In vitro assessment of corrosive properties of titanium as a biomaterial.

Titanium (Ti) is thought to be a highly biocompatible material, and its clinical applications are becoming increasingly frequent. However, there have recently been some clinical papers reporting hypersensitivity and allergic reactions to Ti. The purpose of this study was to assess the corrosive properties of Ti in the intra-oral environment in vitro. Cast pure Ti specimens were immersed in artificial saliva, physiological saline solution, and 128 mmol x L(-1) of lactic, formic and acetic acids for 3 weeks at 37 degrees C with shaking. The colour, weight, surface morphologies and chemical binding state of specimens were observed before and after immersion. Marked discoloration was recognized on the surface of specimens immersed in formic acid, and a significant difference was found between the immersion solutions. Weight changes also varied with solutions; a tendency to increase in formic acid and to decrease in lactic acid. A slight loss was observed in specimens immersed in lactic acid and artificial saliva. The oxide layer composed mainly of Ti-oxide on the surface of the immersed specimens was thinnest in lactic acid, and thickest in formic acid. The present study indicates that both hydrogen evolution type and oxygen diffusion type corrosion on Ti surfaces are possible in the living body. (Our results also confirm some clinical studies reporting Ti accumulation in surrounding tissues and Ti causing allergic reactions.)