What is causing this teenage athlete's recurrent knee pain?

THE CASE: A 16-year-old athlete has had recurrent right knee pain for the past few months. A few months earlier, he had a minor injury to the knee, which caused minimal pain. However, the knee continued to hurt on and off, and he now feels “something inside the knee.” The parents are concerned that the pain will hinder his participation in sports. He has had no knee swelling or redness. He denies pain in any other joint.

On physical examination, he sits comfortably. He has full range of motion of the knee and no crepitation. There is some tenderness on the medial aspect of the femoral condyle. Lachman, anterior drawer, and posterior drawer tests are negative. The knee is stable to varus and valgus stress.

Anteroposterior and tunnel views of the right knee are shown.

What is the cause of this boy’s recurrent knee pain?

•  Fracture of the femoral condyle
•  Osgood-Schlatter disease
•  Osteochondritis dissecans
•  Avulsion fracture of the patella
•  Synovial osteochondromatosis

The correct answer is Osteochondritis dissecans.

Discussion: The radiographs show an osteochondral defect in the lateral aspect of the medial femoral condyle with intraarticular loose bodies (A and B). MRI of the knee, to delineate the extent of the lesion, shows an unstable, stage 4 osteochondral defect that involves the distal medial femoral condyle with at least 3 intra-articular loose bodies (C). In addition, there is small knee joint effusion with changes of mild synovitis.