A new two-site model for hydroxyapatite dissolution in acidic media

Abstract

A physical model featuring two distinct types of dissolution sites for hydroxyapatite (HAP) crystals is presented. One of these sites (site No. 1) is associated with dissolution along c axis dislocations of HAP crystals and is the important site when dissolution occurs into partially saturated solutions. Site No. 2 is active only when dissolution occurs into nearly completely unsaturated solutions, and, because of its greater apparent rate constant, is the more important dissolution site under these conditions. In the physical model block dental enamel or a compressed HAP pellet is represented as a porous matrix of these HAP crystals with interstitial spaces which are permeated by the dissolution medium. The behavior of the model for various situations can be calculated by combining Fick's second law equation for diffusion with the kinetic equations for the behavior of the dissolution sites and solving the resulting boundary value problem. This model is capable of accounting for dissolution kinetics over a range of variation of experimental variables (degree of undersaturation, Ca P ratio, and effective diffusion layer thickness). The model also correctly predicts the conditions necessary for zonal as opposed to surface dissolution: high partial saturation, a viscous dissolution medium, or the presence of a site No. 2 dissolution inhibitor. Electron microscopic studies of dissolution morphology at the single crystal level are also in full agreement with the model. Finally, an examination of the available data on enamel remineralization suggests that the holes formed as a result of site No. 1 dissolution are likely the primary sites for remineralization.