Pseudomalignant myositis ossificans. A pathological study of eight cases.

Eight cases of pseudomalignant myositis ossificans are presented. There was a male predominance and four patients were under 20 years of age. There was no history of trauma, but there was evidence of mild local or general inflammation with positive roentgenograms in two cases. Histologically there was active fibroblastic proliferation, appearing to develop essentially in fibrin deposits with metaplasia leading to the formation of an unusual osteoid. The newly formed mass was surrounded by bone and compressed the surrounding muscle fibers, which were either atrophic or trapped therein. The histological criteria distinguishing this disorder from other forms of myositis ossificans are discussed, and comparisons are drawn with nodular fascitis and proliferative myositis. An association with the latter was observed in one case. This nontumourous growth must be distinguished from osteogenic sarcoma of soft tissue. The differential diagnosis is based on careful histological criteria, which must be supported by clinical and radiological data.