THA HETEROTOPIC OSSIFICATION
Prevention Protocol | Brooker Classification | NSAIDs vs Radiation | High-Risk Groups
BROOKER CLASSIFICATION
Critical Must-Knows
- Indomethacin 75mg daily for 6 weeks is standard NSAID prophylaxis (start within 24 hours of surgery)
- Single-dose radiation (700cGy) within 24-72 hours if NSAIDs contraindicated or high-risk revision
- Major risk factors: previous HO, ankylosing spondylitis, DISH, post-traumatic arthritis, hypertrophic OA, male gender
- DO NOT excise HO before 12-18 months maturation - high recurrence if excised early
- Brooker III-IV causes functional limitation - pain, restricted ROM, difficulty with ADLs
Examiner's Pearls
- "NSAIDs work by inhibiting prostaglandin synthesis preventing osteoblast differentiation
- "Radiation works by inhibiting mesenchymal stem cell differentiation and osteoblast proliferation
- "Combination NSAIDs + radiation NO additional benefit over single modality alone
- "Australian data: prophylaxis reduces severe HO from 20% to 2-5% in high-risk patients
Critical HO Exam Points
High-Risk Groups (MUST Prophylax)
Previous HO = 90% recurrence without prophylaxis. Ankylosing spondylitis, DISH, post-traumatic OA (especially acetabular fractures), hypertrophic OA, male gender, approach-dependent risk (anterolateral higher than posterior).
Timing is Everything
NSAIDs: Start within 24 hours, continue 6 weeks. Radiation: Single dose 700cGy within 24-72 hours post-op OR single dose pre-op (4 hours before incision). Late prophylaxis ineffective.
Brooker Classification (Know Cold)
I = islands in soft tissue. II = gap over 1cm between spurs. III = gap under 1cm. IV = ankylosis. Only III-IV functionally significant. Assess at 6-12 months on AP pelvis radiograph.
Excision Pearls
Wait 12-18 months for maturation (cold bone scan, normal ALP). Re-prophylax at excision (radiation preferred over NSAIDs). Revision THA + HO excision = highest recurrence without prophylaxis.
Quick Decision Guide
| Patient Scenario | Prophylaxis Strategy | Timing | Key Pearl |
|---|---|---|---|
| Primary THA, no risk factors | Routine NSAIDs (indomethacin 75mg daily) | Start within 24h, continue 6 weeks | Reduces severe HO from 7% to 2% |
| Previous HO, ankylosing spondylitis, DISH | NSAIDs OR single-dose radiation (700cGy) | NSAID: start within 24h; Radiation: within 72h post-op | Either modality equally effective in high-risk |
| Revision THA + HO excision | Single-dose radiation (700cGy) preferred | Pre-op (4h before incision) OR within 24h post-op | Recurrence 20-30% without prophylaxis |
| NSAID contraindications (GI bleed, renal) | Single-dose radiation only option | Within 24-72 hours post-op | No increased wound complications |
ASHAMEDHigh-Risk Factors for HO (ASHAMED)
Memory Hook:Be ASHAMED if you forget prophylaxis in these patients - they WILL form HO!
BIGABrooker Classification (B-I-G-A)
Memory Hook:Think B-I-G-A: Brooker classification gets progressively BIGGER and closer until Ankylosis!
PROSPERNSAIDs Mechanism (PROSPER)
Memory Hook:NSAIDs help patients PROSPER after THA by preventing the prostaglandin-mediated bone formation!
Overview and Epidemiology
Heterotopic ossification (HO) is the formation of mature lamellar bone in soft tissues around the hip following total hip arthroplasty. It represents abnormal differentiation of pluripotent mesenchymal stem cells into osteoblasts in response to local and systemic factors.
Why HO Matters Clinically
While mild HO (Brooker I-II) is often asymptomatic and clinically insignificant, severe HO (Brooker III-IV) occurs in 5-20% of patients without prophylaxis and causes substantial morbidity: restricted range of motion (especially internal rotation and flexion), pain with movement, difficulty with sitting and personal hygiene, and in extreme cases (Brooker IV) complete ankylosis requiring revision surgery.
