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BIOMECHANICAL INVESTIGATION OF
THE HEALING EQUINE LINEA ALBA

presented by
Arthur Irving Ortenburger, III

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BIOMECHANICAL INVESTIGATION OF THE
HEALING EQUINE LINEA ALBA

By
Arthur Irving Ortenburger, III

A THESIS

Submitted to
Michigan State University
in Partial Fulfillment of the Requirements
for the Degree of

MASTER OF SCIENCE

Department of Large Animal Clinical Sciences

1986

ABSTRACT

Biomechanical Investigation of the
Healing Equine Linea Alba

By

Arthur Irving Ortenburger, III

Ventral midline incisional hernias in the horse
following celiotomy may limit future use and require
corrective surgery. To provide a better understanding of
the biomechanics of wound healing of the equine abdominal
wall, wound strength was quantitated in terms of ultimate
load and energy absorbance in 30 ponies following celiotomy.
The effects of duration of healing, wound location, and
suture material type were determined by tensometry of the
healing linea alba wound at seven-day intervals. Overall
wound strength decreased in the first seven postoperative
days, then increased rapidly during the next seven. WOund
strength was equivalent to that of the normal linea alba by
the 14th postoperative day, at which time it was also
significantly stronger in the caudal abdominal wound
compared to the cranial region. WOunds closed with gut
suture were significantly stronger at the 28th postoperative

day than those closed with polydioxanone sutures.

ACKNOWLEDGEMENTS

"To save one from a mistake is a gift."

Herbert, 1965

Completion of the project described in this thesis was
made possible by the support and advice of my graduate
committee, Drs. Hubbard and Robinson, and especially by my
graduate advisor, Dr. John Stick. From each of these
teachers my own education has been advanced and the content
of this thesis improved. Dr. Ron Slocombe very kindly
undertook the task of statistical analysis of the data, and
then was most accomplished in explaining to me what I had

done. For all these gifts of knowledge and technique I am

most grateful.

ii

TABLE OF CONTENTS

LIST OF TABLES

LIST OF FIGURES

LIST OF ABBREVIATIONS .
INTRODUCTION

REVIEW OF THE LITERATURE
Incisional Hernias
Wound Healing . . .
WOund Tensometry .

Suture Materials

MATERIALS AND METHODS . . . . . .
Experimental Animals and Groups .
Description of Surgery and Postoperative

Management . . . . . .
Necropsy and Sample Collection .
Specimen Preparation .

Statistical Analysis

RESULTS AND DISCUSSION

Implications for the Surgeon .

CONCLUSIONS AND SUMMARY .

iii

vi

. vii

ll
16

24
24

26
32
32
39

42
64

67

APPENDIX A

Sizes of Suture Material.

APPENDIX B

Schematic of Sample Testing .

APPENDIX C

Filtered Data, With Calculated Units

REFERENCES .

iv

71

72

74
80

10.
ll.
12.

LIST OF TABLES

SIGNALMENT OF EXPERIMENTAL PONIES .

STATISTICS OF ULTIMATE LOAD FOR SUTURE MATERIAL

GROUPS, SUTURES REMOVED . . .

STATISTICS OF ULTIMATE LOAD FOR SUTURE MATERIAL
GROUPS, SUTURES INTACT . . . . . . . . .

STATISTICS OF ULTIMATE LOAD FOR INCISION REGION
GROUPS, SUTURES REMOVED . . . . . . . . .

STATISTICS OF ULTIMATE LOAD FOR INCISION REGION
GROUPS, SUTURES INTACT . . . . . . . .

STATISTICS OF ENERGY ABSORBANCE FOR SUTURE
MATERIAL GROUPS, SUTURES REMOVED

STATISTICS OF ENERGY ABSORBANCE FOR SUTURE
MATERIAL GROUPS, SUTURES INTACT .

STATISTICS OF ENERGY ABSORBANCE FOR INCISION
REGION GROUPS, SUTURES REMOVED . .

STATISTICS OF ENERGY ABSORBANCE FOR INCISION
REGION GROUPS, SUTURES INTACT . . . .

WOUND STRENGTH AS A PERCENTAGE OF NORMAL.
SIZES 0F SUTURE MATERIAL .
FILTERED DATA WITH CALCULATED UNITS

25

43

44

45

46

47

48

49

. 50

55
71
74

\OCXDNChU'l-l-‘LONl—J

t—Ii—ar—ai—a
WNI—IO

14.
15.
16.
17.

18.

LIST OF FIGURES
VISCOELASTIC LOAD-DEFORMATION CURVE .

STRUCTURE OF POLYGLACTIN .
STRUCTURE OF POLYDIOXANONE .

ROTATION OF SUTURE MATERIAL POSITION .

DETAIL OF LINEA ALBA CLOSURE .
DISSECTION 0F WOUND SEGMENT .

WOUND SEGMENT COMPLETELY DISSECTED .

DESIGN OF TISSUE SEGMENT GRIPS

WOUND SEGMENT WITH GRIPS ATTACHED

DISTRACTED WOUND SEGMENT AFTER FAILURE .

EXAMPLE PLOT OF LOAD VERSUS TIME
ULTIMATE LOAD OF WOUNDS OVER POSTOPERATIVE TIME
ENERGY ABSORBANCE OF WOUNDS OVER POSTOPERATIVE

TIME . . . . . . . . . .
POSTMORTEM INCISION REGIONS .
TRANSVERSE FROZEN WOUND SECTION

ENERGY ABSORBANCE OF REGIONAL WOUNDS .

ENERGY ABSORBANCE OF WOUNDS BY SUTURE MATERIAL

TYPE . . . . . . . .
SCHEMATIC OF SAMPLE TESTING

vi

12
22
23
30
31
35
36
37
38
40
41
51

53
56
57
59

62
72

LIST OF ABBREVATIONS

EA Energy absorbance (Failure energy)
IH Incisional hernia

J Joules

Kg Kilogram

LA Linea alba

mg milligram

N Newton

POD Postoperative day

sem Standard error of the mean

UL Ultimate load (Breaking strength)

vii

INTRODUCTION

The indications for gaining surgical access to the
contents of the abdomen are many; in horses they include
sudden anatomic displacement, ischemic diseases and
impaction of the gastrointestinal tract, cesarean section
in the mare, cryptorchidectomy, and cystotomy in males. In
"most cases the preferred approach to the abdomen is by
celiotomy, or longitudinal incision of the linea alba, a
tendinous union of the abdominal musculature and fascia,
which comprises the ventral midline of the abdominal wall.
A major complication in the horse of this approach is
postoperative incisional hernia.

The number of cases of incisional hernia following
celiotomy in horses is probably increasing, owing to an
increase in the number of abdominal procedures performed.
Incisional hernias may prevent use of the horse as an
athlete, may increase the risk of gestation in brood mares,
and constitute a serious cosmetic blemish. When compared
to other domestic species, horses may be more at risk for
developing incisional hernias, due to their propensity to
rest in a standing position, the weight of their abdominal
viscera, the relatively weak suture materials available for

initial closure of the abdominal wall, and the increased

proportion of procedures which result in or follow bacterial
peritonitis.

A feature peculiar to equine incisional hernias is
their late onset when compared to those occurring in humans
and dogs. This point is difficult to support with hard
evidence, but as a clinical impression most of these
hernias seen in horses are first detected in the third and
fourth post-operative weeks; in contrast to humans and dogs
where they are usually noticed at the end of the first
week. As the most commonly used absorbable suture materials
have lost form 50% to 80% of their tensile strength between
the second and third post-operative weeks, it is reasonable
to wonder if there is a delayed wound healing sequence in
the abdominal wall of horses that contributes to incisional
hernia.

The hypothesis that horses have delayed wound healing
of the linea alba was investigated. Ponies were used as an
experimental model. The specific objectives of this study
were: 1) To quantitate wound strength in the postoperative
period, 2) to evaluate the influence of three different
suture materials on wound strength of the linea alba in the
postoperative period, and 3) to learn if there are
differences of wound strength within the length of the

incision.

REVIEW OF THE LITERATURE

"I do not think that, though much has been
written thereon, it is yet adaquately
recognized that the steps in making and in the
repair of an abdominal wound are of the
greatest importance."

Moynihan, 1926

Incisional Hernia

 

The presence of an incisional hernia (IH) is a
significant cause of postoperative morbidity.(1) Most
of what is known of this problem is to be found in the
medical literature from retrospective studies of human
patients requiring abdominal surgery. (2-4) A number of
workers have described the incidence of these hernias,
and have attempted to identify risk factors which predispose
to their formation. Spies et. al. found 438 such hernias
occurring in 34,300 people requiring abdominal incisions.(2)
Bucknall's prospective evaluation of 1129 laparotomies
revealed that 7.42 of these ended in IH.(5) A broad range
is reported when rates of IR are examined: In one review
IH were found to occurr in from 1.62 to 18.62 of human
abdominal surgeries.(6) In general these variations may be
accounted for by differences of detection method, case

selection, and follow-up intervals.

