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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.


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


Osteonecrosis in SONK has no predisposing factors. However, by definition, secondary osteonecrosis of the knee occurs secondary to an insult. SONK is not thought to be caused by bone death but may be caused by osteoporosis and insufficiency fractures 6. Some authors suggest that the primary event leading to spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture.

Radiographic features

It is almost always unilateral, usually affects the medial femoral condyle (but can occasional involve the tibial plateau9) 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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