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This is to certify that the

thesis entitled

MECHANICAL PROPERTIES OF HTRLR)

presented by

Danette Skowronski Taylor

has been accepted towards fulfillment
of the requirements for

Master of Science degree inBiomechanics

 

 

mw

Major professor

Jul 26 1990
Date Y '

 

0-7639 MS U is an Affirmative Action/Equal Opportunity Institution

 

 

UBRARY
Mlchtgan State
University

 

 

 

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MSU Is An Affirmative Action/Equal Opportunity Institution
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MECHANICAL PROPERTIES OF HTRO

BY

DANETTE SKOWRONSKI TAYLOR. D.O.

A THESIS

Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER of SCIENCE

Department of Biomechanics

1990

ABSTRACT
MECHANICAL PROPERTIES OF HTRQ
BY

DANETTE SKOWRONSKI TAYLOR

Attention has been focused on the need for substitutes
for natural bone graft. This research examined the
compressive mechanics of and implant potential of one such
substitute: Hard Tissue Replacement, or HTRO.

HTRO was molded by the manufacturer into dowels
representative of the shape used in the Cloward anterior
spinal fusion. The dowels were tested along axial and
di ametric compress ion. The load-deformat ion curves
generated were analyzed to obtain stiffness. yield load and
peak load. The data from the diametric tests was compared
to the results of similar tests run on natural bone dowels.

A study to explore the amount of bony ingrowth into
HTRO was run in conjunction with the mechanical testing.
Plugs were implanted in canine cervical spines; after a
designated time. the implants were examined histologically
for evidence of ingrowth.

HTRO had yield loads from 74 - 216% higher than
natural bone. Stiffness averaged 31% higher. Results of
the implantation study showed clear signs of bony ingrowth
without gross evidence of inflammation.

ACKNOWLEDGEMENTS

The author wishes to express her appreciation to the
following persons and corporation for their support during
the years of this Master's program:

Dr. R. W. Soutas-Little for his support. ideas and
direction of this project.

U.S. Surgical Corporation. especially Dr. M. Hermes
and Mr. R. Torgerson. for their generous financial
support and supply of materials.

Dr. James Rechtien for his guidance. interest and
friendship during this program.

Dr. Roger Rant and Dr. Charles DeCamp for their
important input and constructive ideas regarding this
project.

Troy R. Taylor and the Skowronski family for their

constant support. encouragement and love during my
years in school.

iii

TABLE OF CONTENTS

gage

ACKNOWLEDGEMENTS ...................................... iii
LIST OF FIGURES ....................................... v
LIST OF TABLES ........................................ vii
SECTION

I. INTRODUCTION ................................... 1

II. LITERATURE SURVEY ............................... 5
III. MATERIALS AND METHODS ........................... 11

Iv. RESULTS ......................................... 25

v. DISCUSSION ...................................... 4o
BIBLIOGRAPHY .......................................... 47
APPENDIX A ........................................... 51

iv

10.

11.

12.

LIST OF FIGURES

Photograph showing structural compari-
son of trabecular bone (left) and HTRO.

Simplified Cloward procedure ...........
HTRP Cloward dowels of varying sizes...

"Stadium" shaped Cloward dowels (left)
and traditional round dowels ...........

Results showing the contrast between
modes of failure of "dry" HTRO plugs
and those soaked in lactated Ringers
solution ...............................

HTRO plug within grips representing
vertebral bodies ...... . ................

Typical load-deformation curve gener-
ated during compression of HTRO ........

Typical load-deformation curve showing
method of calculating stiffness and

yield load. as well as location of peak
load ...................................

Variability seen during axial compres-
sion ...................................

Graphic results of weighing experiment.
All Plugs were kept at 37°C. Plugs
were not allowed to return to room
temperature prior to weighing ..........

Moduli values (N/mmz) calculated after
axial compression. Temperature values
refer to pre-test conditions; all tests
were run at room temperature ...........

Yield loads of natural bone and HTRQ
grafts in diametric compression. All
tests were run at identical strain rates
and at room temperature. Descriptions
listed are pre-test conditions .........

12

15

18

21

23

24

26

27

3O

31

13.

14.

15.

16.

1'7.

Subgross photomicrograph of fused inter-
vertebral space demonstrating porous
HTRG. Note the fracture in the middle
of the graft. There is also a sectioned
bone pin (middle right region). 1. 2.
and 3 refer to zones examined by conven-
tional light microscopy. HSE. 4X ......

Photomicrograph from Zone 1 of figure
13. There is an abrupt transition
from normal. woven trabecular bone
(left) into the pale-staining osteoid
growing within the HTRO. HSE. 18X .....

Photomicrograph from Zone 1 of figure
13: Bony trabeculae growing within

the porous implant material. There
are osteoblasts with rounded nulcei
surrounded by an osteoid matrix. HSE.
72X ....................................

Forces acting on the head and neck .....

Graft shapes used for spinal fusions...

vi

34

36

38
43
45

LI ST OF TABLES

Variables included in primary testing..
Results of weighing experiment .........

