Myositis Ossificans

Myositis ossificans is heterotopic bone formation within muscle, typically after trauma or in association with neurological conditions. The name is a misnomer: there is no inflammation, and the bone forms in muscle, not from muscle.

History. First described in 1883, it was originally thought to be inflammatory and is now recognised as heterotopic ossification. The zoning phenomenon was once described as pathognomonic; it is characteristic, not pathognomonic, and it is absent early.

Who gets it. Young, active adults, with a peak at 20-40 years; the traumatic form predominates in athletes and trauma patients. There is a male predominance, reflecting higher exposure to contact-sport and trauma mechanisms.

Where. It is most common after quadriceps contusions, elbow trauma (especially in children) and hip trauma. In the West Point athlete series (Ryan et al. 1991) it developed in approximately 9% of significant quadriceps contusions.

Risk factors. The features that mark out the patient at risk after a contusion:

• Knee range of motion under 120 degrees
• Football or other contact injury
• Previous quadriceps injury
• Treatment delay over 3 days
• Ipsilateral knee effusion
• Early aggressive massage or stretching

How it forms. Muscle trauma, a contusion with its haematoma, starts a sequence that takes 6-12 months to complete:

1. The haematoma organises into fibrous tissue
2. Fibroblasts undergo metaplasia to osteoblasts
3. Heterotopic bone forms within the muscle
4. The bone matures

The zoning phenomenon. The bone forms from the outside in. The periphery forms first and matures to lamellar bone while the centre is still immature, so the radiograph shows mature bone peripherally and immature bone centrally. Osteosarcoma shows the reverse, mineralising most densely in the centre, and this is the feature on X-ray that separates the two.

Characteristic, not pathognomonic. Zoning develops as the lesion matures, and in the first weeks it is absent. In that early phase both imaging and histology can be non-characteristic and can mimic sarcoma (Savvidou 2021), which is exactly when the misdiagnosis happens. An early film that lacks zoning excludes neither myositis ossificans nor a sarcoma: interval imaging establishes the diagnosis, and a single early film does not.

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The radiographic zoning phenomenon has a microscopic counterpart, and it is what makes the sarcoma trap comprehensible. The lesion is organised into three concentric zones, and the diagnostic danger lies entirely in the centre:

• Central zone, the danger zone. Highly cellular, mitotically active proliferating fibroblasts and immature mesenchymal spindle cells in a myxoid or haemorrhagic background, with reactive but not truly malignant atypia. On a small or early biopsy this is the zone misread as osteosarcoma.
• Intermediate zone. Osteoblasts rimming trabeculae of immature woven bone (osteoid) within a fibrovascular stroma, an orderly transition rather than a permeative one.
• Peripheral zone. Well-organised mature lamellar or trabecular bone, cytologically bland and sharply demarcated from the surrounding muscle. This is the rim seen radiographically.

Why it maps onto imaging. Bone organises from the periphery inward over weeks, so the peripheral bone, the earliest to mature, forms the corticated rim. Extraskeletal osteosarcoma is the opposite: pleomorphic cells produce malignant osteoid throughout the mass, with the densest mineralisation centrally and destructive margins.

Telling it from osteosarcoma. The key exam distinction:

• Myositis ossificans is well-circumscribed with orderly zonal maturation, immature centre to mature periphery; osteosarcoma is disorganised and permeative, with no zonal gradient
• The peripheral bone of myositis ossificans is bland and lamellar, where osteosarcoma produces malignant osteoid from atypical cells throughout
• A subset of myositis ossificans carries a USP6 gene rearrangement, shared with nodular fasciitis and aneurysmal bone cyst and absent in osteosarcoma, which can support the diagnosis in genuinely difficult cases

The practical rule. A biopsy sampled from the immature centre in the first few weeks is the classic error that yields a false osteosarcoma report. If biopsy is truly unavoidable, sample the periphery, and have a musculoskeletal pathologist interpret it together with the trauma history and imaging, never in isolation.

After the quadriceps, the elbow is the classic site, and it carries a distinct, heavily examinable iatrogenic risk. Elbow myositis ossificans follows posterior elbow dislocation, terrible-triad injuries and paediatric supracondylar or forearm fractures. It characteristically forms anteriorly within the brachialis, which lies directly on the anterior capsule and distal humerus, where haematoma from the injury organises, ossifies and blocks the flexion-extension arc.

