Ex Vivo Biomechanical Evaluation of the Canine Cranial Cruciate Ligament Deficient Stifle with Varying Angles of Stifle Joint Flexion and Axial Loads After Tibial Tuberosity Advancement

The tibial tuberosity advancement (TTA) procedure was designed to stabilize the cranial cruciate ligament (CrCL) deficient stifle by advancing the insertion of the patellar tendon. This neutralizes cranial tibial thrust force (CrTT) and prevents cranial tibial subluxation (CTS) during weight bearing. Previous biomechanical investigations have proven that TTA prevents tibial subluxation at a standing angle of 135° stifle extension with a 30% body weight axial load applied. No one has investigated the effects of larger stifle angles and axial loads as well as the effects of TTA upon retropatellar force (RPF), patellar tendon load (PTL) and patellar tendon angle (PTA).

The purpose of this study was to evaluate the effect of varying stifle angulation and axial loads in the CrCL deficient stifle after TTA upon CrTT, RPF, PTL, and patellar PTA as determined by the tibial plateau angle method (PTA^TPA) and the common tangent method (PTA^CT). We hypothesized that TTA would neutralize CrTT at both stifle angles and loads applied. Also, RPF and PTL will be decreased with TTA. We hypothesized that mean PTA^TPA and PTA^CT would be similar. Furthermore, we hypothesized that mean PTA^TPA and PTA^CT would be equivalent to 90° in all groups and that PTA^CT would be less variable than PTA^TPA.

Stifle joints were subjected to differing loading conditions using a constrained limb press model. The CrCL was transected in situ under load. Force sensors allowed direct measurement of CrTT, RPF and PTL. Lateral radiographs were used to assess the PTA^TPA and PTA^CT. Descriptive statistics were used to report CrTT (this force returned to zero point values, defined as the neutral point advancement - NPA). At the NPA, RPF was compared to baseline using a two-tailed sign test. Pair-wise comparisons of PTL as well as between PTA^TPA and PTA^CT were performed utilizing paired t-tests. Comparisons between loading conditions were made with a one-way ANOVA and post hoc Tukey’s test. Equivalence tests were used to test mean PTA^TPA and PTA^CT for equivalence to 90°. Significance was set at a p-value of 0.05.

CrTT returned to baseline values, and RPF and PTL were reduced in all specimens with TTA. At the NPA, PTA^TPA > PTA^CT in two of the three loading conditions, but did not differ in the third. Mean PTA^TPA and PTA^CT varied between loading conditions. The threshold for each of the groups, at which the PTA could be significantly different from 90°, was larger for PTA^TPA than PTA^CT in all groups, as greater variation was observed with PTA^TPA versus PTA^CT.

This study further supports the claim that reduction of CrTT occurs after TTA in the CrCL deficient stifle joint through an alteration of PTA. Additionally, RPF and PTL also decrease after TTA. The PTA^CT may be more accurate than PTA^TPA at reflecting the true PTA.

TTA can stabilize the CrCL deficient stifle joint while effectively reducing RPF and PTL. The PTA^CT method appears to be less influenced by stifle joint flexion angle than PTA^TPA, and may be the preferred technique for preoperative planning.