Tensile strength of arterial prosthetic anastomoses

JOURN.\L

OF

SURCiICAL

TENSILE

RESEARCH,

13, 209-214 (1972)

STRENGTH WESLEY

OF ARTERIAL

S. MOORE,

M.D.,

AND

F.A.C.S.,

ROBERT

PROSTHETIC ALBERT

E. ALLEN,

Moore).

The measurement of collagenase activity by the Wound Healing Laboratory of the University of California Medical Center is gratefully acknowledged. We wish to thank Mr. Charles Joseph Michielsen for the design and construction of the tensile strength machine. Submitted for publication May 16, 1972.

AND METHODS

i Porosity: 5000 cc/cm’/min/water at 120 mm Hg, 70 denier Dacron yarn, 30 needles/in. United States Catheter and Instrument Corporation. 209

Inc. reserved.

F.A.C.S.,

Ten adult mongrel dogs weighing 13-18 kg were anesthetized with intravenous sodium pentothal and connected to respirators. In each dog a piece of infrarenal abdominal aorta 4 cm long was resected and replaced with a knitted Dacron graft (DeBakey type)t 8 mm in diameter. To evaluate separately the effect on anastomosis tensile strength of fibrous tissue ingrowth and encapsulation from the contribution of anastomotic suture material, each dog had either the proximal or distal anastomosis performed with a continuous 4-O chromic catgut suture (average absorption time 3 weeks) and the opposite anastomosis performed with a continuous, nonabsorbable suture of 4-O braided Dacron (Tycron) . Thus, when the catgut was absorbed the tensiIe strength of that anastomosis would be based on ingrowth of fibrous tissue and encapsulation at the site of the anastomosis. The proximal and distal positions of each suture type were placed alt’ernately in the 10 dogs. Four and one-half months following implantation, the dogs were sacrificed and the grafts were removed in continuity with the abdominal aorta. Particular care was taken to prevent disruption of the fibrous tissue capsule surrounding the graft (Fig. 1). The specimens were tested for anastomotic tensile strength, collagen turnover, and fibrous tissue response.

From the Surgical Services, Veterans Administration Hospital, San Francisco. California, and the University of California at San Francisco. *Present address: 2500 Hanover Drive, Lansing, MI. Reprint requests: Veterans Bdministration Hospital, 4150 Clement St.? San Francisco, CA 94121 (Dr.

c 1972 by Academic Press, in any form o9 reproduction

M.D.,

M.D.*

MATERIALS

PROSTHETIC GRAFTS ARE satisfactorily used for reconstruction of large arteries. However, the most effective prosthesis, particularly for replacement of small arteries, has yet to be developed. In the evaluation of new vascular prostheses an important factor to study is the nnastomotic healing characteristics with respect to tensile strength, and the ability of the prosthesis to resist disruption and development of a false aneurysm at the graft-artery junction. In this study we quantitated the anastomotic healing characteristics of a successful, clinically proven, large vessel prosthesis in order to develop a standardized method for m.easuring the characteristics of anastomotic healing that could be applied to newly developed vascular prost,heses. The factors that were evaluated were the relative contribution of anastomotic suture material and fibrous tissue to anastomotic tensile strength, together with the stability of collagen at the graft-arter.v interface.

Copyright All rights

D. HALL,

ANASTOMOSES

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Fig. 1. Dacron prosthesis removed in continuity with proximal and distal the prosthesis is intact, totally covering the graft and both anastomoses.

Tensile Strength Measurements To assess the tensile strength of the anastomoses by a controlled reproducible method a machine was constructed that would progressively increase a vertical loading force on the anastomosis at a constant rate until anastomotic disruption occurred (Fig. 2). Each specimen was separated into a proximal and distal anastomosis by dividing the midportion of the graft in a transverse direction. The anastomoPRESSURE

GAUGE

Fig. 2. The tensile strength unit operates on a hydraulic principle. The components are labeled and a grafkartery segment is attached between the upper and lower transverse bar ready for testing.

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The fibrous

tissue capsule

of

ses were left circumferentially intact and in continuity with the surrounding fibrous tissue capsule. The graft end of the anastomosis was attached to the upper transverse bar of the tensile strength machine. The artery end of the anastomosis was attached to the lower transverse bar. The upper bar was raised by two hydraulic plungers, which were powered by a self-contained compressed air cylinder. The rate of flow, and hence the rate of excursion of the upper transverse bar, was regulated by a valve which was adjusted and kept in a constant position for all measurements. The lower transverse bar, to which the arterial end of the anastomosis was attached, was connected by a plunger to a second hydraulic system, so that as the upper bar moved a disrupting force was applied to the anastomosis. As long as the anastomosis remained intact, the generated force was transmitted to the lower transverse bar, causing compression of the second hydraulic system, which was continuously measured in pounds per square inch (Fig. 3). The pressure generated at the time anastomotic disruption took place was noted on the attached gauge and recorded by a pressure transducer connected to a strip chart recorder. The paper speed of the recorder was set at a constant rate, so that the rate of increasing compression could be verified and the maximum compression generated at the time of anastomotic disruption could be recorded (Fig.

