Calcium hydroxyapatite crystalline structure

Vega, E. D., Pedregosa, J. C. Narda, G. E. 1999. Interaction of oxyvanadium (IV) with crystalline calcium hydroxyapatite surface mechanism with no structural modification. Journal of Physics and Chemistry of Solids, 60, 759-766. 

Bone Toxicity. In addition to its effect on calcium absorption, excess absorbed strontium adversely affects bone development in several ways, leading to the development of rickets in children and young animals. Strontium binds directly to hydroxyapatite crystals, which may interfere with the normal crystalline structure of bone (Storey 1961). In addition, excess strontium may prevent the normal maturation of chondrocytes in the epiphyseal plates of long bones (Matsumoto 1976). Excess strontium apparently interferes with the mineralization of complexed acidic phospholipids that is thought to help initiate the formation of hydroxyapatite crystals in developing bone (Neufeld and Boskey 1994). As a result, affected bone contains an excess of complexed acidic phospholipid and a significantly lower ash weight. 

In their publication, LeGeros and LeGeros (1993) oudine the different apatites, ranging from natural apatite (minerals) to biological (human dentin, enamel, and bone) and synthetic (chemically synthesized) apatite. This publication clearly shows that apatite is a group of crystalline compounds. The most important of these compounds is calcium hydroxyapatite. All the related crystal structures, such as fluoroapatite, chloroapatite, and carbonate apatite are derived from it. 

The crystalline mineral in bones and teeth is generally regarded as an imperfect calcium hydroxyapatite. Apatite minerals, principally calcium fluorapatite, are both abundant and ubiquitous and are the principal source of phosphate for fertilizers. Their abundance is probably an expression of the very high affinity which calcium and phosphate ions have for each other so that it is perhaps not surprising that, on account of its stability, calcium hydroxyapatite has been selected to play an important part, both structurally and physiologically, in many living things. Ions other than calcium, phosphate and hydroxyl are present in the crystallites in which the atomic ratio of calcium to phosphorus departs considerably from the theoretical value of 1-67 (Table 35.1). 

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. 

The level of calcium in solution will depend upon the presence of precipitating anions, notably phosphate and carbonate. Calcium will precipitate as the phosphate to give hydroxyapatite, Caio(P04)6(OH)2, in bones and teeth, and as the phosphate or carbonate to give other structures, including small crystals, or non-crystalline deposits in cells. Small crystals of calcium carbonate, found in the inner ear of some animals, are responsible for the control of balance. Various calcified tissues result from the precipitation of calcium salts, such as hydroxyapatite in the calcification of the aortic wall, and the oxalate in various stones. 

The approach adopted for the interpretation of the spectra of poorly crystalline calcium phosphates was to take the shell model of the crystalline phase having the greatest chemical similarity and progressively simplify and refine the model while maintaining a good fit to the observed spectrum. For the amorphous calcium phosphates, however, it was found that virtually identical shell models resulted from simplification and refinement of either the hydroxyapatite or brushite shell models to give the structure depicted in Fig. 18. 

Crystalline Phosphate Studies. On the basis of the results with triethyl phosphate, a series of calcium phosphates was examined by infrared spectrophotometry. Pertinent properties of these materials are summarized in Table II, and their spectral characteristics are shown in Table III. None of the synthetic hydroxyapatites [Caio(P04)e(OH)2] had the stoichiometric Ca/P ratio of 1.667, although they showed the apatite lattice structure. A typical infrared transmission spectrum (between 1500 and 700 cm.-1) of a dry powder synthetic hydroxyapatite is shown in Figure 1. 

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