Brooker Classification of Heterotopic Ossification

Identifying high-risk patients is critical because prophylaxis is far more effective than treatment of established HO.

Understanding the biology helps explain why prophylaxis works and why timing of excision matters:

• HO is the formation of mature lamellar bone in soft tissues where bone does not normally exist — it is NOT calcification (dystrophic or metastatic) and it is NOT ossification from periosteum.
• The trigger is thought to be osteoprogenitor cells (likely derived from mesenchymal stem cells) that are incorrectly signalled to differentiate along the osteoblastic lineage in response to inflammation and tissue injury.
• Key molecular mediators: Bone morphogenetic proteins (BMP-2, BMP-4), prostaglandin E2, and inflammatory cytokines released at the time of surgery activate the process. This is why COX inhibition (indomethacin) is effective prophylaxis.
• Three phases of HO formation:
  1. Early (0–4 weeks): Inflammatory; soft-tissue swelling, pain, decreased range of motion. Radiographs may be normal. Bone scan shows increased uptake.
  2. Intermediate (4–16 weeks): Osteoid formation; mineralisation begins. Radiographs show faint, fluffy ossification. Alkaline phosphatase is elevated.
  3. Mature (4–12+ months): Organised lamellar bone with cortical and cancellous patterns. Radiographs show well-defined bone. Bone scan uptake decreases. This is the safe window for excision.

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The classification is assessed on a single AP radiograph of the hip (typically a supine or standing AP pelvis). It describes the most severe heterotopic bone formation visible in the soft tissues between the greater trochanter and the femoral shaft/pelvis.

The Brooker system is the most widely used, but other classifications exist:

• Hattrick and Harris (modified Brooker): Adds a grade 0 (no HO) and refines the gap measurement — sometimes used in research trials for greater granularity.
• DeLee classification: Uses CT-based assessment for pre-operative planning of excision; maps the anatomic zone of HO (anterior, posterior, medial, lateral) — more useful for surgical planning than the Brooker AP-only assessment.
• Santiago classification: A CT-based system that grades HO by the quadrant involved and the volume of heterotopic bone — provides three-dimensional information the Brooker system lacks.

In clinical practice and exams, Brooker remains the standard. Know that it has moderate inter-observer reliability (especially distinguishing Grade II from Grade III when the gap is close to 1 cm) and that CT is the investigation of choice for surgical planning.

Prophylaxis Options

Two strategies have Level I evidence for HO prophylaxis after THA:

Surgical Excision

• Indication: Brooker Grade III–IV HO causing functional limitation (restricted flexion less than 60 degrees, impaired gait, difficulty with sitting or hygiene).
• Timing: Delay excision until the HO is mature — typically 12–18 months after index surgery. Maturity is confirmed by a stable Brooker grade on serial radiographs, normalising alkaline phosphatase, and a 'cold' or decreasing uptake on bone scan.
• Approach: Excision through the original surgical approach where possible. Meticulous haemostasis is critical because haematoma promotes recurrence.
• Peri-operative prophylaxis is MANDATORY: Either indomethacin or radiotherapy must be given at the time of excision (typically radiotherapy within 48 hours post-excision) — recurrence rates without prophylaxis are high (up to the original grade or worse).

Global Epidemiology

• HO after THA has an overall incidence of 8–50 percent on radiographs, but only 1–5 percent develop functionally significant (Brooker Grade III–IV) ossification.
• Male patients are 2–3 times more affected than female patients. Bilateral THA in a patient who developed HO after the first side has a recurrence rate over 80 percent without prophylaxis.
• Higher rates reported with lateral and anterolateral surgical approaches compared with the posterior approach, likely due to greater soft-tissue dissection and gluteal muscle trauma.

Guidelines Comparison

Key Practice Variation

• US practice tends toward radiotherapy more commonly for high-risk patients, partly driven by radiotherapy infrastructure availability and the higher rate of cementless stems (where early NSAID use may theoretically impair bony ingrowth, though clinical evidence for this is weak).
• European and UK practice tends to favour indomethacin as first-line prophylaxis due to cost-effectiveness and ease of administration, reserving radiotherapy for recurrent HO or NSAID-intolerant patients.
• Japanese and East Asian registries report slightly lower HO incidence overall, which may reflect differences in surgical approach preferences or genetic factors, though data are limited.