Myositis ossificans. A clinicopathologic study of 21 cases.

This clinicopathologic study concerns 21 patients with myositis ossificans, whose ages ranged from 7 to 81 years (average, 40.3 years). The lesions were located in the thigh (10 cases), upper arm (6 cases), and other sites (5 cases). In all patients the lesion was solitary, and in all but one, it was seen within the muscle. Histologically, the lesions exhibited a wide range of histologic features with different amounts of immature fibroblastic cells, osteoid, cartilage, and young or mature bone accompanied by fibrous connective tissue. They could be classified into three types according to the predominant or most striking histologic features. Type I (6 cases) was characterized by highly cellular areas with islands of osteoid, having occasionally been confused with extraskeletal osteogenic sarcoma. Type II lesions (8 cases) consisted mainly of osteoid and young bone rimmed by osteoblasts, in the occasional presence of cellular areas. Type III lesions (7 cases) were made up almost wholly of mature bone and cartilage surrounded invariably by dense fibrous connective tissue. Prognosis was excellent in 17 patients for whom follow-up information was available. Difference between Type I myositis ossificans and extraskeletal osteogenic sarcoma was briefly described, following an additional review of three cases of the latter.