Osteonecrosis — also called avascular necrosis (AVN) — is a condition where a section of bone dies because its blood supply has been disrupted. In the hip, it most commonly affects the femoral head, the ball at the top of the thigh bone.
Why bone needs a blood supply
Bone is living tissue, constantly being broken down and rebuilt by cells that depend on a steady blood supply for oxygen and nutrients. When that supply is interrupted — from injury, certain medications, or in some cases no identifiable cause at all — the bone in that region can die. Without treatment, the dead bone can eventually collapse under normal loading, changing the shape of the joint surface.
Common causes and risk factors
- Long-term or high-dose corticosteroid use
- Excess alcohol consumption
- Trauma or fracture that damages the local blood vessels
- Certain blood disorders and clotting conditions
- Some cases have no clearly identifiable cause
The stages of osteonecrosis
- Stage 1: Bone changes visible only on MRI, not yet on X-ray. Often no symptoms, or mild discomfort.
- Stage 2: Changes visible on X-ray, but the joint surface itself is still round and intact.
- Stage 3: Early collapse of the bone surface — a subtle flattening visible on imaging, often with new pain on weight-bearing.
- Stage 4: Joint surface collapse has progressed to secondary arthritis, with cartilage damage and joint space narrowing.
Treatment by stage
Early stages (1–2)
Reducing load on the joint, addressing any underlying cause (such as reducing steroid use where medically possible), and in some cases a joint-preserving procedure called core decompression — drilling a small channel into the affected bone to relieve pressure and encourage new blood vessel growth.
Later stages (3–4)
Once the joint surface has collapsed and secondary arthritis has set in, joint-preserving options are far less effective, and hip replacement becomes the more reliable way to restore pain-free movement.
The key message
Osteonecrosis is one of the clearest examples in orthopaedics of why early diagnosis changes the treatment path available to you. A stage 1 finding managed proactively looks very different from the same condition caught at stage 4.