Irrigation with reduced sodium hypochlorite solution concentration using laser‐induced cavitation is effective and safe in rat intraradicular biofilm model

<jats:title>Abstract</jats:title><jats:p>This study aimed to investigate the optimal sodium hypochlorite solution (NaOCl) concentration to effectively remove the root canal biofilm without stimulating periradicular inflammation using coronal laser‐activated irrigation (CLAI). To compare the efficacy of different NaOCl concentrations combined with CLAI in removing the biofilm, an in vivo intraradicular biofilm rat model was used. Root canals were irrigated using an Er:YAG laser with either 5% or 0.5% NaOCl. Biofilm removal efficacy of CLAI was compared to that of conventional needle irrigation using scanning electron microscopy (SEM) and quantitative polymerase chain reaction (qPCR). Histological observation of CLAI‐associated periradicular inflammation was also conducted. In both the 5% and 0.5% CLAI groups, SEM observation showed the opening of the dentin tubules and biofilm removal. qPCR analysis indicated that the residual bacteria counts after cleaning were significantly lower in the 5% and 0.5% CLAI groups than in the conventional needle irrigation and positive control groups (Tukey test, <jats:italic>p</jats:italic> < 0.05), and no significant difference was observed between the 5% and 0.5% CLAI groups (<jats:italic>p</jats:italic> > 0.05). Periapical inflammation in the 5% CLAI group revealed the most severe, including significant neutrophilic and lymphocytic infiltration with abscess formation, while only mild vasodilation was observed in the 0.5% CLAI group. CLAI can remove the biofilm independently of chemical action, which avoids the risks associated with high NaOCl concentrations. Therefore, this root canal irrigation technique ensures safety and effectiveness, promising to contribute to new treatment strategies intended to remove intraradicular biofilm.</jats:p>