• View in gallery

    Photograph of an uncontoured 3.5-mm 6-hole nonlocking TPLO plate used for all 20 canine hind limb specimens evaluated in an ex vivo study to assess the contribution of ARPs and an intact fibula to the compressive strength of 4 TPLO constructs (bone and implants). For each specimen, the plate was contoured to the shape of the proximal aspect of the tibia and secured by the use of 6 nonlocking bicortical screws with the fourth distal screw (first screw in the distal segment) placed in compression fashion. Following completion of the TPLO and prior to mechanical testing, each specimen was evaluated to ensure that the tibial crest had a craniocaudal dimension of ≥ 10 mm; that the TPA was between 0° and 14° and limb alignment had changed < 5°; that the plate was secured to the bone with 6 bicortical screws, each of which was seated within the plate; that there was no visible gap between the plate and bone or at the osteotomy site; that none of the screws crossed the osteotomy site or entered the joint surface; and that no drill holes or screws engaged the fibula.

  • View in gallery

    Mean ± SD load prior to mechanical testing (displacement, 0 mm); at 0.5, 1.0, and 3.0 mm of displacement; and at ultimate failure for canine hind limb specimens that underwent 1 of 4 TPLO constructs (construct in which the ARP was removed [control group; blue line; n = 5], construct in which 1 ARP was left in place [single-pin group; red line; 5], construct in which a second ARP was placed 0.5 cm distal and parallel to the first ARP and both pins were left in place [double-pin group; gray line; 5], and construct in which the ARP was removed and the fibula was cut with a radial saw where the saw intersected the fibula as the cut was extended distally beyond the tibial osteotomy site [cut-fibula group; yellow line; 5]). Following completion of the TPLO, specimens were mounted in a hydraulic testing machine and axially loaded such that a compressive load was applied through the joint surface of the proximal segment. Initially, each specimen underwent 10,000 cycles of cyclic testing at a force equal to 40% of the body weight of the cadaver from which it was obtained. Then, the specimen was tested to failure at a displacement rate of 1 mm/s. Mean load did not differ significantly (P < 0.05) among the 4 groups at any displacement.

  • 1. Johnson JA, Austin C, Breur GJ. Incidence of canine appendicular musculoskeletal disorders in 16 veterinary teaching hospitals from 1980 to 1989. Vet Comp Orthop Traumatol 1994;7:5659.

    • Search Google Scholar
    • Export Citation
  • 2. Wilke VL, Robinson DA, Evans RB, et al. Estimate of the annual economic impact of treatment of cranial cruciate ligament injury in dogs in the United States. J Am Vet Med Assoc 2005;227:16041607.

    • Search Google Scholar
    • Export Citation
  • 3. Tonks CA, Lewis DD, Pozzi A. A review of extra-articular prosthetic stabilization of the cranial cruciate ligament–deficient stifle. Vet Comp Orthop Traumatol 2011;24:167177.

    • Search Google Scholar
    • Export Citation
  • 4. Cook JL, Luther JK, Beetem J, et al. Clinical comparison of a novel extracapsular stabilization procedure and tibial plateau leveling osteotomy for treatment of cranial cruciate ligament deficiency in dogs. Vet Surg 2010;39:315323.

    • Search Google Scholar
    • Export Citation
  • 5. Kim SE, Pozzi A, Kowaleski MP, et al. Tibial osteotomies for cranial cruciate ligament insufficiency in dogs. Vet Surg 2008;37:111125.

  • 6. Bruce WJ, Rose A, Tuke J, et al. Evaluation of triple tibial osteotomy. A new technique for the management of the canine cruciate-deficient stifle. Vet Comp Orthop Traumatol 2007;20:159168.

    • Search Google Scholar
    • Export Citation
  • 7. Raske M, Hulse D, Beale B, et al. Stabilization of the CORA based leveling osteotomy for treatment of cranial cruciate ligament injury using a bone plate augmented with a headless compression screw. Vet Surg 2013;42:759764.

    • Search Google Scholar
    • Export Citation
  • 8. Hoffmann DE, Miller JM, Ober CP, et al. Tibial tuberosity advancement in 65 canine stifles. Vet Comp Orthop Traumatol 2006;19:219227.

  • 9. Arnoczky SP, Tarvin GP, Marshall JL, et al. The over-the-top procedure: a technique for anterior cruciate ligament substitution in the dog. J Am Anim Hosp Assoc 1979;15:283290.

    • Search Google Scholar
    • Export Citation
  • 10. Biskup JJ, Balogh DG, Scott RM, et al. Long-term outcome of intra-articular allograft technique for treatment of spontaneous cranial cruciate ligament rupture in the dog. Vet Surg 2017;46:691699.

    • Search Google Scholar
    • Export Citation
  • 11. Slocum B, Slocum TD. Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Vet Clin North Am Small Anim Pract 1993;23:777795.

    • Search Google Scholar
    • Export Citation
  • 12. Duerr FM, Martin KW, Rishniw M, et al. Treatment of canine cranial cruciate ligament disease. A survey of ACVS diplomates and primary care veterinarians. Vet Comp Orthop Traumatol 2014;27:478483.

    • Search Google Scholar
    • Export Citation
  • 13. Gordon-Evans WJ, Griffon DJ, Bubb C, et al. Comparison of lateral fabellar suture and tibial plateau leveling osteotomy techniques for treatment of dogs with cranial cruciate ligament disease. J Am Vet Med Assoc 2013;243:675680.

    • Search Google Scholar
    • Export Citation
  • 14. Mölsä SH, Hielm-Björkman AK, Laitinen-Vapaavuori OM. Use of an owner questionnaire to evaluate long-term surgical outcome and chronic pain after cranial cruciate ligament repair in dogs: 253 cases (2004–2006). J Am Vet Med Assoc 2013;243:689695.

