Valgus instability as a cause for recurrent lateral patellar dislocation: a new mechanism for patellofemoral instability?

An association between lateral patellar dislocation (LPD) and medial collateral ligament injury (MCL) has recently been demonstrated on MRI. The same valgus injury that disrupts the femoral MCL insertion can also cause a simultaneous MPFL tear at its femoral insertion, due to the close anatomical relationship of both ligamentous structures. Valgus laxity due to MCL-deficiency increases the Q-angle and further adversely affects patellar stability. A knee diagnosed with a severe MCL tear, should be evaluated for patellofemoral instability, as a temporary patella dislocation could have been caused by the same injury and an acute LPD indicating a tear of the MPFL can be associated with a concomitant injury of the MCL. In case of recurrent symptoms of patellofemoral instability in a MCL-deficient knee, both the MPFL and MCL should be managed operatively. An isolated MPFL reconstruction in this knee is likely to fail due to a persistent increase of the Q-angle.