Exostosis (Gr.Exostosis 7008 out of, and bone), an osseous tumor developed on the surface of a bone, originally or eventually continuous with its substance, circumscribed, without interior cavity, having the same structure and life as the bone on which it is found. There are two varieties of this growth: in one the bone, like all other tissues of the system, takes on a morbid development, an eccentric hypertrophy of its substance, forming a well defined tumor on its surface by the mere excess of interstitial osseous deposit; in the other the new ossific matter is deposited originally on the surface, under or between the laminae of the periosteum, separated from the bone at first by cartilage, but afterward becoming consolidated to it in the usual manner of bony processes. The first variety may affect the greater part of a bone, and deserves rather the name of hyperostosis; and the second, by the process of ossification, may be converted into the first; this distinction is of considerable importance in the prognosis and treatment of the affection.

The muscles and soft parts over an exostosis are generally not changed, unless the tumor be of considerable size and in the neighborhood of large nerves and vessels; but the periosteum is almost always thickened, and less adherent to the bone than usual. In the first variety the form is regular, and the bony fibres diverge from the natural direction to enter the tumor, as in other forms of eccentric hypertrophy; in the second variety the form is irregular, often fantastic and rough, and there is an evident base by which it is as it were immovably articulated to the supporting bone, except in very old growths; this base in recent cases is cartilaginous and readily separated, and shows that this kind of exostosis originates from and is nourished by the investing periosteum; it indicates also a method of treatment which has been found successful, by denuding them of their periosteum and causing their necrosis and separation from want of nutrition. If the cartilaginous base rests upon the bone, under the periosteum, the removal of this membrane will cause an exfoliation of the subjacent bone; but if between the laminae of this envelope, a similar operation will effect the fall of the tumor without injury to the surface of the bone; the cartilage soon becomes ossified, and the exostosis forms one body with the bone, resembling the first variety in having no basal line of separation.

In course of time the excessive deposit of phosphate of lime in these growths may convert them into a substance having the appearance, consistence, weight, and polish of ivory.-Among the constitutional causes of exostosis are syphilitic poisoning, the scrofulous diathesis, and the gouty and rheumatic conditions; the immediate cause is inflammation, produced by mechanical or other means, leading to a deposit first of plastic and then of osseous matter, the development being similar to that of normal bone. In some constitutions there is such a disposition to the deposit of ossific matter, that the slightest contusion is sufficient to cause the development of these bony growths, not only on bones but in the substance of tendons and ligaments; and the affection is often hereditary. When the growth takes place in the cavity of a bone, as in the cranial cavity, it has been called enostosis, but with doubtful propriety, because in this case the growth is upon the bone and outside of its structure. The prognosis varies, principally in proportion to the rapidity of the growth, which when very slow may not be much regarded, except when interfering with the functions of some important organ, as a joint, or into the cranial cavity. The treatment also varies with the prognosis.

Often the removal is not a matter of moment, as exostoses may be carried through life without much inconvenience; and the removal may be a hazardous undertaking, as when the tumor encroaches upon a joint whose cavity would become opened by the operation. If the circulation in an important artery is impeded, removal becomes desirable, and should be undertaken when there is reasonable hope of a successful result. Topical applications are often beneficial, and in the earlier stages, in the form of blisters and strong counter-irritants, often effect the removal by absorption. A strong tincture of iodine, or a solution of iodine in iodide of potassium, is often very serviceable. The constitutional treatment, particularly when syphilis has preceded the affection, should not be neglected. Preparations of mercury may be cautiously administered, particularly the iodide, and iodine may be given in combination with potash or soda salts. When much pain is experienced, anodynes may be administered, either by the mouth or topically.