aka Osteochondritis dessicans
Results directly from trauma (repetitive microtrauma) or secondarily from loss of blood supply to an area of subchondral bone, resulting in avascular necrosis. As the necrotic bone is resorbed, the overlying cartilage looses its support. Without its cartilage cover, the bony fragment may become dislodged into the joint.
Occurs in the knee in 75% of cases (lateral aspect of the medial femoral condyle (75%)). M>F 3:1, Average age 10-20, bilateral in 30-40%.
- Plain Film: lucencies in the articular epiphysis (which may have a well-demarcated rim of sclerosis) or loss of sharp cortical line of the articular surface. Should be evaluated for stability by MRI.
- CT: island of subchondral bone demarcated by a rarefied zone.
- MRI: most accurate method for staging lesions (short of arthroscopy).
- I. Marrow edema (stable).
- II. Articular cartilage is breached. Low-signal rim surrounding fragment indicates fibrous attachment(stable).
- III. Pockets of fluid around undetached and undisplaced osteochondral fragment (unstable).
- IV. Displaced osteochondral fragment (unstable).
Conservative treatment (rehabilitation) is recommended for stable (stages I and II) lesions. If symptoms worsen, arthroscopy is recommended to evaluate stability of the lesion.
- Spontaneous Osteonecrosis of the Knee (SONK) - insidious, non-tramautic, weight bearning surface of medial femoral condyle, older patients