changes in tensile strength and knot security of surgical sutures in vivo

7/27/2019 Changes in Tensile Strength and Knot Security of Surgical Sutures in Vivo

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Changes in Tensile Strengthand Knot Security ofSurgical Sutures in VivoJohn B. Herrmann, MD, Worcester, Mass

Changes in tensile strength and knot se-

curity of commonly used surgical suture ma-

terials during the postoperative period were

evaluated by implanting sterile suture loopsin the subcutaneous tissue of rats and rab-

bits. Tensile strength testing after varyingperiods of implantation showed that of the

materials studied, only Dacron maintained

its initial knot security and tensile strength.Silk and cotton showed moderate loss of

strength even in two weeks. Of the absorb-

able sutures, polyglycolic acid sutures were

superior in tensile strength and markedly su-

perior in knot security as compared to cat-

gut. Both materials showed progressive loss

of strength reaching minimal values at about

three weeks.

Theintroduction of a variety of

new suture materials in recent

years has given surgeons the opportu¬nity to select the most appropriatesuture for each specific surgical ap¬

plication. Thorough knowledge of the

properties of the various suture mate¬rials available is essential in makingthese decisions. In a previous report1the in vitro tensile strength and knot

security of currently available mate¬rials has been presented. Perhaps ofmore

importanceto the

surgeonand

his patients are the changes that mayoccur in these important suture prop¬erties during the postoperative pe¬riod. The following experiments were

designed to evaluate the changes in

breaking strength and knot securityin the early postoperative period. The

materials studied were those in most

common use today, including the new

synthetic absorbable suture, poly¬glycolic acid (PGA).

Methods

Young adult Sprague-Dawley strainwhite rats and New Zealand strain white

rabbits were used in the following experi¬ments. All animals were individually cagedand fed a standard laboratory diet withwater ad lib.

Sterile sutures intended for general op¬

erating room use were obtained commer¬

cially. Sterile suture loops for implantationwere prepared by tying the suture around

an 8.2-mm diameter sterile glass rod usingthree squared throws on the knot. Suture

loops were implanted subcutaneously inthe abdominal wall of experimental ani¬

mals through a small midline incision withuse of ether anesthesia and aseptic oper¬

ating room technique.At sacrifice, the loops were carefully dis¬

sected free from surrounding tissue and

immediately placed in a Petri dish on a

saline-soaked sponge to prevent drying.Testing was performed on a commercial

recording tensiometer. Suture loops were

placed individually over right-angle rods

clamped in the jaws of the tensiometerthat were then distracted at a rate of 1.0

cm/min until disruption or knot slippageoccurred. If the knot began to slip, the ten¬

siometer was momentarily stopped, and a

clamp applied to the "rabbit ears" beyondthe knot to prevent further slipping. The

breaking strength of the loop was defined

as the maximum force applied to the loopat the point of disruption. Knot securitylevel is defined as the force applied to the

loop at the point of knot slippage or disrup¬tion (if slippage did not occur). Tensile

strength is breaking strength divided by

cross-sectional area. Cross-sectional area

was calculated from the diameter of thesuture measured with a precision microme¬ter. Effective tensile strength is knot se¬

curity level divided by cross-sectional area.Percent

loss of tensile strength is calcu¬lated on the basis of preimplantation ten¬

sile strength. The methods outlined abovehave been presented in detail in a previouspublication.'

Experiment 1.—Suture loops of size 2-0

Dacron, silk, cotton, PGA, plain catgut,and medium chromic catgut were im¬

planted in each of 40 rats. Animals were

killed in groups of ten at 3, 7, 10, and 14

days. Suture loops were tested as previ¬ously outlined.

Experiment 2.—Suture loops of sizes 0

and 3-0 PGA and medium chromic catgut

from the two major manufacturers (a totalof six loops in each animal) were implantedin 60 rats and ten rabbits. The rats were

killed at 5, 10, 15, 20, 25, and 30 days. The

rabbits were all killed at ten days. The su¬

tures were tested as previously described.

Results

Experiment 1.—Changes in breakingstrength of the suture loops followingimplantation are shown in Fig 1. Da¬

cron retains its strength, but all other

materials, including silk and cotton,

show loss of strength over the two-week period studied. Plain catguthad negligible strength by 14 days.The PGA sutures were superior in

strength to chromic catgut at all in¬tervals studied and were equal in

strength to comparably sized silk and

cotton sutures at 14 days.Conversion of breaking strength

values to actual tensile strengthvalues based on cross-sectional area

accentuated the differences notedabove

(Fig 2)because of the larger

Accepted for publication Dec 1, 1972.From the Department of Surgery, Georgetown

University School of Medicine, and the SurgicalService, Washington Veterans AdministrationHospital, Washington, DC.

