Clara S. S. Goh
BVSc, MS, DACVS-SA
Dr. Goh earned her veterinary degree at Sydney University, Australia. She completed her postgraduate specialty training (rotating internship, surgical residency, and surgical oncology fellowship) at Colorado State University, where she currently holds a faculty position in small animal orthopedic surgery. Her clinical and research interests include cruciate disease, hip dysplasia, and minimally invasive surgery (arthroscopy). Dr. Goh has been an invited speaker and instructor at numerous conferences and surgical labs throughout the United States, supporting continuing education of veterinarians from around the world. She is also the current small animal surgery program chair for the North American Veterinary Community (NAVC).Read Articles Written by Clara S. S. Goh
Cranial cruciate ligament (CrCL) disease continues to be a leading cause of hindlimb lameness in dogs. Multiple biomechanical and biological factors are thought to contribute to CrCL injury.1-4
Stifle radiographs are often taken as part of the routine workup of CrCL cases, but most general practitioners do not measure the tibial plateau angle (TPA). This additional step can be performed with only slight modification of the radiographic procedure. Many conservative and surgical treatment options exist for CrCL disease, and knowledge of a patient’s TPA can help determine the most suitable treatment and aid clients in making the best decision for their pet.
Role of the TPA in the Pathogenesis of CrCL Disease
While a direct association between the TPA as an isolated risk factor in CrCL injury is not established,5,6 studies have shown higher mean TPAs in dogs with CrCL disease compared with healthy dogs7,8 and an increased risk of contralateral CrCL injury.9 Mechanically, cranial tibial thrust during loading of the stifle joint increases in magnitude with increasing TPA and can result in repetitive CrCL microtrauma.10-15 Associated inflammatory changes lead to progressive CrCL injury, pain, and osteoarthritis in affected joints.1,2 Overall, an increased TPA is considered a contributing predisposing factor in CrCL disease, along with other conformational abnormalities such as straight stifle angle and narrowed intercondylar notch.1,2
The lateral stifle projection is most useful in diagnosing CrCL injury, although caudocranial radiographs should also be performed for assessment of osteoarthritis and other abnormalities. For the lateral projection, the patient is positioned in lateral recumbency with the contralateral limb pulled forward (BOX 1).
Conventionally, the stifle and tarsus are positioned at 90° of flexion, although this angle has been found to be less important than achieving a straight, well-centered view.16 A true lateral position is achieved when the femoral and tibial condyles are superimposed (FIGURES 2 AND 3).17
Straight positioning of the tibia is particularly important for proximal tibial landmark identification, as CrCL instability can allow rotation across the stifle joint so that the tibia is rotated while the femur is straight (FIGURE 4).
Care should be taken to center the x-ray beam over the stifle joint with collimation to include the entire tibia (FIGURE 3). This avoids image distortion that may affect TPA landmark assessment (FIGURE 2).18 Centering the x-ray beam too low can lead to overestimation, and too high can lead to underestimation, of the TPA.17
Although image calibration (FIGURE 5) does not change the TPA measurement, as the angle will remain the same regardless of image size, it allows the radiographs to be used for potential surgical planning rather than only diagnostic purposes.
Other radiographic findings such as stifle effusion, osteophyte formation, and cranial tibial subluxation can be considered consistent with CrCL disease or injury (FIGURE 6). Severe stifle osteoarthritis with osteophyte formation around tibial condyles can make TPA landmark identification challenging (FIGURE 7).19 Radiographs of the contralateral “normal” stifle may be useful in such cases, as the TPA should be very similar on both sides. Many dogs have early partial CrCL disease with minimal clinical signs on their “normal” side, so this can also be a useful screening tool of prognostic value.
The tibial plateau is the proximal weight-bearing surface of the tibia. The TPA is defined as the angle between the slope of the medial tibial condyle and the line perpendicular to the mechanical axis of the tibia. The mechanical or functional axis of the tibia joins the center of the weight-bearing surfaces and is identified by the midpoint between the medial and lateral intercondylar tubercles (also called intercondylar eminence) and center of the talocrural joint.13
Although different TPA measurement techniques and use of computed tomography and magnetic resonance imaging in evaluation have been described, the most commonly used radiographic method, outlined here, was originally established by Slocum and Devine.13
Draw the mechanical tibial axis (MTA) between the intercondylar eminence proximally (FIGURE 8) and center of the talocrural joint distally (FIGURE 9).
Draw a second line across the slope of the medial tibial condyle (FIGURES 10 AND 11). This is the tibial plateau line. The cranial and caudal points of this line should be equally distant from the intercondylar eminence.
