Carbon, a promising material in endoprosthetics. Part 1: the carbon materials and their mechanical properties.
Progress in endoprosthetics depends to a large extent on the availability of materials. Carbon is known for its excellent biocompatibility. Carbon materials can also be manufactured with a great variety of properties. The application of carbon materials has been restricted to some special cases like heart valves or tooth roots. Construction of high-loaded endoprosthetic joints has been impossible due to insufficient mechanical strength or manufacturable size of the materials. Three new carbon materials have been developed which seem to offer new possibilities: (1) a high strength isotropic carbon, (2) a silicon carbide/carbon composite (SiC/C) and (3) a carbon fibre reinforced carbon (CFRC). The mechanical properties of these materials were investigated including size effects (length / to thickness d ratio of samples) and manufactured parameters (final heat treatment temperature, fibre volume content and fibre orientation). Flexural strength of the first two materials, which are both isotropic, increases with decreasing l/d ratio, whereas the CFRC behaves contrarily. In the latter case, failure at low l/d ratios is caused by shear failure. The Wöhler diagram of all three materials demonstrates limits of fatigue even after only 10(3) to 10(4) cycles. The fatigue strength of the isotropic C and the SiC/C is between 70 and 80% of the static strength, even in Ringer's solution. The Young's modulus of the isotropic C is comparable to that of femoral bone. The modulus of the SiC/C is still appreciably low. It follows that both isotropic materials are applicable for sliding parts like ball and socket. Their tribological properties are described in a following paper. The CFRC finally combines high fatigue strength (approximately 400 MN/m2) with a moderate Young's modulus (approximately 150 X 10(3) MN/m2). These properties indicate application for shafts and pins.