular cartilage structure via ultrasound in individuals with a history of ACLR at 4 - months (± 2 weeks) and 6 - months (± 2 weeks) pos t - surgery. P ARTICIPANTS Participants were recruited by 4 fellowship tr ained orthopedic surgeons at the Michigan State University sports medicine clinic and on Michigan State U niversity campus via flyers, emails and word of mouth. Participants between the ages of 16 and 3 5 years old with a history of primary, unilateral ACLR and full knee range of motion were included in the study. Participants were excluded from the study if th ey had a previous history of intra - articular knee injury not related to the current ACL injury (i.e. meniscal pathology, articular cartilage pathology, previous ACL injury ) or rheumatoid arthritis. P articipant s were not excluded if they had other surgical procedures (meniscal, articular cartilage or MCL related surgical procedures) completed at the time of ACLR . 106 SAMPLE SIZE ESTIMATION A priori sample s ize estimations were completed using G*Power ( Version 3.1.9.2 , Henrich Heine Universität Düsseldorf, Brunsbuttel, Germany) assuming an alpha level of 0.05 and a statistical power of 0.80. The sample size estimation was based on a large effect ( d =0. 79 ) indicating differences in ultrasound - defined resting medi al f emoral articular cartilage compartmental thickness between the injured limb of individuals on average 3 years post - ACLR and healthy controls which indicated that a minimum of 1 2 participants would be required to detect differences in femoral articular cartilage size over time within this population . 9 PARTICIPANT SCREENING PROCESS ew Board and all participants provided written informed consent before engaging in study activities. Participants under the age of 18 provided informed assent and their parents or guardians provided informed consent prior to engaging in any study related procedures. Previous research suggests that dehydration may negatively impact articular cartilage imaging. 154 Following the consent process, p articipants were required to provide a urine sample to assess (USG) was assessed via an Atago 3730 digital refractometer (ATAGO U.S.A., Inc., Bellevue, WA) . P articipant s w ere considered dehydrated if their USG was greater than 1.025. 185 P articipants who were dehydrated were rescheduled to another day to eliminate hydration status as a conf ounding factor. R ESTING CARTILAGE U LT RASOUND IMAGING ASSESSMENT After providing written informed consent, p articipants sat in a long - seated position (i.e. knees supported by table in extended position ) for 30 minutes to minimize the effects of knee joint loading experienced during activities of daily living prior to the assessment. 7,8 Ultrasound 107 images of anterior femoral articular cartilage were captured in both knees with a Vivid i Q ultrasound machine and 12 L - RS linear probe (GE Healthcare, Boston, MA) with a valid 6,186 and reliable 7 - 9,183 assessment technique . After unloading , participant s were instructed to sit with their back s flat against the wall and bend their contralateral knee to 140° of knee flexion as determined by a manual goniometer . The position of the posterior aspect of the calcaneus of the flexed knee was recorded using a tape meas ure affixed to the table to ensure similar knee positioning between the 4 - month and 6 - month session (Figure 2 A & 2 B ). To image the resting femoral articular cartilage, the ultrasound probe was placed perpendicular to the anterior surface of the femoral condyles and aligned with the most ant erior aspects of the medial and lateral femoral condyles, superior to the patella. 8 A transparency grid placed over the monitor display of the image was used to record the position of the medial femoral condyle, la teral femoral condyle, a nd intercondylar notch to improve reliability of knee images post exercise and between sessions. 8 A total of three images were colle cted on the contralateral limb followed by the involved li mb of participants recovering from ACLR. 6 M ONTH POST - ACLR ASSESSMENT The h ydration screening process and resting ultrasound imaging assessment described at the 4 - month assessment were repeated at the 6 - month assessment. IMAGE PROCESSING Ultrasound i mages were processed using open source Image J software (National Institute of Health, Bethesda , MD). R esting femoral articular cartilage images were randomized by a study team member and processed by 1 blinded rater (C.L.) using built - in measurement tools. Tot al femoral articular cartilage cross - sectional area ( CSA ) was measured as the space between the outer borders of the medial and lateral condyles in addition to the superior synovial - 108 cartilage border and the inferior cartilage - bone border (Figure 3 A). 7 - 9,183 The central point of the femoral articular cartilage was manually identified by the rater as the middle of the synovial - cartilage border of the articular cartilage separating the medial and lateral upslopes (Figu re 3 A ). After identifying the total CSA and the central point of the femoral articular cartilage, the images were processed through a custom MATLAB code (Version 9.2, Mathworks, Natik, MA) to segment the total femoral articular cartilage CSA into medial, i ntercondylar , and lateral compartments . The intercondylar compartment length was define d as the middle 25% of the femoral articular cartilage extending from the manually identified central point (Figure 3 B). The medial compartment length was defined from t he medial border of the intercondylar compartment to the outer border of the medial condyle and the lateral compartment length was defined from the lateral border of the intercondylar compartment to the outer border. M edial , intercondylar, and lateral f emo ral articular cartilage compartmental CSA (mm 2 ) w ere normalized to individual compartment length to calculat e femoral articular compartmental thickness (mm) for each image (Figure 3 B). S TATISTICAL ANALYSIS Participant characteristic differences between 4 - and 6 - month s post - ACLR were assessed with paired t - test. A previous study reported no differences of resting medial and lateral femoral articular cartilage compartmental thickness between the contralateral limb of individuals with a history of ACLR compa red to dominant limb of healthy controls. 9 Therefore, the contralateral limb of the participants with ACLR was us ed as a control limb for statistical analysis in this study. Main effects and interactions for betwee n limb (involved limb and contralateral limb) and time ( 4 and 6 months post - ACLR) differences were assessed using a 2 - way repeated measure analysis of variance ( ANOVA ) . S ignificant interactions were further investigated using a paired 109 sample t - test to iden tify differences between limbs at each time point and within limbs across time . Alpha was set to 0.05 a priori . RESULT S A total of 20 participants ( 10 Male/10 Female, age range = 16 - 33 years old , 10 h amstring graft/9 b one - patellar tendon - bone grafts/1 Allo graft ) participated in this longitudinal study and 100% of participants completed both visit assessments (days between t esting sessions = 5 8 . 5 ± 10. 4 ). There were no differences in BMI of adult participants ( p =0.20) and BMI percentile of adolescent partici pants ( p =0.68) between 4 - and 6 - months post - ACLR (Table 10 ) . There was a significant difference in months since surgery ( p <0.001 ) between the 4 - and 6 - month visit. There were no significant limb main effects, time main time effects, or interactions for an y of the femoral articular cartilage thickness outcomes (Table 11 ). Average between limb and time femoral articular cartilage compartmental thickness differences for all compartments a re reported in T able 12 . M edial, intercondylar, and lateral femoral arti cular cartilage compartmental thickness in the involved and contralateral limbs over time are presented in Figure 8 . M edial, intercondylar and lateral femora l articular cartilage compartmental thickness between limbs at each individual time point are repre sented by boxplots in Figure 9 . 110 Table 10 . Participant and study session characteristics ( Mean ± Standard Deviation ) 4 Month Visit 6 Month Visit p - value BMI for Adults (kg/m 2 , N=10 ) 28.6 [ 20.8 , 39.6 ] 28.9 [ 21.3 , 40.1 ] 0.20 BMI for Adolescents (Per centile , N=10 ) 70.5% [ 21.0% , 83.0% ] 71.5% [ 23.0% , 82.0% ] 0.68 Months Since Surgery (N=20) 4.0 ± 0.3 [ 3.5 , 4.4 ] a 6.1 ± 0.3 [ 5.6 , 6.6 ] a <0.001 a a = p <0.05 111 Table 11 . R esting f emoral a rticular c artilage c ompartmental t hickness (mm) at 4 - and 6 - m onths p os t - ACLR (Mean ± SD ) Compartment 4 Month Visit 6 Month Visit Main Limb Effect p - values Main Time Effect p - values Interaction Effect p - values Involved Limb Contralateral limb Involved Limb Contralateral limb Medial Thickness (mm) 2.04 ± 0.5 9 2.01 ± 0. 39 2. 1 3 ± 0. 5 6 2.0 5 ± 0.37 0. 50 0.2 2 0.4 4 Intercondylar Thickness (mm) 2.5 3 ± 0 .5 2 2.5 4 ± 0. 4 8 2. 61 ± 0. 78 2.5 2 ± 0.4 4 0.7 8 0. 72 0. 49 Lateral Thickness (mm) 2.0 5 ± 0. 29 2.0 8 ± 0.3 2 2. 12 ± 0.3 5 2.0 7 ± 0.3 2 0.9 2 0. 30 0. 24 All main limb and time ef fects, and interactions are not significant ( p >0. 05 ) 112 Table 12 . Resting f emoral a rticular c artilage c ompartmental t hickness (mm) d ifferences b etween l imbs and o ver t ime (Mean ± Standard Deviation) Compartment Between Limb Differences Between Time Differ ences 4 Months 6 Months Involved Limb Contralateral Limb Medial Thickness (mm) 0.02 ± 0.32 0.09 ± 0.47 0.09 ± 0.41 0.03 ± 0.15 Intercondylar Thickness (mm) - 0.01 ± 0.49 0.09 ± 0.84 0.08 ± 0.60 - 0.02 ± 0.17 Lateral Thickness (mm) - 0.03 ± 0.35 0.05 ± 0.37 0.07 ± 0.27 - 0.01 ± 0.07 Larger values indicate greater involved limb cartilage outcomes compared to the contralateral limb or greater cartilage outcome at 6 months compared to 4 months 113 Figure 8 . M edial (A), intercondylar (B), and lateral (C) femoral articular cartilage compartmental thickness in the involved and contralateral limbs from 4 to 6 months post ACLR. Green circles and the solid lines represent the contralateral limb and blac k circles and dotted lines represent the involved limb 114 F igure 9 . M edial (A & B), intercondylar (C & D), and lateral (E & F) femoral articular cartilage thickness between limbs at 4 - and 6 - months post - ACLR. The bottom and top of the vertical line represent the minimum and maximum of the cartilage thickness outc ome. The bottom and top of the box indicate the firs t and third quartile of the cartilage thickness outcomes. The line 115 represents the average cartilage thickness outcome. Green boxes represent the involved limb and gray boxes represent t he contralateral lim b 116 DISCUSSION Individuals with a history of ACLR have a 4 - 6 times elevated risk of developing radiographic presence of knee joint structural changes indicative of osteoarthritis more than 10 years after surgery compared to individuals wi thout a history of knee injury. 203 B iochemical and histological knee joint changes may be present within the first 6 months post - ACLR, 13,150 but i t is unclear if structural changes in the knee joint articular cartilage are present while individuals remain in rehabilitat ive care . Accessible u ltrasoun d machines that can perform assessment s of resting femoral articular cartilage thickness may help identify structural changes or limb differences in individuals with a history of ACLR . 9,148 In this longitudinal study , p articipants recovering from ACLR did not demonstrate significant difference in cartilage thickness between limbs or over ti me within 6 months of surgery. The results of our study suggest that 1.) structural cartilage thickness changes or limb differences may not occur 4 to 6 months after ACLR, 2.) 2 months between longitudinal ultrasound assessment may not be a long enough tim e to capture meaningful structu ral changes in femoral articular cartilage , and 3.) ultrasound assessment may not be a sensitive measure to detect articular cartilage changes or limb differences before 6 months post - ACLR while patients remain in rehabilitat ive care. We hypothesized that the involved limb would demonstrate greater between limb differences compared to the contralateral limb at 6 - months post - ACLR based on an extensive body of literature indicating that individuals consistently experience risk f actors for knee PTOA. The development of PTOA involves complex relationships between metabolic, mechanical, structural pathways . 18 Overall, many wet biomarkers (i.e. serum, urinary, synovial fluid) indicative of cartilage breakdown and pro - inflammatory process es are elevated within the first few months post - ACLR providing evidence of a catabolic knee joint environment. 150 117 Additionally, patients demonstrated altered knee joint bi omechanics 123,204 during the first 4 to 6 months post - ACLR indicating mechanical changes. Therefore, m etabolic and mechanical pathways precede pre - radiographic, structural changes in the knee within the first 6 month s post - ACLR . 148 T he presence asymmetrical knee joint loading, and or elevated pro - inflammatory and cartilage breakdown are mechanical and metabolic biomarkers that may identify individuals who are at risk for develop PTOA earlier than ultrasound post - ACLR . A systematic review suggests that structural changes of the tibiofemoral articular cartilage (i.e. changes in cartilage thickness) are n ot detectable on MRI imaging until 2 years after surgery. 