Surface topography of silicon nitride affects antimicrobial and osseointegrative properties of tibial implants in a murine model.

While silicon nitride (Si3 N4 ) is an antimicrobial and osseointegrative orthopaedic biomaterial, the contribution of surface topography to these properties is unknown. Using a methicillin-resistant strain of Staphylococcus aureus (MRSA), this study evaluated Si3 N4 implants in vitro utilizing scanning electron microscopy (SEM) with colony forming unit (CFU) assays, and later in an established in vivo murine tibia model of implant-associated osteomyelitis. In vitro, the "as-fired" Si3 N4 implants displayed significant reductions in adherent bacteria versus machined Si3 N4 (2.6 × 104 vs. 8.7 × 104 CFU, respectively; p < 0.0002). Moreover, SEM imaging demonstrated that MRSA cannot directly adhere to native as-fired Si3 N4 . Subsequently, a cross-sectional study was completed in which sterile or MRSA contaminated as-fired and machined Si3 N4 implants were inserted into the tibiae of 8-week old female Balb/c mice, and harvested on day 1, 3, 5, 7, 10, or 14 post-operatively for SEM. The findings demonstrated that the antimicrobial activity of the as-fired implants resulted from macrophage clearance of the bacteria during biofilm formation on day 1, followed by osseointegration through the apparent recruitment of mesenchymal stem cells on days 3-5, which differentiated into osteoblasts on days 7-14. In contrast, the antimicrobial behavior of the machined Si3 N4 was due to repulsion of the bacteria, a phenomenon that also limited osteogenesis, as host cells were also unable to adhere to the machined surface. Taken together, these results suggest that the in vivo biological behavior of Si3 N4 orthopaedic implants is driven by critical features of their surface nanotopography.