[Mechanical stability of hip joint endoprosthesis shafts of carbon fiber composite materials]

The use of composite materials in orthopaedic surgery permits the design of implants with varying degrees of stiffness. A system of carbon fibre reinforced epoxy hip stems was investigated and compared to the mechanical safety of metal ones. Special attention has been paid to the fact that torsional moments are the common failure mechanism of composite materials. In total 74 hip stems (three different sizes) have been tested under static and dynamic loading conditions according to ISO 7206. A group of 27 implants was pre-conditioned for 75 days in 80 degrees C Ringer's solution to discover potential degradation effects of absorbed fluids. The carbon fibre hip stems proved to reach run-out loads higher than conventional metal ones. Moisture absorption didn't influence the mechanical properties within the range of the experimental accuracy. The calculated fatigue stress of 750 MPa permits further optimization of stiffness parameters without the risk of implant fracture.