Snapshot A 13-year-old boy presents with left knee pain. He reports that he cannot specifically localize where the pain is in the left knee but has noticed it has progressively worsened over the course of 7 months. He also says that knee swelling typically appears after he plays basketball. On physical exam, there is mild swelling on the left knee. With the left knee in the flexed position with pressure over the medial femoral condyle, there is tenderness to palpation. Range of motion of the knee is intact. An anterior-posterior radiograph of the affected knee demonstrates crescent-shaped radiolucency surrounding subchondral bony fragments. Introduction Clinical definition osteonecrosis of subchondral bone Epidemiology incidence rare 15-29 cases per 100,000 patients demographics males > females age 10-20 years of age risk factors African American race repeated trauma and overuse Etiology unknown but may be due to recurring micro-trauma ischemic injury Pathogenesis initial trauma leads to necrosis which places the patient at risk for chondromalacia fracture necrotic bone absorption note that revascularization is required to heal the subchondrial bone when this revascularization is impaired osteochondritis dissecans ensues Prognosis skeletally immature patients are more likely to have a complete recovery than skeletally mature patients patients with less severe lesions are more likely to have a complete recovery Presentation Symptoms knee, elbow, or ankle pain worsens with physical activity stiffness swelling Physical exam full range of motion is usually found with knee involvement tenderness to palpation crepitus may be found Imaging Radiography indication initial imaging test for patient suspected to have osteochondritis dissecans findings early lesions may appear normal advanced lesions may demonstrate subchondral flattening and radiolucency at the cortical surface Studies Diagnostic criteria diagnosis is based on clinical presentation and plain radiography of the affected area Differential Avascular necrosis Treatment Conservative restrict physical activity, immobilization, and physical therapy indication first-line for less severe osteochondritis dissecans Operative orthopedic surgery indication those who are unresponsive to conservative management severe osteochondritis dissecans Complications May progress to osteoarthritis of the affected joint