The effect on sound human enamel of exposure to calcifying fluids in vitro.

Abstract

Previous work showed that after exposure of areas of human enamel to calcifying fluids <i>in vitro</i> for periods of up to 24 weeks, experimental areas were more resistant to artificial caries than were untreated control areas. In the present experiments, teeth were exposed to calcifying fluids for shorter periods. 30 erupted and 30 unerupted caries-free human permanent teeth were slit in half longitudinally and the cut face on each varnished. One half was randomly selected as experimental and exposed to a calcifying fluid for periods from 1 to 10 days. The controls were exposed to de-ionised water, both fluids being renewed every 24 h. Calcifying fluids were prepared from synthetic hydroxyapatite (Ca/P 1.63) having calcium concentrations of either 1.0, 1.5 or 3.0 mM/l with phosphate at 1.8 or 2.9 mM/l. All calcifying fluids contained 200 mM sodium chloride/l and were used at pH 7.4. In some experiments, fluoride at a level of either 0.05 or 0.50 mM/l was added. After experiment, sections through experimental and control areas were examined with polarised light using an Ehringhaus compensator after imbibition with Thoulet’s medium (n=1.62). It was possible to increase the negative intrinsic birefringence of erupted enamel, to a depth of 20 μm from the enamel surface, by an average of 13.2% relative to control areas. With unerupted enamel, the corresponding average increase in the surface tissue was 18.3%. In addition, an average of 9.8% increase was found at a depth of 100 μm into the enamel on unerupted teeth. The presence of fluoride greatly enhanced the mineralising ability of the calcifying fluid. Ultra-thin sections cut perpendicular to the tooth surface revealed a firmly adherent precipitate on the enamel surface and within small wedge-shaped defects. Apart from changes within the enamel, this surface precipitate may be sufficient to retard caries formation by providing a reserve of calcium phosphate and by acting as a diffusion barrier.