QUIZ: No. 22, 02/08/2025

X ray

MRI

ANS – Mature Myositis Ossificans

Myositis ossificans is heterotopic ossification within soft tissues, usually post-traumatic, but may be idiopathic or associated with neurological injury. The typical radiographic appearance of myositis ossificans is circumferential peripheral calcification with a lucent center (zoning phenomenon) and a radiolucent cleft (string sign) that separates the lesion from the cortex of the adjacent bone. There is usually no periosteal reaction unless the lesion is closely adjacent to bone. A clear separation from adjacent bone is maintained, with a preserved soft tissue or fat plane, helping differentiate it from aggressive bone-forming lesions like osteosarcoma. Treatment is reserved for symptomatic lesions, and surgical resection is usually curative.

Early Phase (0-2 weeks)

- X-ray: Normal or soft tissue swelling
- MRI: Edema, soft tissue mass
- USG: Hypoechoic with mild vascularity

Intermediate Phase (2-4 weeks)

- X-ray: Peripheral rim calcification
- CT: Early mineralization
- MRI: Peripheral low signal rim, central T2 hyperintensity

Mature Phase (>4-6 weeks)

- X-ray: Well-defined ossified mass with zoning
- CT: Mature bone-like appearance
- MRI: Low signal intensity, reduced edema

TEACHING POINTS

v Don’t biopsy early lesions – risk of misdiagnosis as sarcoma.

v Zoning pattern and maturation over time are diagnostic hallmarks.

P.S - A core needle biopsy was performed on this expansile mass and HPE revealed atypical cartilaginous tissue, muscle, and bone fragments consistent with myositis ossificans.