patellar dislocation

Lateral patellar dislocation refers to lateral displacement followed by dislocation of the patella due to disruptive changes to the medial patellar retinaculum.

Epidemiology

Patellar dislocation accounts for ~3% of all knee injuries and is commonly seen in those individuals who participate in sports activities.

Pathology

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) .

Radiographic features

Plain radiograph
MRI

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 images should be scrutinized for the presence of chondral or osteochondral injury, especially if displaced as an intra-articular body, as this may affect surgical management.

The trochlear groove and patella may have abnormal morphology that predisposes to patellar dislocation.

Differential diagnosis

  • acute ACL tear: no medial patellar contusion in this injury
  • direct trauma to lateral knee: normally no patellar contusion