The forced-stretch pitfall. The single greatest avoidable risk is forced passive stretching or manipulation of the stiff, freshly injured elbow. Vigorous passive mobilisation re-injures the muscle, enlarges the haematoma and converts it into heterotopic bone, which is why the old teaching of aggressive passive stretching of the stiff post-traumatic elbow, and of manipulation under anaesthesia in the acute or subacute phase, has been abandoned.

Principles at the elbow. The rules specific to this joint:

• Favour early active and active-assisted range of motion within a pain-free arc; avoid forced passive stretch and early manipulation under anaesthesia
• Consider NSAID or indomethacin prophylaxis after high-risk elbow trauma or surgery in at-risk patients
• Neurogenic elbow ossification (after head injury) matures more slowly and recurs more often than the traumatic form
• Once the lesion is stable (corticated margins, settled ALP, plateaued scan), a symptomatic anterior lesion restricting the arc can be excised under the same maturity rule and post-excision prophylaxis used elsewhere

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Myositis ossificans is classified by its cause and by its maturity.

By cause. Three types are described:

By maturity. Maturity is read from the radiograph, the bone scan and the alkaline phosphatase (ALP) together, and it decides whether excision is safe:

History. Ask about trauma or a contusion in the preceding weeks to months:

• Quadriceps contusion, the most common
• Elbow trauma
• Hip trauma
• Head injury or spinal cord injury, in the neurogenic form

Early, at 3-4 weeks. There is a palpable firm mass in the muscle, range of motion is decreased and activity is painful. Swelling may persist.

Established, at 3-6 months. The mass is firm and fixed within the muscle. Range of motion is limited out of proportion to the injury, passive stretch aggravates the pain, and the lesion may cause functional impairment. By this stage the X-ray shows the zoning phenomenon.

Radiographs. The lesion appears at 3-4 weeks after injury. While immature it has ill-defined margins without cortication; zoning follows, and corticated margins develop over 6-12 months. Corticated margins on X-ray or CT are the most accessible sign that the lesion has stopped changing.

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Bone scan. A three-phase bone scan is hot while the lesion is immature, and uptake falls as it matures. Serial scans are the traditional indicator of maturity, and the traditional rule required the scan to be cold before excision. Mature heterotopic bone is bone and retains some uptake, so the practical criterion is uptake that has fallen to a stable plateau on serial scans rather than literal absence of activity. Waiting for a truly cold scan can mean never operating; a still-rising or still-hot scan means the lesion is active.

CT shows the zoning phenomenon clearly and is the planning study, showing the relationship to the neurovascular structures; 3D reconstruction helps.

MRI shows the soft-tissue involvement and shows muscle oedema early. It may help distinguish the lesion from a tumour, but it is supportive rather than diagnostic.

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Alkaline phosphatase. Serum ALP rises with active ossification and settles as the lesion matures. It is non-specific, so a trend beats a single value: monitor it serially if maturity is uncertain, and read a normal value as support for maturity rather than proof of it.

Differential diagnosis. The mimics, and the error each one invites:

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The early lesion. A lesion under 6 months old, with a hot bone scan and an elevated ALP, is managed conservatively and is not excised, because recurrence is high after excision of an immature lesion:

• NSAIDs may limit progression if caught early
• Physiotherapy for gentle range of motion, avoiding aggressive stretching
• Monitoring with serial X-rays, bone scan and ALP

When to excise. The conventional rule is to wait a minimum of 6 months and ideally 12, on the grounds that excision disrupts the maturation process and the lesion recurs. The widely quoted 50%+ recurrence after early excision is convention, and is not measured by any study cited on this page (see Controversies).

The case for earlier surgery. The rule is now contested. He 2018, the only direct comparison of timing cited on this page, studied the post-traumatic elbow: excision before 12 months (mean 7.4 months) gave better elbow function, in range of motion and Mayo scores, than later surgery, with no difference in recurrence. Judge the lesion by corticated margins and a settled clinical picture, and weigh a stiff, disabling joint against a number on a scan: a disabling joint may justify operating before the scan settles, and the decision increasingly rests on symptoms rather than on a scan.