    • Search Google Scholar
    • Export Citation
  • 15. Christopher SA, Beetem J, Cook JL. Comparison of long-term outcomes associated with three surgical techniques for treatment of cranial cruciate ligament disease in dogs. Vet Surg 2013;42:329334.

    • Search Google Scholar
    • Export Citation
  • 16. Coletti TJ, Anderson M, Gorse MJ. Complications associated with tibial plateau osteotomy: a retrospective of 1,519 procedures. Can Vet J 2014;55:249254.

    • Search Google Scholar
    • Export Citation
  • 17. Krotscheck U, Nelson SA, Todhunter RJ, et al. Long term functional outcome of tibial tuberosity advancement vs. tibial plateau leveling osteotomy and extracapsular repair in a heterogeneous population of dogs. Vet Surg 2016;45:261268.

    • Search Google Scholar
    • Export Citation
  • 18. Boudrieau RJ. Tibial plateau leveling osteotomy or tibial tuberosity advancement? Vet Surg 2009;38:122.

  • 19. Fitzpatrick N, Solano MA. Predictive variables for complications after TPLO with stifle inspection by arthrotomy in 1,000 consecutive dogs. Vet Surg 2010;39:460474.

    • Search Google Scholar
    • Export Citation
  • 20. Taylor J, Langenbach A, Marcellin-Little DJ. Risk factors for fibular fracture after TPLO. Vet Surg 2011;40:687693.

  • 21. Kloc PA, Kowaleski MP, Litsky AS, et al. Biomechanical comparison of two alternative tibial plateau leveling osteotomy plates with the original standard in an axially loaded gap model: an in vitro study. Vet Surg 2009;38:4048.

    • Search Google Scholar
    • Export Citation
  • 22. Bordelon J, Coker D, Payton M, et al. An in vitro mechanical comparison of tibial plateau levelling osteotomy plates. Vet Comp Orthop Traumatol 2009;22:467472.

    • Search Google Scholar
    • Export Citation
  • 23. Thambyah A, Pereira BP. Mechanical contribution of the fibula to torsion stiffness in the lower extremity. Clin Anat 2006;19:615620.

    • Search Google Scholar
    • Export Citation
  • 24. Reif U, Dejardin LM, Probst CW, et al. Influence of limb positioning and measurement method on the magnitude of the tibial plateau angle. Vet Surg 2004;33:368375.

    • Search Google Scholar
    • Export Citation
  • 25. Baroni E, Matthias RR, Marcellin-Little DJ, et al. Comparison of radiographic assessments of the tibial plateau slope in dogs. Am J Vet Res 2003;64:586589.

    • Search Google Scholar
    • Export Citation
  • 26. Slocum B, Slocum T. Tibial plateau leveling osteotomy for cranial cruciate ligament. In: Bojrab M, Ellison GW, Slocum B, eds. Current techniques in small animal surgery. 4th ed. Baltimore: Williams and Wilkins, 1998;12091215.

    • Search Google Scholar
    • Export Citation
  • 27. Windolf M, Leitner M, Schwieger K, et al. Accuracy of fragment positioning after TPLO and effect on biomechanical stability. Vet Surg 2008;37:366373.

    • Search Google Scholar
    • Export Citation
  • 28. Robinson DA, Mason DR, Evans R, et al. The effect of tibial plateau angle on ground reaction forces 4–17 months after tibial plateau leveling osteotomy in Labrador Retrievers. Vet Surg 2006;35:294299.

    • Search Google Scholar
    • Export Citation
  • 29. Piermattei DL, Flo GL, DeCamp CR. Fractures: classifications, diagnosis and treatment. In: Brinker, Piermattei, and Flo's handbook of small animal orthopedics and fracture repair. 4th ed. St Louis: Saunders Elsevier, 2006;129132.

    • Search Google Scholar
    • Export Citation
  • 30. Butler DL, Hulse DA, Kay MD, et al. Biomechanics of cranial cruciate ligament reconstruction in the dog II. Mechanical properties. Vet Surg 1983;12:112118.

    • Search Google Scholar
    • Export Citation
  • 31. Evans R, Horstman C, Conzemius M. Accuracy and optimization of force platform gait analysis in Labradors with cranial cruciate disease evaluated at a walking gait. Vet Surg 2005;34:445449.

    • Search Google Scholar
    • Export Citation
  • 32. Brady RB, Sidiropoulos AN, Bennett HJ, et al. Evaluation of gait-related variables in lean and obese dogs at a trot. Am J Vet Res 2013;74:757762.

    • Search Google Scholar
    • Export Citation
  • 33. Caporn TM, Roe SC. Biomechanical evaluation of the suitability of monofilament nylon fishing and leader line for extra-articular stabilization of the cranial cruciate-deficient stifle. Vet Comp Orthop Traumatol 1996;9:126133.

    • Search Google Scholar
    • Export Citation
  • 34. Leitner M, Pearce SG, Windolf M, et al. Comparison of locking and conventional screws for maintenance of tibial plateau positioning and biomechanical stability after locking tibial plateau leveling osteotomy plate fixation. Vet Surg 2008;37:357365.

    • Search Google Scholar
    • Export Citation
  • 35. Biskup J, Freeman A, Camisa W, et al. Mechanical properties of canine patella-ligament-tibia segment. Vet Surg 2014;43:136141.

  • 36. Puhl JJ, Piotrowski G, Enneking WF. Biomechanical properties of paired canine fibulas. J Biomech 1972;5:391397.

  • 37. Silbernagel JT, Kennedy SC, Johnson AL, et al. Validation of canine cancellous and cortical polyurethane foam bone models. Vet Comp Orthop Traumatol 2002;4:200204.

    • Search Google Scholar
    • Export Citation

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Contribution of antirotational pins and an intact fibula to the ex vivo compressive strength of four tibial plateau leveling osteotomy constructs

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  • 1 Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.
  • | 2 Department of Small Animal Clinical Sciences, College of Veterinary Medicine, University of Tennessee, Knoxville, TN 37996.
  • | 3 Department of Materials Science and Engineering, College of Engineering, University of Tennessee, Knoxville, TN 37996.
  • | 4 Department of Materials Science and Engineering, College of Engineering, University of Tennessee, Knoxville, TN 37996.