Demographics and Risk Distribution
- Age: Older patients slightly higher risk
- Gender: Males 2-3 times higher risk than females
- Indication: Post-traumatic arthritis 50-90%, ankylosing spondylitis 50%, primary OA 3-7%
- Previous HO: 90% recurrence risk without prophylaxis
- Genetic factors: HLA-B27 association in ankylosing spondylitis
Impact on Outcomes
- ROM restriction: Loss of 20-50 degrees flexion/rotation in Brooker III-IV
- Pain: 30-40% of Brooker III patients report pain
- ADL limitation: Difficulty with sitting, shoe tying, toileting
- Revision rate: 1-3% require HO excision for function
- Cost: Prophylaxis cost-effective compared to revision surgery
Pathophysiology and Molecular Mechanisms
The Mesenchymal Stem Cell Hypothesis
HO formation requires three elements: osteogenic precursor cells (mesenchymal stem cells in traumatized tissue), inducing factors (BMPs, inflammatory cytokines), and permissive environment (adequate vascularization, local hypoxia). All three must be present - prophylaxis interrupts the cellular differentiation pathway.
Cellular Cascade
The formation of heterotopic bone follows a predictable sequence initiated at the time of surgical trauma:
Phase 1 (0-48 hours): Inflammation and Induction
- Surgical trauma releases inflammatory mediators (IL-1, IL-6, TNF-alpha)
- Tissue hypoxia and local hematoma formation
- BMPs released from bone debris and soft tissue damage
- Mesenchymal stem cells recruited to trauma site
Phase 2 (2-7 days): Differentiation
- Prostaglandin E2 (PGE2) drives stem cell differentiation toward osteoblasts
- NSAIDs block COX-2 enzyme preventing PGE2 synthesis
- Radiation induces DNA damage in rapidly dividing mesenchymal cells
- Critical window where prophylaxis is effective
Phase 3 (1-6 weeks): Proliferation
- Osteoblast proliferation and matrix production
- Primitive woven bone formation
- Continued NSAID prophylaxis prevents ongoing differentiation
- Late prophylaxis ineffective after osteoblasts established
Phase 4 (6-18 months): Maturation
- Woven bone remodels to mature lamellar bone
- Vascular channels develop
- Bone scan activity decreases
- Alkaline phosphatase normalizes (marker of maturation)
| Mechanism | Target | Timing Window | Clinical Effect |
|---|---|---|---|
| NSAIDs (indomethacin, COX-2 inhibitors) | Prostaglandin synthesis inhibition | Start within 24h, continue 6 weeks | Prevents mesenchymal stem cell differentiation to osteoblasts |
| Radiation (700cGy single dose) | DNA damage in dividing cells | Within 24-72h post-op (or pre-op) | Inhibits mesenchymal cell proliferation and osteoblast function |
| BMP inhibitors (experimental) | Block osteogenic signaling | Early post-operative period | Not yet clinically available - research phase |
Classification Systems
Brooker Classification (Most Widely Used)
The Brooker classification is the gold standard for grading HO after THA. It is based on AP pelvis radiograph findings and correlates well with clinical significance.
| Class | Radiographic Features | Functional Impact | Management |
|---|---|---|---|
| Brooker I | Islands of bone within soft tissues around hip | None - incidental finding | No treatment required, reassurance |
| Brooker II | Bone spurs from pelvis or proximal femur with gap greater than 1cm between opposing surfaces | Minimal - usually asymptomatic | Observation, rarely symptomatic |
| Brooker III | Bone spurs from pelvis or proximal femur with gap less than 1cm | Moderate - reduced ROM (20-50 degrees), may cause pain | Consider excision if symptomatic and mature |
| Brooker IV | Apparent radiographic ankylosis (bone bridging) | Severe - marked ROM restriction, pain, ADL limitation | Excision + prophylaxis if significant functional loss |
Brooker Distinction Points
The key distinction between Brooker II and III is the 1cm gap between opposing bone surfaces. Measure the smallest distance between proximal femoral HO and pelvic HO on AP radiograph. Brooker III-IV are clinically significant because they mechanically block hip motion, whereas I-II do not contact opposing surfaces and thus allow full ROM.