Mechanical properties of HTRP dowels in
diametric compression. Group descrip-
tion refers to pre-testing conditions.
All tests were run at room temperature
and at identical strain rates. follow-
ing protocol used in natural bone allo-
graft testing ..........................

Mechanical properties of trabecular

bone dowels in diametric compres-
sion(22) ...............................

vii

16
28

32

33

INTRODUCTION

Foreign materials have been used as implants to
strengthen bony material since the sixteenth century (10).
Over the years, substances ranging from metal to plastic
and coral to natural bone have been employed in an attempt
to increase the strength and therefore the function of
various bones within the body.

Of this variety of materials, natural bone has
long been considered the "ideal" graft substance for use in
bony structures: natural bone is hydrophilic. osteogenic.
and strong. Additionally, there are three potential
sources of natural bone which can be harvested for use as
graft: xenograft. bone from a species separate from that
which the graft will be used in; allograft. bone from a
different individual of the same species as the recipient;
and autograft, bone taken from one location for use in a
separate location within the same individual (15).

Problems are inherent within each of these types
of grafts. The use of autologous bone requires a second
major surgical procedure to obtain the graft. which can
contribute to an increase in morbidity. Xenograft and
allograft carry a risk of inducing an immunogenic response
as well as the danger of infection. Although sterilization
protocols are required, the procedures commonly used --
lyophiliztion and/or gamma irradiation -- have been shown
to damage the structural integrity of the graft (38).

1

The need for an artificial grafting material.
allowing circumvention of the problems outlined above. is
apparent. However. a successful graft must meet all the
criteria (”I a formidable list. The ideal bone substitute
has been described as being biologically inert, readily
available, easily adaptable to the site in which it is
implanted. and ultimately. replaceable by host tissue, that
is. biodegradable (4. 5. 35). It can be argued that the
biodegradation of a material is not necessary. provided the
other criteria are satisfied and the tissue surrounding the
graft is able to adapt as required by environmental changes
(5).

Presented here are the preliminary results of
research on a synthetic graft material trade-named Hard
Tissue Replacement. or HTRO, with comparison made to the
compressive strength of trabecular bone (Figure 1). This
material is a polymeric composite constructed of poly
methylmethacrylate (PMMA) and poly hydroxyethyl
methacrylate (PI-IEMA). In a patented process. a core of
PMMA is coated with an envelope of PHEMA; the result is a
granular or beaded substance, each head maintaining a
hydrophobic (PMMA) interior and a hydrophilic (PHEMA)
exterior. This material can be left in a loose granular
form or heat-molded into various shapes. FDA approval for
HTRO use in dental applications such as alveolar ridge
augmentation or as a filling within bony defects has

already been obtained (3, 4). Reports from case studies

 

FIGURE 1: Photograph showing structural
comparison of trabecular bone (left) and
HTRO.

3

indicate that HTRO is a light-weight scaffold which bone
easily surrounds and penetrates (3). This suggests that
HTRO may easily adapt to other bony areas where graft is
needed.

This research program used a Cloward plug model.
which was one of the first widely accepted uses of graft
material in the 0.8. The procedure replaces a defective
disc within the cervical spine with a cylindrical dowel of
bone. thereby maintaining disc space and protecting the
nerve roots which exit the spinal cord in the area (14)
(Figure 2).

The graft used must maintain structural function
until fusion of the vertebral bodies surrounding the graft
is complete -- occasionally as long as six months (41). If
the graft fails during fusion. nerve root impingement and
subsequent tisSue damage following collapse of the graft is)
a possibility.

While it is recognized that additional forces such
as torsion and bending also act upon the graft (23). the
initial load that the graft must bear is compression;
therefore. this study was solely a comparison of direct
compressive loads on HTRO and trabecular bone. A variety
of environments were tested in an attempt to simulate

physiologic or ig-vivo surroundings.

Exposed vertebral column (anterior view)

Vertebral disc removed; space tor Cloward
dowel enlarged

 

Aliognrait dowel In place within vertebral
colu n

 

FIGURE 2: Simplified Cloward procedure.
5

LITERATURE SURVEY

From the year 1775, when in France a metal wire
was used to fasten together fragments of a shattered
humerus (42). there have been attempts to improve the
healing process of the human body by the implantation of
foreign substances. However, it was not until aseptic
surgical technique was developed by Lister (32) almost a
full century later that implants were successful in helping
to repair defects within the human structure.

With the development of sterile technique came the
first successful transplant of bone between individuals in
1878 (27). Ten years later. Fraenkel (19) successfully
implanted a plastic material when he used celluloid to fill
large defects in canine skulls.

The onset of the twentieth century brought
increased knowledge about natural bone grafts. This data
included information regarding antigenicity. graft
strength. and controversy over how the grafts were
ultimately incorporated into the body. Nonetheless. it was
not until 1950 that a formalized method for collecting and
preserving bone and other tissues for transplant purposes
was developed by the U.S. Navy (11).