Abstract

OBJECTIVE To assess the contribution of antirotational pins (ARPs) and an intact fibula to the compressive strength of 4 tibial plateau leveling osteotomy (TPLO) constructs (bone and implants).

SAMPLE 20 hind limbs from 10 canine cadavers.

PROCEDURES Each hind limb was assigned to 1 of 4 TPLO constructs (construct in which the ARP was removed, constructs in which 1 or 2 ARPs were left in place, and construct in which the ARP was removed and the fibula was cut). Following TPLO completion, all limbs underwent mechanical testing that included 10,000 cycles of cyclic axial compression followed by testing to failure at a displacement rate of 1 mm/s. Displacement during cyclic testing; load generated at 0.5, 1.0, and 3.0 mm of displacement; ultimate load; and failure type were recorded for each limb. Mean values were compared among the groups.

RESULTS None of the specimens failed during cyclic testing. None of the variables assessed during mechanical testing differed significantly among the 4 groups. During testing to failure, the majority (17/20) of specimens failed as the result of a long oblique fracture through the first screw hole in the distal segment.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that the axial compressive strength and stiffness of a TPLO construct were not significantly affected by the addition of 1 or 2 ARPs or the presence of an intact fibula. These findings appear to support removal of ARPs during uncomplicated TPLOs, but further research is warranted to assess the effect of ARP removal on bone healing and complication rates.

Abstract

OBJECTIVE To assess the contribution of antirotational pins (ARPs) and an intact fibula to the compressive strength of 4 tibial plateau leveling osteotomy (TPLO) constructs (bone and implants).

SAMPLE 20 hind limbs from 10 canine cadavers.

PROCEDURES Each hind limb was assigned to 1 of 4 TPLO constructs (construct in which the ARP was removed, constructs in which 1 or 2 ARPs were left in place, and construct in which the ARP was removed and the fibula was cut). Following TPLO completion, all limbs underwent mechanical testing that included 10,000 cycles of cyclic axial compression followed by testing to failure at a displacement rate of 1 mm/s. Displacement during cyclic testing; load generated at 0.5, 1.0, and 3.0 mm of displacement; ultimate load; and failure type were recorded for each limb. Mean values were compared among the groups.

RESULTS None of the specimens failed during cyclic testing. None of the variables assessed during mechanical testing differed significantly among the 4 groups. During testing to failure, the majority (17/20) of specimens failed as the result of a long oblique fracture through the first screw hole in the distal segment.

CONCLUSIONS AND CLINICAL RELEVANCE Results indicated that the axial compressive strength and stiffness of a TPLO construct were not significantly affected by the addition of 1 or 2 ARPs or the presence of an intact fibula. These findings appear to support removal of ARPs during uncomplicated TPLOs, but further research is warranted to assess the effect of ARP removal on bone healing and complication rates.

Cranial cruciate ligament rupture is one of the most common causes of hind limb lameness in dogs and costs owners > $1 billion on an annual basis.1,2 Repair options are often grouped into extra-articular suture repair methods, such as the lateral fabella suture3 and prosthetic ligament4; tibial osteotomies, such as the TPLO,5 triple tibial osteotomy,6 center of rotation of angulation–based leveling osteotomy,7 and tibial tuberosity advancement8; and intra-articular repair methods, such as the over-the-top procedure9 and allograft placement.10 The TPLO was first described in the early 1990s and has become one of the most common cranial cruciate ligament repair methods performed in large-breed dogs.11,12 The TPLO results in equal, if not better, outcomes, compared with extra-articular suture repair methods.13,14 Similarly, TPLO outcomes and complication rates are comparable to those of other tibial osteotomy repair methods.15–18

Reported overall complication rates associated with TPLO range from 9% to 18.5%, with major complication rates ranging from 1.5% to 6.6%.15,16,19 Major complications associated with TPLO include meniscal injury, infection, tibial crest fracture, tibial shaft fracture, patellar ligament injury, and implant failure. Mechanical weakness can lead to partial loss of proximal segment rotation and development of delayed unions, malunions, or nonunions. The mechanical strength of a repair often relies on proper reduction, proper implant selection, proper implant placement, and maintenance of other natural supportive structures, such as the fibula.20

Although a number of TPLO plates are available, mechanical comparisons have not been performed for all available products,21,22 and selection of TPLO implants is often surgeon and cost dependent. Depending on the size of the patient, implant size is often chosen to optimize the number of screws placed in the proximal segment of the osteotomy. Although subjective, patients that are large, overweight, and overactive often raise concerns about implant failure. Precautions, such as the selection of a broad plate, addition of a second linear plate, use of locking screws (if the plate accepts locking screws), and leaving the ARP in place, can be used.19 Mechanical testing indicates that broad plates and locking plates can withstand higher loads than standard TPLO plates.22

Data are lacking regarding the mechanical benefits of the incorporation of the ARP with the plate fixation of a TPLO. If incorporation of the ARP with the plate fixation increases the strength of the repair, it may be more economical to leave the ARP in place rather than adding locking screws or using a larger implant. Conversely, if the benefits of leaving the ARP in place do not outweigh potential complications, such as pin loosening or breakage, then removal of the ARP should be recommended.

Although fibular fractures are rarely repaired in dogs, the contribution of such fractures to crus mechanics is unknown. In humans, the fibula contributes to the torsional strength of the lower portion of the leg,23 but little is known about its role in veterinary species. Fibular fracture has not been associated with an increase in complications following TPLO in dogs. However, an increase in TPA (ie, loss of leveling, or “rockback”) during healing has been described following TPLO for dogs with concurrent fibular fracture of the ipsilateral limb,20 and the contribution of the fibula to the mechanical properties of TPLO-repaired limbs is unknown. Elucidation of the mechanical importance of the fibula to TPLO-repaired limbs will help clinicians take appropriate actions when that bone is damaged before, during, or after surgery.

