Bone grafts are second only to blood transfusions on America's list of transplants. Artificial bone is becoming an increasing alternative to the two mostcommon 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 surgeryinvolves titanium and cobalt chromium alloys. Bone is very alive and constantly rebuilds itself. Its porus framework is a composition of collagen proteinfibers 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 substance that more closely resembled real bone than the metal alloys, researchers turned to the artificial production of hydroxyapatite.
In the 1960s, Marshall R. Urist, a professor of orthopedic surgery at the University of California, Los Angeles, uncovered the process of bone formation.Improved diagnostic techniques and research into the structure and composition of bone,has led to the development and FDA approval, or pending approval, of a number of synthetic calcium-based bone repair products. The first FDA approval of a synthetic bone implant (or bone void filler) was granted in 1992 to a product called Pro Osteon, a calcium phosphate material that mimics hydroxyapatite manufactured from coral through a thermochemical process developed in the 1970s. Because this material remains porus, it acts as a framework through which cells and blood vessels from the natural bone intertwine. This new bone growth connects both ends of the fracture and the body ultimately reabsorbs the synthetic framework. In over 20,000 procedures using this product, there have been zero rejections. Because of its porosity, however, thismaterial lacks the strength needed for weight-bearing bones.
Richard J. Lagow, a chemist at the University of Texas at Austin, developed away to synthesize hydroxyapatite into a porous and much stronger form suitable for bone replacement, and a denser form similar to tooth enamel. The finished product, a bioceramic called Megagraft 1000, is processed by calcium metal, calcium hydroxide, and phosphoric acid reaction at 700 to 850 degrees Celsius. Not only does its porosity invite invasion by blood vessels and cells that gradually break down the implant as new, natural bone grows, the number of "pores" can be adjusted to match that of the graft recipient, encouraging faster regrowth of natural bone. In collaboration with professor Joel W.Barlow, a chemical engineer at the same university, Lagow devised a computer-guided laser to generate different bone shapes and sizes with the intentionof providing bone "blanks" customized for individual patient needs. Hydroxyapatite is also used as a coating for artificial joints and prostheses to encourage bone to grow and bind tightly to the implant.
1993 saw FDA approval of Collagraft, a hydroxyapatite/tricalcium phosphate and bovine collagen which is mixed with the patients bone marrow. In 1998, Dr. Jay Lieberman, of the University of California, Los Angeles began clinical trials to treat osteonecrosis (bone death) of the hip, by inserting a capsule containing bone-morphogenetic protein (BMP), into an allograft. BMP, discovered by Urist in 1965, occurs naturally in small quantitiesin the bone matrix and imitates the bone development process which occurs during fetal development. John M. Wozney with Genetics Institute in Massachusetts, first cloned BMP in 1988--in 1998, more than 30 BMPs have been cloned. Over the next decade, researchers look to tissue-engineered implants for reconstructive skeletal deformities and cell-based therapies for osteoarthritis andosteoporosis. Carbon fiber composites--materials used in skis and tennis rackets--are also being tested for use because the composites resemble bone in both stiffness and flexibility.