157 The results of the current study suggest that the same may be true of resting ultrasound imaging assessment of femoral articular cartilage thickness withi n the first 6 months post - ACLR . Other MRI imaging associated with articular cartilage compositional changes 155,156 as opposed to structural changes, indicate that femoral articular cartilage proteoglycans may decrease and water content may i ncrease between pre - operative to 6 months post - ACLR, but not 6 months to 12 months after surge ry. 13,16,159 In further support of these conclusions, a cross - sectional study assessing ultrasound measures of femoral articular cartilage in individuals, on average 3 years post - ACLR, reported greater involved lim b medial thickness compared to the contralateral limb. 9 Ultrasound may not be a clinically relevant tool to identify between limb femoral articular cartilage structural differences or changes over time during the f irst 6 months after ACLR . However based on evidence from the ultrasound - based cross - sectional study, 9 future research should utilize longer longitudinal assessments post - ACLR (i.e. 2 - 3 years) to determine when stru ctural changes in femoral articular cartilage are apparent with accessible ultrasoun d assessments and the length of time necessary to detect changes between assessments. Diagnostic u ltraso und machines are accessible at most orthopedic clinics and hospitals and can 118 be used in examination rooms without referral to outside facilities for other imaging modalities . Any type of u ltrasound assessment reduce s exposure to radiation when compared to traditional, diagnostic radiographic imaging, and are easier to use to comple te longitudinal assessments compared to MRIs. While the standard ultrasound assessment technique involves extensive image processing time by the assessor , it still provides a promising valid and reliable image screening of femoral articular cartil age struc ture that should be continued to be researched for appropriate clinical application. A limitation of this study is that the presence of concomitant meniscal surgical procedures or articular cartilage pathologies at time of ACLR surgery were not controlled for despite the fact that they may impact s ynovial knee joint health . Individuals undergoing ACLRs and meniscectomies are 3.54 (95% Confidence Interval = 2.56 - 4.91) more likely to develop knee OA compared to ind ividuals undergoing an isolated AC LR 50 and damage to the articular cartilage such as bone contusions are also associated with the development of tibiofemoral OA after ACLR. 54 Surgical information could only be extracted from 80% (n=16) of participants in this study, but a ppro ximately 56% (n=9) of participants also received a meniscectomy or meniscal repair surgery at the time of ACLR. We were unable to determine who had articular cartilage damage at the time of injury, but previou s study report a s many as 80% of participants s uffer a bone contusion along with the ACL injury. Meniscal and articular pathologies impact a large percentage of patient and could not be controlled for in our analyses. While our sample size was appropriatel y powered to assess differences between limbs a nd across time points in this population, a larger sample size may be necessary to determine the effects of meniscal surgical procedures or articular cartilage pathologies impact resting femoral articular cart ilage thickness. 119 CONCLUSION Individuals within 6 months of ACLR did not demonstrate significant differences in femoral articular cartilage outcomes assessed with ultrasonography between limbs or from 4 to 6 months post - ACLR. Ultrasound may not be a clinic ally relevant assessment tool to identify early synovial joint changes during a time when individuals recovering from ACLR consistently undergo rehabilitative care. Future longitudinal research studies should assess when u ltrasound assessment of resting ar ticular cartilage compartment al thickness can detect between limb differences or changes over time post - ACLR . 120 CHAPTER 5: CUMULATIVE KNEE JOINT LOADING AND FEMORAL ARTICULAR CARTILAGE THICKNESS 4 TO 6 MONTHS POST - ACLR A BSTRACT Altered mechanical knee j oint loading is a contributor to the development of knee post - traumatic osteoarthritis in individuals with a history of anterior cruciate ligament reconstruction ( ACLR ) . Poor knee joint health after ACLR is associated with g reater knee abduction moment, le sser knee extensio n moment, and lesser vertical ground reaction force (vGRF) during the first 50% of stance phase, but a gap exists in the literature understanding its relationship to cumulative knee joint loading (steps/day). The purpose of this longitudi nal study was to a ssess the associations among gait biomechanics , steps per day at 4 months post - ACLR , a nd involved limb femoral articular cartilage compartmental thickness at 6 months post - ACLR. A total of 19 participants ( 9 Male/10 Female, age range=16 - 3 3 years old) recovering from ACLR complet ed mechanical knee joint loading assessment at 4 months and ultrasound assessment of involved limb femoral articular cartilage imaging at 6 months. P were capture d with 3D motion captur e and 2 embedded force plates while walking at a self - selected pace. Participants also wore a physical activity monitor on their hip for seven days during all waking hours to measure steps per day. F emoral articular cartilage images were capture d in partic knee. A semi - automated segmentation processing technique was used to measure medial femoral articular cartilage thickness (mm). The biomechanical outcome with the strongest relationship ( r ) to medial femoral articular cartilage compartment al thickness was entered into the linear regression model in addition to steps per day (6,028± 1592 ) to predict medial femoral articular cartilage thickness. Peak k nee extension moment had the strongest relationship with medial femoral articular cartilage t hickness ( r = 0.45, p =0.047) , while peak knee abduction moment 121 ( r =0.22, p =0.35) and vGRF ( r =0.05, p =0.85) did not demonstrate significant relationships. K nee extension moment and s teps per day 4 months po st - ACLR explained 39% of the variance in involved limb medial femoral articular cartilage thickness 6 months post - ACLR ( p =0.003) . Individuals who complete more steps per day with poor knee sagittal plane gait biomechanics demonstrate poorer structural medial femoral articular cartilage outcomes. At 4 months p ost - ACLR, individuals complete low accumulations of daily steps despite their ability to participate in approved moderate - intensity activities. Poor knee s agittal plane gait biomechanics should be addressed early during rehabilitation before integrating in dividuals into greater levels of activity. 122 INTRODUCTION P ost - traumatic knee osteoarthritis (PTOA) leads to disability in activities of daily living 1 and limits participation in physical activity. 205 A s many as 50% of individuals undergoing anterior cruciate ligament reconstruction ( ACLR ) will rapidly develop radiographic PTOA within 20 years , 3 which is concerning considering the risk of ACL injury and the prevalence of ACLRs in indivi duals under the age of 20 is double that of individuals in any other decade of life. 44 C hanges indicative of declining articular cartilage health include greater water content, 16 lesser proteoglycan content 11,16 and greater or lesser medial tibiofemoral articular cartilage structure (i.e. medial femoral articular cart ilage thickness) 176 are prese nt within the first 6 months post - ACLR and most often within the medial tibiofemoral compartment. 160 Specifically, d iagnostic ultrasound is an emerging valid 6 and reliable 7 technique of assessing pre - radiographic changes in resting knee femoral articular cartilage thickness. Individuals post - ACLR demonstrate greater involved limb medial femoral articular cartilage compartmental thickn ess compared to their contralateral limb which is hy pothesized to be representative of cartilage swelling . 9 D ue to the high risk of knee PTOA in this clinical population, there is a critical need to understand which factors contribute to the initial development and accelerated progression of pre - rad iographic articular cartilage structur al changes after ACLR . Cumulative mechanical loading of the knee joint which include gait biomechanics and daily volume of loading may have significant impact on knee articular cartil age structural changes after ACLR . 15 G reater peak knee abduction moment s , 11 lesser knee extension moment s , 120,121 and lesser vert ical ground reaction force s ( vGRF s ) 16 during the stance phase of walking have been consistently identified af ter ACLR and are associated with biochemical, compositional, and structural changes of the tibiofemoral a rticular cartilage. These gait 123 biomechanical alterations have the potential to change compression and shearing forces at the knee or shift joint contac t forces from the lateral to medial tibiofemoral compartment leaving the articular cartilage unable to ad apt to the different forces acting upon it . 15,202 Recent evidence also suggests that middle - aged individuals who are at high - risk for knee OA and participate in limited da ily volume of loading may demonstrate MRI - imaging changes associated with lesser proteoglycan and greater water femoral articular cartilage content over 4 years. 17 This may impact individuals after ACLR when they are reported to complete less than 1,500 steps per day compared to those without a history of knee injury. 87 While existing evidence suggests gait bi omechanics and volume of loading may individually play a role in PTOA development, a gap in the literature exists regarding the role of cumulative mechanical loadi ng at the knee joint in individuals recovering from ACLR . Mid - to - late rehabilitation post - AC LR marks a crucial period of time when biomechanical factors of mechanical loading may be associated with unhealthy knee articular cartilage structural changes. At 4 months post - ACLR , most patients are integrated back into activities such as running, 206 jumping and potentially cutting 14 which p lace greater biomech anical mechanical loading (shear forces and contact forces) on the knee. 202 At 6 months post - ACLR, greater than 50% of patients have been discharged from rehabilitation , and many health care professionals begin to consider clearing patients for unrestric ted activity. 14 T herefore, interventi ons targeting mechanical loading risk factors should be incorporated before 6 months post - ACLR. T he primary objective of this study is to determine how different characteristics of mechanical knee joint loading contribute to early femor al articular cartilage structure in individuals after ACLR. We hypothesize that greater peak knee abduction moment, lesser peak knee extension moment, and lesser peak vGRF 4 months post - ACLR will be associated wi th 124 greater involved limb femoral articular c artilage compartmental thickness 6 months post - ACLR , but greater peak knee abduction moment will demonstrate the strongest relationship. We also hypothesize that greater peak knee abduction moment and lesser dail y steps at 4 months post - ACLR will predict g reater involved limb medial femoral articular cartilage compartmental thickness 6 months post - ACLR . M ETHODS This longitudinal study tracked individuals from 4 months (± 2 weeks) to 6 months (± 2 weeks) post - ACLR . G ait biomechanics and daily steps counts w ere collected at the 4 - month assessment , and ultrasound assessed femoral articular cartilage thickness was collected at the 6 - month assessment . Institutional Review Board and all participants provided written informed consent before participating in the study. Individuals under the age of 18 and their parents or guardians provided written informed assent and consent, respectively. P ARTICIPANTS Participants were recruited from 4 fellowship trained orthopedic surgeons at the university orthopedic sports medicine clinic . Individuals were included in they were 16 and 30 years old with a primary unilateral ACLR . Participants were also required to be ambulatory and could achieve , full pain - free knee range of motion. Individuals were excluded from the study if they report ed a history of lower extremity injury within the past 6 weeks , more than 1 ACLR, or rheumatoid arthritis. P artici pant s were not excluded if they had concomitant patholo gies or surgical procedures completed at the time of ACL injury or ACLR such as MCL tears, meniscal tears, meniscectomy , meniscal repair, bone marrow lesions, or microfracture surgery. 125 G AIT ASSESSMENT 4 MONTHS POST - ACL R A 10 - camera motion capture analysi s system (Vicon Motion Systems Ltd., UK) and 2 embedded force plate s (Advanced Mechanical Technology, Inc., Watertown, MA) were used to measure lower extremity walking gait kinematic and kinetic data at 240 and 1200 Hz respectively. 207 A total of 8 clusters of retroreflective markers each w ere placed on each (Figure 1 0 A & 1 0 B) . 207 Each cluster ha d 4 retroreflective markers for a total of 32 used in the biomechanical analysis. The proximal and distal joint segments and join t centers (Figure 1 0 A) w ere identified using a stylus , and hip joint centers were calculated using the Bell method. 