Inactivation of bacteria by high-power 940nm laser irradiation

Abstract

Summary Background and objective Diode lasers are used in dentistry for oral surgery or disinfection of root canals in endodontic treatment. The aim of the study was to elucidate the basic mechanism of action in laser-induced root canal disinfection, which could be a photothermal and/or photochemical effect. Materials and methods To find out whether high-power NIR laser bacterial killing is caused by the light itself (photochemical effect or direct bacterial heating by selective absorption) or by a general thermal process, bacterial suspensions of a non-pathogenic strain of Escherichia coli were heated in a water bath and by diode laser (940 nm) each with the same temporal temperature course. Furthermore, bacterial suspensions were irradiated during which the temperature was fixed by iced water. Efficacy of bacterial inactivation was determined using double fluorescence labeling to distinguish vital from dead bacteria. Results Comparison of bacteria-killing efficacy between laser and water bath-induced heating showed no significant differences. The most important parameter was the maximum temperature achieved during heating. Laser irradiation of bacteria at low temperatures (elimination of temperature effect) did not show any bactericidal effect. Conclusion The experiments show that the inactivation of bacteria, at least for E. coli , using high-power 940 nm laser irradiation is solely based on a thermal mechanism. This should be taken into consideration during laser-assisted endodontic disinfection, where efficient bacterial inactivation must be achieved whilst preserving temperature-sensitive neighboring tissue.