Fluoride Release From Biofilm Reservoirs Formed by a Calcium Pretreatment

Objectives: Using calcium before a fluoride treatment has been shown to increase fluoride retention in dental biofilms. However, the kinetics of release of these fluoride reservoirs to interfere in the caries process has not been studied under standardized conditions.
Methods: In three independent experiments (n=6, n=6, n=3), S. mutans biofilms were formed on bovine enamel slabs and cycled through feast (tryptic soy broth containing (TSB) 1% sucrose, 8 h/day) and famine (TSB containing 0.1 mM glucose solution, 16 h/day) media. Between each feast/famine periods, the biofilms were treated with two sequential treatments, one minute each: G1: 150 mM NaCl→0.9% NaCl (negative control, no fluoride); G2: 150 mM NaCl→0.05% NaF (226 ppm F); G3: 150 mM NaCl→0.5% NaF (2,260 ppm F); G4: 150 mM CaCl2→0.05% NaF (226 ppm F). Media collected after each period was analyzed for pH and fluoride concentration (ion-selective electrodes). After 4 days, the biofilm was collected for determination of fluoride concentration. Data was analyzed by nested ANOVA (α=5%).
Results: G4 significantly increased fluoride concentration in the biofilm (66.6±12.0 µg/g) when compared with the other groups (G1:6.1±3.1, G2: 9.3±5.1, G3:13.4±12.7 μg/g), which were not different from each other. Upon pH drops in the feast periods (minimum pH ranging from 4.4 to 4.7, no difference among groups), significantly more fluoride was released into the feast media for G4 (3.1±0.2 µg) when compared with G2 (1.7±0.1 µg), even though they were treated with the same fluoride concentration.
Conclusions: The results indicate that fluoride reservoirs formed in the biofilm by a calcium pre-treatment are released during acid challenges.