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Updated: Feb 13 2024

Spontaneous Osteonecrosis of the Knee (SONK)

  • summary
    • Spontaneous osteonecrosis of the knee (SONK) is an idiopathic condition that leads to the development of a crescent shaped osteonecrosis lesion, mostly commonly in the epiphysis of the medial femoral condyle.
    • Diagnosis can be radiographic for advanced disease but may require MRI in determining the extent of disease.
    • Treatment is generally nonoperative as most cases are self-limiting. Surgical management is indicated for progressive cases that fail conservative management. 
  • Epidemiology
    • Demographics
      • most common in middle age and elderly
      • affects females (>55yo) more frequently than males
    • Anatomic location
      • 99% of patients have only one joint involved
      • usually epiphysis of medial femoral condyle
  • Etiology
    • Pathophysiology
      • may represent a subchondral insufficiency fracture
      • also believed to be caused by a meniscal root tear
      • can occur post-arthroscopically
        • most commonly after partial meniscectomy
          • thought to be related to altered joint biomechanics
  • Presentation
    • Symptoms
      • sudden onset of severe knee pain
      • effusion
      • limited range of motion secondary to pain
      • tenderness over medial femoral condyle
  • Imaging
    • Radiographs
      • recommended views
        • standing AP and lateral of hip, knee and ankle
      • most useful study
      • is helpful to confirm the diagnosis and assist in determining the extent of disease helping guide treatment considerations
      • lesion is crescent shaped
  • Differential
    • Must differentiate from
      • osteochondritis dissecans
        • more common on lateral aspect of medial femoral condyle in adolescent males
      • transient osteoporosis
        • more common in young to middle age men
      • bone bruises and occult fractures
        • associated trauma, bone fragility or overuse
  • Treatment
    • Nonoperative
      • NSAIDs, narcotics, protected weight bearing
        • indications
          • mainstay of treatment as most cases resolve
        • technique
          • physical therapy directed at quadriceps strengthening
        • outcomes
          • initial conservative measure and has shown good results
    • Operative
      • arthroplasty
        • indications
          • when symptoms fail to respond to conservative treatment
        • outcomes
          • successful results reported with TKA (larger lesions or bone collapse) and UKA (smaller lesions) when properly indicated
      • high tibial osteotomy
        • indications
          • when angular malalignment present
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