Osteochondritis dissecans (knee)
Follow-up x-ray of a known knee problem.
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There is flattening and irregularity of the weight-bearing surface of the lateral femoral condyle, consistent with osteochondritis dissecans of the knee. There is subchondral sclerosis, and a lucent area suggesting subchondral cyst formation. X-rays of the other knee (not shown) demonstrated similar changes.
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.
The differential diagnosis includes normal fusing apophysis (painless) or acute osteochondral fracture. In older patients there may be insufficiency fracture or spontaneous osteonecrosis of the knee (SONK).