The effect of Al(III) and fluoride on the rate of dissolution of powdered enamel in 0.1M lactate solution has been studied at pH 4.5 and 5.0. The dissolution rate of synthetic hydroxyapatite (HAP) microcrystals in similar solutions is reported for pH 5.0. A combination of Al(III) and fluoride has a stronger inhibitory effect on dissolution of enamel and HAP at pH 5.0 than that of these substances separately, even though the concentration of free fluoride ions decreases in the presence of Al(III). The effects on dissolution of enamel at pH 4.5 are similar, but less pronounced. It is suggested that Al(III) and its major complexes in slightly acidic solution, being positively charged, are adsorbed at different sites on the mineral surface from fluoride, thereby producing a synergistic effect. When the dissolution process takes place far from equilibrium, inhibitor concentrations greater than about 100μM ( ∼ 3 ppm for Al(III) and ∼ 2 ppm for F−) show significant inhibition under the conditions studied. In such solutions the real concentration of Al3+ is of the order 1μM. Close to equilibrium, the total inhibitor concentration need only be of the order 1μM (i.e. 0.03 ppm for Al(III), 0.02 ppm for F−) for a strong effect. In such solutions, the real concentration of Al3+ is in the nanomolar range.

Longitudinal microradiography has been used to determine inhibiting effects of aluminum (III), Al, and fluoride on mineral loss from slices of bovine enamel exposed to a demineralizing solution 4 h daily for 35 days. Inhibitor treatment was 5 min four times daily. For the remaining time, the samples were immersed in a neutral calcium phosphate solution which allowed neither remineralization nor demineralization. This study indicates that a 1-mM (27 ppm) solution of Al in a 0.1-M lactate solution, pH 5, has an inhibitory effect on the in vitro demineralization of bovine enamel. Application of this solution alternating with 20 mM (380 ppm) fluoride gave the same total inhibition as treatment with 20 mM fluoride alone.