Significant risk factors which have been associated
with development of IH in humans include wound infection,
male gender, ventral midline incisions made in the cranial
abdomen, vertical incisions (those parallel to the linea
alba), increasing age of the patient, obesity,
postoperative cachexia, postoperative coughing, and
diabetes.(2,7) One study also suggests that the longer
the initial incision, the longer and wider will be the
hernia which follows, should one occur.(2) There is recent
awareness that IH may be first noticed long after surgery.
As many as 502 of these hernias may develop more than a
year after surgery, and there are several case reports
describing initial onset from 6 to 12 years
postoperatively. (2,7-9) Ellis found that late hernias,
defined as those occurring more than one year after
surgery, increase the IH rate by 5.82, and that people
in this late group did not have the accepted risk factors
described above associated with onset of their hernias.(7)
However, Harding found that of nine IH risk factors, only
wound sepsis (in the perioperative period) was
significantly associated with later hernias.(3)

Both in humans and canids, the majority of IH do
occur, or at least are first detected, within the first
two to four weeks following surgery.(10,ll) The risk
factors described for dogs are similar to those of humans

including wound sepsis, advanced age, obesity,

chronic cough, and poor surgical technique.(ll)

The prevalence of this major complication of abdominal
surgery has inspired many workers to search for better
methods of treatment and prevention. Research efforts have
concentrated on attempts to define the suture material,
suture pattern, and suture tension which are best able to
prevent IH.(12-l9) Results of these studies are often in
conflict, and usually not directly comparable with each
other due to differences of experimental design. Preston
et. al. may have best summarized the situation by stating:

"A careless surgeon will have poor results by

any method, and a good (careful) surgeon may have
good results with poor methods."(20)

Review of the veterinary literature for information
on IH in horses yields little data useful for comparison
to other species. McIlwraith notes that "incisional
hernias (in horses) may not be apparent in the immediate
postoperative period, and tend to be noticed some time

after surgery."(21)a Milne has stated that delayed

 

a There is general agreement among a number of equine

surgeons questioned on this point. The observation that
such hernias occur late is a common anecdotal statement,
but the literature is devoid of prospective studies which
document this phenomenon. A review of four recent cases of
incisional hernias from Michigan State University,
Veterinary Clinical Center records reveals a mean time of
detection on the 60th postoperative day, with a range of
29-90.

breakdown of the sutured abdominal wound may occur up to
four months postoperatively.(22) There is also a broad
range of suggestions relative to the safest suture material
and suture pattern which should be used for closure of the
equine abdominal wall.(21-25) In general, most authors
suggest use of at least U.S.P. #2 size material, and
interrupted suture patterns. When large incisional hernias
must be repaired in horses, there is agreement by most
equine surgeons that a prosthetic material is advantageous
to cover the defect. Johnson describes the use of
polypropylene mesh for successful repair of IH in
ponies.(26) In an earlier project which employed wound
tensometry, Allen compared the strength of different suture
patterns in equine abdominal wounds. His conclusion was
that a two-row imbricating continuous mattress suture

pattern provided the greatest strength.(27)

Wound Healing

 

Wound healing in mammals may be considered a prolonged
process; there are recognized tissue, cellular, and
molecular events which herald the beginning of this
process, but its ending depends very much on different
objective points of view.(28) For example, the functional
healing of an aseptic surgical skin incision may be
considered complete in as little as eight days, when suture
support is no longer required and epithelialization is

complete. Yet there will be a number of changes

observable in such a wound for at least another three
months, and changes of wound strength may continue for a
period of years.(30,31) The time-course of events in
wound healing are also sufficiently different between
different tissue types that comparisons should be based

on similar tissues. Interspecies differences have been
found between rates of healing for a given tissue type.(32)

Wound healing of connective tissue includes the
descriptive lag, fibroplastic, and maturation phases.(29)
These divisions describe physiologic events which occur
over time, to produce functional scar tissue. In many
wound healing studies, one accepted determinant of healing
is the strength of the scar.(32) Accurate measurement of
wound strength can be difficult, depending on the specific
tissue or anatomic structure being evaluated. The majority
of studies of tissue strength have examined skin or fascia
with rodents as animal models.(33) With these models the
qualitative features of increasing scar strength may be
described, but there is little information detailing
quantitative measurements for equids.(27)

The measured strength of collagenous tissue is
influenced by a number of intrinsic and extrinsic factors.
The intrinsic include collagen types and quantity present,
age, hydration, elastin content, and numerous systemic
aspects of general health and nutrition of the animal.(33)
Reports of differing specific testing procedures suggest

that rate of distension, orientation of the scar and normal

collagen fibers, and the security of tissue-gripping clamps
can produce wide variations of perceived strength of the
wound tissue.(34) In a review of the literature which
describes acquisition of strength by healing skin and
tendons in several species, a number of statements emerge
with which most authors would agree:

1) In the immediate postoperative period,
the sutured skin or abdominal wall has 402 of
its normal, unwounded strength.(35)

2) There is a measurable loss of wound
strength in the first five to seven days.(35)

3) There is a rapid increase in wound
strength from the seventh to the let day after
wounding.(36)

4) The total healing time required for a
scar to approach unwounded levels of strength
varies with species, and is variable from one
individual to the next.(36,37)

5) Scars in tendons never achieve the
tensile strength of the normal, unwounded tissue
being studied.(36)

6) Hollow abdominal viscera with a muscular
wall may attain nearly 1002 of normal strength in
a relatively short time after wounding.(36

7) There is a prolonged period of gradual
increase in wound strength after the fourth
postoperative week, which may continue for longer
that a year.(38)

8) Wound strength may be described in
many ways: the best single methodbappears to
be energy absorbance (EA).(39,40)

 

b When a material sample is tested to destruction by
tensile forces, the area beneath a plot of load (in
Newtons) against deformation (in meters) is the total
energy absorbed by the sample (in Joules) on the way to
mechanical failure.

The local biochemical events which accompany
increasing wound strength reflect increasing content and
molecular changes of collagen fibers.(4l) There are
histochemical and biomechanical difference between the
fibrils of collagen Types I and III, which are the
principle types found in healing wounds.(4l)

The relative proportions of collagen fiber types
found in a maturing scar change slowly with the age of a
wound.(4l) The replacement of the early appearing Type
III collagen by the mechanically stronger Type I collagen
as well as the appearance of elastin fibers, is part of the
general process of the maturation phase of wound
healing.(4l,42) Increased organization of collagen fibers
along lines of greatest local mechanical load, decreasing
fiber solubility, and increased cross-linking within and
between fibers probably continues for a period of months
after a wound is visibly healed on its surface.(42-44)

The biomechanical events during healing of linea
alba incisions have been described in rabbits and
horses.(27,39,45) Important features of these studies
include:

1) The normal unwounded linea alba in

both species is stronger in tension than the

sum of the anatomic layers which give rise to

it (the adjacent abdominal wall).(27,39)

2) The measured strength of the linea
alba will decrease as samples tested are gripped
a greater distance away from the linea alba

itself.(45)

3) Sutures contribute to wound strength
up to the 14th postoperative day in rabbits.(35)

10

4) The strength of the immediate
postoperative sutured linea alba is about 402
of the unwounded linea alba in rabbits, and
varies from 312 to 862 in horses, depending on
the suture pattern used.(27,35)

5) The cranial linea alba is weaker than
its caudal region.(39)

Opinions vary with regard to the ideal site for
abdominal incisions in humans. The goals of access to
particular visceral structures, ease and security of
closure, cosmesis, and prevention of incisional hernia
combined with the changing cross-sectional anatomy of
the several abdominal regions must all be considered.

There is better agreement in the case of equine laparotomy:
The ventral midline is preferred for most procedures.(21,
46,47) Only one description of the biomechanics of ventral
midline celiotomy in horses was found in the veterinary
literature.(27) This project was intended to show the
strength and subsequent healing provided by different
suture patterns and materials used in the abdominal wall.
In Allen's work, the range of variation in healing time and
the small number of samples within groups make
interpretations difficult.(27) Also, the tensometry
employed was the "bursting strength" class, which may be a
source of error.(37) The major findings of this study are
that a two-row imbricating continuous pattern of a
multi-filament synthetic suture resulted in the best short

and long-term wound strength.(27)

11

Surgeons have an interest in the effects of suture
material type on the quality and strength of wound closure.
Certainly there are other properties of suture materials to
consider, but if one should fail to maintain tissue
apposition through the early phases of wound healing, these
other properties become of secondary importance. Two
prospective studies evaluating different suture materials
concluded that material type was not a deciding factor in
the rates of wound sepsis and incisional hernia.(48,49)
These trials did not attempt to describe actual wound
strength with the different materials, and the small number
of cases and short follow-up periods may have prevented
showing significant differences. There has been a good
deal of propriety work designed to quantitate changes in
suture material strength over time, but few of these (as
published) provide information on the strength of the wound

itself.(50,56)

Wound TensometryC

 

Quantitative measurement of the strength of tissues

begins with an understanding of their behavior in tension

 

C There are references in the biomechanics literature

to tissue-tension measuring equipment, not always given
the same generic name. In this thesis, the definitions
found in Dorlands Medical Dictionary, 26th ed., 1981,
and Webster's Third New International Dictionary, 1981,
will be adhered to: Tensimeters measure the pressure of
a gas or vapor, tensiometers measure the surface tension
of a liquid, and tensometers measure the tension of a
solid material.

12

and failure. When compared to materials such as glass and
oil, tissue neither breaks nor flows. The term
"viscoelastic" is applied to this third type of material
response to load.(57) Collagenous structures including
ligaments, fascia, and healing wounds all respond to

progressive loading as shown in Figure 1.

Load,
Newtons

 

Deformation, Meters

Figure l. Viscoelastic load-deformation curve.

The overall shape of this curve reveals that a period
of linear elongation (a), in response to loading is
followed by plastic deformation (b), and ultimately failure
(c). An overloaded tendon stretches, then tears, and
finally breaks. As Evans and Barbenel note, the bulk of
research has concentrated on the behavior of tissue in
terms of sudden, forcible failure.(58) The biomechanics
literature is devoid of studies of healing equine wounds

which describes their behavior in terms of cyclic loading,

13

as may be expected to occur in vivo. The complexity and
cost of equipment to measure simple destructive failure of
a material are considerably less than for equipment which
uniformly loads and unloads a sample for thousands of
cycles.

