Properties of osteoconductive biomaterials: Calcium phosphates

Bone is formed by a series of complex events involving the mineralization of extracellular matrix proteins rigidly orchestrated by cells with specific functions of maintaining the integrity of the bone. Bone, similar to other calcified tissues, is an intimate composite of the organic (collagen and non-collagenous proteins) and inorganic or mineral phases. The bone mineral idealized as calcium hydroxyapatite, Ca-10(PO4)(6)(OH)(2), is a carbonate-hydroxyapatite, approximated by the formula: (Ca,X)(10)(PO4,HPO4,CO3)(6)(OH,Y)(2), where X are cations (magnesium, sodium, strontium ions) that can substitute for the calcium ions, and Y are anions (chloride or fluoride ions) that can substitute for the hydroxyl group. The current author presents a brief review of CaP biomaterials that now are used as grafts for bone repair, augmentation, or substitution. Commercially-available CaP biomaterials differ in origin (natural or synthetic), composition (hydroxyapatite, beta-tricalcium phosphate, and biphasic CaP), or physical forms (particulates, blocks, cements, coatings on metal implants, composites with polymers), and in physicochemical properties. CaP biomaterials have outstanding properties: similarity in composition to bone mineral; bioactivity (ability to form bone apatitelike material or carbonate hydroxyapatite on their surfaces), ability to promote cellular function and expression leading to formation of a uniquely strong bone-CaP biomaterial interface; and osteoconductivity (ability to provide the appropriate scaffold or template for bone formation). In addition, CaP biomaterials with appropriate three-dimensional geometry are able to bind and concentrate endogenous bone morphogenetic proteins in circulation, and may become osteoinductive (capable of osteogenesis), and can be effective carriers of bone cell seeds. Therefore, CaP biomaterials potentially are useful in tissue engineering for regeneration of hard tissues.