EFFECT OF CARBONATE CONTENT AND CRYSTALLINITY ON THE METASTABLE EQUILIBRIUM SOLUBILITY BEHAVIOR OF CARBONATED APATITES

The purpose of this investigation was to assess the applicability of t he metastable equilibrium solubility (MES) concept, previously develop ed in our laboratory, over wide ranges of two independent variables, c arbonate content and crystallinity, and also to examine the influences of these variables on the MES behavior of carbonated apatites (CAPs). The CAP samples were prepared by dicalcium phosphate dihydrate (DCPD) hydrolysis for 48 h in NaHCO3-containing media at 95, 70, and 50 degr ees C. This method of preparation gave CAP samples with varying carbon ate contents and crystallinities. A previously developed technique wit h slight modification was used to determine the MES distribution for e ach of the CAP samples. The equilibration solutions were prepared both with and without extraction of fluoride with hydroxyapatite (HAP). Fr om X-ray diffraction, the full width at half-maximum (FWHM) of the 002 reflection was used as a measure of crystallinity. The findings of th is study showed that each of these preparations possessed an MES distr ibution and therefore provided further support that the MES distributi on is a common phenomenon describing the dissolution behavior of CAPs, regardless of their carbonate content and crystallinities. The crysta llinities of the CAPs decreased and the MES values increased with incr easing carbonate content and decreasing temperature of synthesis. A pl ot of the mean MES value against CAP crystallinity revealed that the m ean MES was a single-valued function of crystallinity; i.e., when crys tallinity was taken into account, there was no additional effect of ca rbonate on the MES. When fluoride was not extracted from the equilibra tion solutions, the MES shifted to lower values. The concept of MES di stribution and its dependence on the crystallinity of CAP may provide insight into the mechanism of dissolution of biological apatites, whic h may be considered imperfect crystalline substances. (C) 1996 Academi c Press, Inc.