As the number of tissues available for
transplantation increased. so too did the rate of infection
following implantation. In 1955 Carr and Hyatt (l3)
hypothesized that greater than ten percent of transplanted

6

grafts at that time acted to introduce infection into the
recipient. This high rate of infection occurred in spite
of the careful tissue banking procedure developed by the
Navy. which eliminated graft procurement from a donor if
the cause of death had contaminated the tissues in any way
(11).

In an effort to combat this rate of infection.
Carr, e_t_: §_1_ (13) proposed freeze—drying or lyophilization
as a method of sterilizing the tissue grafts. While this
treatment prolonged the storage time of the tissue. it was
not effective in reducing the rate of infection. In 1958
DeVries. gt a; (16) suggested the use of radioactive cobalt
(cobalt 60) as a means to sterilize contaminated bone.

With increased handling and processing. the
integrity of bone grafts decreased. Triantafyllou. e_t_ a;
(38) showed that with combined freeze-drying and
sterilization with gamma irradiation the strength of bone
was decreased in most cases to less than 30 percent of
comparable fresh bone specimens. Using these results and
the results of independent tests which confirmed
Triantafyllou's findings. Pelker. gt a_]._ (33) developed a
hypothesis which cautioned that the methods of
sterilization and storage of a graft be carefully examined
prior to its implantation to avoid the possibility of
placing the bone into a location where failure was

inevitable.

As research on natural bone developed. research on
plastics as implant materials was continuing. In 1945.
Elaine (8) successfully implanted a sterile dough of methyl
methacrylate into gaps in the skulls of cats and rabbits.
No pathological tissue reaction occurred and three months
after the implantation ossified tissue was seen in the
space between the meninges and the implanted plastic.

Hamby. g; a; (20) attempted to compete in the
field of neurosurgery when. in 1959, he replaced ruptured
lumbar vertebral discs with methyl methacrylate. Hamby's
patients all recovered successfully. However. no
significant difference was found in either length of
hospital stay or post—operative function when his subjects
were compared to a group of patients with ruptured discs
who underwent traditional treatment of discectomy only.

No further notable developments occurred until the
mid-1970's. While attempting to develop a material
suitable for tooth replacement. Taylor and Smith (36)
determined that porous methyl methacrylate was a suitable
implant in fibrous tissue with a notable lack of tissue
reaction. They did stipulate that the pore size of the
material was to be controlled so that vascular penetration
of the graft could occur.

In 1977, Ashman. e_t_ a; (5) also determined that
porous poly methlymethacrylate was suitable for tissue
implantation. Paralleling Taylor's results. Ashman stated
that the pore size within the implant material appeared to

8

have an effect on the optimal location of that implant.
Ashman's research stated that a one-hundred micron (100 um)
pore size was adequate for connective tissue ingrowth in an
osseous site and a 450 micron (um) size was necessary for
bony ingrowth in the same site.

During this same time period. Ashman. gt; a_l (2)
successfully implanted methyl methacrylate into canine
alveolar ridge spaces following extraction of natural
teeth. In each case. a methyl methacrylate replica of each
tooth removed was implanted immediately into the socket of
each natural tooth. After Six months. there was no
evidence of alveolar ridge resorption; in addition. normal
osteoblastic and osteoclastic activity was seen at the
junction of the alveolar bone and the implants.

Approximately 10 years later. Duff (18) found that
the use of methyl methacrylate in conjunction with bone
screws over a field of not more than three vertebral bodies
was a safe and effective method of providing both immediate
and long—term stabilization for traumatic cervical spine
fracture-dislocation.

With advancing technology. new materials continue
to be introduced as possible substitutes for bone.
Coralline hydroxyapatite. ceramics (calcium hydroxyapatite
and tricalcium phosphate) and new polymeric substances
(Hard Tissue Replacement or HTRO) have been tested as
implants with encouraging results. In each case. when
these materials are implanted. bone has been seen growing

9

within the pores of the implant with little or no
pathological tissue response elicited (3. 4, 7, 22. 23. 35.
37. 39).

As more organisms become resistant to present day
pharmacopaeia and new. often lethal. diseases continue to
evolve it is conceivable that the current practice of
transplanting tissue from one individual to another will be
slowly abolished. As this occurs. it is imperative that
adequate substitutes be developed. This paper looks at the
mechanical properties of one such substitute. HTRO. and
compares these properties with the mechanical profile of
sterilized natural bone in a similar configuration--that of

the Cloward plug dowel.

10

MATERIALS AND METHODS

The material used in this study is an experimental
artificial bone graft manufactured and supplied by the U.S.
Surgical Corporation (Norwalk. CT).

