A detailed study of a commercial zirconia (Y-TZP) aged at 37 degrees C held in Ringer's solution or transplanted in animals or humans is reported. No degradation in strength was found for MOR bars or for hip joint heads subjected to stresses caused by pressing titanium alloy trunnions into the head tapers with loads up to 30 kN. Polished specimens also showed no increase in the monoclinic phase percentage upon aging for 1 year. Small specimens transplanted in animals experience no change in the micro-indentation KIC toughness value, suggesting no degradation. Zirconia heads recovered after 2 years of human implantation showed no loss of strength. The difference between the excellent performances of this ceramic presented here and other studies can be accounted by the microstructural design and quality of the zirconia.

The physical properties of ages zirconia ceramic hip joint heads were analysed, using four zirconia heads retrieved after an implantation time of two to three years. The mechanical properties of the zirconia ceramic and the surface characteristics of the heads, i.e. roughness, microstructure, etc., were compared to those of heads before implantation. No significant change in these properties was observed after two to three years in humans. Nor was calcium phosphate observed to form on the heads.

Standardization of bores and cones for modular hip prosthesis heads has been addressed by both the International Standard Organisation (ISO) and the American Society for Testing and Materials (ASTM) in response to the need to control the conditions of head-trunnion fit that thereby improves implant safety. Standardization of these conditions could help the surgeon avoid fit mismatch, especially during revision surgery. Instituting a system of standardization without taking all critical parameters into consideration, however, could be more dangerous than no system at all. The current article discusses the main taper parameters that directly influence the performance of ceramic head-metallic trunnion assemblies and argue for standardization that integrates all these parameters. Several examples, based on experimental mechanical tests and finite element analysis, are given to support the argument.

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