Heat-treatment-induced Reduction in the Apparent Solubility of Human Dental Enamel

Abstract

Holcomb and Young (1980) have shown a significant increase in human dental enamel (HE) structural order resulting from heat treatment in the temperature range of from 275 to 400'C. Also, previous work in our laboratory had shown dramatic decreases in the initial dissolution rates (IDRs) of both carbonated apatite (CAP) heated at similar temperatures (from 300 to 500°C) and HE exposed to CO2 laser irradiation for which calculated surface temperatures were in this same range. We hypothesize that thermal treatment shifts the apparent solubility distribution profile of HE toward lower apparent solubilities, paralleling the observed increase in crystal structural order and the decrease in IDRs. Powdered HE was heated in a furnace at temperatures ranging from 150 to 500'C for 24 hours. The apparent solubility distributions of both heated and unheated HE powders were measured by equilibration for 24 hours in a series of partially saturated solutions simulating various amounts of HE dissolved in a pH 4.5 dissolution medium. The apparent solubility distribution for the unheated HE showed a peak at KHAP [the ion activity product based on the Ca10(PO4)6(OH)2 stoichiometry] of 10-121.0. Heat treatment shifted the apparent solubility distribution to lower solubilities: The peak KHAP values were ∼10-124.8 at 200°C; ∼10-127.8 at 300°C; and ∼10-129.1 from 400 to 500°C. This ∼8 orders of magnitude decrease in KHAP for HE heated at from 400 to 500°C correlates with the previously observed reduction in the IDR driving force for laser-treated HE experiencing a similar surface temperature. Furthermore, this decrease in the apparent solubility coincides with the dramatic increase in HE structural order occurring in this temperature range, as observed by Holcomb and Young.