Antibacterial Effect of 445 nm and 970 nm Diode Laser Irradiation in a Simulated Lateral Canal Model: An Ex Vivo Study

Persistent Enterococcus faecalis infection in inaccessible dentinal regions contributes to endodontic treatment failure, and adjunctive diode laser disinfection has been proposed to reach these areas, but direct comparisons of 445 nm and 970 nm wavelengths in a standardized lateral canal model are lacking. The aim of this study was to evaluate the antibacterial effect of a diode laser through a 0.5 mm dentin barrier in a clinically relevant ex vivo model and to compare its efficacy in reducing Enterococcus faecalis at wavelengths of 445 nm and 970 nm. Fifteen extracted human anterior teeth were instrumented to ProTaper F3, embedded in acrylic resin, and prepared with a standardized lateral canal 0.5 mm from the main canal. Following steam sterilization, the canals of specimens were inoculated with E. faecalis. Each tooth was exposed to all three conditions (control, 970 nm, 445 nm) across six successive experimental runs, and laser irradiation was performed through root dentin using a SiroLaser Blue diode laser (Dentsply Sirona), yielding 26 run-level measurements per condition overall. For each tooth, log10-transformed CFU/mL values were averaged within each condition prior to statistical analysis, giving one paired value per tooth per condition (n = 15). The mean log10-transformed bacterial load was 4.77 ± 0.12 in the control condition, 4.54 ± 0.11 following 970 nm irradiation, and 4.06 ± 0.75 following 445 nm irradiation. The Friedman test showed a significant overall difference among conditions (χ2(2) = 15.60, p < 0.001; Kendall’s W = 0.52), and Holm-adjusted Wilcoxon signed-rank pairwise comparisons (two-sided α = 0.05) showed significantly lower bacterial recovery following both 445 nm (mean tooth-level relative difference 55.9%; adjusted p = 0.0013) and 970 nm irradiation (38.2%; adjusted p = 0.0013) compared with the control, with significantly lower recovery following 445 nm than 970 nm irradiation (32.1%; adjusted p = 0.0215); these relative differences varied considerably among individual teeth. These findings indicate that diode laser irradiation exerts a wavelength-dependent antibacterial effect beyond the directly irradiated canal, supporting its potential as an adjunct to conventional root canal disinfection.