Spontaneous osteonecrosis of the knee was described as a distinct entity in 1968 (Ahlback et al). The etiology of this lesion remains unknown, although micro-trauma and vascular insufficiency have been suggested as causes. Typically the patient is a female in the sixth and seventh decade, sudden onset is characteristic, and the lesion occurs mainly on the medial femoral condyle. Radiographs show a subchondral radiolucency on the weight-bearing area with subsequent flattening, collapse and sclerosis.