Spontaneous Osteonecrosis of the Knee

Spontaneous osteonecrosis of the knee (SONK), first described by Ahlback in 1968, is characterised by the development of an area of osteonecrosis on the weight-bearing surface of the medial femoral condyle. Osteonecrosis of the knee may however, also occur on the outside of the knee (the lateral femoral condyle) or on the lower leg bone (at the tibial plateau). Contrary to its classification as osteonecrosis, this disease does not seem to be associated with any appreciable amount of bone necrosis (Mears et al. 2009).


The exact cause of the osteonecrosis of the knee is not yet known, but one theory is that a stress fracture, combined with a specific activity or trauma, results in an altered blood supply to the bone. Another theory supposes that a build-up of fluid within the bone puts pressure on blood vessels and diminishes circulation.


Incidence of SONK is generally higher in those aged over 60 and the condition is said to be three times more common in women.


Symptoms include, but are not limited to:
• Sudden pain on the inside of the knee, perhaps triggered by a specific activity or minor injury
• Walking is difficult & weight bearing aggravates the pain.
• Increased pain at night and with activity
• Swelling over the front and inside of the knee
• Localized tenderness over the medial femoral condyle
• Heightened sensitivity to touch in the area
• Limited motion due to pain


When diagnosing, 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 scans will generally show a focal area of high activity. MRI however, is the most useful study for definitive confirmation/ determining the extent of disease, thereby helping to guide treatment considerations.


1 Incipient stage Severe pain, X-ray normal or some focal osteoporosis, Bone Scan & MRI positive. Conservative treatment – NSAIDs, narcotics, protected weight bearing.

Physical therapy directed at quadriceps strengthening.

There is no evidence that other treatment modalities are of any use.

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. For severely symptomatic stage 3 & 4:

Arthroplasty – when symptoms fail to respond to conservative treatment.

High tibial osteotomy – when angular malalignment is present.


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.



SONK photo


Fig. a & b: A 58-year-old male patient initially presented with MRI-detected osteonecrosis in the medial femoral condyle of the left knee.
Fig. c & d: After non-operative treatment protocol approach (inclusive of low-dose
bisphosphonates and high-dose vitamin D administration), the MRI follow-up 16 weeks later showed a remission of the lesion (Breer et al. 2013).


There are a lot of operative and non-operative treatment options described in the literature for SONK. Whereas the latter stages are preferentially treated by different surgical concepts, non-operative strategies certainly compete against operative treatment options for the early stages.


Optimisation of bone metabolism should be part of every conservative or surgical treatment concept (Breer et al. 2013).



Ref: Breer S, Oheim R, Krause M, Marshall RP, Amling M, Barvencik. Spontaneous osteonecrosis of the knee (SONK).
Knee Surg Sports Traumatol Arthrosc 2013 Feb;21(2):340-5. Mears SC, McCarthy EF, Jones LC, Hungerford DS, Mont MA.
Characterization and Patholo gical Characteristics of Spontaneous Osteonecrosis of the Knee. Iowa Orthop J. 2009; 29: 38-42.
http://www.myorthopod.com.au/spontaneous-osteonecrosis-knee.html http://www.orthofracs.com/adult/elective/knee/sonk.html#sthash.EK7y1ih1.dpuf


The comments above are the implicit advice of Workplace Physiotherapy. The views expressed are based on current evidence-based research and accepted best practice approaches. Unless otherwise stated, these comments are not the view of WorkCover NSW or any other professional body. No reproduction or forwarding of this advice is permitted without the consent of the author.


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