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    Schematic depiction of the 3 experimental groups in an ex vivo study conducted to assess the biomechanical strength and gapping characteristics of canine gastrocnemius tendons following experimental tenotomy and tenorrhaphy by means of a core locking-loop suture with the knot at 1 of 3 locations (exposed on the external surface of the tendon [external-exposed knot], buried just underneath the external surface of the tendon [external-buried knot], or buried internally between the apposed tendon ends [internal knot]). Tendons were randomly assigned to the 3 groups (12 tendons/group) and sharply transected 2 cm distal to the musculotendinous junction. All experimental repairs were performed with size-0 polypropylene suture on a swaged V-20, 26-mm, 1/2-circle tapered needle.

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Effect of knot location on the biomechanical strength and gapping characteristics of ex vivo canine gastrocnemius tenorrhaphy constructs

Jessica L. Corrie DVM1, Daniel J. Duffy BVM&S(Hons), MS1, Yi-Jen Chang BVetMed, MS1, and George E. Moore DVM, PhD1
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  • 1 From VCA Aurora Animal Hospital, Aurora, IL 60506 (Corrie); Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC 27607 (Duffy, Chang); and Veterinary Administration, College of Veterinary Medicine, Purdue University, West Lafayette, IN 47906 (Moore).

Abstract

OBJECTIVE

To evaluate the effect of knot location on the biomechanical strength and gapping characteristics of ex vivo canine gastrocnemius tenorrhaphy constructs.

SAMPLE

36 cadaveric gastrocnemius tendons from 18 adult dogs.

PROCEDURES

Tendons were randomly assigned to 3 groups (12 tendons/group) and sharply transected and repaired by means of a core locking-loop suture with the knot at 1 of 3 locations (exposed on the external surface of the tendon, buried just underneath the external surface of the tendon, or buried internally between the apposed tendon ends). All repairs were performed with size-0 polypropylene suture. All constructs underwent a single load-to-failure test. Yield, failure, and peak forces, mode of failure, and forces required for 1- and 3-mm gap formation were compared among the 3 knot-location groups.

RESULTS

Mean yield, failure, and peak forces and mean forces required for 1- and 3-mm gap formation did not differ significantly among the 3 groups. The mode of failure also did not differ significantly among the 3 groups, and the majority (33/36 [92%]) of constructs failed owing to the suture pulling through the tendinous substance.

CONCLUSIONS AND CLINICAL RELEVANCE

Final knot location did not significantly affect the biomechanical strength and gapping characteristics of canine gastrocnemius tenorrhaphy constructs. Therefore, all 3 evaluated knot locations may be acceptable for tendon repair in dogs. In vivo studies are necessary to further elucidate the effect of knot location in suture patterns commonly used for tenorrhaphy on tendinous healing and collagenous remodeling at the repair site.

Abstract

OBJECTIVE

To evaluate the effect of knot location on the biomechanical strength and gapping characteristics of ex vivo canine gastrocnemius tenorrhaphy constructs.

SAMPLE

36 cadaveric gastrocnemius tendons from 18 adult dogs.

PROCEDURES

Tendons were randomly assigned to 3 groups (12 tendons/group) and sharply transected and repaired by means of a core locking-loop suture with the knot at 1 of 3 locations (exposed on the external surface of the tendon, buried just underneath the external surface of the tendon, or buried internally between the apposed tendon ends). All repairs were performed with size-0 polypropylene suture. All constructs underwent a single load-to-failure test. Yield, failure, and peak forces, mode of failure, and forces required for 1- and 3-mm gap formation were compared among the 3 knot-location groups.

RESULTS

Mean yield, failure, and peak forces and mean forces required for 1- and 3-mm gap formation did not differ significantly among the 3 groups. The mode of failure also did not differ significantly among the 3 groups, and the majority (33/36 [92%]) of constructs failed owing to the suture pulling through the tendinous substance.

CONCLUSIONS AND CLINICAL RELEVANCE

Final knot location did not significantly affect the biomechanical strength and gapping characteristics of canine gastrocnemius tenorrhaphy constructs. Therefore, all 3 evaluated knot locations may be acceptable for tendon repair in dogs. In vivo studies are necessary to further elucidate the effect of knot location in suture patterns commonly used for tenorrhaphy on tendinous healing and collagenous remodeling at the repair site.