Identified by the name Hard Tissue Replacement®.
or more commonly HTRO. this material is the result of a
patented process which envelopes a hydrophobic inner core
(poly methylmethacrylate or PMMA) with a hydrophilic shell
(poly hydroxyethyl methacrylate or PHEMA). The result of
this procedure is a myriad of beads in varying sizes. each
with a Pm core and PHEMA outer shell. The beads can be
sorted according to mesh size or left as a random mix. The
material can be packed into cavitations in bony tissue as
loose beads or heat molded into varying shapes and used as
a solid graft. This study used material which had been
segregated loosely into two main bead sizes: 20/24 mesh and
30/40 mesh. The tests were performed using HTRO which had
been molded into dowel shapes (Figure 3).

The mechanical testing of this material was done
in conjunction with an implant study using a Cloward
anterior approach in large (> 50 pounds) mongrel dogs. The
goal of the implant study was to determine if bone would
grow into the I-ITRO; however. prior to the first
implantation it was clear that a great deal of information

regarding this material would be necessary.

11

 

FIGURE 3: HTRO Cloward dowels of varying
sizes.

12

To gain this preliminary information. varying
sizes of HTRO plugs were tested in compression. The
purpose of examining a large variety of plugs was to
determine any change in mode of failure with varying size.
In addition. the assortment of dimensions was deemed
necessary following Xéray investigation of canine cervical
spines. The results of the radiographic inquiry' showed
marked variability of canine spines and a lack of
predictability of spine size given external Characteristics
of each dog examined.

Visual inspection of each group of plugs concluded
that the HTRO was essentially consistent in a 20% diametric
compression throughout the range of sizes. Given this
knowledge. the first implant of an HTRO plug was performed.

The initial implantation delineated several
differences between canine and human cervical spines which
were not apparent during the preliminary protocol
research. The horizontal state of the canine cervical
spine and the propensity of animals to resume motion almost
immediately following surgery created a requirement to
stabilize the graft plugs -- a requirement not covered in
the original Cloward anterior approach description. The
initial modification made was to insert a 0.45 french
diameter Kirshner wire (k-wire) through the vertebral body
cephalad to the graft and into the HTRO plug. Subsequent
implants utilized a new plug shape -— the "stadium" shape
-- which was designed to increase the proximity of the plug

13

with the vertebral bodies (Figure 4). Other Changes in
testing protocol. such as state of graft hydration. were
developed following failure of implanted grafts.

The data which was collected during the
preliminary phases was used to modify and improve the
implantation protocol. and was not analyzed or used in
statistical comparisons. Table 1 lists the varying
configurations tested initially in conjuction with the
implantation study. Note that all testing was done at
identical strain rates and at room temperature.

Additional experimentation included the following
tests:

1. A group of grafts was subjected to vertical
axis compression in an effort to determine the
corresponding expansion of the diameter of each plug. This
information was to be used to calculate a Poisson's ratio
of HTRO.

2. A study involving the ability of HTRO to
withstand cyclic loads was attempted in an effort to
simulate cyclic loading patterns within canine cervical
spines during physiologic load bearing. 13 x 10 mm
"stadium" shaped plugs were soaked in lactated Ringers
solution for a minimum of 2 hours prior to testing. Two
hours was chosen following experimental protocol used for
natural bone graft (29). Each plug was then secured within
the testing grips and a k-wire was inserted. following the
implant study.

14

 

 

 

t _--.AAA .4

unmCl' “ 2l 3 ‘ 4 5 6 7 I 9 l

 

FIGURE 4: "Stadium" shaped Cloward dowels
(left) and traditional round dowels.

15

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16

The first cyclic test used repeated deformations
of a specific magnitude. 2.0 mm. following the criteria
used in the single ramp tests. was selected as the
deformation that the plug was to undergo on a repeated
basis for 2 hours.

The second cyclic test was done under load-control
conditions. subjecting the plug to loads of not more than
150 lb over a two hour period. The load of 150 lb was
arbitrarily chosen based on a combination of the minimal
acceptable load needed for human cervical spine graft (1500
N (28)) and the knowledge that the loads generated by dogs
within the cervical spine was not expected to be as great
as those loads generated within human spines.

3. An experiment to determine the ability of
HTRG to absorb fluid was also completed. Prior to this
test. a time of 2 hours had been selected as the minimum
amount of time required to absorb a maximal amount of
fluid. based on results which show the time required by
lyophilized natural bone to rehydrate is 2 hours (29). The
importance of this was seen after a brief experiment
comparing plugs that were ”dry" with those that had soaked
for an arbitrary period of time (Figure 5).

Three plugs were isolated with dessicant (Sigma
Chemical Corporation. St. Louis. Mo.) for 4 hours and then
weighed individually on a Mettler analytical balance.
After the weights were recorded. the plugs were each placed
in separate jars of lactated Ringers solution and incubated

17

n. _ arm ULSESXCAHT

5-. TEMPLRATU “'1

EU <

 

24 HOUR LACTATED RINGER'S SOLUTION

ROOM TEMPERATURE

 

 

FIGURE 5: Results showing the contrast
between modes of failure of "dry" HTRO
plugs and those soaked in lactated
ringers solution. This graphic
demonstration prompted an experiment to
analyze how much fluid molded HTRO would
absorb.

