Tendon injury poses a unique clinical challenge because even with optimal treatment and therapy, the healed tendon does not recover its biochemical and mechanical properties to preinjury levels.1 Patellar tendon ruptures are rare musculoskeletal injuries in dogs.2,3 Ruptures of the patellar tendon are particularly detrimental because of inability to extend the stifle joint, which is attributed to the loss of the quadriceps mechanism.4–6 Patellar tendon rupture is most commonly described as occurring in the midsubstance region,7 but in the authors' experience, rupture at the proximal aspect also occurs.
Rapid surgical intervention in dogs with ruptured patellar tendons is crucial to ensure rapid healing without gap formation and allow for a return in tensile strength and function with minimal scar formation.8 Several techniques for tenorrhaphy in dogs have been previously described.2,3,5,8–10 However, repair of patellar tendon rupture near its origin at the patellar apex may prove challenging because there can be minimal tendon substance remaining and minimal bone stock to facilitate a secure reattachment.11
Previously described techniques for tenorrhaphy such as the 3-loop pulley, Bunnel-Mayer, Krackow, and Kessler locking-loop patterns can be modified to attach tendon directly to bone.8,12,13 Bone tunnels are a primary way of securing the attachment, but the need for comparatively large suture sizes to achieve the required strength results in large bone tunnels that increase the potential for patellar fracture. However, compared with similar-diameter monofilament sutures, UHMWPE suture provides twice the tensile strength and a higher resistance to elongation14; thus, similar results can be achieved with smaller-diameter suture and smaller bone tunnels.
The purpose of the study reported here was to compare use of a 3LSLS technique with use of the previously described M3LP technique12 to repair rupture of the proximal aspect of patellar tendons in limbs from canine cadavers. We hypothesized that tendons repaired with the 3LSLS technique would require a significantly greater force load to develop a 1-mm gap, a 3-mm gap, and failure of the repair than would tendons repaired with the M3LP technique.
Materials and Methods
Sample
The use of cadavers in this study was approved by the Institutional Animal Care and Use Committee of the University of Florida. Paired hind limbs were collected from 6 mixed-breed adult dogs of similar size that weighed 20 to 30 kg and were euthanized for reasons unrelated to the study. Whole cadavers were stored at −20°C prior to thawing and tissue collection. Once collected, tissues were stored at 0°C until use; all tissues were tested within 72 hours of harvesting. A limb from each pair was randomly assigned (by means of coin toss) to be repaired by the 3LSLS or M3LP technique.
Specimens were prepared by removal of all soft tissues except for the distal aspect of the quadriceps femoris muscle group, patella, patellar tendon, and tibial tuberosity from each limb. The patellar tendon was then transected with a scalpel blade near its origin on the patella. This site of transection was chosen to simulate rupture of the tendon at its proximal aspect. The transected edge of the tendon was marked with black ink to increase its visibility during testing. Transections and repairs were performed by a veterinary surgeon (MDJ).
3LSLS repair technique
The 3LSLS pattern was modified from the pattern used with a proprietary systema for repair of human calcaneal (Achilles) tendon rupture and from a method previously used for repair of patellar tendon avulsion in a dog.15 For this technique, size-2 UHMWPE suture was used with the manufacturer-supplied jig and suturing kita for patellar tendon repair. The repair for this study was modified by securing the ruptured tendon to the origin via bone tunnels rather than placing the 3LSLS on both sides of a midsubstance tear. The jig was placed over the distal segment of the transected patellar tendon, and size-2 UHMWPE suture was threaded through the jig and tendon in accordance with manufacturer recommendations. A total of 3 sutures were placed in the distal segment of the patellar tendon: 2 simple sliding sutures and 1 self-locking suture (Figure 1).