Introduction

Multiple strategies currently exist for sutured repair of tendon lacerations in dogs.14 Variations in suture pattern,1,5,6 suture material,79 and the use of epitendinous sutures to augment tendon repair methods3,7,10,11 have been investigated in dogs and shown to affect the biomechanical properties of the final repair. The strength and integrity of a tenorrhaphy construct is also affected by the method used for suture knotting and its resultant effect on knot security.12 Knot strength is dependent on the technique used for suture knotting by the surgeon, suture material and composition, and number of throws used to complete the knot.1317 A decrease in knot strength because of surgeon inexperience or poor execution can lead to suture failure as a result of suture breakage or loosening of the knot, with consequent unraveling and construct elongation.13 In fact, the knot has been identified as the weak point along the length of a suture line.18,19 Concerns associated with knot placement include the bulk and size of the knot, an increase in construct stiffness, knot-induced friction and its effect on glide function, and development of peritendinous adhesions caused by knot irritation.1922

The location (extratendinous vs intratendinous) of the knot in a tenorrhaphy may affect the extent of suture interaction with surrounding tissues and the amount of foreign material exposed on the tendon surface.2023 Minimizing the presence of suture material on the tendon surface is particularly important in human patients to decrease the occurrence of finger triggering. Discomfort in the palm during movement of the affected digits, caused by hypertrophy and inflammation of the tendon-sheath interface, which can limit the range of motion of the affected digits during active flexion. However, intratendinous knot placement takes longer to complete than extratendinous knot placement and decreases the load required for 2-mm gap formation and ultimate strength of the tenorrhaphy.20,24 Knowledge regarding the effect of final knot location on the biomechanical strength of tenorrhaphy constructs is important prior to in vivo assessment of knot placement on tendon healing and collagenous remodeling at the tenorrhaphy site.

Currently, there is a paucity of information regarding the effect of knot location (extratendinous vs intratendinous) on the biomechanical properties of tenorrhaphy constructs. The primary objective of the study reported here was to assess the effect of knot location (external-exposed, external-buried, or internal) on the biomechanical strength and gapping characteristics of ex vivo canine gastrocnemius tenorrhaphy constructs. The null hypothesis was that the biomechanical properties and gap formation characteristics would not differ among the 3 knot-location (construct) groups.

Materials and Methods

Samples

Live animals were not used in the study; therefore, it was deemed exempt from review by the North Carolina State University Institutional Animal Care and Use Committee. Results of a sample size calculation that involved the use of findings from a pilot project conducted in preparation for the study indicated that 12 tendons/construct group would be sufficient to detect a mean ± SD load difference of 22 ± 5 N between groups with 95% confidence (ie, α = 0.05) and 80% power.

Thirty-six hind limbs were sequentially harvested from 18 musculoskeletally normal adult (> 1-year-old) mixed-breed canine cadavers that weighed between 25 and 30 kg. All dogs were euthanized at a local animal shelter by IV infusion of sodium pentobarbital (Euthasol) for reasons unrelated to the study. Dogs with a history of orthopedic disease or evidence of musculoskeletal abnormalities on the basis of results of a focused orthopedic examination performed by 1 investigator (DJD) were excluded from the study.

Cadavers were stored at room temperature (21 °C) after euthanasia. Within 4 hours after death, each hind limb was removed from the cadaver at the level of the distal metaphysis. All soft tissues were removed from each hind limb except the musculotendinous origin of the gastrocnemius muscle at the supracondylar eminence on the caudodistal aspect of the femur and the musculotendinous contributions of the paired gastrocnemius muscles extending to their insertion on the proximocentral aspect of the tuber calcanei. Specimens were dissected as described.25 Following dissection, each specimen was individually wrapped in gauze laparotomy sponges that were soaked with saline (0.9% NaCl) solution, placed in an impervious plastic bag (Ziploc 1-gallon bags; SC Johnson & Son Inc) with the contralateral limb from the same cadaver, and stored at –20°C until experimental testing. Prior to experimental manipulation, specimens were allowed to thaw for 10 hours at room temperature by use of a validated technique.26

