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toms from pressure on the cranial nerves may also occur.

Osteopetrosis (Albers-Schonberg disease, marble bone disease). Osteopetrosis, a hereditary disease transmitted as a recessive mutation, is another bone disease characterized by normal bone formation but by absence of bone resorption, causing the bones to become thick and dense.

Osteopetrosis generally affects the bones of the extremities but can involve the entire skeleton. It is characterized by too much calcific tissue. The cortices become thickened and the individual bony trabeculae increase in girth as a result of the ever-increasing bone formation that is not counterbalanced by the normal resorption processes. Eventually the marrow spaces become encroached upon, leading to paucity of blood-forming tissue, with consequent myelophthisic anemia. Fractures are common because of increased fragility caused by the hyper-calcification.

The deformities caused by osteopetrosis are for the most part confined to the shafts of the long bones, making them shorter than normal, thick, and deformed, with their ends expanding to resemble somewhat the shape of the Erlenmeyer flasks. How-ever, numerous cases involving the skull have been described. Gross thickening occurs at the base of the skull, with the normal markings of the skull being lost and the diploe being effaced. The cranial foramina may be encroached upon and narrowed, causing the nerves passing through them to become compressed. This can lead to impairment of vision and hearing.

Radiographically, the skull shows a great deal of radiopacity and density and has been described as a "bladder of lard." The normal bone structure is effaced and the medullary cavities are indistinguishable from the crest of the bone.

Involvement of either jaw is always bilateral, al-though the maxilla, including the facial portions, is usually affected more often than the mandible.

Osteoporosis. Osteoporosis is a disease usually found in older patients that involves various bones of the skeletal system. It also occurs in the jaws of younger people when teeth have been missing for many years. It is characterized by enlargement of the bone spaces and the haversian canals, which causes the bones to become weak and brittle.

Osteoporosis of the vertebrae is a normal finding in most elderly people. It often occurs after pro-longed bed rest caused by a debilitating illness or can occur as a result of prolonged adrenocortical steroid

Fig. 6-43. A lesion elsewhere in the body may metastasize to the alveolar bone. Here the patient has adenocarcinoma. (From Kolas, S., and others: Radiographic patterns of resorption seen in some gnathodental hard-tissue disturbances, Dent. Clin. N. Amer., November, 1968.)

therapy. In any event, the bone in severe cases would not be strong enough to support implants.

Bone neoplasms. Benign and malignant growths are not uncommon in the jaws. Benign tumors tend to be local and self-containing, whereas malignant tumors spread easily by extension or by a few cells breaking away from the main mass to lodge else-where in the body. A malignant tumor of the jaw may originate there, but this is most unusual. Such a tumor usually spreads to the jaw by extension of a cancer of the gingiva or the buccal mucosa or by metastasis of a malignant lesion of the lung, breast, kidney, prostate, thyroid gland, or another bone (Fig. 6-43) . While a malignant tumor is always serious, a benign tumor may not be unless it is precancerous or continues to grow and limits normal functioning of the area. The implantologist would be taking a great responsibility upon himself if he alone were to diagnose the type and malignancy of a tumor. Although many tumors have a characteristic appearance in radiographs, accurate diagnosis depends on biopsy and histologic studies.

The neoplasms discussed here are not limited to the bones of the skull or the mandible; they may occur in any bone of the body. Bone tumors arising solely from the dental lamina or its derivatives or common, nonodontogenic jaw tumors are discussed more extensively in Diseases of the Teeth and Sup-porting Bone, p. 203.

Benign tumors. Benign bone tumors include osteomas, chondromas, fibrous dysplasia, benign