Anatomy and biomechanics of patellar instability

Patellar stability is governed by unique anatomy and biomechanics, which greatly influence treatment strategies in the setting of instability. The patella is stabilized by the following 4 factors: lower extremity alignment, articular geometry, dynamic muscular stabilizers, and passive ligamentous stabilizers. Lower extremity malalignment results in increased lateral forces on the patella contributing to instability. Lower extremity malalignment can also result dynamically from poor neuromuscular control. The lateral trochlear geometry resists lateral translation and when hypoplastic contributes to instability. The primary muscular stabilizer of the patella is the vastus medialis obliquus. It inserts on the patella at an oblique angle relative to the anatomic axis of the femur, and stabilizes the patella with medial and posterior forces. The medial patellofemoral ligament is the primary ligamentous stabilizer of the patella and guides it into the trochlear groove during early knee flexion. The medial patellofemoral ligament is injured in nearly all cases of patellofemoral dislocation and is considered the essential lesion in recurrent patellofemoral instability. The anatomy and biomechanics of patellar instability provide a framework to properly evaluate and treat patellar instability.