Photographs depicting the 3LSLS suture pattern as performed in a study to compare use of this technique with use of the M3LP technique to repair complete rupture of patellar tendons in limbs from 6 canine cadavers. The patellar tendon (PT) was transected near its origin at the patella, and its attachment at the tibial tuberosity (TT) was left intact; a commercially available kit with a jig (not shown) for repair of human calcaneal tendon rupture was used for suture placement. A—The distal sliding suture (white; 1) was placed first, and the suture used to create the self-locking component (blue; 2) was placed proximal to the first suture. Two loop-end sutures (green and white-striped; 3a and 3b) were temporarily placed proximal to the second suture for use in passing the self-locking suture back through the tendon. The most proximal suture (black and white-striped; 4) was a simple sliding suture. B—Each end of the self-locking suture (2) was threaded through the ipsilateral loop-end suture (3a and 3b), and the loop-end sutures were pulled through the tendon (directions shown with red arrows), bringing the self-locking suture ends with them. C—Each end of the self-locking suture (2) was then pulled through the loop created by passing the opposite end back through the tendon to complete the self-locking component; the simple sliding sutures (1 and 4) are labeled for identification. The tendon was subsequently secured with the sutures appropriately tensioned.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photographs depicting the 3LSLS suture pattern as performed in a study to compare use of this technique with use of the M3LP technique to repair complete rupture of patellar tendons in limbs from 6 canine cadavers. The patellar tendon (PT) was transected near its origin at the patella, and its attachment at the tibial tuberosity (TT) was left intact; a commercially available kit with a jig (not shown) for repair of human calcaneal tendon rupture was used for suture placement. A—The distal sliding suture (white; 1) was placed first, and the suture used to create the self-locking component (blue; 2) was placed proximal to the first suture. Two loop-end sutures (green and white-striped; 3a and 3b) were temporarily placed proximal to the second suture for use in passing the self-locking suture back through the tendon. The most proximal suture (black and white-striped; 4) was a simple sliding suture. B—Each end of the self-locking suture (2) was threaded through the ipsilateral loop-end suture (3a and 3b), and the loop-end sutures were pulled through the tendon (directions shown with red arrows), bringing the self-locking suture ends with them. C—Each end of the self-locking suture (2) was then pulled through the loop created by passing the opposite end back through the tendon to complete the self-locking component; the simple sliding sutures (1 and 4) are labeled for identification. The tendon was subsequently secured with the sutures appropriately tensioned.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photographs depicting the 3LSLS suture pattern as performed in a study to compare use of this technique with use of the M3LP technique to repair complete rupture of patellar tendons in limbs from 6 canine cadavers. The patellar tendon (PT) was transected near its origin at the patella, and its attachment at the tibial tuberosity (TT) was left intact; a commercially available kit with a jig (not shown) for repair of human calcaneal tendon rupture was used for suture placement. A—The distal sliding suture (white; 1) was placed first, and the suture used to create the self-locking component (blue; 2) was placed proximal to the first suture. Two loop-end sutures (green and white-striped; 3a and 3b) were temporarily placed proximal to the second suture for use in passing the self-locking suture back through the tendon. The most proximal suture (black and white-striped; 4) was a simple sliding suture. B—Each end of the self-locking suture (2) was threaded through the ipsilateral loop-end suture (3a and 3b), and the loop-end sutures were pulled through the tendon (directions shown with red arrows), bringing the self-locking suture ends with them. C—Each end of the self-locking suture (2) was then pulled through the loop created by passing the opposite end back through the tendon to complete the self-locking component; the simple sliding sutures (1 and 4) are labeled for identification. The tendon was subsequently secured with the sutures appropriately tensioned.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
The patella was then isolated, and 2 axial tunnels were drilled with a 1.5-mm bit such that they were entirely within the patella and did not converge with each other (Figure 2). The free ends of the 3 sutures that were previously placed through the distal segment of the patellar tendon were threaded through each of the 2 axial tunnels and passed through a 3.5-mm oblong titanium button, and the ends of each suture (identified by color) were tied together with 3 square knots. The excess suture ends were cut 4 to 5 mm from the knot.
