OBJECTIVETo study the clinicopathologic features, diagnosis and differential diagnosis of myositis ossificans (MO).

METHODSThe clinical features, radiologic results and pathologic findings of 15 cases of MO (including biopsy and surgical specimens) were analyzed. The hematoxylin and eosin sections were reviewed under light microscope. Immunohistochemical staining for S-100 protein, vimentin, desmin, actin and osteonectin was performed.

RESULTSThe age of the patients ranged from 12 to 46 years. The male-to-female ratio was 11:4. Thirteen cases were located in the parosteum of long bone or subperiosteal soft tissue. The remaining two cases occurred in iliac region and palm, respectively. Five patients had history of injury, while 2 patients had operation before. Four patients had no history of trauma and the remaining one had unknown clinical history. Histologically, zonation pattern was not conspicuous in 10 biopsy cases and 8 corresponding surgical specimens. On the other hand, zonation pattern was observed in 5 biopsy cases and 7 corresponding surgical specimens. Follow up revealed relapses in two patients. Immunohistochemical study showed various degree of positivity for vimentin, desmin, actin and osteonectin. S-100 protein was focally positive in 2 of the cases. The Ki-67 index varied from 1% to 10%.

CONCLUSIONCorrect diagnosis of MO relies on correlation of clinical features, radiologic examination and pathologic findings.