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Development of a patient-specific TMJ prosthesis2559compared to industrial grade 5 Ti6Al4V. Grade 23 is also most frequently used as a starting powder during the additive manufacturing of titanium implants.Besides Ti’s excellent strength and manufacturability, it also boasts a high corrosion resistance. This is thanks to the presence of a thin (1.5 – 10 nm in thickness) but stable oxide film on the surface which minimizes the release of metal ions from the bulk.(28,29) This layer is mainly composed of amorphous TiO2 with small amounts of suboxides TiO and Ti2O3 near the metal/oxide interface, and depending on the alloying elements, traces of Al2O3, V2O3 or V2O5, …(30,31) The nearly-stoichiometric structure of TiO2 with few ionic defects/vacancies makes this compound an excellent barrier for ionic migration from the bulk metal to the environment.(32) As a result and in contrast to other bioinert implant materials, Ti alloy implants are not encapsulated by fibrous tissue. Even in particulate form, tissue activation remains weak because of this protective layer.(33) However, the presence of other metal oxides in the passive film on the Ti6Al4Valloy does raise some concerns. Although Al2O3 has never been associated with toxicity or allergy after orthopedic biomaterial degradation,(34) vanadium oxide can cause allergic reactions,(35) as well as toxicity at low concentrations and with continuous exposure.(36) Moreover, the presence of alloying metal ions (Al, V) having a different valence than the host metal (Ti) can alter the ionic transport across the oxide layer. Whereas the stable Al2O3 decreases the anion vacancies thereby enhancing the barrier function of TiO2, vanadium oxide dissolves from the passive film creating vacancies that enable ionic transport and therefore increase metal ion release.(32)Despite the limited Ti ion release from Ti based implants, it can still be a problem for certain percentage of the patient population. Prospective skin patch testing of orthodontic patients who wear titanium- and nickelcontaining appliances demonstrated a nickel allergy prevalence of 14% and a titanium allergy prevalence of 4%.(37) It is thus imperative to subject potential candidates for TMJ replacement to skin patch testing for titanium hypersensitivity. However, a standardized patch test is not Nikolas de Meurechy NW.indd 255 05-06-2024 10:14