Myositis ossificans is a benign process, characterised by heterotopic ossification, usually within muscle, following trauma.
Description
Myositis ossificans (MO) is a benign, self-limiting condition characterised by heterotopic ossification, usually in the large muscles of the limbs. This condition is typically associated with a history of trauma, often minor, and is most commonly found in active young adults.
Pathogenesis
The exact pathogenesis of MO is unclear, but it is thought to result from aberrant healing following muscle injury, leading to fibroblastic proliferation and subsequent ossification. The trauma provokes an intense inflammatory response, causing muscle fibres to be replaced by highly vascular fibroblastic tissue. Over time, this progresses to form bone.
Subtypes
MO is generally classified into two subtypes:
- Myositis ossificans traumatica: The most common type, associated with trauma.
- Myositis ossificans progressiva: A very rare, genetic condition leading to ossification of muscles, tendons, and ligaments.
Epidemiology, Risk Factors & Associations
- Associated with trauma, often minor (80% cases)
- Most common in active young adults, particularly males
- Frequently occurs in athletes due to repetitive microtrauma
Clinical Features
Presents with localised pain, swelling, and decreased range of motion. A palpable mass may be noted on physical examination several weeks after the inciting event.
Complications
There is no known risk of malignant transformation. However, the ossification can lead to significant functional impairment if located near a joint.
Pathological Features
Histopathology
- Macroscopic: Early lesions are soft and bloody, while mature lesions are firm with a chalky white cut surface due to calcification.
- Microscopic: Characterised by zonal pattern of maturation from the centre outwards. Central areas show primitive mesenchymal cells and osteoblasts, whereas peripheral zones show mature lamellar bone.
Serology
Not typically applicable to this condition.
Biochemistry
Not typically applicable to this condition.
Radiological Features
General Features
- Post-traumatic heterotopic ossification in muscle (classically quadriceps, brachialis); zonal maturation with peripheral mature bone and immature central stroma (“zoning phenomenon”—key discriminator from malignancy).
- Temporal evolution:
- 0–2 wks oedematous mass
- 2–4 wks peripheral mineralisation appears
- 6–8 wks organised rim
- 2–6 mo cortical-like shell, symptoms settle.
- Lesion is intramuscular, often with a fat/cleavage plane from adjacent cortex and no cortical destruction (helps exclude osteosarcoma).
XR
- Early (≤2–3 wks): Soft-tissue swelling ± faint peripheral amorphous calcification; may be normal.
- Subacute (3–6 wks): Peripheral, well-defined calcified rim with lucent centre (maturing periphery).
- Late (>6–8 wks): Cortical-like ossified shell, well-circumscribed, sometimes laminated; stable or shrinking size.
CT
- Best for mineralisation pattern
- Shows peripheral trabecular/cortical ossification and low-attenuation centre; sharply demarcated cleft from adjacent bone.
- No cortical breach or marrow invasion (supports benignity).
MRI
- Very early: Bulky T2 hyperintense oedematous mass with avid enhancement; mineralisation may be occult. Can mimic sarcoma (use history and repeat imaging).
- Zonal signal: Low-signal peripheral rim on all sequences (mature bone) with intermediate/high-signal centre (immature fibroblastic tissue/haemorrhage).
- Blooming on GRE/SWI from calcification/haemosiderin periphery.
- Intact cortex, no intramedullary tumour, only adjacent soft-tissue/muscle oedema.
- Decreasing oedema/enhancement over weeks; increasing peripheral low-signal rim thickness.
US
- Early: Hypoechoic intramuscular mass with hypervascular peripheral rim on Doppler; posterior enhancement.
- Evolving: Echogenic peripheral arc/rim with posterior shadowing as ossification develops; central softer echotexture.
- Late: Well-defined calcified shell, minimal internal flow; dynamic scan shows lesion separate from cortex.
Diagnosis
Diagnosis is primarily based on characteristic clinical and imaging features. Look for zoning (peripheral maturity) and a cleft from bone; repeat imaging in 2–4 wks shows maturation if uncertain. If needed, histological confirmation can be obtained, but is typically avoided due to risk of exacerbating the condition. Do not biopsy too early (≤4 wks) if classic evolution—histology can look worrisome and lead to overdiagnosis..
Differential Diagnosis
- Osteosarcoma: A malignant bone tumour which can also present with periosteal reaction and soft tissue mass. However, it usually has a more aggressive appearance on imaging, lacks the zonal pattern of maturation, and typically occurs in younger patients.
- Soft tissue sarcoma: Presents as a soft tissue mass, often with calcification. However, the pattern of calcification is typically more haphazard, and it lacks the zonal maturation pattern.
Management
Conservative management with rest, non-steroidal anti-inflammatory drugs (NSAIDs), and physiotherapy is usually sufficient. Surgical intervention is typically reserved for lesions causing significant functional impairment.