Loop modification of the traditional three-loop pulley pattern improves the biomechanical properties and resistance to 3-mm gap formation in a canine common calcanean teno-osseous avulsion model

Daniel J. Duffy Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC

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 BVM&S, MS, MRCVS, DACVS-SA, DECVS
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Weston L. Beamon Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC

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Yi-Jen Chang Department of Clinical Sciences, College of Veterinary Medicine, North Carolina State University, Raleigh, NC

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George E. Moore Veterinary Administration, College of Veterinary Medicine, Purdue University, West Lafayette, IN

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 DVM, PhD

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Abstract

OBJECTIVE

To compare the biomechanical properties and gapping characteristics following loop modification of a 3-loop-pulley (3LP) pattern in an ex vivo canine common calcaneal tendon (CCT) avulsion repair model.

SAMPLE

56 skeletally mature hindlimbs from 28 canine cadavers.

PROCEDURES

The CCTs were randomized to 1 of 4 experimental groups (n = 14/group) then sharply transected at the teno-osseous junction. Groups consisted of a 3LP, 4-loop-pulley (4LP), 5-loop-pulley (5LP), or 6-loop-pulley (6LP) pattern with loops placed 60° apart using size-0 polypropylene. Yield, peak, and failure loads, construct stiffness, loads to produce a 3-mm teno-osseous gap, and failure mode were evaluated and compared between groups.

RESULTS

Yield (P = 0.001), peak (P < 0.001), and failure loads (P < 0.001), construct stiffness (P < 0.001), and loads to 3-mm gap formation (P = 0.005) were all significantly greater for 6LP compared to all other groups. Mode of failure did not differ among groups (P = 0.733) with 75% (42/56) of repairs failing by mechanism of core sutures pulling through the tendinous tissue. Pattern modification by increasing the number of loops increased the repair site strength by 1.4, 1.6, and 1.8 times for 4LP, 5LP, and 6LP compared to 3LP, respectively.

CLINICAL RELEVANCE

Increasing the number of suture loops compared to a traditional 3LP repair is a relatively simple technique modification that significantly increases teno-osseous repair site strength and loads required to cause 3-mm gap formation. The results of this study justify further focused investigation of increasing the number of suture loops in vivo for teno-osseous CCT repair in dogs.

Abstract

OBJECTIVE

To compare the biomechanical properties and gapping characteristics following loop modification of a 3-loop-pulley (3LP) pattern in an ex vivo canine common calcaneal tendon (CCT) avulsion repair model.

SAMPLE

56 skeletally mature hindlimbs from 28 canine cadavers.

PROCEDURES

The CCTs were randomized to 1 of 4 experimental groups (n = 14/group) then sharply transected at the teno-osseous junction. Groups consisted of a 3LP, 4-loop-pulley (4LP), 5-loop-pulley (5LP), or 6-loop-pulley (6LP) pattern with loops placed 60° apart using size-0 polypropylene. Yield, peak, and failure loads, construct stiffness, loads to produce a 3-mm teno-osseous gap, and failure mode were evaluated and compared between groups.

RESULTS

Yield (P = 0.001), peak (P < 0.001), and failure loads (P < 0.001), construct stiffness (P < 0.001), and loads to 3-mm gap formation (P = 0.005) were all significantly greater for 6LP compared to all other groups. Mode of failure did not differ among groups (P = 0.733) with 75% (42/56) of repairs failing by mechanism of core sutures pulling through the tendinous tissue. Pattern modification by increasing the number of loops increased the repair site strength by 1.4, 1.6, and 1.8 times for 4LP, 5LP, and 6LP compared to 3LP, respectively.

CLINICAL RELEVANCE

Increasing the number of suture loops compared to a traditional 3LP repair is a relatively simple technique modification that significantly increases teno-osseous repair site strength and loads required to cause 3-mm gap formation. The results of this study justify further focused investigation of increasing the number of suture loops in vivo for teno-osseous CCT repair in dogs.