Indications for excision. Excision is for a lesion that is:

• Stable: corticated margins, ALP settled, bone scan plateaued, size no longer growing
• Symptomatic: pain, ROM limitation or functional impairment
• Unresponsive to conservative management

An asymptomatic mature lesion is observed. It may resolve or remain stable, and is excised only if it becomes symptomatic.

Prophylaxis after excision. Prophylaxis is mandatory, and is planned before the operation. The two options:

• Indomethacin 75mg daily for 6 weeks, starting on the first postoperative day
• Radiation 700-800 cGy as a single fraction within 72 hours

The conventional figures are recurrence of 50%+ without prophylaxis and 10-20% with it. The trials comparing indomethacin with radiation found no significant difference between them; the Evidence Base sets out how firm that equivalence is.

Planning. CT with 3D reconstruction defines the extent of the lesion, its margins and its relationship to the neurovascular structures, and the approach is planned from it.

Excision. Complete excision with meticulous haemostasis is the aim:

1. Use an extensile approach if needed, identify the lesion and protect the neurovascular structures
2. Identify the margins, distinguishing the lesion from normal muscle
3. Excise all the heterotopic bone, preserving viable muscle
4. Achieve meticulous haemostasis, because haematoma promotes recurrence
5. Consider a suction drain
6. Repair muscle if needed and close in layers

Nerve injury. The nerves are at risk during excision. Identify them early and dissect carefully; neurolysis may be required.

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Prophylaxis runs alongside rehabilitation:

Recurrence. The rates below are the conventional figures rather than ones measured on this page:

Prognostic factors. Recurrence is influenced by:

• Maturity: mature lesions recur less
• Prophylaxis, which dramatically reduces recurrence
• Completeness of excision: incomplete excision increases recurrence
• Location: some sites have higher recurrence

Function. Most patients have a good functional recovery. Range of motion is expected to improve and pain relief is usually achieved, although a recurrence may require repeat surgery.

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Global epidemiology:

• Traumatic myositis ossificans is the dominant form worldwide and is closely tied to contact-sport participation and high-energy trauma, so reported frequency tracks local sporting culture and trauma burden rather than geography.
• Significant quadriceps contusions carry roughly a 9% risk (athlete cohorts); the elbow is the next most common site, particularly after paediatric supracondylar/forearm injuries and forceful reduction.
• Neurogenic heterotopic ossification (after traumatic brain or spinal cord injury) is a distinct, related entity that is more common in regions with high road-trauma and conflict-injury volumes.

Side-by-side guideline themes:

The 50% figure. The "50% recurrence if excised immature" figure comes from older surgical case series, not modern controlled data. The principle of waiting for maturity is sound, but the precise recurrence rate is uncertain and likely site- and technique-dependent.

Bone scan or serial imaging. Historic teaching mandates a cold three-phase bone scan before excision, but many units now rely on stable, fully corticated lesions on serial radiographs or CT plus a normalised ALP. There is no high-level trial defining the single best maturity test.

Is excision necessary at all? Conservative series (e.g. Simon et al. 2016) show most athletes return to full activity despite persisting ossification, which challenges routine excision. Surgery is increasingly reserved for genuinely mechanical, function-limiting lesions.

Prophylaxis in traumatic myositis ossificans. The RCT evidence for NSAID and radiotherapy equivalence derives from acetabular-fracture and arthroplasty heterotopic ossification and is extrapolated to excision of myositis ossificans. Direct trial data in traumatic myositis ossificans are lacking.

NSAID or radiotherapy. The trials found no significant difference in efficacy, so the real debate is NSAID gastrointestinal and renal risk and the effect on fracture and soft-tissue healing, against radiotherapy's cost, access and theoretical malignancy concern in young patients. That balance favours NSAIDs in most healthy young trauma patients.

Related pages: Heterotopic Ossification for the broader entity, of which post-traumatic myositis ossificans is the intramuscular form and from which most of the prophylaxis evidence on this page is borrowed. For the malignancies this lesion is mistaken for — the reason an early biopsy is dangerous — see Extraskeletal Osteosarcoma, Parosteal Osteosarcoma, High-Grade Surface Osteosarcoma and Osteosarcoma. Fibrodysplasia Ossificans Progressiva is the genetic mimic in which biopsy and surgery actively provoke the disease, and Elbow Stiffness and Contracture covers the joint where excision timing matters most.