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

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