Bone formation in a rectal inflammatory polyp

Heterotopic bone formation (osseous metaplasia) is rarely detected in the gastrointestinal tract. Most of reported cases are associated with malignant lesions. We herein report a case of osseous metaplasia in a rectal inflammatory polyp and a review of the literature on suggested mechanisms for its aetiology. A 39-year-old man visited our hospital with a chief complaint of melena. Total colonoscopy revealed a slightly reddish subpedunculated polyp, about 12 mm in diameter, in the lower rectum. Endoscopic resection was performed. Histologically, several foci of heterotopic bone formation were found. From the review of the literature, all of the polyps described were larger than 10mm in diameter, 55.6% showed inflammatory changes, and 62.5% were detected in the rectum. Osteogenic stimulation was considered to be a result of the inflammatory process. As our inflammatory polyp was located in the rectum, the pathogenesis could be a reactive change stimulated by the repeated local trauma, or be on a peculiar characteristic of the rectal mucosa itself.