In ankylosing spondylitis with severe hip flexion deformity (30° fixed flexion deformity) and ankylosis of the hip joint, total hip arthroplasty is the definitive treatment. The specific technical challenge unique to AS patients undergoing THR compared to osteoarthritis patients is:

• A Increased risk of intraoperative femoral fracture due to osteoporosis from steroid use
• B Prosthetic infection risk is lower in AS patients
• C Accurate acetabular component positioning is difficult because the natural bony landmarks are obliterated; there is also an increased risk of heterotopic ossification and perioperative seizures from the ankylosed position ✓
• D Cementless fixation is contra-indicated in AS

Explanation

In ankylosing spondylitis, the acetabular landmarks (acetabular rim, transverse ligament) are obliterated by bony ankylosis, making component positioning by standard landmarks unreliable — computer navigation or intraoperative imaging is recommended. Patients with AS also have spinal rigidity (eliminating the pelvis's compensatory pelvic tilt mechanism) making cup alignment targeting uniquely challenging. Heterotopic ossification risk is substantially higher in AS (up to 40%) due to the osteoproliferative disease state — prophylaxis with indomethacin or single-fraction radiotherapy is essential. Patients chronically flexed in the ankylosed position may also develop neurological complications when the hip is mobilised intraoperatively.

Reference: Maheshwari Essential Orthopaedics, 6th ed.

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Written and medically reviewed by the StethoPrep medical team.