Internal Rotation, Varus, and Anterior Femoral Component Malalignments Adversely Affect Patellofemoral Joint Kinematics in Patellofemoral Arthroplasty.

Background: Patellofemoral arthroplasty (PFA) is reported to provide nearly normal PF joint kinematics but only with adequate surgical techniques. This study evaluated the effects of various femoral component settings on patellar component biomechanics. Methods: A dynamic musculoskeletal computer simulation analyzed normal knee and standard PFA models, as well as 8 femoral component malposition models: 5° internal or external rotation, 5° valgus or varus, 5° extension or flexion, and 3-mm or 5-mm anterior positioning. Mediolateral patellar translation, lateral patellar tilt, and contact force and stress at the PF joint were measured in each model during gait. Results: The patella in the standard PFA model was shifted up to 5.0 mm laterally near heel off and was tilted up to 3.0° laterally at heel strike compared to the normal knee model. The patella in the external rotation model translated more laterally in the direction of the femoral component setting than in the standard model. However, in the internal rotation and varus alignment models, the patellar lateral shift occurred largely in the opposite direction of the femoral component setting. The patella in most models was tilted in the same direction as the femoral component setting. The PF contact force was increased, especially in the anterior femoral position models, by up to 30 MPa compared with 20 MPa in the standard model. Conclusions: Internal rotation, varus, and anterior femoral component settings during PFA should be avoided to reduce postoperative complications, whereas external rotation might be appropriate only for cases with lateral patellar instability.