Myositis Ossificans (MO) is a form of heterotopic ossification where bone tissue forms within muscle or other soft tissues. It exists in two distinct forms:
- Myositis Ossificans Circumscripta (Nonhereditary)
- Myositis Ossificans Progressiva (Hereditary), also called Fibrodysplasia Ossificans Progressiva (FOP)
1. Myositis Ossificans Circumscripta (Nonhereditary Form)
Etiology and Risk Factors
- Typically occurs following blunt muscle trauma (e.g., contusion, strain, or hematoma)
- Common in young adults and athletes
- Risk increases with premature return to activity post-injury
- Most affected sites: quadriceps femoris, brachialis, and deltoid
Clinical Presentation
- Appears within 1–2 weeks post-trauma
- Localized pain, swelling, warmth, and restricted joint movement
- May present with a firm, tender mass over the muscle
- ESR and alkaline phosphatase levels may be elevated
- Symptoms gradually improve, unlike osteosarcoma (pain worsens over time)
Imaging Findings
- Plain Radiographs: Normal early (first 1–2 weeks), then show peripheral calcification progressing centripetally by 2–6 weeks
- CT Scan: Demonstrates well-defined peripheral ossification with central lucency
- Bone Scan: Increased uptake in the lesion <3 weeks post-injury
- Ultrasound: May detect early soft-tissue changes
- Key Differentiation:
- MO: Peripheral-to-central calcification
- Osteosarcoma: Central-to-peripheral calcification
Histopathology
- Early phase: Proliferation of undifferentiated mesenchymal cells infiltrating muscle
- 2–3 weeks: Peripheral osteoid formation, with immature fibroblasts centrally
- Mature lesion: Peripheral lamellar/woven bone, central fibrous tissue, occasional cartilage component
Management
- Conservative management is first-line:
- Rest and immobilization
- NSAIDs (e.g., indomethacin) for pain and to limit ossification
- Gentle active ROM exercises once inflammation subsides
- Avoid passive stretching—may exacerbate the condition
- Surgical excision:
- Considered only after 9–12 months (when lesion matures)
- Indicated if function is impaired or pain persists
2. Myositis Ossificans Progressiva (Fibrodysplasia Ossificans Progressiva, FOP)
Etiology
- Rare, autosomal dominant genetic disorder
- Caused by mutations in the ACVR1 gene (activin A receptor type I)
- Results in progressive heterotopic ossification of connective tissue
- Triggered even by minor trauma, intramuscular injections, or viral illnesses
Clinical Features
- Onset in early childhood
- Progressive restriction of movement due to ossification
- Characteristic malformed great toes (hallux valgus or short great toes)
- Episodes of painful soft tissue swellings precede ossification
- Ossification follows a predictable anatomical pattern (cranial-to-caudal, axial to appendicular)
Complications
- Respiratory compromise (due to ossification of chest wall)
- Malnutrition and difficulty with oral intake (jaw fixation)
- Profound physical disability over time
Diagnosis
- Clinical features + genetic testing (ACVR1)
- Imaging: Extensive soft tissue ossification
- Avoid biopsy or trauma as it may exacerbate ossification
Management
- No definitive cure
- Prevent trauma, intramuscular injections, or surgical interventions
- Short courses of corticosteroids during flare-ups
- Use of bisphosphonates, NSAIDs, or experimental therapies (e.g., palovarotene) under investigation
Key Differentiating Features Between MO and Osteosarcoma
| Feature | Myositis Ossificans | Osteosarcoma |
|---|---|---|
| Age | Teens/young adults | Teens/young adults |
| Trauma history | Common | Rare |
| Pain progression | Improves over time | Worsens over time |
| Calcification pattern | Peripheral → central | Central → peripheral |
| Location | Diaphysis, soft tissue | Metaphysis, bone |
| Imaging | Shell-like ossification | Sunburst pattern, Codman’s triangle |
| Histology | Zonal ossification, benign | Malignant osteoid, atypia |