The objectives of the study reported here were, first, to assess whether the ARP adds to the compressive strength of a TPLO construct (bone and implants) and whether use of a second ARP further contributes to the compressive strength of the repair and, second, to quantify the contribution of an intact fibula to the TPLO construct in a cadaveric model. We hypothesized that the addition of 1 or 2 ARPs would not significantly add to the ex vivo mechanical axial strength of a TPLO repair and that fracture of the fibula would significantly decrease the compressive strength of a TPLO repair.

Materials and Methods

Sample

The study involved the use of 10 pairs of cadaveric hind limbs from skeletally mature dogs that weighed between 20 and 40 kg. Dogs were euthanized for reasons unrelated to the study, and the hind limbs were obtained in accordance with University of Tennessee institutional guidelines regarding the acquisition and use of cadavers for research purposes. Each hind limb was harvested by means of a midfemoral osteotomy, and then all soft tissues were removed from the limb except for the patellar ligament and the capsule, collateral ligaments, and intra-articular structures of the stifle joint. The specimens were wrapped in gauze soaked with saline (0.9% NaCl) solution and stored at −80°C until tested.

Study design

Each limb was thawed at room temperature (approx 22°C) for 3 hours on the day of testing. Prior to testing, mediolateral and craniocaudal radiographic images were obtained of each limb. The mediolateral images were acquired with the stifle joint at a 90° angle to the tarsus and the digits, tarsus, and stifle joint in contact with the table; the position of the limb was adjusted until radiographic superimposition of the femoral and tibial condyles was achieved as described.24 If a radiographically evident osseous abnormality was detected in any limb, that limb and the contralateral hind limb harvested from the same dog (ie, pair of hind limbs) were excluded from the study. The TPA of each limb was measured on the mediolateral radiographic image as described.25 For a limb to be included in the study, the tibia had to be large enough to accommodate a 3.5-mm 6-hole TPLO platea (Figure 1) while a minimum of a 10-mm craniocaudal dimension of the tibial tuberosity crest was maintained at the level of the distal attachment of the patellar ligament.

Figure 1—
Figure 1—

Photograph of an uncontoured 3.5-mm 6-hole nonlocking TPLO plate used for all 20 canine hind limb specimens evaluated in an ex vivo study to assess the contribution of ARPs and an intact fibula to the compressive strength of 4 TPLO constructs (bone and implants). For each specimen, the plate was contoured to the shape of the proximal aspect of the tibia and secured by the use of 6 nonlocking bicortical screws with the fourth distal screw (first screw in the distal segment) placed in compression fashion. Following completion of the TPLO and prior to mechanical testing, each specimen was evaluated to ensure that the tibial crest had a craniocaudal dimension of ≥ 10 mm; that the TPA was between 0° and 14° and limb alignment had changed < 5°; that the plate was secured to the bone with 6 bicortical screws, each of which was seated within the plate; that there was no visible gap between the plate and bone or at the osteotomy site; that none of the screws crossed the osteotomy site or entered the joint surface; and that no drill holes or screws engaged the fibula.

Citation: American Journal of Veterinary Research 79, 6; 10.2460/ajvr.79.6.621

Limbs deemed appropriate for testing were randomly assigned to 1 of 4 treatment groups by means of a random number generator. The parameters for the random number generator were set to ensure that each group contained an approximately equal number of right and left limbs and that paired limbs were not assigned to the same group. The limbs in group 1 (control group; n = 5) underwent a TPLO construct in which the ARP was removed. The limbs in group 2 (single-pin group; n = 5) underwent a TPLO construct in which a single ARP was left in place. The limbs in group 3 (double-pin group; n = 5) underwent a TPLO construct in which a second ARP was placed up to 0.5 cm distal and parallel to the first ARP, and both pins were left in place. The limbs in group 4 (cut-fibula group; n = 5) underwent a TPLO construct in which the ARP was removed and the fibula was cut with a radial saw where the saw intersected with the fibula as the cut was extended distally beyond the tibial osteotomy site.

TPLO

One board-certified veterinary surgeon (JJB) performed all TPLO constructs by use of a standard TPLO radial osteotomy procedure and a 24-mm radial saw.b The proximal segment of the tibia was rotated to a target TPA of 5°,26 and that angle was held in rotation with a 0.062-in Kirschner wire. Any visible osteotomy gap was eliminated before pin placement. The pin was placed within 0.5 cm of the patellar ligament attachment, parallel to the joint surface, across the osteotomy, and through the caudal cortex of the proximal tibial fragment. A 6-hole TPLO plate was contoured to the shape of the proximal tibial fragment and secured with 6 nonlocking bicortical screws with the fourth distal screw placed in compression fashion. For the limbs assigned to the double-pin group, a second ARP was placed up to 0.5 cm distal and parallel to the first ARP.

After completion of the TPLO procedure, orthogonal radiographic images of each limb were obtained and assessed by a board-certified veterinary surgeon (JJB). Each TPLO construct had to meet the following criteria before mechanical testing: a TPA between 0° and 14°, all screws engaging both cortices (ie, bicortical screws), no screw crossing the osteotomy or entering the stifle joint surface, no drill holes observed in and no screws engaging the fibula, no visible gap between the tibia and TPLO plate, all screws seated into the plate, a change in limb alignment of < 5°, no visible osteotomy gap, and a tibial crest ≥ 10 mm. Additionally, for the limbs in the single-pin and double-pin groups, the ARPs had to engage 2 cortices, cross the osteotomy site once, and be within 10° parallel to the stifle joint surface (and each other for limbs in the double-pin group).