208 A right - handed Euler sequence was used to calculate ankle, knee and hip joint angles. Motion analysis software (Innovative Sports Training, Inc., Chicago, IL) was used to capture and process kinematic a nd kinetic data. 207 Data were filtered with a 4 th order low pass Butterworth filter with a cut - off of 12 Hz for kinematic data and 1 2 0 Hz for kinetic data. Prior to kinematic and kinetic assessment, participants we re asked to walk along a 6 - meter track between 4 TF100 timing gates (TracTronix, Belton, MO ) to provide real - time 201 Participants complete d 10 practice trials to become fa miliar with the task and determine average habitual gait speed. Participants were asked to complete 5 successful gait trials on each leg collected by the motion capt ure system . A trial w as ed the force plate s , the participant d id not stutter step or change stride length to load on the force plate and the participant walk ed within ± 5% of average habitual gai t speed that was previo usly determined . Stance phase during gait was identified between initial contact (vGRF>10 N) and toe - off (vGRF<10 N) of the reconstructed limb . The primary outcomes extracted for analysis were peak internal k nee 126 abduction moment , pea k internal knee extensi on moment and peak vGRF during the first 50% of the stance phase of the involved limb and averaged together for each participant. Greater negative values indicate greater internal knee extension moment, and greater positive values in dicate greater knee abd uction moment or vGRF. 127 Figure 1 0 . A.) Clusters with 4 retroreflective markers each were placed on the thoracic and lumbar regions. The red circles over the medial and lateral knee joint line represent stylus placement used to ide ntify the knee joint center. B. ) Clusters with 4 retroreflective markers each we re place on the outside of the right thigh and shank, and on top of the right foot. Cluster placement was identical on the left leg. 128 D AILY STEP COUNT MONITORING 4 MONTHS POST - ACLR Participants were given an Actigraph Link activity monitor (Actigraph, LLC , Pensacola, FL) after the completion of the 4 - month gait assessment . P articipant s were instructed to wear the monitor on their right hip for 7 days during all waking hours exc ept during sleeping or during water activities. During the 7 - day period, participants were asked to fill out a daily activity log recording time of day activity was performed, the type of activity, duration of the activity and perceived intensity of th e ac tivity. R aw tri - axial acceleration data were downloaded to Actilife steps per day . Accelerometer data collection and analysis methods are described in detail in Ta ble 1 3 based adequate reporting methods described by Montoye et al. 209 129 Table 1 3 . Accelerometer d ata c ollection and a nalysis m ethods Items to Report Methods Model of Accelerometer Actigraph Link Data Collection Sampling Rate 30 Hz 87 Data Analysis Epoch Length 60s epoch 74 Place of Accelerometer Right Hip 74 Number of participants receiving accelerometer 1 9 Accelerometer distribution method Received i n - person, returned in - person or in the mail Days of data collection at each time point 7 days Criteria for defining non - wear of accelerometer Minimum Length: 90 minutes Sma ll Window Length: 30 minutes Spike Tolerance: 2 minutes 76 Number of valid days and number of minutes per day of accelerometer data needed to be included in analysis 48 0 minutes per day 76 Accelerometer data PA outcome of interest and interpretat ion method S teps per day Number of participants non - compliant or had accelerometer malfunction issues 1 130 U LTRASOUND ASSESSMENT OF FEMORAL ARTICULAR CARTILAGE THICKENSS 6 MONTH S POST - ACLR Participants returned for a second study session 6 months post - ACLR. Before the ultrasound assessment, a p articipant s hydration status via urine specific gravity (USG) was assessed with an Atago 3730 digital refractometer (ATAGO U.S.A., Inc., Bellevue, WA) . Dehydration may impact articular cartilage imaging assessment and should be eliminated as a potential confounding factor. 154 I f participants USG exceed 1.025 then they w ere rescheduled until their USG was below 1.025 . 185 Ultrasound assessment was completed by a single assessor using the Vivd i Q System (General Electric Company, Boston, MA) with 12 L - RS linear probe with a sampling rate of 1 2 MHz. Upon arrival, participants were asked to sit in a long sitting position for 30 minutes to neutralize any effects from prior activity . 8 After 30 minutes, the participant was asked to align his or her back against the wall and place his or her surgical knee into 140° of knee flexion determined by a manual goniometer . 8 The linear probe was placed perpendicular to the surface of the anterior femoral articular cartilage aligned horizontally between the inner most portions of the medial and lateral condyle s and superior to the patella. 8 A horizontal grid over the real - ti me image of the ultrasound cartilage was used to center the intercondylar notch in relation to the grid and record the positions of the media l condyle and lateral condyle based on the grid coordinates. 8 Once correct placement was achieved, a screenshot of the articular cartilage was recorded. The coordinates of the three landmarks were used for all imag ing assessments of th e involved knee . IMAGE PROCESSING The methods descr ibed have high intra - session reliability (ICC = 0.98 - 0.99). 8 Randomized i mages will be processed using open source Image J (National Institute of Health, 131 Bethesda, MD) to calculate average medial femoral articular cartilage thickness and medial cross - sectional area. 8 Total cross - sectional area ( CSA ) was measured by 1 blinde d rater (C.L.) using built - in measurement tools. Total CSA was defined as the space between the outer b orders of the medial and lateral condyles in addition to the superior synovial - cartilage border and the inferior cartilage - bone border (Figure 3 A ). 7 - 9,183 The central point of the femoral articular cart ilage was ma nually identified by the blinded rater as the middle of the synovial - cartilage border of the articular cartilage separating the medial and lateral upslopes (Figure 3 A). After identifying the total CSA and the central point of the femoral articu lar cartilag e, the images were processed through a custom MATLAB code (Version 9.2, Mathworks, Natik, MA) to segment the total femoral articular cartilage CSA into medial and middle sections. The middle segment length was defined as the inner 25% of the fe moral articu lar cartilage extending from the manually identified central point (Figure 3 B). The medial segment length was defined from the medial border of the middle segment to the outer border of the medial. Average medial thickness (mm) normalized to segment length and CSA (mm 2 ) were calculated for each image of the femoral articular cartilage (Figure 3 B). 132 STATISTICAL ANALYSIS r product moment correlation coefficients were used to evaluate and select the kinetic gait parameter ( peak internal knee abducti on moment , peak internal knee extension moment, or peak v GRF within the 50% of the stance phase ) with the strongest relationship to average medial femoral articular cartilage thickness deformation . This was done to redu ce the number of potential predictors and to prevent multicollinearity between the predictor variables . A total of 2 predictor cumulative loading outcomes were used to predict each femoral articular cartilage deformation explanatory outcomes, separately. A l inear regression model with forward entry w as used to assess the ability of daily step counts and the selected gait outcome to predict involved limb medial femoral articular cartilag e compartmental thickness . SAMPLE SIZE ESTIMATION An a priori sample size estimations was determined using op en - source software, G*Power ( Version 3.1.9.2 , Henrich Heine Universität Düsseldorf, Brunsbuttel, Germany ). An effect size of 0.44 was calculated from a previous study reporting that knee extension moment durin g gait explained 31.8% of the variance in ultra sound - defined femoral articular cartilage CSA . 186 Based on t his calculation, a sample of 29 participants is necessary t o achieve a power of 80% and alpha level of 0.05 using 2 predictor variables. R ESULTS A total o f 20 participants (10 Male/10 Female, age range = 16 - 33 years old) participated in this longitudinal study , and 95 % (N=19) of participants completed all components of both visit assessments (days between testing sessions = 58. 9 ± 10.4). A single participan t (male , age=25 years old, 4 - month BMI= 39.6 kg/m 2 , 6 - month BMI=40.1 kg/m 2 ) was removed from the analysis because the participant did not meet minimum requirements for physical activity 133 monitor wear time . There were s ignificant differences between months si nce surgery , but not BMI or BMI percentile between the 4 - and 6 - month visi t (Table 1 4 ). Average mechanical loading assessed 4 months post - ACLR and medial femoral articular cartilage compartmental thickness assessed 6 months post - ACLR are reported in Table 1 5 . Correlation matrix relationship results between gait biomechanical outcomes 4 - months post - ACLR and medial femoral articular cartilage compartmental thickness 6 - months post - ACLR are reported in Table 1 6 . Lesser involved limb knee extension moment asses sed 4 months post - ACLR was significantly correlated with g reater involved limb medial femoral articular cartilage compartmental thickness ( r = 0.45, p =0.047) assessed 6 months post - ACLR . Involved limb knee abduction moment ( r =0.22, p =0.35) and vGRF ( r =0.05, p =0.85) assessed 4 months post - ACLR were not significantly correlated with involved limb medial femoral articular cartilage compartmental thickness assessed 6 months post - ACLR (Figure 1 1 ). Based on this analysis, i nvolved limb knee extension moment and ste ps per day were entered as predicto r variables in the linear regression analysis for involved limb medial femoral articular cartilage compartmental thickness . Lesser peak knee extension moment ( R 2 =0.20, p =0.02) and greater average steps per day ( R 2 =0.19, p =0.04) assessed 4 months post - ACLR significantly predicted greater involved limb medial femoral articular cartilage compartmental thickness assessed 6 months post - ACLR ( R 2 =0.39, p =0.03 ). Fig ure 1 2 illustrates the relationship between observed medial femoral articular cartilage thickness and medial femoral articular cartilage thickness values predicted by lesser knee extension moment and greater steps per day. 134 Table 1 4 . Participant and study sessi on characteristics (N=19) 4 Month Visit 6 Month Visit p - value BMI for Adults (kg/m 2 ) 29.1 ± 6.1 [20.8, 38.9] a,d 29.4 ± 6.2 [21.3, 39.4] a,d 0.27 BMI for Adolescents (Percentile) 70.5 [21.0, 83.0] b,c 71.5 [23.0, 82.0] b,c 0.68 Months Since Surgery 4.0 ± 0.2 [3.5, 4.3] a 6.1 ± [5.6, 6.6] a <0.001 Days Monitor Worn 6.2±1.5 [4,10] a - - Time Monitor Worn 5 , 304.6±2 , 165.3 [ 2578.0, 11,781.0 ] a - - a = reported as Mean ± Standard Deviation [Minimum, Maximum]; b = reported as Median [Minimum, Maximum]; c = based on N=9; d = based on N=10; e = p <0.05 135 Table 1 5 . Mechanical l oading o utcomes 4 months p ost - ACLR and m edial f emoral a rticular c artilage compartmental t hickness 6 m onths p ost - ACLR Predictor and Explanatory Outcomes (Mean ± Standard Deviation) vGRF (Nm/kg) 1.12 ± 0.08 Internal Knee Extension Moment (Nm/kg) - 0.24 ± 0.11 Internal Knee Abduction Moment (Nm/kg) 0.09 ± 0.04 Steps/ D ay 6085 ± 1592 Medial Thickness (mm) 2.13 ± 0.57 136 Table 1 6 . Correlation matrix between gait biomechanical outcomes 4 - mo nths post - ACLR and medial femoral articular cartilage compartmental thickness 6 - months post - ACLR Knee Extension Moment (Nm/kg) Knee Abduction Moment (Nm/kg) vGRF (Nm/kg) Medial Thickness (mm) Knee Extension Moment (Nm/kg) - - - - Knee Abduction Moment (Nm/kg) r = - 0.08, p = 0.75 - - - vGRF (Nm/kg) r = - 0.19, p = 0.41 r = 0.56, p = 0.01* - - Medial Thickness (mm) r = 0.45, p = 0.047* r = 0.22, p = 0.35 r = - 0.05, p = 0.85 - *=relationship is statistically significant, p <0.05 137 Figure 1 1 . Relations hips between mechanical knee joint loading outcomes during gait at 4 months post - ACLR and average medial femoral articular cartilage thickness at 6 months post - ACLR . Negative values indicate greater knee extension moment, but positive values indi cate great er knee abduction moment and vGRF. 138 Figur e 1 2 . Association between o bserved and predicted involved limb medial femoral articular cartilage thickness 139 DISCUSSION Altered m echanical knee joint loading is a modifiable risk factor in the accelerated developme nt of PTOA in individuals recovering from ACLR. 18 Poor patterns of mechanical loading identified through greater peak knee abduction moment, lesser peak knee extension moment, and greater peak vGRF during gait are related to biochemical, histological, and structural changes indicative of PTOA development. 11,16,159,175 However, the relationship between knee joint health mechanical knee joint loading that takes into account the cyclical nature of loading or how often the knee is loaded or not loaded throughout the day has not been explored in individuals with a history of ACLR. In our study, l esser knee extension moment and greater steps per day at 4 months post - ACLR w ere associated with greater medial femoral articular cartilage thickness 6 months post - ACLR . These findings indicate that individuals who return to greater volumes of ambulatory activity with unresolved sagittal plane kinetic alterations may demonstrate greater involved limb changes to medial articular cartilage stru cture within 6 months of ACLR. Articular cartilage thinning is a f undamental signs of radiographic knee OA in later phases of disease progression. 147 Conversely, early in the d evelopment of knee joint OA, medial femoral articular cartilage may thicken as a precursor to later degenerative changes. G reater thickness of tibiofemoral articular cartilage during the initiation phase of knee OA development may result from greater water content and cartilage swelling. 210 Interestingly, this pattern has been reported on MRIs approximately 2 years after AC LR. 211,212 These findings are also consistent with u ltrasound assessment of femoral articul ar cartilage which identified cartilage thickening in the involved limb compared to the contralateral limb o n average 3 years after ACLR . 9 In our study, i ndividuals with altere d gait biomechanics who t ook more aver age daily 140 steps demonstrate d greater medial femoral articular cartilage thickness which may be an indicator of poor structural changes in the knee joint . Compared to previous literature, this longitudinal study took a unique and comprehensive approach to u nderstanding cumulative mechanical knee joint loading by including gait biomechanics and the amount of cyclical loading together. Individuals in our study demonstrate a complex relationship between knee sagittal plane kinetics indicative of limb underloadi ng and daily cyclical loading activity which would be considered ideal i f evaluated in isolation . These results highlight the multifactoria l and multidirectional aspects of mechanical knee joint loading that interact to influence articular cartilage struct ure which has not been previously reported. Understanding these interact ive relationships may help better guide impairment - based rehabilitation approaches since individuals with a history of ACLR may demonstrate different alterations in gait kinetics and v olumes of loading. Contrary to our findings, Teng et al. reported that greater peak external knee flexion moment during gait assessed 6 months post - ACLR is associated with declining proteoglycan content (i.e. greater T1 rho relaxation times) in the medial femoral condyle assessed via MRI at 1 and 2 years post - AC LR . 