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.
Patients often recall an acute onset of severe pain without significant trauma.
By definition, secondary osteonecrosis of the knee occurs secondary to an insult. SONK is not thought to be caused by bone death but instead by osteoporosis and insufficiency fractures, with histopathologically proven origins in weakened trabeculae and applied microtraumatic forces 6,13. It is now accepted that spontaneous osteonecrosis of the knee is a subchondral insufficiency fracture that has further collapsed 13.
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.
In the later stages features seen include:
- flattening of the medial femoral condyle
- subchondral radiolucent focus
- complicating subchondral fracture with periosteal reaction
Features can vary depending on the stage, and are best characterised on T2-weighted and proton density-weighted sequences. The following criteria apply to lesions without overlying cartilage abnormalities:
- subchondral bone plate fracture 13
- in the weight-bearing area of the involved condyle
- subtle flattening or a focal depressive deformity
- an irregular, discontinuous hypointense line in the subarticular marrow, representing callus and granulation tissue
- there may be a fluid-filled cleft within the subchondral bone plate (poor prognostic factor) 13
- excavated defect of the articular surface (advanced cases)
- focal subchondral area of low signal intensity subjacent to the subchondral bone plate representing local ischaemia (considered most important in early lesions and a specific MRI finding 12)
- this area shows no enhancement on post-contrast; if it is thicker than 4 mm or longer than 14 mm, the lesion may be irreversible and may evolve into irreparable epiphyseal collapse and articular destruction
- appears as a thickened subchondral bone plate, which represents a fracture with callus and granulation tissue and secondary osteonecrosis in the subarticular region 13
- ill-defined bone marrow oedema and a lack of peripheral low signal intensity rim as seen in osteonecrosis and bone infarcts
Treatment and prognosis
Prognosis varies from complete recovery to total joint collapse 2. Treatment can either be operative or non-operative, with initial treatment often conservative and consisting of analgesia and protected weight bearing. Subchondral hypointense fracture lines tend to resolve with conservative therapy. In more advanced cases, subchondroplasty (where a bone substitute is injected) may be considered.
Possible considerations include:
History and etymology
It was first systematically described by Ahlback in 1968 2
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