Myositis Ossificans

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

Historical context:

• First described in 1883
• Originally thought to be inflammatory
• Now recognized as heterotopic ossification
• "Zoning phenomenon" described as pathognomonic

Epidemiology:

• Most common after: Quadriceps contusions, elbow trauma (especially in children), hip trauma
• Develops in approximately 9% of significant quadriceps contusions (West Point athlete series, Ryan et al. 1991)
• Peak age: Young, active adults (20-40 years); the traumatic form predominates in athletes and trauma patients
• Male predominance, reflecting higher exposure to contact-sport and trauma mechanisms
• Risk factors: Severe contusion (knee ROM under 120 degrees), football/contact injury, previous quadriceps injury, treatment delay over 3 days, ipsilateral knee effusion, and early aggressive massage/stretching

Pathophysiology: Myositis ossificans results from:

• Muscle trauma (contusion, hematoma)
• Hematoma organization into fibrous tissue
• Metaplasia of fibroblasts to osteoblasts
• Heterotopic bone formation within muscle
• Maturation process over 6-12 months

Zoning Phenomenon:

• Peripheral zone: Mature lamellar bone (forms first)
• Central zone: Immature woven bone (forms later)
• Pathognomonic on X-ray
• Distinguishes from osteosarcoma (reverse pattern)

Maturation Process:

Key Pathophysiological Concept:

• Bone forms from outside in (periphery first)
• This is opposite to osteosarcoma (center first)
• Maturation takes 6-12 months
• Early excision disrupts this process and causes recurrence

History:

• Recent trauma or contusion (weeks to months)
• Quadriceps contusion (most common)
• Elbow trauma
• Hip trauma
• Head injury or spinal cord injury (neurogenic)

Physical Examination:

Key Clinical Signs:

• Palpable mass: Firm, fixed within muscle
• ROM limitation: Out of proportion to injury
• Pain with stretch: Aggravated by passive stretch
• Functional impairment: May limit activities

Radiographs:

• Appears at 3-4 weeks after injury
• Zoning phenomenon: Mature bone peripherally, immature centrally
• Pathognomonic - distinguishes from osteosarcoma
• Maturation: Corticated margins develop over 6-12 months

Three-Phase Bone Scan:

• Early (immature): Hot (increased uptake)
• Mature: Cold (no uptake)
• Most important for maturity assessment
• Must be cold before excision

CT Scan:

• Shows zoning phenomenon clearly
• Useful for surgical planning
• Assesses relationship to neurovascular structures
• 3D reconstruction helpful

MRI:

• Shows soft tissue involvement
• May help distinguish from tumor
• Not diagnostic but supportive
• Shows muscle edema early

Laboratory:

• ALP (alkaline phosphatase): Elevated early, normalizes with maturity
• Normal ALP confirms maturity
• Monitor serially if uncertain

Differential Diagnosis

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Note: Surgical excision is only performed when the lesion is mature and symptomatic. This section describes the technique for excising mature myositis ossificans.

Recurrence:

• Most common complication
• 50%+ if excised before maturity
• 10-20% if mature with prophylaxis
• 50%+ if mature without prophylaxis
• Prevention: Wait for maturity, use prophylaxis

Nerve Injury:

• Risk during excision
• Identify nerves early
• Careful dissection
• May require neurolysis

After Excision:

Prophylaxis:

• Indomethacin: 75mg daily for 6 weeks (start post-op day 1)
• OR Radiation: 700-800 cGy single fraction within 72 hours
• Both equally effective
• Mandatory - reduces recurrence from 50%+ to 10-20%

Recurrence Rates:

Prognostic Factors:

• Maturity: Mature lesions have lower recurrence
• Prophylaxis: Dramatically reduces recurrence
• Complete excision: Incomplete excision increases recurrence
• Hemostasis: Hematoma promotes recurrence
• Location: Some locations have higher recurrence

Functional Outcomes:

• Most patients have good functional recovery
• ROM improvement expected
• Pain relief usually achieved
• Recurrence may require repeat surgery

• The "50% recurrence if excised immature" figure comes from older surgical case series, not modern controlled data. The principle - wait for maturity - is sound, but the precise recurrence rate is uncertain and likely site- and technique-dependent.
• Bone scan vs serial imaging for maturity: historic teaching mandates a cold three-phase bone scan before excision, but many units now rely on stable, fully corticated lesions on serial radiographs/CT plus normalised ALP. There is no high-level trial defining the single best maturity test.
• Is excision even necessary? Conservative series (e.g. Simon et al. 2016) show most athletes return to full activity despite persisting ossification, challenging routine excision. Surgery is increasingly reserved for genuinely mechanical, function-limiting lesions.
• Prophylaxis after excision of traumatic MO: the RCT evidence for NSAID/radiotherapy equivalence derives from acetabular-fracture and arthroplasty HO, then extrapolated to MO excision. Direct trial data in traumatic myositis ossificans are lacking.
• NSAID vs radiotherapy choice: efficacy is equivalent; the real debate is NSAID gastrointestinal/renal risk and effect on fracture/soft-tissue healing versus radiotherapy cost, access, and theoretical malignancy concern in young patients - favouring NSAIDs in most healthy young trauma patients.

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:

Registry & trial evidence:

• No dedicated myositis-ossificans registry exists; the strongest comparative data come from heterotopic-ossification prophylaxis RCTs in acetabular-fracture and hip-arthroplasty cohorts (see Evidence Base), which consistently show NSAID and single-fraction radiotherapy to be equivalent.
• Arthroplasty registries (NJR, AJRR, AOANJRR, SHAR) track heterotopic ossification only indirectly via revision/stiffness data, reinforcing that severe HO is uncommon when at-risk patients receive prophylaxis.

High- vs limited-resource practice variation:

• Where bone scintigraphy, MRI/CT, and radiotherapy are readily available, maturity is confirmed objectively before excision and radiotherapy is an option for prophylaxis.
• In limited-resource settings, serial plain radiographs (corticated margins, stable size) plus normalising alkaline phosphatase guide maturity, and oral NSAID prophylaxis is preferred for cost, access, and simplicity.