Osteogenic potential of Sr-doped calcium phosphate hollow spheres in vitro and in vivo

Treatment of osteoporotic fractures with conventional surgical methods is associated with a high rate of complications. Intense search for new treatment options includes development of specific biomaterials aimed to be part of the surgical armamentarium. Strontium doped calcium phosphate spheres (SrCPS) is a new material that might be of interest due to the influence on osteoclast and osteoblast activity. In the present study, we successfully constructed hollow spherical SrCPS particles with a diameter of ~700 nm and shell thickness of ~150 nm. The Sr content was about 20 wt %. Cell viability and cytotoxicity were investigated in vitro with concentrations from 0 to 1000 μg/mL of SrCPS in medium extract in a day chase study. The in vivo biocompatibility was tested in a delayed bone-healing model in a rat vertebral defect by histology, μCT, and nanoSPECT. The SrCPS showed no toxicity in vitro with comparable cell number in all concentrations. Increased metabolism was seen in the cell viability study in cells exposed to 400 and 600 μg/mL. SPECT showed good biocompatibility with no local adverse effects and an increased osteoblast activity as compared to adjacent vertebra. SrCPS implantation induced bone formation and resulted in complete resorption and defect consolidation.