A 22-year-old athlete has a painful firm thigh mass six weeks after a blunt contusion. Radiograph shows a mass with dense peripheral ossification and a relatively lucent centre, separated from the underlying femoral cortex by a clear zone. The most likely diagnosis is:

• A Parosteal osteosarcoma
• B Myositis ossificans traumatica ✓
• C Periosteal chondroma
• D Calcified haematoma

Explanation

Myositis ossificans traumatica matures from outside inward, so the periphery shows well-formed mature lamellar ossification while the centre remains lucent and cellular. This zoning phenomenon is the key feature distinguishing it from parosteal osteosarcoma, in which maturation is most advanced centrally and the lesion is attached to the cortex without a cleavage plane. The clear zone separating the mass from the femoral shaft further supports myositis ossificans. Biopsy of the immature central zone can be misread as sarcoma, so correlation with history and imaging is essential.

Reference: Grainger and Allison's Diagnostic Radiology, 7th ed.

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Written and medically reviewed by the StethoPrep medical team.