On the structural safety of ceramic hipjoint heads.

Abstract

For safety and life-time predictions of ceramic hip-joint heads the stress distributions are calculated by the method of finite elements. Permanent "frozen" stresses caused by a relative sliding motion on the conical interface between head and stem are considered as well as the stresses due to alternating external loads distributed over different areas on the sphere. Based on fracture mechanics maximum allowable flaw depths for given life times are evaluated using these stress distributions and the known slow crack growth behaviour of different bioceramics. Under normal loading conditions a flaw depth up to 500 mym can be accepted for an appropriate life time, whereas assuming an accumulation of disadvantageous conditions the flaws should not exceed 100 mym. For detecting such flaws a proof-testing configuration is discussed