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Spontaneous osteonecrosis of the knee

Spontaneous osteonecrosis of the knee, also known as Ahlback disease, SONK or even SPONK has similar appearances to osteochondritis dissecans of the knee but is found in an older age group.

Osteonecrosis in this setting has no predisposing factors. However, by definition, secondary osteonecrosis of the knee occurs secondary to an insult.

SONK is not caused by bone death but may be caused by osteoporosis and insufficiency fractures.6


SONK is seen more frequently in women (M:F 1:3), and affects older patients, typically over the age of 55.

Radiographic features

It is almost always unilateral, usually affects the medial femoral condyle and is often associated with a meniscal tear


Can vary from complete recovery to total joint collapse 2.

Differential diagnosis

History and etymology

It was first systematically described by Ahlback in 1968 2

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