Experimental procedures

A random number generator (Research Randomizer version 4.0; Urbaniak GC, Plous S) was used to randomly allocate each tendon specimen to 1 of 3 experimental groups (12 tendons/group), with care taken to ensure that the tendon specimens obtained from the same cadaver were not assigned to the same group. A No. 10 scalpel blade was used to manually transect each specimen in the transverse plane 2 cm distal to the musculotendinous junction. Each specimen was transected on a hard surface to ensure that the tenotomy was perpendicular to the length of the tendon and consistent among the specimens. Following the tenotomy, the cut surface of the distal tendon segment was held adjacent to a calibrated ruler (Sterile Surgical Ruler; Medline) that was positioned parallel to and 5 cm from the transected tendon edge and photographed (iPhone SE Camera; Apple Inc). One investigator (Y-JC) measured the cross-sectional area of each tendon segment by use of an imaging software program (ImageJ version 1.5; NIH).

All experimental repairs were performed with size-0 polypropylene suture (Surgipro; Covidien Ltd) on a swaged V-20, 26-mm, 1/2-circle tapered needle. The size of the suture material used was selected on the basis of the results of another study,27 which indicated that core suture size is positively associated with the biomechanical strength of a tenorrhaphy construct. One board-certified veterinary surgeon (DJD) experienced in tendon repair performed all tenorrhaphy constructs. The specimens were sutured under surgical lighting with the aid of optical magnification (4.5X Surgical loupes; Surgitel). Subjectively equal tension was applied to each suture strand during pattern completion to ensure removal of all slack from the construct prior to the knot being tied. The surgeon also took care to ensure that the transected ends of the tendon were in apposition with no tissue bunching at the repair site.

All tenorrhaphies were performed by the placement of a core locking-loop suture through the center of the tendinous substance as described.28 For the external-exposed and external-buried knot groups, the first suture bite was taken 15 mm from the transected end of the proximal tendon segment and passed transversely from the dorsomedial to dorsolateral surface of the segment. Then a suture bite was taken through center of the tendinous substance, beginning 12 mm from the transected end of the proximal tendon segment, passing longitudinally into the center of the tendinous substance of the distal tendon segment, and exiting on the dorsolateral surface of the distal tendon segment 15 mm from the transected end. The third and fourth suture bites were the same as the first and second bites except that the longitudinal suture passed from the distal to the proximal tendon segments (Figure 1). For the internal knot group, the first bite was initiated at the transected edge of the proximal tendon segment, then passed longitudinally through the center of the tendinous substance to exit on the dorsomedial surface of the proximal tendon segment 15 mm from the transected end. The second and third bites were the same as the first and second bites described for the external-exposed and external-buried knot constructs. The fourth bite was initiated 12 mm from the transected end of the distal tendon segment and passed longitudinally through the center of the tendinous substance to exit at the transected edge of the distal tendon segment.

Figure 1
Figure 1

Schematic depiction of the 3 experimental groups in an ex vivo study conducted to assess the biomechanical strength and gapping characteristics of canine gastrocnemius tendons following experimental tenotomy and tenorrhaphy by means of a core locking-loop suture with the knot at 1 of 3 locations (exposed on the external surface of the tendon [external-exposed knot], buried just underneath the external surface of the tendon [external-buried knot], or buried internally between the apposed tendon ends [internal knot]). Tendons were randomly assigned to the 3 groups (12 tendons/group) and sharply transected 2 cm distal to the musculotendinous junction. All experimental repairs were performed with size-0 polypropylene suture on a swaged V-20, 26-mm, 1/2-circle tapered needle.

Citation: American Journal of Veterinary Research 82, 12; 10.2460/ajvr.21.03.0038

The location of suture pattern initiation governed the final location of the knot. For all constructs, the suture pattern was ended with a square knot followed by 3 additional suture throws, and the suture was cut 3 mm from the completed knot. For each construct in the external-exposed knot group, the knot was tied on the dorsomedial aspect of the tendon surface (Figure 1). For each construct in the external-buried knot group, immediately prior to pattern completion, a No. 15 scalpel blade was used to make a 6-mm-long, 2-mm-deep incision in the dorsomedial aspect of the tendon substance. Prior to tying of the knot, the suture was embedded within the incision in the tendon substance so that the knot would not be visible on the tendon surface after completion. For each construct in the internal knot group, the knot was located within the repair site and was not visible upon completion. All tendon specimens were kept moist with saline solution, which was applied by use of a spray bottle as necessary to prevent tissue desiccation throughout construct creation and biomechanical testing.

