Patellar dislocation accounts for ~3% of all knee injuries and is commonly seen in those individuals who participate in sports activities.
Patellar dislocation most commonly results from a twisting motion, with the knee in flexion and the femur rotating internally on a fixed foot (valgus-flexion-external rotation) .
The following features are noted:
- medial retinacular abnormalities (ranging from strain to complete disruption) with adjacent periligamentous edema and hemorrhage
- lateral displacement of patella (not necessarily seen in transient dislocation)
- medial patellar contusion +/- corresponding lateral femoral condyle contusion
- joint effusion
The presence of an abnormal medial patellar retinaculum should suggest the diagnosis of transient lateral patellar dislocation .
The trochlear groove and patella may have abnormal morphology that predisposes to patellar dislocation.
- acute ACL tear: no medial patellar contusion in this injury
- direct trauma to lateral knee: normally no patellar contusion