Interface of synthetic inorganic biomaterials and bone regeneration

Synthetic inorganic calcium phosphate compounds have been utilized clinically to fill the bone defect. Sintered hydroxyapatite (HA) and beta-tricalcium phosphate (β-TCP) bone substitutes are known to be biocompatible and osteoconductive. Furthermore β-TCP is bioresorbable whereas HA is not. Non-sintered synthetic octacalcium phosphate (OCP) has been shown to enhance bone regeneration accompanied by the conversion into the hydrolyzed apatitic product in situ and the biodegradation. There are still controversies over the chemical nature of the first mineral crystals formed in bone, dentin, and cementum. However, OCP has been advocated to be a precursor of biological apatite in these mineralized tissues because its existence could explain the non-stoichiometry of the mineral crystals in their compositions. The surface of the implants as well as the forming mineral crystals is continuously exposed to biological constituents, such as extracellular matrices, inorganic biominerals, and cellular components. The conversion into the apatite from inorganic precursor phases, such as OCP, could include a biomimetic process regarding the interaction at the crystal surfaces in biological environments. It appears that the ideal artificial scaffold to regenerate bone is resorbable, bioactive, and further activates host osteoprogenitor cells by itself. The present paper focuses on the surface of synthetic inorganic biomaterials and bone regeneration by synthetic precursors of HA, such as OCP, amorphous calcium phosphate (ACP) and dicalcium phosphate (DCP). The analyses include mechanism of OCP hydrolysis into apatite, experimentally stimulated bone formation by the implantation, and adsorption of biomolecules onto the calcium phosphate compounds, of particular interest in reactive bone induction with calcium phosphate implants.