Biomechanical testing

All biomechanical tests were performed at room temperature by use of a materials tensile testing machine (Model 5967 Universal Testing System; Instron Inc). A 4.5-mm-diameter tunnel was drilled across the femoral condyle so that the proximal aspect of the construct could be securely mounted to the testing machine with a 4-mm-diameter bolt attached to a 3-D–printed test jig. A bone-fixation clamp was used to firmly secure the foot to the testing machine in a manner that positioned the calcaneus at 90° relative to the base of the construct. Once a construct was securely mounted in the testing machine, the force was zeroed, and the construct was preloaded to 2 N to remove any slack from the construct and ensure a consistent resting length among constructs. The testing system was recalibrated to zero before data collection to ensure testing began under consistent conditions for all constructs. Each construct specimen was distracted until failure by the application of tension at a constant rate of 20 mm/min. Data were collected at a frequency of 100 Hz by use of biomechanical software (Bluehill 3; Instron Inc). A digital camera (Lumix DMC-FZ200; Panasonic Corp) was positioned 20 cm from and level with the tenorrhaphy site to record each test so gap formation could be monitored and measured. A metric ruler was positioned axially and immediately adjacent to the mounted construct within the camera’s field of view. An automated triggering system was used to synchronize the digital camera with the load data.

Following test completion, computer software (Matlab R2018b; Mathworks) was used to generate load displacement curves from which biomechanical parameters of interest were evaluated. Yield force was defined as the first deviation from linearity in the initial flat portion of the load displacement curve. Failure force was defined as the point at which the suture failed or pulled through the tendinous tissue, and peak force was defined as the greatest load recorded for a construct. For each specimen, the mode of failure (suture failure or pull through the tendinous tissue) was recorded on the basis of visual assessment at the time of testing and review of the video recording by 1 investigator (Y-JC).

Gaps were defined as the smallest distance measured at the center of the tendon repair site. Gaps were measured by use of a digital caliper that was calibrated to a ruler of known size within an imaging software program (ImageJ version 1.5; NIH). The video recording of each test was evaluated frame by frame to identify the precise time at which 1- and 3-mm gaps became visible at the repair site. This information was then cross-referenced with the load data to determine the force (load) required to create 1- and 3-mm gaps in each specimen.

Statistical analysis

Outcomes of interest included the forces at yield, failure, and peak; forces required to create 1- and 3-mm gaps; and mode of failure. The respective data distributions of continuous variables were assessed for normality by the Shapiro-Wilk test. All continuous variables were normally distributed and were summarized as the mean ± SD. Mixed linear models were used to compare means among the 3 construct groups. Each model included a mixed effect to account for the fact that both hind limbs from each cadaver were assessed. The Bonferroni adjustment was used for post hoc pairwise comparisons when necessary. The Pearson χ2 test of association was used to compare the mode of failure among the 3 construct groups. All analyses were performed with commercial statistical software (SAS version 9.4; SAS Institute Inc), and values of P < 0.05 were considered significant.

Results

Experimental constructs

After transection and repair of the gastrocnemius tendon, all specimens underwent biomechanical testing without any observed procedural errors. Therefore, all 36 tendon specimens were included in the analysis. The mean cross-sectional area for all 36 tendon specimens was 0.10 ± 0.01 cm2 and did not differ significantly between left and right hind limbs (P = 0.841) or among construct groups (P = 0.136).

Biomechanical testing

The mean force at yield (P = 0.481), failure (P = 0.406), and peak (P = 0.315) did not differ significantly among the 3 construct groups (Table 1). All 12 specimens within each construct group developed 1- and 3-mm gaps prior to failure. The mean force required to create 1-mm (P = 0.391) and 3-mm (P = 0.518) gaps did not differ significantly among the 3 construct groups.