16 Participants in the Teng et al. study were older (30.6±8.6 years old) a nd had lower BMI (23.9±2.7) 16 which ar e both risk factors that negatively impact knee joint health 10,213 which may account for the differences in these results compared to the current study . Volume of activity may also impact th is relationship. While g ait biomechanics were the primary predictor outcomes, the amount of activity participants completed at 6 months post - ACLR (i.e. self - reported or accelerometer - based) were not reported in the Teng. et al. study. 16 B ased on our findings, we speculate that the relationships between gait kinetics and knee articular cartilage health may be moderated by 141 the relationships between gait kinetics, volume of l oading, and MRI - based tibiofemoral T1 rho re laxations time assessments in individuals within the first 6 months to a year post - ACLR. In our study, lesser knee extension moment during the stance phase of gait was moderately related to thicker medial femor al articular cartilage. t is described as adopting lesser internal knee extension moment and knee flexion excursion during the stance phase of gait . This gait strategy is prevalent in the involved limbs of individuals at 6 and 12 months post - ACLR 214 and is associated in lesser medial tibiofemoral contact forces. 215 In this ca se, lesser knee extension moment may reduce medial tibiofemoral contact forces an d underload the femoral articular cartilage. It is hypothesized that lesser knee extension moment may contribute to the development of PTOA . 15 The framework suggests that compartments of the articular cartilage tha t were once acclimated to a certain degree of shear and compressive forces of loading may experience lesser mechanical load due t o gait pattern alterations d uring the initiation phase of PTOA after ACLR . 4,15,18 As a result, the medial femoral articular cartilage compartment may not adapt and may be unprepared to accept greater accumulations of cyclical mechanical loading (i.e. greater steps per day) . The concept of underloading is supported by previous research repo rt ing that i ndividuals who developed radiographic medial knee PTOA 5 years after ACLR, demonstrated lesser tibiofemoral contact forces at 6 months after surgery compared to individuals who did not develop PTOA. 216 Therefore, addressing knee extension moment underloading using p romising real - time biofeedback gait retraining interventions which single session 217 and may have implications for improving biochemical indicators of cartilage breakdown. 178 142 In our study, volume of activity was a significant predictor of knee articular cartilage struc ture. In addition to knee articular cartilage health, l ow volume of loading may impact other aspects of their health. For example, p hysical activity participation has various impacts on an 64 and reducing the risk of developing a myriad of chronic, non - communicable diseases. 65 Adults who meet national ate - vigorous physical activity) achieve approximately 7,000 steps per day. 218 Regardless of in jury history, m en and women over the age of 20 (n=3,725) and adolescents b etween 12 and 19 years old (n=2,610) who participated in the NHANES study completed an average of 9,685 219 and 8 , 225 - 11,660 steps per day , respectively . 220 In comparison, i ndividuals 4 months post - ACLR in our study only completed an average of 6,085 daily steps and 74% (n=14) of those individuals did not achieve 7 ,000 steps per day . During mid to late phases of ACLR rehabilitation, many individuals are completing lesser daily steps compared to the general population and m ay not be accumulating adequate steps per day necessary to achieve recommended levels of physical activity. A c ross - sectional stu d y also report ed that individuals complete less steps per day compared to healthy controls as long as 5 years after ACLR 87 indicating that this trend may continue even once patients receive physician clearance for return to unrestricted activity. Recovery from ACLR may promote physical inactivity behavior during rehabilitation which may co ntinue even after individuals no longer have physical activity restrictions. While individuals recovering from ACLR have sports activity or even jogging restrictions at 4 months, this does not reduce their ability to parti cipate in approved ambulatory acti vities that increase step accumulation throughout the day or achieve recommended weekly physical activity guidelines . Once high - risk gait biomechanics have been addressed in rehabilitation, health care providers should encourage 143 increasing moderate - intensi ty physical activity participation to meet national recommendations and educate patients on how to achieve these goals through approved ambulatory activities. Future research should determine if specific daily step accumulation recommendations or step - base d goals can help maintain knee joint health and reduce the risk of developing functional impairments. BMI is a signif icant predictor of knee OA in middle - aged individuals with and without a history of knee injury. 142 A limitation of this study is that BMI was not contro lled for in the statistical analysis. BMI is a challenging outcome to control when including adolescents and adults because BMI is measured differently in these populations. Appropriate BMI reporting in a dolescents is calculated as a percentile compared to national percentiles based on age and sex. 102 In or der to limit the number of predictors and covariates in our linear regression, A strength of this study i s its use of accelerometers to measure daily activity in free - living settings co mpared to self - reported activity . However, in order to fully understand the amount of underloading that occurs daily, sedentary behavior may be a better outcome. Actigraph monitors worn at the hip are ade quate measures of activity, but other wearable accel erometers (i.e. A ctivpals attached to the thigh) may better capture length of time in positions of sedentary behavior such as lying down or sitting. 221 Future research should assess how underloading measured through sedentary behavior relates to knee joint health after ACLR. C ONCLUSION Greater volume of load ing with high - risk biomechanics in individuals recovering from ACLR is associated with poor structural changes in the medial femoral articular cartilage. Individuals who walk with lesser knee extens ion moment may underload the knee joint reducing 144 the abili ty of the medial femoral articular cartilage compartment to adapt to greater steps per day. During mid - late phases of rehabilitation, individuals recovering from surgery are completing low amounts of steps per day that may reduce their ability to achieve n ational physical activity recommendations. Once high - ris k gait biomechanics are addressed, health care providers should encourage greater daily activity and educate patients on how they can increase their activity through approved ambulatory activities . 145 CHAPTER 6 : SUMMARY AND CONCLUSIONS SUMMARY The purpose of this dissertation was to 1.) establish intra - rater and inter - rat er reliability of a standard resting and novel post - loading after 3,000 steps ultrasound assessment of femoral articular cartilage st ructure in healthy individuals , 2.) assess between limb and time differences of medial, intercondylar and lateral femoral a rticular cartilage structure in individuals 4 and 6 months post - ACLR, 3.) determine the ability of cumulative mechanical knee joint l oading assessed 4 months post - ACLR to predict medial femoral articular cartilage structure assess 6 months post - ACLR in ind ividuals recovering from surgery. RELIABILITY OF RESTING CARTILAGE AND CARTILAGE RESPONSE TO LOADING ULTRASOUND ASSESSMENT S OF FEMOR AL ARTICULAR CARTILAGE A total of 30 participants (age= 21.8 ± 3.8 years, gait speed = 1.3 ± 0.2 m/s) completed both sessions of the observational laboratory study. P articipants completed on average 3,009 steps in 28.4 minutes during the loading protocol i n the first visit. During the loading protocol in the second visit, participants completed on average 3008 steps in 28.4 minutes. Resting cartilage assessment of femoral articular cartilage structure demonstrated excellent intra - rater ( ICC 2 .k = 0 .99 ) and tes t - retest ICC 2. k = 0 .97 - 0.99) reliability . Cartilage response to loading ultrasound assessment demonstrated good to excellent intra - rater reliability (ICC 2. k = 0 .84 - 0.95), but poor test - retest reliability (ICC 2. k = - 0.36 - 0.46). Resting cartilage ultrasound assess ment can be reliably used to assess between processing sessions and over multiple study visits , but the cartilage response to loading ultrasound assessment should not be used when assessing outcomes over multiple study visits . Femoral arti cular cartilage m ay no t consistently deform in healthy participants due to step - tracking equipment with poor reliabil ity resulting in underloading or 146 overloading beyond the recommended optimal range (3,00 steps) . The pattern or cyclical nature of articular cartilage deform ation response at less than 1,000 steps or greater than 5,000 steps is unclear and should be assessed in future research. Regardless with the current knowledge , r eliable step - tracking devices should be used in future studies to en sure optimal loading durin g the loading protocol for consistent cartilage deformation. LONGITUDINAL ULTRASOUND ASSESSMENT OF FEMORAL ARTICULAR CARTILAGE THICKNESS 4 TO 6 MONTHS POST - ACLR A total of 20 participants recovering from ACLR completed the second longitudinal study ( 10 Mal es/10 Females, age range=16 - 33 years old, 10 hamstring graft/10 bone - patellar tendon - bone graft/1 allograft) at 4 - and 6 - months post - ACLR (days between testing sessions = 58.5±20.4) . There were no significant limb main effe cts ( p range=0.50 - 0.92) , time mai n effects ( p range=0.22 - 0.72) , or interactions ( p range=0.24 - 0.49) for any of the femoral articular cartilage thickness outcomes . Individuals may not have femoral articular cartilage structural limb differences or changes within the first 6 months pos t - ACL R . Standard ultrasound assessment of femoral articular cartilage structure may not be able to detect limb differences or changes across time during mid - late periods of rehabilitation . ASSOCIATIONS BETWEEN CUMULATIVE KNEE JOINT LOADING AND MEDIAL FEMORAL AR TICULAR CARTILAGE THICKNESS WITHIN 6 MONTHS POST - ACLR A total of 19 participants with a history of ACLR ( 9 Male/10 Female, age r ange =16 - 33 years old ) completed the longitudinal study at 4 and 6 months post - ACLR including knee joint mechanical loading ass essments at 4 months post - ACLR . Participants walked approximately 6085 steps per day (standard deviation = 1592) wore the physica l activity monitor for an average of 6.2 days (standard deviation = 1.5) and 5304.6 minutes per day (standard deviation=11,781. 0) . 147 Involved limb medial femoral articular cartilage compartmental thickness at 6 months post - ACLR was significantly associated with involved limb knee extension moment ( r =0.45, p =0.047), but not knee abduction moment ( r =0.05, p =0.85) and vGRF during the f irst 50% of stance phase ( r =0.22, p =0.35) during gait at 4 months - post ACLR. Greater involved limb knee extension mom ent ( p =0.02) and greater steps per day ( p =0.04) assessed 4 months - post ACLR predicted greater involved limb medial femoral articular cartil age compartmental thickness at 6 months post - ACLR (R 2 =0.39, p =0.03) . Cumulative knee joint loading is associated with femoral articular cartilage structure and volume of activity may moderate the relationship between gait biomechanics and knee joint health . After ACLR, individuals with poor knee gait alterations that underl oad the knee joint may be u nable to adapt to greater volume of loading resulting in femoral articular cartilage swelling. Health care providers should use evidence - based interventions ear ly during rehabilitation to target individuals with lesser knee exten sion moment as individuals are integrated back into weight - bearing activities. LIMITATION S A limitation of both stud ies is that we did not consider how stage of skeletal maturity may impa ct femoral articular cartilage changes in our adolescent participants . Most individuals undergoing ACLR are less than 20 years old . 44 Therefore, it was imperative to include adolescents in our first and second stud y to include a sample reflective of the general population. Individuals under the age of 20 may not have reached the highest stage of skeletal maturity . Adolescents without a MRI - ass essed tibiofemoral articular cartilage thickness increases over a year (boys = 0.8%, girls = 1.4%) . 135 It is unclear if these changes can be detected over a 2 - month period using standard ultrasound assessment or if synovial joint injury such as ACL tears impact articular cartilage 148 growth. A total of 5 adolescents without a history of knee injury and 10 adolescents with a history of ACLR were included in the first and second study, respectively. We did not assess ske letal maturity in our participants in either study , but this may be an important consideration for future research. Future studies should determine if changes in skeletal maturity affect longitudinal standard ultra sound assessment of femoral articular cart ilage structure. A second limitation of the second study is that concomitant injuries or surgical procedures were not included as a covariate in the statistical analyses. Concomitant injuries and surgical procedures consistently accompany ACL tears and A CLRs. 45 Meniscal or chondral damage (i.e. bone bruise s, bone marrow lesions) are relevant pathologies that increase the risk of PTOA in individuals with a history of ACLR. 50 ,54 Meniscal i njuries occur in 60% of individuals with ACL tears an d may be surgically treated with meniscectomy or meniscal repairs. 