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Biomaterials in TMJ replacement773Second, it should be noted that while wrought CoCrMo has better wear properties than cast CoCrMo, the Christensen system made use of the latter.(18) However, CoCrMo-on-UHMWPE implants have shown excellent wear properties, and when used in articulation with UHMWPE during hip simulator testing, Ti6Al4V exhibits a 35% higher wear rate than CoCrMo.(41,48) As such, CoCrMo-on-UHMWPE is currently considered the gold standard for low friction articular components in orthopedic and TMJ TJR systems.(18,38,40,41)In vitro and in vivo performanceWhen designing a prosthesis, it is important to rigorously test the selected materials for all the properties discussed above, not only by standard material characterization techniques but also by using both in vitro and in vivo tests, as the biological environment can have a significant impact on the material’s performance. The importance of in vivo wear testing, for example, has been clearly established for hip joint prostheses, in which it has become clear that the wear rate can be much higher than that encountered during in vitro testing.(12) With respect to TMJ implants, the use of Proplast in the Vitek-Kent prosthesis serves as a clear example of how in vitro testing without thorough in vivo experimentation is insufficient to allow the appropriate choice of materials.(6,12,23,34,37,48) Materials Currently Used in Total Temporomandibular Joint ProsthesesAlthough current implant devices make use of materials that have been thoroughly tested and evaluated in the field of orthopedic surgery, many different and much less well-suited materials have been used throughout the history of TMJ TJR. For example, some of the first screws or even prostheses, such as the condylar design by Flot et al., were made out of a stainless steel alloy.(6,34) While several stainless steel alloys, such as 316L, have low production costs, good availability, and good tensile strength and fatigue properties, they are not suitable TMJ implantation materials. This is at least partially because they are susceptible to corrosion and crevice formation, and they are biotolerant rather than Nikolas de Meurechy NW.indd 77 05-06-2024 10:14