Table 1

Mean ± SD forces at yield, failure, and peak and forces required for 1- and 3-mm gap formation for 36 canine gastrocnemius tendons that underwent ex vivo tenotomy and tenorrhaphy by means of a core locking-loop suture with the knot at 1 of 3 locations (exposed on the external surface of the tendon [external-exposed knot; n = 12], buried just underneath the external surface of the tendon [external-buried knot; 12], or buried internally between the apposed tendon ends [internal knot; 12]).

Construct groupYield force (N)Failure force (N)Peak force (N)Force required for 1-mm gap formation (N)Force required for 3-mm gap formation (N)
External-exposed knot26.52 ± 7.3235.92 ± 8.7236.86 ± 8.5216.10 ± 6.0622.49 ± 5.63
External-buried knot25.07 ± 10.4138.17 ± 7.8539.31 ± 7.26514.19 ± 5.0320.16 ± 6.08
Internal knot25.67 ± 7.6432.92 ± 5.0733.65 ± 3.74711.27 ± 5.1618.53 ± 5.41

Yield force was defined as the first deviation from linearity in the initial flat portion of the load displacement curve. Failure force was defined as the point at which the suture failed or pulled through the tendinous tissue. Peak force was defined as the greatest load recorded for each construct. The mean value did not differ significantly (P < 0.05) among the 3 construct groups for any of the forces listed.

The majority (33/36 [92%]) of experimental specimens failed because the suture pulled through the tendinous substance. The remaining 3 (8%) specimens failed as a result of suture breakage. The mode of failure did not differ significantly (P = 0.758) among the 3 construct groups.

Discussion

On the basis of the results of the present ex vivo study, we accepted (failed to reject) the null hypothesis that the knot location (exposed on the external surface of the tendon [external-exposed knot], buried just underneath the external surface of the tendon [external-buried knot], or buried internally between the apposed tendon ends [internal knot]) of a locking-loop suture used for tenorrhaphy would not significantly affect the biomechanical strength and gapping characteristics of repaired canine gastrocnemius tendons. This information can be used by veterinary surgeons to help guide informed suture use and decrease the detrimental effects that may be caused by the placement of excessive suture bulk external to the repair.

At the present time, there is controversy regarding the location of suture knot placement and its effect on the strength of a primary tenorrhaphy. The veterinary literature contains conflicting results regarding the effect of final knot location on the tensile strength of repaired tendons.20,24,29,30 Results of the present study agreed with the results of an ex vivo study by Chang et al24 involving porcine flexor digitorum profundus tendons. In that study,24 tendons were repaired with 4-0 monofilament nylon suture in a Lim-Tsai pattern with the knots placed externally (extratendinous) or internally (intratendinous) at the tenorrhaphy site. However, the results of the present study contrasted with those of Aoki et al29 and Komatsu et al.20 In the Aoki et al29 study, canine flexor digitorum profundus tendons underwent ex vivo tenotomy and repair by use of a Savage-type technique in which 2, 4, or 6 strands of 4-0 monofilament nylon suture crossed the repair site, and the knots were placed in an extratendinous or intratendinous location. The results of that study29 suggested that the tensile strength of the repair was greater when the knots were placed in an extratendinous versus intratendinous location, regardless of the number of suture strands crossing the repair site, and when a single knotting rather than a double knotting technique was used. In the Komatsu et al20 study, bovine gastrocnemius tendons underwent ex vivo tenotomy and repair by use of 2-0 polyester or polyester-coated braided polyethylene suture placed by means of a side locking-loop technique with the knot positioned externally on the locking loop, internally between the tendon ends, or externally but buried between locking loops in a slit made into the tendon with a scalpel. Results of that study20 suggested that positioning the knot externally but burying it in a superficial slit made between the locking loops provided the greatest tensile strength and led to the development of the fewest gaps at the repair site.

