Total plaque fluoride is in the range 5–10 mg/kg (ppm) on a wet-weight basis. The variability of literature data on plaque fluoride is partly ascribed to analytical problems, many assays being close to or below the concentration detection limit of the fluoride electrode. A change in classification of plaque fluoride compartments is necessary, since recent work indicates that there are two pools of plaque F: <5% of the total F is in plaque fluid as the free ion, and the large remaining portion of total plaque F is designated as bound F, with the total F being >95% extractable by cold 0.5 mol/L perchloric acid.
Sources of plaque fluoride include the diet, saliva, and crevicular fluid; enamel is unlikely to be a regular source for plaque F unless it is either coated daily with labile fluoride compounds, such as calcium fluoride, or released by demineralization.
The location and nature of plaque bound F are not established, but the present evidence is consistent with an intracellular location. Bound F may be released by acids produced in plaque during sugar fermentation, but it is unlikely to reach ion concentrations high enough for sufficient time periods to exert significant inhibition of plaque acidogenesis. Epidemiological evidence showing correlations between pooled plaque F concentrations and caries prevalence in the plaque donors does not exclude the possibility of coincidental effects of water F on both caries and plaque F concentrations.
The action of F in reducing caries has been considered to depend on a multiplicity of effects on enamel and dental plaque, but the range of F concentrations used for such effects in dental plaque spans five orders of magnitude. Of these, only F effects in enhancing enamel remineralization and reducing the solubility rate for enamel mineral appear likely to be of practical importance. Cationic fluoride preparations containing metal ions can reduce plaque formation in the short term, but the effect is manifested mainly by the cation rather than by the F. However, progress has been made in formulation of rinse solutions which can raise plaque fluoride levels (as well as calcium and phosphate) long enough to reduce plaque acidogenicity and its potential to demineralize enamel during fermentation.
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