One of the promising applications of Er. YAG laser is disinfection of residual bacterial biofilm and microcolony in the root canal. However, little information is available regarding the influences of laser irradiation on residual intra-canal bacteria near the root apex. This in vivo study focused on the effects of Er: YAG laser irradiation on the viability of residual bacteria in the root canal around the root apex. Sixty samples from the root canals of 38 patients with chronic periapical periodontitis were examined and used to identify and quantify bacteria using a conventional bacterial culture technique. Forty-two root canals were irradiated by Er: YAG laser, and samples were obtained before and after irradiation. and the other 18 samples from non-irradiated teeth were used as controls. In addition, 5 other samples were observed for viability or death of bacterial cells using a live/dead stain and fluorescence microscope. Before irradiation, 14 bacterial species, detected in 15 of 42 samples examined, were identified. The average of detected bacterial cell number was 37.7 CFU. After irradiation, no bacteria was isolated in 9 of the 15 samples in which bacteria was detected before irradiation. Six bacterial species were isolated from the 6 samples after irradiation. In these 6 samples, the mean cell number detected was 1.7 CFU. From the microscopic observation, viable cells were found in 3 of the 5 samples examined before irradiation. In 1 of these 3 samples after irradiation, viable cells were found, but no dead cells were detected. The results of the present study suggest that Er: YAG laser irradiation against residual bacteria in the canal near the root apex was useful. It is considered that use of the Er: YAG laser in infected root canal treatments could be expanded, if the laser could be irradiated with a wide angle, and if the shape of the chip could be improved.