Nevertheless, the accepted standard of wound
tensometry is sudden breakage and measurement of a
momentary force sustained (ultimate load (UL), breaking
strength, or very loosely, "tensile strength" or the energy
absorbed by a tissue sample (wound) from the application of
inexorable but constant displacement. The devices to
produce such failure either require gross dissection of the
the tissue to allow clamping to the jaws of a machine, or
entrapment of the entire wound over a source of gas or
fluid pressure to produce outward rupture. Each system has
its advantages and disadvantages, and it may be helpful to
list the requirements of the ideal wound testing apparatus.

1) The wound should be tested in situ, in
normal position relative to surrounding tissues.

2) The wound should be kept alive, with its
blood supply intact through testing.

3) Testing cycles should produce only
normal (for the specific gross structure) force
vectors, varying in scale or frequency instead of
direction.

4) Testing equipment should be able to
detect and announce the beginning of plastic
deformation of tissue. This would provide an
experimental endpoint without destructive testing.

5) The ability to accomplish testing in a
sterile field might permit survival of the
subject animal (probably anesthetized during
testing).

14

Howes et. a1. evaluated the strength of healing rectus
abdominus muscle sheath in dogs by excision of wound
samples and mounting them in a commercial Scott thread
testing machine.(55) Botsford used two testing systems for
determination of strength of guinea pig skin wounds.(59)
The first was a simple screw mechanism that interposed a
small spring scale between an attachment to one side of the
skin wound and the traveler on a screw. The second (for
stronger wounds) was a lever arm which was attached to the
skin on one end and supported a small bucket of mercury on
the other. A known amount of mercury was allowed to run
into the bucket, and the force produced calculated from
knowledge of the moment arms and density of mercury. This
particular work is also interesting for being an early
description of survival wound testing, though it seemed to
produce a high standard error in the data (by today's
standards). At the completion of testing an anesthetized
subject, the wound was reclosed and the guinea pig allowed
to recover. Wolarsky used a small box with the abdominal
wall of rats fastened over an opening in one side.(60)

Air was pumped into the box and pressure measured with a
manometer, failure of the linea alba wound was recorded
by an observer. Comparisons between samples were based
on pressure to failure, and not in terms of linear tension.

Allen and Schannen adapted Wolarsky's method by using

larger and stronger materials to permit testing of sutured

linea alba from horses.(27,6l) Cinematographic recording

15

of the pressure on the air pump at the time of visible
rupture of the abdominal wall was used to calculate the
linear force perpendicular to the incision line. Peacock
disputes the use of burst methods to calculate wound
strength by showing that differences between radii of
normal tissue and the healing tissue produce differing
calculated failure force.(37) Even if the different radii
are measured, the tension on the unstressed tissue section
has not been. Abrahams used an early Instron Testor, which
allowed cyclic force application to an excised tissue
sample (but not a healing wound), with automated analog
recording of load cell (strain gauge) voltages in
connection with the time and sample displacement.(62) In
this system, the specimen may be "cycled" while immersed in
physiologic fluid, usually Ringer's solution.

The effects of sample storage until actual testing has
been addressed by several workers. Viidik describes the
alterations of tensile strength in rabbit cranial cruciate
ligament after refrigeration, freezing, and formalin
fixation.(63) Slight changes in one of five experimental
variables studied followed storage by every method. The
fact that some of the significant changes reflected
increased tissue strength while others showed decreasing
strength may be explained by the application of
inappropriate statistical analysis (multiple iterations of
Student's t-test within a group comparison, which would

increase the likelihood of a false significant difference).

16

Viidik recommended that all mechanical testing of tissue
should be done immediately after collection.(63) Matthews
concluded that there was no significant change in the
stress-strain curves of cat tendon after storage by
freezing, although the elastic modulus did decrease after
freezing.(64) He concluded that data in biomechanics
should not be pooled from fresh and frozen samples. Woo
et. al. looked at the effects of freezing rabbit ligaments
at -20 degrees Celsius for three months and found that
there were no changes in stress-strain behavior, but the
frozen ligaments did have a lower ultimate load and modulus

of elasticity.(65)

Suture Materials

 

Selection of suture size and material type is a
judgement the surgeon makes at the time of surgery, and
will be influenced by numerous factors of patient and
procedure. In general, the size of a suture is described
in the United States by an arbitrary number system which
specifies the maximum and minimum diameter, and a minimum
breaking strength for each number-size from ten-ought
(10-0) up to number 7. These sizes and their equivalent
metric sizes and diameters are listed in Appendix A.
Exceptions include slight differences in diameter for gut
suture, and the sizing of metallic wire is by Browne and
Sharpe wire gauge.(66) The choice of suture size by the

surgeon is based on a subjective estimate of the strength

17

of the tissue to be apposed, the expected tension which
must be supported by the suture line, and the principle
that the foreign material remaining in a healing wound
should be kept to a minimum.

The available combinations of chemical composition and
physical form of suture materials provides a wide range of
choice of sutures for a given application. Crane lists the
currently manufactured materials, with their physical and
surgical properties.(67) The physical properties of the
ideal suture have been listed many times: Uniform diameter,
high tensile strength, pliable, good knot security, low
cost, and easily sterilized, to name but a few.(66) It
should also evoke the least inflammatory, immunogenic, and
carcinogenic tissue reaction, and not support sepsis.(67)
The ideal suture material should also be absorbed at a
constant rate after healing has progressed, or be
encapsulated without complications.(67) Since no material
yet devised has all these properties, and the materials
available have differing combinations of these as well as a
few undesirable properties, many workers have attempted to
learn which product is most suitable for a given
application.(l9,50,52-54,56,68-77)

The anatomic structures which must heal following
celiotomy or laparotomy are fibrous tendon or fascia, and
may be expected to have slower rates of healing as measured
by wound strength than skin or visceral tissue.(78)

Sutures placed in the abdominal wall may also be exposed to

18

accelerated degradation due to septic peritonitis. The
specific requirements for a material used in closure of the
linea alba of a horse include large size, high tensile
strength, minimal tissue reactivity, persistence in the
presence of septic peritonitis, and failure to promote the
formation of fistulous suture tracts. McIlwraith suggests
that polyglactind is to be preferred over chromic gut for
its greater strength; he also describes synthetic
nonabsorbable monofilament material as acceptable.(21)
While synthetic monofilament nonabsorbable materials are
quite strong, they may promote formation of suture fistulae.
Shires notes that recovery from anesthesia may be the most
critical moment of the abdominal wall closure in horses.(47)
He suggests that either absorbable or nonabsorbable sutures
may be used, and prefers a simple interrupted pattern.
Milne advocates the use of a large (U.S.P. #5) braided
polyester material for closure of the ventral abdominal
wall.(46) Huskamp reports that with the use of double
strands of #2 polyglycolic acid8 (PGA) in 790 procedures on
horses, the rate of wound herniation was less than 12, but
duration of follow-up is not given.(79) Although the
veterinary literature contains reports of problems with
each material described, there appear to be fewer problems

associated with monofilament nylon, polypropylene and

 

Vicryl - Ethicon, Somerville, NJ.

Dexon - Davis and Geek, Manata, PR.

19

stainless steel. The polymers are criticized for poor
knot security, and the wire suture for difficulty in
handling.

There are a number of reports which describe the
specific liability of different materials to produce a
given adverse effect, including sepsis, decreased wound
strength, and incisional hernia.(50,66,78,80) Edlich et.
al. developed a model for contaminated suture material in
mice. All material samples were treated with different
dilutions of Staphylococcus aureus or Escherichia coli and
the implanted suture strands and implant site evaluated on
the fourth POD for signs of sepsis.(81) They found that
there was significantly less infection associated with PGA
suture material than with chromic gut, less with
monofilament nylon and polypropylene than with stainless
steel, and less with multifilament nylon than any other
multifilament nonabsorbable material, including polyesters,
silk, cotton, and stainless steel. Edlich's work led him
to conclude that the chemical composition of a suture, and
not its physical form, is the single most important
property associated with infection. He also found that
nylon and polypropylene consistently had the lowest
infection rates, and that of the absorbable materials PGA
was least prone to infection. He proposes that nylon and
PGA have bacteriostatic degradation products to explain
these findings, although this has only been shown in

vitro.(81) Bucknall et. al. completed a series of

20

experiments to show the relative in vitro adherence of S.
aureus culture to different suture materials, and of the
effect of suture/wound infections on wound strength of the
incised linea alba of rats.(77) His findings include:

1) Braided silk will retain three times as
many bacteria as monofilament nylon.

2) Braided nylon retains virtually the same
number of bacteria as braided silk.

3) Braided PGA retains more bacteria than
monofilament nylon, but less than braided silk.

4) There was no significant difference in
wound strength between infected and uninfected
wounds.

5) Bacteria were found by scanning electron

microscopy between fibers of braided nylon and
silk up to 70 days after implantation.

Bucknall concludes that synthetic nonabsorbable
material should be used for closure of the abdominal wall.
Barham found that PGA would result in stronger wounds in
irradiated rabbit muscle fascia and in bladder wounds, and
that there was less histologic reaction surrounding PGA
when compared to chromic gut sutures.(68) Craig et. al.
compared PGA and polyglactin (PGL) for material strength
when implanted in rat gluteal muscle.(53) Their findings
that PGL was stronger than PGA at all time intervals out to
the 35th post-implantation day are consistent with another
project which compared strength of the dry unimplanted
materials.(51) Allen compared bursting strength of sutured

equine linea alba, and found that a multifilament synthetic

21

polyamide material was stronger than doubled strands of
gut suture, but did not use comparable experimental
groups.(27)

For this study, chromic gut, polyglactin, and
polydioxanone sutures were used in a comparison of wound
strength of the healing linea alba in ponies. Their
chemical composition, physical structure, and
characteristics and suture materials are described below.

