Fluoride Incorporation and Apatite Solubility

Abstract

The classical theory on the solubility of ionic compounds has been extended to solid solutions and deviations from stoichiometry by combination of thermodynamical and solid state chemical principles. Theory predicts that the incorporation of foreign ions in enamel apatite can change its solubility product over several orders of magnitude in several ways. Experimental data on the composition of enamel indicate that the incorporation of F^- ions will stabilize the enamel crystals by the apparent diminution of Na⁺ and CO₃^2- incorporation. The additional effect may be that a change in the type of the main ternary apatite in enamel apatite crystals, carbonatoapatite into hydroxyapatite, is induced by incorporation of a relatively small amount of F^- ions. It is estimated that these effects in combination lower the solubility product of enamel apatite, calculated as Koha, by 5–10 orders of magnitude. The present theory can be used to screen other single or combined substitutions for their possible anticariogenic effect. Kinetic factors determining the success of prevention by ion incorporation are discussed.

Crystallinity Patterns of Fluoridated Hydroxyapatites before and after Incubation in Acidic Buffer Solution

“…The equilibrium relations given by McCann [1968], Driessens et al [1973Driessens et al [ , 1979 and Verbeeck et al [1980] explain why there is enrichment of fluoride ions during recrys tallization or a cycle of dissolution and re precipitation reactions. However, these re lations are insufficient to explain quanti tatively the variation of fluoride content seen in this study.…”

“…Similar complex patterns of variation with fluoride content were observed in the crystallinity of the original fluoridated apatites irrespective of the tem perature of synthesis. The same tendency was also found after the incubation, although the crystallinity of 60-FHAp increased along the c-axis and the crystallinity of 80-FHAp decreased along the a-axis.It is well known that fluoride ions are readily taken up from solution into apatite crystals [Brudevold et al, 1963;Driessens, 1973; Moreno el al., 1974 Moreno el al., , 1977, We have previously reported [Okazaki et al, 1981] that the crystallinity of fluoridated hy droxyapatites changes irregularly with in creasing fluoride content, and does not parallel the solubility and dissolution rate behavior. It can be speculated that fluo ride loss and uptake during demineraliza tion and remineralization in acidic solu tion may have a significant effect on the properties of fluoridated apatites.…”

Mechanisms of Acid Dissolution of Biological and Synthetic Apatite Crystals at the Lattice Pattern Level

“…Additional evidence of fluoride action as a stabilizing factor [Driessens, 1973;Bugg, 1974] and as a factor increasing crystallinity [Katz et al, 1970] is obtained from the pres ent high resolution TEM and X-ray dif fraction investigation of human enamel powders. It certainly would be a mistaken notion to assert that either fluorapatite or hydroxyapatite are present in crystallites.…”

“…Other factors are known to play a part in dissolution phe nomena, such as the chemical composition and the crystallinity of the crystals, which are more or less certainly related. It has thus been established that incorporation of fluoride ions to the lattice acts as a stabiliz ing factor [Driessens, 1973;Bugg, 1974] and increases its crystallinity [Katz el al., 1970], In the same way, it is known that Mg stabilizes some amorphous phases [Trautz and LeGeros, 19621 and has an opposite ef fect to fluoride [Tomazic et al, 1976].…”

In vitro Fluoride Uptake in Human Dental Enamel from Various Fluoride Solutions

“…An increase in fluoride content of dental enamel as hydroxyfluorideapatite theoreti cally causes a significant decrease of the sol ubility of the enamel [Driessens, 1973] or renders the apatite kinetically less active [Hagen, 1975], Treatment of enamel pow der with solutions containing approximate ly 100 ppm or more of F~ is known to result in formation of calcium fluoride as well as fluorideapatite [Leach, 1959]. The higher level of fluoride in the solutions employed in this study makes formation of calcium fluoride in the enamel likely.…”