Myositis ossificans with atypical clinical, radiographic, or pathologic findings: a review of 23 cases.

Myositis ossificans is a relatively rare, well-defined entity. The 23 cases sent for consultation to two of us (L.V.A. and A.N.) were reviewed. Clinical, radiologic, and microscopic information was reexamined, and special attention was given to features infrequently seen in typical myositis ossificans. Average age at presentation was 23 years, but 1 patient was younger than 2 years, and 2 were younger than 10 years. Due to the uncommon location of 15 lesions and an unusual presentation in 5, the correct diagnosis was not obvious in these cases. A history of trauma was elicited in 8 patients, denied by 7, and was not queried in the remainder. Radiologic studies raised the possibility of a malignant bone-forming tumor in at least three instances; myositis ossificans was originally diagnosed in 6 cases radiologically. In 8 cases, histologic evidence suggested malignancy, including osteosarcoma, either parosteal or extraosseous, in 6. Other diagnoses included epithelioid sarcoma and callus formation. Presentation of these variations from the norm highlights the importance of recognizing the evolution of a nonneoplastic fibro-osseous and cartilaginous entity in which conservative treatment is curative.