Treatment of Myositis Ossificans

Radiographic appearance (mature MO)

Mature myositis ossificans of the proximal right femur — note the well-organized peripheral calcifications with a central lucent area, characteristic of mature MO. — ROSEN's Emergency Medicine

1. Localized (Traumatic) Myositis Ossificans

Conservative (First-line)

• Activity modification / rest: The mass tends to subside spontaneously over several months if the causative activity is stopped. — Adams & Victor's Principles of Neurology, 12th ed.
• Observation is the primary recommended treatment in most orthopaedic references. — Miller's Review of Orthopaedics, 9th ed.
• NSAIDs (especially Indomethacin): Used prophylactically after high-risk trauma or surgery.
  • Indomethacin 25 mg orally three times daily, or SR 75 mg once daily for 6 weeks is the standard regimen.
  • Efficacy is debatable and it may increase the fracture nonunion rate — use cautiously in perioperative settings. — Miller's Review of Orthopaedics, 9th ed.

Prophylactic Radiation Therapy

• 600–800 cGy (6–8 Gy) as a single fraction, given 24 hours before or up to 72 hours after surgery
• Prevents proliferation/differentiation of mesenchymal cells into osteoprogenitor cells
• Equal to indomethacin in effectiveness, with better compliance
• Preoperative radiation is preferred for patients at very high risk (e.g., Paget disease, prior MO; incidence of HO after THA in Paget disease ~50%) — Miller's Review of Orthopaedics, 9th ed.

Surgical Excision

• Indicated only when the lesion is mature — confirmed by radiographic evidence of sharp demarcation and trabecular pattern
• Timing: Wait at least 6–12 months after injury; for THA-related HO, delay at least 6 months
• For patients with traumatic brain injury, timing is critical: wait 3–6 months with evidence of bone maturation; recurrence after resection is likely if neurologic compromise is severe
• Treatment is excision of "problematic" HO once no further growth is evident (stable on serial radiographs, "quiet" bone scan, or >1 year from injury) — Miller's Review of Orthopaedics, 9th ed.
• Adjuvant radiation therapy after excision helps prevent recurrence — Miller's Review of Orthopaedics, 9th ed.

2. Generalized (Progressive) Myositis Ossificans — FOP

Pharmacological

• Bisphosphonates (diphosphonates): Etidronate (EHDP, 10–20 mg/kg orally) inhibits calcium phosphate deposition; has been reported to cause regression of new swellings and prevent calcification. — Adams & Victor's Principles of Neurology, 12th ed.
• Prednisone: Useful in calcinosis universalis (closely related condition); worth attempting given the unclear relationship. — Adams & Victor's Principles of Neurology, 12th ed.
• Palovarotene (RAR-γ agonist): Reduces new heterotopic ossification by ~60% vs. historical controls. FDA-approved in 2023 for females ≥8 years and males ≥10 years. Note: risk of premature epiphyseal closure in younger children. — Harrison's Principles of Internal Medicine, 22nd ed. (2025)
• Garetosumab (REGN2477): Anti-activin A monoclonal antibody — currently in clinical trials. — Harrison's, 22nd ed.

What NOT to do in FOP

• Surgical removal of ectopic bone is contraindicated — trauma from surgery precipitates formation of new areas of heterotopic bone
• Local anesthetic injections can trigger frozen jaw (trismus)
• Low-calcium diet, glucocorticoids, and bisphosphonates have largely been ineffective in halting progression of established ossification in FOP. — Harrison's, 22nd ed.

Key Differentiating Point for Imaging

• MO: calcification begins at the periphery, progresses inward
• Osteosarcoma: calcification begins centrally first — ROSEN's Emergency Medicine

Recent Evidence