Spontaneous osteonecrosis of the knee (SONK)
Sponteous osteonecrosis of the knee (SONK) is a cause for sudden onset swelling and pain to the to the knee without any precipitating injury. It can often catch clinicians out and be mistaken for knee arthritis or meniscal pathology. It is more common on the medial side of the knee, and in women typically over the age of 50.
The aetiology is not completely understood but it is now thought to primarily stem from a subchondral insufficiency fracture (SIF), but is not strictly termed a pathological fracture. There is usually with some evidence of underlying poor bone quality.
Clinical Presentation
- A 45-year-old woman presenting with spontaneous left knee swelling and pain
- No trauma or injury
- Non-smoker
- DH – Depo-Provera
- PMH – Nil of note
- SH – Lawyer
- FH – Nil of note
- Examination left knee
- BMI 37
- Marked Genu Valgus
- Moderate effusion
- Decreased range of movement
- Tenderness lateral joint line
Investigation
- Bloods
- Vitamin D – 40
- TFTs – Normal
- Fasting glucose – Normal
- MRI
- High signal changes in lateral femoral condyle T2
- No obvious subcondral fracture line T1
- No cartilage damage in the knee
- No meniscal injury or tear
- DEXA – T score hip –1.1
Diagnosis
Spontaneous osteonecrosis of the knee, in this case likely associated with;
- Depo Provera usage
- Genu Valgus and overload in the lateral compartment of the knee (SONK more common medial compartment)
- High BMI
- Stressful occupation and disrupted sleep patterns
- Background of normal bone mineral density
Management
Medical;
- Stopped Depo-Provera and switched to Mirena Coil for contraception
- Started on Vitamin D supplementation (60IU/week, recheck in 3 months)
- Lateral Compartment off-loader brace
Rehabilitation;
- Acute phase – Pool based rehabilitation to begin (e.g. walking lengths), isometric quadricep exercises
- Graduated strength-based loading programme
- Aerobic activity – static exercise bike to help improve metabolism and reduce weight
Key learning points – SONK
- Beware of knee pain and swelling that has presented spontaneously without injury
- MRI investigation of choice over x-ray
- Consider biomechanical inputs such as;
- Valgus/varus alignement of the knee
- Meniscal pathology (associated with root tears)
- Consider metabolic inputs such as;
- Smoking and alcohol use
- Low Vitamin D
- Medications that may affect bone health (e.g Depo Provera)
- Treatment includes;
- Optimised loading and strength-based recovery
- Improving bone health
- Surgical consideration for uni-compartmental knee replacement or TKR if ensuing necrosis (a minority of patients)
