Introduction: The patellar tendon angle (PTA), describing the relationship of the patellar tendon to the tibial plateau, is biomechanically significant for canine stifle stability. The crossover point, at which the cranial cruciate ligament (CrCL) becomes the primary stifle stabilizer, has been posited to occur at a PTA of 90° (Montavon et al. (2002), Apelt et al. (2007) and Tepic (2013)). This PTA occurs at a joint angle of 90° (Dennler et al. (2006)). In the absence of the CrCL, the medial meniscus is presumed to act as a stifle stabilizer. The influence of the medial meniscus on the crossover point and cranial tibial subluxation (CTS) in the CrCL deficient canine stifle has not been described previously.The present study evaluated CTS, PTA and joint angle fluoroscopically during stifle movement from flexion to full extension in a cadaveric biomechanical model.

Materials and methods: Sagittal stifle stability was evaluated in right pelvic limbs from canine cadavers under three different test situations (intact, CrCL deficient and medial meniscal release (MMR)). Dogs were euthanized for reasons unrelated to the study, owner approval given for research use and institutional ethical approval obtained. Stifles showing preexisting signs of CrCL disease by palpation, arthrotomy or x-ray were excluded. Limbs were mounted on a custom-made frame, which permitted controlled movement of the tibia relative to the femur, and loaded to simulate constant quadriceps and gastrocnemius forces (147N and 49N respectively). Metal beads were inserted at the CrCL attachment sites to act as markers for measurement of CTS. Fluoroscopic recordings were obtained as stifles were fully extended from full flexion. Following recording of the intact stifle, the CrCL was sectioned and the recording repeated. Finally, MMR was performed prior to a third recording.Measurements of CTS, PTA and joint angle were made by two observers. Measurement reliability within and between observers was assessed using intraclass correlation coefficients (ICC).

Results: Nine stifles were included in the study, with a mean tibial plateau angle of 29° (± 2°).In intact stifles, marker separation increased 3.7 mm (± 1.2 mm) from flexion to full extension: the majority of this , 2.3 mm (± 0.7 mm), occurring above a PTA of 90°.In CrCL deficient stifles, significant (p<0.05) CTS occurred at joint angles between 95° and 100°, corresponding to an intact stifle PTA between 95.5° and 97°. The mean maximal CTS in CrCL deficient stifles was 52.8% of intact marker separation distance, occurring at an average joint angle of 130°, equivalent to an intact PTA of 109°.After MMR, CTS initiated significantly earlier in extension, at a joint angle of 65°, corresponding to an intact PTA of 86°.The PTA in intact stifles was linearly related to joint angle. Due to CTS, PTA increased less rapidly with increasing joint angle in the CrCL deficient and MMR tests. Measurement reliability for CTS and PTA was good, with ICC values for intra- and interobserver agreements >96%.

Conclusion: The model used in this study has advantages over previous static studies of being able to rapidly test multiple joint angles under constant quadriceps and gastrocnemius loads. We demonstrated that statistically significant CTS in the CrCL deficient stifle initiated at a PTA of about 95° and a joint angle of about 95°, supporting current theoretical evidence. The maximal CTS in the CrCL deficient stifle occurred at a joint angle of 130°, corroborating findings by Kanno et al. (2014). MMR resulted in an earlier initiation of CTS, suggesting a stabilizing role for the medial meniscus, but did not cause a significant change in the maximal amount of CTS.