Age / Sex : 35 / M

Chief complaint : Right forearm pain (no trauma history)


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Two weeks later, you can see the final diagonosis with a brief discussion of this case (Please submit only one answer)

Courtesy : 정보미 (Bo Mi Chung), 중앙대학교 병원 (Chung-Ang University Hospital)

Myositis ossificans at supinator muscle

Discussion

Findings
Initial MRI: T2 high, T1 low SI lesion at supinator muscle
Follow up MRI: Increased size of T2 heterogeneously high SI mass with marked perilesional soft tissue swelling. T2 low SI foci and rim. Heterogeneous enhancement.
Follow up plain radiography: a mass with peripheral calcified rim and periosteal reaction

Discussion
Myositis ossificans (MO) or heterotopic ossification is characterized by abnormal heterotopic bone formation involving striated muscle, tendons, ligaments, fasciae, and aponeuroses.

Immature lesions maybe intensely edematous and poorly defined, resembling an aggressive neoplasm, or may have a thick enhancing rim, mimicking an abscess.

Intermediate lesions exhibit a predominantly cellular core of active fibroblasts, resembling nodular fasciitis, before developing better-defined margins and faint marginal calcifications, typically 2–4 weeks after injury. Such early calcifications are difficult to appreciate at MR imaging and are best demonstrated at CT.

Mature lesions, typically 6 or more weeks after injury, demonstrate the distinctive zonal organization of myositis ossificans, with a peripheral ring of mature ossification at its margins sur-rounding a central cellular core. At US, acoustic shadowing develops at the osseous interface at this stage, obscuring the center of the lesion. With further maturation, the cellular central core evolves into mature fat-containing marrow that is clearly recognized at MR imaging.

References
MR Imaging of Muscle Trauma: Anatomy, Biomechanics, Pathophysiology, and Imaging Appearance. Radiographics 2018; 38:124-148.