Biomechanical Rationale and Rational Planning for TPLO

Introduction: TPLO has been a main driver behind the progress of modern veterinary orthopedics. However, the rationale for TPLO planning has seen little scrutiny. In Slocum’s TPLO force diagram, muscle forces on the tibia are omitted. With muscle forces considered, the stifle joint force on the tibia is approximately parallel to the patellar tendon and in most cases, due to co-contraction, inclined even more cranially. The inherent error in TPLO planning due to the ‘mechanical axis’ concept is ~10 degrees, but outcomes have proved satisfactory. Introduction of the 5-degree optimal tibial plateau angle (TPA) has partially compensated for this error, but there is a geometrical cause that— when understood—can provide an objective foundation for planning.

Materials and Methods: Radiographs of 20 stifle joints in extension were used to determine and compare angles by the conventional method and by the rational method that accounts for convexity of both tibial and femoral condyles.

Results: TPLO correction conventionally measured resulted in 2.9 degrees average over correction. Standard deviation of the difference was 3.9; the maximum overcorrection was 8.4 and the maximum under-correction was 5.5 degrees.

Discussion/Conclusion: Two errors in the conventional TPLO planning—one in the presumed direction of the joint force and the other in treating the tibial condyles as a planar surface instead as convex—cancel each other—accidentally and partially. While the average error of conventional planning is relatively modest, individual over or under corrections could have significant clinical consequences.

Acknowledgement: There was no proprietary interest or funding provided.