Mechanical testing

After radiographic images were acquired, the limb specimens were stripped of all remaining soft tissues, the femur was disarticulated, and a mid-diaphyseal tibial osteotomy was performed so that the cut end of the tibia could be potted in a quick-setting casting resinc with at least 5 to 10 mm between the distal end of the TPLO plate and the potting material. Each specimen was mounted in a hydraulic testing machined and axially loaded such that a compressive load was applied through the joint surface of the proximal segment. Initially, each specimen underwent 10,000 cycles of cyclic testing at a force equal to 40% of the body weight of the cadaver from which it was obtained. Then, the specimen was tested to failure at a displacement rate of 1 mm/s. Each mechanical test was video recorded to assess bone or implant movement or failure. Following completion of mechanical testing, each specimen was dissected to determine the mode of failure.

Data analysis

For each of the 4 groups, outcomes of interest during cyclic testing included the number of failures, modes of those failures, and mean extent of axial displacement (ie, shortening of the specimen along the long axis of the bone), and outcomes of interest during testing to failure included the mean loads at 0.5, 1.0, and 3.0 mm of displacement as well as the mean ultimate failure load and stiffness and mode of failure. Mean axial displacement during cyclic testing; loads at 0.5, 1.0, and 3.0 mm of displacement during testing to failure; extent of displacement and load at ultimate failure; and preoperative and postoperative TPAs were compared among the 4 groups by means of 1-way multivariate ANOVA. All analyses were performed with statistical software,e and values of P < 0.05 were considered significant.

Results

For 1 limb during performance of the TPLO construct, the craniocaudal dimension of the tibial tuberosity crest was < 10 mm at the level of the distal attachment of the patellar ligament after the radial cut was made. That limb and the contralateral hind limb from the same cadaver were excluded from the study and replaced with another pair of hind limbs. For all 20 study limbs, the mean ± SD preoperative TPA was 26.6 ± 1.8°, and postoperative TPA was 8.2 ± 1.9°. The mean preoperative TPA (P = 0.86) and postoperative TPA (P = 0.20) did not differ significantly among the 4 groups.

None of the limbs failed during cyclic testing. The mean ± SD axial displacement during cyclic testing was 0.54 ± 0.19 mm for the control group, 0.43 ± 0.15 mm for the single-pin group, 0.37 ± 0.13 mm for the double-pin group, and 0.40 ± 0.23 mm for the cut-fibula group and did not differ significantly (P = 0.35) among the 4 groups.

During testing to failure, the mean load did not differ among the 4 groups at 0.5, 1.0, or 3.0 mm of displacement or at ultimate failure (Figure 2). The mean ± SD stiffness at ultimate failure was 781.7 ± 173.4 N/mm for the control group, 853.2 ± 273.7 N/mm for the single-pin group, 1,054.3 ± 151.7 N/mm for the double-pin group, and 621.6 ± 274.1 N/mm for the cut-fibula group and did not differ significantly among the 4 groups.

Figure 2—
Figure 2—

Mean ± SD load prior to mechanical testing (displacement, 0 mm); at 0.5, 1.0, and 3.0 mm of displacement; and at ultimate failure for canine hind limb specimens that underwent 1 of 4 TPLO constructs (construct in which the ARP was removed [control group; blue line; n = 5], construct in which 1 ARP was left in place [single-pin group; red line; 5], construct in which a second ARP was placed 0.5 cm distal and parallel to the first ARP and both pins were left in place [double-pin group; gray line; 5], and construct in which the ARP was removed and the fibula was cut with a radial saw where the saw intersected the fibula as the cut was extended distally beyond the tibial osteotomy site [cut-fibula group; yellow line; 5]). Following completion of the TPLO, specimens were mounted in a hydraulic testing machine and axially loaded such that a compressive load was applied through the joint surface of the proximal segment. Initially, each specimen underwent 10,000 cycles of cyclic testing at a force equal to 40% of the body weight of the cadaver from which it was obtained. Then, the specimen was tested to failure at a displacement rate of 1 mm/s. Mean load did not differ significantly (P < 0.05) among the 4 groups at any displacement.

Citation: American Journal of Veterinary Research 79, 6; 10.2460/ajvr.79.6.621

The most common mode of failure was a long oblique fracture through the first screw hole in the distal segment (n = 17). Two specimens in the single-pin group failed owing to a fracture in the proximal segment, and 1 specimen in the double-pin group failed owing to a fracture in both the proximal and distal segments.

Discussion

In the present study, no significant differences were identified among the 4 TPLO constructs during cyclic testing or testing to failure. Therefore, we accepted (ie, failed to reject) the hypothesis that the addition of 1 or 2 ARPs would not significantly add to the ex vivo axial strength of a TPLO repair and rejected the hypothesis that an intact fibula would significantly increase the strength of a TPLO repair.

Although no significant differences were identified among the 4 groups for any biomechanical variable evaluated in the present study, some general trends were observed. The double-pin construct appeared to be the strongest. The limbs in the double-pin group had the lowest mean displacement during cyclic testing, and during testing to failure, the mean load for the double-pin group was at least 100 and 200 N greater than that for any other group at 0.5 and 1.0 mm of displacement, respectively. The cut-fibula construct appeared to be the weakest, as evidenced by the fact that the mean load for that group was lowest at 0.5 and 1.0 mm of displacement during testing to failure. The potential clinical implications of those findings are unknown. However, only 5 limbs were evaluated for each construct, so type II error was possible, and significant differences might have been observed had a larger number of limbs been evaluated for each group.

