Theories on the mode of action of fluoride in reducing dental decay.

Several properties of the fluoride ion at physiological concentrations are known which may be concerned with its actions in reducing dental caries. T h e properties that have been most thoroughly studied as a basis of the anticaries action are: (I) the incorporation of fluoride in the hydroxyapatite of the calcified tissues, both during and after their formation, thus increasing the proportion of the less soluble fluoroapatite; (2) a tendency to favour the precipitation of mineral matter from solutions saturated with calcium phosphate, such as saliva; (3) the inhibition and, in a few instances, the slight stimulation of certain enzyme systems. These effects are not mutually exclusive and any or all of them may contribute to the reduction of caries. Other actions have been suggested but have not yet been studied sufficiently fully to be established. T h e possibility of undiscovered effects which may prove to be important cannot, of course, be dismissed. Reactions of fluoride with hydroxyapatite McCann & Bullock (1955) and Leach (1959) concluded from experiments with powdered enamel and dentine that with sodium fluoride solutions containing concentrations below 75-100 p.p.m. the most important reaction with hydroxyapatite was an exchange with hydroxyl to form Auoroapatite. With higher concentrations several other reactions were found to occur such as exchange with carbonate, the precipitation on the crystal surface of calcium and magnesium fluorides and possibly the adsorption of sodium fluoride. Analysis of fluorosed tissues has shown an inverse relationship between the fluoride and carbonate of bone, suggesting that, even at concentrations present in tissue fluid, some replacement of carbonate can occur. T h e difference between the effects on enamel of high and low concentrations of fluoride suggests that the nature of the effect on caries may depend on the means of administering the fluoride. Topical application of 2% sodium fluoride solution or dentifrices containing 0.1 yo flueride produce such high concentrations that they may reduce caries in a different way from the minute concentrations of fluoride ions likely to be present in the mouth from water containing I p.p.m. Effect of fluoride on solubility of enamel and dentine. Treatment of bone, tooth or calcium phosphate with fluoride solutions over a wide range of concentration, beginning at less than I p.p.m., can be readily shown to reduce their solubility. It has therefore long been supposed that a reduction in the solubility of enamel and