Macrophages Orchestrate Peri-implant Inflammation and Direct Healing Following Implant Colocation

Objectives: Surface modifications such as roughness and energy have increased osseointegration rates and decreased healing time clinically. Macrophages are among the first cells to interact with the implant and produce a microenvironment to favor healing and osseointegration or fibrous encapsulation. Our aim was to determine the effects of implant surface wettability and chemistry on the peri-implant microenvironment and the role of macrophages on MSC recruitment and cytokine production.
Methods: Hydrophobic or hydrophilic-microrough Ti or TiZr implants were placed intra-femorally in 10-week male C57Bl/6 mice. Implants were harvested 3D or 7D post-implantation and mRNA for inflammation, matrix-remodeling, and chemotaxis genes surrounding implants were measured and compared to control mouse bone-marrow. In a second study, phagocytes were ablated via clodronate liposomes in mice receiving hydrophobic-microrough or hydrophilic-microrough TiZr. Implants and blood were harvested at 1D, 3D, or 7D from clodronate-mice, PBS-mice, and control-mice. Implant-adherent MSC:phagocyte population was determined via flow cytometry and circulating cytokines were quantified via Multiplex ELISA (n=6 mice/variable; significance: 2-fold regulation/control or ANOVA-Tukey, p<0.05).
Results: Pro- and anti-inflammatory markers were upregulated at 3D on hydrophilic implants, while mRNA on hydrophobic implants was mostly pro-inflammatory. By 7D, inflammatory markers were down-regulated and matrix-remodeling and MSCs-chemotaxis markers were upregulated on hydrophilic-surfaces with the highest change on TiZr implants. Anti-inflammatory cytokines were reduced at all times with macrophage ablation. Implant-adherent MSCs were dramatically reduced after macrophage ablation at all times tested.
Conclusions: Implant surface characteristics modified in vivo inflammatory response. Microrough-hydrophilic TiZr implants highly upregulated anti-inflammatory factors by 3D and resolved the inflammatory environment by 7D. Ablation resulted in lower MSC recruitment and decreased anti-inflammatory markers suggesting that these events are driven by macrophages. Our results demonstrate that surface modifications can control macrophage polarization and affect MSC recruitment and subsequent bone formation around the implant.