Avascular Necrosis of the Femoral Head
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Direct answer
Avascular necrosis of the femoral head is death of subchondral bone from an interrupted blood supply, most often linked to corticosteroids, alcohol, sickle cell disease and femoral neck fracture. The MRI double-line sign on T2-weighted images is the most useful early sign, while the crescent sign appears once the bone begins to collapse. Small lesions without collapse are treated with joint-preserving surgery, and established collapse with arthritis needs arthroplasty.
What you must remember
- Steroid and alcohol: high-dose or prolonged corticosteroids and heavy alcohol intake are common non-traumatic causes in adults, and both are frequent in Indian practice.
- Sickle cell disease: vaso-occlusion infarcts the femoral head, classically in young patients with the disease.
- Trauma: a displaced femoral neck fracture disrupts the retinacular blood supply, making it the commonest traumatic cause.
- Other causes: systemic lupus erythematosus, Gaucher disease, caisson disease and pancreatitis with hyperlipidaemia are recognised associations.
- Double-line sign: a low-signal sclerotic border around a bright band on T2-weighted MRI, indicating necrotic bone.
- ARCO staging: stage I has a normal radiograph with positive MRI, the crescent sign marks subchondral collapse, and stage IV is arthritis.
- Management: core decompression suits small early lesions, osteotomy or grafting suits intermediate disease, and arthroplasty suits advanced collapse.
A staged approach to a painful hip
Avascular necrosis is silent until the bone collapses, so the diagnosis depends on early imaging and on documenting the cause. The five steps below match the investigation and the treatment to the stage of the disease.
- Establish the cause. Document steroid dose, alcohol history, sickle status, trauma and a lupus screen before choosing a line of treatment.
- Image early. A plain radiograph of both hips is followed by MRI when the film is normal and the suspicion is high, because early marrow changes show only on MRI.
- Stage the lesion. ARCO staging defines the size, location and collapse of the lesion, and that stage decides the operation.
- Choose joint preservation for small lesions. Core decompression, with or without bone grafting, suits lesions without collapse.
- Offer arthroplasty when collapse or arthritis is established. Total hip arthroplasty is the standard operation for advanced disease.
Joint-preserving surgery works best before collapse, which is why early MRI matters more than a plain radiograph in a young patient with steroid exposure. Bilateral disease is common with steroid causes, so both hips are imaged.
Where the exam tries to catch you
Most lost marks come from trusting a normal film or misreading the stage.
- Trap: a normal plain radiograph is taken to exclude avascular necrosis in a patient with steroid exposure and groin pain. Correct: the radiograph is often normal early, so MRI is needed when clinical suspicion is high.
- Trap: a collapsed femoral head with arthritis is offered core decompression. Correct: core decompression suits early lesions without collapse, so advanced disease needs arthroplasty.
- Trap: the double-line sign is read as a fracture line. Correct: it is the sclerotic border of necrotic bone on T2-weighted MRI, not a fracture.
Frequently asked questions
Which investigation is most sensitive for early avascular necrosis of the femoral head?
MRI is the most sensitive investigation for early avascular necrosis of the femoral head, detecting marrow changes before the radiograph shows anything. The double-line sign on T2-weighted images is a classic MRI finding in this stage.
What is the double-line sign in avascular necrosis?
The double-line sign is a serpiginous low-signal border around a bright band on T2-weighted MRI, indicating the sclerotic margin of necrotic bone. It helps distinguish avascular necrosis from simple marrow oedema.
Which drug exposure causes avascular necrosis of the femoral head?
High-dose or prolonged corticosteroid therapy is a classic cause of avascular necrosis of the femoral head, and heavy alcohol intake is another. Steroid-induced disease can be bilateral, so both hips are imaged.
What ARCO stage describes a collapsed femoral head with arthritis?
Advanced disease with joint-space narrowing and arthritis is ARCO stage IV. Subchondral collapse, marked by the crescent sign, belongs to an earlier collapse stage, and the final stage is defined by degenerative change.
Why is core decompression used in early avascular necrosis?
Core decompression lowers intraosseous pressure and may encourage revascularisation of the necrotic area in early lesions. It works best before subchondral collapse, so timing is critical.