A patient with ankylosing spondylitis develops severe hip flexion contracture of 40° bilaterally. Total hip replacement in ankylosing spondylitis has a unique concern about the position of acetabular component. The risk is:
- A Heterotopic ossification leading to stiffness and re-ankylosis, risk mitigated by indomethacin or single-dose radiation prophylaxis ✓
- B Cement penetration into the cancellous bone causing aseptic loosening
- C Avascular necrosis of the femoral head due to altered blood supply in AS
- D Periprosthetic fracture due to osteoporotic bone in AS
Explanation
Heterotopic ossification (HO) is the most feared specific complication of THR in ankylosing spondylitis, occurring in up to 50% without prophylaxis compared to 3–5% in primary OA. The inflammatory milieu of AS dramatically increases HO risk. Prophylaxis consists of either indomethacin 25 mg TDS for 6 weeks post-operatively or a single dose of local radiotherapy (6–8 Gy) within 72 hours. Brooker Grade III/IV HO can result in complete re-ankylosis. Additionally, positioning is challenging because the pelvis is often in fixed flexion requiring altered acetabular component angulation, risking malpositioning.
Reference: Maheshwari Essential Orthopaedics, 6th ed.
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Written and medically reviewed by the StethoPrep medical team.