Clinical Assessment
History - Key Questions
- Pain: Location (anterior groin, lateral), character (mechanical vs inflammatory)
- ROM limitation: Which movements restricted (flexion, internal rotation, abduction)
- ADL impact: Difficulty with sitting, shoe tying, toileting, sexual function
- Timeline: When did symptoms develop (early vs late)
- Previous HO: History of HO in contralateral hip or previous surgery
- Risk factors: Ankylosing spondylitis, DISH, post-traumatic arthritis, burns, spinal cord injury
Examination Findings
- ROM: Measure active and passive flexion, internal rotation, abduction
- End-feel: Hard block to motion (bone) vs soft tissue restriction
- Palpation: Firm mass in soft tissue (especially anterolateral)
- Gait: Antalgic or Trendelenburg pattern
- Functional tests: Ability to sit in chair, tie shoes, cross legs
- Compare to contralateral: Asymmetry suggests HO vs stiffness
Differential Diagnosis of Post-THA Stiffness
NOT all post-THA stiffness is HO. Differential includes: component malposition (excessive anteversion/retroversion, high offset), adhesive capsulitis, psoas impingement, abductor deficiency (compensatory stiffness), infection (stiffness + pain + systemic signs), instability (patient guards against provocative positions). Always assess for these before attributing stiffness to HO alone.
Physical Examination Technique
ROM Assessment Protocol:
- Flexion: Supine, hip and knee flexed - measure angle between thigh and trunk (normal 120 degrees)
- Internal rotation: Prone or sitting, knee flexed 90 degrees, rotate foot laterally (normal 30-40 degrees)
- External rotation: Prone or sitting, knee flexed 90 degrees, rotate foot medially (normal 40-50 degrees)
- Abduction: Supine, stabilize pelvis, abduct hip (normal 40-50 degrees)
- Adduction: Supine, cross leg over midline (normal 20-30 degrees)
- Extension: Prone or Thomas test position (normal 10-20 degrees)
Brooker III-IV Typical Patterns:
- Flexion limited to 70-90 degrees (vs normal 120)
- Internal rotation 0-10 degrees (vs normal 30-40)
- Hard end-feel to motion (bone block)
- Pain with forced motion
Investigations
Diagnostic Imaging Protocol
Assess for early HO formation. May show subtle soft tissue ossification. Too early for Brooker classification (bone not yet mature). Compare to immediate post-operative radiograph.
Brooker classification. Bone more mature, pattern clearer. AP pelvis shows full extent of HO. Lateral hip assesses anterior vs posterior location. Document ROM at this visit.
CT: 3D reconstruction shows exact HO location, relationship to prosthesis, bone maturity. Bone scan: "Cold" scan (no increased uptake) confirms maturation - safe to excise. Hot scan = immature = high recurrence if excised.
Normal ALP suggests maturation complete. Elevated ALP indicates active bone formation - delay excision. Trend ALP over 3-6 months. Return to baseline suggests maturity.
Radiographic Features to Report
- Brooker grade (I-IV)
- Location: Anterior, lateral, posterior, medial
- Proximity to components: Impinging on acetabular rim, femoral neck
- Maturation: Corticated margins suggest maturity
- Progression: Compare to previous radiographs
- Bilateral: Check contralateral hip for HO
Advanced Imaging Indications
- CT scan: Pre-operative planning for HO excision (3D mapping)
- Bone scan: Assessing maturation before excision (must be cold)
- MRI: Rarely needed - if suspecting soft tissue pathology
- Fluoroscopy: Dynamic assessment of impingement
- Serial ALP: Trending bone formation activity
Prevention Strategies
NSAID Prophylaxis (First-Line for Most Patients)
Non-selective NSAIDs and selective COX-2 inhibitors are equally effective at preventing HO. The choice depends on patient risk factors, contraindications, and local availability.
| Drug | Dose | Duration | Notes |
|---|---|---|---|
| Indomethacin | 25mg TDS (75mg total daily) | Start within 24h, continue 6 weeks | Most studied, gold standard, higher GI side effects |
| Celecoxib (COX-2 inhibitor) | 200mg BD (400mg total daily) | Start within 24h, continue 6 weeks | Lower GI risk, cardiovascular precautions in high-risk |
| Naproxen | 500mg BD (1000mg total daily) | Start within 24h, continue 6 weeks | Alternative if indomethacin not tolerated |
| Diclofenac | 50mg TDS (150mg total daily) | Start within 24h, continue 6 weeks | Widely available, intermediate GI risk |
NSAID Contraindications
Absolute contraindications: Active peptic ulcer disease, recent GI bleed, severe renal impairment (eGFR less than 30), aspirin-sensitive asthma. Relative contraindications: History of peptic ulcer (use PPI cover), hypertension (monitor), heart failure, anticoagulation (increased bleeding risk). Consider radiation prophylaxis instead if contraindications present.
