Fluoride effect on demineralization of dental enamel and root dentine

Objectives: Fluoride dentifrice is effective to control enamel caries, but considering cervical caries it should be also effective to simultaneously control root dentin caries. Since there are limited studies comparing the fluoride effect on these dental substrates, this study evaluated the effect of fluoride on enamel and root dentin demineralization during the validation of a cariogenic biofilm model for this purpose. Methods: Biofilms of S.mutans UA159 were grown on bovine enamel and root dentine saliva-coated slabs (n=12) immersed in culture media (one slab of each substrate in the same plate well). Eight times a day the biofilms were subjected to 10% sucrose and 2x/day they were treated with one of the following solutions:  purified water (control) and fluoridated solutions at 150, 450 or 1350 ppm F, simulating brushing with dentifrice from 600 to 5000 ppm F.  After 96 h, demineralization and fluoride uptake were assessed on slabs surface. Demineralization was determined by percentage of surface hardness loss (%SHL) and fluoride formed in slabs was quantified after acid etching extraction. Data were analyzed by two-way ANOVA and regression analysis. Results: The results showed fluoride dose-response effect for both dental substrates, decreasing %SHL (r²=0.63 at p=0.001 and r²=0.82 at p=0.0001, respectively for enamel and dentine). Demineralization was higher on dentine than on enamel (p<0.05); also,  a 3x-higher fluoride concentration (1350 ppm F) was necessary to have on dentine the same reduction of demineralization found for enamel at 450 ppm F. Fluoride uptake for treatments control and 150, 450 e 1350 ppm F, respectively, was greater (p<0.05) in dentine (1105.2±332.6; 3896.5±965.0; 4697.8±1113.9 and 6133.3±1280.8 µg F/g) than in enamel (256.4±65.8; 439.4±200.0; 588.2±223.4 and 760.7±241.9 µg F/g). Conclusions: The model was validated and the findings suggest that higher fluoride concentration is necessary to control caries on dentine than on enamel.