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.