Pseudomalignant myositis ossificans: heterotopic new-bone formation without a history of trauma.

Circumscribed heterotopic new-bone formation without a history of trauma is termed pseudomalignant myositis ossificans due to the diagnostic confusion of this benign lesion with malignant lesions. In 20 per cent of the published cases in which there were diagnostic problems, the patient underwent a radical procedure. Roentgenographically, pseudomalignant myositis ossificans is a circumscribed, radiopaque lesion with a central lucent zone that is separated from the underlying cortex by a radiolucent line. Histologically, a characteristic zoning pattern of peripheral maturation is present, the central proliferating zone usually causing the diagnostic problems. The ultrastructure of the lesion shows a regular mineralization front but an abnormal collagen periodicity of 300 angstroms. Excision is indicated for diagnosis and for relief of mechanical block or pain.