Photographs showing the method for securing the transected end of patellar tendons to the patellar apex by use of the 3LSLS technique. A—Two axial bone tunnels were drilled through the patella (shown with pins inserted into the tunnels). B—After the free ends of the 3 sutures extending from the lateral aspects of the patellar tendon were passed through the ipsilateral 1.5-mm axial tunnels, they were passed through a 3.5-mm oblong titanium button, and the ends of each individual suture were secured with 3 square knots/suture. This photograph was obtained following mechanical testing, which can be verified by the black- and white-suture loop as well as the fact that the tendon is not seen apposed to the patellar apex.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photographs showing the method for securing the transected end of patellar tendons to the patellar apex by use of the 3LSLS technique. A—Two axial bone tunnels were drilled through the patella (shown with pins inserted into the tunnels). B—After the free ends of the 3 sutures extending from the lateral aspects of the patellar tendon were passed through the ipsilateral 1.5-mm axial tunnels, they were passed through a 3.5-mm oblong titanium button, and the ends of each individual suture were secured with 3 square knots/suture. This photograph was obtained following mechanical testing, which can be verified by the black- and white-suture loop as well as the fact that the tendon is not seen apposed to the patellar apex.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photographs showing the method for securing the transected end of patellar tendons to the patellar apex by use of the 3LSLS technique. A—Two axial bone tunnels were drilled through the patella (shown with pins inserted into the tunnels). B—After the free ends of the 3 sutures extending from the lateral aspects of the patellar tendon were passed through the ipsilateral 1.5-mm axial tunnels, they were passed through a 3.5-mm oblong titanium button, and the ends of each individual suture were secured with 3 square knots/suture. This photograph was obtained following mechanical testing, which can be verified by the black- and white-suture loop as well as the fact that the tendon is not seen apposed to the patellar apex.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
M3LP repair technique
The patellar tendon on the opposing limb was repaired by use of an M3LP method as described elsewhere.12 For this technique, a single strand of size-0 monofilament polypropylene suture was used in each specimen. Briefly, the patella was isolated, and a single transverse bone tunnel was drilled through the patella with a 1.5-mm drill bit at the approximate level of the midpatella. Suture was passed through the tendon substance a total of 3 times with each pass of the suture oriented perpendicularly through the tendon and approximately 60° from the last passage. After each pass through the patellar tendon, the suture was passed through the transverse patellar bone tunnel in a medial to lateral direction such that the knot of the completed suture pattern was tied on the lateral aspect of the tendon. The pattern was completed with 3 square knots, and the excess suture was trimmed 4 to 5 mm from the last knot.
Mechanical testing
All mechanical testing was performed with a biaxial servohydraulic MTS.b Each specimen was secured to the upper aspect of the testing device by placement of the quadriceps muscle in a custom soft-tissue clamp that consisted of 2 plates of ridged aluminum connected by 4 bolts that were hand tightened with wing nuts. The tibial tuberosity was secured to the base of the MTS by transverse placement of two 2.38-mm-diameter pins through the bone with a custom jig (Figure 3).
Photograph of a specimen (distal part of the quadriceps femoris, patella, patellar tendon, and tibial tuberosity after transection of the patellar tendon and subsequent repair by the 3LSLS technique) secured to a biaxial servohydraulic MTS and viewed from the cranial aspect. The custom soft-tissue clamp is on the quadriceps femoris at the top of the image, and 2 pins (2.38-mm diameter) are placed through the tibial tuberosity via a custom jig for testing. A ruler was placed adjacent to the tendon for measurement of gap formation at the repair site during mechanical force testing.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photograph of a specimen (distal part of the quadriceps femoris, patella, patellar tendon, and tibial tuberosity after transection of the patellar tendon and subsequent repair by the 3LSLS technique) secured to a biaxial servohydraulic MTS and viewed from the cranial aspect. The custom soft-tissue clamp is on the quadriceps femoris at the top of the image, and 2 pins (2.38-mm diameter) are placed through the tibial tuberosity via a custom jig for testing. A ruler was placed adjacent to the tendon for measurement of gap formation at the repair site during mechanical force testing.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Photograph of a specimen (distal part of the quadriceps femoris, patella, patellar tendon, and tibial tuberosity after transection of the patellar tendon and subsequent repair by the 3LSLS technique) secured to a biaxial servohydraulic MTS and viewed from the cranial aspect. The custom soft-tissue clamp is on the quadriceps femoris at the top of the image, and 2 pins (2.38-mm diameter) are placed through the tibial tuberosity via a custom jig for testing. A ruler was placed adjacent to the tendon for measurement of gap formation at the repair site during mechanical force testing.
