Hydroxyapatite crystal structure

Brown, W.E. (1962) Octacalcium phosphate and hydroxyapatite crystal structure of... 

FIGURE 13.5 Schematic of hydroxyapatite crystal structure (a) hexagonal (b) monoclinic. 

Degradation and complete removal of the organic component of bone is accompanied by mass loss of about 20-29% [9, 28-30, 32], emission of carbon dioxide (identified by mass spectrometry coupled with thermogravimetric analysis [7, 31]), and the release of water, associated in some studies to the bone organic phase decomposition [7] or loss of water from the hydroxyapatite crystal structure [30, 43]. Mass loss varies depending on the place of sampling [19,43], tissue type (cortical or cancellous) [30] or tissue health state [31,43]. 

Analcite (NajOAljOj SiO I O), a cubic crystal structure, is formed at high temperatures. It is similar to acmite and also invariably is found beneath sludges of hydroxyapatite or serpentine or under porous deposits of iron oxides. 

By far the most abundant phosphate mineral is apatite, which accounts for more than 95% of all P in the Earth s crust. The basic composition of apatite is listed in Table 14-2. Apatite exhibits a hexagonal crystal structure with long open channels parallel to the c-axis. In its pure form, F , OH , or Cl occupies sites along this axis to form fluorapatite, hydroxyapatite, or chlor-apatite, respectively. However, because of the "open" nature of the apatite crystal lattice, many minor substitutions are possible and "pure" forms of apatite as depicted by the general formula in Table 14-2 are rarely found. 

In the presence of fluoride, calcium ions have been found to be more firmly anchored than in pure hydroxyapatite [67]. This enhances the overall resistance to dissolution. Thus, the presence of a thin stable film of fluorapatite on the surface of hydroxyapatite crystals has two effects, namely (i) resistance to diffusion and dissolution of the anion and (ii) firmer binding of calcium ions into the surface. Both of these make the resulting apatite structure more resistant to dissolution, regardless of the pH of the external medium, and they thereby increase the resistance of the mineral phase to the onset of caries. 

Hydroxyapatite (with some carbonate inclusions) is the most stable of the possible calcium phosphate salts that can be formed under physiological conditions. However, it is not the most rapid one to form. Instead, octacalcium phosphate (OCP) will precipitate more readily than hydroxyapatite. This led Brown in 1987 to propose that, as the kinetically favoured compound, OCP precipitates first, and then undergoes irreversible hydrolysis to a transition product OCP hydrolyzate [68]. This hypothesis is consistent with the observation that enamel comprises hydroxyapatite crystals that have the long, plate-like morphology that is generally considered characteristic of OCP crystals [69]. Overall, it seems that enamel crystals, with their elongated form, result from early precipitation of OCP, which forms a template on which hydroxyapatite units grow epitaxially [70,71]. This leads to enamel mineralisation with the observed thin, ribbon-like structure of crystals. 

More recently, it has been shown that topical fluoride preparations do not lead to fluoridation of the hydroxyapatite crystal [181]. Rather they form a calcium fluoride-like substance that is deposited onto the tooth surface and dissolves when the local pH is lowered [182]. The resulting dissolution adjacent to the tooth surface provides a source of soluble fluoride that can be incorporated into the mineral structure, and thus augment remineralisation. 

The apatite family of minerals is a common feature to many of the minerals shown in Table 3. In nature, the apatite mineral structure conforms to the 6/m class of minerals with hexagonal crystal structure and the generic formula Me5(X04)3Z where Me is Ca, Sr, Ba, Cd, and Pb (typically), X = P, As, V, Mn, and Cr and Z = OH, F, Cl, and Br. In addition to caibon-ate apatite, chloroapatite, chloropyromorphite, fluoroaptite, fluoropyromorphite, hydroxyapatite, and hydroxypyromorphite, the family includes abukumalite ((Ca,Th,Ce)5(P04, Si04)3(0H,F)),... 

Hydroxyapatite (HAP), with basically the same crystal structure as Ca-deficient, carbonate-containing hydroxyapatite, is compatible with and reactive in a live human body. However, sintered HAP prepared by treating fine HAP particles under elevated temperature and pressure has insufficient mechanical properties, in particular fracture toughness, which greatly limits its commercial applicability. It is rarely implanted alone. On the other hand, zirconia, particularly partially stabilized zirconia (PSZ),... 

Selvig, K. A. The crystal structure of hydroxyapatite in dental enamel as seen with the electron microscope. J. Ultrastructural Res. 41, 369 (1972)... 

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. 

It was stated that hydrated calcium monohydrogen phosphate in amorphous or cryptocrystalline form is a potential precursor in the formation of hydroxyapatite because the structural position of Ca2+ on (010) and (110) crystal planes of both minerals essentially correspond to one another492. These planes of calcium ions could easily serve as transition boundaries with little distortion of crystal structure the same holds true for octacalcium phosphate or defect apatites. Thus apatite may form from amorphous or microcrystalline calcium monohydrogen phosphate possible via octacalcium phosphate or defect apatites. This process may already start inside the matrix vesicles and continue during extravesicular activities. 

The crystal structures of lead hydroxyapatites at different degrees of cadmium substitution for lead (4.9, 10.0, 22.4 and 29.0% Cd atoms) have been investigated by X-ray powder pattern fitting and P MAS NMR. The different environments of the phosphate group demonstrated by P NMR spectroscopic data have been discussed in relation to structural data. 

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. 

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. 

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