Heterotopic ossifying tumors in head and neck and heterotopic ossification and calcification

The present review introduces heterotopic bone formation, which may be classified into osteoma in the tongue, lipoma with osseous and chondromatous changes, fibrous dysplasia ossificans progressiva (FOP), myositis ossificans traumatica (MOT), Albright's syndrome, vascular ossification in atherosclerotic lesions, ectopic ossification of periodontal epulis, stromal bone formation in the tumors and soft tissue osteogenic tumors (extraskeletal bone tumors). Those ectopic ossifications are unusual conditions, and in the head and neck area, the tongue is the most frequent site of heterotopic osteoma or chondroma.The biologic mechanism of heterotopic ossification may be similar to those of normal ossification, however, pre-existing changes or the transformation into osteogenic cells from non-osteogenic stem cells might be important roles. The myogenic cells in skeletal and smooth muscles suggest they are frequently transformed to osteogenic cells compared to those of mesenchymal cells. Those processes will involve growth promotion factors. Ectopic ossification also mediates epithelial cells, keratinocytic BMP directly induced to trigger ossification, in the case of calcifying epithelioma Malherbe. Classic bone induction theory may also involved epithelial BMP.