Draw a line perpendicular to the MTA. The TPA is the angle between this line and the tibial plateau (FIGURE 12).
Generally, agreement on radiographic interpretation of CrCL disease is high20; however, inter- and intraobserver variability exists in TPA measurement.19,21 As mentioned, poor stifle positioning, failure to center the beam on the stifle, and severe osteoarthritis can hinder accurate TPA measurement.18,19 Variability in selection of the caudal reference point on the medial tibial condyle, particularly in the vertical direction, can result in significantly different TPA measurements.19,22 Nonetheless, for general practitioners, even a close estimate of the TPA can be of great value in guiding clinical decision making for an individual patient.
FIGURES 13–15 are examples of well-positioned stifle radiographs of dogs with CrCL injury and their TPA measurements. In each, the proximal tibia has been enlarged to highlight TPA landmarks.
Therapeutic Decision Making
The most common choices for CrCL treatment are conservative therapy; extracapsular suture stabilization (ExCap); tibial osteotomy procedures, including tibial plateau leveling osteotomy (TPLO) and center of rotation of angulation (CORA)–based leveling osteotomy (CBLO); and tibial tuberosity advancement (TTA). When guiding clients on the best treatment choice for their pet, the author considers TPA among other factors such as owner goals, pet athleticism, stifle instability, and financial constraints.
Various case series have documented TPAs between 23.5° and 29° in dogs with CrCL disease.5,23-26 However, in a significant number of dogs the TPA exceeds 30°; in some cases, it can exceed 35°. These cases are referred to as high TPA and excessive TPA, respectively.27,28
It is the author’s opinion that dogs with higher-than-average TPAs are poor candidates for conservative therapy and ExCap surgery. Mechanically, a higher TPA puts more stress on the stifle’s natural passive and active stabilizers and makes the ExCap suture more likely to prematurely loosen or fail.29 Recent studies have found that dogs of smaller breeds (e.g., West Highland white terrier) with CrCL disease have higher TPAs than their larger-breed counterparts.8,28,30 This may explain why some small dogs deemed suitable for conservative management or ExCap surgery based on other considerations have failed to achieve a successful outcome with these choices.
When considering TTA surgery, the higher the TPA, the larger the TTA must be to achieve a neutral 90° angle between the tibial plateau and patella tendon.31 The author would not recommend TTA for a patient with a higher-than-average TPA, as this end goal is challenging to achieve with the limited cage implant sizes available.
Although studies comparing CrCL surgical techniques are difficult to interpret due to low case numbers and unique patient populations, TPLO appears to be the more favorable choice for dogs with high to excessive TPAs.27,28,32-34 TPLO appears to be successful in most of these cases as long as the resultant postoperative TPA is below 14°,27 with 6° as the postoperative goal in routine TPLO cases.14 Some dogs with an excessive TPA may be better suited to other tibial osteotomies such as modified cranial closing wedge (CCW)35,36 or a combination of TPLO or CBLO and CCW,37,38 which allow for leveling of the plateau while maintaining a more physiologic resultant tibial conformation (FIGURE 16).
Taking the extra time and effort to measure the TPA in patients with CrCL disease is a valuable additional step. This allows the TPA to be added to the list of considerations used to advise clients on the ideal CrCL treatment option for their pet.
1. Cook JL. Cranial cruciate ligament disease in dogs: biology versus biomechanics. Vet Surg. 2010;39(3):270-277. doi:10.1111/j.1532-950X.2010.00653.x
2. Whitehair JG, Vasseur PB, Willits NH. Epidemiology of cranial cruciate ligament rupture in dogs. JAVMA. 1993;203(7):1016-1019.
3. Duval JM, Budsberg SC, Flo GL, Sammarco JL. Breed, sex, and body weight as risk factors for rupture of the cranial cruciate ligament in young dogs. JAVMA. 1999;215(6):811-814.
