Remineralization Of Synthetic Hydroxyapatite Powders In Fluoride Solutions (demineralization, F-treatment).

Abstract

The purpose of this study was to develop a base of understanding regarding the remineralization behavior of dental enamel in acidic fluoride solutions. The approach was to investigate the reminerali- zation behavior of synthetic hydroxyapatite powders under similar experimental conditions and to relate these findings to those obtained with bovine dental enamel. Crystal suspension experiments were designed to investigate the influences of varying the hydroxyapatite (HAP) seed prepara- tions and seed concentrations (slurry densities) upon the extent of remineralization and the nature of depositing phase (chemical stoi- chiometry and transmission electron microscopy). These studies were conducted at pkFAP = 108 (where pkFAP is minus the loga- rithm of the ion activity product, a(,Ca)2+('10) a(,PO(,4))3-('6) a(,OH)-('2)), Ca/P (molar ratio) = 1.0 and fluoride (F) = 10 ppm at pH 4.5 in 0.10 M acetate buffers. These solution conditions were known to give relatively rapid and complete remineralization of artificial lesions in bovine teeth and HAP pellets. It was concluded from these experiments that, at pkFAP = 108, the remineralization behavior was comparable for the different seed preparations studied. The results were consistent with fluorapatite (FAP) deposition upon the seeds being the predominant process and all crystal dimensions were observed to grow substan- tially. In a second set of studies the kinetics of HAP powder reminer- alization provided data for developing a quantitative understanding of the tooth remineralization behavior. These data directly related the crystal suspension remineralization behavior with changes in the tooth lesion chemistry and mineral densities with time, and deductions were made regarding possible ion activity product gradients within the lesion and how these may change with time. The principal outcomes were that the HAP powder remineralization kinetics data can explain or can help explain the essentially com- plete remineralization of bovine tooth when pkFAP = 108 and the phenomenon of simultaneous demineralization/remineralization which takes place in the bovine tooth when pkFAP > 112. Finally, the influence of the presence of carbonate in apatite on the crys- tal dissolution behavior was assessed. The dissolution rate of carbonate containing HAP (C-HAP) in the presence of fluoride was controlled by an apparent solubility corresponding to an ion activity product that is 4 to 5 orders of magnitude greater than that for HAP.