18

at 37°C. Subsequent weights were taken at 4. 8. 20. 24. 48
and 72 hours. The weights were graphed to visually
estimate the time when the plugs ceased to absorb
additional fluid. This was then determined to be the
minimum soaking time for HTRO plugs.

4. Axial compression tests to compare stiffness
at varying deformation rates:

Fifty-four plugs measuring 14 mm x 20 mm length
were randomly divided into three groups. Each group was
placed into a different environmental condition: a)
exposed to room air at room temperature (21°C). b) hydrated
for 24 hours in lactated Ringers solution at room
temperature. and c) hydrated for 24 hours at simulated body
temperature (37°C). All plugs were returned to room
temperature before testing and all tests were run at room
temperature. Prior to compression. silicone vacuum grease
was applied to the ends of each plug to eliminate and
effects. such as barrelling. caused by friction.

Six plugs from each condition were tested at three
different constant strain rates. Each plug was deformed
1.25 an or 6.25%. The initial strain rate was 0.6254/sec.
Subsequent strain rates tested were 10 times the initial
rate or 6.25%lsec. and 20 times the initial rate or
12.5%/sec. The unloading pattern was not recorded.

5 . A f inal exper iment was performed to
mechanically compare HTRO dowels to natural bone graft.
Information regarding testing protocol. size of the natural

19

bone dowels and results for comparison.*were taken from
studies of the strength of bone dowels following various
sterilization techniques (21. 29).

Eighteen plugs of HTRO which had been manufactured
in a single lot were used in this study. Each plug was
first weighed on a Mettler analytical balance and then
measured for maximum and minimum diameters and length. The
manufactured size was 14 mm diameter X 20 mm length;
however. all plugs showed some variability in size. Nine
plugs were placed in covered jars containing lactated
Ringers solution and nine plugs were isolated in covered
glass jars with dessicant. Three "wet" plugs and three
"dry" plugs were incubated at 37°C; the remaining plugs
were kept at room temperature. The plugs were left for 24
hours—-the minimum time required for HTRQ to equilibrate in
fluid. After 24 hours. the plugs incubated at 37°C were
allowed to return to room temperature. and all plugs were
then subjected to 20% (2.8 mm) diametric compression over
10 sec. along the greatest measured diameter.

All of the diametric tests described above were
completed using equipment which simulated the loading
produced in an in-gilg specimen; that is. each plug was
loaded between two grips representing vertebral bodies in a
vertical position (Figure 6). All test grips were
constructed of stainless steel. The tests to determine

axial compression modulus were completed using stainless

20

 

 

 

 

 

 

 

 

 

FIGURE 6: HTRO plug within grips representing
vertebral bodies.

21

steel plates. and were not modelled after an anatomical
configuration.

The tests were run on an Instron Servohydraulic
materials testing machine. All except one of the testing
protocols required stroke control to be used: the exception
was the cyclic test performed under load control. Load and
deformation data were recorded. (”1 a Nicolet digital
oscilloscope and stored as voltage measurements on floppy
disks. The data was then transferred to an IBM PC Clone
and converted to load-deformation curves (Figure 7).

From 'the load-deformation. curves. the stiffness
and yield load of each specimen was determined. Stiffness
was defined as the slope of the linear portion of the
curves. Yield load was defined as the intersection of a
line offset 2% from the linear portion of the curve with
the curve. Peak load was determined by converting the
voltage value corresponding to the maximum point on the

Nicolet load curve to Newtons (Figure 8).

22

LOAD (N)

LOAD VS. D EF 0 RM ATIO N

CLOWARD TEST; PLUG 2, 24H SOAK. ROOM TEMP

1 2000
1 1000
10000

9000

7000 /
5000

5000

Mv‘v" 9" ’ "'°

4000

O
3000 i/
2000 /

1000 /

o IllllIlIlIlIlITllllIll7rIlIIIIITITr—VTTII]

0.0 1.0 2.0 3.0 4.0

DEFORMATION (MM)

FIGURE 7: Typical load-deformation curve
generated during compression Of HTRG.

23

LOAD (N)

5000

9500

4000

.3500 ;

3000-

2500

I500

1000

500:

LOAD VS. DEFORMATION

 

 

 

 

 

 

 

 

2 2°
_; 5 /° Peak load
- pg
.9 / ["I/ 3“
~: K Yield load l
11 .
: ‘11—— Slope = Silliness
c‘.’ .__—_1 _
.____,.,.// /
IfIY'TTWl rrTIrrrTlrfirrlfIrIIITIIIITfi—Tl
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5' 4.0

oeroamrlou (ram

FIGURE 8: Typical load—deformation curve

showing method of calculating stiffness
and yield load. as well as location Of
peak load.

24

RESULTS

1. Poisson's ratio: The plugs were seen to
compress in a variable fashion during a minimal sustained
axial load (Figure 9). Given this variability. it was not
possible to accurately measure resultant diametric
expansion nor to calculate a Poisson's ratio for this
material using this method.

