HEAT-TREATMENT-INDUCED REDUCTION IN THE APPARENT SOLUBILITY OF HUMAN DENTAL ENAMEL

Holcomb and Young (1980) have shown a significant increase in human de ntal enamel (HE) structural order resulting from heat treatment in the temperature range of from 275 to 400 degrees C. Also, previous work i n our laboratory had shown dramatic decreases in the initial dissoluti on rates (IDRs) of both carbonated apatite (CAP) heated at similar tem peratures (from 300 to 500 degrees C) and HE exposed to CO2 laser irra diation for which calculated surface temperatures were in this same ra nge. We hypothesize that thermal treatment shifts the apparent solubil ity distribution profile of HE toward lower apparent solubilities, par alleling the observed increase in crystal structural order and the dec rease in IDRs. Powdered HE was heated in a furnace at temperatures ran ging from 150 to 500 degrees C for 24 hours. The apparent solubility d istributions of both heated and unheated HE powders were measured by e quilibration for 24 hours in a series of partially saturated solutions simulating various amounts of HE dissolved in a pH 4.5 dissolution me dium. The apparent solubility distribution for the unheated HE showed a peak at K-HAP [the ion activity product based on the Ca-10(PO4)(6)(O H)(2) stoichiometry] of 10(-121.0). Heat treatment shifted the apparen t solubility distribution to lower solubilities: The peak K-HAP values were similar to 10(-124.8) at 200 degrees C; similar to 10(-127.8) at 300 degrees C; and similar to 10(-129.1) from 400 to 500 degrees C. T his similar to 8 orders of magnitude decrease in K-HAP for HE heated a t from 400 to 500 degrees C correlates with the previously observed re duction in the IDR driving force for laser-treated HE experiencing a s imilar surface temperature. Furthermore, this decrease in the apparent solubility coincides with the dramatic increase in HE structural orde r occurring in this temperature range, as observed by Holcomb and Youn g.