Citation: American Journal of Veterinary Research 80, 4; 10.2460/ajvr.80.4.335
Once the specimen was secured in the MTS, the clamp side of the specimen was distracted to a load of 3N to remove slack from the system. The clamp side was then distracted at 25 mm/min until failure. Failure was described as the point at which the repair failed owing to breakage of the suture material or slippage of the suture through the tendon tissue as determined by a drop in the applied force.16 This drop was measured with the MTS software, and failure by suture breakage was identified as a sharp drop in the load-displacement curve.
Each test was filmed with a digital video camera. A metric scale was placed alongside each tendon to monitor the gap formation at the repair site. Time from start of the test until formation of a 1-mm gap, formation of a 3-mm gap, and failure (suture breakage or tissue rupture) was measured according to the recorded time on the MTS during distraction.16 Force required to create a 1-mm gap, a 3-mm gap, and repair failure was measured in Newtons. The mode of failure and gap size at failure were also recorded.
Statistical analysis
The force at gap formation and the distance between repair segments at the time immediately before complete failure were identified by visual analysis of the controlled distraction. The force at gap formation or failure was established by matching the time at visual recognition of the event to the time recorded for force data. Visual observations were made by 1 observer and were performed in triplicate. Data analyses were completed with statistical software.c
Anderson-Darling and Grubbs tests were performed, and the data were found to be normally distributed with no outliers. The amount of force required to produce the predetermined gap sizes, amount of force that resulted in repair failure, and size of the gap at the repair site at failure were compared between techniques with paired t tests. Values of P < 0.05 were considered significant.
Results
The mean ± SD amount of force required to produce a 1-mm gap in patellar tendon repairs performed with the 3LSLS and M3LP techniques was 13.4 ± 4.1 N and 15.8 ± 5.98 N, respectively; the results did not differ significantly (P = 0.44). Force at formation of this gap size ranged from 8.7 to 18.7 N for the 3LSLS technique and from 8.5 to 24.0 N for the M3LP technique. There was also no significant (P = 0.44) difference detected in the amount of force required to create a 3-mm gap (mean ± SD, 43.1 ± 8.5 N and 47.4 ± 10.1 N for repairs performed with the 3LSLS and M3LP techniques, respectively); results for individual limbs ranged from 25.7 to 51.5 N for the 3LSLS technique and from 35.7 to 60.6 N for the M3LP technique. However, repairs performed with the 3LSLS technique required a significantly (P = 0.02) higher load to produce complete failure (mean ± SD, 266 ± 85.6 N), compared with those performed by the M3LP technique (135 ± 70 N). The load at failure for individual limbs ranged from 140.1 to 354.1 N and from 42.4 to 215.1 N for the 3LSLS and M3LP techniques, respectively. The gap size measured immediately before failure was significantly (P = 0.03) greater for repairs made with the 3LSLS technique than for those made with the M3LP technique (mean ± SD, 15.34 ± 4.2 mm vs 8.74 ± 4.4 mm). Measurements of gap size at this point ranged from 8.6 to 17.8 mm and from 3.2 to 13.6 mm for 3LSLS-repaired and M3LP-repaired tendons, respectively.
None of the 6 patellar tendon repairs performed with the 3LSLS technique failed because of suture breakage; all of these specimens had tissue failure with sutures pulled through the tendon, and 2 specimens also had the suture and titanium button pull through a portion of the patella. All specimens with repairs performed by the M3LP technique also had some degree of tissue failure, with 2 of 6 failures resulting from pull-through of the sutures alone; the remaining 4 specimens had sutures pull through the tendon to some degree before the suture broke.
