Abstract: Objective: To compare the antimicrobial effects of Er:YAG laser combined with different concentrations of sodium hypochlorite (NaClO)irrigants on enterococcus faecalis. Methods: Seventy-five single-root teeth were selected and contaminated with enterococcus faecalis for 4 weeks.The models were randomly divided into 5 groups: 5.25 % NaClO(group A),Er:YAG laser + 5.25 % NaClO (group B), 1 % NaClO(group C), Er:YAG laser + 1 % NaClO (group D),and untreated (group E).Group E was used to evaluate the success of models and the baseline number in the root canal. The amount of bacteria in the root canal of all models after treatments were measured by counting the colony-forming units (CFU) in the culture dishes and bacteria reduction was analyzed by means of Chi-square test. Results: Group A, B and D showed great antimicrobial effects and reduced the CFU to 0.But group C could not kill all enterococcus faecalis.Four of 15 culture dishes of group C were empty and without any bacterial colony, but the rest were not. The antimicrobial effects between group A, B and D were the same and significantly better than group C. Conclusion: Low concentrations of NaClO do not have ability to kill all enterococcus faecalis but high concentrations of NaClO shows great antimicrobial effects. Er:YAG laser combined with low concentrations of NaClOirrigants has strong effects on disinfection of enterococcus faecalis.

Key words: Er:YAGlaser , Sodium hypochlorite (NaClO), Enterococcus faecalis , Antimicrobial effects