Heterotopic Ossification
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Direct answer
Heterotopic ossification means mature lamellar bone forming inside soft tissue, most often around the hip after arthroplasty or acetabular surgery and after head or spinal cord injury. The exam favourite is the Brooker classification, which grades peri-hip ossification from isolated bone islands to a fused, ankylosed hip. Prophylaxis with a short course of NSAIDs, or a single dose of radiotherapy given early after surgery, is standard in high-risk patients.
What you must remember
- Definition: mature bone in soft tissue outside the skeleton, a reactive metaplastic process rather than a neoplasm.
- Commonest setting: the hip after total hip arthroplasty or open reduction of acetabular fractures.
- Neurogenic and other settings: head injury, spinal cord injury and burns, where the risk follows the injury and prolonged immobility.
- Brooker class I: isolated bone islands in soft tissue around the hip; class IV is bony ankylosis of the joint.
- Early markers: bone scan uptake appears before mature bone is seen on radiograph, and serum alkaline phosphatase rises in active disease.
- Prophylaxis: NSAIDs such as indomethacin, used for a limited course because they can impair bone union, or early single-dose radiotherapy.
- Timing of excision: delayed until the lesion has matured, judged by a normal bone scan and alkaline phosphatase, because immature excision recurs.
Brooker grades and what they mean for surgery
Brooker's classification grades ectopic bone around the hip by how close it comes to the joint. The grade tells the surgeon whether function is threatened and whether excision is worth planning.
| Brooker class | Radiographic picture | Hip movement | Usual handling |
|---|---|---|---|
| I | Isolated bone islands in soft tissue | Normal | Observation |
| II | Spurs from pelvis or femur, at least 1 cm apart | Mildly reduced | Observation, excision if disabling |
| III | Spurs narrowing the joint gap to under 1 cm | Clearly reduced | Excision after maturation |
| IV | Bony ankylosis of the hip | Fixed | Excision, often with arthroplasty |
Classes III and IV are the grades that justify excision planning, and only once the lesion has matured. Class I and II lesions are usually left alone because they rarely restrict the hip enough to matter.
Serial radiographs and a bone scan show whether the lesion is still active, which decides the timing of any operation. Class IV hips often need excision together with arthroplasty, because the joint surfaces must be rebuilt.
Where the exam tries to catch you
Most lost marks come from confusing heterotopic ossification with other bone-forming lesions of soft tissue.
- Trap: treating every peri-hip bone spur as a sarcoma Correct: heterotopic ossification is reactive and histologically mature, with no atypia, so it is not malignant.
- Trap: using radiotherapy to treat established heterotopic bone Correct: radiotherapy is a prophylactic measure given early after surgery, and it does not remove mature bone.
- Trap: excising the bone before it matures Correct: surgery waits for maturity, because early excision leads to recurrence.
Frequently asked questions
Which hip procedures carry the highest risk of heterotopic ossification?
Heterotopic ossification after hip surgery is most common after total hip arthroplasty and open reduction of acetabular fractures. Men and patients with ankylosing spondylitis or diffuse idiopathic skeletal hyperostosis are at higher risk.
What does the Brooker classification of heterotopic ossification grade?
The Brooker classification grades heterotopic ossification around the hip by the extent of the new bone and the gap between opposing bone surfaces. Class I is isolated bone islands, and class IV is bony ankylosis of the hip. The grade guides whether excision is considered.
Which drugs prevent heterotopic ossification after hip surgery?
Non-steroidal anti-inflammatory drugs, most often indomethacin, are given for a limited course to prevent heterotopic ossification after hip surgery in high-risk patients. Single-dose radiotherapy given early after surgery is the other established option. Both carry trade-offs, so the choice depends on the individual patient.
When is heterotopic ossification excised?
Heterotopic ossification is excised only after the bone has matured, judged by a normal bone scan and a normalised serum alkaline phosphatase. Surgeons usually wait for several months, because excision of immature bone tends to recur. Surgery is offered for functional loss rather than for appearance alone.