<prompt> The primary mechanism of action of fluoride on topical application is -Conversion of hydroxyapatite to fluoroapatite by replacing the OH- ionsInhibition of plaque bacteriaForm a reservoir in salivaImprovement in tooth morphology  <response>: Ans. is a i.e., Conversion of hydroxy-apatite to fluro-apatite by replacing the OH ions When hydroxyapetite is exposed to low fluoride concentrations about 1 ppm a layer offluorapetite forms on the hydroxyapetite crystals. The thin layer governs the rate of enamel dissolutin and reduces rate of caries progression. o During caries attack, the enamel is exposed to a pH of about 5.5 or lower, thereby leading to its dissolution. o The presence of fluoride reduces the solubility of enamel by promoting the precipitation of hydroxyapetite and phosphate mineral. 