Traumatic knee injury

A 44 year old man injured his left knee while walking his dog in the park. He collided with two dogs who were chasing a ball; the main impact was on the anteromedial aspect of the left knee. Immediately after the injury he was in intense pain, the knee was swollen, and he was unable to bear weight. At presentation to the emergency department, examination showed a large effusion at the knee joint and tenderness over the anteromedial aspect of the joint. The range of movement at the knee was 30-60° and the knee was locked.

Findings on anterior and posterior draw testing were normal and valgus and varus stress testing did not identify ligamentous laxity. Lateral gliding of the patella caused considerable discomfort. Initially, he underwent radiography of the knee (figs 1⇓ and 2⇓). Subsequent magnetic resonance imaging (MRI) showed a classic pattern of bone marrow oedema along the medial patella facet and lateral femoral condyle, which is typical of a transient dislocation of the patella.