2. Cyclic testing: The first plug. subjected to
repeated deformations of specific magnitude (2 mm) for two
hours. crumbled shortly after one hour of testing. The
second plug. tested under load-control conditions and
exposed to loads of not more than 150 lbs over a two hour
period. was intact at the end of the testing period and
showed no sign of failure or fracture.

3. Fluid absorption: The plugs continually
absorbed fluid (indicated by an increasing weight) until 24
hours had elapsed. At 72 hours past the start of fluid
immersion. the weight of each plug was not significantly
different from that of 24 hours (Figure 10).

4. Axial compression: The plugs which were
designated as "dry" were able to withstand a higher total
load than the groups of wet conditioned plugs at each of
the different strain rates. There is also evidence that
higher loads were obtained at the strain rate of ten times
that of the intitial strain rate. This pattern does not
continue as the Strain rate increases to twenty times the
original rate.

25

 

FIGURE 9: Variability seen during axial
compression.

26

fiIC)I-al'lh

.‘

Z

03

853

U)
C)
Q

703

(n
\n
O

 

PLUG l:
PLUG 2:
PLUG 3:

a
0 o
O
o x
x x
x * e

X

0

 

a 8 12 15 20 24 28 32 35 00 44 08 52 56 60

TIME IN HOURS

FIGURE 10: Graphic results of weighing
experiment. A11 plugs were kept at
37°C. Plugs were not allowed to return
to room temperature prior to weighing.

27

68

68 72

PLUG PLUG PLUG

1 2 3

TIME
(HOURS)

0 704.0 mg 712.5 mg 681.1 mg

4 763.1 mg 764.7 mg 737.9 mg

8 774.3 mg 793.3 mg 751.1 mg
20 783.8 mg 792.5 mg 774.5 mg
24 790.4 mg 801.8 mg 773.4 mg
48 793.2 mg 801.3 mg 806.7 mg
72 778.6 mg 807.8 mg 793.8 mg

TABLE 2: Results of weighing experiment.

28

A material modus value (E) was calculated from the
values of stress and strain. Under all conditions. it can
be seen that the moduli values obtained for strain rates of
ten and twenty times the initial rate are similar to each
other. but much larger than the values calculated for the
initial strain rate (Figure 11).

5. Comparison to natural bone: All plugs behaved
in a manner consistent with that of cancellous bone; that
is. each sample demonstrated an.initial linear elasticity.
a plastic yield point and a final increased resistance to
load when plotted on a typical load-deformation curve.

Mean yield load values obtained for each specimen
averaged from 74 -190% higher than corresponding yield
loads obtained for fresh bone dowels. Similarly. mean peak
load values ranged from 195 - 216% higher than the highest
reported value for the natural dowels. while mean stiffness
values averaged 31% higher than the average reported value
for natural bone (21). Paired tetests indicate that these
values are significant at p=.001 (Figure 12).

All HTRO grafts which were implanted within canine
cervical spines showed signs of breakage or failure in the
plugs: however. no dog showed neurologic deficit.
indicating that even after breaking. the material retained
some strength. In addition. all plugs which were examined
after eight weeks of implantation showed signs of bony

ingrowth into the graft material (Figures 13. 14 and 15).

29

MODULI VALUES FOR EACH CONDITION

N/mrn2

Initial 10 x initial 20 x initial

strain rate rate -

mm
Dry Mean E 369 442 432
ammhs

q‘ 505 313 :53

Wet
Samples Mean E 279 325 336
(21.000) 0' 29.6 15.5 25.9
grime: Mean E 254 290 299
(37.0%) 0‘ 30 25.1 33.3

0" denotes standard deviation

FIGURE 11: Moduli values (N/mmz) calculated
after axial compression. Temperature
values refer to pre-test conditions; all
tests were run at room temperature.

30

wm— 'WELDLOADS

YIELD LOAD son _

(NEWTONS) 5M -
I

"ACCEPTABLE LOAD” 1500- ----------------------

 

 

FRESH FROZEN LYOPHHJZED
IHRADIATED IRRADIATED mm? mm ”7““ “TR-DRY
21° C 37° c 21° C 370 c

FIGURE 12: Yield loads Of natural bone and
HTRO grafts in diametric compression.
All tests were run at identical strain
rates and at room temperature.
Descriptions listed are pre-test
conditions.

31

GROUP

Lactated Ringer's
Solution - 24 R
Room Temperature

Lactated Ringer's
Solution - 24 H
37°C

Dessicant - 24 a
Room Temperature

Dessicant - 24 a
37°C

TABLE 3:

SPECIMEN NUMBER

GMOUNH UNH OM‘UNH

UNH

YIELD LOAD

4700
4500
5000
5200
5000
4730

4500
5000
4600

7550
7000
7900
8250
7500
8300

7800
7900
7800

222222 222 222222

PEAK LOAD

8950
8650
9750
9750
8800
8600

222222

9300'
9100
9000

9600
9750
9500
9460
9400
9430

8950
8950
8800

Mechanical properties of HTRP

dowels in diametric compression. Group
description refers to pre-testing

conditions.

