INVITRO CORROSION TESTING OF TITANIUM SURGICAL IMPLANT ALLOYS - APPROACH TO UNDERSTANDING TITANIUM RELEASE FROM IMPLANTS

The excellent corrosion resistance of titanium and its alloys to physiological chloride solutions is well documented. Occasionally, however, titanium compounds have been found in tissue adjacent to titanium implants. These findings were reported to be unrelated to wear processes, suggesting that either the metal or its passive film was dissolving. The unpredictability of these findings further suggests that preimplantation surface treatments and/or variations in the physiological environment may be factors. To determine a mechanism by which titanium can be released from an implant a study was initiated which employed electrochemical techniques, Auger electron spectroscopy (AES), and replica transmission electron microscopy (RTEM). Specifically, the purpose was to characterize the passive film on titanium and Ti‐6Al‐4V, and to determine if there is dissolution of the film or metal in a static unstressed state. Passive film behavior of commercially pure titanium, Ti‐6Al‐4V and nitrided Ti‐6Al‐4V was studied by anodic polarization and pulse potentiostatic capacitance techniques in Ringer's solution at 37°C with and without physiological additions of several amino acids. Solution Po2, pH, and specimen surface finish were varied to include all probable in vivo conditions. In all tests, potentiostatic anodic polarization of each material yielded potential‐current density curves which showed passivity over the entire experimental range. This range greatly exceeded the oxygen‐reduction reversible electrode potential. No breakdown potentials were observed. The passive current density was not significantly affected by varying the experimental parameters. AES as well as charge and capacitance measurements showed that the electrochemical reaction which occurred was growth of the passive film. The data indicated that the effective electrochemical area of each specimen was less than its geometrical area, and was dependent on both pH and surface finish. The findings of this study showed that, under static conditions, titanium and Ti‐6Al‐4V should withstand exposure to physiological chloride solutions at body temperature indefinitely. Furthermore, the findings are consistent with a model of the implant surface inwhich the naturla air oxide that initially forms on an abraded implant surface is comprised of microscopic oxide needles. It is proposed that (i) the needless canbe broken off or dissolved in vivo, providing the source of the titanium sometimes found in tissue adjacent to an implant, and (ii) use of certain pretreatments may remove the needles prior to surgery, resulting in the more common case in which titanium is nto found in the tissue. Copyright © 1979 John Wiley & Sons, Inc.