Calcium phosphates Calcium phosphate-based ceramics constitute

Calcium phosphates Calcium phosphate-based ceramics constitute Calcium phosphates Calcium phosphate-based ceramics constitute, at present, the preferred bone substitute in orthopedic and maxillofacial surgery. They are very similar to the .mineral phase of the bone, by their structure and/or their chemical composition :Calcium phosphates usually found in ceramics are ;hydroxyapatite (HAP): Ca10(PO4)6(OH)2 – ;tricalcium phosphate β (β TCP): Ca3 (PO4)2 – .mixtures of HAP and β TCP – Ceramic Materials 855 Name and chemical formula Abbreviation Type of materials and utilization ,Hydroxyapatite Ca10(PO4)6(OH)2 HAP ceramics ,plasma sprayed coatings ,composites drug carrier Calcium deficient apatites ns-HAP Low temperature coating composites, drug carrier α and β tricalcium phosphates Ca3(PO4)2 ,α and β TCP ceramics, composites ,plasma coating cements drug carrier Dicalcium phosphate dihydrate CaHPO4, 2H2O DCPD cements Anhydrous dicalcium phosphate CaHPO4 DCPA cements Octocalcium phosphate Ca8(PO4)4 HPO4)2, 5H2O( OCP cements Tetracalcium phosphate Ca4(PO4)2O TTCP cements Amorphous calcium phosphate ACP cements, drug carrier low temperature coating Table 12.1. Calcium phosphates used as biomaterials .These ceramics are bioactive and can be degraded to various degrees The stoichiometric HAP, characterized by an atomic ratio Ca/P = 1.67 and a hexagonal structure, is the nearest relative of biologic apatite crystals. Moreover, the HAP is the least soluble and the least resorbable calcium phosphate. When an HAP ceramic is implanted in a bone site, the bone tissue formation is observed on its contact (osteoconduction) (see Figure 12.1). Besides, in certain conditions, calcium phosphate ceramics can induce the formation of bone tissue in ectopic sites. HAP implants appear in the form of dense ceramics or with variable porosity or again, in .the case of prostheses, as thin coatings deposited by plasma sprayed on a metal Bioceramics 501 Figure 12.1. Micro-radiography of a section of an HAP-DCPD composite showing the invasion of DCPD by bone tissue The β TCP, characterized by an atomic ratio Ca/P = 1.5, is perfectly biocompatible and bioresorbable. Like HAP, it is capable of developing a chemical .bond with the bone and to stimulate its growth, but its resorption is more rapid It is difficult to make pure HAP or β TCP and biphasic materials HAP-β TCP have been developed initially by accident and later deliberately; they combine the physicochemical properties of each of the compounds. These can be advantageously .]used to prepare materials with controlled resorption and bone substitution [DAC 90 The presence of pores in materials provides anchor points for the bone and improves the mechanical quality of the bone/implant interface; the increase in the .specific surface further encourages cell colonization and the revascularization While calcium phosphate-based bioceramics are excellent materials for bone reconstruction, they have a low mechanical strength (less than that of the bone), not lending themselves to machining. This resistance diminishes while porosity .increases, making the utilization of very porous implants very delicate Even if the HAP remains the most important calcium phosphate, from an industrial point of view, the development of low temperature processes, particularly those concerning mineral cements and coating on metal, have led to the utilization and emergence of other calcium phosphates which are more reactive. Table 12.1 .lists the different calcium phosphates used as biomaterials Ceramic Materials 85, The development of calcium phosphate-based ceramics at high temperature requires taking into account the thermal stability of these compounds. We can distinguish two schemes of decomposition according to the temperature: irreversible decompositions (condensation of hydrogenophosphate ions, decomposition of carbonate ions, of hydroxide ions, etc.) at low temperature (150–1,000?C) and reversible decomposition (decomposition of the apatite into TCP, TTCP and lime) at .)high temperatures (T > 1,000?C Oxides and hydroxides .4,.,.6.7 As discussed in the previous section, it would seem that the layer of hydrated