Standardisation of the description of patellofemoral motion and comparison between different techniques.

Patellofemoral motion is significant clinically, yet in the literature many different methods and terminologies are used, thus making comparison between studies difficult. We review and explain the different methods used for the description of patellofemoral joint motion, compare these methods by experimentation, and propose a standardised method. We found three main methods for describing patellar motion: motion of the patella about femoral body fixed axes, about patellar body fixed axes, and a combination of these. Description about femoral body fixed axes does not make sense clinically. Description about patellar body fixed axes is straightforward, yet the definition of these axes is prone to error due to the lack of anatomical landmarks. The combination method makes most sense clinically and uses more easily found anatomical landmarks. Patellar flexion varied by up to 26% when describing the motion about different axes. Tilt and shift were highly sensitive to the choices of coordinate systems and the axes of motion. The pattern of rotation was consistent between all methods; however, differences between the methods increased with patellar flexion. We propose the description of patello-femoral motion in terms of shift (along a femoral medial-lateral axis), tilt (about the patellar long axis), rotation (about a floating patellar anterior-posterior axis) and flexion (about the femoral medial-lateral axis).