OrthoNet

SONK
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.

Treatment
- 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.

Treatment
- 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!