The common calcanean tendon (CCT) in dogs consists of the musculotendinous contributions from the paired gastrocnemius, biceps femoris, gracilis, semitendinosus, and superficial digital flexor muscles. Paired gastrocnemius and accessory tendons insert on the proximocental or proximomedial aspect of the calcaneus, respectively. In a retrospective study of 45 dogs with CCT mechanism disruption, Corr et al1 determined that 60% (27/45) of injuries occurred at the enthesis onto the tuber calcanei. In 70% (31/45) of dogs, multiple tendons were involved compared to only 20% (9/45) where only the gastrocnemius tendon alone was affected.1 Tendon rupture occurring at the enthesis occurs with variable degrees of avulsion of components of the CCT and can vary regarding patient signalment, chronicity, and level of degenerative pathology.1 Disease affecting the CCT can lead to severe musculotendinous dysfunction and resultant gait impairment dependent on the tendons affected.1,2 Surgical repair involving apposition of avulsed tendon ends to their boney insertion on the calcaneus provides restoration of the tendon working length and limb function, while conservative management typically results in improper healing, scar tissue formation, and predisposition to reinjury or repair failure.3,4 Repair of gastrocnemius tendon (GT) and accessory tendon avulsion from the tuber calcanei primarily involves the use of suture placed through bone tunnels to anchor the CCT to the calcaneus. This is typically followed by rigid postoperative immobilization to reduce strain placed upon the repair during loading during the gait cycle.5,6 Long-term prognosis is reported to be good; however, in a study of working dogs diagnosed with complete or partial CCT tears, only 70% (7/10) returned to full work following repair using a Locking Loop (LL) pattern and talocrural immobilization.7 Tarsal immobilization techniques are typically instituted for 5 to 9 weeks to allow sufficient time for collagenous remodeling and ingrowth, repair, and reorganization while resisting loads applied through the CCT.8,9 It should be noted, however, that isometric muscle contraction contributes to persistent force placed on the CCT at the teno-osseous junction thus necessitating a strong repair of the tendon to bone.10,11 Tendinous healing only leads to 80% of original tendon strength, further emphasizing the requirement for a strong repair.12

The 3-loop pulley (3LP) is considered by many small animal surgeons to be the suture pattern of choice for CCT repair. Over time refinements and modifications in suture repair techniques have led to adjunctive adaptations such as the use of polypropylene mesh, suture anchors, and epitendinous sutures to increase repair site strength and decrease the occurrence of gap formation between tendon ends.6,13,14 The 3LP is superior compared to LL repairs in canine biomechanical studies resulting in increased tensile strength and resistance to gap formation.9,13,15 In one study,15 loads required to produce 2-mm gap formation were greater for 3LP compared to LL patterns. This aforementioned study15 concluded that 3LP patterns provided greater repair site protection and support, less tendinous distortion and less collagen matrix constriction when loading of the tendon substance was performed. When sequential loading and tension is applied to musculotendinous unit repaired with a 3LP pattern, suture loops glide through tendinous tissues in a pulley-like fashion so that equal load is distributed by each of the 3 loops.15 Tension redistribution occurs more easily if each loop is sequentially tightened during pattern completion, and the ends of the tendon are approximated prior to the suture knotting. Modification of the 3LP by increasing the number of suture loops anchored distally through a bone tunnel drilled in the calcaneus may lead to improvements in the biomechanical properties of the repair and resistance of the teno-osseous junction to gap formation. Clinically, this may translate to improved collagenous ingrowth and teno-osseous integration, with the goal of earlier return of repair site strength. Currently, there is a paucity of information in the veterinary literature regarding the effect of suture loops and 3LP pattern modification for CCT repair in dogs. Information gained from such studies may lead to changes in repair techniques and pattern refinement with the goal of improving surgical outcomes. The objective of this study was to compare the biomechanical properties and gapping characteristics following loop modification of a 3-loop-pulley (3LP) pattern in a canine CCT avulsion repair model. Our hypothesis was that increasing the number of suture loops would increase the biomechanical properties and loads to gap formation at the enthesis of experimental CCT repairs.

Materials and Methods

Cadaver collection

Twenty-eight skeletally mature canine cadavers > 1 year of age were obtained from multiple local animal shelters after consented donation from January 2020 to March 2021. Dogs weighed 25 to 32 kg, were medium to large breed, and were obtained within 2 hours of euthanasia (pentobarbital sodium solution, 1 mL/5 kg body weight, IV) for reasons unrelated to this study. Cadaver sex was not recorded. Dogs were determined to be grossly normal at the time of specimen collection with no evidence of concurrent orthopedic disease affecting the hindlimbs based on a focused orthopedic examination performed by one of the study authors (D.J.D). This study was deemed exempt from requiring an institutional animal care and use committee protocol by Department of Clinical Sciences, North Carolina State University, due to specimen acquisition following euthanasia and secondary use of cadaveric tissues.