45 Individuals undergoing m eniscectomies have a 3.5 times increase d risk of PTO A within 10 to 25 years post - ACLR . 50 Furthermore, greater chondral damage demonstrates a moderate association with greater radiographic presence of tibiofemoral PTOA within 6 years post - ACLR. 54 W e were only able to retrieve the surgical records of (N=16) participants which decreases our sample size . At the time of surgery, approximately 63% of the participants (N=10) had a meniscal injury, 19% of participants (N=3) had a meniscectomy , and 35% of p articipants (N=7) had a meniscal repair. Of the 16 participants, we were unable to access any of the patie nts initial imaging records to determine the presences of bone bruises or bone marrow lesions. However, all surgical records reported that the tibiofe moral articular cartilage was intact for all 16 patients at the time of surgery. Future studies should ass ess the effects of meniscal injury, meniscal surgical procedures, and chondral damage (i.e. bone bruises or bone marrow lesions) on femoral articular cartilage thickness assessed with the standard resting ultrasound technique. 149 A third limitation of the se cond study is that we did not control for body mass index (BMI) for adults or BMI percentile for adolescents in the linear regression analysis. BMI is a risk factor for the development of knee OA. 142 However, th is is primarily reported in middle - aged individuals regardless of injury history. 142 Individuals with a history of ACLR represent a unique population the develops accelerated knee OA, because the ACL tears are most likely to occur in individuals below the age of 20. 44 It is unclear if BMI percentile in adolescents increases the risk of kne e OA development. Performing separate linear regression analysis for adolescents and adults to con trol for BMI percentile versus BMI would reduce our sample size to 10 participants for each regression. Regardless, knee joint biomechanics were normalized to body mass. Future research should determine if BMI or BMI percentile affect standard resting ultr asound femoral articular cartilage thickness in individuals with a history of ACLR . S TRENGTHS ACCESSIBLE ASSESSMENT TOOLS FOR HEALTH CARE PROFESSIONALS T he second study incorporate d the use of accessible ultrasound machines and clinically relevant volume o f activity assessments. Early identification of articular cartilage degeneration is key in secondary prevention efforts, but radiographic and MRI - based assessments may not identify pre - radiographic articular cartilage changes or are difficult to utilize fo r longitudinal assessments which limits their feasibility for early screenings for individuals at risk for PTOA development . Diagnostic ultrasound machines are ubiquitously available in o rthopedic clinics and hospitals . W ith more research , the standard ult ra sound assessment of femoral articular cartilage structure has the potential to be integrated into healthcare clinics to identify individuals at risk for developing knee OA . The ultrasound technique utilized in th e second study has not been applied in thi s population during early periods of recovery after ACLR making the study 150 novel. Additionally , we utilized research - grade activity tracking technology for valid data collection to provide better context about volume of activity. However, consumer - grade act ivity per day can easily be assessed and mod ified by clinicians and patients through consumer - grade devices such as FitBit monitors or smart watches . INTEGRATIVE APPROACH TO UNDER STANDING KNEE JOINT MECHANICAL LOADING The second study takes a multifaceted approach to addressing the effects of loadi ng on knee articular cartilage health early after ACLR. Traditionally, knee joint gait biomechanics and volume of loading have been ass essed individually after ACLR , 87,118 but considering these factors conjointly provides a more comprehensive assessment of contributors to knee loading during activity. The results of this study provide the first st ep in a line of research which characterize s th e effects of under - or over - loading behavior on articular cartilage health during critical points of the recovery process following ACLR to slow or mitigate the rapid development of PTOA commonly observed in t his at - risk population . Both volume of loading and magnitude of loading (gait biomechanics) are associated with poor knee articular cartilage structure within the first 6 months post - ACLR . Both facets of mechanical knee joint loading should be considered i n clinical treatment and future research as a m ore comprehensive risk factor. By identifying which load - related factors are associated with poor knee articular cartilage structure , we may be able to develop and implement safe, progressive walking - based pro tocols during recovery with the goal of promoti ng physical activity related behaviors and maintaining healthy knee articular cartilage. 151 CLINICAL IMPLICATIONS AND FUTURE RESEARCH The utility of ultrasound assessment for knee joint health is promising but requires future research before health care providers adopt the technique in clinical practice. First, the resting cartilage assessment semi - automated processing remains time - consuming . This is despite the reduc tion in processing that only requires manual segmentation of the total CSA and improvemen ts in calculating thickness across the entire articular cartilage compartments. 182 Adv ancing to a fully - automated processing techn ique would enhance the clinical feasibility of the technique considering the ubiquitous prevalence of ultrasound machines in orthopedic clinics. Secondly, ultrasound assessments of pre - radiographic knee joint hea lth may be more impactful when utilizing a g rading system of multiple outcomes similar to radiographic assessments of knee osteoarthritis (i.e. Kellgren - Lawrence Classification). 147,222 In addition to articular car tilage thickness, ultrasound may be used to assess synovitis, meniscal extrusion, joint effusion, and cartilage echo - intensity. 8,222 Outcome measure scoring systems assessed via ultrasound have been recommended to screen individuals with radiographic evidence of knee osteoarthritis, 222 but multifaceted ultrasound assessments of pre - radiographic changes early in knee osteoarthritis development have not been established. Future research should incorporate the resting cartilage assessment along with other valid and reliable ult rasound - base d knee joint health outcomes (i.e. meniscal extrusion and cartilage echo - intensity) to potentially establish an adequate screening to identify individuals at risk for knee osteoarthritis development. To our knowledge, this is the first study to longitudina lly assess femoral articular cartilage using the standard resting ultrasound technique after ACLR. Rehabilitation after ACLR is may completed within this 6 - month period. 14 This study has a longitudinal design to better understand the relationship between factor s of loading during rehabilitation and knee articular 152 cartilage joint health when many individuals have returned to activity . Gait biomechanics have been the focus of mechanical knee joint loading and PTOA research post - ACLR, but the linear regression resu lts of the t hird manuscript suggest that volume of loading is also a key contributing factor for researchers and clinicians to consider moving forward . Extensive literature indicates that there are consistent relationships between gait biomechanics and bi ochemical, compositional, and radiographic changes in knee joint health post - ACLR . 11,120,175 Future research assessing knee joint health and volume of loading should also consider incorporating wet inflammatory and cartilage metabolism biomarkers, MRI of T1 rho and T2 relaxation times, and x - ray assessments to better understand th e strength of these relationship s . While novel, these results are focused on a specific period of time post - ACLR (< 6 mont hs). Extensive l ongitudinal assessments may help clarify if the relationship between volume or loading and knee joint health are time dependent post - surgery, and if the direction of the relationships change based on time. For example, individuals with wors e symptoms post - ACLR demonstrate lesser vGRF at 6 months, but greater vGRF at 12 months during walking compared to individuals with fewer symptoms. 126 Longer longitudinal assessments may help determine if relationships bet ween volume of l oading and knee joint health change in a similar way to gait biomechanics. Additionally, r esearchers assessing mechanical knee joint loading should consider the multifactorial nature of this pathway in PTOA development . Future research shou ld incorporate c onsistent assessments of cyclical loading and expand their assessment to understand other loading aspects such as intensity of loading (i.e. moderate to vigorous physical activity) or rate of loading (i.e. cadence or gait speed) post - ACLR. The results of the third manuscript also suggest that volume of loading is an important clinical assessment. Clinicians should consider including volume of loading assessments in day - 153 to - day practice to understand how much cyclical loading is occurring daily and how this relates to achieving or not achieving physica l activity guidelines. Volume of loading can be measured clinically through consumer grade technology (i.e. pedometers, Fitbits, smart watches) . At 4 - months post - ACLR, patients demonstrate low step counts post - A CLR which may impact their ability to meet na tional physical activity guidelines. Clinicians should consider educating patients about the impact of physical inactivity on long - term health and encourage individuals to increasing their physical activity part icipation within the context of their restric tions post - surgery. Future research should explore interventions aimed at increasing step counts using consumer grade technology such as accelerometers and goal - setting phone apps for clinicians to provide as ev idence - based interventions for their patients CONCLUSIONS Resting cartilage u ltrasound assessment of femoral articular cartilage structure demonstrates better measurement properties when assess ing cartilage structural across multiple study visits when com pared to the cartilage response to loading ultrasound assessment technique in individuals without a history of knee injury . Resting cartilage ultrasound assessment of f emoral articular cartilage structure cannot detect involved and contralateral limb diffe rences or changes across time within the first 6 months after ACLR indicating that knee articular cartilage structural changes may not occur while patients typically engage in knee rehabilitatio n. Therefor e, this may be a beneficial time to address modifia ble risk factors such as altered mechanical knee joint loading that are associated with the accelerated development of individuals with a history of ACLR before structural changes begin to occur. Volume of mechanical knee joint loading may moderate the rel ationships between knee gait biomechanics and poor knee articular cartilage structur e. Specifically, individuals demonstrating lesser knee 154 extension moment who walk greater steps per day demonstrate greate r medial femoral articular cartilage thickness whic h is associated with cartilage swelling. Individuals with lesser knee extension moment gait biomechanics should undergo evidence - based interventions to address poor gait biomechanics during the early phase s of rehabilitation before they are introduced to g reater levels of daily activity . 155 R EFERENCES 156 REFERENCES 1. Cross M, Smith E, Hoy D, et al. The global burden of hip and knee osteoarthritis: estimates from the global burden of disease 2010 study. Ann Rheum Dis. 2014;73(7):1323 - 1330. 2 . Furner SE, Hootman JM, Helmick CG, Bolen J, Zack MM. Health - related quality of life of US adults with arthritis: analysis of data from the behavioral risk factor surveillance system, 2003, 2005, and 2007. Arthritis Care Res (Hoboken). 2011;63(6):788 - 799. 3. Luc B, Gribble PA, Pietrosimone BG. Osteoarthritis prevalence following anterior cruciate ligament reconstruction: a systematic review and numbers - needed - to - treat analysis. J Athl Train. 2014;49(6):806 - 819. 4. Andriacchi TP, Koo S, Scanlan SF. Gait m echanics influence healthy cartilage morphology and osteoarthritis of the knee. J Bone Joint Surg . 2009;91 Suppl 1:95 - 101. 5. Eckstein F, Hudelmaier M, Putz R. The effects of exercise on human articular cartilage. J Anat . 2006;208(4):491 - 512. 6. Nare do E, Acebes C, Moller I, et al. Ultrasound validity in the measurement of knee cartilage thickness. Ann Rheum Dis. 2009;68(8):1322 - 1327. 7. Harkey MS, Blackburn JT, Davis H, Sierra - Arevalo L, Nissman D, Pietrosimone B. Ultrasonographic assessment of med ia l femoral cartilage deformation acutely following walking and running. Osteoarthritis Cartilage. 2017;25(6):907 - 913. 8. Harkey MS, Blackburn JT, Hackney AC, et al. Comprehensively a ssessing the a cute f emoral c artilage r esponse and r ecovery after w alkin g and d rop - l anding: An u ltrasonographic s tudy. Ultrasound Med Biol . 2018;44(2):311 - 320. 9. Harkey MS, Blackburn JT, Nissman D, et al. Ultrasonographic a ssessment of f emoral c artilage in i ndividuals w ith a nterior c ruciate l igament r eco nstruction: A c ase - c ontrol s tudy. J Athl Train. 2018;53(11):1082 - 1088. 10. Bodkin SG, Werner BC, Slater LV, Hart JM. Post - traumatic osteoarthritis diagnosed within 5 years following ACL reconstruction. Knee Surg Sports Traumatol Arthrosc. 20 20;28(3):790 - 796. 11. Pfeiffer S J, Spang J, Nissman D, et al. Gait m echanics and T1rho MRI of t ibiofemoral c artilage 6 m onths p ost ACL r econstruction. Med Sci Sports Exerc. 201 9;51(4):630 - 639. 157 12. Culvenor AG, Collins NJ, Guermazi A, et al. Early knee osteoarthritis is evident one year following anterior cruciate ligament reconstruction: a magnetic resonance imaging evaluation. Arthritis Rheum . 