biologic apatite

Biologic and Synthetic Apatite
The name “apatite” describes a family of compounds having similar structure (hexagonal system) in spite of a wide range of compositions. These minerals have the general formula M10(PO4)6X2, where M could be one of several metals (usually calcium, Ca), P is most commonly phosphorus (P), and X is commonly hydroxide (OH) or a halogen such as fluorine (F) or chlorine (Cl).
Biologic apatites are the inorganic phases of calcified tissues (teeth and bones). the inorganic phases of bones and teeth are basically calcium hydroxyapatite. Enamel, dentin, and bone apatite differ in crystallinity, reflecting crystal size (Fig. 2) and concentrations of minor constituents mainly Mg and CO3. The crystal size (nanometers) of biological HA is much smaller than can be produced in synthetic HA. Some commercial HA biomaterials are derived from biologic materials (e.g., processed human bone, bovine bone derived, hydrothermally converted coral or derived from marine algae) Biologic apatites are usually calcium-deficient (i.e., with Ca/P molar ratio less than the stoichiometric value of 1.67 obtained for pure HA, Ca10(PO4)6(OH)2, it have been idealized as calcium HA
Synthetic Hydroxyapatite (HA) is a calcium phosphate based biomaterials whose stoichiometric formula corresponds to Ca10(PO4)6(OH)2 with (Ca/P=1.67) , it is the most stable phase of various calcium phosphates; it is stable in body fluid and in dry or moist air up to 1200°C and does not decompose and has shown to be bioactive due to its resorbable behaviour. It is similar to bone mineral and is widely used as a filler, spacer and bone graft substitute. Hydroxyapatite can be prepared in either dense or macroporous forms. Porous HA is osteoconductive (The phenomenon of new bone formation on the surfaces of bioactive ceramics) and biocompatible; it resorbs with time but the degradation rate is slow. Non-porous or dense HA is considered to be non-biodegradable because of its very low degradation rate in body fluids. Thus, porous hydroxyapatite has now replaced the dense hydroxyapatite form for most biomaterial applications. Studies on synthetic apatites in the last 30 years were motivated by development of calcium phosphate-based biomaterials for bone repair, substitution, and augmentation and as scaffolds for tissue engineering in bone and teeth regeneration. The reason for developing HA biomaterials was their similarity in composition to the biologic apatite and bone mineral. Synthetic HA can be made by solid-state reactions or by precipitation or hydrolysis methods and subsequent sintering at high temperatures, usually 1,000 °C and above. Synthetic apatites can also be prepared using hydrothermal, and sol-gel methods. Apatite nanocrystals are obtained when prepared by precipitation or hydrolysis at lower temperatures (25–60)°C. Synthetic apatite crystals approximating the size of human enamel apatite may be obtained by precipitation or hydrolysis methods with reaction temperature (80-90) °C.. Sintering of HA at temperatures above 1200 °C results in thermal decomposition of apatite forming other calcium phosphates such as ?-TCP and ?-TCP. Studies on synthetic apatites showed that substitutions for Ca, PO4, or OH ions in the apatite structure result in changes in lattice parameters and crystallinity (reflecting crystal size) and dissolution properties. These calcium phosphate bioceramics include HA, ?-TCP, BCP, bovine bone-derived apatite (unsintered and sintered), and coral-transformed apatite. Commercial HA biomaterials are usually prepared by precipitation at high pH and subsequent sintering at about 1,000–1,100 °C. The different preparations and origin (synthetic vs. biologic) are reflected in the difference in their initial crystallinity reflecting crystal size and their dissolution rates, increasing in the order HA << coralline HA < bovine bone apatite (sintered) << bovine bone apatite (unsintered) HA << BCP << ?-TCP.