Enamel dissolution in relation to fluoride concentrations in the fluid of dental plaque-like layers of precultured Streptococcus sobrinus

Abstract

Two experiments were conducted to find out what concentration of fluoride (F), when added as NaF to the fluid of an in vitro caries model, would be sufficient for full protection of the underlying enamel during a long (20 or 40 h) fall in ‘plaque’ pH. An existing caries model with bovine enamel and Streptococcus sobrinus ‘plaque’ was employed, while the fluid phase was initially either fully or partially saturated with tricalcium phosphate. In the model with fully saturated and more strongly buffered fluid, a 10 parts/10 6 addition of F to the fluid phase caused no inhibition of the pH fall. In the model with partially saturated and weakly buffered fluid, prevention of the pH fall increased along with increase of added F from 2 to 20 parts/10 6. In each model, the 10 parts/10 6 addition of F was associated with complete prevention of the increase of calcium (Ca) in the fluid phase. In the partially saturated, weakly buffered model, findings on the inorganic P of the fluid phase agreed with findings on Ca. The findings for microhardness of the enamel surface indicated a complete prevention of the enamel softening through the 10 parts/10 6 F additive. Enamel F content was elevated by the fermentation, and even more so when F was added with sugar to the fluid phase. The Ca content of Strep. sobrinus cells was reduced when incubated with sucrose for 40 h and, especially, with sucrose and F. Thus in the second experiment, prevention of the pH fall, the saturation factors, the release of bacterial Ca and the increase of enamel F, all caused or promoted by the 10 parts/10 6 F additive, could jointly contribute to the full enamel protection.