Hydroxyapatite chemical composition

There are a number of questions that must be answered ia order to maximize the beaefits of hydroxyapatite, eg, how thick the coatiag should be how should the coatiag be appHed to the implant is the chemical composition oa the implant the same as before the appHcatioa what is the nature of the titanium or titanium aHoy hydroxyapatite interface, how strong is it, and by what means is it bonded and what is the interfacial microstmcture ... 

Fig. 6.1 Diversity of hydroxyapatite focusing on chemical composition. Hydroxyapatite is an adaptable crystalline structure, so substitution of other elements and vacancies are frequently observed.

The inorganic phase of bones or teeth is mainly hydroxyapatite (HA), and deviation in Ca/P ratio from common HA (Ca/P = 1.667) is explained by the presence of amorphous phosphates3). The biogenetic HA resembles in size crystals of HA prepared by precipitation from aqueous solutions. The chemical composition of biominerals is similar to HA. However, crystals in bone, dentine and enamel can vary... 

Hydroxyapatite (HAP), the molecular formula of which is Ca5(P04)3(0H) or Caio(P04)6(OH)2, is the major inorganic constituent in bone, teeth, etc. in the human body. HAP has essentially the same chemical composition and crystalline structure as those of human bone and so has good bio-compatibility. For a long time, it has been widely used as a sclerotin material in setting broken bone, filling teeth, etc. [215]. In addition, HAP can also be used as a food additive and moisture-sensitive element, etc. 

An important development of orthopaedic implants has been the development of artificial composites, which when combined with biomolecules will induce osteogenesis. The properties of hydroxyapatite (HA) materials have been studied over the years with a view to increasing elasticity. Because biocompatibility has been a problem when HA has been combined with various polyethylenes and polysulphones, polyhydroxyalkanoates (PHAs) and its chemical composites, a polymer of hydroxybutyric acid (PHB), copolymers of hydroxybutyric acid and... 

Among the most important properties of hydroxyapatite as a biomaterial is its excellent biocompatibility. Hydroxyapatite appears to form a direct chemical bond with hard tissues [Piattelli andTrisi, 1994]. On implantation of hydroxyapatite particles or porous blocks in bone, new lamellar cancellous bone forms within 4 to 8 weeks [ Bajpai and Fuchs, 1985]. Scanning electron micrograph (500 x) of a set and hardened hydroxyapatite-cysteine composite is shown in Figure 39.7. The composite sets and hardens on addition of water. 

Table 2.1 Main calcium phosphate compounds. The first six compounds precipitate at room temperature in aqueous systems. The last six compounds are obtained by thermal decomposition or thermal synthesis. The five columns contain the name, the corresponding chemical formula, the Ca to P molar ratio, the mineral name, and the typical acronym, respectively. When x > 0 in the chemical composition of precipitated hydroxyapatite, one talks also about calcium-deficient hydroxyapatite (CDHA). Generally, x= 1 so that in most cases CDHA has the composition Ca9(HP04) (P04)60H...

Hydroxyapatite has the chemical composition Caio(P04)6(OH)2. In the body, its composition slightly differs from this formula (with the resulting material frequently called being dahlhte), for some calcium ions are replaced by other ions, and fluorine ions replace some of the (OH) ions. Because of their small size, it is rather difficult to determine the exact shape of the hydroxyapatite crystals in bone, which may also be different in different bones. Typically, they are platelet-shaped, with a thickness of only 5 nm and an edge length between 20 nm and 100 nm. These platelets are situated between the tropocollagen molecules (see figure 9.16). 

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... 

This variation in chemical composition, whilst the basic lattice remains unaltered, requires explanation. The diffuse nature of the X-ray diffraction pattern was attributed, some fifty years ago, to the size of the crystals which are so small that each one contains only a few hundred unit cells. More recent studies by both X-ray diffraction and electron microscopy have suggested that, in addition to the minute hydroxyapatite crystals, bone mineral contains a substantial proportion of small particles which do not give a systematic diffraction pattern but only diffuse scattering. This component, amorphous calcium phosphate (ACP), has been estimated to form as much as 68% of the mineral in the femur of very young rats, some 35% in adult rats and about 40% in human adults. When prepared in the laboratory, it exists as minute spheroids, about 20 nm in diameter, each with a denser outer shell enclosing a less dense central portion. Little is known about the arrangement of ions within them, except that the structure is not apatite. The Ca P ratio is about 1-5. When such an amorphous component is present, it will give the mineral a lower overall Ca P ratio than that of an ideal calcium hydroxyapatite (1-67). 

Examples of Raman spectra from three different types of human coronary artery are shown in Fig. 17. The top spectrum was obtained from a sample of nonatherosclerotic (normal) coronary artery, the middle from a noncalcified atheromatous plaque, and the bottom from a calcified plaque. The spectra from these different artery types are distinct and provide clear features for the determination of the chemical composition and for a histological classification of the arterial wall. For example, the normal coronary artery spectrum is dominated by protein features such as the amide I and III modes at 1650 and 1250 cm respectively, and the CH2 bending modes at —1450 cm In noncalcified atheromatous plaques, spectral features of cholesterol and cholesterol esters constitute the major part of the spectrum. The symmetric stretch at 960 cm of the calcium hydroxyapatite phosphate group dominates the spectrum of calcified plaques. 

Wang M, Bonfield W (2001) Chemically coupled hydroxyapatite-polyethylene composites structure and properties. Biomaterials 22 1311-1320... 

---