Osteochondritis dissecans typically affects the lateral surface of the medial femoral condyle in adolescent males. The weight-bearing surfaces of the lateral femoral condyle, tibia or patella may also be involved. Spontaneous healing is usual unless there is an unstable fragment. Signs of instability include large size (>1cm), cyst-like lesions beneath a fragment, contrast beneath a fragment on contrast arthrography, and loose body. OCD may be bilateral in 20-25% of cases.