Idiopathic Spontaneous Osteonecrosis of the Knee (SONK)

Femoral Condyle

– typically 60 year old woman (3:1 female to male) with spontaneous severe pain in the knee, most often medially.
– often give history of severe pain, worse at night, gradually gets better.

Staging: 1. radiographs normal – in some, they never get past this stage
2. slight flattening – may be very subtle
3. radiolucency surrounding sclerotic area of subchondral bone
4. radiiolucency surrounded by a definite sclerotic halo of variable thickness and density
5. arthritic changes – narrowing of joint space, sclerosis, osteophytes

– bone scan and MRI are critical investigations; most importantly, to define the SIZE OF LESION – this is the most important factor in prognosis.
– basically, patients with small lesions (50% of condylar width) do poorly, with progressive disability, increasing pain, deformity, arthritis.

– initially, conservative; make sure you define the extent of the lesion
– surgical treatment has not been shown to be effective at altering natural history – the ultimate prognosis is determined by the size of the lesion.
– in young patient, can try doing a proximal tibial osteotomy to unload the area, with or without a core decompression
– in older patient, might as well do a unicompartmental knee replacement or total knee.
* the decision regarding osteotomy or arthroplasty follows the same guidelines as for the arthritic patient.
Tibial Plateau

– probably underdiagnosed
– often interpreted as meniscal damage – these patients probably get arthrocopy!
– most often in the medial tibial plateau
– again, bone scan and MRI are important investigations.

– there is no effective surgical treatment
– there is nothing to support core decompression
– conservative treatment is the mainstay – arthroplasty if they find this intolerable

Take home message – the size of the lesion is the key!

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