Chromic gut is an absorbable, highly processed natural
material. It may be obtained from the submucosa of sheep
intestine, or the serosal layers of bovine intestine.(66)
Sequential treatment with formaldehyde, cleaning solutions,
and chromic salt solutions produce a soft, supple strand
which must be stored in alcohol. While possessing excellent
handling qualities, gut loses strength rapidly in vivo, and
produces a severe foreign body reaction in the patient. To
quote Crane: "[Chromic gut is] a traditional nonabsorbable
(sic) that is being replaced by synthetic absorbable
products in many practices."(67) The two features of this
material which lessen its reliability in surgery are the
unpredictable loss of strength which occurs both within an
individual animal and between different species, and that
knot security is poor.(66) Gut is recognized as likely to
result in considerable fibrosis and scarring, due to its
high tissue reactivity.

Polyglactin is a complex copolymer of glycolide and
lactide.(82) Its chemical formula is (02H202)m(C3H402)n

22

and the structural formula is shown in Figure 2.

Figure 2. Structure of polyglactin.f

As surgical suture material it consists of a very fine
continuous fibers braided into a somewhat stiff strand with
high strength and good knot security. It is metabolized in
the tissue by enzymatic hydrolysis at a constant rate. It
retains 502 of its tensile strength at the 15th
postoperative day, and is considered to be to be abrasive
when drawn through the tissue being sutured. It is
remarkably difficult to cut, at least in the larger sizes.
Polyglactin also evokes minimal tissue reaction, and is

completely absorbed by 90 days in vivo.(53)

 

f From Craig, et. al., Surgery, Gynecology, and
Obstetrics, 1975; 141:1-10.

23

Polydioxanone (PDS) is a relatively new suture material
which, like gut and PGL, is also absorbable. PDS differs
in being a monofilament form, and has a chemical formula of

(C4H603)n'(83) Its structural formula is shown in Figure 3.

1:1H Hi?
o-c-c-o-c-C
H

Figure 3. Structure of polydioxanone.g

Malnatii et. al. describe this material as having very
low drag when drawn through tissues.(7l) Ray et. a2.
suggest that this material is exceptionally strong, and
retains its strength for a longer time than other
absorbable materials.(84) PDS is hydrolyzed in a manner
similar to PGL, but the process is less rapid due to its
monofilament form. There is little early tissue reaction,

and none seen by postoperative day 91.(84)

 

g From Ray et. al., Surgery, Gynecology and Obstetrics,
1981: 153:497-507.

24

MATERIALS AND METHODS

Experimental Animals and Groups

 

Thirty-six adult ponies were used in this project,
equally divided between six experimental groups which were
defined by duration of postoperative healing. Surgery was
not performed on group one ponies which served as controls
to provide strength of the normal linea alba; group two
ponies were killed immediately after surgery, before wound
healing could begin. Ponies in groups three, four, five,
and six were maintained after surgery to allow normal wound
healing to proceed for 7, 14, 21, and 28 days respectively.
After a complete physical exam revealed no serious disease
problems, ponies were preconditioned by vaccination for
Eastern and Western equine encephalomyelitis, influenza,
and for tetanus. Each was also treated with pyrantel
pamoate at 6.6 mg/kg before being turned out to a 20 acre
pasture for at least 30 days. All ponies were fed on
pasture and provided access to salt blocks and water ad
Zibitum. All ponies were identified by name and a numbered
collar. Each was randomly assigned to one of the six
experimental groups to distribute the variability which may
have resulted from differences of age, weight, and gender.
The signalment and grouping of ponies is described in

Table 1.

25

 

Table 1. Signalment of experimental ponies.

Case No. Name No. Age Sex* Weight** Breed Group
800131 Danny 93 2 m/c 195 Hackney 1
800123 Maria 98 12 f 177 Grade 1
800134 Mudd 92 2 f 134 Grade 1
800143 Wilma 55 4 f 211 Grade 1
800142 Toby 68 2 m/c 141 Grade 1
800129 Holly 72 20 f 145 Shetl. 1
800148 A-H 69 2 m/c 209 Hackney 2
800114 Barney 82 20 m/c 195 Grade 2
800118 Ginger 78 12 f 318 Grade 2
800146 Sarah 91 11 f 202 Grade 2
800138 Peggy 95 9 f 175 Grade 2
800147 Waldo 76 6 m/c 218 Grade 2
800113 Arlo 85 5 m/c 173 Grade 3
800076 Godmoth 56 16 f 255 Grade 3
800153 Grandpa 77 25 m/c 185 Grade 3
800137 Otto 83 9 m/c 193 Grade 3
800128 Daphne 96 14 f 173 Shetl. 3
800115 Wendy-O 83 17 f 177 Grade 3
800119 Floyd 62 2 m/c 193 Grade 4
800150 Norma 61 4 f 184 Grade 4
800139 Zelda 59 12 f 150 Shetl. 4
800152 Adolph 74 9 m/c 165 Grade 4
800151 Snuffy 70 15 m/c 213 Grade 4
800126 Blondie 86 4 f 184 Grade 4
800132 Katie 73 11 f 164 Shetl. 5
800015 Minerva 60 20 f 332 Grade 5
800135 Norma 67 12 m/c 164 Grade 5
800141 Pedro 97 16 m/c 191 Grade 5
800133 Marvin 64 10 m/c 184 Grade 5
800117 Delbar 75 2 m/c 145 Grade 5
800149 Vern 65 3 m/c 205 Hackney 6
800147 Reuben 57 4 m/c 175 Grade 6
800125 Mickey 89 2 m 184 Grade 6
800127 Beau 71 3 m 218 Hackney 6
800120 Gus 63 ll m/c 184 Shetl. 6
800121 Bonnie 80 2 m/c 164 Grade 6
* M - Male, F - Female, M/C - Gelding

** Weight in Kilograms

26

Description of Surgery and Postoperative Management

 

All ponies except those in group one were brought into
a box stall in turn, and kept there for the day before and
two days after surgery. Ponies were prepared on the day
before surgery by clipping hair in a wide area over the
ventral abdomen, from the xiphoid to the inguinal region.
Each was given a single dose of procaine penicillin-G, at
22,000 iu/kg, and benzathine penicillin at the same dose by
intramuscular injection three hours before induction of
anesthesia. Ponies were given a single dose of five
million i.u. sodium penicillin intravenously at the time of
initial skin incision, and 4.4 mg/kg phenylbutazone
intravenously at the completion of surgery. Feed was
withheld for 12 hours prior to surgery. On the day of
surgery each pony was prepared by grooming and introduction
of a 14 gauge Teflon intravenous catheter.h Xylazinei was
given as the preanesthetic sedation at 0.77 mg/kg
intravenously, and a few minutes later induction of
anesthesia accomplished with a fast intravenous drip of a
mixture of 2 grams thiamylal sodiumj in 500 ml of 102
guaifenesin, given to effect. Standard procedures for
endotracheal intubation and maintenance on halothane

anesthesia in a semi-closed system

 

Angiocath, Deseret Medical Inc., Sandy, Utah.
Rompun, Mobay Corporation, Shawnee, Kansas.

j Bio-Tal, Bioceutic Co., St. Joseph, Missouri.

27

were used following induction. Ponies were positioned in
dorsal recumbancy and the ventral abdomen prepared by
scrubbing the skin with an iodophor scrubk preparation and
with 702 propanol. Final skin preparation was with iodophor
solutionl. Standard methods of draping and aseptic
procedures were used throughout surgery.

A 30cm sharp scalpel skin incision was made on the
ventral midline and continued through the superficial
fascia to expose the deep fascia of the external sheath of
the rectus abdominus muscle. Hemorrhage of superficial
vessels was controlled with electrocautery. The linea alba
was divided with a scalpel for a distance of at least
27cm, craniad from the umbilicus. The length of the linea
alba between the umbilicus and to a point five centimeters
from the apex of the xiphoid cartilage was considered to
have three regions of equal length, designated the cranial,
middle, and caudal regions. Therefore, linea alba incisions
were longer in larger ponies, to allow later comparison of
wound strength by regions proportional to the size of the
pony. The peritoneum was bluntly divided and the abdominal
viscera then briefly explored in an attempt to locate major
anatomic or physiologic disease processes, or pregnancy.

Closure of the linea alba was accomplished using four

 

k Betadine Surgical Scrub, Purdue Frederick Co., Norwalk,
Connecticut.

1 Betadine Solution, Purdue Frederick Co., Norwalk,
Connecticut.

28

different suture materials in a simple interrupted pattern.
All sutures were placed one centimeter from the cut margin
of the linea alba and sutures were 1.5 cm apart. All
sutures were U.S.P. #2 size swaged onto 3/8ths circle
reverse cutting needles. Monofilament nylon was used for
closure of the umbilicus, the cranial incision end, and
wherever spacing sutures were required between regions.
Polyglactin, medium chromic gut, and polydioxanone sutures
were each used in turn, at least six simple interrupted
sutures of a given material were used within a particular
region before the next was begun. The sequence of placement
for these three materials was alternated to ensure that each
was present twice in each of the cranial, middle and caudal
thirds of the incision, in each operated group of six
ponies. This alternation within groups is shown in Figure
4. Two adjacent sutures of each of the three test materials
were placed through a loop of 28 gauge stainless steel
suture wire, the twisted ends of which were left in a
subcutaneous pocket. This wire "tag" was placed to allow
later identification of a suture material pair at the time
of tissue segment dissection. Detail of suture placement is
shown in Figure 5. At this point, the number, type, and
relative position of all sutures placed in the abdominal
wall were carefully identified and noted on a form by an
assistant. Closure of the superficial layers proceeded

using size 2/0 polypropylenem in a simple continuous pattern

 

Prolene, Ethicon Inc., Somerville, New Jersey.

29

in the subcutaneous tissue, and staplesn for apposition of
the skin. A dry sterile dressing was applied, and the
wound bandaged with an adhesive elastic material.

