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.