Photothermal Effect of 970 nm Diode Laser Irradiation on Enterococcus faecalis Biofilms in Single-Rooted Teeth Ex Vivo

<b>Objective</b>: The aim of this study was to evaluate the photothermal effect of a 970 nm diode laser on <i>Enterococcus faecalis</i> biofilms. <b>Methods</b>: 72 extracted human single-rooted teeth were prepared, sterilized, and inoculated with <i>Enterococcus faecalis</i> to establish a two-week-old biofilm. The specimens were divided into six groups (n = 12): Group 1 (G1)-negative control (PBS-no laser), Group 2 (G2)-positive control (1% NaOCl rinse-no laser), Group 3 (G3)-a 970 nm laser in 1.5 W pulse mode, Group 4 (G4)-a 970 nm laser in 2 W pulse mode, Group 5 (G5)-a 970 nm laser in 1.5 W continuous mode, Group 6 (G6)-a 970 nm laser in 2 W continuous mode. Bacterial viability was evaluated using the LIVE/DEAD BacLight kit and analyzed by flow cytometry (FCM). Temperature changes on the root surface during irradiation were analyzed using a K-type thermocouple. Data were statistically analyzed using one-way ANOVA and Tukey's multiple comparison test (α = 0.05). <b>Results</b>: Bacterial viability was significantly reduced after laser irradiation in continuous mode using 1.5 W (21% of live bacteria) and 2 W (14% of live bacteria). When the pulsed mode was applied, the reduction in bacterial viability was less, with a mean survival of 53% (1.5 PF, whereas 29% of bacteria survived after 2 W irradiation). <b>Conclusions</b>: The 970 nm diode laser at 2 W continuous mode effectively reduced the viability of <i>E. faecalis</i> biofilms in root canals without causing unacceptable temperature rises at the root surface.