Exostosis is a phenomenon of exophytic growth of compact bone, and the oral exostosis, so called a torus is usually found in the lingual or labial areas of mandible and maxilla, and the hard palatal area. It is not a pathological nor tumoral formation but a localized bony protuberance relevant to developmental and environmental origins. However, the pathogenesis of exostosis has not been clearly elucidated so far. In the present study total 51 cases of oral exostosis were examined by radiological, histological, and immunohistochemical methods to observe the osteogenetic potential existed in the sclerosed bony tissue of exostosis. Particularly, the unilateral mandibular exostoses occurred in the vicinity of mandibular primary growth centers which were more prominent than the contralateral ones in the radiological observation. Histologically the peripheral area of exostosis was composed of lamellated bone and covered with periosteal tissue which showed sparse osteoblastic activity, while the central area of exostosis was composed of thickened and anastomosed trabecular bones with small amount of marrow connective tissue. The latter stained blue in Masson trichrome method, while the former stained red. The immunohistochemical reactions of BMP-2, bFGF, CMG2, and TGF-β1 were clearly positive in the central trabecular bones, while almost negative in the peripheral cortical bones of exostosis. These findings may indicate that the central area of exostosis is less mineralized than the peripheral area of exostosis, and the former expresses different osteogenetic proteins to produce bony tissue contrary to the latter. Therefore, it is suggested that the strong osteogenetic potential in the central area of exostosis be relevant to the growth potential of mandibular and maxillary primary growth centers and play an important role for the latent expansile growth of exostosis in adult life.

Ⅰ. Introduction
 Ⅱ. Materials and methods
 Ⅲ. Results
 Ⅳ. Discussion
 Ⅴ. References

• 김연숙(Department of Dental Hygiene, College of Health Sciences, Cheongju University) | Yeon Sook Kim correspondence