Biomechanics of patellofemoral joint prostheses.

We describe the biomechanics of the patellofemoral joint and of patellofemoral joint prostheses. It seemed that the designs and associated instruments that were studied had little research and development and that soft tissue realignment procedures often were needed because of pre-existing pathology. Factors that help these unicompartmental implants to integrate with the surrounding knee structures are discussed, particularly articular geometry, contact areas, forces and fixation. The desirability of patellar area contact geometry rather than axisymmetric buttons with point or line contact is discussed, in relation to their use in younger more active patients. Bone preservation relating to revision to total knee arthroplasty also is considered. Data on the articular geometry, stability and tracking of the designs studied are presented, showing that although most parameters are returned within normal limits, problems such as erratic subluxation-reduction effects may occur if the patellar component catches on the edge of the femoral component during knee flexion-extension. Through our results, we suggest that further implant and instrumentation research and development are needed before clinical trials leading to widespread use.