2015;67(4):946 - 955. 13. Pedoia V, Su F, Amano K, et a l. Analysis of the articular cartilage T1rho and T2 relaxation times changes after ACL re construction in injured and contralateral knees and relationships with bone shape. J Orthop Res . 2017;35(3):707 - 717. 14. Greenberg EM, Greenberg ET, Albaugh J, Store y E, Ganley TJ. Rehabilitation p ractice p atterns f ollowing a nterior c ruciate l igament r ec onstruction: A s urvey of p hysical t herapists. J Orthop Phys Ther . 2018;48(10):801 - 811. 15. Andriacchi TP, Mundermann A, Smith RL, Alexander EJ, Dyrby CO, Koo S. A framework for the in vivo pathomechanics of osteoarthritis at the knee. Ann Biomed E n g . 200 4;32(3):447 - 457. 16. Teng HL, Wu D, Su F, et al. Gait characteristics associated with a gre ater increase in medial knee cartilage T1rho and T2 relaxation times in patients undergoing anterior cruciate ligament reconstruction. Am J Sports Med. 2017;45(14) :3262 - 3271. 17. Lin W, Alizai H, Joseph GB, et al. Physical activity in relation to knee ca rtilage T2 progression measured with 3 T MRI over a period of 4 years: data from the Osteoarthritis Initiative. Osteoarthritis C artilage. 2013;21(10): 1558 - 66 . 18 . Andriacchi TP, Mundermann A. The role of ambulatory mechanics in the initiation and progression of knee osteoarthritis. Curr Opin Rheumatol . 2006;18(5):514 - 518. 19. Halilaj E, Hastie TJ, Gold GE, Delp SL. Physical activity is associated with changes in k nee cartilage microstructure. Osteoarthritis Cartilage. 2018;26(6):770 - 774. 20. Chou MC, Tsai PH, Huang GS, et al. Correlation between the MR T2 value at 4.7 T and relative water content in articular cartilage in experimental osteoarthritis induced by AC L transection. Osteoarthritis Cartilage. 2009;17(4):441 - 447. 21. Vignon E, Arlot M, Hartmann D, Moyen B, Ville G. Hypertrophic repair of articular cartilage in experimental osteoarthrosis. Ann Rheum Di s . 1983;42(1):82. 22. Calvo E, Palacios I, Delgado E, et al. Histopathological correlation of cartilage swelling detected by magnetic resonance imaging in early experimental osteoarthritis. Osteoarthritis Cartilage. 2004;12(11):878 - 886. 23. Giuliani JR, Kilcoyne KG, Rue JP. Anterior cruciate ligament ana tomy: a review of the anteromedial and posterolateral bundles. J Knee Surg. 2009;22(2):148 - 154. 158 24. Siegel L, Vandenakker - Albanese C, Siegel D. Anterior cruciate ligament injuries: anatomy, physiology, biomechanics, and management. Clin J Sport Med. 2012 ;22(4):349 - 355. 25. Travascio F, Jackson AR. The nutrition of the human meniscus: A computational analysis investigating the effect of vascular recession on tissue homeostasis. J Biomech. 20 17;61:151 - 159. 26. Joseph AM, Collins CL, Henke NM, Yard EE, F ields SK, Comstock RD. A multisport epidemiologic comparison of anterior cruciate ligament injuries in high school athletics. J Athl Train. 2013;48(6):810 - 817. 27. Mountcastle SB, Posner M, Kragh JF, Jr., Taylor DC. Gender differences in anterior cruciat e ligament injury vary with activity: epidemiology of anterior cruciate ligament injuries in a young, athletic population. Am J Sports Med . 2007;35(10):1635 - 1642. 28. Sanders TL, Maradit Kre mers H, Bryan AJ, et al. Incidence of a nterior c ruciate l igament t ears and r econstruction: A 21 - y ear p opulation - b ased s tudy. Am J Sports Med. 2016;44(6):1502 - 1507. 29. Gornitzky AL, Lott A, Yellin JL, Fabricant PD, Lawrence JT, Ganley TJ. Sport - specific yearly risk and incidence of anterior cruciate ligament tears in high school athletes: a systematic review and meta - analysis. Am J Sports Med. 2016;44(10):2716 - 2723. 30. Mehl J, Diermeier T, Herbst E, et al. Evidence - based concepts for prevention of knee and ACL injuries. 2017 guidelines of the ligament committee of the German Knee Society (DKG). Arch Orthop Trauma Surg. 2018;138(1):51 - 61. 31. Padua DA, DiStefano LJ, Hewett TE, et al. National Athletic Trainers' Association p osition s tatement: Prevention of a nterior c ruciate l igament i njury. J Athl Train. 2018;53(1) :5 - 19. 32. Webster KE, Hewett TE. Meta - analysis of meta - analyses of anterior cr uciate ligament injury reduction training programs. J Orthop Res . 2018 ;36(10):2696 - 2708 . 33. Yoon KH, Yoo JH, Kim KI. Bone contusion and associated meniscal and medial colla teral ligament injury in patients with anterior cruciate ligament rupture. J Bone Joint Surg . 2011;93(16):1510 - 1518. 34. Brophy RH, Zeltser D, Wright RW, Flanigan D. Anterior c ruciate l igament r econstruction and c oncomitant a rticular c artilage i njury: In cidence and t reatment. Arthroscopy. 2010;26(1):112 - 120. 35. Dare D, Rodeo S. Mechanisms of p ost - t raumatic o steoarthritis a fter ACL i njury. Curr Rheumatol Rep. 2014;16(10):448. 159 36. Kaeding CC, Léger - St - Jean B, Magnussen RA. Epidemiology and diagnosis of anterior cruciate ligament injuries. Clin Sport Med. 2017;36( 1):1 - 8. 37. Br J Sports Med . 2007;41(Suppl 1):i47 - i51. 38. Kiapour AM, Demetropoulos CK, Kiapour A, et al. Strain response of the anterior cruciate ligament to uniplanar and multiplanar loads during simul ated landings: implications for injury mechanism. Am J Sports Med . . 2016;44(8):2087 - 2096. 39. Mancini EJ, Kohen R, Esquivel AO, Cracchiolo AM, Lemos SE. Comparison of ACL strain in the MCL - deficient and MCL - reconstructed kne e during simulated landing in a cadaveric model. Am J Sports Med . 2017;45(5):1090 - 1094. 40. Carlson VR, Sheehan FT, Boden BP. Video analysis of anterior cruciate ligament (ACL) injuries: a systematic review. JBJS reviews. 2016;4(11):e5 - e5. 41. Hewett TE, Myer GD, Ford KR, et al. Bi omechanical measures of neuromuscular control and valgus loading of the knee predict anterior cruciate ligament injury risk in female athletes: a prospective study. Am J Sports Med . 2005;33(4):492 - 501. 42. Quatman CE , Hewett TE. The anterior cruciate lig ament injury controversy: is "valgus collapse" a sex - specific mechanism? Br J Sports Med. 2009;43(5):328 - 335. 43. Sigward SM, Pollard CD, Havens KL, Powers CM. Influence of sex and maturation on knee mechanics during side - step cutting. Med Sci Sports Exer c. 2012;44(8):1497 - 1503. 44. Mall NA, Chalmers PN, Moric M, et al. Incidence and trends of anterior cruciate ligament reconstruction in the United States. Am J Sports Med. 2014;42(10):2363 - 2370. 45. Herzog MM, Mars hall SW, Lund JL, Pate V, Mack CD, Spa ng JT. T Trends in incidence of ACL reconstruction and concomitant procedures among commercially insured individuals in the United States, 2002 - 2014. Sports Health . 2018;10(6):523 - 531. 46. Houck DA, Kraeutler MJ, Vidal AF, McCarty EC, Bravman JT, Wolcott ML. Variance in anterior cruciate ligament reconstruction graft selection based on patient demographics and location within the multicenter orthopaedic outcomes network cohort. J Knee Surg. 2018;31(5):472 - 478. 47. Maletis GB, Inacio MC, Funahashi TT. An alysis of 16,192 anterior cruciate ligament reconstructions from a community - based registry. Am J Sports Med . 2013;41(9):2090 - 2098. 48. Albright J, Lepon AK, Mayer S. Anterior cruciate liga ment reconstruction in pediatric and adolescent patients using qu adriceps tendon autograft. Sports Med Arthrosc Rehabil . 2016;24(4):159 - 169. 160 49. Capin JJ, Khandha A, Zarzycki R, Manal K, Buchanan TS, Snyder - Mackler L. Gait mechanics after ACL reconstruction differ according to medial meniscal treatment. J Bone Joint S urg . 2018;100(14):1209 - 1216. 50. Claes S, Hermie L, Verdonk R, Bellemans J, Verdonk P. Is osteoarthritis an inevitable consequence of anterior cruciate ligament reconstruction? A meta - analysis. Knee Surg, Sports Traumatol, Arthrosc. 2013;21(9):1967 - 1976. 51. Eriksen EF. Treatment of bon e marrow lesions (bone marrow edema). Bone ke y R ep. 2015;4:755 - 755. 52. Dunn WR, Spindler KP, Amendola A, et al. Which preoperative factors, including bone bruise, are associated with knee pain/symptoms at index anterior cruciate ligament reconstruction ( ACLR)? A Multicenter Orthopaedic Outcomes Network (MOON) ACLR Cohort Study. Am J Sports Med. 2010;38(9):1778 - 1787. 53. Zhang L, Hacke JD, Garrett WE, Liu H, Yu B. Bone bruises associated with anterior cruciate ligament injury as indicators of injury mechanism: A systematic review. Sports Med . 2019;49(3):453 - 462. 54. Keays SL, Newcombe PA, Bullock - Saxton JE, Bullock MI, Keays AC. Factors involved in the development of osteoarthritis after anterior cruc iate ligament surg ery. Am J Sports Med . 2010;38(3):455 - 463. 55. Boks SS, Vroegindeweij D, Koes BW, Hunink MG, Bierma - Zeinstra SM. Follow - up of posttraumatic ligamentous and meniscal knee lesions detected at MR imaging: systematic review. Radiology. 2006; 238(3):863 - 871. 56. Wiggins AJ, Grandhi RK, Schneider DK, Stanfield D, Webster KE, Myer GD. Risk of Secondary Injury in Younger Athletes After Anterior Cruciate Ligament Reconstruction: A Systematic Review and Meta - analysis. The American journal of sport s medicine. 2016;4 4(7):1861 - 1876. 57. Paterno MV, Rauh MJ, Schmitt LC, Ford KR, Hewett TE. Risk of secondary injury in younger athletes after anterior cruciate ligament reconstruction: A systematic review and meta - analysis. Am J Sports Med . 2014;42(7):1567 - 1573. 58. Sch ilaty ND, Nagelli C, Bates NA, et al. Incidence of second anterior cruciate ligament tears and identification of associated risk factors from 2001 to 2010 using a geograph ic database. Orthop J Sports Medicine . 2017;5(8):2325967117724196. 59. Maletis GB, Inacio MC, Funahashi TT. Risk factors associated with revision and contralateral anterior cruciate ligament reconstructions in the Kaiser Permanente ACLR registry. Am J Sports Med . 2015;43(3):641 - 647. 161 60. Kaeding CC, Aros B, Pedroza A, et al. Allograft v ersus autograft anterior cruciate ligament reconstruction: Predictors of failure from a MOON prospective longitudinal cohort. Sports Health . 2011;3(1):73 - 81. 61. Piercy KL, Troiano RP, Ballard RM, et al. T he physical activity guidelines for americans. JA MA. 2018;320(19):2020 - 2028. 62. Telama R, Yang X, Viikari J, Valimaki I, Wanne O, Raitakari O. Physical activity from childhood to adulthood: a 21 - year tracking study. Am J P rev M ed. 2005;28(3):267 - 273. 63. Palomaki S, Hirvensalo M, Smith K, et al. Doe s organized sport participation during youth predict healthy habits in adulthood? A 28 - year longitudinal study. Scand J Med Sci Sports. 2018;28(8):1908 - 1915. 64. Schuch FB, Vancampfort D, Firth J, et al. P hysical a ctivity and i ncident d epression: A m eta - a nalysis of p rospective c ohort s tudies. Am J Psychiatry . 2018;175(7):631 - 648. 65. Lee IM, Shiroma EJ, Lobelo F, Puska P, Blair SN, Katzmarzyk PT. Effect of physical inactivity on major non - communicable dis eases worldwide: an analysis of burden of disease and life expectancy. Lancet. 2012;380(9838):219 - 229. 6 6. Marx RG, Stump TJ, Jones EC, Wickiewicz TL, Warren RF. Development and evaluation of an activity rating scale for disorders of the knee. Am J Sport s Med . 2001;29(2):213 - 218. 67. Kuenze C, Cadmus - Bertram L, Pfieffer K, et al. Relationship between physical activity and clinical outcomes after ACL reconstruction. J Sport Rehabil. 2018:1 - 8. 68. Wright RW. Knee injury outcomes measures. J Am Acad Orthop Surg . 2009;17(1):31 - 39. 69. Briggs KK, Lysh olm J, Tegner Y, Rodkey WG, Kocher MS, Steadman JR. The reliability, validity, and responsiveness of the Lysholm score and Tegner activity scale for anterior cruciate ligament injuries of the knee: 25 years later. Am J Sports Med . 2009;37(5) :890 - 897. 70. van Poppel MN, Chinapaw MJ, Mokkink LB, van Mechelen W, Terwee CB. Physical activity questionnaires for adults: a systematic review of measurement properties. Sports Med . 2010;40(7):565 - 600. 71. Rangul V, Holmen TL, Kurtze N, Cuypers K, M idthjell K. Rel iability and validity of two frequently used self - administered physical activity questionnaires in adolescents. B MC Med Res. 2008;8. 72. Lee PH, Macfarlane DJ, Lam TH, Stewart SM. Validity of the I nternational Physical Activity Questionnaire Short Form ( IPAQ - SF): a systematic review. Int J Behav Nutr Phys Act. 2011;8:115. 162 73. Chomistek AK, Yuan C, Matthews CE, et al. Physical Activity a ss essment with the ActiGraph GT3X and d oubly l abeled w ater. Med Sci Sports Exerc. 2017;49(9):1935 - 1944. 74. Migueles JH, Cadenas - Sanchez C, Ekelund U, et al. A Accelerometer data collection and processing criteria to assess physical activity and other outc omes: A systematic review and practical considerations. Sports Med. 2017 ;47(9):1821 - 1845. 75. John D, Freedson P. A ctiGraph and Actical physical activity monitors: a peek under the hood. Med Sci Sports Exerc. 2012;44(1 Suppl 1):S86 - 89. 76. Choi L, Liu Z, Matthews CE, Buchowski MS. Validation of accelerometer wear and nonwear time classification a lgorithm. Med Sci Spo rts Exerc. 2011;43(2):357 - 364. 77. Freedson PS, Melanson E, Sirard J. Calibration of the Computer Science and Applications, Inc. accelerometer. Med Sci Sports Exerc. 1998;30(5):777 - 781. 78. Sasaki JE, John D, Freedson PS. Validation and comparison of A ctiGraph activity monitors. J Sci Med Sport. 2011;14(5):411 - 416. 79. Matthews CE, Ainsworth BE, Thompson RW, Bassett DR, Jr. Sources of variance in daily physical activity levels as measured b y an accelerometer. Med Sci Sports Exerc. 2002;34(8):1376 - 1381 . 80. Matthews CE, Freedson PS, Hebert JR, et al. Seasonal variation in household, occupational, and leisure time physical activity: longitudinal analyses from the seasonal variation of blood cholesterol study. Am J Epidemiol . 2001;153(2):172 - 183. 81. O'Brien MW, Wojcik WR, Fowles JR. Medical - grade physical activity monitoring f or measuring step count and moderate - to - vigorous physical activity: Validity and reliability study. JMIR mHealth uHealth . 2018;6(9):e10706. 82. Shirazi CP, Israel HA, Kaar SG. Is the Marx Activity Scale reliable in patients younger than 18 years? Sports Health . 2016;8(2):145 - 148. 83. Iversen MD, von Heideken J, Farmer E, Rihm J, Heyworth BE, Kocher MS. Va lidity and comprehensibility of physical activity scales for children wit h sport injuries. J Pediatr Orthop . 