Evidence for NSAIDs
- Efficacy: Reduces severe HO (Brooker III-IV) from 20% to 2-5% in high-risk patients
- Timing critical: Must start within 24 hours - delayed prophylaxis ineffective
- Duration: 6 weeks optimal (shorter duration less effective, longer no additional benefit)
- All NSAIDs equally effective: No single agent superior, choice based on side effect profile
- No effect on osseointegration: Does NOT impair bone-implant interface or fracture healing at standard doses
These prevention strategies work by interrupting the early stem cell differentiation cascade.
Management Algorithm
Non-Operative Management (Brooker I-II, Asymptomatic III)
Most HO does not require treatment. Observation with reassurance is appropriate when HO is asymptomatic or minimally symptomatic.
Indications for Observation
- Brooker I-II: Asymptomatic (90% of cases)
- Brooker III: Minimal symptoms, acceptable ROM for ADLs
- Brooker IV: Patient declines surgery, medical comorbidities prohibit surgery
- Immature HO: Less than 12 months post-op (wait for maturation)
- Stable over time: No progression on serial radiographs
Conservative Measures
- Physiotherapy: ROM exercises (gentle, within pain limits)
- Analgesia: NSAIDs for symptom control (NOT for regression)
- Activity modification: Avoid extreme positions
- Serial monitoring: Radiographs at 3, 6, 12 months
- Patient education: Natural history, maturation timeline
HO Does NOT Regress
A common misconception is that established HO can regress with physiotherapy or NSAIDs. Once mature lamellar bone has formed, it does NOT resorb spontaneously. NSAIDs prevent NEW formation but do NOT reverse existing HO. Physiotherapy maintains ROM but does NOT reduce bone mass. Surgical excision is the ONLY method to remove HO.
This approach avoids unnecessary surgery in the majority of patients who have minimal functional impact.
Surgical Technique - Heterotopic Ossification Excision
Surgical Approach Selection
The approach for HO excision depends on the location of the heterotopic bone and the original THA approach used.
Approach Selection Principles
- Use previous incision when possible (avoid multiple scars)
- Posterior HO: Posterior approach (Kocher-Langenbeck modification)
- Anterior HO: Anterior or anterolateral approach
- Circumferential HO: May need extensile approach or staged procedures
- Always identify neurovascular structures before excising HO
Pre-operative Imaging
- CT scan with 3D reconstruction: Essential for surgical planning
- Map HO location: Anterior, lateral, posterior, medial zones
- Neurovascular relationships: Sciatic nerve, femoral vessels at risk
- Proximity to components: Avoid damaging acetabular cup or femoral stem
- Bone maturity: Corticated margins on CT suggest maturity
Patient Positioning
Setup for Posterior Approach (Most Common)
Lateral decubitus position on bean bag or hip positioner. Operative hip up. Pelvis stabilized with anterior and posterior supports. Contralateral leg flexed and padded.
Dependent leg: Pillow between legs, protect peroneal nerve at fibular head. Axilla: Ensure no pressure on brachial plexus. Greater trochanter: Pad to avoid skin pressure.
Landmarks: Expose from iliac crest to mid-thigh, anterior superior iliac spine (ASIS) to posterior superior iliac spine (PSIS). Free draping: Allow hip flexion, rotation for assessment.
This positioning allows access to posterior and lateral HO while preserving ability to assess ROM intraoperatively.