Tendon preparation

Paired hindlimbs were serially dissected to individual components of the CCT. The paired GT and accessory tendon were isolated using a combination of sharp and blunt dissection. The origin of the GT and accessory tendon onto the dorsocentral and proximomedial aspect of the calcaneus, respectively, were carefully preserved. The superficial digital flexor tendon and surrounding retinaculum surrounding the calcaneus were removed to the periosteum using a No. 10 scalpel blade. Distally, the pes was preserved following sharp talocrural joint disarticulation. Proximally, the superficial digital flexor tendon was transected at its myotendinous junction. A femoral osteotomy was performed immediately proximal to the femoral trochlear groove using a saw. Paired hindlimbs were then wrapped in saline soaked gauze, placed in resealable freezer bags, and stored at −20 °C. Limbs were allowed to thaw at room temperature (21 °C) for 12 hours prior to testing.16

Surgical tendon repair

Thawed hindlimbs were randomly assigned to 1 of 4 equally sized groups using a random number generator with 14 hindlimbs/group. Limbs originating from the same cadaver were controlled from being placed into the same group. Constructs were serially repaired in a randomized manner using repair assignment data sheets generated using computed randomizer software. A single investigator (W.L.B) performed all surgical repairs under the close observation and instruction of a board-certified surgeon (D.J.D). At the level of the enthesis of the GT and accessory tendon, a No. 10 scalpel blade was used to perform a complete transverse tenotomy immediately proximal to the calcaneal tuberosity. Following tendinous transection, the pes was held in a mechanical vice and an oscillating bone saw (DePuy Synthes; Johnson & Johnson) was used to create an osteotomy performed from a point at the caudodistal calcanean sulcus to a point 0.5 cm craniodistal to the calcanean tuberosity (Figure 1). The osteotomy was performed in a mediolateral plane as would be performed during clinical cases to remove peritendinous osteophyte or enthesiophyte formation and generate a flat healthy bone bed for tendinous reattachment. A single transverse bone tunnel was then drilled using a 2.0 drill bit, 10 mm distal to the center of the calcaneal osteotomy and equidistant from the cranial and caudal calcaneal bone margins, respectively.17 A metric surgical ruler was used to ensure consistency for points of drill bit entry and exit among specimens. The pes was then removed from the vice and tendons repaired ensuring anatomic alignment of the CCT to the calcaneus. Tendinous reattachment was performed in all groups using US Pharmacopeia size-0 polypropylene suture. Each suture strand was removed from the sterile packaging, and the swaged-on needle was removed by cutting the suture at an angle of approximately 45°. A 20-gauge hypodermic needle was used to facilitate suture passage through the transverse bone tunnel and CCT in all groups. The 3LP group was performed as previously described9,15 and consisted of 3 suture loops passing through the body of both the GT and accessory tendon. Distally the suture was passed through the transverse bone tunnel in a latero-medial direction, through components of the CCT, and then back through the bone tunnel with bites through the tendon taken at 5, 10, and 15 mm, respectively, from the cut tendon end. The 4-loop-pulley (4LP) group consisted of 4 suture loops with the fourth loop placed 5 mm proximal to the third loop. The 5-loop-pulley (5LP) group consisted of 5 suture loops with the fifth loop placed 5 mm proximal to the fourth suture loop. Lastly, the 6-loop-pulley (6LP) group consisted of 6 suture loops with the sixth loop placed 5 mm proximal to the fifth respective loop. Suture orientation through respective bone tunnels remained consistent during pattern completion regardless of group assignment. A modified suture pattern was used for all repairs with loops placed 60° apart from the point of initial tendon penetrance with sequential loops placed at a measured distance of 5 mm between each loop. It should be noted that due to the inherent nature of suture passes in the 4LP, 5LP, and 6LP groups, suture loops were orientated in the same plane. In all groups, sutures were subjectively tensioned to remove any slack from the repair while closely opposing the distal cut surface of the CCT to the calcaneal osteotomy, while ensuring that bunching of the tendon ends did not occur. A square knot followed by 3 additional throws was used and suture cut 3-mm from the knot.

