Recurrent Dislocation of the Patella: Definition and Concept
Recurrent dislocation of the patella is a condition in which the patella repeatedly dislocates, usually laterally, from the trochlear groove during knee movement, after a previous episode of patellar dislocation. The dislocation may be complete or may spontaneously reduce, leaving recurrent episodes of subluxation, giving way, apprehension or anterior knee pain. In clinical practice, recurrence generally implies two or more documented episodes of patellar dislocation; however, terminology is not absolutely uniform, and some patients have recurrent instability without a clearly witnessed complete dislocation.
The usual direction is lateral, because the normal lateralizing forces produced by the quadriceps and the valgus alignment of the lower limb are opposed medially by the medial patellofemoral ligament (MPFL), the medial retinaculum and the geometry of the trochlea. The MPFL is the principal soft-tissue restraint to lateral displacement in early knee flexion, particularly from full extension to approximately 30 degrees. With increasing flexion, the patella becomes progressively engaged in the trochlear groove, and bony stability becomes more important.
Recurrent dislocation may follow a single traumatic first dislocation with healing failure of the medial stabilizers, or may result from pre-existing anatomical risk factors. Important factors include trochlear…