Fig. 3~. Magnified S1w of the graft-artery segment cla111ped tc II! upper and lower tranxverse bar before 8.p~~II ation of the disrupting force. 3b. Force is a tug applied to the graftarlery anastomosis !G bte the pulling up of the lower transverse lxn ~ahich produces compression of the second:l.rl hydraulic system as recorded on the premr t’ gauge. SC. Anastomotic disruption and tht: Illessure generated at the time of rupture.

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manifested by a measurable level of collagenase; conversely, if collagen were relatively stable in the area of the anastomosis, collagenase activity would be minimal or absent. Histological

0

4

8

12

16

18

TIME ( seconds) Fig. 4. A linear relationship was demonstrated between applied force and time. The anastomosis ruptured in 7 set at a generated pressure of 45 psi. When the rupture occurred the trace became horizontal, since no additional pressure was being generated.

Studies

Longitudinal sections through the artery and the corresponding end of the graft at each anastomosis were removed after the tensile strength studies. The tissue was fixed in formalin, sectioned and stained for microscopic examination. The microscopic sections were examined for differences in the fibrous tissue response at the catgut anastomosis as compared to the Dacron suture anastomosis, and to compare the histological appearance of the artery adjacent to the two anastomoses. RESULTS

ot- I 0

IO

I

/ 20

30

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I 50

I 60

I 70

I so

PRESSURE (Ibs/inch21

Fig. 6. Calibration data comparing pressure in pounds per square inch to force in pounds. The t test for linear correlation was t = 6.58, indicating a confidence level of linearity in excess of 99.9%. By using this curve, pressure data were directly converted to the applied force necessary to disrupt the anastomosis. 4). The compression system was calibrated by a spring scale and the relationship between pressure and force was found to be linear; therefore, a reading in pressure could be directly converted to a reading in pounds of force. The tensile strength of the anastomosis was defined as amount of force required (measured in pounds) to break the anastomosis (Fig. 5). Collagen

Turnover

After tensile strength was measured, sections of artery adjacent to the proximal and distal anastomoses were assayed for collagenase activity by the method of Gross and Lapiere [l]. This was done on the assumption that a high turnover rate of collagen would be

The force required to produce anastomotic disruption and the time to achieve that force are shown in Table 1. The mean force required to disrupt the catgut suture anastomosis was 8.8 pounds. The mean force required to disrupt the Dacron sutured anastomosis was 16.4 pounds. Disruption of the catgut sutured anastomosis always occurred at the graft-artery junction. Disruption of the Dacron-sutured anastomosis was usually caused by tearing of the artery proximal to the anastomosis, or by sutures pulling through and disrupting the arterial wall. Table 1. Force and Time

Required

to Produce

Anastomotic Disruption Catgut-Sutured Anastomosis Dog

1 2 3 4 5 6 7 8 9 10

Tensile Strength Force (pounds)

Dacros-Sutured Anastomosis

Time to D$iss,t

Tensile S;;t;h

Time to Dtr’p’

(set)

(pounds)

(set)

10.8 4.4 23.0 9.2 2.8 20.8 10.3 2.8 18.6 1.0 6.9 16.6 7.5 2.4 15.0 7.2 2.4 9.1 6.7 1.0 14.1 8.3 14.1 4.8 12.5 10.8 16.6 Dog No. 10 died on the 4th postoperative

27.2 22.8 7.2 13.2 15.6 4.8 12.8 10.4 17.6 day.

IMOORE,

HBLL,

AND

ALLEN:

The time-force relationship of anastomotic disruption is shown in Fig. 6. A t test of linear correlation indicated a confidence level of linearity greater than 99.9%, confirming that the tensile strength testing device produced an anastomotic force that increased at a constant rate with respect to time. In reviewing the individual tensile strength data, it was apparent that a range existed for both the catgut-sutured anastomoses and the Dacron-sutured anastomoses. Figure 7 compares the tensile strength of the Dacron and catgut anastomoses in each animal. A t test of linear correlation indicated a confidence level of linearity in excess of 99.9%. This suggests that the variability in tensile strength at both anastomoses was directly related to each animal’s fibrous tissue response, since this was the only healing variable common to both anastomoses. An analysis of collagenase failed to show enzyme activity in any of the anastomoses. This would indicate that collagen at the anastomotic site is relatively stable with apparently little or no collagen turnover. Individual histologic studies showed no difference between the catgut- and Dacron-sutured anastomoses with respect to fibrous tissue response or changes in the architecture of the artery adjacent to the anastomosis. These data support the view that the tensile strength of the catgut anastomosis reflects the fibrous tissue contribution to the total tensile strength of a permanently sutured anastomosis.