Figure 1
Figure 1

Diagrammatic illustration of 4-loop-pulley (4LP), 5-loop-pulley (5LP), and 6-loop-pulley (6LP) modification of a traditional 3-loop pulley (3LP) pattern in an ex vivo canine common calcanean teno-osseous avulsion repair model evaluated with the use of 56 cadaveric hindlimbs of 28 skeletally mature dogs > 1 year of age euthanized for unrelated reasons. A—The insertions of the paired gastrocnemius (GT) and accessory tendon (AT) onto the calcaneal (C) tuberosity. The red line represents a simulated clinical osteotomy. B, C, D, and E—Illustrations of 3LP, 4LP, 5LP, and 6LP, respectively. The scale immediately adjacent to the tendon reflects 0.5-cm increments, correlating with suture passage. Blue lines represent suture and orange numbers represent each suture bite of the 3LP while purple numbers represent additional loops within each pattern modification. The green line labeled “s” is the initiation of suture pattern placement. A transected illustration of the repair site is shown to the right of each pattern. Light blue and purple dotted arrows represent the direction of suture passage, and orange numbers indicate the order of loop placement at the initial area of suture passage. AT= accessory tendon. C = Calcaneus. Cd = Caudal. Cr = Cranial. GT = Gastrocnemius tendon. L = Lateral. M = Medial. T = talus. Tc = talocrural joint.

Citation: American Journal of Veterinary Research 83, 8; 10.2460/ajvr.21.09.0139

Biomechanical testing

Repaired constructs were serially loaded into a materials testing machine (Instron, Norwood, MA). A 4.5-mm bone tunnel was drilled transversely in a mediolateral plane across the femoral condyle to facilitate proximal attachment of the test specimen to the custom 3-D printed jig mounted on the crosshead of the machine. Distally the pes was secured using a modified bone clamp (SKU-1652-1; Sawbones). The calcaneal osteotomy was then positioned to orient the cut surface of the calcaneus perpendicular to the base of the testing machine at the start of each test so the direction of applied load was axially aligned with the teno-osseous construct. Surgical repairs were preloaded to 2 N to achieve a consistent resting length and remove any slack. Repaired constructs were subsequently distracted until failure at a rate of 20 mm/minute with data collected at a frequency of 100 Hz. Assessed biomechanical parameters included assessment of yield, peak, and failure loads (N), displacement (mm), and construct stiffness (N/mm). A software program (Matlab R2018b, Mathworks) was used to assess outcome measures performed by a single investigator (W.L.B). Yield load was defined as the first identifiable peak followed by a small observable decrease in the load-displacement (LD) curve. Peak load was defined as the greatest load experienced during each test. Failure load was defined as the load applied prior to a sudden acute decrease (> 50%) in the LD curve or the point of construct failure. Lastly, stiffness was defined as the extent to which repaired constructs resisted deformation when load was applied, calculated at 50% to 80% of yield load over the elastic region of the LD curve. Mode of failure was recorded during specimen testing and confirmed following review of the video recordings by a single investigator (W.L.B). A high-definition digital camera recorded each test at 50 frames/second and was synchronized following the defined preload being reached using an automated triggering system. Gelberman et al17 showed that gap formation of > 3 mm resulted in reduction in normal return of strength and stiffness over time. Because of these prior findings we elected to examine 3-mm gap formation as an assessed outcome measure. Gap formation was assessed using the minimum distance (millimeters) at the center if the teno-osseous CCT junction to assess for development of a 3-mm gap. A digital caliper was calibrated against the ruler of known length within the video (ImageJ; National Institutes of Health, Bethesda, MD). Exact time points at which 3-mm gaps were visualized were recorded and cross-referenced with machine load data to calculate the load at which a 3-mm gap occurred. If failure occurred prior to the formation of an identifiable gap, then this was recorded as “no gap.”

Statistical analysis

A preliminary study was performed to refine the study methodology, bone tunnel drilling technique and respective suture patterns for each group. An a priori power analysis determined a sample size of ≥ 13 tendons/group would provide an ≥ 80% power to detect a mean difference of 30 ± 8 N at a 5% alpha error rate in independent measures. Preliminary study data were not included in the final statistical model. Data were assessed for parametric distribution using the Shapiro-Wilk test for normality. Continuous variables were normally distributed and described as mean ± SD. Differences in group means were assessed using mixed linear models. Group was included in each model as a fixed effect, with limb side from each dog as the subject to account for repeated measures. An autoregressive covariance structure was used. If the effect of group was significant, pairwise comparisons of least square means were conducted with Bonferroni adjustments for multiple comparisons. Proportional distributions in failure mode were compared between loop groups using the Pearson chi-square test of association. All analyses were performed using commercial software (SAS, version 9.4; SAS Institute Inc). Values of P < 0.05 were considered statistically significant.

