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Chapter 392showed significantly higher wear rates compared to UHMWPE, when used in low-conformity designs such as a knee TJR device. Despite the absence of cytotoxic effects observed with PEEK composite wear debris, the conclusions of Wang et al.(91), Grupp et al.(94) and Brockett et al.(95) are of extreme importance when considering the use of (CFR-) PEEK as a bearing material for a TMJ TJR device. As stated by Mercuri et al.(13), the functional anatomy of the knee is far less constrained than that of the hip joint. The same can be said for the TMJ, which in this aspect is far more comparable to the knee joint than to the hip joint. As such, based on the available orthopedic literature, CFRPEEK should be considered unsuitable as a bearing surface for a TMJ TJR system, though research and simulations designed specifically for TMJ TJR are necessary to make more definite conclusions. Alumina-zirconia compositesWith respect to tribological properties, ceramic materials outperform metals and polymers. As such, the bioinert ceramics, alumina (Al2O3) and zirconia (ZrO2), are widely applied as articulating surfaces in orthopedic joint replacements. While Al2O3 has been in use in hip joints since the 1970s, ZrO2 was first introduced into the field of orthopedic surgery around 1980; its fracture toughness and flexural strength are superior to those of Al2O3.(96) Indeed, ZrO2 displays a high resistance to crack propagation owing to stress-induced phase transformation at the crack tip, which is accompanied by a volumetric expansion that induces compressive stresses.(97) As a result of this exceptional balance of toughness and strength, ZrO2 steadily gained popularity, and excellent success was reported (failure rates as low as 0.002%).(97) In 2001, however, because of deviations in thermal processing during manufacturing, particular batches of ZrO2 femoral heads experienced accelerated aging, resulting in high fracture rates in vivo. The inevitable withdrawal of these batches from the market led to a loss of confidence in ZrO2 and focus shifted to using metal-on-metal implants.(48,96,98) The problem with thermal processing regretfully highlighted one of the main concerns about ZrO2 ceramics, which is its high sensitivity to aging, also known as low temperature degradation (LTD), in the presence of Nikolas de Meurechy NW.indd 92 05-06-2024 10:14