Patellar Dislocation
Lateral displacement of the patella from the trochlear groove, usually after a twisting injury, with rupture of the medial patellofemoral ligament.
Overview
Acute lateral patellar dislocation typically occurs when a valgus and external rotation force is applied to a flexing knee. The medial patellofemoral ligament (MPFL), the primary restraint against lateral translation, is disrupted in over 90 percent of first-time dislocations. Anatomical risk factors (trochlear dysplasia, patella alta, increased TT-TG distance, valgus alignment) predispose to recurrence.
Epidemiology
Incidence is approximately 5 to 7 per 100,000 overall, rising to 29 per 100,000 in the 10 to 17 year age band. Adolescent females are disproportionately affected. After a first episode, recurrence rates range from 15 to 44 percent and are higher in the skeletally immature and in those with anatomical risk factors.
Symptoms
Patients describe the patella going out of place, often with spontaneous relocation on knee extension. Immediate haemarthrosis, medial tenderness, apprehension on lateral translation, and a positive J-sign on active extension are typical. Osteochondral fragments produce mechanical symptoms.