Fabrication and biological evaluation of hydroxyapatite ceramics including bone minerals

Biological apatite present in the bones and teeth of mammals contains various minerals, which create numerous nanoscale defects in their crystal structures. Substitution of the ions of these minerals into hydroxyapatite [Ca10(PO4)6(OH)2; HAp] induces considerable strain and various defects in the crystal structure of HAp. Although autogenous bone and synthetic HAp ceramics have been used clinically as bone grafts, autografting generally has better clinical results than artificial-bone grafting. In the present study, we fabricated HAp ceramics including bone minerals (bone HAp ceramics) as model materials to clarify the relationship between the nanoscale defect structure and bioactivity of the biological apatite. We also implanted bone HAp ceramics in the tibiae of rabbits, along with standard HAp ceramics without bone minerals (pure HAp ceramics) as a control, and examined the biological response of the living hard tissue to the implants histologically. The single-phase HAp and carbonate ion content of the bone HAp ceramics could be maintained by sintering at 1000°C for 5 h under a flow of carbon dioxide gas. The inclusion of trace elements and changes in the lattice constants were confirmed, and Raman spectroscopy indicated the presence of defects. Biological evaluation showed significantly more newly formed bone around the bone HAp ceramics at 4 weeks than around the pure HAp ceramics. These results demonstrated that bone HAp ceramics that include trace minerals and nanoscale defect structures may promote early-stage bone formation.