Influence of polymers for use in saliva substitutes on de- and remineralization of enamel in vitro.

A number of polymers which have previously been tested for their applicability as thickening agents in saliva substitutes were studied in vitro for their caries-protective properties. These were: polyacrylic acid, carboxymethylcellulose, xanthan gum, guar gum, hydroxyethylcellulose and porcine gastric mucin. The polymers were tested for their effects on: (1) growth of hydroxyapatite crystals in a supersaturated calcium phosphate solution, (2) dissolution of hydroxyapatite crystals in 50 mM acetic acid, pH 5.2 and (3) demineralization and remineralization of bovine enamel in a pH-cycling model. Growth of hydroxyapatite crystals was strongly inhibited by polyacrylic acid and carboxymethylcellulose at very low concentrations (0.005% w/v). Other polymers displayed lower inhibition of hydroxyapatite crystal growth. Hydroxyapatite dissolution was inhibited by all polymers except by hydroxymethylcellulose and xanthan gum. This occurred both in the presence of the polymers as well as after a 30-min preincubation. In the pH-cycling experiment, bovine enamel specimens with preformed lesions were alternately exposed to a demineralization buffer and a remineralization buffer containing the polymers hydroxyethylcellulose, carboxymethylcellulose, xanthan gum, polyacrylic acid, or porcine gastric mucin. A remineralization buffer containing 1 ppm NaF was used as a positive control. Under the experimental conditions, the control experiment without additives resulted in a net mineral loss (30.6 mumol Ca/cm2 after 14 days of pH cycling). In the presence of 1 ppm NaF, a small mineral gain was observed (8.6 mumol/cm2). All polymers largely inhibited further demineralization (1.2-12.3 mumol/cm2) except polyacrylic acid which, inhibited of its high calcium-binding capacity, caused demineralization, especially in the remineralization buffer (17.1 mumol/cm2). In conclusion, polymers tested in this study, except the polyacrylic acid, reduced the demineralization of enamel in vitro. The precise mechanism of the protective effect is not clear but it is speculated that formation of an absorbed polymer layer on the hydroxyapatite or enamel surface may provide protection against acidic attacks.