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Chapter 386β-stabilizing alloying elements, such as Nb, zirconium (Zr), Mo, Ta, and iron (Fe), instead of Al and V.(48,59,68) While β-Ti alloys have been used since the 1960s (mainly for aerospace applications), specialized biocompatible alloys were only first developed in the 1990s.(69) The quaternary Ti-Nb-Zr-Ta (TNZT) alloys are currently considered to be among the most promising alloys to replace cp Ti and Ti6Al4Vin implant applications.(68) These β-Ti alloys exhibit higher corrosion resistance than the older Ti alloys because of the presence of Nb, Zr, and Ta, which form more stable oxides (especially in comparison with V). The presence of Nb2O5, ZrO2, or Ta2O5 strengthens the TiO2 passive film, effectively reducing the release of metal ions.(48) Another important asset of these new alloys is their lower elastic modulus, compared with cp Ti and Ti-6Al-4V. The two most common TNZT alloys, Ti–29Nb–13Ta–4.6Zr and Ti–35Nb–7Zr–5Ta, have elastic modulus values of 65 GPa and 55 GPa, respectively, which are a closer match to the modulus of cortical bone.(48,59)An important drawback of β-Ti alloys, however, is their lower fatigue strength, compared with that of Ti-6Al-4V. One possible strategy to improve fatigue strength uses thermal treatment to induce the formation of finely dispersed α and ω phases throughout the β matrix. Overall, β-Ti alloys have a good heat treatment response, meaning that, depending on the thermal processing, it is possible to fine-tune their mechanical properties. As such, a combination of enhanced strength (including fatigue strength) and fracture toughness can be obtained, yet at the expense of a low elastic modulus.(48,59,70) Alternatively, particles such as Y2O3, SiO2/ZrO2, SrO, and CeO2 can be added to the alloy, as these cause dispersion strengthening of the material.(59,70)Furthermore, although the friction wear characteristics of Nb-containing β-Ti alloys are superior to those of cp Ti or Ti6Al4Vbecause of the lubricating properties of Nb2O5, the total wear resistance is still too low for using these alloys in articulating joint surfaces.(48,68) It should also be noted that although in vitro studies have shown that phagocytic cells are stimulated more by Ti6Al4Vwear debris than by Ti-6Al-6Nb or Ti-13Nb-13Zr debris(19), further long-term research concerning Nikolas de Meurechy NW.indd 86 05-06-2024 10:14