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Spontaneous Osteonecrosis of the Knee - SONK

Also known as: Ahlback’s disease

Definition

  • A condition of unknown aetiology characterized by the development of an area of osteonecrosis on the weight-bearing surface of the medial femoral condyle.
  • First described by Ahlback in 1968.

Epidemiology

  • Affects people older than 65
  • Three times more common in women

Aetiology

  • Unknown
  • The medial femoral condyle may be particularly at risk because it is supplied by an end-artery, namely a nutrient artery from the superior medial geniculate artery.

Clinical presentation

  • History
    • Sudden onset of severe deep-seated pain in the knee.
    • Walking is difficult & weight bearing aggravates the pain.
    • The knee is relatively comfortable at rest
    • often significant night pain.
  • Examination
    • small effusion
    • good range of motion in the joint.
    • localized tenderness over the medial femoral condyle.
    • Stressing the medial femoral condyle in flexion is painful.
    • rarely the lateral femoral condyle may be involved.

Radiology

Xray

  • The plain X-ray may be normal or may show a characteristic area of osteonecrosis on the weight-bearing surface of the medial femoral condyle.

Bone Scan

  • If the plain X-ray is normal a bone scan is the next step & this will show a focal area of high activity.

MRI

  • MRI scan will also show abnormality before the plain X-ray.
  • T1
    • discrete area of low intensity signal on the medial femoral condyle.
  • T2
    • a low signal intensity area in the centre of the lesion plus high-intensity about the margins secondary to oedema

Staging

Staging of SONK
Stage Description  
1 incipient stage
  • Severe pain
  • X-ray normal or some focal osteoporosis
  • Bone scan & MRI positive
2 avascular stage
  • Pain
  • X-ray: radiolucent oval shadow on the medial femoral condyle with some flattening of the articular surface.
  • Bone scan & MRI positive
3 collapsed stage
  • Pain
  • X-ray: collapse of the subchondral bone plate with a calcified plate & clear sclerotic halo
4 degenerative stage
  • Severe pain with or without deformity
  • X-ray: shallow concave articular surface with secondary OA changes, a narrowed joint space & varus deformity.

Natural history

  • Stages 1 & 2 may recover spontaneously. 
    • There is no good evidence that any treatment, whether it be arthroscopy, drilling, curettage or whatever changes the natural history.
  • Stages 3 & 4 have a poor prognosis, with progression to severe OA almost inevitable.
  • The important variables other than stage are the area of the lesion & the condylar width ratio.
  • An area of <3.5 cm2 is associated with a better prognosis.
  • A ratio of the diameter of the lesion to the width of the condyle of less than 0.4 also has a better prognosis.

Differential diagnosis

  1. Osteochondritis dissecans
  2. Osteonecrosis-like syndrome of the knee
  3. Medial compartment OA
  4. Degenerate medial meniscus
    • These patients will to tend to benefit from intra-articular steroids & local anaesthetic whereas patients with SONK will not
  5. Secondary osteonecrosis of the femoral condyle
    • This is important because is responds well to drilling & decompression
  6. Stress fractures
  7. Other conditions
    • Tumour
    • Infection
    • Pes anserinus bursitis etc

 

Treatment

  • Stage 1 & 2
    • conservative treatment
    • there is no evidence that other treatment modalities are of any use
  • Stage 3 & 4
    • Arthroplasty, either unicompartmental or total for severely symptomatic stage 3 & 4

 

 

Webpage Last Modified: 14 October, 2011