The medial patellar retinaculum is the fibrous expansion on the medial side of the knee joint connecting the medial patellar border to the medial femoral epicondyle and the medial tibial condyle. Its deepest and most important component is the medial patellofemoral ligament (MPFL, from the medial patella to the medial epicondyle saddle point), which provides approximately 50-60% of the lateral restraining force preventing lateral patellar dislocation. The medial patellotibial ligament (MPTL) provides additional inferior restraint.
Primary passive medial restraint preventing lateral patellar translation and dislocation; the MPFL is the primary checkrein against lateral patellar displacement from 0-30 degrees of knee flexion when the patella is not yet engaged in the trochlear groove.
The MPFL (deepest medial retinaculum component) is torn in virtually every first-time lateral patellar dislocation: the medial retinaculum stretches and the MPFL avulses from the medial patellar border or less commonly from the medial epicondylar attachment. Bone bruising at the medial patella and lateral trochlea on MRI is pathognomonic. Recurrent patellar instability from MPFL insufficiency is treated by MPFL reconstruction using a gracilis or semitendinosus autograft from the medial epicondyle saddle point to the medial patella isometric point.
First-time lateral patellar dislocation tears the medial patellofemoral ligament from its patellar attachment, producing medial patellar bone bruising and medial retinaculum haematoma on MRI; recurrence rate after first dislocation is 15-20% in adults but increases to 50% in adolescents, with MPFL reconstruction indicated after the second dislocation or in high-risk anatomy with trochlear dysplasia.
