Fluoride Concentration Required to Prevent Citric Acid Erosion of Enamel

Concentrations of fluoride as low as 0.5 ppm have been shown to be effective for stabilizing healthy tooth enamel against erosive attacks from plaque-like lactic/acetic acid solutions (Carey et al. IADR 2008).  Citric acid has the ability to chelate calcium and therefore is more erosive than lactic or acetic acid.  Objectives: To determine the relationship between citric acid concentration and the amount of fluoride required to stabilize the enamel surface during an erosive attack.  Methods:  Sound enamel samples were thinned using 600-grit abrasive paper to ~100 µm. The cut surfaces were protected with a layer of x-ray transparent epoxy and a piece of a TEM grid was embedded in the epoxy.  A static erosive challenge was used where enamel slices (n≥3 for each condition) were exposed to citric acid solutions (0.3 % or 1.0%) at pH 3.9 with varying amounts of fluoride (0, 25, 50, or 100 ppm F) for 4 h at room temperature with stirring.  The enamel slices were evaluated for erosion by comparing x-ray microradiographs taken before and after the experiments.    Results: 

Two-way ANOVA found the strength of the citric acid and the dose of fluoride to be significant (p ≤ 0.05).  The interaction between the acid strength and fluoride dose was not significant (p ≥ 0.05).  Conclusions: Fluoride ion has a stabilizing effect on the surface of enamel against citric acid induced erosion.  More fluoride ion is needed to stop the erosive effect of the more concentrated citric acid solution.  Supported by NIH Grant R01DE14707, ADAF and NIST.