Analysis of Chemisorbed Tribo-Film for Ceramic-on-Ceramic Hip Joint Prostheses by Raman Spectroscopy

Polycrystalline Diamond as a Potential Material for the Hard-on-Hard Bearing of Total Hip Prosthesis: Von Mises Stress Analysis

Due to polymeric wear debris causing osteolysis from polymer, metal ions causing metallosis from metal, and brittle characteristic causing fracture failure from ceramic in the application on bearing of total hip prosthesis requires the availability of new material options as a solution to these problems. Polycrystalline diamond (PCD) has the potential to become the selected material for hard-on-hard bearing in view of its advantages in terms of mechanical properties and biocompatibility. The present study contributes to confirming the potential of PCD to replace metals and ceramics for hard-on-hard bearing through von Mises stress investigations. A computational simulation using a 2D axisymmetric finite element model of hard-on-hard bearing under gait loading has been performed. The percentage of maximum von Mises stress to respective yield strength from PCD-on-PCD is the lowest at 2.47%, with CoCrMo (cobalt chromium molybdenum)-on-CoCrMo at 10.79%, and Al₂O₃ (aluminium oxide)-on-Al₂O₃ at 13.49%. This confirms that the use of PCD as a hard-on-hard bearing material is the safest option compared to the investigated metal and ceramic hard-on-hard bearings from the mechanical perspective.

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.