Titanium Release from Implants: A Proposed Mechanism

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Abstract
The excellent corrosion resistance of titanium surgical implant alloys is well documented. However, titanium compounds have been occasionally found in tissues adjacent to titanium implants, and these findings were reported to be unrelated to wear processes. The purpose of this study was to determine a mechanism by which titanium is released from an implant. The corrosion behavior of commercially pure titanium and Ti-6Al-4V was studied by electrochemical means, namely, anodic polarization and differential capacitance techniques. Tests were performed in Ringer's solution at 37°C. Solution pO2, pH, and surface finish of the specimen were varied to include all probable in vivo conditions. The corrosion resistance of commercially pure titanium and Ti-6Al-4V was not significantly altered by variations in pH, pO2, or surface finish. The electrochemical reaction which occurred was growth of the metal's protective passive film. The data indicated that the effective area of a specimen was less than its apparent surface area and was dependent on pH and surface finish. These findings suggest a model of the implant surface in which the natural air oxide that initially forms is comprised of microscopic oxide needles at surface irregularities plus a smooth planar oxide. It is proposed that the oxide needles can be broken off or dissolved in vivo, providing the source of the titanium sometimes found in tissue adjacent to an implant. Use of certain pretreatments may remove the needles prior to surgery, resulting in the more common case in which titanium is not found in the tissue.