Ultramicro Analysis of the Fluid in Human Enamel During in vitro Caries Attack by Hydrochloric Acid

Abstract

An in vitro experimental caries model is described in which an ultramicro system of analysis was used during simulated decay to study the composition of fluid in micro wells drilled within human enamel. In this experiment, hydrochloric acid was used as the demineralizing agent because the results could be directly compared to recent bench-scale studies using a two-compartment diffusion cell. The two sets of results agree in principle and generally confirm an earlier ‘diffusion-controlled’ caries model. Furthermore, the results are contrary to caries models based on the assumption that the rate-controlling process is release of ions from the enamel crystal surfaces because: (1) The solutions at various points in the lesion remained satured during the attack, although a highly demineralized lesion was used in the experiment, and a high degree of undersaturation was used to drive the demineralization. (2) The Ca/P ratio in the lesion solution increased during the demineralization. (3) The calcium and phosphate concentrations increased during the demineralization above their initial values although there was no concentration gradient for hydrochloric acid in this system. (4) Small membrane potentials developed during the demineralization, indicating that the tooth surface was permselective. (5) The directions of the changes in the concentrations of all the ions in solution could be predicted from the permselectivity using the ‘diffusion-controlled’ model. These results have major implications relative to the design of theoretical physicochemical models employed to elucidate the mechanism of caries.

The Hydroxyapatite Ion Activity Product in Acid Solutions Equilibrated with Human Enamel at 37°C

“…White and Nancollas [1977] and Driessens et al [1986] concluded that at least the majority of enamel has a solubility close to that of H A . Fluid from microwells in enamel sections undergoing demineralization with acid from the natural surface had values of IHA closed to those reported here [Vogel et al, 1987[Vogel et al, . 1988.…”

Metastable Equilibrium Solubility Distribution of Carbonated Apatite as a Function of Solution Composition

“…The success of the present approach to determine the best surface complex governing the CAP dissolution rested heavily upon the accuracy of the ion activity product calculations, and there were concerns related to some of the literature-reported equilibrium constant values ͑K CaAc ϩ , K CaH2 PO 4 ϩ , and K CaHPO 4 o ͒ and the selection of the activity coefficients. A study has been recently completed in our laboratory to establish best values for the equilibrium constants and activity coefficients for these IAP calculations (30).…”

“…The surface complex as envisaged by us is an interfacial entity formed at the surface of the equilibrating domain of CAP due to the rapid exchange of ions at the surface with those in the ambient solution. The substitution of CO 3 2Ϫ by PO 4 3Ϫ could lead to the surface complex assuming the HAP stoichiometry, although the quantitative value of MES would still be driven by the energetics of the underlying crystal domain.…”

“…Such studies have included investigations pertinent to the formation of dental caries (1)(2)(3)(4) and to the dynamics of bone mineral (5)(6)(7)(8)(9). Despite efforts over many decades, as summarized in the following, a comprehensive understanding of solubility behavior of apatites is still lacking, even for wellcharacterized synthetic apatites.…”

Notes on the Dissolution of Human Dental Enamel in Dilute Acid Solutions at High Solid/Solution Ratio

“…Undersaturation with respect to enamel apatite has often been suggest ed to be the rate-controlling factor in the caries process. Vogel et al [1987] observed that the fluid in their micro wells in lesions was saturated with respect to HAp. in agreement with our original supposition.…”