Following recovery from anesthesia, each animal was
bandaged with a heavy elastic bandageo which was removed on
the seventh postoperative day (POD). Phenylbutazonep was
given at 4.4 mg/kg per 03 every 12 hours beginning 12 hours
after surgery for a total of 3 treatments. After the
second POD, ponies were moved out into a sand lot for
another five days, at which point those in groups with
survival times longer than one week were returned to

pasture.

 

n Proximate II, Ethicon Inc., Somerville, New Jersey.

0 Elastikon Elastic Tape, Johnson and Johnson, New

Brunswick, New Jersey.

p Butazolidin Paste, Burrough Wellcome Co., Kansas City,
Missouri.

30

 

 

 

 

 

 

' I .0. V .. ' v 'I'. '/ 7
"- ‘ . I. .0...
: 32-. '-‘ - - I
F . ' o... ‘0' 0'. 7/ I
' . . .' I : :o.'.....
. "" An. A l‘:'l..
1'7?» - '.o‘1 .
'.'. A A 0'1 A A

  

 

     

'1‘. '
yu".'."'~"'"" a . u . .0 . .
h.\. ~o: .-.o..... :f...l. :g.o.. .I... z. ’0’. . .a.-

.. ‘..n....-o .o.|0l0
oo'\tn0n°0~1A'4AAA'IuLOQA'AA'

 

 

 

 

 

' I v I' . v I'
I I '
.
”AA A ALLA LL
vv ‘ vvvi‘v '1 v v y-
l .-
A A I A AA A
V ' '.. Y‘ooro:.o?:2.‘.'o"..i .I.I.V.‘..‘ .~:~
’n.'°'.'.o'. 0~'¢'00":e:.... 0 .'~ ‘
"'."a 'o.$.o:'.i.ofi .'.: '0

 

 

 

 

;';.. ..._'.°.

'3 -"- - .' - "- 1‘3- :Z'Ziifr's'fi:Zita-2:1. 7

.::o..o......‘..u "~’o..... .. '0‘.
o' no '...t o'.oo'oo
o..oo""' .0. ..I0 0

 

 

 

 

 

 

 

Figure 4. Rotation of suture material position using
group six as an example. Each suture material type was
present twice in each region of the incision within a
group. The horizontal black lines represent the linea alba

incision.

 

- Gut

 

 

 

 

 

O - Polyglactin

 

 

 

C .0
.0

o.- .,
\
Q

- Polydioxanone

.0
.32

 

 

 

31

 

 

 

 

 

 

 

 

H

 

J J
/
fir-F———-————'—‘”

\'

<=:‘;'::'-'-“

Figure 5. Detail of linea alba closure. The wire 100p
incorporating two adjacent sutures, one for each suture
material type, is in the subcutaneous space.

32

Necropsy and Sample Collection

 

On the appropriate POD euthanasia was performed with
intravenous pentobarbital sodiumq, given at 85 mg/kg.
Necropsy began with removal of the skin covering the
ventral abdomen. The gross appearance of the healing
incision was noted, and a wide section of the full-thickness
abdominal wall, with the sutured incision at its center was
removed. The abdomen and its contents were then examined
to identify gross signs of abnormal healing or adhesions.

A plastic sheet marked with the pony's name, number, date
of necropsy, and the orientation of the sample (cranial
end) was applied to the 20 cm by 30 cm rectangular slab of
the abdominal wall. The slab was placed in two heavy
plastic bags, and immediately put in a flat position into a
freezer at -60 degrees Celsius. Throughout the procedure
of necropsy and sample storage, care was taken not to put

undue tension on the healed linea alba.

Specimen Preparation and Mechanical Testing

 

The preparation and mechanical testing of the segments
from all ponies was a batch process, and followed an
average freezer storage time of 45 days. At this time the
abdominal wall specimens were taken in random order to a

dissection room adjacent to the testing area. The

 

q Fatal-Plus, Vortech Pharmaceuticals, Dearborn, Michigan.

33

still-frozen abdominal wall was sectioned on a bandsaw to
yield six, two-cm-wide sections of the incision line, each
section containing two sutures of a given material type.
Three of these sections also had one of the stainless steel
wire loops present on the superficial surface. A numbered
spring-clip was immediately applied to each section, for
purposes of later identification. The six sections thus
obtained from each pony were processed as a group, beginning
with thawing in Ringers' lactated solution at room
temperature. Each section was then dissected in a manner to
discard the peritoneum, retroperitoneal fat, and rectus
abdominus muscle, shown in Figure 6. This left a section 2
cm wide with an appearance similar to that shown in Figure
7. For sections which were to be tested without sutures,
the pair of sutures were cut at this time.

Each section was tested in turn by the attachment of a
grip system, loading into a servo-hydraulic mechanical
testing devicer, and recording the force-deformation curve
obtained. The grips employed for this project were prepared
by bonding a carborundum grit cloth with epoxy cement to
steel blocks, and clamping these with each side of the
abdominal wall section in a "sandwich" (Figure 8), by means

of adjustable locking pliers, as shown in Figure 9.

 

r Instron Testor, Instron Corporation, Canton,

Massachusetts.

34

The testing apparatus included the Instron testor,
computers and a videocamera and recording system.t After
positioning the prepared specimen into the Instron testor,
any slack in the tissue was removed by adjustment of the
hydraulic ram, the sample was identified to the videocamera
by a numbered card, and the computer-driven test program was
initiated. The tissue section was distracted to failure at
a constant rate of one cm/sec for a total distance of 5
centimeters. An example is shown in Figures 10 and 11.
During this time a magnetic disk recorder recorded the time
courses and force generated from a load-cell to which the
section was attached and the motion of the hydraulic ram.
These force and motion values were also displayed on an
oscilloscope“, the image of which was photographed at the
completion of testing a tissue section. The vidoecamera
recorded the visual appearance of wound distraction with a
time signal initiated by the computer. At the completion of
testing all tissue sections, the numerical data on magnetic
disks was transferred to PDP 11/23 disk files for automated
processing. The testing sequence and recording of data is

depicted in a schematic shown in Appendix B.

 

S PDP 11/23, Digital Equipment Corporation, West Concord,

Massachusetts.

t Circon Color Microvideo Camera, Model MV9M330, Circon
Corporation, Santa Barbara, California.

u Nicolet Oscilloscope, Nicolet Test Instruments, Madison
Wisconsin.

35

 

Figure 6. Dissection of a wound segment. Prior to
testing, the internal (i) and external (e) rectus sheath
layers were isolated, and the peritoneum (p),
retroperitoneal fat (f), and rectus muscle (m) were
discarded. The scale on the right indicates centimeters.

36

 

Figure 7. Wound segment completely dissected. Only the
internal and external rectus sheath, linea alba (arrow), and
sutures remain.

37

 

 

 

 

 

 

 

Figure 8. Design of tissue segment grips.

38

 

Figure 9. Wound segment with grips attached. Four
locking pliers serve to grip tightly the grit-coated steel
blocks and tissue "sandwich".

39

Data processing programs written at a later time
enabled specific retrieval and printing of the values shown
in Appendix C. The individual data points, obtained every
1/200 second, were plotted as load versus stroke curves by
automated method. Videotape recordings of the testing
procedure were reviewed at a later time to assess the
location of section breaking (failure) and to learn if
slipping of the tissue from the grips occurred.

For each experimental group the ultimate load (UL) and
energy absorbance (EA) were compared against duration of
post0perative healing, region of the abdomen, and suture

material type.

Statistical Analysis

 

Multiple iterations of a computer-assisted block
analysis of variance (ANOVA) were used to assess
statistical significancev. The dependent variables for
each of two procedures were either UL or EA, against the
factors of healing time and region of the abdomen or suture
material. Data was converted by log transformation prior

to entry into the ANOVA program. The requirement for

significance was a critical p<0.05.

 

V Michigan State University Cyber 750 job numbers TB06320
and TB13238.

40

 

Figure 10. Distracted wound segment after failure.
This section was from a group three pony; the two
polyglactin sutures have not broken seven days after surgery
(arrows).

41

 

 

300‘ <—291 new'rous
/ 5.54 JOILES
LOAD “m' /
newrous //
o; 2///////////////////////III,.,.
TIME. secouos

ultimate load of a wound segment is the maximum force
sustained, in Newtons (N). The energy absorbance is the
total work performed to cause failure, and is the shaded
area under this curve, if seconds of time are converted to
meters of displacement, in units of Joules (J).

RESULTS AND DISCUSSION

"Tensile strength is the most important
parameter of an incisional wound, both
to the patient and to the surgeon."

Peacock, 1984

Data was lost due to recording failure at the time of
testing for six of the 198 tissue sections studied. Data
for these sections was reconstructed from the oscilloscope
photographs and entered into the appr0priate computer files
manually. The numerical results of section testing for
ultimate load in Newtons (N) and energy absorbance in
Joules (J) are listed in Appendix C. The group means (x),
standard deviations (sd), and standard errors of the mean
(sem) are shown in Tables 2-9.

When wound strength in terms of UL is plotted against
duration of healing (postoperative time) as shown in Figure
12, the overall sigmoid shape is apparent. This feature of
wound strength dynamics was seen for all circumstances of
UL and EA (Figure 13) regardless of suture material, region
of the linea alba, or whether or not the sutures were
present at the time of strength testing. When wound
strength is evaluated in terms of energy absorbance the
results are identical to those described for UL. The

strength of the wound immediately following surgery and on

42

43

Table 2. Statistics of ultimate load for suture
material groups, sutures removed,in Newtons.

X sd sem
GROUP: 1
Vicryl
PDS n/a
Gut
All
GROUP: 2
Vicryl
PDS n/a
Gut
All
GROUP: 3
Vicryl 32 10 4
PDS 32 20 8
Gut 33 ll 5
All 34 14 3
GROUP : 4
Vicryl 215 54 22
PDS 256 88 36
Gut 205 53 22
All 225 67 16
GROUP: 5
Vicryl 273 101 41
PDS 291 93 38
Gut 330 83 34
All 298 90 21
GROUP: 6
Vicryl 345 117 48
PDS 369 182 74
Gut 340 68 28

All 351 124 29

44

Table 3. Statistics of ultimate load for suture
material groups, sutures intact,in Newtons.

