Myositis Ossificans

Myositis ossificans is a benign, self-limited formation of heterotopic bone within muscle, usually after blunt trauma. The usual form is myositis ossificans traumatica, most often in the quadriceps (especially the vastus intermedius) and brachialis in athletes (football, rugby, martial arts), and in adolescents and young adults aged 15 to 30 years. A form without a history of trauma and a progressive genetic form (fibrodysplasia ossificans progressiva) also exist.

It follows a deep haematoma after a contusion, a muscle tear, or an elbow dislocation in the case of brachialis. Predisposing factors are repeated trauma to the same area, early aggressive massage or stretching, delayed treatment of haematoma, and returning to sport too early.

Complications are persistent pain, loss of joint range of motion (knee flexion after thigh injury, elbow extension after brachialis injury), muscle weakness, nerve compression, and, rarely, confusion with a sarcoma, particularly parosteal osteosarcoma.

Pathophysiology.

A haematoma in muscle triggers inflammation and the differentiation of mesenchymal cells into osteoblasts and chondroblasts. Maturation follows a zonal pattern: a central immature zone of fibroblasts and proliferating cells, a mid zone of osteoid, and a peripheral zone of mature lamellar bone (the zoning phenomenon, which distinguishes it from a sarcoma, where the centre is mature).

The bone appears on radiographs at 3 to 6 weeks, matures by 5 to 6 months, and often shrinks over time.

After a blunt injury the patient has a painful, swollen and bruised muscle, with a tender mass that becomes firmer and warmer over 1 to 2 weeks. A palpable hard, tender mass develops in the muscle, with pain that fails to settle after 2 to 3 weeks, and loss of range of motion (reduced knee flexion to under 90 degrees in the quadriceps).

Examine muscle strength, the active and passive range, neurovascular function, and the skin. Warning features of an alternative diagnosis are a mass without trauma, rapid growth, night pain and systemic upset.

• Radiographs: normal in the first 2 weeks; at 3 to 4 weeks, fluffy calcification with a peripheral rim of mature bone and a lucent centre (the zoning phenomenon). A cleavage plane separates the lesion from the adjacent bone.
• Ultrasound shows early haematoma and calcification at 1 to 2 weeks; MRI is nonspecific early, and later shows a mass with oedema (mimicking a sarcoma).
• CT shows the peripheral shell of mature bone.
• Alkaline phosphatase may be raised (peaks at 2 to 3 months). Biopsy only if the diagnosis is uncertain, because early biopsy can be mistaken for sarcoma.
• Genetic testing (ACVR1) if multiple lesions occur without trauma.

Differential diagnosis.

• Parosteal osteosarcoma and periosteal osteosarcoma.
• Soft tissue sarcoma and osteochondroma.
• Calcific tendinitis and heterotopic ossification after surgery or spinal injury.
• Haematoma, abscess and fibrodysplasia ossificans progressiva.

Prevention after a muscle contusion.

• Rest, ice, compression and elevation for 24 to 48 hours, with the knee held in flexion (for a thigh contusion) to limit haematoma.
• Avoid heat, massage, aggressive stretching and ultrasound over the injury in the first weeks.

Established myositis ossificans.

• Rest and protection for 1 to 3 weeks, then gentle painless active range-of-motion exercises and isometric strengthening, increasing as pain allows.
• Physiotherapy to restore range and strength; return to sport when the range of motion is full and strength is symmetrical (usually 3 to 6 months).
• Ibuprofen 400 mg orally every 8 hours with food for up to 5 to 7 days (maximum 1200 mg daily); avoid with eGFR under 30 mL/min/1.73 m2, peptic ulcer, heart failure or anticoagulant use. Longer NSAID courses are used by some centres to limit bone formation, but evidence is limited.
• Surgical excision is delayed for 6 to 12 months until the lesion matures, and done only for persistent pain, mechanical block or nerve compression. Early excision leads to recurrence.
• Extracorporeal shock wave therapy is used in some centres, with limited evidence.

Operative antibiotic prophylaxis. Cefazolin 2 g intravenously (3 g when weight is 120 kg or over) within 60 minutes before incision as a single dose (repeat after 4 hours or blood loss over 1500 mL); with severe penicillin allergy, vancomycin 15 mg/kg intravenously starting 90 to 120 minutes before incision as a single dose, with levels and creatinine checked in prolonged cases.

Analgesia.

• Paracetamol 1 g orally every 6 hours as needed for up to 5 to 7 days (maximum 4 g daily; 3 g daily when weight is under 50 kg, with chronic alcohol use or hepatic impairment).
• Oral morphine 5 to 10 mg every 4 hours as needed for severe pain for up to 3 days (start 2.5 to 5 mg with age over 75 or eGFR under 30 mL/min/1.73 m2; avoid exceeding 60 mg in 24 hours without senior review); monitor sedation and respiration and prescribe a laxative.

Monitoring. Range of motion and pain at each visit in the first 6 weeks. Radiographs at 4 to 6 weeks and 3 months to document maturation (the lesion should be smaller and better defined). Review sooner if the mass grows, which should prompt MRI and biopsy.

Referral and escalation. Refer a mass that grows, has no history of trauma, is not zoned, or causes severe loss of motion to orthopaedic oncology. Refer persistent symptoms at 6 months for consideration of excision. Urgent referral for neurovascular compression.

Special groups.

• Children: myositis ossificans is uncommon under age 10; consider non-accidental injury and bleeding disorders.
• Spinal cord injury and head injury: neurogenic heterotopic ossification is a related condition with its own prophylaxis.
• Athletes: avoid early return to contact; protect with padding.
• Kidney impairment: avoid ibuprofen and reduce morphine doses.