Results

Left and right hindlimbs were equally distributed among all groups (P > 0.99). No specimens were excluded during cadaveric collection, repair, or construct testing.

Biomechanical testing

Yield load significantly differed among groups (P = 0.001). Mean yield load of the 5LP and 6LP was significantly greater compared to the 3LP group, respectively (P < 0.023). Yield load of the 4LP, 5LP, and 6LP were 1.2 times, 1.4 times, and 1.6 times greater than 3LP, respectively. Peak loads significantly differed among groups (P < 0.001). Mean peak load of the 6LP was significantly greater compared to all other experimental groups (P < 0.001). Mean peak load of the 4LP was significantly lower compared to the mean yield load of the 6LP (P = 0.002) Peak loads of the 4LP, 5LP, and 6LP were 1.4 times, 1.6 times, and 1.8 times greater than 3LP, respectively. Failure loads differed among groups (P < 0.001). Mean failure load of the 6LP was significantly greater compared to all other groups (P < 0.001). Mean failure load of the 4LP was significantly lower than the mean failure force of the 6LP (P = 0.002). Failure loads of the 4LP, 5LP, and 6LP were 1.4 times, 1.6 times, and 1.8 times greater than 3LP, respectively. Stiffness differed among groups (P < 0.001). Construct stiffness of the 6LP was significantly greater compared to 3LP and 4LP (P < 0.001 and P < 0.0084, respectively). Stiffness of the 4LP, 5LP, and 6LP was 1.1 times, 1.3 times, and 1.6 times greater than 3LP, respectively (Table 1).

Table 1

Mean ± SD yield, peak, and failure force and stiffness measurements obtained by the use of constructs to test 3, 4, 5, and 6-loop pulley (3LP, 4LP, 5LP, and 6LP, respectively) patterns in an ex vivo canine common calcanean teno-osseous avulsion repair model evaluated with the use of 57 cadaveric hindlimbs of 28 skeletally mature dogs > 1 year of age euthanized for unrelated reasons.

Suture pattern Yield force (N) Peak force (N) Failure force (N) Stiffness (N/mm)
3LP 166.37 ± 36.65a 174.99 ± 31.62a 174.81 ± 31.70a 6.02 ± 1.01a
4LP 203.77 ± 49.95a 246.61 ± 35.62b 243.16 ± 36.47b 6.88 ± 1.28a
5LP 235.31 ± 74.10b 280.99 ± 59.99b 280.89 ± 60.09b 7.56 ± 1.71a
6LP 262.61 ± 66.45b 317.21 ± 34.29c 313.07 ± 35.05c 9.36 ± 2.41b

The means ± SDs are the least squares estimates from the statistical analysis.

a-c

Values with different superscripts within a column differ significantly (P < 0.01).

Gap formation data

Loads to 3-mm gap formation at the teno-osseous junction significantly differed among groups (P = 0.005). Mean load to create a 3-mm gap in the 6LP was significantly greater compared to 3LP and 4LP groups (P = 0.011 and P = 0.032, respectively). In 2 of 14 constructs in the 3LP group, there was suture breakage prior to identification of a 3-mm gap. A gap was identified in all 4LP, 5LP, and 6LP constructs prior to failure. Loads required to cause 3-mm gapping of the 4LP (148.47 ± 33.98 N), 5LP (187.77 ± 45.27 N), and 6LP (199.15 ± 46.23 N) were 1.0, 1.3, and 1.4 times greater than the 3LP (143.87 ± 19.86 N) group, respectively, and results for 5LP and 6LP were significantly (P < 0.05) greater than those for 3LP and 4LP.

Mode of construct failure

Mode of failure did not differ among experimental group (P = 0.733). In 75% (42/56) of experimental repairs, construct failure occurred by suture pulling through the tendon substance. This can be compared to 25% (14/56) of constructs that failed due to failure of the core suture strand itself. No sutures failed at the level of the knot or from the bone at the level of the transverse bone tunnel. Failure distant to the repair site was not observed in any of these experimental repairs.