X sd sem
GROUP: 1
Vicryl
PDS n/a
Gut
All
GROUP: 2
Vicryl 108 34 14
PDS 97 54 22
Cut 92 27 11
A11 99 38 9
GROUP: 3
Vicryl 113 46 21
PDS 92 33 14
Gut 52 34 14
All 84 44 11
GROUP: 4
Vicryl 264 106 43
PDS 260 70 29
Cut 269 64 26
All 264 77 18
GROUP: 5
Vicryl 334 104 42
PDS 298 60 24
Gut 312 56 23
All 315 74 17
GROUP: 6
Vicryl 325 139 57
PDS 362 165 68
Gut 336 102 42

All 341 131 31

45

Table 4. Statistics of ultimate load for incision
region groups, sutures removed,in Newtons.

X sd sem
GROUP: 1
Region 1 226 98 40
2 301 92 37
3 278 94 38
Total 269 91 22
GROUP: 2
Region 1
2 n/a
3
Total
GROUP: 3
Region 1 33 13 5
2 35 12 5
3 34 20 9
Total 34 14 3
GROUP: 4
Region 1 283 65 27
2 197 58 23
3 195 39 16
Total 225 67 16
GROUP: 5
Region 1 295 109 44
2 341 104 42
3 259 34 14
Total 298 90 21
GROUP: 6
Region 1 463 107 44
2 307 111 46
3 283 71 29

Total 351 124 29

46

Table 5. Statistics of ultimate load for incision
region groups, sutures intact,in Newtons.

X sd sem
GROUP: 1

Region 1 198 67 27

2 299 94 39

3 278 94 38

Total 258 92 22
GROUP: 2

Region 1 120 . 54 22

2 93 26 11

3 84 23 9

Total 99 38 9
GROUP: 3

Region 1 104 53 22

2 84 35 14

3 59 37 17

Total 84 44 11
GROUP: 4

Region 1 327 47 19

2 259 88 36

3 207 40 16

Total 264 77 18
GROUP: 5

Region 1 279 66 27

2 337 84 34

3 329 67 27

Total 315 74 17
GROUP: 6

Region 1 343 198 81

2 362 97 39

3 318 91 37

Total 341 131 31

 

47

Table 6. Statistics of energy absorbance for suture
material groups, sutures removed,in Joules.

X sd sem
GROUP: 1
Vicryl
PDS n/a
Gut
All
GROUP: 2
Vicryl
PDS n/a
Gut
All
GROUP: 3
Vicryl 0.4 0.2 0.1
PDS 0.3 0.2 0.1
Gut 0.3 0.1 0.1
All 0.3 0.2 0.1
GROUP: 4
Vicryl 2.6 0.5 0.2
PDS 3.0 1.2 0.5
Gut 2.6 1.2 0.5
All 2.7 1.0 0.2
GROUP: 5
Vicryl 3.3 1.4 0.6
PDS 3.1 0.5 0.2
Gut 4.1 0.7 0.3
All 3.5 1.0 0.2
GROUP: 6
Vicryl 4.6 1.1 0.4
PDS 5.9 3.6 1.5
Gut 4.9 1.8 0.7
All 5.1 2.4 0.6

48

Table 7 Statistics of energy absorbance for suture
material groups, sutures intact,in Joules.

X sd sem
GROUP: 1
Vicryl
PDS n/a
Gut
All
GROUP: 2
Vicryl 1.2 0.4 0.2
PDS 1.3 0.6 0.3
Gut 1.0 0.5 0.2
All 1.3 0.5 0.1
GROUP: 3
Vicryl 1.6 1.1 0.4
PDS 1.3 0.5 0.2
Gut 0.6 0.6 0.2
All 1.2 0.8 0.2
GROUP: 4
Vicryl 3.0 1.1 0.4
PDS 3.0 0.7 0.3
Gut 3.2 1.1 0.4
All 3.0 0.9 0.2
GROUP: 5
Vicryl 4.1 1.3 0.5
PDS 3.3 1.1 0.5
Gut 4.2 0.8 0.3
All 3.9 1.1 0.3
GROUP: 6
Vicryl 4.9 2.5 1.0
PDS 4.7 1.6 0.6
Gut 4.5 1.8 0.7
All 4.7 1.9 0.5

 

49

Table 8 Statistics of energy absorbance for incision
region groups, sutures removed,in Joules.

X sd sem
GROUP: 1
Region 1 2.9 1.0 0.4
2 3.9 2.3 0.9
3 3.6 1.3 0.5
Total 3.5 1.6 0.4
GROUP: 2
Region 1
2 n/a
3
Total
GROUP: 3
Region 1 0.3 0.2 0.1
2 0.3 0.2 0.1
3 0.3 0.2 0.1
Total 0.3 0.2 0.1
GROUP: 4
Region 1 3.2 0.9 0.4
2 2.9 1.0 0.4
3 2.0 0.8 0.3
Total 2.7 1.0 0.2
GROUP: 5
Region 1 2.8 0.9 0.4
2 4.1 0.9 0.4
3 3.5 0.9 0.4
Total 3.5 1.0 0.2
GROUP : 6
Region 1 5.9 2.9 1.2
2 4.3 2.6 1.1
3 5.2 1.3 0.5
Total 5.1 2.4 0.6

50

Table 9 Statistics of energy absorbance for incision
region groups, sutures intact,in Joules.

X sd sem
GROUP: 1

Region 1 3.2 1.1 0.5

2 3.9 2.3 0.9

3 3.0 0.9 0.3

Total 3.4 1.5 0.4
GROUP: 2

Region 1 1.2 0.5 0.2

2 1.3 0.5 0.2

3 1.3 0.5 0.2

Total 1.3 0.5 0.1
GROUP: 3

Region 1 1.6 1.2 0.5

2 1.0 0.6 0.3

3 0.9 0.4 0.1

Total 1.2 0.8 0.2
GROUP: 4

Region 1 3.6 1.0 0.4

2 3.1 0.8 0.3

3 2.5 0.8 0.3

Total 3.0 0.9 0.2
GROUP: 5

Region 1 3.8 0.9 0.4

2 3.9 1.4 0.6

3 3.9 1.4 0.6

Total 3.9 1.1 0.3
GROUP: 6

Region 1 4.2 2.2 0.9

2 5.5 2.0 0.8

3 4.4 1.4 0.6

Total 4.7 1.9 0.5

51

 

 

 

I/ y I 1 l

 

O 7 14 21 28
DAYS POST-OP
Figure 12. Ultimate load of wounds as a function of

postoperative time. The solid line (o———————o) represents
strength of the wound with sutures intact. The broken line
(o——uu———o) shows strength of wounds from which the sutures
were removed. Error bars are equal to one standard error
of the mean, and those with asterisks have group means
significantly different from the other means at the same
postoperative time.

 

52

postoperative day seven was significantly different (lower)
than wound strength on the 14th, let, and 28th
postoperative day. The stength of the linea alba with
sutures was significantly greater than the same wound
without sutures on the day of surgery and on the seventh
POD. From the 14th postoperative day through to the 28th,
wound strength with or without suture support is not

significantly different from that of the normal linea alba

 

by either strength variable (UL or EA). There is a trend .
of wound strength exceeding that of the unwounded structure,
but at the experimental endpoint of 28 days healing, this
difference was not significant. It is interesting to note
that for both variables the strength of the wound without
sutures becomes stronger when compared to the wound with
sutures between the let and 28th postoperative day,
although the difference is not significant. There are two
likely explanations for this finding: First, that
dissection of the individual tissue sections to a depth
necessary to cut the embedded sutures may have weakened the
fibrous wound, or second, that the vertical penetration of
adjacent fascia and bridging of the wound by suture
material was a site of stress concentration, which allowed
distraction forces to act on collagen fiber orientation in
an unfavorable direction.

The overall time effects of equine linea alba healing
are similar to those shown by other species. Peacock

states that:

53

EA.

 

 

 

JOULES
(° ‘1‘
O 7 14 21 28
DAYS POST-OP
Figure 13. Energy absorbance of wounds as a function of

postoperative time. The solid line (o———-———o) represents
strength of the wound with sutures intact. The broken line
(o——-—-—o) shows strength of wounds from which sutures were
removed. Error bars are equal to one standard error of the
mean, and those with asterisks have group means
significantly different from the other mean at the same
postoperative time.

54

"It has been observed that the rate at which all
wounds gain strength is approximately the same during the
first 14 to 21 days after wounding."(29)

However, an unusual feature of the data presented here

is the rapidity with which wound strength exceeded that of
normal. Nelson and Dennis found similar rates of healing
in the rabbit external rectus sheath, but final wound
strength even at 42 days did not reach that of the normal
rectus sheath in the same animal.(35) Wound strength from
the ponies in this project is shown as a percentage of
normal linea alba in Table 10. There is a major difference
in these figures when compared to those of rabbits, in that
strength of the normal unwounded structure is exceeded by
that of the wound in as little as 28 days. It is difficult
to know if this interspecies difference is real, or if it
results from attempts to compare dissimilar information.
Differences of experimental design, testing procedures
(burst strength versus breaking strength) and statistical
analysis prevent valid interspecies comparison. For the
larger species used in this project there is an increased
total amount of suture material to act as a foreign
substance, inciting increased fibrosis within the local
wound environment. It is apparent that the overall picture
of early wound strength (less than 30 days) is similar for
ponies as has been described in rats and rabbits.(33)

At the time of necropsy, and when tissue sections were
dissected for testing, there was seen to be considerable

fibrosis and gross thickening of the external rectus sheath

55

 

Table 10. Wound strength as a percentage of normal.
Group No. Postoperative Time Sutures Intact UL2 EA2
1 Controls n/a 100 100
2 Postoperative yes 38 38
no 0 0
3 7 days yes 32 35
no 13 9
4 14 days yes 100 87
no 85 78
5 21 days yes 119 113
no 113 101
6 28 days yes 129 136
no 133 148

 

56

 

Figure 14. Postmorten incision regions. The skin has
been removed from the ventral abdomen of pony number 59, 14
days after surgery, following euthanasia. At surgery the
linea alba was divided into three regions in proportion to
the individual pony size, as shown in the figure. The
cranial region (P) was closed with polydioxanone, the middle
region (V) with polyglactin, and the caudal region with

(G) with chromic gut.

