Heterotopic Ossification after Hip Arthroplasty

Heterotopic ossification (HO) — the formation of mature lamellar bone in soft tissues where bone does not normally exist — is a recognised complication of total hip arthroplasty (THA), occurring in varying degrees in approximately 10–50% of patients after THA on routine radiographic screening, though clinically significant HO (causing pain and restricted motion) affects only approximately 5–15%. HO forms in the periarticular musculature and fascial planes around the hip, most commonly in the anterior and lateral soft tissues. Severe HO can cause significant restriction of hip motion — particularly flexion and abduction — and may require surgical excision in refractory cases.

• Pathophysiology: HO is driven by osteogenic precursor cells (mesenchymal stem cells) in the periarticular soft tissues that are triggered to undergo ectopic ossification by the local surgical trauma and inflammatory milieu; bone morphogenetic proteins (BMPs — particularly BMP-2 and BMP-7) released from disrupted bone and soft tissue during surgery act as potent osteoinductive signals; the surgical approach, degree of soft tissue trauma, and bleeding into the wound all influence the osteogenic stimulus; the resulting bone forms as mature lamellar bone (not woven bone or calcification) — hence the term `ossification` rather than `calcification`; it takes approximately 12–18 months to fully mature (confirmed by falling uptake on radionuclide bone scan)
• Risk factors: prior HO (the strongest predictor — approximately 90% recurrence risk without prophylaxis after excision); hypertrophic osteoarthritis as the indication for THA (prominent osteophytes indicate a hyperactive osteogenic response); ankylosing spondylitis (HO is particularly severe and aggressive in AS); prior hip or pelvic surgery or radiotherapy; male sex; diffuse idiopathic skeletal hyperostosis (DISH); Paget`s disease of bone; neuromuscular disease (spinal cord injury, traumatic brain injury — the highest risk population for HO outside arthroplasty)

• NSAIDs — indomethacin: the most widely used pharmacological prophylaxis; indomethacin 25 mg three times daily for 6 weeks post-operatively; inhibits prostaglandin synthesis via COX inhibition — prostaglandins are key mediators of osteoblast differentiation and the osteogenic response to surgical trauma; evidence shows approximately 50–75% reduction in clinically significant HO with indomethacin prophylaxis; concerns — GI side effects (dyspepsia, ulceration); renal impairment; possible impairment of fracture healing and bone ingrowth of cementless implants (NSAIDs inhibit bone healing — theoretical concern in the context of cementless THA; this is debated and most evidence suggests that 6 weeks of NSAIDs does not impair cementless fixation in standard cases); alternative NSAIDs (celecoxib, diclofenac) are also used
• Radiotherapy: single-fraction external beam radiotherapy (7 Gy) administered within 72 hours of surgery (pre- or post-operatively) is an effective alternative to NSAIDs; particularly used in high-risk patients (prior HO, AS, DISH, hypertrophic OA, prior RT, patients who cannot take NSAIDs); radiotherapy inhibits osteogenic precursor cells in the periarticular tissues; the dose of 7 Gy is subthreshold for wound healing complications but sufficient to ablate local osteogenic progenitors; theoretical cancer risk (very low with single 7 Gy fraction); must be given within 72 hours (osteogenic cascade begins rapidly after surgical trauma); prophylactic RT and NSAIDs are broadly equivalent in efficacy for standard-risk patients; RT is preferred for high-risk patients
• Both prophylaxis options can be combined in the highest-risk patients (e.g., prior severe HO requiring excision with recurrence risk ~90%)

• Timing of excision: HO must be fully mature before surgical excision; immature HO has a high recurrence rate after excision; maturity is confirmed by: (1) radiographic appearance — mature lamellar bone with cortical margins on plain X-ray (as opposed to fluffy `cloud-like` immature calcification); (2) radionuclide bone scan — falling or normalised uptake indicates maturation (typically 12–18 months post-surgery); (3) biochemical markers — normalised alkaline phosphatase (elevated during active HO formation); excision before maturation (typically before 12–18 months) leads to recurrence in ~80% of cases
• Excision technique: surgical approach dictated by the location of the HO (anterior or lateral); the HO is excised en bloc where possible; neurovascular structures (sciatic nerve, femoral neurovascular bundle) are at risk in severely ankylosed cases — intraoperative neurophysiological monitoring is recommended for class III–IV HO excision; prophylaxis (NSAIDs or RT) must be commenced immediately post-excision (within 72 hours for RT) to prevent recurrence; without prophylaxis, recurrence after excision approaches 80–90%

• HO after THA: ectopic lamellar bone in periarticular soft tissues; 10–50% radiographic incidence; clinically significant 5–15%; Brooker I–II usually asymptomatic; Brooker III–IV causes pain + restricted motion + potential ankylosis
• Brooker classification: I (islands of bone — no bridging); II (spurs, gap >1 cm); III (spurs from pelvis AND femur, gap <1 cm); IV (apparent bony ankylosis)
• Risk factors: prior HO (strongest predictor — ~90% recurrence without prophylaxis); hypertrophic OA; ankylosing spondylitis; DISH; male sex; Paget`s; neurological disease (SCI, TBI)
• Pathophysiology: BMPs released by surgical trauma → osteoinduction of mesenchymal stem cells → mature lamellar bone; matures over 12–18 months
• Prophylaxis — indomethacin: 25 mg TDS × 6 weeks; COX inhibition → reduced prostaglandin → reduced osteoblast differentiation; ~50–75% reduction in clinically significant HO; GI + renal side effects; theoretical concern re cementless bone ingrowth (debated)
• Prophylaxis — radiotherapy: single fraction 7 Gy; within 72 hours of surgery; ablates osteogenic precursors; equivalent efficacy to NSAIDs; preferred in high-risk patients or NSAID contraindication; very low cancer risk at this dose
• Excision timing: MUST wait for maturity; confirm with bone scan (normalised uptake) + X-ray (cortical margins) + normalised ALP; typically 12–18 months; excision before maturity → 80% recurrence
• Post-excision prophylaxis mandatory: NSAID or RT within 72 hours after excision; without prophylaxis → 80–90% recurrence; can combine both in highest-risk cases
• HO vs calcification distinction: HO = mature lamellar bone (organised cortical structure on X-ray); calcification = dystrophic calcium deposition (amorphous, no cortex); clinically and radiographically distinct