Pediatric Patellar Dislocation

Etiology:
— Trauma resulting in lateral displacement followed by dislocation of patella due to disruptive changes to medial patellar retinaculum
— Often due to sports injury
Imaging Radiograph: Rarely imaged acutely and usually imaged post reduction
— Pre reduction: Lateral displacement of patella on Merchant view
— Post reduction: Medial soft tissue swelling, small avulsion fragments (sliver sign), joint effusion, hemarthrosis can be seen as fluid-fluid level, often associated with femoral trochlear dysphasia which appears flattened on sunrise view
DDX:
Complications:
Treatment: Often spontaneously reduces
Clinical: Relatively common

Radiology Cases of Patellar Dislocation

Radiograph of lateral patellar dislocation — AP (above) and Merchant (below left) radiographs of the right knee show lateral displacement of the patella and a small avulsion fragment (sliver sign) medial to the patella. Merchant radiograph of the right knee post reduction (below right) shows appropriate reduction of the patella along with medial soft tissue swelling.

Radiograph of patellar dislocation — AP (above left) and lateral (above right) radiographs of the knee show the patella to be dislocated laterally and posteriorly and there is a small knee effusion. AP (below left) and lateral (below right) radiographs of the knee obtained post reduction show the patella now to be in its correct position.

Radiograph of patellar dislocation — AP radiograph of the knee (left) shows lateral dislocation of the patella which is confirmed on the Merchant view (right). The lateral view (center) also shows the patella is not in its normal location.