Comparisons of immersion and electrochemical properties of highly biocompatible Ti-15Zr-4Nb-4Ta alloy and other implantable metals for orthopedic implants

Metal release from implantable metals and the properties of oxide films formed on alloy surfaces were analyzed, focusing on the highly biocompatible Ti-15Zr-4Nb-4Ta alloy. The thickness and electrical resistance (R-p) of the oxide film on such an alloy were compared with those of other implantable metals. The quantity of metal released during a 1-week immersion test was considerably smaller for the Ti-15Zr-4Nb-4Ta than the Ti-6Al-4V alloy. The potential (E-10) indicating a current density of 10 mu A cm(-2) estimated from the anodic polarization curve was significantly higher for the Ti-15Zr-4Nb-4Ta than the Ti-6Al-4V alloy and other metals. Moreover, the oxide film (4-7 nm thickness) formed on the Ti-15Zr-4Nb-4Ta surface is electrochemically robust. The oxide film mainly consisted of TiO2 with small amounts of ZrO2, Nb2O5 and Ta2O5 that made the film electrochemically stable. The R-p of Ti-15Zr-4Nb-4Ta was higher than that of Ti-6Al-4V, i.e. 0.9 Omega cm(2) in 0.9% NaCl and 1.3 Omega cm(2) in Eagle's medium. This R-p was approximately five-fold higher than that of stainless steel, which has a history of more than 40 years of clinical use in the human body. Ti-15Zr-4Nb-4Ta is a potential implant material for long-term clinical use. Moreover, E-10 and R-p were found to be useful parameters for assessing biological safety.