Fact-sheet: Myositis ossificans
Updated on 09/27/2019 at 9:13 AM
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Definition
Heterotopic, non-neoplastic proliferation of bone and cartilaginous tissue within the soft tissues of the skeleton
Benign, solitary, ossified soft tissue mass developing within a skeletal muscle
History of trauma reported in half of cases
Adolescents and young adults
Can affect any muscle in the body (limb muscles in 80%)
Evolution in 3 stages:
Initial stage (pseudo-inflammatory) S1-S3: mesenchymal proliferation, highly cellular and pleomorphic, arising from the periphery of the lesion
Intermediate stage (pseudo-tumoral) S4-S8: transformation of mesenchymal cells into osteoblasts depositing osteoid tissue, also from the periphery;
Maturation stage (healing) S9-S12: transformation into well-differentiated lamellar bone, with the appearance of fatty marrow in the intertrabecular spaces
=> zonal distribution: centripetal ossification of the lesion. In the subacute phase, the lesion is characterized by a highly polymorphic center (vascularized, without calcification) surrounded by a peripheral shell of more or less mature bone (the reverse of osteosarcoma)
Clinical features
Initial symptoms: pain, tension, soft tissue mass that may enlarge but regresses to become more circumscribed after 2 to 3 weeks of evolution
Leaves behind a small indurated mass
80% of cases: muscles of the appendicular skeleton (buttock, thigh, scapular region, arm)
Laboratory findings
Possibility of a biological inflammatory syndrome early in the course, which may lead the diagnosis astray
Radiography
Initially normal, radiographs show fine calcifications after 2 to 6 weeks of evolution
Faint at first then lamellar, they are located at the periphery of the lesions and are followed by centripetal ossification
Late in the course, it may become incorporated into the adjacent cortex
Ultrasound
May demonstrate the zonal phenomenon before calcifications appear on radiographs or CT
In the acute phase: heterogeneous appearance of the area, with fasciculated hypoechoic zones resembling muscle fibers
The appearance of clusters of muscle fibers oriented along their normal longitudinal axis is a good positive diagnostic finding (rounded areas on transverse sections and elongated, parallel areas on longitudinal sections)
In the subacute phase: hypoechoic mass with a hyperechoic center. Calcifications appear at the periphery (posterior acoustic shadowing)
CT
Faint then lamellar calcifications, demonstrated earlier than on radiographs
Confirms the absence of connection between the lesion and the adjacent bone
Hypodense center of the lesion and edema of the adjacent soft tissues may be demonstrated early in the course
MRI
MRI appearance depends on stage:
- Initially: poorly defined ovoid mass, hypo- or isointense on T1, heterogeneous and hyperintense on T2, with peripheral enhancement, NONSPECIFIC
- Over time: peripheral ring and central hypointense areas: reflect mineralization
- Mature lesions: well-defined heterogeneous masses, fat signal with peripheral hypointense ring
- possible fluid-fluid levels: intralesional hemorrhage
Nuclear medicine
Intense focal nonspecific uptake
Management
Repeat imaging (radiographs and US) every 2 weeks to visualize the progressive, centripetal appearance of calcifications
Treatment: local excision in case of functional impairment
Differential diagnosis
Initial phase:
- sarcoma
- abscess
- muscle necrosis
- hematoma
- necrotic metastasis
