Spontaneous osteonecrosis of the knee (SONK)

Discussion:

MRI features are suggestive of spontaneous osteonecrosis of the knee (SONK) involving the medial femoral condyle. It is usually a diagnosis in an older age group (>55 years) with a female predilection and usually unilateral, mostly affecting medial tibial plateau. This type of knee disease occurs with no history of trauma, however being considered as an insufficiency fracture on top of osteoporosis. It is often associated with a meniscal tear.

MRI suggestive features are:

  • location: in the weight-bearing area, usually the medial femoral condyle
  • subtle flattening / focal depressive deformity
  • an irregular, discontinuous hypointense line in the subarticular marrow 
  • ill-defined bone marrow edema and lack of peripheral low signal intensity rim as seen in osteonecrosis and bone infarcts
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