Tensile Stress Analyses of the Hip Joint Endoprosthesis Ceramic Head with Real Shape Deviations

In vivo fracture of ceramic heads of total hip joint endoprostheses has been stated in a not negligible number of patients in the Czech Republic hospitals, some time ago. The reliability of the ceramic head is based on the Weibull weakest link theory. The probability of the failure depends on the tensile stress values in the head, on the volume in which stress acts and on the material parameters of the used bioceramics. The stress analyses of the ceramic heads were realized using computational modelling - FEM system ANSYS under ISO 7206-5 loading. The maximum values of the tensile stress in the ceramic head are significantly influenced on the shape deviations of conical surfaces of the head and the stem cones. There are two types of the shape deviations – the first is macro deviations (different angle of the taper of the stem cone and the head cone), the second is micro shape deviations (the stochastic distribution of unevenness of cone contact surfaces in order micrometers). The macro and micro shape deviations of the real cones were carried out using the IMS-UMPIRE measuring equipment. By computational modelling it has been proved that the character and the size of shape deviations on a conical contact area between the stem and the head of hip joint endoprosthesis have a pronounced influence on the character and the value of stress in the head and, hence, on the probability of the head’s failure. Ignorance of this influence can lead to unforeseen failures of ceramic heads and thus to cast doubt on the trust-worthiness of a certain type of total hip joint endoprosthesis and of surgical treatment.