Causes / typical injury mechanism: Spontaneous osteonecrosis of the knee (SONK) is a rare condition that most commonly presents in elderly females. SONK is an acute injury that most often occurs following microtrauma (may be chronic and reach a "breaking point") or increased activity. It should be distinguished from secondary osteonecrosis of the knee and postarthroscopic osteonecrosis of the knee, which have identifiable causes. See avascular necrosis of knee for more information.

Classic history and presentation: Sudden onset of severe knee pain and effusion in an elderly woman with a history of osteopenia / osteoporosis. The knee pain is unilateral in > 99% of cases and confined to the medial compartment in approximately 95% of cases.

Prevalence: The exact prevalence is unknown. SONK is likely underdiagnosed. Many patients with end-stage osteoarthritis may have had occult SONK that went undiagnosed and progressed to outright osteoarthritis. In patients older than 50 years and older than 65 years with acute medial compartment knee pain, the incidence of SONK is 3.4% and 9.4%, respectively.

• Age – Patients are most likely older than 60 years.
• Sex / gender – SONK is 3 times more common in women.

Risk factors: Female sex, older age, and osteopenia / osteoporosis.

Pathophysiology: The exact pathophysiology of SONK is unknown. It may represent a subchondral insufficiency fracture, but others believe it is caused by a meniscal root tear.

Grade / classification system:

Koshino Classification

1. Knee symptoms with normal radiographs.
2. Flattening of the weight-bearing surface and subchondral radiolucencies surrounded by osteosclerosis of the affected condyle.
3. Extension of radiolucencies with subchondral collapse.
4. Degenerative phase with osteosclerosis and osteophyte formation surrounding the condyle.