All tests were run at room

temperature and at identical strain

rates following protocol used in natural

bone allograft testing.

32

STIFFNESS

7500
7667
8000
8667
8000
7000

8000
7666
7333

10333
10667
10000
11333

9667
12000

11333
11667
10667

N/mm
N/mm
N/mm
N/mm
N/mm
N/mm

N/mn
N/mm
N/mm

N/mm
N/mn
N/mn
N/mn
N/mn
MADE

MAID
Nhla
N/un

SPECIMEN NUMBER

18687A
186878
19787A
197878
20087A
200878
20187/38A
20187/398
20587A
205878
20887/4A
20887/48

TABLE 4:

dowels in diametric compression (22).

2300
2150
1840
1950
2720
3110
2150
2400
2050

700
3400
2600

YIELD LOAD

222222222222

2509
2410
2079
2016
3091
3745
2697
2822
2182

819
3544
2769

PEAK LOAD

222222222222

Mechanical properties of bone

33

4500
3600
5400
5200
5650
5050
4300
4050
6500
1900
4950
5100

STIFFNESS

N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm
N/mm

FIGURE 13: Subgross photomicrograph of
fused intervertebral space
demonstrating porous HTRO. Note
the fracture in the middle of the
graft. There is also a sectioned
bone pin (middle right region). 1.
2. and 3 refer to zones examined by
conventional light microscopy.

H83. 4X.

34

 

35

FIGURE 14: Photomicrograph from Zone 1
of Figure 13. There is an abrupt
transition from normal. woven
trabecular bone (left) into the
pale-staining osteoid growing
within the HTRO. 88E. 18X.

36

 

37

FIGURE 15: Photomicrograph from Zone 1
of Figure 13: Bony trabeculae
growing within the porous implant
material. There are osteoblasts
with rounded nulcei surrounded by
an osteoid matrix. HSE. 72X.

38

 

39

DISCUSSION

The results of the ip—m testing show a great
deal of consistency. Why. then. was it impossible to
obtain a Poisson's ratio of this material? The most valid
explanation is the porosity of this material. When each
plug was crushed along its axis. theoretically only the
beads nearest the compression arm would have been exposed
to the load generated. The pores between the beads would A
have acted as a "cushion" for the remainder of the plug and
absorbed the majority of the compression by simply
collapsing. This would explain the deformation seen only
nearest the compression arm during the experiments to
determine Poisson's ratio.

A second question that must be considered is:
Given the alleged identical features of each manufactured
group of plugs. why was so much variation seen -- both in
visual inspection of the plugs and in plug weights? (This
variation was most obvious during the weighing
experiment.) One possible explanation is that. given the
slight variation in bead sizes within the grafts. there
must be a corresponding variation in porosity for each
plug. Recall that the manufacturer did not separate bead
sizes beyond a crude 20/24 mesh or 30/40 mesh size. This
would imply a corresponding "chance" distribution of beads
within the molded plugs. leading to a wide variety of dead
space among the plugs.

40

The results of the axial compression tests were as
expected: the material stiffened as the strain rate rate
increased. and then appeared to begin to level off at a
constant stiffness. The "dry" samples were seen to be
stiffer in all forms of compression. indicating a greater
likelihood of brittle failure. Similar to the failure mode
seen in natural bone prior to rehydration (29).

Given the variability described above. are the
results which Show HTRO to be significantly (p < 0.001)
stronger than natural bone accurate? The author has
confidence that the results are correct. In each case of
diametric compression the material was deformed to an
extent which eliminated all pores between beads and
therefore tested only the material itself. By inspection
of the raw data (Appendix A). it is possible to see a
"knee" or flat portion of each deformation curve prior to
the generation of any load on the material. This flat
portion is believed to be the collapse of the pore or dead
space within each plug. following which is the curve which
represents the properties of HTRO accurately.

The results outlined above indicate the HTRO is
significantly stronger than natural bone in compressive
states. This is desirable during the initial stages of a
graft implant when brittle failure is most likely to
occur. And. because some grafts can take as long as six
months to become fully incorporated. long term strength is
a necessity as well (40). HTRG does not undergo

41

biodegradation within the body. but instead retains its
structure (3). Thus. it can be theorized that HTRO will
demonstrate long term strength and stability.

HTRO is easily sterilized by common methods
without damage to its inherent structure. In. addition.
HTRP can be kept in a hydrated state indefinitely as. after
twenty-four hours. it will absorb no more liquid. This
demonstrates an ease of handling and storage which is
currently not seen when working with natural bone.

At this time. the standard for grafting material
remains natural bone allograft. However. the results
described here indicate that the synthetic grafting
material known as HTRO appears to be an adequate substitute.

While these i_n-mg9_ results Show that HTRO is
consistently stronger than natural trabecular bone in
diametric compression. the parallel implantation studies
did not replicate these conclusions. In contrast. the
implantation studies frequently met failure due to graft
breakage. This was completely' unexpected following the
in—yiggg experiment results.

