| Summary: | Ceramic materials are creating much interest for use in orthopaedics as the modular component in hip replacement operations. The bioceramic, 3 mol % yttria-stablised zirconia (3Y-TZP), or biograde zirconia, is currently undergoing clinical trials. As materials used for hip replacements are exposed to cyclic stresses during gait, it is important to understand the long term performance of the material under cyclic loads. In order to determine the long-term fatigue behaviour of biograde zirconia, it was necessary to perform laboratory tests on prepared specimens of the material. Moreover, the programme sought to determine whether the fatigue life of the material was affected by loading parameters and by exposure to a simulated physiological environment. The fatigue tests involved subjecting disc-shaped specimens of 3Y-TZP in the 'as-fired' condition to cyclic biaxial stresses in a concentric ring apparatus, designed specifically for the research programme. Under the present test conditions, the material underwent a definite cyclic fatigue effect but the behaviour was not affected by loading frequency nor by exposure to a simulated physiological environment. A rise in the fatigue limit was recorded when testing the material at a higher stress ratio. The most detrimental flaws in the 3Y-TZP specimens were those caused by green-machine tearing. The remaining failure-initiating flaws were hole/agglomerate pairs, inclusions and machining/handling flaws. Fatigue crack growth from the pre-existing flaws resulted in accumulated damage until the size of the damaged area was critical for specimen failure. Based on laboratory test data, it would appear unlikely that zirconia bioceramic femoral heads would fail from fatigue during their implantation life, although it is paramount that component testing and/or clinical trials are undertaken to provide conclusive evidence.
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