Carbonated hydroxyapatite

Kokubo et al. [16,17] showed that the hydroxyapatite formation on the surfaces of bioactive materials in the living body can be reproduced even in an acellular protein-free simulated body fluid (SB F) with ion concentrations nearly equal to those of human blood plasma. This indicates that the hydroxyapatite layer is formed through chemical reaction of the bioactive glass with the surrounding body fluids. The formed layer consists of carbonated hydroxyapatite with small crystallites and low crystallinity, which is similar to bone hydroxyapatite. Hence the bioactivity of a material can be evaluated even in vitro by examining the hydroxyapatite formation on its surface in SBF. 

The collagen fibers leave small compartments where apatite nanocrystals are deposited during a controlled biomineralization process [20]. The collagen acts as a structural framework in which plate-like nanocrystals of carbonated hydroxyapatite (CHA) are embedded to strengthen the bone. The chemical formula of biological CHA can be represented as follows ... 

M. Vignoles, G. Bonel, D.W. Holcomb, R.A. Young, Influence of preparation conditions on the composition of type B carbonated hydroxyapatite and on the localization of the carbonate ions, Calcif. Tissue Int. 43 (1988) 33-40. 

The biologically most relevant calcium phosphates are dahllite [carbonate hydroxyapatite, (Na,Ca)10(PO4,CO3)6(OH)2] francolite [carbonate fluoroapatite] hydroxyapatite [Ca10(PO4)6(OH)2]... 

About twenty different skeletal minerals are reported from organisms7,8 however, only four are common (1) aragonite, (2) calcite, (3) dahllite = carbonate hydroxyapatite, and (4) opal. The remaining minerals depicted in Fig. 1 are either trace constituents or occur only in a few isolated species. It is for this reason that the article concentrates on carbonate, phosphate and silica deposition in plants and animals. For reviews on general aspects of biomineralization and discussions on individual taxonomic classes see Ref.9-47 ... 

People at risk of osteoporosis, e.g. elderly housebound persons, must maintain an adequate intake of calcium and vitamin D. Calcium dietary supplementation (Ca gluconate, carbonate, hydroxyapatite, citrate, maleate) reduces nett bone loss where intake may be inadequate, i.e. below 800 mg/d, and ergocaldferol 10 micrograms (400 units) by mouth corrects dietary vitamin D deficiency. 

Minerals of phosphorites are quite simple when compared with those of rock phosphates of magnesium, aluminium and iron. They consist essentially of apatites, whitlockite, monetite and brushite. However, francolite or dahllite (carbonate fluorapatite and carbonate hydroxyapatite) probably comprise more than 99% of the phosphatic constituent of the average phosphorite. 

The hydroxyapatite crystals in bone and teeth are imperfect due to other anions and cations, especially magnesium, chloride, carbonate, and fluoride ions. Carbonate (C032-) is the most important. At low carbonate contents (<4% by weight), a carbonate ion replaces a phosphate ion in the crystal ( A site substitution), but at higher contents (>4% by weight) it replaces a hydroxide ion ( B site substitution). Either substitution slightly shortens and fattens the crystal ( c or a axes increase) and increases solubility. In contrast, if hydroxide ions are present, they can be replaced by fluoride, which decreases apatite solubility (Sect. 16.2.1). Crystallographic analyses indicate that, in bone and dentin, phosphate is often replaced by carbonate, whereas in enamel it is more often replaced with chloride (Cl1-). Carbonated hydroxyapatite is critical for enamel development (see Sect. 9.5.3). 

Its topography reveals the unique structure consisting of aligned prisms or rods with 5pm diameter that extent approximately perpendicular from the dentin-enamel junction towards the tooth surface. Each rod consists of tightly packed carbonated hydroxyapatite crystals with very high aspect ratio. Nano-indentation studies revealed a pronounced anisotropy as the stiffness differs parallel and perpendicular to the rod extension. Even so, different fibre orientation on a micro level as shown in Figure 3.4b account for a quasi-isotropic behaviour. 

---