Complications
| Complication | Incidence | Risk Factors | Management |
|---|---|---|---|
| NSAID GI toxicity | 5-10% dyspepsia, 1-2% ulcer | Age over 65, previous ulcer, H. pylori, anticoagulation | PPI co-prescription, switch to COX-2 inhibitor, or radiation |
| NSAID renal impairment | 2-5% transient dysfunction | Pre-existing CKD, dehydration, ACE inhibitors, age over 75 | Monitor creatinine, hydrate, avoid if eGFR under 30 |
| NSAID cardiovascular events | 1-2% (COX-2 inhibitors) | Previous MI, stroke, uncontrolled hypertension | Use lowest effective dose, shortest duration, consider radiation |
| HO recurrence post-excision | 20-30% with prophylaxis, 80% without | Early excision (less than 12 months), incomplete excision, no prophylaxis | Wait for maturation, complete excision, radiation prophylaxis |
| Nerve injury during excision | 2-5% (sciatic nerve most common) | Extensive posterior HO, adherent to nerve, revision surgery | Careful dissection, nerve identification, accept residual HO if needed |
| Infection after excision | 1-3% | Revision surgery, diabetes, prolonged operative time | Standard surgical site infection management, antibiotics |
Avoid Early Excision
The most common error leading to HO recurrence is excising HO before it has matured (less than 12 months post-op). Immature HO contains active osteoblasts and mesenchymal cells that will rapidly re-form bone after excision. Wait for: (1) Cold bone scan, (2) Normal ALP, (3) Corticated radiographic margins, (4) Minimum 12-18 months post-op. Patience prevents recurrence.
Postoperative Care and Rehabilitation
NSAID Prophylaxis Timeline
Start NSAID immediately (indomethacin 25mg TDS or celecoxib 200mg BD). Administer with food to reduce GI upset. Consider PPI if high GI risk. Monitor for allergic reaction.
Continue NSAID daily. Monitor for GI symptoms (dyspepsia, nausea). Check renal function if at risk (baseline creatinine). Encourage compliance (common to discontinue early). Standard THA rehabilitation proceeds.
Continue NSAID daily. Review for side effects at follow-up visit. ROM exercises progressing. Weight-bearing as per THA protocol. Reassurance that GI symptoms usually improve.
Complete 6-week course. Stop NSAID at 6 weeks (no taper needed). Beyond 6 weeks no additional benefit. Consider repeat creatinine if renal concerns. First radiograph at 6 weeks.
Patient Education Points
- Duration: Must complete full 6 weeks (stopping early reduces efficacy)
- Timing: Take with food to reduce stomach upset
- Side effects: Notify if severe abdominal pain, black stools, reduced urine
- Interactions: Avoid other NSAIDs, anticoagulants (discuss with surgeon)
- Alternatives: If intolerable, contact surgeon (can switch to radiation)
Monitoring Protocol
- Week 1: Phone follow-up (tolerating medication?)
- Week 2: Review renal function if high-risk
- Week 6: Clinical review, stop NSAID, first radiograph
- Month 3: Radiograph to assess early HO formation
- Month 6-12: Final radiograph, Brooker classification
This protocol ensures optimal prophylaxis compliance while monitoring for complications.
Outcomes and Prognosis
| Intervention | Key Outcomes | Predictors of Success |
|---|---|---|
| NSAID prophylaxis | Reduces severe HO from 20% to 2-5%, well-tolerated in 90% | Compliance (6 weeks), started within 24h, appropriate dose |
| Radiation prophylaxis | Equal efficacy to NSAIDs, no wound complications | Appropriate timing (within 72h), correct dose (700cGy), adequate field |
| HO excision | 30-50 degree ROM improvement, 70-80% pain relief, 20-30% recurrence | Maturity (over 12 months), complete excision, radiation prophylaxis, appropriate patient selection |
| Observation (mild HO) | Stable over time, minimal symptoms, high satisfaction | Brooker I-II, asymptomatic, patient reassurance, ROM maintenance |
Predictors of Poor Outcome
Factors associated with poor outcomes: Early excision (less than 12 months = 80% recurrence), no prophylaxis at excision (recurrence rate doubles), incomplete excision (residual HO = continued symptoms), unrealistic expectations (cannot restore normal hip ROM in all cases), nerve injury during excision (2-5% permanent deficit). Optimal outcomes require patient selection, timing, technique, and prophylaxis.