We used an osteotomy-contact model (ie, no osteotomy gap) for the present study because we believed it was the most clinically relevant model. Other studies21,22 that investigated the compressive strength of TPLO constructs used models with osteotomy gaps of ≥ 3 mm (ie, gap models). Although gap models allow the contribution of the implant to the construct strength to be evaluated independent of bone, they are not clinically relevant because a visible gap at the osteotomy site is likely to be reduced or corrected intraoperatively for clinical patients. Moreover, we did not believe that ARPs would function well if a large gap was present. However, use of a load-sharing model such as the osteotomy-contact model introduced variation not observed in gap models owing to differences in bone quality, bone apposition, and osteotomy compression among limbs. In the present study, bone quality was assessed visibly and radiographically, but more objective assessments of bone quality, such as histologic evaluation and dual x-ray absorptiometry, were not performed. Bone apposition is inherently decreased after a TPLO, and lower degrees of rotation are beneficial for biomechanical stability.27 Given that the mean preoperative and postoperative TPAs did not differ significantly among the 4 groups of the present study and all limbs had a clinically acceptable TPA (0° to 14°)28 prior to mechanical testing, it was unlikely that bone apposition varied significantly among the limbs of the 4 groups. Variation in osteotomy compression was minimized by the elimination of any visible gap before plate application. For all 4 TPLO constructs, axial compression was increased by the application of the first screw in the distal segment (ie, fourth screw overall) in compression. Such an arrangement can shift the osteotomy by approximately 1 mm,29 although variations likely exist for different implants. The compressive load between the proximal and distal segments was not measured; therefore, variability between cortical contact and compression could not be quantified, and variation in compression could have caused differences in the measured compressive strength.

For the limbs evaluated in the present study, cyclic loading simulated most repeatable stresses and strains that occur during physiologic conditions. In dogs, the peak vertical force for a hind limb is approximately 25% to 40% of the animal's body weight when it is walking.30,31 Thus, for a 25-kg dog at a walk, the peak vertical force for the hind limbs would be approximately 60 to 100 N. However, peak vertical force increases as limb velocity increases. In another study32 that involved dogs with body weights that ranged from 24 to 32 kg, the mean peak vertical force for the hind limbs was 214 and 229 N when dogs were trotted at velocities of 1.8 and 2.5 m/s, respectively. For each of the 4 TPLO constructs evaluated in the present study, mechanical test results suggested that the axial motion of a repaired stifle joint would be < 0.5 mm for clinical patients when at a walk. Some of the displacement observed during cyclic testing might have been caused by the specimens settling into the testing apparatus and potting material, but < 0.02 mm of displacement was observed during the first 20 cycles for most specimens.

When dogs are playing or exercising vigorously, the hind limbs may incur forces upward of 400 to 600 N.33 A limitation of the present study was that the maximum load during cyclic testing was set at 40% of the cadaver's body weight, and the specimens were exposed to high loads only during testing to failure.34 The specimens evaluated in the present study were tested to failure at a displacement rate of 1 mm/s, which, although slower than the physiologic displacement rate, allowed for visual assessment of the mode of failure.35 The cut-fibula construct may allow > 1 mm of axial displacement during vigorous activity, which could increase the risk for complications and may explain the loss of TPA observed for TPLO-repaired joints when a fractured fibula is present.20 Results of the present study indicated that all 4 TPLO constructs evaluated had the capacity of limiting axial displacement to ≤ 3 mm during a single vigorous event. The amount of motion tolerable at a TPLO osteotomy site that will still allow healing is unknown, but the results of this study suggested that restricted motion is likely to be important, especially in patients with a fibular fracture.

In the present study, the mean ± SD postoperative TPA was 8.2 ± 1.9° even though the target postoperative TPA was 5°. The difference between the target and actual postoperative TPA might have been caused by inaccuracy in the radiographic measurements and intraoperative rotational measurements and surgeon error. The variation for postoperative TPA was small, which suggested that the cause of the discrepancy was consistent among limbs. The postoperative TPAs achieved for the limbs evaluated in this study would likely be acceptable for clinical patients because results of another study28 indicate that the clinical outcome is similar for patients with postoperative TPAs of 0° to 14°.

In 2 previous studies,21,22 a synthetic bone-gap model was used to evaluate TPLO constructs. In the Bordelon et al22 study, the strength of 4 TPLO plates was assessed, and the mean stiffness at failure for nonlocking and locking plates was 3,432 and 3,222 N/mm, respectively, which was substantially greater than that observed for each of the 4 TPLO constructs evaluated in the present study. Conversely, in the Kloc et al21 study, 3 TPLO plates (2 nonlocking and 1 locking) were evaluated with a tibia-gap model, and the mean stiffness at failure was 258 and 383 N/mm for the nonlocking plates and 486 N/mm for the locking plate, which was substantially less than that observed for the constructs of the present study. The discrepancies in the stiffness of the TPLO constructs of those 2 studies21,22 and the present study were likely caused by differences in bone quality and loading parameters as well as the extent of plate contouring. Consequently, a direct comparison of the results of the present study with the findings of those 2 studies21,22 would be invalid, and additional studies are necessary to compare the strength among TPLO constructs that use locking plates and those that use nonlocking plates with ARPs.

To our knowledge, the contribution of the fibula to the mechanical crus strength of dogs has not been elucidated. It is generally accepted that an intact fibula contributes to the overall stability of a TPLO repair, although that contribution has not been quantified. Furthermore, it is unknown whether a fibular fracture should be repaired if fractured during a TPLO procedure or another crus fracture repair. The load-versus-deformation curves generated for the 4 TPLO constructs evaluated in the present study revealed that, early on, an intact fibula generated higher loads than a cut or fractured fibula at displacements up to 1 mm, but that load difference was not significant. As the displacement increased > 1 mm, the cut-fibula construct appeared to be stronger than the control (intact fibula) construct. We suspect that when displacement exceeded 1 mm, there was compression of the entire construct, and the tibial segments were fully engaged in load sharing, which overshadowed or masked the role of the fibula. In human patients, the mechanical rotational strength of the crus decreases by 5% when the fibula is cut and by 11% when it is removed,23 which is a significant change. In dogs, the rotational strength of isolated fibulas has been assessed.36 To our knowledge, the bending or axial strength of the canine fibula has not been evaluated, and further studies are warranted to assess the contribution of the fibula to the bending and rotation of the hind limbs of dogs.

