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.

Epidemiology

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

Pathology

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 occasionally involve the tibial plateau 9) and is often associated with a meniscal tear

MRI

Features can vary dependant on the stage:

  • ill-defined bone marrow oedema and a lack of peripheral low signal intensity rim as seen in avascular necrosis and bone infarcts
  • a focal subchondral area of low signal intensity adjacent to the subchondral bone plate and representing local ischaemia (considered is a specific MRI finding 12); this area shows no enhancement on post-contrast
  • deformity of the subchondral bone plate (flattening or focal depression) in the weight-bearing area of the involved condyle 

Treatment and prognosis

Prognosis varies from complete recovery to total joint collapse 2. Treatment can either be operative or nonoperative.

Differential diagnosis

History and etymology

It was first systematically described by Ahlback in 1968 2

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