Long-Term Follow-Up Data
5-Year Outcomes After Excision:
- ROM improvement maintained in 75% (some late loss due to capsular contracture)
- Pain relief sustained in 80% (improved from 70-80% at 1 year)
- Recurrence stabilizes at 20-30% (most occurs within 2 years)
- Patient satisfaction remains high (80%) if appropriate selection
- Revision rate for recurrent HO: 5% (second excision has higher recurrence)
Registry Data (AOANJRR):
- HO excision performed in 1-3% of THA cases nationally
- Higher rates in post-traumatic arthritis cohort (8-10%)
- Revision for HO more common in males (M:F ratio 3:1)
- No difference in outcomes between ceramic and polyethylene bearings
Evidence Base and Key Trials
Indomethacin vs Radiation for HO Prevention (Randomized Trial)
- RCT comparing indomethacin 75mg daily × 6 weeks vs radiation 700cGy single dose
- No difference in severe HO rates: Indomethacin 6% vs Radiation 5%
- Both significantly better than historical controls (no prophylaxis 25% severe HO)
- Indomethacin: 12% discontinued due to GI side effects
- Radiation: No wound complications, no increased infection
COX-2 Inhibitors for HO Prophylaxis (Prospective Cohort)
- Prospective cohort: celecoxib 200mg BD × 6 weeks vs historical indomethacin controls
- Severe HO rates: Celecoxib 4% vs Indomethacin 5% (not statistically different)
- GI side effects: Celecoxib 6% vs Indomethacin 18% (p less than 0.01)
- No difference in wound complications or infection rates
- Patient compliance better with celecoxib (92% vs 78% completed 6 weeks)
Timing of Radiation Therapy for HO Prevention (Multicenter RCT)
- RCT: pre-operative radiation (4h before surgery) vs post-operative (24-72h after) vs no prophylaxis
- Severe HO: Pre-op 3%, Post-op 4%, No prophylaxis 23% (p less than 0.001)
- No difference between pre-op and post-op radiation timing
- No wound complications in radiation groups
- Single 700cGy dose as effective as fractionated 1000cGy
HO Excision Timing and Recurrence (Retrospective Series)
- Retrospective review of 50 HO excisions after THA
- Excision under 12 months: 82% recurrence; 12-18 months: 25% recurrence; over 18 months: 12% recurrence
- All recurrences occurred in patients who did NOT receive radiation prophylaxis at excision
- Bone scan maturity (cold scan) better predictor than time alone
- Incomplete excision associated with 100% recurrence
Australian Registry Data on HO After THA (AOANJRR)
- HO excision performed in 1.2% of all THA cases nationally
- Post-traumatic arthritis cohort: 8.7% HO excision rate (highest indication)
- Anterolateral approach: 2.1% excision rate vs posterior approach: 0.8%
- Male gender: 1.8% vs female: 0.7% (M:F ratio 2.6:1)
- 85% of surgeons report routine NSAID prophylaxis use; 15% selective use
Exam Viva Scenarios
Practice these scenarios to excel in your viva examination
Scenario 1: Primary THA HO Prevention (Standard, ~2-3 min)
"You are planning a primary total hip arthroplasty for a 62-year-old male with primary osteoarthritis. He has hypertrophic osteophytes on radiographs. What is your strategy for heterotopic ossification prophylaxis?"
Scenario 2: Previous HO Management (Challenging, ~3-4 min)
"A 58-year-old male had a right total hip arthroplasty 3 years ago and developed Brooker grade III heterotopic ossification. He now requires a left total hip arthroplasty for symptomatic osteoarthritis. His right hip is pain-free but has reduced flexion to 80 degrees and internal rotation 5 degrees. How do you manage HO risk for the planned left THA?"
Scenario 3: Symptomatic HO Excision (Critical, ~3-4 min)
"A 55-year-old male is 14 months post-THA for post-traumatic arthritis following an acetabular fracture. He has Brooker grade IV heterotopic ossification with hip flexion limited to 60 degrees and internal rotation 0 degrees. He has significant difficulty sitting and toileting. AP pelvis radiograph shows extensive anterior and lateral HO bridging the femur and pelvis. How do you manage this patient?"
MCQ Practice Points
Pathophysiology Question
Q: What is the primary mechanism by which NSAIDs prevent heterotopic ossification? A: NSAIDs inhibit cyclooxygenase (COX) enzymes, preventing prostaglandin E2 (PGE2) synthesis. PGE2 is a critical mediator of mesenchymal stem cell differentiation into osteoblasts. By blocking PGE2 production during the early post-operative period (0-6 weeks), NSAIDs prevent the cellular cascade that leads to heterotopic bone formation. This is why timing is critical - NSAIDs must be started within 24 hours to interrupt the initial stem cell differentiation phase.
