Problems and complications in knee joint endoprosthetics from a material-testing point of view

The possibilities of using different material combinations for the replacem ent of knee joint endoprostheses are very limited. Therefore, cobalt-chromi um-molybdenum alloys are currently being used for femoral components and ul tra-high molecular weight polyethylene for tribologically stressed tibial c omponents. Titanium alloys can be considered for the tibial component only. For tribological reasons, it is not possible to use a femoral component mad e of titanium unless it has a corresponding coating. As far as the design i s 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 ha nd, however, the forces and strains are rather high and therefore a certain material thickness is necessary in order to avoid fatigue fracture. Regard ing polyethylene, the same known principle must be taken into consideration here - not to avoid using the so-called floating design, i.e., the polyeth ylene components should always have a supporting limitation, or cold flow m ight occur. The tribological behaviour of polyethylene is restricted in cas es of constrained tibia plateaus in so far as the linear or punctual contac t of the initial run-in phase leads to correspondingly high surface pressur e 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 lon g-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 s imulator results in this respect have been promising so far.