Reprint requests to Department of Surgery,Worcester City Hospital, 26 Queen St, Worces-

ter, Mass 01610 (Dr. Herrmann).

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* 5-

c

î?m

S¬

Oc

•XL

Size 2-0 Suture LoopsSubcutaneous Implants

3 7 10

Days After Implantation

14

Fig 1.—Changes in knot pull breakingstrength of size 2-0 suture loops over a two-week period following subcutaneous im¬

plantationin rats.

80-

EE to

ù:60-

£)

C<1)

5 40-

(Ôc

ì 20-

O)

Size 2-0 Suture LoopsSubcutaneous Implants-

Dacron-Silk

.

Chromic Catgut- PGA

-Plain Catgut

-Cotton

J + 1 SE

3 7 10


Fig 2.—Changes in tensile strength(breaking strength divided by cross-sec¬

tional area).

Size 2-0 Suture LoopsSubcutaneous Implants- Dacron- Silk

.

Chromic Catgut

-

PGA

-

Plain Catgut- Cotton

±1 SE

Fig 3.—Effect of implantation on knotfailure level.

3 7 10





Size O

-PGA-Chromic Catgut A

-Chromic Catgut

10 15 20 25


Size 3-0

— PGA--Chromic Catgut A

-Chromic Catgut

10 15 20


25

Fig 4.—Comparison of breaking strengthof two samples of chromic catgut and PGAsutures after implantation. Left, Size 0 ma¬

erial; right, size 3-0 material.

Fig 5.—Comparison of breaking strengthof two samples of chromic catgut and PGAsutures (sizes 0 and 3-0) in the rat and rab¬

bit at 10 days. Asterisk denotes > .05.

7

i

Izi

IE

D Rat

K//d Rabbit

10 Days

LiChromic ChromicCatgut Catgut A

Size 0

PGA Chromic ChromicCatgut Catgut A

Size 3-0

PGA

diameter of catgut.Determination of knot security

level (Fig 3) showed that Dacron, silk,

cotton, and PGAsutures retained ex¬

cellent knot security with three

squared throws on the knot followingimplantation. Plain and chromic cat¬

gut, on the other hand, showed a

marked loss of knot security even at

three days.Experiment 2.—This experiment was

performed to extend the period of ob¬

servation for the absorbable suture

materials, to compare different sizes

of these materials, to compare chro¬

mic catgut from the two major manu¬

facturers, and to extend observations

into a different species. The results of

loop breaking strength determina¬tions in the rat up to 30 days are

shownin

Fig4. The PGA sutures re¬

tain their strength superiority over

comparable sizes of chromic catgutduring the first 20 days. Polyglycolicacid sutures appear to have a pre¬dictable rate of strength loss re¬

gardless of size with an end point ofabout 25 days. Chromic catgut loses

strength more rapidly in the early pe¬riod with the end point decreasingwith smaller sizes of the suture. No

significant difference was noted be¬tween the two sources of chromic cat¬

gut used in this study.

No significant differences were

noted in the loss of strength of chro¬mic catgut in the rat as compared to

the rabbit at ten

days.A slight, but

significant, increased strength losswas noted for PGA in the rabbit as

compared to the rat both in the 0 and

3-0 sizes (Fig 5).

Comment

The purpose of a surgical suture is

to maintain approximation of tissues

until the healing process has pro¬

gressed to the point where artificial

support is no longer necessary for the

wound to resist normal stresses. Be¬

yond this point, sutures serve no use-




ful purpose and may, in fact, be thesource of irritation or serve as a nidusfor persistent infection. Thus, theideal suture should persist and main¬tain tensile strength until the tissuehas healed sufficiently, and then dis¬appear. Unfortunately, requirementsof various tissues differ widely from a

few days (muscle, skin, and subcuta¬neous tissue), weeks to months (fasciaand tendon), to indefinite periods(vascular prostheses). No one suturewill therefore be able to meet the cri¬teria for an "all-purpose" materialeven from this narrow point of view.

Until recently, catgut—both plainand chromicized—has been the onlytruly absorbable material available.It is made from collagen obtainedfrom the intestinal submucosa of

sheepand cattle.

