Improving Bearing Surfaces of Artificial Joints

The requirements for materials to be used for bearing surfaces in joint replacement are corrosion resistance in the body environment, reliability, hardness, and stiffness. Most important is coverage of the bearing surface with a stable oxide layer, so that the articulating surfaces can be lubricated by the synovial fluid. The synovial fluid, being a protein, can degenerate during frictional heating. This can be avoided if the materials used for wear couples have very good thermal conductivity. Bioceramics for joint replacement have been used since the 1970s. Alumina ceramics reduce the wear rate and solve the problem of implant loosening (osteolysis). Although the in vivo fracture rate for alumina parts is very small, further improved reliability is demanded. Alternative materials may be non-oxide ceramics, zirconia ceramics, or hard coatings on metals. Advanced non-oxide ceramics, such as SiC and Si3N4, are not suitable for bearing surfaces in knee- and hip-joint replacement because the surface oxide formed is SiO2, which chips off. Y-TZP zirconia does not have adequate phase stability. All hard coatings tried hitherto (TiN, DLC) have not been good enough. Alumina matrix composite (AMC) is a new type of bioceramics. AMC offers excellent tribological properties, no frictional heating, improved mechanical strength and fracture toughness, thus more in vivo reliability. So far test results have been very promising.