Calcium phosphate compounds are abundant in nature and in living systems. Calcium phosphates in the form of substituted apatites are present as minerals (e.g., calcium hydroxyapatite, HA; fluorapatite, FA; carbonate fluorapatite or staffelite, CFA; chlorapatite, ClA, Mg-substituted tricalcium phosphate or whitlockite, beta-TCMP). Biologic apatites which constitute the principal inorganic phase in normal calcified tissues (e.g., enamel, dentin, bone) are carbonatehydroxyapatite, CHA; except in the case of some fish enameloids in which the mineral phase is carbonate substituted fluorapatite, CFA. In some pathological calcifications (e.g., urinary stones, dental tartar or calculus, calcified soft tissues - heart, lung, joint cartilage) mixed calcium phosphate phases co-exist with each other or with other calcium compounds (e.g., calcium carbonate, calcium oxalates). These calcium phosphates include: amorphous calcium phosphate, ACP; dicalcium phosphate dihydrate (DCPD); octacalcium phosphate (OCP); beta-TCMP; and CHA.

Calcium phosphate ceramics have now gained acceptance as bone substitute materials in dentistry and medicine. Albee reported the first successful medical application in human in 1920 and the first dental application in animals was reported in 1975 by Nery et al. At the present time, dental and medical applications of commercial calcium phosphate ceramics or calcium phosphate-based composites include: repair of bony defects, repair of periodontal defects, maintenance or augmentation of alveolar ridge, ear implant, eye implant, spine fusion, adjuvant to uncoated implants.

Commercialization of calcium phosphates, principally HA and beta-TCP, was spearheaded by Jarcho, deGroot and Aoki in the 1980's. To date, commercial calcium phosphate ceramics include, HA (from synthetic or natural origin), beta-TCP, biphasic calcium phosphate or BCP (an intimate mixture of HA and beta-TCP) and unsintered apatite (AP). HA is used also as the source material for plasma spraying coating on commercial orthopaedic and dental implants using the plasma-spray technique. Other calcium phosphates (ACP, DCPD, anhydrous dicalcium phosphate or DCPD, tetracalcium phosphate or TTCP) are used in the manufacture of calcium phosphate cements. The calcium phosphate bioceramics as bone substitute materials more closely approximate the properties and composition of the bone mineral. These biomaterials, compared to autografts or allografts, have the advantages of unlimited supply, low cost and absence of immunogenicity.

HA of natural origin are those derived from coral or from bovine bone. Coral-derived hydroxyapatite or coralline HA is prepared by the hydrothermal conversion of coral (a calcium carbonate in the calcite form) in the presence of ammonium phosphate. Bovine-bone derived hydroxyapatite are obtained by removing the organic matrix with or without subsequent sintering, HA from natural origin differ from synthetic HA in composition, crystal morphology (size and shape) and physico-chemical properties. Synthetic HA, beta-TCP and BCP are prepared by precipitation or hydrolysis and subsequent sintering; the type of calcium phosphate ceramic obtained depending on the Ca/P molar ratio of the unsintered material.

Coatings on commercial orthopaedic and dental implants are obtained by plasma-spraying technique. The plasma-sprayed HA coating, although obtained from HA, consists principally of HA and ACP with minor concentrations of alpha-TCP, beta-TCP, TTCP and sometimes CaO. It is speculated that the stability of the coating may also affect the stability of the implant. Resorption of the coating depends on the ACP/HA ratio of the coating: the higher the ratio, the more soluble or less stable is the coating. Coated implants combine the strength of the metal (Ti or Ti alloy) and the bioactivity of the calcium phosphate. Such coatings are reported to enhance and accelerate skeletal fixation.

Calcium phosphate cements consist of solid and liquid components. The solid components may consist of one or more calcium compounds (e.g., ACP, DCPD, DCPA, TTCP) or other calcium compounds (calcium carbonate, calcium sulfate or gypsum). The liquid component may be inorganic (e.g., phosphate or saline solution) or organic acids (e.g., lactic acid, tartaric acid, succinic acid, etc.). The setting products may be apatitic or non-apatitic which eventually transforms to bone apatite-like in vivo.

The many desirable properties of calcium phosphate bioceramics have been documented, including similarity in composition to bone mineral; bioactivity (ability to form bone apatite-like or carbonate hydroxyapatite on their surfaces and ability to develop a direct adherent and strong bonding with the bone tissue); ability to promote cellular function and expression; ability to form a direct uniquely strong interface with bone; and osteoconductivity (ability to provide a scaffold or template for the formation of new bone). In addition, calcium phosphate ceramics with the appropriate three-dimensional geometry are able to bind and concentrate bone moprhogenetic proteins in circulation and may become osteoinductive (capable of osteogenesis) and can be effective carries of bioactive peptide or bone cell seeds and are therefore potentially useful in tissue engineering for regeneration of hard tissues.

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