Figure 5. Differential host cell responses to MRSA contaminated “as fired” vs. machined Si₃N₄ implants in bone over time.

Sterile L-shaped “as fired” and machined Si₃N₄ implants (n=3) were exposed to USA300LAC culture as described in Figure 1, surgically implanted into the tibiae of 6-week-old, female Balb/c mice, removed on the indicated day post-op, and processed for SEM (A-X). Representative ×125 images of the center of the L-shaped implants are shown (bar = 100μm), and higher power images of regions containing host material are shown at ×1,000 (bar =100μm). Of note is the highly reactive “as fired” Si₃N₄ implant surface, which displayed a progressive transformation from a MRSA infected surface, to and osseous surface over time as follows. Day 1 bacterial biofilm (inset image ×5,000; bar = 1μm) is attacked by inflammatory cells (B; ×2,000 bar =2μm); Day 3 bacterial pod (arrow in D; ×5,000; bar = 1μm) is elevated from the surface by inflammatory cells anchored with fibrin cables; Day 5 eradicated bacteria are replaced by mixture of round cells (macrophages) and the appearance of a few polygonal cells (arrows in F; ×1,000; bar = 10μm); Days 7, 10 & 14 show polarized osteoblastic cells (H, J, L, ×1,000). In contrast, machined Si₃N₄ implant surfaces were less reactive with both bacteria and host cells as follows. Day 1 bacteria are rarely found in biofilm (N; ×5,000; bar = 1μm); Day 3 no bacteria are found, and few activated macrophages are present on the surface (P; ×3,000; bar = 2μm); Days 5, 7, 10 & 14 incomplete coating by red and white blood cells.