The present study had some limitations. Mechanical testing was performed in only 1 direction, and outcome was measured only as load and axial displacement. Although the osteotomy-contact model used minimized variation, it did not allow us to comment on the ability of the TPLO constructs to counteract bending and rotational forces. We also did not measure derotation, another clinically relevant variable. Only 1 type of plate was evaluated. The grade of stainless steel used for the plate and how it was manufactured (eg, cold working vs casting) can substantially affect implant strength21; therefore, further studies are necessary to compare the outcome for stifle joints repaired with different TPLO plates with and without ARPs or an intact fibula. The hind limbs evaluated in the present study were harvested from 10 canine cadavers, and the bone quality may have varied among the specimens. That variability could have been minimized by the use of synthetic bone, which has mechanical properties similar to that of canine cancellous bone.37 However, because of the complex 3-D shape of the proximal aspect of the tibia, such as its triangular shape, narrowing of the epiphysis to metaphysis, and subtle surface irregularities, cadaver bones were considered a better model. Future studies may involve the use of 3-D printed bone models that would minimize or eliminate the variability associated with cadaver bones. Finally, we did not evaluate a TPLO construct that included 1 or 2 ARPs in addition to a cut fibula.

Results of the present ex vivo study indicated that the compressive strength of a TPLO construct with a nonlocking plate was not significantly affected by the addition of 1 or 2 ARPs. Additionally, the fibula did not significantly contribute to the axial compressive strength or stiffness of the TPLO constructs evaluated in this study. These findings can be used to help support the decision to remove ARPs during uncomplicated TPLO repairs, although further studies are warranted to assess the effect of ARP removal on bone healing and complication rates.

Acknowledgments

Supported by the Companion Animal Fund at the University of Tennessee College of Veterinary Medicine.

ABBREVIATIONS

ARP

Antirotational pin

TPA

Tibial plateau angle

TPLO

Tibial plateau leveling osteotomy

Footnotes

a.

Unity cruciate plate, New Generation Devices, Glen Rock, NJ.

b.

Crescentic osteotomy saw blade, New Generation Devices, Glen Rock, NJ.

c.

Smooth-Cast 300Q, Smooth-On Inc, Macungie, Pa.

d.

ElectroPuls E1000, Instron Norwood, Mass.

e.

SAS, version 9.4, SAS Institute Inc, Cary, NC.

References

  • 1. Johnson JA, Austin C, Breur GJ. Incidence of canine appendicular musculoskeletal disorders in 16 veterinary teaching hospitals from 1980 to 1989. Vet Comp Orthop Traumatol 1994;7:5659.

    • Search Google Scholar
    • Export Citation
  • 2. Wilke VL, Robinson DA, Evans RB, et al. Estimate of the annual economic impact of treatment of cranial cruciate ligament injury in dogs in the United States. J Am Vet Med Assoc 2005;227:16041607.

    • Search Google Scholar
    • Export Citation
  • 3. Tonks CA, Lewis DD, Pozzi A. A review of extra-articular prosthetic stabilization of the cranial cruciate ligament–deficient stifle. Vet Comp Orthop Traumatol 2011;24:167177.

    • Search Google Scholar
    • Export Citation
  • 4. Cook JL, Luther JK, Beetem J, et al. Clinical comparison of a novel extracapsular stabilization procedure and tibial plateau leveling osteotomy for treatment of cranial cruciate ligament deficiency in dogs. Vet Surg 2010;39:315323.

    • Search Google Scholar
    • Export Citation
  • 5. Kim SE, Pozzi A, Kowaleski MP, et al. Tibial osteotomies for cranial cruciate ligament insufficiency in dogs. Vet Surg 2008;37:111125.

  • 6. Bruce WJ, Rose A, Tuke J, et al. Evaluation of triple tibial osteotomy. A new technique for the management of the canine cruciate-deficient stifle. Vet Comp Orthop Traumatol 2007;20:159168.

    • Search Google Scholar
    • Export Citation
  • 7. Raske M, Hulse D, Beale B, et al. Stabilization of the CORA based leveling osteotomy for treatment of cranial cruciate ligament injury using a bone plate augmented with a headless compression screw. Vet Surg 2013;42:759764.

    • Search Google Scholar
    • Export Citation
  • 8. Hoffmann DE, Miller JM, Ober CP, et al. Tibial tuberosity advancement in 65 canine stifles. Vet Comp Orthop Traumatol 2006;19:219227.

  • 9. Arnoczky SP, Tarvin GP, Marshall JL, et al. The over-the-top procedure: a technique for anterior cruciate ligament substitution in the dog. J Am Anim Hosp Assoc 1979;15:283290.

    • Search Google Scholar
    • Export Citation
  • 10. Biskup JJ, Balogh DG, Scott RM, et al. Long-term outcome of intra-articular allograft technique for treatment of spontaneous cranial cruciate ligament rupture in the dog. Vet Surg 2017;46:691699.

    • Search Google Scholar
    • Export Citation
  • 11. Slocum B, Slocum TD. Tibial plateau leveling osteotomy for repair of cranial cruciate ligament rupture in the canine. Vet Clin North Am Small Anim Pract 1993;23:777795.

    • Search Google Scholar
    • Export Citation
  • 12. Duerr FM, Martin KW, Rishniw M, et al. Treatment of canine cranial cruciate ligament disease. A survey of ACVS diplomates and primary care veterinarians. Vet Comp Orthop Traumatol 2014;27:478483.

    • Search Google Scholar
    • Export Citation
  • 13. Gordon-Evans WJ, Griffon DJ, Bubb C, et al. Comparison of lateral fabellar suture and tibial plateau leveling osteotomy techniques for treatment of dogs with cranial cruciate ligament disease. J Am Vet Med Assoc 2013;243:675680.

    • Search Google Scholar
    • Export Citation
  • 14. Mölsä SH, Hielm-Björkman AK, Laitinen-Vapaavuori OM. Use of an owner questionnaire to evaluate long-term surgical outcome and chronic pain after cranial cruciate ligament repair in dogs: 253 cases (2004–2006). J Am Vet Med Assoc 2013;243:689695.