2016;36(3):278 - 283. 84. Kowalski KC, Crocker PR, Kowalski NP. Convergent validity of the physical activity questionnaire for adolescents. Pediatr E xerc S ci. 1997;9(4):342 - 352. 85. Dunn WR, Spindler KP. Predictors of activity level 2 years after anterior cruciate ligament recons truction (ACLR): a Multicenter Orthopaedic Outcomes Network (MOON) ACLR cohort study. Am J Sports Med. 2010;38(10):2040 - 2050. 163 86. Ardern CL, Taylor NF, Feller JA, Webster KE. Fifty - five per ce nt return to competitive sport following anterior cruciate lig ament reconstruction surgery: an updated systematic review and meta - analysis including aspects of physical functioning and contextual factors. Br J Sports Med. 2014;48(21):1543 - 1552. 87. Bell DR, Pfeiffer KA, Cadmus - Bertram LA, et al. Objectively measured physical activity in patients after anterior cruciate ligament reconstruction. Am J Sports Med. 2017;45(8):1893 - 1900. 88. Tengman E, Brax Olofsson L, Nilsson KG, Tegner Y, Lundgren L, H ager CK. Anterior cruciate ligament injury after more than 20 years: I. Physical activity level and knee function. Scand J Med Sci Sports. 2014;24(6):e491 - 500. 89. Kuenze C, Lisee C, Pfeiffer KA, et al. Sex differences in physical activity engagement aft er AC L reconstruction. Phys Ther Sport. 2018;35:12 - 17. 90. Tudor - Locke C, Craig CL, Aoyagi Y, et al. How many steps/day are enough? For older adults and special populations. Int J Behav Nutr Phy . 2011;8(1):1. 91. Lisee C LA, Birchmeier T, O'Hagan K, Kuenze C. Quadriceps strength and volitional activation after anterior cruciate ligament reconstruction (ACLR): A systemtic - review and meta - analysis. Sports H ealth. 20 19;11(2):163 - 179 . 92. Kemnitz J, Wirth W, Eckstein F, Ruhdorfer A, Culvenor AG. Longitudinal c hange in thigh muscle strength prior to and concurrent with symptomatic and radiographic knee osteoarthritis progression: data from the Osteoarthritis Initiative. Osteoarthritis Cartilage. 2017;25(10):1633 - 1640. 93. Wang D, Jones MH, Khair MM, Miniaci A. Patient - reported outcome measures for the knee. J Knee Surg. 2010;23(3):137 - 151. 94. Pua YH, Ho JY, Chan SA, Khoo SJ, Chong HC. Associations of isokinetic and isotonic knee strength with knee function and acti vity level after anterior cruciate ligament reconstruction: a prospective cohort study. Knee. 2017;24(5):1067 - 1074. 95. Hootman JM, Macera CA, Ham SA, Helmick CG, Sniezek JE. Physical activity levels among the general US adult population and in adults wi th and without arthritis. Arthritis R heum. 2 003;49(1):129 - 135. 96. Wallis JA, Webster KE, Levinger P, Taylor NF. What proportion of people with hip and knee osteoarthritis meet physical activity guidelines? A systematic review and meta - analysis. Osteoarthritis Cartilage. 2013;21(11):1648 - 1659. 9 7. Richmond SA, Fukuchi RK, Ezzat A, Schneider K, Schneider G, Emery CA. Are joint injury, sport activity, physical activity, obesity, or occupational activities predictors for osteoarthritis? A systematic review. J Orthop Sports Phys Ther . 2013;43(8):515 - B51 9. 164 98. Qin J, Barbour KE, Nevitt MC, et al. Objectively m easured p hysical a ctivity and r isk of k nee o steoarthritis. Med Sci Sports Exerc. 2018;50(2):277 - 283. 99. Lee J, Chang RW, Ehrlich - Jones L, et al. Sedentary behavior and physical function: object ive evidence from the Osteoarthritis Initiative. Arthritis Care Res (Hoboken). 2015;67(3):366 - 373. 100. Wang Y, Simpson JA, Wluka AE, et al. Is physical activity a risk factor for primary knee or hip replacement due to osteoarthritis? A prospective cohor t s tudy. J R heumatol. 2011;38(2):350 - 357. 101. Timmins KA, Leech RD, Batt ME, Edwards KL. Running and knee osteoarthritis: A systematic review and meta - analysis. Am J Sports Med . 2017;45(6):1447 - 1457. 102. Prevention CfDCa. Overweight and Obesity. 2017 ; https://www.cdc.gov/obesity/index.html . Accessed February 7, 2019. 103. Prospective Studies C, Whitlock G, Lewington S, et al. Body - mass index and cause - specific mortality in 900 000 adults: collaborative a nalyses of 57 prospective studies. Lancet. 200 9;373(9669):1083 - 1096. 104. Sridhar MS, Jarrett CD, Xerogeanes JW, Labib SA. Obesity and symptomatic osteoarthritis of the knee. J Bone Joint Su rg . 2012;94(4):433 - 440. 105. Pamukoff DN, Dudley RI, Vakula MN, Blackburn JT. An evaluation of the heel stri ke transient in obese young adults during walking gait. Gait Posture. 2016;49:181 - 183. 106. Pamukoff DN, Lewek MD, Blackburn JT. Greater vertical loading rate in obese compared to normal weight young adults. Clin B iomech. 2016;33:61 - 65. 107. Wohlfahrt - Veje C, Tinggaard J, Winther K, et al. Body fat throughout childhood in 2647 healthy Danish ch ildren: agreement of BMI, waist circumference, skinfolds with dual X - ray absorptiometry. E J C lin N utr. 2014;68(6):664 - 670. 1 08. Ogden CL, Fryar CD, Carroll MD, Flegal KM. Mean body weight, height, and body mass index, United States 1960 - 2002. Adv D ata. 2004(347):1 - 17. 109. Hales CM, Carroll MD, Fryar CD, Ogden CL. Prevalence of obesity among adults and youth: United States, 2015 - 2016. NCHS Data Brief . 2017(288): 1 - 8. 110. Ogden CL, Carroll MD, Kit BK, Flegal KM. Prevalence of obesity and trends in body mass index among US children and adolescents, 1999 - 2010. J AMA . 2012;307(5):483 - 490. 165 1 11. Burns EA, Collins AD, Jack RA, 2nd, McCulloch PC, Lintner DM, Harris JD . Trends in the body mass index of pediatric and adult patients undergoing anterior cruciate ligament reconstruction. Orthop J Sports Med . 2018;6(4):2325967118767398. 12. Harput G, Guney - Deniz H, Ozer H, Baltaci G, Mattacola C. Higher body mass index ad versely affects knee function after anterior cruciate ligament reconstruction in individuals who are recreationally active. Clin J Sports Med . 2018;18 [Epub ahead of print] . 113. Reyes C, Leyland KM, Peat G, Cooper C, Arden NK, Prieto - Alhambra D. Associa tion between overweight and obesity and risk of clinically diagnosed knee, hip, and hand osteoarthritis: A population - based cohort study. Arthritis Rheum . 2016;68(8):1869 - 1875. 114. Pietrosimone B, Kuenze C, Hart JM, et al. Weak associations between body mass index and self - reported disability in people with unilateral anterior cruciate ligament reconstruction. Knee Surg Sports Traumatol Arthrosc. 2018;26( 5):1326 - 1334. 115. Cox CL, Huston LJ, Dunn WR, et al. Are articular cartilage lesions and meniscus tears predictive of IKDC, KOOS, and Marx activity level outcomes after anterior cruciate ligament reconstruction? A 6 - year multicenter cohort study. Am J S ports Med . 2014;42(5):1058 - 1067. 116. Filbay SR, Ackerman IN, Russell TG, Crossley KM. Return to sp ort matters - longer - term quality of life after ACL reconstruction in people with knee difficulties. Scand J Med Sci Sports. 2017;27(5):514 - 524. 117. Kuenze C, Hertel J, Saliba S, Diduch DR, Welt man A, Hart JM. Clinical thresholds for quadriceps assessment after anterior cruciate ligament reconstruction. J Sport Rehabil. 2015;24(1):36 - 46. 118. Hart HF, Culvenor AG, Collins NJ, et al. Knee kinematics and joint moments during gait following anterior cruciate ligament reconstruction: a systematic review and me ta - analysis. Br J Sports Med. 2016;50(10):597 - 612. 119. Robertson DGE, Caldwell GE, Hamill J, Kamen G, Whittlesey SN. Research methods in biomechanics 2nd edition. Human Kinetics; 2013. 120. Hart HF, Collins NJ, Ackland DC, Cowan SM, Crossley KM. Gait c haracteristics of p eople with l ateral k nee o steoarthritis after ACL r econstruction. Med Sci Sports Exerc. 2015;47(11):2406 - 2415. 121. Shimizu T, Samaan MA, Tanaka MS, et al. Abnormal biomechanics at 6 months are associated with cartilage degeneration at 3 years after anterior cruciate ligament reconstruction. Arthroscopy . 2018;35(2):511 - 520 166 1 22. Khandha A, Manal K, Wellsandt E, Capin J, Snyder - Mackler L, Buchanan TS. Gait mechanics in those with/without medial compartment knee osteoarthritis 5 years a fter anterior cruciate ligament reconstruction. J Orthop Res . 2017;35(3):625 - 633. 123. Slater LV, Hart JM, Kelly AR, Kuenze CM. Progressive changes in walking kinematics and kinetics a fter anterior cruciate ligament injury and reconstruction: A review an d meta - analysis. J Athl Train. 2017;52(9):847 - 860. 124. Pietrosimone B, Blackburn JT, Harkey MS, et al. Greater mechanical loading during walking is associated with less collagen turno ver in individuals with anterior cruciate ligament reconstruction. Am J Sports Med . 2016;44(2):425 - 432. 125. Johnston CD, Goodwin JS, Spang JT, Pietrosimone B, Blackburn JT. Gait biomechanics in individuals with patellar tendon and hamstring tendon anter ior cruciate ligament reconstruction grafts. J Biomech. 2019;82:103 - 10 8. 126. Pietrosimone B, Seeley MK, Johnston C, Pfeiffer SJ, Spang JT, Blackburn JT. Walking g round r eaction f orce p ost - ACL r econstruction: Analysis of t ime and s ymptoms. Med Sci Sports Exerc. 2019;51(2):246 - 254. 127. Cisternas MG, Murphy L, Sacks JJ, S olomon DH, Pasta DJ, Helmick CG. Alternative methods for defining osteoarthritis and the impact on estimating prevalence in a US population - based survey. Arthritis Care Res (Hoboken) . 2016;68(5):574 - 580. 128. United States Bone and Joint Initiative: The Burden of Musculoskeletal Diseases in the United States (BMUS). 2014; 3rd: https://www.boneandjointburden.org/ . Accessed May 25, 2019 . 129. W. Herzog FS. Biological Materials - Articular Cartilage. In: Benno M. Nigg WH, ed. Biomechanics of the Musculo - Ske letal System. 2nd ed. Ann Arbor, Michigan: Wiley; 1999:95 - 109. 130. Pamela K. Levangie CCN, Michael Lewek. Joint Structure and Function: A Comprehensive Analysis. 5th ed: F.A. David Company; 2019. 131. Buckwalter JA, Mow VC, Ratcliffe A. Restoration of injured or degenerated articular cartilage. J Am Acad Ortho Surg . 1994;2(4):192 - 201. 132. Li G, Yin J, Gao J, et al. Subchondral bone in o steoarthritis: insight into risk factors and microstructural changes. Arthritis R es T her. 2013;15(6):223 - 223. 133 . Felson DT, McLaughlin S, Goggins J, et al. Bone marrow edema and its relation to progression of knee osteoarthritis. Ann I ntern M ed. 2003;139(5):330 - 336. 167 1 34. Eckstein F, Boeth H, Diederichs G, et al. Longitudinal change in femorotibial cartilage thick ness and subchondral bone plate area in male and female adolescent vs. mature athletes. Ann Anat . 2014;196(2 - 3):150 - 157. 135. Jones G , Ding C, Glisson M, Hynes K, Ma D, Cicuttini F. Knee articular cartilage development in children: a longitudinal study o f the effect of sex, growth, body composition, and physical activity. Pediat r R es. 2003;54(2):230 - 236. 136. Vos T, Allen C, Arora M, et al. Global, regional, and national incidence, prevalence, and years lived with disability for 310 diseases and injurie s, 1990 2015: a systematic analysis for the Global Burden of Disease Study 2015. Lancet. 2016;388(10053):1545 - 1602. 137. Losina E, Paltiel AD, Weinstein AM, et al. Lifetime medical costs of knee osteoarthritis management in the United States: impact of e xtending indications for total knee arthroplasty. Arthritis Care Res (Hoboken). 2015;67(2):203 - 215. 138. Alk an BM, Fidan F, Tosun A, Ardicoglu O. Quality of life and self - reported disability in patients with knee osteoarthritis. Mod R heumatol. 2014;24(1) :166 - 171. 139. Lane NE, Brandt K, Hawker G, et al. OARSI - FDA initiative: defining the disease state of osteoarthritis. Osteoarthritis Cartilage. 2011;19(5):478 - 482. 140. O'Neill TW, McCabe PS, McBeth J. Update on the epidemiology, risk factors and dise ase outcomes of o steoarthritis. Best Prac Res Cl Rh. 2018;32(2):312 - 326. 141. Bedson J, Croft PR. The discordance between clinical and radiographic knee osteoarthritis: a systematic search and summary of the literature. BMC Musculoskelet Disord. 2008;9:1 16. 142. Dillon CF, Rasch EK, Gu Q, Hirsch R. Prevalence of knee osteoarthritis in the United States: arthritis data from the Third National Health and Nutrition Examination Survey 1991 - 94. J Rheumatol . 2006;33(11):2271 - 2279. 143. Parsons C, Fuggle NR, Edwards MH, et a l. Concordance between clinical and radiographic evaluations of knee osteoarthritis. Aging Clin Exp Res . 2018;30(1):17 - 25. 144. Muthuri SG, McWilliams DF, Doherty M, Zhang W. History of knee injuries and knee osteoarthritis: a meta - analy sis of observatio nal studies. Osteoarthritis Cartilage. 2011;19(11):1286 - 1293. 145. Lie MM, Risberg MA, Storheim K, Engebretsen L, Oiestad BE. What's the rate of knee osteoarthritis 10 years after anterior cruciate ligament injury? An updated systematic review. Br J Sports Med. 2019. 1 46. Ackerman IN, Kemp JL, Crossley KM, Culvenor AG, Hinman RS. Hip and k nee o steoarthritis a ffects y ounger p eople, t oo. J Orthop Sporst Phys Ther . 2017;47(2):67 - 79. 168 147. Kohn MD, Sassoon AA, Fernando ND. Classifications in brief: Kellgren - Lawrence classification of osteoarthritis. Clin Orthop Relat Res . 2016;474(8):1886 - 1893. 148. Kraus VB, Burnett B, Coindreau J, et al. Application of biomarkers in the development of drugs intended for the treatment of osteoarthritis. Osteoarthritis Cartilage. 20 11;19(5):515 - 542. 149. Wright RW. Osteoarthritis classification scales: Interobserver reliability and arthroscopic correlation. J Bone Joint Surg . 2014;96(14):1145 - 1151. 150. Harkey MS, Luc BA, Golightly YM, et al. Osteoarth ritis - related biomarkers following anterior cruciate ligament injury and reconstruction: a systematic review. Osteoarthritis Cartilage. 2015;23(1):1 - 12. 151. Chmielewski TL, Trumble TN, Joseph AM, et al. U rinary CTX - II concentrations are elevated and ass ociated with knee pain and function in subjects with ACL reconstruction. Osteoarthritis Cartilage. 2012;20(11):1294 - 1301. 152. Dam EB, Loog M, Christiansen C, et al. Identification of progressors in osteoa rthritis by combining biochemical and MRI - based m arkers. Arthritis R es T her. 2009;11(4):R115. 153. Wáng Y - physics principles and applications in knee and intervertebral disc imagin g. Quant I maging M edicine S urg. 2015;5(6):858 - 885. 