Relationship between demineralization events in dental enamel and the pH and mineral content of plaque.

Acid production in dental plaque from fermentation of dietary carbohydrate does not necessarily lead to demineralization of the underlying enamel. In the past it has been understood that pH buffering by plaque constituents must be overcome to allow the pH to fall, and then the fall must be of sufficient magnitude to exceed a critical pH value, i.e. the point where plaque fluid is just saturated with respect to enamel mineral. An evaluation of the literature suggests, however, that the critical pH is not a fixed value. It changes slowly as enamel mineral solubility changes with repeated pH cycling. Further, plaque mineral ion sinks, in which ions are removed from solution (first described by Luoma in 1964) and ion reservoirs from which ions are added to solution, modulate the critical pH value during plaque pH fall and rise so that its exact value is difficult to predetermine. A solid phase calcium phosphate ion reservoir in plaque may saturate plaque fluid with respect to enamel mineral continuously as the pH falls so that the critical pH is never exceeded. Common ion repression of enamel mineral dissolution is likely to be effective in the order pH greater than Ca greater than P. Plaque fluoride influences enamel dissolution in more than one way. Simultaneous dissolution of hydroxyapatite and reprecipitation of fluorapatite, a process which results in apparent dissolution repression, is probably the most important mechanism initially. This process may coat individual enamel crystals with a F-rich layer so that, while the total F content is rather low, its effective solubility is more like that of fluorapatite. Fluoride in plaque fluid may then repress enamel mineral dissolution by common ion repression. If fluoride action follows this sequence efforts to build F into enamel clinically would be just as important as attempts to maintain F levels in plaque and saliva.