Adolescent Girl With Knee Pain

Osteochondritis dissecans 

The radiograph reveals a lucency in the subchondral area of the lateral aspect of the right medial femoral condyle. This is diagnostic of osteochondritis dissecans, a necrotic bone lesion of unknown etiology, which occurs in adolescent athletes and can occur in adults. It is rare, with an incidence of about 20 per 100,000 people.1 It is most commonly observed in the knee, especially in the lateral aspect of the medial femoral condyle, and is second most commonly observed in the ankle, usually in the talar dome. It often leads to separation of a bony fragment, resulting in a disrupted joint line and a loose body in the joint. Orthopedic referral is mandatory. Most cases are staged with magnetic resonance imagery (MRI) (Figure 2). Lesions meeting MRI criteria for stability are treated with a period of immobilization and close observation. Lesions thus deemed to be unstable are treated operatively with debridement, pinning, resection, or bone grafting. This patient required surgery. By the time of the operation, the fragment had displaced and was found in the suprapatellar bursa. The donor site was debrided and the fragment was reimplanted with absorbable nails.

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Figure 2. Coronal T1-weighted MRI image reveals a very large osteochondral (ie, involving both cartilage and bone) defect of the medial femoral condyle (arrow). Used with permission of Daniel J. Pallin, MD, MPH, Division of Emergency Medicine, Children's Hospital Boston, and Department of Emergency Medicine, Brigham and Women's Hospital, Boston, MA.

This case of mild, subacute trauma also reminds us to think outside the box. The minor trauma 1 week before presentation was not relevant to the underlying disease process. Anchoring to the history of trauma would have led to misapplication of the Ottawa Knee Rule2 and possibly false reassurance. Osteochondritis dissecans can certainly be managed with immobilization and orthopedic referral, and without radiographs. However, if the patient failed to follow up, the emergency physician could have been blamed for the eventual displacement of the fragment, described above.