Myositis ossificans: radiologic evaluation of two cases with diagnostic computed tomograms.

Although most physicians associated myositis ossificans with recent, acute trauma, only 40%-60% of patients give such a history. The appearance of a soft tissue mass without a clear history of trauma may suggest a diagnosis of sarcoma, especially because results of a biopsy of the central portion of an area of myositis ossificans may yield immature, undifferentiated tissue resembling a sarcoma. Pain and rapid growth of a mass are more usual in myositis ossificans than in sarcomas, and careful inquiry may reveal stretching injury or chronic trauma associated with normal, vigorous, physical activities. Recognizing the characteristic histologic zoning phenomenon (immature tissue centrally surrounded by more mature tissue and a peripheral shell of benign bone) during the biopsy procedure permits the correct diagnosis of myositis ossificans. Plain radiographs or conventional tomograms may reflect this histologic zoning by demonstrating the typical, mature, outer shell of bone. Although additional radiographic studies are not usually necessary, they may be obtained when the mass is suspected to be a sarcoma. In two patients computed tomographic scans clearly demonstrated well-defined, peripheral shells of mature bone, diagnostic of myositis ossificans.