Atraumatic osteonecrosis of the patella

Atraumatic secondary osteonecrosis of the patella is a rare entity and has been reported in only several case reports. The purpose of this study was t o define the clinical and radiographic characteristics and outcome of this disease. Osteonecrosis of the patella was found in 25 knees in 19 patients. The mean age was 42 years (range, 21-63 years). Eighteen (95%) patients ha d greater than 2 g of lifetime corticosteroid exposure, and six (32%) patie nts had systemic lupus erythematosus. Osteonecrosis was found in the superi or pole of the patella in all 23 knees (17 patients) that had magnetic reso nance imaging. Osteonecrosis was apparent on plain radiographs in five knee s (20%) in four patients, and only one knee (4%) in one patient had collaps e of the posterior articulating surface of the patella. One (4%) patient pr esented with anterior knee pain localized to the patella. At a mean followu p of 4 years (range, 2-18 years), none of the patients had an intervention that focused on treating the patellar lesion. Patellar osteonecrosis is cha racterized bg patients with coincident lesions of the distal femur and the proximal tibia and lesions localized to the superior pole of the patella. I t is a nonprogressive disease that does not warrant surgical exploration.