Effect of Fluoride on Titanium Discoloration and Elution under Biofilm

Objectives: We reported that fluoride did not enhance the corrosive properties of the titanium surface under artificial biofilm (JADR, 2015). This study aimed to compare the effect of fluoride on titanium under artificial biofilm with that in the buffer solution, with consideration for environmental pH.
Methods: An experimental well was made of polymethyl methacrylate with mirror-polished pure titanium at the bottom. Streptococcus mutans NCTC10449 were packed into the well as artificial biofilm and incubated after the addition of 1% glucose with and without fluoride (0, 225 or 900 ppm F) at 37°C for 90 min. In addition, pH under the biofilm was monitored by a miniature pH electrode. In immersion test, the titanium samples were immersed in the fluoride solution (0, 225 or 900 ppm F, pH 4.2 or 6.5) for 90 min. Before and after incubation or immersion, color difference value (ΔE*ab) and gloss of the titanium surfaces were determined using a spectrophotometer (CM-700d, Konica Minolta, Tokyo, Japan). The amounts of titanium eluted into biofilm or immersion solutions were measured by inductively-coupled plasma mass spectrometry (Agilent 8800, Agilent, Tokyo, Japan).
Results: Only after immersion in fluoride solution containing 900 ppm F at pH 4.2, ΔE*ab was significantly increased to 15.06±5.35 (p<0.05); gloss was significantly decreased to 216.67±268.47 (p<0.01); and the increase of titanium was detected (125.63±113.60 ppm). On the other hand, after the incubation under biofilm, there was no significant difference in ΔE*ab, gloss and the amounts of titanium eluted, irrespective of the concentration of fluoride. The final pH values with 0, 225 and 900 ppm F were 4.17±0.18, 6.53±0.06 and 6.54±0.12, respectively.
Conclusions: The present study indicates that fluoride induced corrosion of titanium in acidic environment. However, under artificial biofilm, fluoride did not enhance the titanium corrosion, because fluoride could prevent bacteria from producing acidic conditions.