per year. This may be a problem in itself since the bearing surfaces are often less than 10 mm thick and may deform as they get thinner. The wear may also cause problems, as inflammation can be caused by increased quantities of polyethylene wear particles in the synovial fluid.

There are many controversies. Much of the research effort of the orthopedic-community is directed to studying and improving joint replacement. The main controversies are
* the best or most appropriate bearing surface