Heterotopic ossification: review of histologic findings and tissue distribution in a 10-year experience.

Heterotopic ossification (HO) within tissues involved by a pathologic process is a well-recognized phenomenon. It is most frequently observed in atherosclerotic plaques, in soft tissue around joints, and in the central nervous system. Less frequently, carcinomas and some benign neoplasms will undergo heterotopic ossification. We performed a retrospective review of our experience with HO over a 10-year period to determine the frequency and tissue site distribution of heterotopic ossification. A computerized review of surgical pathology records of approximately 126,000 reports revealed 85 cases in which heterotopic ossification, ectopic bone or metaplastic bone was specifically mentioned in the surgical pathology diagnosis. Twenty-two cases were neoplasms of non-osseous tissues, and 63 cases were non-neoplastic lesions. Immunohistochemical staining for bone morphogenic proteins (BMP) 1, 4, and 6 was performed. Fourteen cases showed staining for BMP-1, 22 cases showed staining for BMP-4, and five cases showed weak staining for BMP-6. HO is a relatively infrequent finding and is more commonly seen in degenerative and reparative conditions than in neoplasms.