57

 

Figure 15. Transverse frozen wound section. Section (A)
was from the cranial incision of pony number 59, containing
PDS. Note the thicker scar in the caudal region, (B), where
chromic gut was used to close the wound.

58

surrounding the incision line. This may be seen in the
photographs in Figures 14 and 15. Especially in the region
containing chromic gut sutures, it was sometimes difficult
to clamp the full thickness of the scar surrounding the
incision into the tissue-holding grips. This resulted in
slipping of some of the specimens before complete failure
had occurred (since only a small part of the total collagen
mass was in contact with the grip surfaces). While the
data shows at least a high minimum strength, it does not
reflect what might be an actually greater strength than
recorded. It is at this last healing interval that the
standard error of the mean is the largest for nearly all
experimental groups.

Incision region effects are described by the graph in
Figure 16. Wound strength immediately following surgery
and at the 28th postoperative day healing endpoint is
nearly identical for the caudal, middle, and cranial
regions. However, the caudal region is stronger than the
cranial in the acute phase of wound healing from days 7
through 21, although this difference is significant only on
the 14th postoperative day. This finding is similar to
that described by Nilsson when he evaluated the strength of
the rabbit linea alba in a craniocaudal axis.(45) Why
animals require a stronger caudal abdominal wall than
cranial or heal the caudal abdomen more quickly, at least
with regard to wound strength, is not known.

In this project the degree of gross fibrosis covering

59

EA.

 

 

 

JOULES ‘
O 7 14 21 28
DAYS POST-OP
Figure 16. Energy absorbance of regional wounds. The

solid line (o———————o) represents the cranial segment of
the linea alba; the broken line (o——~—-——o) shows strength
of the caudal region. The middle region of the incision is
not shown. The error bars are equal to one standard error
of the mean, and those with asterisks have group means
significantly different from the other mean at the same
postoperative time. The normal datum represents the mean
strength of the linea alba without regard to location.

60

the external rectus sheath was found to be associated with
the location of the gut suture, not with the region of the
abdomen. If the mass of the collagen found in healing
wounds does not account for the greater strength of the
caudal region, then the explanation may lie in local
differences of fiber orientation and cross-linking. This
finding may serve to explain the clinical observation that
incisional hernias tend to occur more often in cranial
incisions in humans or at the cranial limit of an incision
in horses.(2)W Comparing the normal anatomy of the two
regions, there is an increased proportion of muscle inserting
closely to the LA in the cranial region. The caudal region
is a broader collection of the fibrous aponeuroses of the
abdominal muscle layers, which meet in a flattened curve on
the midline. The sharper angle formed at the midline by
the left and right sides of the abdominal wall in the
cranial region may also experience increased cyclic loading
by the lateral expansion of the thorax with normal inspiratory
effort. Proximity to the internal attachments of the
diaphragm may also produce excessive motion or forces of
distraction on wounds in the cranial abdominal wall.

Suture material effects (Figure 17) were only compared in

 

w In a review of five cases of incisional hernia

following celiotomy at the University of Missouri Equine
Center, all were located at the cranial limit of the
abdominal wall incision.

61

groups 3, 4, 5, and 6, as group one had no sutures, and in
group two there was no breaking of sutures in those section
tested. Wounds sutured with gut were found to be
significantly stronger only in terms of EA than those
closed with PDS on the 28th postoperative day. This
information suggests that the increased fibrosis elicited
by chromic gut is capable of absorbing more total energy on
the way to failure, but does not confer any advantage as
far as increasing the breaking strength. This finding is
in agreement with Peacock's observation that ". . . we
actually measure the strength of the strongest
component."(29) Donaldson in a prospective trial comparing
PGA, chromic gut, and polypropylene in 231 human laparotomy
patients found no significant difference in the rates of
wound dehiscence or incisional hernia.(86)

Interpretation of the results of this thesis requires
awareness of their source: The in vitro testing procedures
used here cannot be considered to mimic the loads and
cycling of the connective tissue scar in viva. Virtually
all wound strength research is designed on the basis of a
single observation (for each tissue sample) which follows
application of a sudden overwhelming stress. The normal
physiologic stresses encountered by the wound may be very
different. In the case of the healing linea alba, the
constant motion required by inhalation, the frequent
occasions to use the abdominal press, changes of posture,

and even whole body activity probably all combine to

62

E.A.

 

 

 

JOILES
o 7 14 . 21 28
DAYS POST-OP
Figure 17. Energy absorbance of wounds by suture

material type. The solid (o———————o) represents wound
strength of the linea alba closed with chromic gut, the
broken line (o—--—-—o) wounds closed with polyglactin, and
the dashed line (o—n--—uo) wounds closed with
polydioxanone. The error bars are equal to one standard
error of the mean, and those with asterisks have group means
significantly different from the other means at the same
postoperative time.

63

produce gradual elongation of the scar. It seems
reasonable to suppose that the early scar, composed of Type
III collagen in a low—strength, disorganized mass is most
susceptible to gradual elongation in response to these
momentary high-stress forces. Later, when replacement by
Type I collagen fibers aligned in the direction of local
physiologic stress forces has occurred, elongation of the
scar may be arrested. While there have been several risk
factors identified from which later development of an
incisional hernia may follow, the pathophysiology is not at
all clear.

A common problem in tensometry of the linea alba is
the finding that this structure (or its mature wound after
sufficient healing has occurred) is stronger than the
entire thickness of the abdominal wall only a few centimeters
away. The conclusion is often drawn that the wound or
linea alba is stronger than the lateral abdominal fascia.
The problem remains: 1) Wound strength is still unknown,
and 2) The conclusion of strength is based on sudden
failure of the testing method. The appearance of incisional
hernias (especially late hernias) is a clue that cyclic
loading over time is, to the wound, a force more difficult
to resist. Perhaps the healing linea alba is stronger than
the more lateral abdominal wall after 21 days because it
needs to be. We have come full circle to the problems of
adequate tissue gripping for this anatomic structure and

the need for more physiologic (cyclic) wound testing.

 

64

The problem of designing a system of tissue grips that
can firmly hold the complex cross-section of muscle and
fascia which is the abdominal wall will probably be solved.
A clamp for large tendons has been described which
circulates a refrigerant through its hollow structure,
freezing the tendon between its jaws.(87) This clamp has
exerted in excess of 13,000 Newtons without slippage of the
tendon. Mechanical testing equipment is available which
may be programmed to load a tissue sample in a controlled
cycle. It would still be desirable to maintain the section
"alive" throughout the process of controlled mechanical

fatigue, a far more difficult problem to solve.

Implications for the Surgeon

 

The surgeon should interpret with caution the findings
described in this thesis. For example, it would be
premature to believe that abdominal wounds have regained
normal strength by the 14th postoperative day, and allow
strenuous activity by a patient at that time. In fact, it
may be true that celiotomy wounds withstand severe,
isolated stress (distraction forces) after the second week
following surgery quite well. The question of how well
such early wounds withstand repeated distraction forces
remains unanswered. Similarly, many accepted guidelines of
suture selection argue against the use of chromic gut
suture. Synthetic absorbable sutures now available are far

stronger, retain their strength longer, are less prone to

65

inciting wound sepsis or sterile abscesses, and tend to
evoke less severe inflammation of the wound. It was
previously thought possible that polydioxanone would
contribute sufficiently to wound strength beyond the 14th
postoperative day (and beyond the time afforded by other
absorbable suture materials) that it would provide a long
term advantage. Instead, the conclusion which must be
drawn is that sutures do not contribute to wound strength
in the pony's abdominal wall beyond the 13th postoperative
day. The persistence of foreign material (suture) across
the healing wound may act as a stress-riser, and actually
weaken the collagenous scar.

The needs of the surgical task at hand will continue
to dictate the appropriate site for incision. If the
cranial celiotomy wound is more at risk for later
herniation, as suggested by the results of this project and
clinical observations, then perhaps those cases which
require cranial incision should be managed more
conservatively in the postoperative period. At least the
data obtained here would suggest that there may be greater
risk of wound dehiscence of cranial incisions than those
placed in the caudal region, up through the 20th

postoperative day.

.' 4...;

66

The results and conclusions described here are taken
from an experimental model (ponies). Pending further clinical
or laboratory studies which examine the dynamics of wound
strength of the linea alba in full-sized horses, it seems
reasonable to apply the findings described above to surgical

practice involving all equids.

CONCLUSIONS AND SUMMARY

"Surgeons are pragmatic creatures, more
interested in departures that offer
real benefits to patients than in those
which satisfy statisticians."
Evans and Pollack,
1984

In this project wound healing in ponies has been
examined and has been defined from the particular need for
strength and functional closure of the abdominal wall.

This is not the only valid criteria of healing, but may be
considered the most important for this particular anatomic
structure. A narrow definition of wound strength was
applied to the dynamics of tissue healing in the abdominal
wall of ponies. The influences of anatomic site and
different suture materials were also investigated.