ARTERIAL

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ANASTOMOSES

25-

5-

0 0

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FORCE7

Ibs)

I 25

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Fig. 6. The force required for anastomotic disruption is compared to the time required to develop that force. The t test of linear correlation was t = 8.719, or a confidence level in excess of 99.9%. This confirms our requirement that the increase in force developed by the tensile strength unit must be linear with respect to time. *At time of anastomosis disruption;

**time required to produce disrupting force.

DISCUSSION This study indicates that the major contribution t’o anastomotic tensile strength is made by the suture material. The Dacron-sutured anast,omoses were usually stronger than the artcry itself and were at least twice as strong as t#heanastomoses dependent upon host fibrous tissue response in the absence of permanent suture material. These results support our clinical observations in patients with anastomotic false aneurysms. False aneurysms occurred only in patients in whom silk sutures had been used for the anastomosis. At the time of operation the original silk sutures were disrupted or totally absorbed. The fibrous bridge between graft and artery had dilated to form

0

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STRENGTH (Ibs.

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3’0

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ANASTOMOSES

force)

Fig. 7. The tensile strength of the Dacron anastomosis is compared to the tensile strength of the catgut anastomosis in each animal. The t test of linear correlation was equal to 6.595, indicating a linear confidence level in excess of 99.9%. These data demonstrate that variability in anastomotic tensile strength was directly related to the fibrous tissue response at the anastomosis.

an aneurysm [2]. However, in dogs the fibrous tissue response did add further strength to the sutured anastomosis, since there were individual differences in anastomotic strength. The

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animals in which the anastomosis sutured with the catgut had greater tensile strength also had correspondingly greater tensile strength in the Dacron-sutured anastomosis. This variability in total tensile strength can only be accounted for by differences in quality of the fibrous tissue response. The technique developed for testing anastomotic tensile strength in which the relative contributions of fibrous tissue and the suture material are independently measured has proved to be an accurate method which could be used in evaluating any new prosthetic graft. This method of testing utilized the entire circumference of the anastomosis, as opposed to testing small strips of an anastomosis [3]. The use of anastomotic strips may lead to inaccuracies of measurement due to division of the sutures, variability in strip size, and disruption of the fibrous tissue capsule. The method also involves the application of progressively increasing force at a constant rate, which enables tensile strength to be tested under reproducible conditions. This is preferable to producing stress by the manual addition of weights to one end of the anastomosis, since forces applied in this manner are present for a variable period of time and occur in sudden increments [4]. Finally, the technique of using an absorbable suture at one anastomosis to isolate the contribution to tensile strength by the fibrous tissue response is more satisfactory than the technique of individually cutting and removing suture material at the time of testing. This is because the removal of suture material from an anastomosis also necessitates disruption of the fibrous capsule, giving an erroneous impression of the extent of contribution of fibrous tissue to tensile strength [5]. The absence of collagenase activity at the sites of anastomosis indicates the stability of collagen at the anastomotic interface and sug-

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gests that the continued presence of this graft material is in relative harmony with the host vessel. SUMMARY A method of accurately measuring the tensile strength of an arterial prosthetic anastomosis is described. By performing one anastomosis with an absorbable suture and the other with a Dacron suture, the contribution of fibrous tissue ingrowth and encapsulation was separately measured 4.5 months following implantation and was found to be only one-half as strong as the permanently sutured anastomosis. We concluded from the study that a permanent suture was ultimately the most important factor in anastomotic tensile strength because of the two-fold increase in tensile strength over fibrous tissue alone, and the fact that the anastomosis with permanent sutures was stronger than the artery itself. When the graft was removed no collagenase was found at the anastomosis, suggesting that collagen at the graft--artery interface is relatively stable and that little or no collagen turnover is taking place. REFERENCES Gross, J., and Lap&e, C. M. Collagenolytic activity in amphibian tissues: A tissue culture assay. Proc. Nut. Acad. Sci. 48:1014, 1962. 2. Moore, W. S., and Hall, A. D. Late suture failure in the pathogenesis of anastomotic false aneurysms. Ann. Surg. 172 : 1064, 1970. 3. Hohf, Robert P. Tensile strength of the arterial prosthesis anastomosis during healing. Ann. Surg. 1.

156 :805, 1962.

4. Kottmeier, C. A., and Wheat, M. W. Jr. Strength of anastomoses in aortic prosthetic grafts. Amer. Surg.

31:128,

1965.

5. Edwards, W. S., Dalton, D. Jr., and Quattlebaum, R. Anastomoses between synthetic graft and artery. A study of tensile strength. Arch. Surg. (Chiago) 86:477,1963.