Osteogenesis effects of strontium-substituted hydroxyapatite coatings on true bone ceramic surfaces in vitro and in vivo

To develop bioactive bone graft materials that can induce rapid bone regeneration, a novel biomaterial was synthesized by coating true bone ceramic (TBC) substrates with strontium-substituted nano-hydroxyapatites (SrHA) (Sr concentrations of 0%, 10%, 40%, 100%) through a sol–gel dip-coating approach. All coated TBC scaffolds retained the inherent natural trabecular structure, porosity, compressive strength and simultaneously possessed a micro/nanotopography SrHA layer on the substrate surface. The dimension of the deposited crystal increased and the density of the deposited apatite particles became sparse with increasing Sr content, but a unique HA crystalline phase was observed under all conditions. The modified TBC scaffolds significantly enhanced the adhesion, proliferation, and osteogenic differentiation of MC3T3-E1 osteoblasts in vitro. Particularly, the Sr10-TBC group (10 mol% Sr2+ in apatite coating) revealed the highest osteogenic efficacy over the other groups. Three-dimensional CT imaging and histological evaluations on a bilateral critical-sized rabbit radial defect model for 12 weeks showed significant bone formation in the Sr10-TBC implants. The new bone area ratios of the Sr10-TBC group were significantly higher than that of the TBC group. Additionally, Sr10-TBC implants showed faster degradability compared with raw TBC implants during the 12 weeks of implantation. The results indicate that TBC modification with 10% SrHA coating stimulated osteogenesis and could be a promising biomaterial for future bone defect regeneration.