Although portionsof the suture, particularly the knots,may become encapsulated and persistfor long periods of time, the suture is

generally broken down by proteolytictissue enzymes and absorbed. This

process may take six months or

longer. Loss of strength occurs muchmore rapidly, however. The process of

catgut absorption is accompanied byan inflammatory reaction that hasbeen noted by many investigators.2 ' '

Another truly absorbable suture is

now available. This material, poly¬glycolic acid, is a synthetic polyesterpolymerized from glycolic (hydroxy-acetic) acid and is available as a

braided suture. This material is ab¬sorbed by simple hydrolysis in 60 to90 days without the inflammatory re¬

action evoked by catgut.A distinction must be made be¬

tween the rate of absorption of a su¬

ture material and the rate of tensile

strength loss of that material. The

terms are not

interchangeable.Al¬

though the rate of absorption is ofsome importance with regard to late

suture complications such as sinus

tracts and granulomas, the rate oftensile strength loss is of much

greater importance to the surgeonconsidering the primary function of a

suture-maintaining tissue approxi¬mation during healing.

Of the materials evaluated in this

study, only the synthetic polyestermaterial Dacron retained tensile

strength even during the relativeshort span of the experiment. Bothsilk and cotton, generally referred to

as "nonabsorbable" materials loststrength during the initial two weeksof implantation (cotton, 40%; silk,30%). This has been reported by otherauthors.4 Over longer intervals, actual

fragmentation and loss of substancehave been observed histologically.Nylon has also been reported to lose

strength after prolonged implanta¬tion.4 In the clinical setting a disturb¬ing incidence of anastomotic false an¬

eurysms has been noted followinginsertion of vascular

prostheses usingsilk suture material. This has been at¬tributed to suture failure, and the re¬

sults presented above would supportthis concept. It is apparent, therefore,that only permanent materials, suchas Dacron, should be used for such ap¬

plications.Considerable controversy has

arisen regarding the rate of tensile

strength loss of absorbable sutures.This has become of particular interestsince the introduction of the syn¬

thetic PGA suture to challenge cat¬gut, the traditional absorbable mate¬rial. The rapid loss of strength of

plain catgut is well-known and con¬

firmed by the present study. Previ¬

ously published studies comparingchromic catgut and PGA sutures haveshown that PGA loses most of its

strength by 14 to 21 days. Resultswith chromic catgut have rangedfrom 14 days2·5 to two months.3 This

disparity was one of the factors that

promptedthe present

investigation.Our current study would indicate thatchromic catgut has about the same

period of strength retention as PGA.

Although the source of catgut used inthe previous studies was not speci¬fied, we were unable to detect anysignificant differences in material ob-

tained from the two major manufac¬

turers. Similarly, only minor differ¬ences were noted in the two different

experimental animals used in the

present study. It is therefore unlikelythat either the source of the materialor the experimental animal accountsfor the wide variation reported. The

difference in experimental method re¬

mains the most likely source of thedifference. It is interesting that thein vivo human studies of Haxton*1tend to support the lower range forchromic catgut (8 to 14 days).

Of perhaps more significance is themarked loss of knot security exhib¬ited by both plain and chromic catgutin the presence of body fluids. This

phenomenon was not evident in theother materials studied. Since all su¬

tures are tied in clinical

practice,maintenance of tissue approximationis dependent not only on suture

strength but on knot security as well.

This investigation was supported by VeteransAdministration Research Funds.

Nonproprietary andTrade Names of Drug

Polyglycolicacid—Dexon.

References

1. Herrmann JB: Tensile strength andknot security of surgical suture materials.Am Surg 37:209-217, 1971.

2. Herrmann JB, Kelly RJ, Higgins GA:Polyglycolic acid sutures: Laboratory andclinical evaluation of a new absorbable su-

ture material. Arch Surg 100:486-490,1970.3. Postlethwait RW: Polyglycolic acid

surgical suture. Arch Surg 101:489-494,1970.

4. Postlethwait RW, et al: Wound heal-

ing: II. An evaluation of surgical suturematerial. Surg Gynecol Obstet 108:555-566,1959.

5. Katz AR, Turner RJ: Evaluation oftensile and absorption properties of poly-glycolic acid sutures. Surg Gynecol Obstet131:701-716, 1970.

6. Haxton H: Surgical Techniques. Bris-tol, England, John Wright & Sons Ltd,1970, p 65.

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changes in tensile strength and knot security of surgical sutures in vivo

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