Osteochondritis dissecans (OCD) is a disease process where a segment of the articular cartilage and the underlying subchondral bone begins to separate from the osteocartilagenous tissue. Many factors are believed to be involved in the development of osteochondritis dissecans; including trauma, genetics, and prolonged ischemia. Direct trauma could create a transchondral fracture; however, the predilection of OCD for the posterolateral portion of the medial femoral condyle suggests indirect trauma as a more likely cause. In the knee, OCD occurs in the medial femoral condyle 75% of the time.

Osteochondritis dissecans may occur if there is a fracture that extends into the subchondral bone in which revascularization across the fracture line does not properly occur, thus causing nonunion. While ischemia has been well studied as a possible cause of osteochondritis dissecans, Rogers and Gladstone’s recent study places less emphasis on ischemia as a cause. Their publication showed abundant anastomoses to intramedullary cancellous bone at the distal femur.

If left untreated, a lesion will often progress through a series of four stages (Berndt/Harty classification):
   
•   Stage I consists of a small compressed, nondisplaced fragment.
•   Stage II consists of a partially detached osteochondral fragment.
•   Stage III lesions are the most common and consist of a completely detached fragment that is nondisplaced.
•   Stage IV lesions consist of a completely detached fragment that is displaced from the crater bed. This is also termed a loose body.

Symptoms of osteochondritis dissecans may include pain, stiffness, weakness, and a mechanical sensation of popping or clicking. Sometimes, one can feel a loose body sensation if a fragment has separated from a condyle. These symptoms are often intermittent and associated with exertion. Patients should be asked when and how long the pain is occurring. If the pain is constant, one should suspect a loose body in the knee capsule. On physical exam, an effusion and lack of full knee extension on the affected side may often be noted.

Cooper et al. suggest specific criteria for surgical intervention in children, including: symptoms that have persisted for 6-12 months, if radiographic indices predict inadequate healing with conservative measures, if skeletal maturity will occur within 6 months, or if loose bodies are present. Also, if a loose body is found incidentally in a weight bearing joint of an asymptomatic patient, surgery should be considered because these lesions may lead to early degenerative joint disease.