Classification Question
Q: What is the key distinguishing feature between Brooker grade II and grade III heterotopic ossification? A: The distance between opposing bone surfaces: greater than 1cm = Brooker II; less than 1cm = Brooker III. Both grades involve bone spurs projecting from the pelvis or proximal femur, but Brooker III has spurs that are close enough (less than 1cm gap) to mechanically impinge and restrict range of motion. Brooker II maintains a gap greater than 1cm and is usually asymptomatic. This distinction is critical because Brooker III-IV are functionally significant and may require treatment, while I-II are typically observed.
Prevention Question
Q: A patient with ankylosing spondylitis requires THA. What is the most appropriate HO prophylaxis strategy? A: Either NSAIDs (indomethacin 75mg daily × 6 weeks) OR single-dose radiation (700cGy) - both are equally effective in high-risk patients. Ankylosing spondylitis is a very high-risk condition for HO (50% severe HO without prophylaxis). The choice depends on patient factors: if NSAID contraindications (GI, renal), use radiation; if radiation unavailable or patient preference, use NSAIDs. There is NO additional benefit to combining both modalities. Key is to ensure whichever modality is chosen is given at the correct dose and timing.
Timing Question
Q: At what minimum time point post-THA is it safe to excise heterotopic ossification, and how do you confirm maturation? A: Minimum 12-18 months post-operatively, confirmed by cold technetium-99m bone scan and normalized alkaline phosphatase. Excising immature HO leads to 80% recurrence because active osteoblasts and mesenchymal stem cells remain viable and rapidly re-form bone. Mature HO shows corticated margins on radiograph, no increased uptake on bone scan (cold scan), and normal serum ALP. Time alone is insufficient - maturation must be confirmed biochemically and scintigraphically. For Brooker IV, wait 18-24 months to ensure complete maturation.
Excision Question
Q: A patient undergoes HO excision 15 months post-THA. What prophylaxis should be given, and what is the expected recurrence rate? A: Single-dose radiation 700cGy (preferred) given pre-operatively or within 24 hours post-operatively; recurrence rate 20-30% with radiation, 50-80% without. Radiation is more effective than NSAIDs at preventing recurrence after HO excision, likely because the local environment is already primed for bone formation and radiation directly targets proliferating osteoblasts. NSAIDs can be used but have higher recurrence. Complete excision of all HO is essential - incomplete excision dramatically increases recurrence. Even with optimal technique and prophylaxis, recurrence occurs in 20-30% of cases.
Evidence Question
Q: What are the key findings from randomized trials comparing NSAIDs to radiation for HO prophylaxis? A: Equally effective (both reduce severe HO from 20% to 2-5%), no difference in wound complications or infection rates, choice based on patient factors and logistics. Multiple RCTs have shown equivalent efficacy. NSAIDs have GI and renal side effects (10-15% discontinuation rate) but are simple to prescribe. Radiation has no systemic side effects but requires radiation oncology coordination. Combination therapy (NSAIDs + radiation) provides NO additional benefit over single modality. Patient contraindications and institutional resources guide choice.
Australian Context and Medicolegal Considerations
AOANJRR Data and National Practice
- Prophylaxis rates: 85% of Australian surgeons use routine NSAID prophylaxis for all THA
- COX-2 inhibitors: 60% of prophylaxis prescriptions (increasing preference vs traditional NSAIDs)
- HO excision rate: 1.2% nationally; 8.7% in post-traumatic arthritis cohort
- Approach variation: Anterolateral 2.1% excision rate vs posterior 0.8%
- Gender disparity: Male HO excision rate 1.8% vs female 0.7%
- State variation: Higher routine prophylaxis in Victoria (92%) vs other states (80-85%)
PBS and Medication Access
- Indomethacin: PBS-listed for HO prophylaxis (25mg capsules)
- Celecoxib: PBS-listed with special authorization for HO prophylaxis (200mg capsules)
- Typical prescription: Indomethacin 25mg TDS × 6 weeks (authority script needed for full course)
- PPI co-prescription: Often PBS-listed concurrently (esomeprazole, pantoprazole)
- Radiation: Medicare rebate item 15000 (external beam radiotherapy) covers HO prophylaxis
- Cost-effectiveness: Prophylaxis costs AU$50-200 vs HO excision AU$15,000-25,000
Medicolegal Considerations
Key documentation requirements for informed consent:
HO Risk Discussion: Document that patient was counseled about HO risk factors (if present) and prevention options. Failure to offer prophylaxis in high-risk patients (previous HO, ankylosing spondylitis, post-traumatic arthritis) is defensible only if contraindications exist and documented.