Discussion

In this study, we investigated the biomechanical properties and gap formation to 3 mm at the teno-osseous junction following modification of the number of suture loops in a canine CCT avulsion repair model. The results of this study demonstrate that loop modification by increasing the number of suture loops through the CCT increased repair site strength while increasing loads required to cause 3-mm gapping. We therefore accepted our hypothesis. Increasing the number of suture loops anchored distally in through a calcanean bone tunnel is a relatively simple technique modification that requires further evaluation for use in CCT repair in dogs.

Several human and veterinary studies1822 have shown that improvements in tensile strength can be achieved by increasing the number of suture strands or suture loops traversing the tenorrhaphy. As a result, many techniques used for distal extremity tendon repair in humans have led to an increase in pattern complexity and resultant use of multistrand repair patterns.19,20,22 In a study using an ovine tendon model Al-Qattan et al20 demonstrated an approximately 2-fold increase in repair site strength when increasing from a 2 to 6 strand repair method. Recent literature has demonstrated similar findings in veterinary studies21,23 evaluating mid body repair following laceration of canine tendons. Our study demonstrated improvements in the biomechanical properties and repair site strength by increasing the number of suture loops. Compared to the 3LP, increasing the number of loops increased the tensile repair site strength by 1.4, 1.6, and 1.8 for 4LP, 5LP, and 6LP, respectively. Increasing the number of suture loops also indirectly increases the points of suture purchase and the number of strands through the tendon. In the veterinary literature,15 3LP repairs have been shown to be biomechanically superior compared to LL patterns using the same sized suture material. Due to the fact that the studies5,6,9,24 controlled other extraneous variables, these findings likely relate to the 3 loops leading to 6 strands crossing the repair site in the 3LP compared to only 2 strands utilized for LL (Kessler) patterns. One potential drawback associated with increasing the number of suture loops using a 6LP is a greater number of needle passes made into the GT and accessory tendon. However, the results of prior studies5,9,13,2527 have indicated that increases in tenorrhaphy strength afforded by the implemented suture pattern outweigh potential drawbacks associated with an increased volume of suture on the tendons surface or a greater number of needle punctures made into the body of the tendon.

The results of this study are in agreement with prior studies evaluating 3LP in canine hindlimb models.5,6,9 Interestingly, our data regarding the biomechanical properties exceed those of previously reported 3LP data in dogs.13 The increase in loads seen likely relate to the size of core suture used28 and the fact that the accessory tendon was included within the final repair. Complete rupture involving all components of the CCT is the most common clinical presentation seen clinically in 27% to 43% (12/45; 12/28) of dogs with a talocrural hyperflexion seen on orthopedic examination.1,8 Increasing suture size is positively correlated with overall tensile strength of canine tendon repair both in human29 and veterinary publications.28,30 Prior publications5,6,9,13 have shown that core 3LP repairs alone do not resist the calculated force generated within the CCT apparatus during ambulation in dogs. Moores et al9 theorized through mathematical calculations, that the force generated in the CCT of a 30-kg dog at the trot to be 399 N. Although the 6LP in our study was > 300 N, loads prior to failure were lower than this theorized force; however, it is much higher than those previously reported in dogs. This is a simple technique modification that may significantly increase the tensile strength of these experimental repairs. Surgical repair without the concurrent use of talocrural immobilization for up to 9 weeks may predispose repairs to failure;8 therefore, we still recommend and advocate for continued repair site protection based on the results of our study, even when using the 6LP. It should be noted, however, that the exact force borne by the individual components of the CCT is unknown and warrants further investigation.

