Biomaterials
What is the main reason why β titanium alloys such as the Ti-Nb-Ta-Zr system have been developed for implants that fix bone?
- ARetaining the BCC β phase down to room temperature makes the Young's modulus lower than that of Ti-6Al-4V and reduces stress shielding
- BThe β phase has an HCP structure with strong atomic bonding, which raises the Young's modulus above that of Ti-6Al-4V and prevents implant deformation
- CThe β phase prevents a passive film from forming on the titanium surface, which makes chemical bonding to bone easier
Show answer and explanation
Answer: A. Retaining the BCC β phase down to room temperature makes the Young's modulus lower than that of Ti-6Al-4V and reduces stress shielding
Titanium takes the HCP α phase at low temperature and the BCC β phase at high temperature, and adding large amounts of β-stabilizing elements such as Nb, Ta and Mo retains the β phase even at room temperature. β alloys have a Young's modulus of about 55–80 GPa, lower than that of the α+β alloy Ti-6Al-4V (about 110 GPa) and closer to that of bone (about 10–30 GPa), so they reduce stress shielding. Nb, Ta and Zr are also considered to have low cytotoxicity, with the added advantage of avoiding V and Al. It is the α phase that is HCP; calling the β phase HCP confuses the crystal structures. Raising the stiffness of an implant works in the direction of strengthening stress shielding. β alloys also form a passive film on their surface and keep their corrosion resistance, and losing the film would only increase ion release, not lead to bonding with bone.