Topical fluoride is used to prevent and treat dental caries. Acidulated phosphate fluoride, or APF, preparations, which have been accepted by the American Dental Association since 1968, contain 1.23 percent fluoride ion (12,300 parts per million, or ppm). The fluoride ion is derived from sodium fluoride and hydrogen fluoride and is acidified by the addition of 0.1 molar phosphoric acid. The resulting pH is between 3 and 4. Some APF formulations are thixotropic, which means that they liquefy when the tray is pushed into place on the teeth and return to gel form once the tray is in place
APF gels and foams have replaced solutions. The maximum recommended dose is 5 milliliters per tray. APF preparations should be applied for four minutes; the rationale behind this treatment time is that it was used in all the clinical studies that demonstrated an anti-caries effect associated with APF. The advantage of foam is that less fluoride is applied to the tray, thus decreasing the amount of fluoride that is swallowed. A study by Whitford and colleagues8 concluded that APF foam and gel are equivalent with respect to fluoride uptake; and that only one-fifth the volume of foam is required to adequately cover the teeth.
Acute oral exposure to sodium fluoride causes hypocalcemia (causing tetany and ventricular fibrillation), hyperkalemia and gastrointestinal disturbance. Fluorosis is the result of chronic exposure.9
MECHANISM OF ACTION
The mechanism of action of fluoride that results in the reduction of dental caries is not completely understood. There is evidence that fluoride inhibits the demineralization of enamel, enhances remineralization and inhibits bacterial enzymes. The bacteria that cause caries produce acid through the fermentation of carbohydrates, which can dissolve the carbonated hydroxyapatite mineral of teeth, a process called “demineralization.” Incorporation of fluoride into apatite crystals reduces the dissolution of apatite during acid attacks.
Fluoride concentrated in plaque and saliva interrupts the caries process by inhibiting the demineralization and enhancing the remineralization of enamel. Caries result from the bacterial metabolism of carbohydrates, which produce acids that solubilize the carbonated hydroxyapatite of teeth. When fluoride is present in sufficient concentrations, it adsorbs to the crystal surface, attracting calcium and phosphate ions to form a remineralized acid-resistant fluorapatite structure.1,2 The fluoride incorporated into teeth during development is not thought to be a major factor in preventing caries; rather, it is the daily presence of fluoride in plaque and saliva that plays a significant role.2 Therefore, regular exposure to fluoride in drinking water, toothpastes and topical preparations is important for caries prevention.
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