[Knee endoprosthesis: selection and requirements of materials]

The possibilities of using different material combinations for the replacement of knee joint endoprostheses are very limited. Therefore, cobalt-chromium-molybdenum alloys are currently being used for femoral components and ultra-high molecular weight polyethylene for tribologically stressed tibial components. Titanium alloys can be considered for the tibial component only. For tribological reasons, it is not possible to use a femoral component made of titanium unless it has a corresponding coating. As far as the design is concerned, problems arise from the fact that, on the one hand, there is a demand for the smallest possible size or resection height. On the other hand, however, the forces and strains are rather high and therefore a certain material thickness is necessary in order to avoid fatigue fracture. Regarding polyethylene, the same known principle must be taken into consideration here--not to avoid using the so-called floating design, i.e., the polyethylene components should always have a supporting limitation, or cold flow might occur. The tribological behaviour of polyethylene is restricted in cases of constrained tibia plateaus in so far as the linear or punctual contact of the initial run-in phase leads to correspondingly high surface pressure consequently overstressing the polyethylene. In order to improve this, mobile meniscal bearings are used and the surface pressures achieved here can be endured by the polyethylene even over a long-term. The extent to which the new so-called cross-linked polyethylene can be used in knee joint endoprosthetics is currently being tested, and the simulator results in this respect have been promising so far.