Sian B Jones

Effects of pH and acid concentration on erosive dissolution of enamel, dentine, and compressed hydroxyapatite

The aims of this study were to determine the effects of pH and acid concentration on the dissolution of enamel, dentine, and compressed hydroxyapatite (HA) in citric acid solutions (15.6 and 52.1 mmol l(-1) ; pH 2.45, 3.2, and 3.9), using a pH-stat system. After an initial adjustment period, the dissolution rates of enamel and HA were constant, while that of dentine decreased with time. The dissolution rate increased as the pH decreased, and this was most marked for enamel. To compare substrates, the rate of mineral dissolution was normalized to the area occupied by mineral at the specimen surface. For a given acid concentration, the normalized dissolution rate of HA was always less than that for either dentine or enamel. The dissolution rate for dentine mineral was similar to that for enamel at pH 2.45 and greater at pH 3.2 and pH 3.9. The concentration of acid significantly affected the enamel dissolution rate at pH 2.45 and pH 3.2, but not at pH 3.9, and did not significantly affect the dissolution rates of dentine or HA at any pH. The variation in response of the dissolution rate to acid concentration/buffer capacity with respect to pH and tissue type might complicate attempts to predict erosive potential from solution composition.

Efficacy of desensitizing dentifrices to occlude dentinal tubules.

Dentine hypersensitivity occurs when patent dentinal tubules are subjected to external stimuli, with pain being reduced by products that occlude tubules. This study compared the efficacy of a recently developed arginine-containing dentifrice, two established strontium-based products, and a fluoride control to occlude tubules when subjected to acid challenge. Dentine specimens with patent tubules were divided into four groups that were treated with a slurry consisting of one of the pastes mixed with stimulated human saliva. Treated specimens were further subdivided and soaked in 0.3% citric acid for 10 s, 30 s, 2 min, 5 min or 10 min. Tubule occlusion on representative scanning electron microscopy images was scored by blind review. All three desensitizing pastes offered good tubule occlusion, which was maintained to varying degrees following acidic challenge. After immersion in acid for 10 and 30 s, the strontium acetate- and arginine-containing pastes almost fully occluded tubules, but only the strontium acetate paste retained this level of occlusion after immersion in acid for 2 min, with strong statistical evidence that this paste occluded more tubules than the other pastes after immersion in acid for 2 or 5 min. This suggests that strontium acetate pastes may be the most effective at reducing dentine hypersensitivity.

Inhibition of erosive dissolution by sodium fluoride: Evidence for a dose-response

OBJECTIVES The aim of this study was to investigate the protective effects of sodium fluoride solutions and commercial mouthrinses on hydroxyapatite (HA) dissolution in citric acid in vitro, with and without a salivary pellicle. METHODS A rapid-throughput HA solubility-reduction model was employed in which HA dissolution was quantified using ion chromatography. Two HA substrates were selected, a high-resolution powder and 80 μm diameter beads, and studied in the presence and absence of a salivary pellicle (pooled human saliva, 2 h). Immediately prior to acid exposure, substrates were exposed to one of a number of pre-treatments that included aqueous fluoride (F(-)) solutions and commercially available mouthrinses with F(-) concentrations of 0-450 μg/g (as NaF). Dissolution reduction was calculated relative to a deionised water negative control. RESULTS For aqueous solutions and mouthrinses, a fluoride dose-response was observed with a plateau around 100 μg/g F(-) for both HA substrates, with or without pellicle. Concentrations as low as 10 μg/g F(-) significantly reduced HA dissolution. The HA substrate had little impact on the fluoride dose-response, and the fluoride was equally effective in the presence of a pellicle as in its absence. CONCLUSIONS Fluoride significantly reduced HA dissolution at concentrations of 10 μg/g and higher. A fluoride dose-response was seen at low concentrations. This study illustrates the use of a powerful rapid-throughput HA solubility-reduction model for investigating HA dissolution in citric acid in the presence of dissolution inhibitors. CLINICAL SIGNIFICANCE A single exposure to fluoride solutions with fluoride concentrations and exposure time representative of brushing or rinsing with mainstream oral care products was shown to significantly inhibit HA dissolution under conditions relevant to dental erosion. A similar efficacy was observed in the presence and absence of salivary pellicle.

The Effect of Monoalkyl Phosphates and Fluoride on Dissolution of Hydroxyapatite, and Interactions with Saliva

The aims were to investigate the effect of monoalkyl phosphates (MAPs) and fluoride on dissolution rate of native and saliva-coated hydroxyapatite (HA). Fluoride at 300 mg/l (as NaF) inhibited dissolution of native HA by 12%, while potassium and sodium dodecyl phosphates (PDP, SDP), at 0.1% or higher, inhibited dissolution by 26-34%. MAPs, but not fluoride, also showed persistence of action. MAPs at 0.5% and fluoride at 300 mg/l were then tested separately against HA pre-treated with human saliva for 2 or 18 h. Agents were applied with brushing to half the specimens, and without brushing to the other half. In control (water-treated) specimens, pre-treatment of HA with human saliva reduced dissolution rate on average by 41% (2 h) and 63% (18 h). Brushing did not have a statistically significant effect on dissolution rate of saliva-coated specimens. In brushed specimens, fluoride significantly increased the inhibition due to 2- or 18-hour saliva pre-treatment. It is hypothesised that brushing partially removes the salivary film and allows KOH-soluble calcium fluoride formation at the surfaces of HA particles. Inhibition was reduced by PDP in 2-hour/non-brushed specimens and in 18-hour/brushed specimens. PDP did not affect dissolution rates in the remaining groups and SDP did not affect dissolution rate in any group. Possible reasons for these variable results are discussed. The experiments show that pre-treatment with saliva can significantly modify results of tests on potential anti-erosive agents and it is recommended that saliva pre-treatment should be a routine part of testing such agents.