Is Surface Metastability of Today’s Ceramic Bearings a Clinical Issue?

Abstract

Recent studies on zirconia-toughened alumina (ZTA) evidenced that in vivo aged implants display a much higher monoclinic zirconia content than expected from in vitro simulations by autoclaving. At the moment, there is no agreement on the source of this discrepancy: Some research groups ascribe it to the effect of mechanical impact shocks, which are generally not implemented in standard in vitro aging or hip walking simulators. Others invoke the effect of metal transfer, which should trigger an autocatalytic reaction in the body fluid environment, accelerating the kinetics of tetragonal-to-monoclinic transformation in vivo. Extrapolations of the aging kinetics from high (autoclave) to in vivo temperature are also often disputed. Last, Raman spectroscopy is by far the preferred method to quantify the amount of monoclinically transformed zirconia. There are, however, many sources of errors that may negatively affect Raman results, meaning that the final interpretation might be flawed. In this work, we applied Raman spectroscopy to determine the monoclinic content in as-received and in vitro aged ZTA hip joint implants, and in one long-term retrieval study. We calculated the monoclinic content with the most used equations in the literature and compared it with the results of X-ray diffraction obtained on a similar probe depth. Our results show, contrary to many previous studies, that the long-term surface stability of ZTA ceramics is preserved. This suggests that the Raman technique does not offer consistent and unique results for the analysis of surface degradation. Moreover, we discuss here that tetragonal-to-monoclinic transformation is also necessary to limit contact damage and wear stripe extension. Thus, the surface metastability of zirconia-containing ceramics may be a non-issue.

Advantages and developments of Raman spectroscopy for electroceramics

“…Raman spectroscopy has been successfully used in electroceramics also for spatially resolved phase analyses, such as polytype detection in SiC 97,98 , solid solution studies in AlN-SiC composites 99 , and the determination of heterophase mixtures in the relaxor ferroelectric Pb(In 1/2 Nb 1/2 )O 3 -Pb(Mg 1/3 Nb 2/3 )O 3 -PbTiO 3 100 . Quantitative analyses of phase segregation, similar to the determination of monoclinic fraction in zirconia ceramics 101,102 , however, require the presence of suitable standards with a known phase fraction 103 . Such standards are generally hard to establish in electroceramics.…”

Current status and future potential of wear-resistant coatings and articulating surfaces for hip and knee implants

“…The hydrolytic long-term stability of zirconia-based material remains a concern however [ 171 , 172 ], and would have to be thoroughly investigated before being an option for biomedical implants.…”