154. Mosher TJ, Dardzinski BJ. Cartilage MR I T2 relaxation time mapping: overview and applications. Seminars M usculoskelet R adiol. 2004;8(4):355 - 368. 155. Regatte RR, Akella SV, Lonner JH, Kneeland JB, Reddy R. T1rho relaxation mapping in human osteoarthritis (OA) cartilage: comparison of T1rho w ith T2. J Magn R eson I maging. 2006;23(4):547 - 553. 156. Hatcher CC, Collins AT, Kim SY, et al. Relationship betwe en T1rho magnetic resonance imaging, synovial fluid biomarkers, and the biochemical and biomechanical properties of cartilage. J Biomech. 2017 ;55:18 - 26. 157. Van Ginckel A, Verdonk P, Witvrouw E. Cartilage adaptation after anterior cruciate ligament inju ry and reconstruction: implications for clinical management and research? A systematic review of longitudinal MRI studies. Osteoarthritis Cart ilage. 2013;21(8):1009 - 1024. 158. Li X, Benjamin Ma C, Link TM, et al. In vivo T(1rho) and T(2) mapping of articular cartilage in osteoarthritis of the knee using 3 T MRI. Osteoarthritis Cartilage. 2007;15(7):789 - 797. 169 1 59. Kumar D, Su F, Wu D, et al. F rontal plane knee mechanics and earl y cartilage degeneration in people with anterior cruciate ligament reconstruction: A longitudinal study. Am J Sports Med . 2018;46(2):378 - 387. 160. Li X, Kuo D, Theologis A, et al. Cartilage in anterior cruciate ligamen t - reconstructed knees: MR imaging T1 {rho} and T2 -- initial experience with 1 - year follow - up. Radiology. 2011;258(2):505 - 514. 161. Li H, Chen S, Tao H, Chen S. Quantitative MRI T2 relaxation time evaluation of knee cartilage: comparison of meniscus - intact and - injured knees after anterior cr uciate ligament reconstruction. Am J Sports Med . 2015;43(4):865 - 872. 162. Van Ginckel A, Verdonk P, Victor J, Witvrouw E. Cartilage status in relation to return to sports after anterior cruciate ligament reconstruction . Am J Sports Med . 2013;41(3):550 - 55 9. 163. Ostergaard M, Court - Payen M, Gideon P, et al. Ultrasonography in arthritis of the knee. A comparison with MR imaging. Acta R adiol. 1995;36(1):19 - 26. 164. Liu B, Lad NK, Collins AT, et al. In vivo tibial carti lage strains in regions of cartilage - to - cartilage contact and cartilage - to - meniscus contact in response to walking. Am J Sports Med . 2017;45(12):2817 - 2823. 165. Vanwanseele B, Lucchinetti E, Stussi E. The effects of immobilization on the characteristics of articular cartilage: cu rrent concepts and future directions. Osteoarthritis Cartilage. 2002;10(5):408 - 419. 166. Cotofana S, Buck R, Wirth W, et al. Cartilage thickening in early radiographic knee osteoarthritis: a within - person, between - knee comparis on. Arthritis Care Res (Hob oken). 2012;64(11):1681 - 1690. 167. Nomura M, Sakitani N, Iwasawa H, et al. Thinning of articular cartilage after joint unloading or immobilization. An experimental investigation of the pathogenesis in mice. Osteoarthritis Carti lage. 2017;25(5):727 - 736. 168. Maldonado DC, Silva MCPd, Neto SE - R, de Souza MR, de Souza RR. The effects of joint immobilization on articular cartilage of the knee in previously exercised rats. J Anat . 2013;222(5):518 - 525. 169. Vincent TL, Wann AKT. M echanoadaptation: articular cartilage through thick and thin. J P hysio l . 2018 ;597(5);1271 - 1281. 70. Pietrosimone B, Troy Blackburn J, Harkey MS, et al. Walking speed as a potential indicator of cartilage breakdown following anteri or cruciate ligament re construction. Arthritis Care Res (Hoboken) . 2016;68(6):793 - 800. 170 171. Patterson BE, Culvenor AG, Barton CJ, et al. Worsening knee osteoarthritis features on magnetic resonance imaging 1 to 5 years after anterior cruciate ligament reconstruction. Am J Spor ts Med . 2018:363546518789685. 172. Thoma LM, Snyder - Mackler L, Risberg M, White DK. Trajectories of weight gain in young adults following anterior cruciate ligament rupture: the delaware - Oslo ACL cohort study. Osteoarthritis Cartilage. 2019;27:S274 - S275. 173. Blackburn JT, Pietrosimone B, Harkey MS, Luc BA, Pamukoff DN. Quadriceps function and gait kinetics after anterior cruciate ligament reconstruction. Med Sci Sports Exerc. 2016;48(9):1664 - 1670. 174. Oiestad BE, Juhl CB, Eitzen I, Thorlund JB. Knee extensor muscle weakness is a risk f actor for development of knee osteoarthritis. A systematic review and meta - analysis. Osteoarthritis Cartilage. 2015;23(2):171 - 177. 175. Pietrosimone B, Loeser RF, Blackburn JT, et al. Biochemical markers of cartilage metabolism are associated with walkin g biomechanics 6 - months following anterior cruciate ligament reconstruction. J Orthop Res . 2017;35(10):2288 - 2297. 176. Pamukoff DN, Montgomery MM, Holmes SC, Moffit TJ, Garcia SA, Vakula MN. Association between gait m echanics and ultrasonographic measures of femoral cartilage thickness in individuals with ACL reconstruction. Gait Posture. 2018;65:221 - 227. 177. Hosseini A , Van de Velde S, Gill TJ, Li G. Tibiofemoral cartilage contact biomechanics in patients after rec onstruction of a ruptured anterior cruciate ligament. J Ortho p Res . 2012;30(11):1781 - 1788. 178. Luc - Harkey BA, Franz JR, Hackney AC, Blackburn JT, Padua DA, Pietrosimone B. Lesser lower extremity mechanical loading associates with a greater increase in s erum cartilage oligomeric matrix protein following walking in individuals with anterior cruciate ligament reconstruction. Clin Biomech . 2018;60:13 - 19. 179. Zhang W, Doherty M, Peat G, et al. EULAR evidence - based recommendations for the diagnosis of knee osteoarthritis. Ann Rheum Dis. 2010;69(3):483 - 489. 180. Eckstein F, Cicuttini F, Raynauld JP, Waterton JC, Peterfy C. Magnetic resonance imaging (MRI) of ar ticular cartilage in knee osteoarthritis (OA): morphological assessment. Osteoarthritis Cartilage. 2006;14 Suppl A:A46 - 75. 181. Favero M, Ramonda R, Goldring MB, Goldring SR, Punzi L. Early knee osteoarthritis. RMD Open. 2015;1(Suppl 1):e000062 - e000062. 182. Lisee C, McGrath ML, Kuenze C, et al. Novel semi - automated ultrasound segmentation techniqu e for assessing avergae regional femoral articular cartilage thickness. J Sport Rehabil. IN PRESS. 171 183. Harkey MS, Blackburn JT, Davis H, Sierra - Arevalo L, Nissman D, Pietrosimone B. The association between habitual walking speed and medial femoral carti lage deformation following 30minutes of walking. Gait Posture. 2018;59:128 - 133. 184. Cotofana S, Eckstei n F, Wirth W, et al. In vivo measures of cartilage deformation: patterns in healthy and osteoarthritic female knees using 3T MR imaging. Eur Radiol. 2 011;21(6):1127 - 1135. 185. Armstrong LE, Kavouras SA, Walsh NP, Roberts WO. Diagnosing dehydration? Blend evidence with clinical observations. Curr Opin Clin Nutr Metab Care . 2016;19(6):434 - 438. 186. Schmitz RJ, Wang HM, Polprasert DR, Kraft RA, Pietros imone BG. Evaluation of knee cartilage thickness: A comparison between ultrasound and magnetic resonance i maging methods. Knee. 2017;24(2):217 - 223. 187. Pfeiffer SJ, Davis - Wilson HC, Pexa B, et al. Assessing s tep c ount - d ependent c hanges in f emoral a rticu lar c artilage u sing u ltrasound. J Med Ultrasound. 2019. [Epub Ahead of Print] 188. Koo TK, Li MY. A g uideline of s electing and r eporting i ntraclass c orrelation c oefficients for r eliability r esearch. J Chiropr Med . 2016;15(2):155 - 163. 189. Riemann BL, L ininger MR. Statistical Primer for Athletic Trainers: The essentials of understanding measures of reliability and minimal important change. J Athl Train . 2018;53(1):98 - 103. 190. Bland JM, Altman DG. Agreement between methods of measurement with multiple observations per individual. J Biopharm Stat . 2007;17(4):571 - 582. 191. Zou GY. Sample size formulas for estimating intraclass correlation coefficients with precision and assurance. Stat Me d . 2012;31(29):3972 - 3981. 192. Pamukoff DN, Montgomery MM, Moffi t TJ, Vakula MN. Quadriceps f unction and k nee j oint u ltrasonography after ACL r econstruction. Med Sci Sports Exerc. 2018;50(2):211 - 217. 193. Lee SW, Patel J, Van Dien C, et al. The transverse infrapatellar view: A new ult rasound technique to measure dist al femoral cartilage thickness. J Ultrasound Med 2019. [Epub ahead of print] . 194. Spannow AH, Stenboeg E, Pfeiffer - Jensen M, Herlin T. Ultrasound measurement of joint cartilage thickness in large and small joints in health y children: a clinical pilot stud y assessing observer variability. Pediatr R heumatol O nline J . 2007;5:3. 172 1 95. Eckstein F, Ateshian G, Burgkart R, et al. Proposal for a nomenclature for magnetic resonance imaging based measures of articular cartilage in osteoarthritis. Osteoarthritis Car tilage. 2006;14(10):974 - 983. 196. Spannow AH, Pfeiffer - Jensen M, Andersen NT, Herlin T, Stenbog E. Ultrasonographic measurements of joint cartilage thickness in healthy children: age - and sex - related standard reference values. J R heumatol. 2010;37(12):25 95 - 2601. 197. Fokkema T, Kooiman TJ, Krijnen WP, CP VDS, M DEG. Reliability a nd v alidity of t en c onsumer a ctivity t rackers d epend on w alking s peed. Med Sci Sports Exerc. 2017;49(4):793 - 800. 198. Sanders JO, Qiu X, Lu X, et al. The uniform pattern of gr owth and skeletal maturation during the human adolescent growth spurt. Sci Rep . 2017;7(1):16705. 199. Anderson M. Use of the Greulich - Pyle "Atlas of Skeletal Development of the Hand and Wrist" in a clinical context. Am J P hys A nthropol. 1971;35(3):347 - 35 2. 200. Oo WM, Bo MT. Role of u ltrasonography in k nee o steoarthritis. J C lin Rheumatol . 2016;22(6):324 - 329. 201. Pfeiffer S, Harkey MS, Stanley LE, et al. Associations between slower walking speed and T1rho magnetic resonance imaging of femoral cartila ge following anterior cruciate ligament reconstruction. Arthritis Care Res (Hoboken) . 2018;70(8):1132 - 1140. 202. van Rossom S, Smith CR, Thelen DG, Vanwanseele B, Van Assche D, Jonkers I. Knee joint loading in healthy adults during functional exercises: Implications for rehabilitation guidelines. J Orthop Sports Phys Ther . 2018;48(3):162 - 173. 203. Poulsen E, Goncalves GH, Bricca A, Roos EM, Thorlund JB, Juhl CB. Knee osteoarthritis risk is increased 4 - 6 fold after knee injury - a systematic review and m eta - analysis. Br J Sports Med. 2019;53(23):1454 - 1463. 204. Lepley AS, Kuenze CM. Hip and knee kinematics and kinetics during landing tasks after anterior cruciate ligament reconstruction: A systematic review and meta - analysis. J Athl Train. 2018;53(2):144 - 159. 205. Davis MA, Ettinger WH, Neuhaus JM, Mallon KP. Knee osteoarthritis and physical functioning: evidence from the NHANES I Epidemiologic Followup Study. J Rheumatol . 1991;18(4):591 - 598. 206. Rambaud AJM, Ardern CL, Thoreux P, Regnaux J - P, Edouar d P. Criteria for return to run ning after anterior cruciate ligament reconstruction: a scoping review. Br J Sports Med. 2018;52(22):1437. 173 207. Birchmeier T, Lisee C, Geers B, Kuenze C. Reactive strength index and knee extension strength characteristics a re predictive of single - leg hop performance after anterior cruciate ligament reconstruction. J Strength Cond Res . 2019;33(5):1201 - 1207. 208. Bell AL, Pedersen DR, Brand RA. A comparison of the accuracy of several hip center location prediction methods. J Biomech. 1990;23(6):617 - 621. 209. Montoye AHK, Moore RW, Bowles HR, Korycinski R, Pfeiffer KA. Reporting accelerometer methods in physical a ctivity intervention studies: a systematic review and recommendations for authors. Br J Sports Med. 2018;52(23):1 507 - 1516. 210. Buck RJ, Wyman BT, Hellio Le Graverand MP, Hudelmaier M, Wirth W, Eckstein F. Osteoarthritis may not be a one - way - road of cart ilage loss comparison of spatial patterns of cartilage change between osteoarthritic and healthy knees. Osteoar thritis Cartilage. 2010;18(3):329 - 335. 211. Su F, Hilton JF, Nardo L, et al. Cartilage morphology and T1rho and T2 quantification in ACL - reconstructed knees: a 2 - year follow - up. Osteoarthritis Cartilage. 2013;21(8):1058 - 1067. 212. Williams A, Winalski CS, Chu CR. Early articular cartilage MRI T2 c hanges after anterior cruciate ligament reconstruction correlate with later changes in T2 and cartilage thickness. J Orthop Res . 2017;35(3):699 - 706. 213. Lane AR, Harkey MS, Davis HC, et al. Body m ass i ndex a nd t ype 2 c ollagen t urnover in i ndividuals a ft er a nterior c ruciate l igament r econstruction. J Athl Train. 2019;54(3):270 - 275. 214. Davis - Wilson HC, Pfeiffer SJ, Johnston CD, et al. Bilateral g ait 6 and 12 m onths p ost - a nterior c ruciate l igament r econstruc tion c ompared with c ontrols. Med Sci Sports Exerc. 2020;52(4):785 - 794. 215. Saxby DJ, Bryant AL, Modenese L, et al. Tibiofemoral c ontact f orces in the a nterior c ruciate l igament - r econstructed l nee. Med Sci Sports Exerc. 2016;48(11):2195 - 2206. 216. Well sandt E, Gardinier ES, Manal K, Axe MJ, Buchanan TS, Snyder - Mackler L. Decreased knee joint loading associated with early knee osteoarthritis after anterior cruciate ligament injury. Am J Sports Med . 2016;44(1):143 - 151. 217. Luc - Harkey BA, Franz JR, Blac kburn JT, Padua DA, Hackney AC, Pietrosimone B. Real - time biofeedback can increase and decrease vertical ground reaction force, knee flexion excursion, and knee extension moment during walking in individuals with anterior cruci ate ligament reconstruction. J Biomech. 2018;76:94 - 102. 218. Tudor - Locke C, Leonardi C, Johnson WD, Katzmarzyk PT, Church TS. Accelerometer steps/day translation of moderate - to - vigorous activity. Prev M ed. 2011;53(1 - 2):31 - 33. 174 219. Schuna JM, Johnson WD, Tudor - Locke C. Adult self - r eported and objectively monitored physical activity and sedentary behavior: NHANES 2005 2006. Int J Behav Nutr Phys . 2013;10:126 - 126. 220. Tudor - Locke C, Johnson WD, Katzmarzyk PT. Accelerometer - determine d steps per day in US children and youth. Med Sci Sports Exerc. 2010;42(12):2244 - 2250. 221. Kozey - Keadle S, Libertine A, Lyden K, Staudenmayer J, Freedson PS. Validation of wearable monitors for assessing sedentary behavior. Med Sci Sports Exerc. 2011;43(8):1561 - 1567. 222. Oo WM, Linklater JM, Hunter DJ. Imaging in knee osteoarthritis. Curr O pin R heumatol. 2017;29(1):86 - 95.