Abstract
ABSTRACTZirconia in Zirconia toughened alumina ceramic hip replacements exists in an unstable state and can transform in response to stress giving the material improved fracture toughness. Phase transformation also occurs under hydrothermal conditions such as exist in vivo. To predict the hydrothermal aging that will occur in vivo accelerated aging procedures have been used, but validation of these models requires the study of retrieved hip joints. Here 26 retrievals are analysed to determine the degree of phase transformation in vivo. These were compared with virgin heads, heads that had undergone the accelerated aging process and heads wear tested to 5 million cycles in a hip simulator. Monoclinic content and surface roughness were measured using Raman spectroscopy and white light interferometry respectively. The monoclinic content for retrieved heads was 28.5% ± 7.8, greater than twice that in virgin, aged, or wear tested heads and did not have a significant correlation with time, contrary to the predictions of the hydrothermal aging model. The surface roughness for retrieved heads in the unworn area was not significantly different to that in virgin, aged, or unworn areas of wear tested heads. However in worn areas of the retrieved heads, the surface roughness was higher than observed in wear simulator testing. These results indicate that current testing methodologies do not fully capture the operational conditions of the material and the real performance of future new materials may not be adequately predicted by current pre‐clinical testing methods. © 2017 The Authors. Journal of Orthopaedic Research Published by Wiley Periodicals, Inc. on behalf of Orthopaedic Research Society 35:2781–2789, 2017.
Highlights
Zirconia toughened alumina (ZTA) composite ceramics aim to combine the fracture toughness of zirconia and the wear resistance and chemical stability of alumina.[1]
The tetragonal zirconica can undergo a transformation to a monoclinic phase along with a volume increase of between 3% and 4% that induces compressive stresses and inhibits crack propagation.[5]. Generally beneficial this transformation can be triggered by a moist environment such as that found in the hip, in a process known as low temperature degradation (LTD).[5]
Our study indicated that wear simulator parts did not replicate the increased surface roughness, nor the level of monoclinic transformation observed in retrievals
Summary
Zirconia toughened alumina (ZTA) composite ceramics aim to combine the fracture toughness of zirconia and the wear resistance and chemical stability of alumina.[1]. The current methodology to predict the phase transformation that will occur in vivo is based on the assumption that the growth of monoclinic nuclei is thermally activated.[8] Chevalier et al.[7] and Pezzotti et al.[9] have conducted a series of tests measuring the transformed monoclinic fraction as a function of time at elevated temperatures. Based on the reported activation energy in[8] an accelerated aging protocol, that would simulate the in vivo conditions, was suggested in which 1 h of accelerated aging for ZTA is expected to be equivalent to 3.9 years in vivo.[1,8] This protocol, adopted by the ISO standard 6474-2:2012,11 is to autoclave the ceramic at 134 ̊C, under two bars water steam, for 10 h to simulate 39 years of in vivo conditions There may be some discrepancy due to the difference in measurement techniques as both Raman spectroscopy and X-ray diffraction (XRD) are used by different authors[13,14,15] to assess monoclinic content
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