Zirconia Phase Transformation in Zirconia-Toughened Alumina Ceramic Femoral Heads: An Implant Retrieval Analysis

Ceramic Wear Particles: Can They Be Retrieved In Vivo and Duplicated In Vitro?

BACKGROUND

Little is known about retrieved zirconia platelet toughened alumina (ZPTA) wear particles from ceramic-on-ceramic (COC) total hip arthroplasty. Our objectives were to evaluate clinically retrieved wear particles from explanted periprosthetic hip tissues and to analyze the characteristics of in vitro-generated ZPTA wear particles.

METHODS

Periprosthetic tissue and explants were received for 3 patients who underwent a total hip replacement of ZPTA COC head and liner. Wear particles were isolated and characterized via scanning electron microscopy and energy dispersive spectroscopy. The ZPTA and control (highly cross-linked polyethylene and cobalt chromium alloy) were then generated in vitro using a hip simulator and pin-on-disc testing, respectively. Particles were assessed in accordance with American Society for Testing and Materials F1877.

RESULTS

Minimal ceramic particles were identified in the retrieved tissue, consistent with the retrieved components demonstrating minimal abrasive wear with material transfer. Average particle diameter from in vitro studies was 292 nm for ZPTA, 190 nm for highly cross-linked polyethylene, and 201 nm for cobalt chromium alloy.

CONCLUSION

The minimal number of in vivo ZPTA wear particles observed is consistent with the successful tribological history of COC total hip arthroplasties. Due to the relatively few ceramic particles located in the retrieved tissue, in part due to implantation times of 3 to 6 years, a statistical comparison was unable to be made between the in vivo particles and the in vitro-generated ZPTA particles. However, the study provided further insight into the size and morphological characteristics of ZPTA particles generated from clinically relevant in vitro test setups.

Damage analysis of retrieved BioloxⓇdelta components used in hard and soft bearings

This retrieval study included 43 Biolox delta explants (18 CoC, 25 CoP). Implants were examined macroscopically, whereby damage was evaluated using a semi quantitative scoring system. Confocal microscopy was used to examine wear related damage patterns of the articulating surfaces. Scanning electron microscopy (SEM) with energy-dispersive X-ray spectroscopy (EDS) was used to analyze wear marks on the implant surface and wear debris in periprosthetic tissue samples. Raman spectroscopy and X-ray diffraction (XRD) were used to quantify monoclinic zirconia fractions. On all components, in vivo wear resulted predominantly in different damage patterns caused by metal transfer. In CoC bearings stripe wear was additionally detected, and some implants underwent severe damage due to component breakage. The wear scores were higher for CoC components, with no differences between the scores for CoC heads and liners. Wear features caused comparable roughening on implants from CoC and CoP bearings. SEM studies demonstrated that most wear marks were caused by metal debris released from implant components. Grain pull-out was observed in stripe wear regions. Monoclinic phase shift was observed in a similar quantity on components from CoP and CoC bearings. The increase of monoclinic zirconia content around metal deposits was minimal and was more pronounced in areas of stripe wear. The results of this study indicate, that ZTA components in general undergo minimal wear in both, CoC and CoP bearings, however, it is more pronounced in the former. Metal deposits, as the most common wear feature, have no significant effect on monoclinic phase transition. STATEMENT OF SIGNIFICANCE: In this paper, we classify all damage patterns macroscopically according to an established scoring system and assess them regarding surface roughness (confocal microscopy) and monoclinic phase content (Raman spectroscopy) in order to derive the severity for patients. We compare hard-hard and hard-soft bearings and relate damage patterns with metal transfer based on SEM/EDS examinations. Furthermore, we work out correlations between patient-specific data, cause of revision and the physical condition of each individual sample Our cohort consists of 43 Biolox delta retrievals, a comparatively large quantity. In addition, we address current topics such as metal transfer and, based on the classification of damage patterns, provide incentives and/or meaningful focal points for further research.

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

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.