Immediate and Sustained Reduction in Hydroxyapatite Dissolution Rate by Fluoride

Objectives: Dental erosion is a problem of considerable clinical concern and there is substantial interest in the development of oral healthcare products as preventive treatments for erosion. Fluoride is included in most toothpaste and mouthrinse formulations and has significant anti-cariogenic activity. The aim of this study was to investigate the effect of a fluoride pre-treatment on HA dissolution in an erosion-like regime. A real-time method was used to determine the effect of fluoride concentration on HA dissolution for a sustained period following fluoride exposure.

Methods: The HA dissolution rate in 14 mM citric acid, pH 3.20, was measured using a pH stat system over an assay period of 30 min. The baseline dissolution rate was calculated as the mean of three successive assays performed on the same disc. The HA discs were subsequently immersed in gently stirred 300 mg/L or 1000 mg/L fluoride solutions (as NaF) for 120 s followed by rinsing in deionised water. Discs were then subject to a series of sequential assays until the baseline dissolution rate was recovered.

Results: Exposure to 300 mg/L fluoride resulted in a reduction in HA dissolution rate immediately after treatment, but the rate reverted to the baseline value after the first acid exposure. Exposure to 1000 mg/L fluoride resulted in a greater reduction in HA dissolution rate, and the inhibitory effect was sustained over the period of two successive assays, a total acid exposure time of 60 min. The HA dissolution rate had reverted to its baseline level by the third assay.

Conclusion: Exposure to sodium fluoride solution reduced the rate of HA dissolution in citric acid. The magnitude and persistence of the inhibitory effect was influenced by fluoride concentration.

Research supported by GlaxoSmithKline