It was determined that there is a rapid gain of
strength in the healing linea alba between the 7th and 28th
days, preceded by a slight decrease in strength in the
first postoperative week. This pattern of increasing wound
strength is substantially the same as has been described in
other species. The rate of wound strength increase in
ponies may be faster than that seen in rabbits, based on

per cent of strength of the normal linea alba. The finding

that most wounds analyzed in this study had a final strength

67

68

greater than that of the linea alba in the normal control
ponies is not difficult to account for. On gross inspection
of the thickness of the fibrous scar at the time of sample
dissection, the greater thickness of the healed incisions
compared to the normal rectus sheath and linea alba was
obvious. Allen obtained similar results in horses with
different suture patterns and testing methods than were
used here, finding that even as early as the 12th
postoperative day the wound strength may equal or exceed
that of the normal structure.(27) Quantitative measurement
of scar mass was not a design feature of this project, but
should certainly be considered in future research in this
area. Energy absorbance, or failure energy, is the more
sensitive indicator of wound strength and was able to
discern significant differences in the data not detectable
by ultimate load (breaking strength).

The wound in the caudal abdomen regained strength more
quickly than did the cranial wound, although by the 28th
postOperative day they were similar. The physiologic
mechanism which is responsible for this is not known,
although it has been described in other species. This may
be an artifact resulting from the particular methods
employed to grip and distract collagenous tissue. It seems
unlikely that the process of wound healing is capable of
such "fine-tuning" within a given anatomic structure. The
finding of decreasing strength in the cranial limit of

ventral abdominal incisions is perhaps supported by clinical

69

events: That incisional hernias in horses are usually in
the cranial part of the wound made earlier.

Lastly, the use of chromic gut suture in ponies is
attended by extensive fibrosis of the superficial surface of
the external rectus sheath. The total mass of scar tissue
covering the incision site may explain the fact that at the
end of the 4th postoperative week, these wounds were
significantly stronger than those containing PDS, the least
tissue-reactive suture material. Of greater concern to the
surgeon is the fact that on the 7th postoperative day,
those regions closed with chromic gut had lost an

appreciable part of their strength.

APPENDICES

71

 

 

 

APPENDIX A

TABLE 11. Sizes of suture materials

USP Sizes B & S Wire
Metric TExtiles Gut Gauge
0.1
0.2 10-0
0.3 9-0
0.4 8-0
05 7-0 8-0 41
0.7 6-0 7-0 38-40
1.0 5-0 6-0 35
1.5 4-0 5-0 32-34
2.0 3-0 4-0 30
3.0 2-0 3-0 28
3.5 0 2-0 26
4.0 1 0 25
5.0 2 1 24
6.0 3 & 4 2 22
7.0 5 3 20
8.0 6 4 19
9.0 7 18

72

APPENDIX B

 

 

 

 

 

 

00015

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 18. Schematic of sample testing.

Data from each wound sample was recorded on magnetic disks,
including load, time signal, and position of the distracting
hydraulic piston. The visual appearance of wound failure

was recorded by videocamera on tape with the same time
signal mark.

73

APPENDIX B

 

 

 

 

 

8

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Figure 18, continued. Schematic of sample testing.

The data from all wound segments tested was processed to
produce plots of load versus distance, ultimate load
(breaking strength), and energy absorbance (failure energy).

74

APPENDIX C

TABLE 12. Filtered data, with calculated units. GROUP 1

 

 

Ultimate Energy
Pony: DANNY Load, N Absorbed, J
Section Suture
1 194 3.7
2 243 3.1
3 291 2.9
4 408 3.2
439 4.2
6 377 3.5
Pony: HOLLY
Section
1 120 1.0
2 102 1.2
3 145 1.2
4 260 2.1
5 283 1.9
6 133 2.4
Pony: MARIA
Section
1 200 3.5
2 241 3.1
3 237 2.5
4 219 3.1
5 180 2.3
6 297 3.1
Pony: MUDD
Section
1 175 3.9
2 291 4.2
3 390 7.4
4 427 8.4
5 263 3.7
6 259 6.1
Pony: TOBY
Section
1 175 4.0
2 326 2.5
3 394 5.9
4 246 3.2
5 192 2.8
6 168 2.9
Pony: WILMA
Section
1 321 3.3
2 391 3.4
3 247 3.7
4 243 3.3
5 308 2.9
6 202 3.4

75

 

 

TABLE 12, continued. GROUP 2
Ultimate Energy

Pony: A-H Load, N Absorbed, J

Section Suture
1 V 165 1.5
2
3 G 102 2.0
4
5 P 79 1.4
6

Pony: BARNEY

Section
1 V 65 0.4
2
3 G 52 0.6
l;
5 P 46 0.3
6

Pony: GINGER

Section
1 G 133 1.2
2
3 P 126 1.7
4
5 V 113 1.3
6

Pony: PEGGY

Section
1 P 192 1.7
2
3 V 113 1.4
l;
5 G 99 1.6
6

Pony: SARAH

Section
1 V 110 1.3
2
3 G 81 0.9
l;
5 P 83 1.9
6

Pony: WALDO

Section
1 P 56 0.8
2
3 V 83 1.2
l;
5 G 82 1.3
6

76

 

 

TABLE 12, continued GROUP 3
Ultimate Energy

Pony: ARLO Load, N Absorbed, J

Section Suture
1 G 110 1.7
2 G 46 0.5
3 P 108 1.2
4 P 48 0.5
5 V 722 0.8
6 V 580 0.4

Pony: DAPHNE

Section
1 P 52 0 4
2 P 18 0 1
3 V 102 l 2
4 V 41 0 3
5 G 48 0 8
6 G 28 0 3

Pony: GODMOTHER

Section
1 V 170 2.2
2 V 39 0.5
3 G 29 0.3
4 G 37 0.3
5 P 114 1.3
6 P 66 0.4

Pony: GRANDPAW

Section
1 P 103 1 7
2 P 40 0 3
3 V 70 0 8
4 V 32 0 6
5 G 14 0 3
6 G 30 O 2

Pony: OTTO

Section
1 G 37 0.3
2 G 41 0.3
3 P 125 2.0
4 P 37 0.2
5 V 72 1.2
6 V 31 0.4

Pony: WENDY-O

Section
1 V 153 3.5
2 V 16 0.1
3 G 71 0.4
4 G 14 0.1
5 P 47 1.1
6 P 13 0.1

77

 

 

TABLE 12, continued GROUP 4
Ultimate Energy

Pony: ADOLPH Load, N Absorbed, J

Section Suture
1 P 325 3.2
2 P 389 3.2
3 V 248 2.8
4 V 177 2.8
5 G 226 3.8
6 G 180 1.6

Pony: BLONDIE

Section
1 V 406 4.7
2 V 286 2.8
3 G 256 3.6
4 G 256 4.6
5 P 213 2.4
6 P 186 1.7

Pony: FLOYD

Section
1 V 354 3.9
2 V 212 2.4
3 G 374 3.0
4 G 148 2.4
5 P 157 2.0
6 P 183 1.6

Pony: NORMA

Section
1 G 293 4.6
2 G 224 3.3
3 P 326 3.5
4 P 280 3.3
5 V 187 2.1
6 V 203 3.4

Pony: SNUFFY

Section
1 G 275 2.3
2 G 269 2.3
3 P 231 4.0
4 P 178 2.5
5 V 272 2.8
6 V 267 2.4

Pony: ZELDA

Section
1 P 307 2.6
2 P 319 4.3
3 V 117 1.8
4 V 142 1.8
5 G 187 1.6
6 G 150 1.3

78

 

 

TABLE 12, continued GROUP 5
Ultimate Energy

Pony: DELBAR Load, N Absorbed, J

Section Suture
1 P 405 5.5
2 P 365 2.9
3 V 246 2.5
4 V 356 5.4
5 G 326 3.6
6 G 271 4.3

Pony: KATIE

Section
1 G 239 4.1
2 G 234 2.7
3 P 314 2.6
4 P 240 2.8
5 V 336 5.2
6 V 212 2.8

Pony: MARVIN

Section
1 P 254 3 2
2 P 435 3 5
3 V 495 5 1
4 V 410 3 9
5 G 392 3 9
6 G 291 4 8

Pony: MINERVA

Section
1 G 258 3.5
2 G 339 4.1
3 P 310 2.9
4 P 192 3.6
5 V 413 5.5
6 V 265 3.7

Pony: NORMAN

Section
1 V 222 3.1
2 V 269 1.7
3 G 345 5.8
4 G 464 4.5
5 P 249 2.5
6 P 223 2.4

Pony: PEDRO

Section
1 V 294 3.2
2 V 127 2.1
3 G 309 4.4
4 G 381 4,4
5 P 257 2.8
6 P 293 3.1

79

 

 

TABLE 12, continued GROUP 6
Ultimate Energy

Pony: BEAU Load, N Absorbed, J

Section Suture
l V 444 5.6
2 V 461 5.6
3 G 377 5.6
4 G 341 8.4
5 P 392 4.7
6 P 330 6.6

Pony: BONNIE

Section
1 V 145 2.1
2 V 396 3.9
3 G 399 6.0
4 G 263 4.2
5 P 250 4.0
6 P 288 6.6

Pony: GUS

Section
1 G 155 1.9
2 G 322 3.8
3 P 211 2.5
4 P 122 0.9
5 V 243 1.9
6 V 203 3.6

Pony: MICKEY

Section
1 P 369 6.0
2 P 635 11.2
3 V 439 6.0
4 V 461 5.9
5 G 374 6.4
6 G 386 5.4

Pony: REUBEN

Section
1 G 277 2.8
2 G 447 3.9
3 P 282 4.1
4 P 318 3.0
5 V 218 5.0
6 V 208 5.0

Pony: VERN

Section
1 P 669 7.0
2 P 519 7.0
3 V 462 8.6
4 V 338 3.5
5 G 431 4.4
6 G 283 3.8

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81

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