Osteochondritis dissecans of the knee.

Osteochondritis dissecans is a separation of an articular cartilage subchondral bone segment from the remaining articular surface. This is a different entity from osteonecrosis, with which osteochondritis dissecans is commonly confused. In osteochondritis dissecans, the fragment separates from a vascular normal bony bed, while in osteonecrosis, the fragment typically is more peripheral and separates from an avascular bony bed. Osteochondritis dissecans is more common in adolescents and young adults, with the knee, elbow, and ankle being the most common sites. Injury to an area of fairly tenuous blood supply is the most likely cause. Treatment is typically nonoperative for stable lesions and operative for unstable lesions. Most patients do well with no long-term sequelae, but this depends on a variety of factors including the location and size of the lesion, patient age, and treatment.