Physical and Chemical Considerations of the Role of Firmly and Loosely Bound Fluoride in Caries Prevention

Historically, there has been considerable debate concerning the roles of loosely bound (calcium fluoride) and firmly bound (fluorapatite) fluoride for caries prevention. Research now shows that fluorapatite (FAP) is a finite reaction product of enamel/apatite fluoridation with or without CaF₂ formation, suggesting that CaF₂ always be considered as a supplement to, rather than a substitute for, FAP formation. In the presence of low levels of fluoride in the solution phase, the crystallization of hydroxyapatite is enhanced, while the corresponding dissolution is retarded. Fluoride in the bulk FAP or CaF₂ solid phase, in contrast, has limited impact on crystal growth or dissolution kinetics. Both FAP and CaF₂ can provide F to the solution phase to enhance remineralization and retard demineralization of enamel HAP crystallites. The FAP provides most of this F under low pH conditions, while CaF₂ provides F at neutral or lower pH. The reactivity of fluoride on sound and carious enamel differs significantly. Carious enamel acquires more fluoride, acquires it more quickly, and itself acts as a source of retained fluoride in comparison with the more limited reactivity of sound enamel. Overall, the most important question concerning fluoride reactivity relates to its efficiency in enhancing remineralization or retarding demineralization processes. This is influenced not only by the reaction products, e.g., loosely or firmly bound fluoride, but also by the nature of the enamel substrate and frequency of application of the topical fluoridating agent. Inasmuch as the reactivity of bulk HAP is dominated by surface layers of FAP material, the debate over usefulness of various fluoride reaction products solely on a chemical level is no longer critical. Instead, all factors influencing the efficiency of a fluoridating regimen must be considered in the development of improved systems for caries prevention.