    • Search Google Scholar
    • Export Citation
  • 15. Christopher SA, Beetem J, Cook JL. Comparison of long-term outcomes associated with three surgical techniques for treatment of cranial cruciate ligament disease in dogs. Vet Surg 2013;42:329334.

    • Search Google Scholar
    • Export Citation
  • 16. Coletti TJ, Anderson M, Gorse MJ. Complications associated with tibial plateau osteotomy: a retrospective of 1,519 procedures. Can Vet J 2014;55:249254.

    • Search Google Scholar
    • Export Citation
  • 17. Krotscheck U, Nelson SA, Todhunter RJ, et al. Long term functional outcome of tibial tuberosity advancement vs. tibial plateau leveling osteotomy and extracapsular repair in a heterogeneous population of dogs. Vet Surg 2016;45:261268.

    • Search Google Scholar
    • Export Citation
  • 18. Boudrieau RJ. Tibial plateau leveling osteotomy or tibial tuberosity advancement? Vet Surg 2009;38:122.

  • 19. Fitzpatrick N, Solano MA. Predictive variables for complications after TPLO with stifle inspection by arthrotomy in 1,000 consecutive dogs. Vet Surg 2010;39:460474.

    • Search Google Scholar
    • Export Citation
  • 20. Taylor J, Langenbach A, Marcellin-Little DJ. Risk factors for fibular fracture after TPLO. Vet Surg 2011;40:687693.

  • 21. Kloc PA, Kowaleski MP, Litsky AS, et al. Biomechanical comparison of two alternative tibial plateau leveling osteotomy plates with the original standard in an axially loaded gap model: an in vitro study. Vet Surg 2009;38:4048.

    • Search Google Scholar
    • Export Citation
  • 22. Bordelon J, Coker D, Payton M, et al. An in vitro mechanical comparison of tibial plateau levelling osteotomy plates. Vet Comp Orthop Traumatol 2009;22:467472.

    • Search Google Scholar
    • Export Citation
  • 23. Thambyah A, Pereira BP. Mechanical contribution of the fibula to torsion stiffness in the lower extremity. Clin Anat 2006;19:615620.

    • Search Google Scholar
    • Export Citation
  • 24. Reif U, Dejardin LM, Probst CW, et al. Influence of limb positioning and measurement method on the magnitude of the tibial plateau angle. Vet Surg 2004;33:368375.

    • Search Google Scholar
    • Export Citation
  • 25. Baroni E, Matthias RR, Marcellin-Little DJ, et al. Comparison of radiographic assessments of the tibial plateau slope in dogs. Am J Vet Res 2003;64:586589.

    • Search Google Scholar
    • Export Citation
  • 26. Slocum B, Slocum T. Tibial plateau leveling osteotomy for cranial cruciate ligament. In: Bojrab M, Ellison GW, Slocum B, eds. Current techniques in small animal surgery. 4th ed. Baltimore: Williams and Wilkins, 1998;12091215.

    • Search Google Scholar
    • Export Citation
  • 27. Windolf M, Leitner M, Schwieger K, et al. Accuracy of fragment positioning after TPLO and effect on biomechanical stability. Vet Surg 2008;37:366373.

    • Search Google Scholar
    • Export Citation
  • 28. Robinson DA, Mason DR, Evans R, et al. The effect of tibial plateau angle on ground reaction forces 4–17 months after tibial plateau leveling osteotomy in Labrador Retrievers. Vet Surg 2006;35:294299.

    • Search Google Scholar
    • Export Citation
  • 29. Piermattei DL, Flo GL, DeCamp CR. Fractures: classifications, diagnosis and treatment. In: Brinker, Piermattei, and Flo's handbook of small animal orthopedics and fracture repair. 4th ed. St Louis: Saunders Elsevier, 2006;129132.

    • Search Google Scholar
    • Export Citation
  • 30. Butler DL, Hulse DA, Kay MD, et al. Biomechanics of cranial cruciate ligament reconstruction in the dog II. Mechanical properties. Vet Surg 1983;12:112118.

    • Search Google Scholar
    • Export Citation
  • 31. Evans R, Horstman C, Conzemius M. Accuracy and optimization of force platform gait analysis in Labradors with cranial cruciate disease evaluated at a walking gait. Vet Surg 2005;34:445449.

    • Search Google Scholar
    • Export Citation
  • 32. Brady RB, Sidiropoulos AN, Bennett HJ, et al. Evaluation of gait-related variables in lean and obese dogs at a trot. Am J Vet Res 2013;74:757762.

    • Search Google Scholar
    • Export Citation
  • 33. Caporn TM, Roe SC. Biomechanical evaluation of the suitability of monofilament nylon fishing and leader line for extra-articular stabilization of the cranial cruciate-deficient stifle. Vet Comp Orthop Traumatol 1996;9:126133.

    • Search Google Scholar
    • Export Citation
  • 34. Leitner M, Pearce SG, Windolf M, et al. Comparison of locking and conventional screws for maintenance of tibial plateau positioning and biomechanical stability after locking tibial plateau leveling osteotomy plate fixation. Vet Surg 2008;37:357365.

    • Search Google Scholar
    • Export Citation
  • 35. Biskup J, Freeman A, Camisa W, et al. Mechanical properties of canine patella-ligament-tibia segment. Vet Surg 2014;43:136141.

  • 36. Puhl JJ, Piotrowski G, Enneking WF. Biomechanical properties of paired canine fibulas. J Biomech 1972;5:391397.

  • 37. Silbernagel JT, Kennedy SC, Johnson AL, et al. Validation of canine cancellous and cortical polyurethane foam bone models. Vet Comp Orthop Traumatol 2002;4:200204.

    • Search Google Scholar
    • Export Citation

Contributor Notes

Address correspondence to Dr. Biskup (biskupj@oregonstate.edu).