4. Doom M, de Bruin T, de Rooster H, van Bree H, Cox E. Immunopathological mechanisms in dogs with rupture of the cranial cruciate ligament. Vet Immunol Immunopathol. 2008;125(1-2):143-161. doi:10.1016/j.vetimm.2008.05.023
5. Reif U, Probst CW. Comparison of tibial plateau angles in normal and cranial cruciate deficient stifles of Labrador retrievers. Vet Surg. 2003;32(4):385-389. doi:10.1053/jvet.2003.50047
6. Venzin C, Howard J, Rytz U, et al. Tibial plateau angles with and without cranial cruciate ligament rupture: comparison between different dog populations and a wolf population. Vet Comp Orthop Traumatol. 2004;17(4):232-236. doi:10.5167/uzh-63115
7. Seo BS, Jeong IS, Piao Z, et al. Measurement of the tibial plateau angle of normal small-breed dogs and the application of the tibial plateau angle in cranial cruciate ligament rupture. J Adv Vet Anim Res. 2020;7(2):220-228. doi:10.5455/javar.2020.g413
8. Aertsens A, Rincon Alvarez J, Poncet CM, Beaufrère H, Ragetly GR. Comparison of the tibia plateau angle between small and large dogs with cranial cruciate ligament disease. Vet Comp Orthop Traumatol. 2015;28(6):385-390. doi:10.3415/VCOT-14-12-0180
9. Muir P, Schwartz Z, Malek S, et al. Contralateral cruciate survival in dogs with unilateral non-contact cranial cruciate ligament rupture. PloS One. 2011;6(10):e25331. doi:10.1371/journal.pone.0025331
10. Warzee CC, Dejardin LM, Arnoczky SP, Perry RL. Effect of tibial plateau leveling on cranial and caudal tibial thrusts in canine cranial cruciate-deficient stifles: an in vitro experimental study. Vet Surg. 2001;30(3):278-286. doi:10.1053/jvet.2001.21400
11. Reif U, Hulse DA, Hauptman JG. Effect of tibial plateau leveling on stability of the canine cranial cruciate-deficient stifle joint: an in vitro study. Vet Surg. 2002;31(2):147-154. doi:10.1053/jvet.2002.31041
12. Fujita Y, Hara Y, Ochi H, et al. The possible role of the tibial plateau angle for the severity of osteoarthritis in dogs with cranial cruciate ligament rupture. J Vet Med Sci. 2006;68(7):675-679. doi:10.1292/jvms.68.675
13. Slocum B, Devine T. Cranial tibial thrust: a primary force in the canine stifle. JAVMA. 1983;183(4):456-459.
14. 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(4):777-795. doi:10.1016/s0195-5616(93)50082-7
15. Janovec J, Kyllar M, Midgley D, Owen M. Conformation of the proximal tibia and cranial cruciate ligament disease in small breed dogs.
Vet Comp Orthop Traumatol. 2017;30(3):178-183. doi:10.3415/VCOT-16-07-0115
16. Aulakh KS, Harper TAM, Lanz OI, Daniel GB, Werre SR. Effect of stifle angle on the magnitude of the tibial plateau angle measurement in dogs with intact and transected cranial cruciate ligament. A cadaveric study. Vet Comp Orthop Traumatol. 2011;24(4):272-278. doi:10.3415/VCOT-10-09-0131
17. Reif U, Dejardin LM, Probst CW, DeCamp CE, Flo GL, Johnson AL. Influence of limb positioning and measurement method on the magnitude of the tibial plateau angle. Vet Surg. 2004;33(4):368-375. doi:10.1111/j.1532-950X.2004.04053.x
18. Grierson J, Sanders M, Guitan J, Pead M. Comparison of anatomical tibial plateau angle versus observer measurement from lateral radiographs in dogs. Vet Comp Orthop Traumatol. 2005;18(4):215-219.
19. Fettig AA, Rand WM, Sato AF, Solano M, McCarthy RJ, Boudrieau RJ. Observer variability of tibial plateau slope measurement in 40 dogs with cranial cruciate ligament-deficient stifle joints. Vet Surg. 2003;32(5):471-478. doi:10.1053/jvet.2003.50054
20. Bogaerts E, Van der Vekens E, Verhoeven G, et al. Intraobserver and interobserver agreement on the radiographical diagnosis of canine cranial cruciate ligament rupture. Vet Rec. 2018;182(17):484. doi:10.1136/vr.104523
21. Baroni E, Matthias RR, Marcellin-Little DJ, Vezzoni A, Stebbins ME. Comparison of radiographic assessments of the tibial plateau slope in dogs. Am J Vet Res. 2003;64(5):586-589. doi:10.2460/ajvr.2003.64.586
22. Ocal MK, Sabanci SS. Effect of anatomic variation in caudal tibial plateau on the tibial plateau angle in dogs: a cadaveric study. J Small Anim Pract. 2013;54(10):537-540. doi:10.1111/jsap.12132
23. Morris E, Lipowitz AJ. Comparison of tibial plateau angles in dogs with and without cranial cruciate ligament injuries. JAVMA. 2001;218(3):363-366. doi:10.2460/javma.2001.218.363
