Hydroxyapatite biological apatite

R.A. Young, Biological apatite vs hydroxyapatite at the atomic level, Clin. Orthop. 113 (1975) 249-262. 

Ideally, hydroxyapatite has the formula mentioned above. The synthetic material usually contains fewer than 10 Ca-ions and more than 2 OH-ions per crystal unit. Important differences in crystal structure, composition and specific surface exist between synthetic and biologic apatite. These differences result from the processing method of the raw materials and the synthetic method used. 

The substitution by other ions reduces the theoretical stoichiometric Ca/P ratio of 1.67 of hydroxyapatite to values for bone-like apatite of less than 1.6 (LeGeros, 1991), even as low as 1.4 (Weng etal., 1994). This non-stoichiometry of biological apatite can be described by the approximate formula (Young and Holcomb, 1982 Hattori and Iwadate, 1990 Liu et al., 2001). 

Hydroxyapatite Ca5(P04)30H is the main mineral constituent of bones and teeth. The chemical composition of biological apatite is somewhat different from the above given formula. They are usually defined as non-stoichiometric apatites. This non- stoichiometry is caused by the presence of different ions in very small or in... 

Mention has already been made of substitution within the crystal lattice. A foreign ion, provided that it is similar in size to the ion which it replaces, may exchange for a normal hydroxyapatite constituent. This process is called heteroionic exchange, whereas the exchange of like ion for like is called isoionic exchange. There are, thus, two ways in which ions other than calcium, phosphate or hydroxyl may become part of the structure of biological apatite. Firstly,... 

Difficulties arise, not so much in explaining why mineralization readily and regularly occurs in certain tissues, but rather why other tissues, which resemble them in many ways, do not normally mineralize. Thus it is relatively easy to explain how crystals of a very sparingly soluble substance such as hydroxyapatite can be formed in bone, on the basis of the concentrations of calcium and phosphate ions present in blood these are sufficiently high to permit small crystals of biological apatite to grow at the expense of ions in solution. It is more difficult to appreciate why, under apparently similar conditions, a tissue such as skin which, like bone, contains... 

It now remains to explain the apparent paradox that the ionic product for calcium and monohydrogen phosphate ions in serum exceeds the solubility product of both hydroxyapatite and biological apatite and yet crystals of these substances are not formed in the bloodstream. Of the several reasons for this, the most basic one can be demonstrated by a simple experiment. 

The other ceranfic widely used are phosphate salts of calcium, with the chosen phase usually being hydroxyapatite. This material is conventionally prepared by thermal methods at temperatures well in excess of 1000°C. As a result of their preparation at high temperatures, the salts are carbonate free and are made up of much larger and more perfect crystals than those found in biological apatite minerals including bone. The imperfect crystalline structure of bone mineral leads to the natural material being soluble and reactive with respect to body fluids. In contrast, the synthetic materials are much less reactive than those found in living tissue and problems with biocompatibility can arise. 

A variation of calcium phosphate is the major component of bones and teeth in all vertebrates including humans. These calcium phosphates are usually referred to collectively as biological apatites, which are nonstoichiometric compounds based on pure apatites, Ca5(P04)3X, where X can be fluorine (F), chlorine (Cl), or hydroxyl (OH). (These are called fluoro-, chloro-, and hydroxyapatite, respectively.) In biological apatites the calcium cations can be replaced with varying amounts of strontium, magnesium, sodium, and potassium ions, and the phosphate anions can be replaced with hydrogen phosphates and carbonates. 

Human teeth are also composed primarily of biological apatite. The outer two layers of a human tooth consist of enamel on the outside and dentine underneath that. Dentine and bones are very similar in composition and mechanical properties, but enamel is almost pure hydroxyapatite, Ca5(P04)30H. Dental enamel is the hardest part of the human body. In addition, the hardness of dental enamel is enhanced by the presence of fluoride ions in place of the hydroxides. (Thus we see... 

Spevak, L. et al (2013) Fourier transform infrared spectroscopic imaging parameters describing acid phosphate substitution in biologic hydroxyapatite. Calcif. Tissue Int, 92 (5), 418 -428. Penel, G. et al. (1998) MicroRaman spectral study of the P04 and C03 vibrational modes in synthetic and biological apatites. Calcif. Tissue Int, 63 (6), 475 -481. 

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 ... 

Interest in the nature of interactions between shortchain organic surfactants and large molecular weight macromolecules and ions with hydroxyapatite extends to several fields. In the area of carles prevention and control, surfactant adsorption plays an important role in the Initial states of plaque formation (1-5) and in the adhesion of tooth restorative materials ( ). Interaction of hydroxyapatite with polypeptides in human urine is important in human biology as hydroxyapatite has been found as a major or minor component in a majority of kidney stones ( 7). Hydroxyapatite is used in column chromatography as a material for separating proteins (8-9). The flotation separation of apatite from... 

R.Z. Legeros, Biological and synthetic apatites, in P.W. Brown, B. Constantz (Eds.), Hydroxyapatite and Related Materials, CRC Press, Boca Raton, 1994, pp. 3-28. 

Nevertheless, fluoride does lead to a reduction in the solubility of hydroxyapatite in aqueous solution, even in the absence of trace levels of fluoride in solution, and hence can be seen to have an effect in the solid state as well [57], Apatites are complex and diverse materials which have the general formula Caio(P04)eX2 (X = F, Cl, OH) and they represent a crystallographic system, in which there can be considerable replacement of species. Thus, with little or no change in the dimensions of the crystal lattice, there can be exchanges of OH for F, Ca + for Sr +, and PO4 for CO and all of these are known to occur in biological systems. Natural hydroxyapatite, for example, is often partially carbonate substituted [58]. 

LeGeros RZ (1994) Biological and synthetic apatites. In Brown PW, Constanz B (eds) Hydroxyapatite and related materials. CRC Press, Boca Raton, pp 3-28 Kaflak-Hachulska A, Samoson A, Kolodziejski W (2003) H MAS and H—> P CP/MAS NMR study of human bone mineral. Calcif Tissue Int 73 476-486... 

The first and primary protective effect of fluoride is due to its strong, spontaneous reaction with metal ions. Biologically, the most important of these ions is the calcium ion, large amounts of which interact with phosphate to form bones and teeth. Studies show that fluoride reduces apatite solubility in acids by an isomorphic replacement of hydroxide ions with fluoride ions to form fluoro-hydroxyapatite and difluoro-apatite (Fig. 16.6a). 

---