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Biomaterials in TMJ replacement79358.9–69.5 weight (wt) % Co, 27.0–30.0 wt % Cr, 5.0–7.0 wt % Mo, and up to 2.5 wt % Ni, the two FDA-approved systems make use of a newer wrought ASTM F1537 CoCrMo alloy with similar Co, Cr, and Mo content but a Ni content below 1 wt %.(7,9,55–57) Thermomechanical processing of these wrought alloys results in improved mechanical properties and wear resistance.(42) The elastic modulus remains equally high for both types of CoCrMo alloys and is around 210 GPa. As a result, CoCr and CoCrMo alloys are prone to stress shielding.(48)As discussed earlier, the Nexus CMF (Christensen) system provided a metal-on-metal articulation, whereas in the two FDA-approved systems, a metal-on-polyethylene bearing is applied. However, clear concerns have been reported regarding the use of metal-on-metal systems in TMJ replacements. Multiple reports in the recent literature have described the need for explantation of the Christensen system because of reactions caused by metallic debris.(3,47). While the absolute wear volume might be lower in metal-on-metal systems, more reactions to the wear debris are seen. Wolford et al.(18) reported that patients fitted with a metal-onmetal system exhibited significantly elevated body levels of Co and Cr. In comparison, patients fitted with a TMJ Concepts prosthesis showed no signs of UHMWPE or metallic debris, indicating good wear characteristics of the CoCrMo–UHMWPE combination in TMJ articulation. The same conclusion was reached by Westermark et al.(58) after histologic analysis of soft tissues surrounding Biomet and TMJ concept prostheses. These findings are further supported by several in vitro and in vivo studies showing that CoCr particles can exert toxic effects in the exposed tissues. McGregor et al.(54) found through animal studies that there is sufficient evidence for the carcinogenicity of metallic Co and limited evidence for the carcinogenicity of Co alloys; they classified Co-containing implants as possibly carcinogenic for humans.Titanium alloysGrowing concerns regarding the toxicity and stress shielding risks of CoCr alloys have stimulated the use of Ti alloys in TJR. Although the strength and fatigue resistance are somewhat lower than for CoCr, they are still sufficient for load-bearing implant applications and Ti alloys have Nikolas de Meurechy NW.indd 79 05-06-2024 10:14