E5 Chemical Balance

Topic

The chemical balance of the oral environment determines whether the tooth's hard tissues dissolve—a process known as demineralization—or undergo repair—a process known as remineralization.

This chemical balance is determined by comparing two quantities: the actual concentration of calcium, phosphate, and hydroxyl ions dissolved in saliva against the concentration of these same ions that would exist if the saliva were in exact equilibrium with the hydroxyapatite of the dental enamel. When the actual concentration of dissolved ions is lower than this equilibrium concentration, the dental enamel dissolves; when the actual concentration exceeds the equilibrium concentration, mineral precipitation onto the enamel occurs—that is, remineralization.

Fluoride shifts this chemical balance in favor of remineralization: it replaces the hydroxyl ions within the dental enamel's hydroxyapatite crystals to form fluorapatite. Fluorapatite is a crystal with an equilibrium concentration in saliva that is several orders of magnitude lower than that of the original hydroxyapatite. Because this equilibrium concentration is much lower, fluorapatite resists acid dissolution even when acidity levels are two to three pH units lower than those that would dissolve the original hydroxyapatite.

Saliva constantly contributes to this chemical balance by supplying calcium and phosphate to replenish the ions lost by the dental enamel, thereby keeping the actual concentration of dissolved ions as close as possible to—or even above—the equilibrium concentration, favoring remineralization over demineralization. In addition to supplying minerals, saliva regulates the pH of the oral environment—primarily through a buffer system composed of bicarbonate, carbon dioxide, and carbonic acid—by neutralizing acids that would otherwise shift the oral environment's chemical equilibrium toward demineralization.

The speed at which fluoride is incorporated into dental enamel to form fluorapatite, and the effectiveness of that fluoride in preventing dental caries, depend on the concentration of free fluoride available in the saliva: the higher the concentration of free fluoride in the saliva, the faster fluoride is incorporated into the enamel and the greater the anti-caries efficacy achieved. Thus, the comparison between actual ion concentration and the equilibrium concentration with hydroxyapatite, the mineral supply and buffering capacity of saliva, and fluoride availability are the factors that collectively determine the direction in which the chemical equilibrium of the oral environment—described at the beginning—shifts.

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