Discussion
Results of the present study showed no significant differences between patellar tendon repairs performed by use of the 3LSLS technique with size-2 UHMWPE suture and those performed by use of the M3LP technique with size-0 monofilament polypropylene suture in regard to the amount of force required to create a clinically relevant gap between the severed tendon end and its point of reattachment. One study17 of dogs with transected and repaired flexor tendons indicated that the maximum allowable gap for tendon repair that results in successful tendon healing is 3 mm. The similarity in force needed for gap formation in the present study may have represented a limitation in tendon tissue holding strength; both suture strengths might exceed that of the tendon. Additionally, the lack of detectable difference in these measures may have been attributable to the small sample size resulting in a type 2 statistical error.
The range of forces placed on patellar tendons in dogs has not been established. A study was performed to examine strain on the patellar tendon in adult goats.18 In goats, the force measured in the patellar tendon ranged from 200 N during standing to 1,000 N at a trot; goats in that study18 weighed 57 to 67 kg (vs 20 to 30 kg for dogs from which our study sample was collected). On the basis of our ex vivo findings, neither repair technique used in the present study would be expected to be able to withstand those type of forces, reinforcing the recommendation2,3,10,19–21 for secondary and tertiary techniques to protect the primary repair method used in patellar tendon repair.
Our results indicated that, with the suture materials used in this study, the 3LSLS technique may better resist catastrophic failure of patellar tendon repair by requiring a higher load to induce failure and sustaining a larger gap immediately prior to the point of failure, compared with the M3LP technique. Although the gap immediately prior to failure was substantially greater than that previously shown to allow healing (3 mm),17 the higher load required for failure of these repairs in our study may be relevant in situations that involve unruly patients or noncompliant owners. The lack of failure as a result of suture breakage in repairs performed with the 3LSLS technique suggested a resilience to catastrophic failure with forces that caused suture breakage in repairs made with the M3LP technique. However, most tendon repair techniques include use of an internal splint spanning from the patella to the tibial tuberosity to reduce direct strain on the tenorrhaphy and prevent complete failure.2,3,10,19–21 In this manner, the biomechanical testing in the present study may not have reflected potential differences in outcome between the 2 repair methods in a clinical setting with augmented techniques for tenorrhaphy protection, such as internal splinting, coaptation, or transarticular external fixation.
The modes of failure were noticeably different between techniques. The finding that forces required to induce 1- and 3-mm gap formation were similar between techniques and that pull-through of sutures occurred to some degree in all specimens and was the sole mode of failure in 8 of 12 indicated that when the suture remains intact the 2 techniques are likely to have similar outcomes. That 4 of 6 repairs performed with the M3LP technique involved suture breakage and the maximum load at failure was significantly lower when this technique was used might indicate that the use of this pattern with size-0 monofilament polypropylene suture results in a repair that is more susceptible to catastrophic failure at lower forces, compared with that achieved by use of the 3LSLS technique and UHMWPE suture.
Because the 3LSLS technique involves the use of 3 separate sutures, a complete failure would require a 3-level failure; this is in contrast to the M3LP technique that relies on 1 continuous strand, so that any failure in the loop would result in complete failure of the repair. Additionally, 2 of the 3LSLS repairs had concurrent damage to the patella at the time of tendon tissue failure. Bone tunnel widening has been reported22 to develop during the immediate postoperative period when UHMWPE was used for hip luxation repair in dogs but was not found to be progressive. Although the exact cause for widening was not identified in that report,22 it is possible that infection was a factor because braided sutures can create a better nidus for infection than monofilament sutures.22 However, this was not a cause of tunnel widening in previous reports.15,22 The possibility of UHMWPE suture use widening the tunnels or of the suture becoming infected in patients cannot be disregarded. In regard to the 2 specimens that failed with the suture and button pulling through the patella, the result was attributed to the tunnel being placed too close to the articular side of the patella and leaving an inadequate amount of bone stock. This indicated that care should be taken to not place the tunnel too close to the articular aspect of the patella. In our study, size-0 monofilament polypropylene suture was selected for use with the M3LP technique to allow the use of similar bone tunnel diameters between the 2 treatment groups. Use of a larger-gauge monofilament suture material was considered as a means of better matching suture strength between methods; however, this would have required a larger-diameter patellar bone tunnel and could potentially have caused excessive trauma to the tendon tissue during placement.