There are several reasons as to why the grafts may
have failed within the ig-Lilq setting including the fact
that the grafts were only tested in compression. Forces
within a living being will include torsion and bending
(Figure 16). either of which may have contributed to the

failures.

42

 
  

A A

 
 
  

21
/
cv~ LL?)
‘ ’ L \. fi‘j

I
'r

_\ _-._
I FLEXION ' I [EXTENSION-I

 

— —-— -- u.—

 

/-//_\
r7 \

l/ \\ ‘

r ' ,"\ I

1' v I . '

I

L___ l
,__.——-——-1 . / “ _.._.-..__...
i ““5"“- \ / \ /[ROIAIION l
l SENDING ‘ _.__. 1-4

FIGURE 16:

Forces acting on the head and
neck.

43

A second reason. perhaps more significant. are the
mechanical problems found with the cylindrical shape of the
Cloward plug as reported by Keblish.. g}; a; (25) in 1967.
These problems were reported clinically' and had enough
impact so that at least one major hospital abandoned
entirely the Cloward shape in 1964.

There are three main shapes of bone graft used for
spinal fusion: cylindrical (Cloward). horseshoe (Smith-
Robinson) and strut (Bailey-Badgley) (Figure 17). White.
9; g; (40) studied the load—bearing capabilities of each of
these graft shapes immediately following implantation.
Their results indicate that the Smith-Robinson horseshoe
shaped graft was mechanically strongest. This conclusion
is based in part on the shape of the graft -- shown to be
strongest -- and in part on the fact that the cortical
endplates of the vertebral bodies are not destroyed during
implantation of the graft.

Given this information and the results of the
research conducted for this thesis. it would appear that
HTRO may still succeed in an 11.1fm setting. If
modifications reflecting the increased strength of the
Smith-Robinson graft are coupled with the known compressive
strength of HTRO. the result may well be a graft which is
strong. easily sterilized. and adaptable. More study is
needed to quantify these thoughts; however. as additional

information becomes known. it is likely that artificial

44

Smith-Robinson (horseshoe)

 

FIGURE 17: Graft shapes used for spinal
fusions.

45

graft will replace natural bone as the standard.
guaranteeing sterility and function in a variety of

applications.

46

10.

11.

BIBLIOGRAPHY

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1974. pp 180-185.

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332-341.

Ashman. A. and P. Bruins. "HTR'" for Edentulous Ridge
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Ashman. A. and P. Bruins. "Prevention of Alveolar
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Ashman. A. and M. Moss. ”Implantation of Porous PMMA
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pp 657-665.
Bell. G. and S. Bailey. "Anterior Cervical Fusion for
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Bright. R.. G. Friedlander and K. Sell. "Tissue
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47

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Cloward . R. "The Anter ior Approach for Removal of
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Crenshaw. A. . ed. Campbell ' s Operative Orthopaedics .
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DeVries. P.. C. Badgley and J. Hartman. "Radiation
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Ducheyne. P.. L. Hench. A. Kagan. M. Martens. A.
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Duff. T. "Surgical Stabilization of Traumatic
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Methylmethacrylate - Long Term Results in 26
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1986: PP 39-44.

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Journal of Orthopaedic Research. Vol. 5. No. 1,
1937. pp 114-121.

48

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28.

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33.

Holmes. R.. V. Mooney. R. Bucholz. A. Tencer. "A
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252-262.
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1986. pp 232-245.

Keblish. K. and K. Keggi. "Mechanical Problems of the
Dowel Graft in Anterior Spinal Fusion." Journal
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MacEwen. W. "The Osteogenic Factors in the Development
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Mertz. R.. V. Hodgson. L. Thomas. G. Nyquist. "An
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Mukherjee. Joia. "The Effects of Sterilization and
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Ono. K. and K. Tada. "Metal Prosthesis of the
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49

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35.

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37.

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42.

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Shimazaki. K. and V. Mooney. "Comparative Study of
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as an Implant Material." Journal of Biomaterials
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467-479.

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Triantafyllou. N.. E. Sotiropoulous. J.
Triantafyllou. "The Mechanical Properties of the

Lyophilized and Irradiated Bone Grafts." Acta
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Uchida. A.. S. Nade. E. McCartney. W. Ching. "Bone
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White. A.. J. Jupiter. W. Southwick. M. Panjabi. "An
Experimental Study of the Immediate Load Bearing
Capacity of Three Surgical Constructions for
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21-28.

White. A.. and M. Panjabi. "The Role of Stabilization
in the Treatment of Cervical Spine Injuries."
Spine. Vol. 9. No. 5. 1984. pp 512-522.

Williams and Roaf. Implants in Surgery. (Saunders.
Philadelphia). 1973.

50

APPENDIX A

APPENDIX A: RAW DATA FROM DIAMETRIC
COMPRESSION TESTS

51

 

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