The Role of Fluoride in Caries Reversal
Dental caries is an infectious disease caused by the complex interaction of cariogenic (caries-causing) bacteria with carbohydrates (i.e., sugars) on the tooth surface over time. Cariogenic bacteria metabolize carbohydrates for energy and produce organic acids as byproducts. The acids lower the pH in the plaque biofilm.47
The hydroxyapatite of tooth enamel is primarily composed of phosphate ions (PO43–) and calcium ions (Ca2+). Under normal conditions, there is a stable equilibrium between the calcium and phosphate ions in saliva and the crystalline hydroxyapatite that comprises 96% of tooth enamel. When the pH drops below a critical level (5.5 for enamel, and 6.2 for dentin), it causes the dissolution of tooth mineral (hydroxyapatite) in a process called demineralization. When the natural buffer capacity of saliva elevates pH, minerals are reincorporated into the tooth through the process of remineralization.47
The initial stage of the caries process results in white spot formation, a result of acid penetration and solubilization of some (but not all) of the subsurface mineral (Figure 11A). Left untreated, this subsurface damage can progress to a point where the crystal can no longer provide sufficient support to the enamel surface structure, and the surface collapses (cavitates).
The caries process can be affected in several ways. One of the most effective methods to prevent caries is by promoting remineralization and slowing down demineralization. This can be accomplished with fluoride therapy. It is widely accepted that the regular use of fluoride, such as in dentifrice and drinking water, is extremely effective at preventing dental caries. In 1999, the US Center for Disease Control (CDC) issued a statement that water fluoridation is one of the 10 most important public health measures of the 20th century.48 Fluoride’s presence in low concentration and high frequency is more effective at preventing caries than high levels of fluoride used in low frequency. Because water fluoridation is not available in many countries, dentifrice is considered to be one of the most important sources of fluoride globally.
When fluoride is present in oral fluids (i.e., saliva), fluorapatite, rather than hydroxyapatite, forms during the remineralization process. Fluoride ions (F–) replace hydroxyl groups (OH–) in the formation of the apatite crystal lattice (Figure 11B), resulting in a stronger, fluoridated tooth mineral (fluorapatite). Fluorapatite is less soluble than hydroxyapatite, even under acidic conditions. Because fluorapatite is less soluble than hydroxyapatite, it is also more resistant to subsequent demineralization when acid challenged.
Caries is generally considered to be a sub-surface phenomenon. With fluoride treatment, a non-cavitated lesion can be remineralized with fluorapatite and have greater resistance to subsequent demineralization than hydroxyapatite. Even at very low concentrations, fluoride is effective as an anticaries agent.49
In the US, there are three commonly used sources of fluoride in oral care products; sodium fluoride (NaF), sodium monofluorophosphate (SMFP) and stannous fluoride (SnF2). All three of these fluoride sources provide the important F- ion, which both inhibits demineralization and promotes remineralization of damaged tooth mineral. In addition, SnF2 is considered to have unique properties, as it provides efficacy against bacterial acids in addition to its fluoridating benefits.
A) Demineralization – the caries formation process. Damage occurs in subsurface regions of the enamel, leaving an intact outer layer on the enamel surface. B) Remineralization – the caries reversal process. Caries is reversed through the process of remineralization, in which calcium, phosphate and fluoride are incorporated in the areas damaged due to demineralization processes, resulting in a stronger, fluoridated mineral.