Osteotomy or bony debridement of the proximal tuber calcanei is frequently performed intraoperatively to remove sclerotic bone and peritendinous enthesiophyte formation and encourage the development of a healthy bone bed to promote teno-osseous integration.1,8 Decreasing the formation of a 3-mm gap between the distal tendon ends and the calcaneus is clinically important.1,29,31,32 Close apposition of the tendon ends to bone during this protracted period of healing is critical to allow the progression of teno-osseous ingrowth and eventual collagenous integration into the calcaneus.11,17,33 This is typically followed by a period of controlled exercise and incremental load application postoperatively in dogs.1,8 Limited information currently exists regarding the influence of teno-osseous repair methods in dogs. Evaluation of techniques for CCT anchorage to the calcaneus with the aim to decrease gap formation and to promote tendinous healing is important to guide intraoperative decision-making. Use of a 6LP significantly increased the loads required to cause the development of a 3-mm gap at the enthesis thus supporting increasing the number of suture loops and suture strands that cross the repair site. Our results support the results of prior investigators where increasing the number of strands increased loads required to cause a 3-mm gap.1922,34 Due to the fact that each sequential loop in the 6LP is offset by 60° there is complete engagement of the suture to the tendon for 360° around the total circumference at the enthesis. It is important to realize that the number of suture loops and thus the number of strands crossing the repair is not the only important factor to consider during tendinous repair.21,35 Other factors such are core suture size,28 depth of core suture placement,36 use of an epitendinous suture,37,38 and increasing the relative distance of suture bite purchase from the transection site39 have all been identified as important considerations during surgical tendon reconstruction. Sequential increases in each of these evaluated variables results in increased tensile strength, stiffness, and resistance of the repair to the development of 3-mm gap formation.15,26,37,40 Based on the results of our study, the 6LP repair pattern may translate in vivo to improve CCT healing. These speculative hypotheses, however, must be interpreted with caution as further studies are necessary to determine the clinical benefit of 6LP repair methods over 3LP.

Our results are in agreement with the results of studies by Moores et al6,11 where the majority of repairs failed by the core suture pulling through the tendinous tissue. In 75% (42/56) of constructs in our study, regardless of the number of suture loops used, repairs failed by mechanism of suture pull through. As there were no failures at the knot or trans-osseous bone tunnel, these results may be explained by greater force distribution and load sharing between the circumferentially placed suture loops and collagen fibers composing the tendon. We hypothesize that use of additional suture loops, as used in 5LP and 6LP repairs, predispose the tendon-suture interface to represent the weakest part of the sutured construct. We postulate that increasing the bite distance from the enthesis may also allow a greater degree of load distribution and remove areas of stress concentration placed on individual suture loops placed close to the enthesis. This may be advantageous during clinical cases due to the chronic degenerative changes that affect the integrity of collagen fibers and the suture holding capacity of tendinous tissues at the distal CCT. Perceptively, we noted that when suture loops of the 6LP were tensioned under incremental loading, the circumferential nature of the offset suture loops also had the benefit of resisting distal tendon deformation with pull through observed at greater loads compared to 3LP. It should be noted that pattern modification such as increasing the number of locking loops, changes in suture material,41 suture size,28,36 tendon type,37 or components of the CCT1,2 affected may all affect the applicability of these results.

We recognize some important limitations of this study. The cadaveric nature of these experimental constructs cannot accurately replicate clinical scenarios where conditions of vascular compromise, inflammatory tendinopathy, or chronic degenerative change may negatively affect or alter the biomechanical properties of the final repair. We also cannot account for the effect of patient age, concurrent metabolic disease or endocrinopathies, or other systemic comorbidities that may have influenced pathology of the CCT. Tenotomy creation was performed in a controlled manner by sharp iatrogenic transection, which contrasts CCT encountered clinically where fraying and damage to the tendon occurs. It should be noted, however, that the senior author will frequently debride the distal end of the CCT to remove scar tissue and create a flat tendon surface for calcaneal attachment during clinical cases. We utilized an incremental distraction to failure protocol to simulate acute overloading of the repair in the immediate postoperative period and to allow for meaningful comparison to prior investigators.6,9,13,15 A further limitation relates to the assessment of gap formation. During review of the obtained camera footage, progressive gap formation between the distal end of the CCT and calcaneal osteotomy site was easily identified. However, at times calculation using the imaging software was hampered as the tendon ends were not always parallel and the length of gap not always uniform in nature. We do not believe, however, that the results or conclusions drawn from them were affected with variability minimized using a single trained investigator. Lastly, due to same investigator performing both the construct repairs and failure testing, it was not possible completely mask group assignment. This may have lead to the potential for unintentional operational and observer bias.

In conclusion, incrementally increasing the number of sutures loops resulted in improvement of the biomechanical properties of the repair while increasing the loads required to cause 3-mm gap formation. Of patterns examined, 6LP was superior to all other evaluated constructs in all assessed parameters. Pattern modification by increasing the number of suture loops compared to a traditional 3LP repair is a relatively simple technique modification that significantly increases repair site strength. The results of this study warrant further focused investigation of loop medication for teno-osseous repair following CCT avulsion in dogs.

Acknowledgments

Authors received no sources of funding for the purpose of this study and declare that there were no conflicts of interest.

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