Surface Roughness as a Determinant of the Immunological Tissue Response to Additively Manufactured Titanium Implants

Background/Aim: Surface roughness strongly affects host responses to titanium implants. Additive manufacturing (AM) by selective laser melting (SLM) allows controlled surface generation but the immune consequences of AM-derived roughness remain unclear. Materials and Methods: Three AM titanium disks with distinct roughness were analyzed by scanning electron microscopy and profilometry. At 10 and 30 days after subcutaneous implantation of disks in rats, tissue responses were evaluated according to ISO 10993-6. A novel phagocyte accumulation index (PAI) was applied, integrating mononuclear infiltration, multinucleated giant cells, coverage, and morphology. Results: All specimens induced only mild to moderate inflammation, confirming biocompatibility. Histology showed fibrosis, macrophage persistence, and rare multinucleated giant cells, without intergroup differences by ISO scoring. In contrast, the PAI detected significant differences: high-roughness specimens showed the highest values at day 30, significantly exceeding those of medium-roughness surfaces (p<0.05), while low-roughness surfaces were intermediate in value. Conclusion: The PAI revealed surface roughness-dependent differences in phagocytic activity not captured by ISO scoring. High-roughness AM surfaces promoted the strongest response, highlighting AM and its process parameters as a biological determinant and PAI as a sensitive tool for biomaterial evaluation.