Enamel chemistry
Enamel mottling apart, the fact that fluoride affects dental caries has been confirmed by many well-controlled studies of topical fluoride agents and studies of the posteruptive caries-preventive mechanisms of fluoride (for review, see Fejerskov et al, 1996a, b).
For many years, it was believed that incorporation of fluoride into enamel increased the resistance of the tooth to dissolution and that the surface enamel fluoride concentration could be a marker of tooth resistance or susceptibility to caries. This was determined by enamel biopsy; a small area of enamel was dissolved or abraded away and then analyzed for fluoride content. A number of variations on this technique have been described and used to associate enamel fluoride with tooth resistance.
For ease of access, the enamel biopsy was taken from buccal smooth surfaces, which are not susceptible to caries. Smooth-surface caries arises mainly on approximal areas below the contact point of the molars and premolars, which are impossible to biopsy in vivo. In studies on enamel surface fluoride, it is assumed the biopsy actually gives a reliable measure, but the fluoride concentration can vary markedly from one small area to the next, even within the same tooth (Weatherell et al, 1977). It would appear, therefore, that statistically significant relationships between surface enamel fluoride and caries cannot be demonstrated on an individual basis, and in studies conducted some decades ago it is not surprising that no consistent results were reported.
It is highly likely that in the populations studied, largely in the United States, other fluoride mechanisms have been far more important, for example, the action of fluoride on plaque and bacterial metabolism, coupled with its acknowledged role in remineralization. Fluoride in surface enamel may have a slow-release or reservoir function, releasing fluoride ions to retard enamel demineralization and enhance remineralization. Simple biopsy of enamel for fluoride remains, therefore, only a very weak marker of tooth resistance on both an individual and a population basis.
The relationship between other trace elements in whole enamel and caries prevalence has also been evaluated. In the largest of these studies (almost 500 enamel samples) more than 30 trace elements were evaluated. The results showed significant positive correlations with the caries prevalence of the tooth donor for manganese, copper, and cadmium; significant negative correlations were found for aluminum, iron, selenium, and strontium (Curzon and Croker, 1978). Studies based on whole enamel do not, however, serve as a good indicator of tooth resistance on an individual basis. Because caries is initiated at the enamel surface, surface trace element concentrations would, as with fluoride, be more appropriate.
It is now recognized this is a very dynamic process, with fluoride and other trace elements passing in and out of the surface enamel. The total concentration of an element in enamel is therefore less important than its availability at the tooth surface during periods of cariogenic challenge. Simple analysis of enamel, surface or whole, is therefore no longer considered relevant to tooth resistance.
Nor is solubility directly related to dental caries. In addition, the solubility of enamel is dependent not only on fluoride content but also on other factors, such as carbonate content, many trace elements, and various other inorganic components. For example, experiments have shown that test samples of enamel from sharks, consisting of almost 100% fluorapatite, are more easily decayed than is well-matured enamel from elderly humans (Ogaard et al, 1988).
