Silicon Nitride Induces Osteoconduction Via Activated Mitochondrial Oxidative Phosphorylation and Neovascularization

Silicon Nitride (Si3N4: SiN) is a thermodynamically stable ceramic material with excellent mechanical properties and wear/corrosion resistance for industrial applications. SiN is also proposed for orthopedic and dental implant applications because of its enhanced osteoconduction. However, the biological mechanism of SiN-induced bone formation has not been fully elucidated. In this study, SiN significantly increased in vitro mineralization of human bone marrow mesenchymal stromal cells (BM-MSC) and in vivo peri-implant bone volume in mouse femurs over conventionally used titanium (Ti) implants. RNA sequencing of BM-MSC cultured on SiN disc revealed that the functional gene clusters associated with mitochondrial oxidative phosphorylation were significantly elevated over the Ti disc groups. SiN in aqueous solution releases ammonium/ammonia, which may provide a source for glutamine-dependent energy production. It was confirmed that BM-MSC upregulated the glutamate-ammonia ligase (GLUL) expression with osteogenic condition. In addition, SiN increased the expression of functional gene clusters involving vascular formation. The upregulation of HIF1a in vitro and the increased VEGFR3-positive blanching vascular structures in vivo supported that SiN-induced neovascularization. This study has uncovered an important mechanism that SiN stimulates osteoconduction through unique glutamine-driven mitochondrial oxidative phosphorylation and establishes oxygen and nutrient supply by neovascularization, leading to stable osseointegration.