Prophylaxis Consent: Document discussion of NSAID risks (GI, renal, cardiovascular) vs radiation (no increased wound/infection risk, gonadal shielding). Patient refusal of prophylaxis should be documented.
Excision Consent: If excising HO, must document discussion of: (1) Recurrence risk 20-30%, (2) Nerve injury risk 2-5%, (3) Realistic ROM expectations, (4) Alternative of observation if minimal symptoms. Excising immature HO (less than 12 months) without documenting urgent indication is difficult to defend if recurrence occurs.
Adverse Outcomes: If severe HO develops despite prophylaxis (2-5% even with prophylaxis), documentation of appropriate prophylaxis dose, timing, and duration protects against litigation. If patient developed GI bleed on NSAIDs, documentation of pre-prescription risk assessment and PPI consideration is critical.
Common litigation scenarios: (1) No prophylaxis offered in high-risk patient who develops severe HO, (2) NSAID-related GI bleed without documented risk assessment, (3) Early HO excision (less than 12 months) leading to recurrence, (4) Nerve injury during HO excision without documented consent discussion.
ACSQHC Guidelines (Australian Commission on Safety and Quality in Health Care)
- VTE Prophylaxis: NSAIDs for HO do NOT replace mechanical or pharmacological VTE prophylaxis
- Antimicrobial Stewardship: HO does NOT increase infection risk; standard THA antibiotic prophylaxis unchanged
- Patient Safety: NSAID prescribing requires documented renal function, GI risk assessment
- Medication Reconciliation: Ensure NSAIDs do not interact with anticoagulants (warfarin, DOACs)
THA Heterotopic Ossification
High-Yield Exam Summary
High-Risk Factors (ASHAMED)
- •Ankylosing spondylitis (50% severe HO without prophylaxis)
- •Spinal cord injury (neurologic HO risk)
- •Hypertrophic osteoarthritis (large osteophytes predict HO)
- •Acetabular fracture/post-traumatic OA (50-90% risk)
- •Male gender (2-3× higher risk than females)
- •Extensive soft tissue damage (revision THA, difficult exposures)
- •DISH - diffuse idiopathic skeletal hyperostosis
Brooker Classification
- •I = Islands of bone in soft tissue (asymptomatic, observe)
- •II = Spurs with gap greater than 1cm (usually asymptomatic)
- •III = Spurs with gap less than 1cm (may restrict ROM, consider excision)
- •IV = Ankylosis (significant functional loss, excision often needed)
- •Assess on AP pelvis at 6-12 months post-op
Prophylaxis Protocol
- •NSAIDs: Indomethacin 75mg daily × 6 weeks (start within 24h)
- •COX-2 inhibitors: Celecoxib 200mg BD × 6 weeks (lower GI risk)
- •Radiation: 700cGy single dose within 24-72h post-op (or pre-op)
- •Both modalities equally effective (reduce severe HO from 20% to 2-5%)
- •NO benefit from combining NSAIDs + radiation
- •Previous HO = 90% recurrence without prophylaxis (radiation preferred)
Excision Pearls
- •Wait 12-18 months minimum (Brooker III) or 18-24 months (Brooker IV)
- •Confirm maturation: cold bone scan + normal ALP + corticated margins
- •Pre-op CT with 3D reconstruction for surgical planning
- •Complete excision essential (incomplete = recurrence)
- •Radiation prophylaxis 700cGy at excision (preferred over NSAIDs)
- •Recurrence: 20-30% with prophylaxis; 50-80% without; 82% if excised before 12 months
- •Nerve injury risk 2-5% (sciatic most common)
- •ROM improvement: 30-50 degrees; Pain relief: 70-80%
Complications to Counsel
- •NSAID GI toxicity: 5-10% dyspepsia, 1-2% ulcer (use PPI in high-risk)
- •NSAID renal impairment: 2-5% (monitor creatinine, avoid if eGFR under 30)
- •NSAID CV events: 1-2% with COX-2 inhibitors (caution in cardiac patients)
- •HO recurrence post-excision: 20-30% with prophylaxis, 80% without
- •Nerve injury during excision: 2-5% (sciatic - permanent deficit possible)
- •Infection after excision: 1-3% (standard wound infection management)