Osteoma is a tumor composed of bone. It may be a result of inflammatory processes of the periosteum or be a distinct new growth.

If developing from a bone-forming tissue, is called an homologous osteoma.

If arising in a tissue that is not bone forming, is called a heterologous osteoma. The latter are found in the meninges, lung, and parotid gland.

An osteoma is a hard, bony, rounded, and more or less tabulated growth. Microscopically it presents quite typically the normal structure of bone. May be composed of spongy or compact new bone, osteoma spongiosum and osteoma durum.

Fig. 50. - Hyaline Chondroma. Oc. 2; ob. 3 (McFarland).

If the growth is small, circumscribed and flat, and arising from pre-existing bone, it is called an osteophyte. If irregular and projecting, an exostosis.

Occurs most commonly at the epiphyses of long bones. Is benign. May be in combination with cartilage, fibrous tissue, fat, or sarcoma, in which latter case it is malignant.