An unusual case of patellar dislocation. A case report and literature review.

CASE REPORT We present a case of a 57 year old female whose patella became locked above a femoral osteophyte. She attended A & E at the Royal Victoria Hospital, Belfast. She had been pushing a chair forward under a table with her left knee when she felt a 'pop' inside it followed by severe pain. She was unable to bend or weightbear on the knee. Her left knee was previously asymptomatic with no recent trauma. She had a right knee arthroscopy and trimming of medial meniscus three years ago. On examination the left knee was locked in full extension. There was a moderate sized effusion and the left patella was positioned higher than its counterpart. There was a prominent vertical sulcus immediately below the inferior pole of the left patella extending towards the tibial tuberosity. On palpation there was pain around the inferior pole of patella and overlying the patellar tendon. She could not actively straight leg raise. Active and passive knee flexion caused severe pain. Examination of collateral and cruciate ligaments was not possible due to inability to flex the knee. The knee lacked the typical boggy swelling of a true patellar tendon rupture but this was the clinical diagnosis. Radiographic investigation demonstrated a high-riding patella on A-P view. The lateral view showed an osteophyte on the inferior margin of the patellar articular surface locked over an osteophyte on the superior aspect of the femoral articular surface (Fig. 1). The patella could no longer track freely downwards, explaining the patient's symptoms. Reduction was easily achieved under intravenous analgesia. The limb was flexed at the hip by supporting the heel causing a relative hyperextension at the knee. The interlocking osteophytes of patellar and femoral articular Fig]1. Lateral radiograph of knee demonstrating interlocking osteophytes of patellar and femoral articular surfaces.