Formation and structure of Ca-deficient hydroxyapatite

Abstract

When amorphous calcium phosphate (ACP) was transformed to crystalline hydroxyapatite (HA) in a series of aqueous slurry concentrations ranging from low to high, the higher slurry concentrations produced more Ca-deficient HA as measured by Ca/P ratio and heat-produced pyrophosphate. We feel that the excess solution phosphate produced in the higher slurry transformations results in lower Ca/P ratio HA. It has been suggested that an ACP is the precursor to bone apatite. Regulation of the in vivo ACP slurry concentration could then control the stoichiometry and, therefore, the metabolic activity of bone apatite. X-ray radial distribution function (RDF) analyses showed that CO3(2-) substitution in HA creates far greater structural distortions than do Ca deficiencies. The latter, however, do produce small, but observable, structural distortions when compared to stoichiometric HA. It now seems clear that the RDF of bone apatite can be modeled by a synthetic, Ca-deficient, CO3(2-)-containing HA.

Failure to detect an amorphous calcium-phosphate solid phase in bone mineral: A radial distribution function study

“…However, this suggestion by Blumenthal et al [34,35] is quite a different proposal from the ACP theory as originally proposed, which postulated and reported that a stable ACP phase exists in bone mineral and that this phase is the major solid phase present in young bone, constituting up to 79% and more of the total calcium-phosphate mineral phase. Projecting from the ACP theory to the initial bone mineral deposited, one would predict that virtually 100% of the mineral phase would be in the form of ACE Gay [36] has suggested that the noncrystalline mineral she observed in electron micrographs of frozen sections of rapidly mineralizing bone is ACE and that it represents the first phase deposited during the mineralization.…”

“…Although there does not appear to be any direct evidence for the existence of an ACP solid phase in bone, it is possible that a small amount of ACP does occur as a transient intermediate in the formation of bone mineral [34,35]. However, this suggestion by Blumenthal et al [34,35] is quite a different proposal from the ACP theory as originally proposed, which postulated and reported that a stable ACP phase exists in bone mineral and that this phase is the major solid phase present in young bone, constituting up to 79% and more of the total calcium-phosphate mineral phase.…”

Hydroxylapatite nanoparticles: fabrication methods and medical applications

“…The aqueous precipitation process yields calciumdeficient HAp (CDHA) via intermediate phases (precursors): amorphous calcium phosphate (ACP) is formed over a broad range of precipitation conditions [85][86][87][88][89][90][91][92][93][94][95] and octacalcium phosphate (OCP) was formed in some cases [46,85,96,97]. Alternatively CDHA may precipitate without going through an ACP precursor [98].…”

An intermediate state in hydrolysis of amorphous calcium phosphate

“…The dependence of this conversion process on the solution environment has been extensively studied [12,13]. In all the latter studies [10][11][12][13], the conversion of ACP is considered as a single process without an intermediary. The present work, therefore, was undertaken to study the nature of the hydrolysis processes in previously prepared ACE with emphasis on the formation and effects of an OCP-like intermediary.…”