Idiopathic osteonecrosis of the knee.

Idiopathic osteonecrosis of the knee is typically a disease of elderly patients characterised by acute localised pain of sudden onset. Radiography in the early stages is negative, but scintigraphy is positive in the first few weeks, showing a localised "hot spot". At a later stage radiography becomes positive, showing subchondral rarefaction, collapse, osteochondral sequestration and reactive perifocal sclerosis. The subchondral bone is clearly necrotic but covered by intact cartilage which subsequently fractures at the edges of the crater to form a separated fragment. At the same time, resorption phenomena appear in the necrotic bone together with the production of immature new bone. Conservative treatment is effective in the majority of cases in which the affected area is small. Larger areas demand surgical intervention, the most effective being corrective tibial osteotomy. In very severe cases, however, total prosthetic replacement is necessary.