24. Wilke VL, Conzemius MG, Besancon MF, Evans RB, Ritter M. Comparison of tibial plateau angle between clinically normal greyhounds and Labrador retrievers with and without rupture of the cranial cruciate ligament. JAVMA. 2002;221(10):1426-1429. doi:10.2460/javma.2002.221.1426
25. Fox EA, Dycus DL, Leasure CS, Fox HA, Canapp Jr SO. Average tibial plateau angle of 3,922 stifles undergoing surgical stabilization for cranial cruciate ligament rupture. Vet Comp Orthop Traumatol. 2020;33(3):167-173. doi:10.1055/s-0039-3401811
26. Todorović AZ, Lazarević Macanović MV, Mitrović MB, Krstić NE, van Bree HJJ, Gielen IMLV. The role of tibial plateau angle in canine cruciate ligament rupture: a review of the literature. Vet Comp Orthop Traumatol. 2022;35(6):351-361. doi:10.1055/s-0042-1750316
27. Duerr FM, Duncan CG, Savicky RS, Park RD, Egger EL, Palmer RH. Comparison of surgical treatment options for cranial cruciate ligament disease in large-breed dogs with excessive tibial plateau angle. Vet Surg. 2008;37(1):49-62. doi:10.1111/j.1532-950X.2007.00348.x
28. Witte PG, Scott HW. Tibial plateau leveling osteotomy in small breed dogs with high tibial plateau angles using a 4-hole 1.9/2.5 mm locking T-plate. Vet Surg. 2014;43(5):549-557. doi:10.1111/j.1532-950X.2014.12202.x
29. Havig ME, Dyce J, Kowaleski MP, Reynolds LR, Budsberg SC. Relationship of tibial plateau slope to limb function in dogs treated with a lateral suture technique for stabilization of cranial cruciate ligament deficient stifles. Vet Surg 2007;36(3):245-251. doi:10.1111/j.1532-950X.2007.00258.x
30. Macias C, McKee WM, May C. Caudal proximal tibial deformity and cranial cruciate ligament rupture in small-breed dogs. J Small Anim Pract. 2002;43(10):433-438. doi:10.1111/j.1748-5827.2002.tb00009.x
31. Butterworth S, Maddox T. Relationship between tibial plateau angle and tibial tuberosity advancement requirement in 175 dogs with cruciate deficient stifles. In: BSAVA Congress Proceedings 2015. British Small Animal Veterinary Association; 2015:453.
32. Tikekar A, De Vicente F, McCormack A, et al. Retrospective comparison of outcomes following tibial plateau levelling osteotomy and lateral fabello-tibial suture stabilisation of cranial cruciate ligament disease in small dogs with high tibial plateau angles. N Z Vet J. 2022;70(4):218-227. doi:10.1080/00480169.2022.2052992
33. Krotscheck U, Nelson SA, Todhunter RJ, Stone M, Zhang Z. 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(2):261-268. doi:10.1111/vsu.12445
34. Gordon-Evans WJ, Griffon DJ, Bubb C, Knap KM, Sullivan M, Evans RB. Comparison of lateral fabellar suture and tibial plateau leveling osteotomy techniques for treatment of dogs with cranial cruciate ligament disease. JAVMA. 2013;243(5):675-680. doi:10.2460/javma.243.5.675
35. Guénégo L, Vezzoni A, Vezzoni L. Comparison of tibial anatomical-mechanical axis angles and patellar positions between tibial plateau levelling osteotomy (TPLO) and modified cranial closing wedge osteotomy (AMA-based CCWO) for the treatment of cranial cruciate ligament disease in large dogs with tibial plateau slopes greater than 30° and clinically normal Labradors retrievers. BMC Vet Res. 2021;17(1):368. doi:10.1186/s12917-021-03094-3
36. Terreros A, Daye RM. Modified cranial closing wedge osteotomy to treat cranial cruciate ligament deficient stifles with excessive tibial plateau angles: Complications, owner satisfaction, and midterm to long-term outcomes. Vet Surg. 2020;49(6):1109-1117. doi:10.1111/vsu.13431
37. Talaat MB, Kowaleski MP, Boudrieau RJ. Combination tibial plateau leveling osteotomy and cranial closing wedge osteotomy of the tibia for the treatment of cranial cruciate ligament-deficient stifles with excessive tibial plateau angle. Vet Surg. 2006;35(8):729-739. doi:10.1111/j.1532-950X.2006.00217.x
38. Schlag AN, Peycke LE, Hulse DA. Center of rotation of angulation-based leveling osteotomy combined with a coplanar cranial closing wedge ostectomy to manage cranial cruciate ligament insufficiency in dogs with excessive tibial plateau angle. Vet Surg. 2020;49(6):1125-1131. doi:10.1111/vsu.13480