Bone grafts are second only to blood transfusions on America's list of transplants. Artificial bone is becoming an increasing alternative to the two most common types of bone grafts or repairs: the autografts, in which a piece of bone is taken from elsewhere in the patient's body (usually the pelvis), or the allograft bone transplanted from cadavers; the Red Cross maintains a bone bank for this purpose. A third type of bone-replacement surgery involves titanium and cobalt chromium alloys. Bone is very alive and constantly rebuilds itself. Its porus framework is a composition of collagen protein fibers running through hydroxyapatite, a mineral that makes up about 70% of living bone. Due to the fact that there is little "spare" bone in the body for use in autografts, possible rejection by the recipient's immune system and the risk of transmitted diseases from cadaver bones, and to find a...