The strength of knotted suture material may change over time in vivo. In an in vivo study,30 flexor tendons of dogs were repaired with 5-0 braided polyester suture by use of a 4-strand modified Savage technique with the knots located internally between the tendon ends or externally at the repair site followed by the implementation of a passive range-of-motion physical therapy protocol and mechanical testing performed at 1, 3, and 6 weeks after surgery. Results indicated that the tensile strength was greater for repairs with externally placed knots, compared with repairs with internally placed knots initially, but did not differ between the 2 knot locations at 6 weeks after surgery.30 The difference in tensile strength on the basis of knot location initially after surgery was attributed to the adverse effects of internally placed knots on collagen production and remodeling at the repair site.30 In the present ex vivo study, the tenorrhaphies were performed with size-0 polypropylene suture placed in a core locking-loop pattern, and the mean tensile strength did not differ significantly among constructs with an external-exposed knot, external-buried knot, or internal knot. The disparities in results between the present ex vivo study and the aforementioned in vivo study30 are likely the result of differences in the tendon type and suture type, size, and pattern used for the tenorrhaphy. Polypropylene suture was purposefully chosen for use in the present study because it is frequently used for tendon repair in clinical patients and to facilitate comparison of the results with findings of other similar studies without worrying about suture composition adding to interstudy variability. Additional in vivo investigation is warranted to further elucidate the effect of knot location on tendon repair strength and healing in clinical patients and to evaluate the role of postoperative physical rehabilitation on tendon healing.

We investigated the force required to create 1- and 3-mm gaps at the repair site and the forces at yield, failure, and peak for the tenorrhaphy constructs of the present study. The mean forces required to create 1- and 3-mm gaps did not differ significantly among the 3 construct groups. That finding differs from the results of the previously described Chang et al24 study. In that study,24 the force required to create 2-mm gaps in constructs with externally placed knots was significantly greater than that for constructs with internally placed constructs. The apparent discrepancy in results between the present study and the Chang et al24 study was likely attributable to differences in species, tendon type and size, and the suture type and pattern used. Additional comparative studies are needed to further investigate the relationship between knot location and subsequent gap formation.

The majority (33/36 [92%]) of tenorrhaphy constructs in the present study failed owing to the suture pulling through the tendinous substance, and only 3 (8%) constructs failed because the suture broke. The mode of failure did not differ significantly among the 3 construct groups of the present study. In the Chang et al24 study, all experimental constructs failed as a result of suture breakage. The discrepancy between the 2 studies was most likely caused by differences in the suture type and size used in the constructs (size-0 polypropylene in the present study vs 4-0 monofilament nylon in the Chang et al24 study). The mode-of-failure results for the present study were similar to those of other ex vivo studies11,25,27 in which suture of similar size (size-0 or 2-0) was used for the tenorrhaphy constructs. As the size of the suture used for tendon repair increases, there comes a point when the tensile strength of the suture exceeds that of the suture-tissue interface resulting in the suture pulling through the tendinous substance.

The present study had important limitations, including its ex vivo design, lack of tendon and suture preconditioning, and lack of objective tension evaluation during testing, which could have led to small inconsistencies between specimens. In vivo studies are necessary to assess the effects of infection, tissue reactivity, inflammation, fibrosis, and location of knot placement on tendon blood supply. Additionally, we did not investigate glide function as it relates to tenorrhaphy suture knotting. Finally, the tendon specimens evaluated in this study were obtained from a homogenous population of dogs. Results might vary for specimens obtained from dogs of various breeds, tendons other than gastrocnemius tendons, and tenorrhaphy constructs in which a different suture material or suture size was used. Suture size and knot size could affect in vivo tendon architecture, blood supply, and subsequent progression of normal healing, particularly for tenorrhaphy techniques with buried or internal knot locations.

Results of the present ex vivo study indicated that the biomechanical strength and gapping characteristics of canine gastrocnemius tendon specimens that underwent experimental tenotomy and repair by use of a core locking-loop method did not differ on the basis of final knot location. Therefore, knots exposed on the external tendon surface, buried just underneath the external tendon surface, or buried internally between the 2 apposed tendon ends may all be acceptable for tendon repair in dogs. In vivo studies are necessary to determine the relevance of these findings and the effect of final knot location on tendinous healing and collagenous remodeling at the repair site.

Acknowledgments

This work was performed at the North Carolina State University College of Veterinary Medicine.

No third-party funding or support was received in connection with this study or the writing or publication of the manuscript. The authors declare that there were no conflicts of interest.

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Contributor Notes

Address correspondence to Dr. Duffy (djduffy@ncsu.edu).