Statement of the Problem:Enterococcus faecalis(E. faecalis) is one of the most important microorganisms in the evaluation of the antibacterial effects of intra-canal medications due to its ability to penetrate dentinal tubules and form biofilms. Calcium hydroxide, as the most common intra-canal medication, has little effect on this bacterial species. In contrast, it is hypothesized that nano scale hydroxide particles are more effective due to their smaller size and higher surface-to-volume ratio.Purpose:This study aimed to investigate the antimicrobial effect of nano-calcium hydroxide on the four- and six-week-old intra-canalE. faecalisbiofilms.Materials and Method:In thisin vitrostudy, seventy maxillary single-canal premolar teeth were used. After cleaning and preparing the root canals, the samples were placed in
vials containingE. faecalissolution in which the culture medium was changed daily. Each group was divided into three subgroups (n=20) in terms of the
antimicrobial material used as the intra-canal medication including subgroup 1: nano-calcium hydroxide, subgroup 2: calcium hydroxide, and subgroup 3: phosphate-buffered saline
solution (control group). The antimicrobial property was measured by counting colony-forming units (CFU). The data were analyzed with Mann-Whitney U and Kruskal–Wallis tests.
Statistical significance was set atp< 0.05.Results:The mean CFU in the six-week-old biofilm group was significantly higher than that in the four-week-old biofilm (p= 0.003). A comparison between the subgroups showed a significant decrease in CFU in the six-week-old biofilm in the nano-calcium hydroxide subgroup compared to that in the calcium hydroxide subgroup (p= 0.002). However, the decrease was not significant in the four-week-old biofilm group (p= 0.06).Conclusion:Under the limitations of the present study, the antimicrobial properties of nano-calcium hydroxide were higher than conventional calcium hydroxide on mature biofilm, whereas the antimicrobial properties were not clinically and significantly different on immature biofilm.