Calculation of Intercrystalline Solution Composition during in Vitro Subsurface Lesion Formation in Dental Minerals † ZEREN WANG*, JEFFREY L. FOX* X ,ARIF A. BAIG*, MAKOTO OTSUKA ‡ , AND WILLIAM I. HIGUCHI* Received December 20, 1994, from the * Department of Pharmaceutics and Pharmaceutical Chemistry, School of Pharmacy, University of Utah, Salt Lake City, UT 84112, and ‡ Kobe Pharmaceutical University, Kobe, Japan. Accepted for publication September 20, 1995 X . Abstract 0 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 inter- crystalline solution composition. Introduction The caries-preventive role of fluoride has been intensively investigated since epidemiological studies 1 clearly revealed a general inverse relationship between the fluoride concentra- tion in public drinking water and caries incidence. Recent studies have shown that solution fluoride (not necessarily fluoride incorporated into a crystalline phase) protects teeth from acid attack, even though the concentration of fluoride in solution is low. 2-8 It has also been shown 7,9-12 that the presence of low solution fluoride concentrations can lead to a subsurface pattern of lesion formation in dental enamel, as opposed to gross cavitation. This phenomenon can be at- tributed to the fluoride concentration gradient in the inter- crystalline solution, and this intercrystalline solution compo- sition gradient can further yield a gradient of the tendency for mineral to dissolve locally in the enamel. The direct validation of this hypothesis requires a technique which can determine the microenvironmental composition in the inter- crystalline solution during the demineralization process. Such a technique has not previously been available. Vogel et al. 13 have developed a microdrill method combined with fluoride microanalysis. However, this technique is not applicable to monitoring the pore solution concentration of fluoride in the microenvironment during demineralization, as a function of position and time. Techniques to determine the composition of the solid mineral as a function of position and time are now readily available. The amount of fluoride associated with the mineral can be determined by an electron microprobe technique 14 and the mineral density can be measured by an X-ray micro- radiography method. 15 Therefore, based on the measured composition of solid mineral as a function of time and position, a model independent data analysis (MIDA) technique, previ- ously established in our laboratory 16,17 can be applied to calculate the intercrystalline solution composition as a func- tion of position and time in the microenvironment during demineralization. This method provides an indirect way to determine the intercrystalline solution composition, which is a prerequisite to a comprehensive understanding of the mechanism of subsurface lesion formation. The purpose of this study was to refine and validate this technique. The technique was then applied to bovine teeth and to compressed disks of carbonated apatite (CAP) in in vitro subsurface lesion formation experiments to determine the pore solution composition during demineralization and to determine the dissolution driving force which controls the dissolution of dental mineral in the presence of a low level of solution fluoride. An Intercrystalline Solution Composition Model for Subsurface Lesion Formation The Physical ModelsThe intercrystalline solution com- position model for subsurface lesion formation was established by Bergstrom 9,10 and Chu. 16 It is schematically illustrated in Figure 1 and is similar to models reported previously. 18,19 The diffusion and surface reaction processes of an additional species, fluoride, are also taken into account. † This paper was evaluated and reviewed through the office of Associ- ated Editor David J. W. Grant. X Abstract published in Advance ACS Abstracts, November 1, 1995. Figure 1sThe scheme of the intercrystalline solution composition model for subsurface lesion formation. © 1996, American Chemical Society and 0022-3549/96/3185-0117$12.00/0 Journal of Pharmaceutical Sciences / 117 American Pharmaceutical Association Vol. 85, No. 1, January 1996