Enhanced cortical bone formation ability and antibacterial properties of silica-substituted octacalcium phosphate blocks through silver loading

Some major challenges faced in current bone substitutes are infection and poor bone regeneration capacity. Skeletal diseases, the associated postoperative infections, and the risk of bone regeneration failure are increasing because of the increase in the global aging population. We have developed materials for bone regeneration that were based on octacalcium phosphate (OCP), which is a highly biocompatible major inorganic component of immature bones, using the ionic insertion method. Herein, we developed functional OCP blocks by introducing silica and Ag in the OCP crystal structure and evaluated their bone regeneration capacity. Similar to OCPs containing only Ag, Ag-substituted OCP-silica blocks (OCP-silica:Ag blocks) exhibited crucial contact antimicrobial activity against Staphylococcus aureus. The bone regeneration capacity of OCP-silica:Ag blocks implanted into bone defects created in rabbit femurs was comparable with that of OCP-silica blocks, with a remarkably higher bone attachment. In particular, they exhibited a critical cortical bone regeneration, which was not observed when OCP-silica blocks were used. These results reveal that, unlike the Ag phosphate coating on the surface of carbonate apatite introduced in previous studies, the Ag addition to OCP crystals introduces antibacterial properties and enhances the OCP bone regeneration capacity.