The use of UHMWPE allowed for placement of 2 axial patellar bone tunnels to distribute tensile forces parallel to the pull of the quadriceps with the intent of decreasing the risk of patellar fracture. The axial bone tunnels also allowed for direct apposition of the free tendon end to the apex of the patella. The titanium button was used to mitigate subsequent suture laxity by distributing forces over a larger area and to increase forces necessary to result in suture pull-through at the proximal aspect of the patella. In this study, the button was placed below the quadriceps tendon, and this placement creates the potential for the button to come in contact with the femur. Therefore, future applications should include a modification to place the button within the quadriceps tendon, as was described in a case report where this technique was used.15
A tenorrhaphy with the M3LP technique as typically described and performed in our study results in suture material and potentially the knot coming in direct contact with the articular cartilage at the distal aspect of the femur, which could result in irritation and injury.12,14 In the 3LSLS technique, the sutures are placed lateral to the patellar tendon, and the knots are created dorsal to the patella where they are secured at the button. This placement may be beneficial as it can prevent direct contact with the articular cartilage that would result in abrasion and damage to the articular surface. The effective and safe use of UHMWPE suture in a M3LP technique might be challenging to achieve because of its potential to cause damage to the articular surface during direct contact and because of increased tissue drag leading to inadequate tissue apposition or damage created in the effort to achieve tissue apposition.
Use of braided nonabsorbable suture is generally considered to cause an increased risk of infection, compared with the use of nonabsorbable monofilament suture.23 The nature of the present study precluded comment on the risk of infection, although one of the authors (MDJ) has used the 3LSLS technique with UHMWPE suture successfully in a dog with patellar tendon avulsion.15 In a retrospective study24 of dogs with tarsal ligament injury, an increased incidence of complications was found with use of either nylon or UHMWPE implants, and 5 of 8 limbs stabilized with prosthetic ligaments developed major complications owing to infection. On the basis of those findings, caution should be exercised whenever nonabsorbable implants are used.
Several limitations of the present study prevent direct extrapolation of the results to a clinical situation. Because the study was performed with cadaveric specimens, we cannot comment on the effect of the studied techniques on tissue healing, blood supply to the tissue, or strength of the healed tendon. Because of the small sample size, small differences between the 2 techniques may not have been detectable. Additionally, we tested the repairs by application of load-to-failure rather than cyclic testing. Cyclic testing of cadaveric soft tissue may not be representative of the clinical situation; however, in vivo repairs are likely subject to substantial cyclic loading during walking. The 3 types of repairs performed in this study may behave differently under cyclic loading, but whether and how this would affect results is unknown. It is also important to note that to differentiate the effects of technique from those attributable to suture material differences, 2 more groups would have been required, and that investigation was beyond the scope of the present study. We cannot, however, rule out that the greater in load required for failure of repairs performed with the 3LSLS technique, compared with that for repairs made by use of the M3LP technique, resulted from strength of the UHMWPE suture.
Future studies should compare the 3LSLS technique to the modified M3LP technique for repair of patellar ligament rupture in limbs with internal splints applied to assess the primary tenorrhaphy's resistance to gap formation and failure under such conditions. The greater absolute force required to cause failure of patellar tendon repairs with the 3LSLS technique, compared with repairs performed with the M3LP technique, may suggest clinically relevant advantages for tendon repair in dogs, and in vivo studies appear warranted.
Acknowledgments
Funded by the University of Florida College of Veterinary Medicine Small Animal Clinical Sciences Departmental funds.
Matthew Johnson is a consultant for Arthrex Vet Systems but has no direct financial connection or benefit from use of the products in this study.
The authors thank Dr. Diego Sobrino and CJ Travers for technical assistance and data collection.
ABBREVIATIONS
| 3LSLS | Three-level self-locking suture |
| M3LP | Modified 3-loop pulley |
| MTS | Materials testing system |
| UHMWPE | Ultrahigh-molecular-weight polyethylene |
Footnotes
Percutaneous Achilles Repair System (PARS), Arthrex, Naples, Fla.
858 Mini Bionix, MTS Systems Corp, Eden Prairie, Minn.
Minitab, version 17.1.0, State College, Pa.
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