The influence of fluoride on the demineralisation of calcium apatites

Abstract

Dental caries is a disease characterised by the dissolution of the hydroxyapatite component of the tooth by acids in the oral environment. The early caries process can be inhibited by the presence of fluoride but the preventive mechanism involved is unresolved. The present study was designed to investigate the role of apatite-incorporated fluoride, apatite-surface-adsorbed fluoride, and free solution fluoride in hydroxyapatite dissolution under laboratory conditions. Hydroxyapatite was prepared by aqueous precipitation. Partially-fluoridated hydroxyapatite was produced by adding sodium fluoride solutions of varying concentration during the reaction. Surface-adsorbed fluoridated hydroxyapatite was produced by stirring hydroxyapatite in a sodium fluoride solution. Dissolution of powder and pellets was studied at different temperatures using 0.01 moldm-3 acetate buffers at 0.1 moldm-3 ionic strength and pH 5.0 and 3,0. Pellets were made by compressing hydroxyapatite with polyethylene powder. The relative importance of chemical processes (acid reactivity and solubility) and diffusion processes in the complex dissolution reaction was determined. The reaction of acid with powdered and pelletised hydroxyapatite was very rapid in the initial stages of dissolution. There was no observable temperature effect. The chemical processes were the same under the various pH and temperature conditions and were unaffected by the presence of fluoride. A high fluoride concentration in the apatite slowed down the diffusion component of the reaction in the later stages of dissolution. Surface-adsorbed fluoridated hydroxyapatite with a high fluoride content had the properties of fluorapatite. Incorporated and adsorbed fluoride lowered the total solubility of the apatite. Dissolution increased when pH was decreased but fluoride still caused a reduction in rate at the lower pH. The increase in reaction rate was proportional to the increase in unionised acetic acid concentration. The different forms of hydroxyapatite (powder and pellet) influenced the reaction rate. The diffusion process was dominant much earlier for pellet dissolution (from approximately 10 minutes) than powder dissolution (after 1 hour). Acid reactivity and solubility effects were more significant in powder dissolution but insignificant in pellet dissolution compared to diffusion. The acid reactivity and diffusion activation energies for both powder and pellet dissolution were very low and were unaffected by fluoride. Changes in reaction rate in the diffusion-controlled part of the reaction were due to changes in the frequency factor, A, of the Arrhenius equation which is dependent on the diffusion of unionised acid. Low concentrations of fluoride (<1000ppm) in the apatite had little effect on dissolution but small amounts of fluoride in the buffer (<10ppm) did reduce hydroxyapatite dissolution. This is caused by calcium fluoride formation on the apatite surface which inhibited the diffusion process. The overall conclusion of this study is that free fluoride present at the enamel surface at less then 10ppm is more effective in reducing dissolution than greater concentrations (100 times) of apatite-incorporated or surface-adsorbed fluoride.