Introduction: Dental caries and apical periodontitis are ones of the most prevalent chronic diseases and involve infection by cariogenic and endodontic bacteria. It can be said that the most required to cure is disinfection. We hypothesized that NB-UVB could be used for intraoral disinfection without affecting cells, and that it could be used in combination with TiO2 to disinfect complex root canals. The objectives were to investigate the effects of UV on dental pulp cells and oral bacteria and to evaluate the enhancement effect of a photocatalyst on bactericidal effects of UV irradiation. Methods: UV irradiation devices of UVB (310, 285 nm) and UVC (265 nm) were prepared. Cell proliferation and cytotoxicity assays after UV irradiation were performed using human dental pulp cells. The antibacterial activity of UV irradiation was investigated in Streptococcus mutans, Staphylococcus aureus, Enterococcus faecalis, Actinomyces naeslundii, Porphyromonas gingivalis, and Fusobacterium nucleatum. A curved simulated root canal with plastic training block was used to evaluate the effect of combined UV and TiO2 treatment. Data were analyzed by one-way ANOVA (p < 0.05) followed by Tukey’s post hoc tests (p < 0.05). Results: Human dental pulp cell proliferation was decreased by 265 nm, 285 nm, and 310 nm UV irradiation, although 310 nm UV irradiation did not show cytotoxic effects on these cells. Oral bacterial growth was suppressed following UV irradiation at 285 nm and 265 nm. Viability of all bacteria significantly decreased with UV irradiation. In the curved simulated root canal, viability of E. faecalis in UV irradiation at 285 nm with long taper fibers was significantly decreased in the 300 s irradiation group. E. faecalis proliferation was inhibited by combined UV irradiation and TiO2 application with long taper fibers in the curved simulated root canal. Conclusions: The wavelength of UVB and UVC showed bactericidal effects on oral bacteria including caries-related bacteria and apical periodontitis–related bacteria while NB-UVB did not. UVB with longer fibers was more effective in disinfection on E. faecalis in curved simulated root canal, and the combined use of photocatalyst further improved the disinfection effect.
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