Calculation of Intercrystalline Solution Composition during in Vitro Subsurface Lesion Formation in Dental Minerals†

Abstract

Applications of a novel technique to calculate intercrystalline solution composition during enamel demineralization are presented. Bovine tooth enamel blocks and carbonated apatite (CAP) compressed disks were demineralized in an in vitro subsurface lesion system. The demineralization medium was a 0.1 M acetate buffer at pH 4.5, containing calcium, phosphate, and fluoride (0.5 ppm). Mineral samples were demineralized for various times, and fluoride profiles and mineral density profiles of these samples were determined by electron microprobe and X-ray microradiography, respectively. A model independent data analysis (MIDA) technique uses these data along with the differential equations for mass transfer and permits calculation of the local intercrystalline solution composition profiles inside the porous mineral matrix as functions of time and position. The invariance in diffusivity with time as calculated in the analysis was taken as an indicator of the physical reasonableness of the method. Current outcomes suggest that it is the sharp gradient of fluoride concentration in the intercrystalline solution which causes the formation of subsurface lesions. Since the driving force for mineral dissolution is a function of solution composition, a gradient of this driving force is consequently formed. Using a compressed disk of carbonated apatite powder as a model for block enamel excluded the possibility of the existence of a gradient of mineral composition which could also cause a gradient of the driving force for mineral dissolution. An FAP surface complex hypothesis is consistent with the current view that fluoride in the intercrystalline solution has a stronger inhibition effect on the dissolution of mineral than does fluoride in the mineral phase. With the help of the MIDA technique, calculated results indicate that the mechanism of the formation of subsurface lesions is dynamically controlled by the intercrystalline solution composition.

Directional/acidic dissolution kinetics of (OH,F,Cl)‐bearing apatite

“…Dissolution studies of apatite in water and acidic media have been focused on the solubility products,5 reaction kinetics and mechanisms,6–8 effects of impurities on the kinetics,9 the formation of a self‐inhibition calcium‐rich layer,10 and the dissolution rate at various pH 5, 11–13…”

Metastable Equilibrium Solubility Behavior of Carbonated Apatites in the Presence of Solution Fluoride

“…These investigators proposed that an FAP surface complex, formed via an exchange of F − for OH − ions on the crystal surface, may have accounted for the dissolution retardation. The more recent studies of Wang et al (27) have provided further support for this hypothesis.…”

Scanning microradiographic study on the influence of diffusion in the external liquid on the rate of demineralization in hydroxyapatite aggregates

“…(25) and W ang et al . (26) who regarded HAP as diffusing as a molecular species without ionization in mathematical models of the dissolution of enamel.…”