sively because of the likely importance of the incorporated The purpose of this investigation was to assess the applicability carbonate ion in the dental caries process. The carbonate of the metastable equilibrium solubility (MES) concept, previously content of human dental enamel is in the range of 2 to developed in our laboratory, over wide ranges of two independent 4% by weight (1). The presence of carbonate as the major variables, carbonate content and crystallinity, and also to examine impurity anion has been found to decrease the stability of the the influences of these variables on the MES behavior of carbonapatite structure, increase its solubility, and thereby render it ated apatites (CAPs). The CAP samples were prepared by dicalmore susceptible to acid attack (2-7). The effects of carboncium phosphate dihydrate (DCPD) hydrolysis for 48 h in ate substitution in apatites are manifested as changes in the NaHCO 3 -containing media at 95, 70, and 50ะC. This method of lattice parameters, decreases in crystal size, increases in cryspreparation gave CAP samples with varying carbonate contents and crystallinities. A previously developed technique with slight tal strain and disorder, and changes in crystal morphologies modification was used to determine the MES distribution for each (2,5,6,(8)(9)(10)(11)(12). The properties of carbonated apatites are of the CAP samples. The equilibration solutions were prepared also influenced by the conditions under which the synthesis both with and without extraction of fluoride with hydroxyapatite is carried out. Numerous studies (5, 6, 10) have shown that (HAP). From X-ray diffraction, the full width at half-maximum carbonated apatites synthesized at different temperatures and (FWHM) of the 002 reflection was used as a measure of crystallin-pH exhibit different physicochemical properties, such as deity. The findings of this study showed that each of these preparagree of crystallinity and dissolution characteristics. tions possessed an MES distribution and therefore provided fur-The variability in the properties of CAP due to either ther support that the MES distribution is a common phenomenon the incorporation of impurity ions during synthesis or the describing the dissolution behavior of CAPs, regardless of their conditions of synthesis itself may be related to the imperfeccarbonate content and crystallinities. The crystallinities of the CAPs decreased and the MES values increased with increasing tions in the crystalline structure of the material. These impercarbonate content and decreasing temperature of synthesis. A plot fections may result in a metastable substance with variable of the mean MES value against CAP crystallinity revealed that solubility properties. Recent studies by Hsu et al. (13) have the mean MES was a single-valued function of crystallinity; i.e., shown that CAPs synthesized by the hydrolysis of anhydrous when crystallinity was taken into account, there was no additional dicalcium phosphate (DCP) and human dental enamel exeffect of carbonate on the MES. When fluoride was not extracted hibit metastable equilibrium solubility (MES) behavior. Alfrom the equilibration solutions, the MES shifted to lower values. though the MES phenomenon had previously been noted by The concept of MES distribution and its dependence on the crys-Brown (14) in dissolution studies involving human dental tallinity of CAP may provide insight into the mechanism of dissoenamel, there had been no attempts until recently (13) to lution of biological apatites, which may be considered imperfect quantitatively describe the MES concept. The results of the crystalline substances.