To evaluate the antibacterial effect of sonic and ultrasonic activated irrigation on bacterial reduction of a dual-species biofilm in root canals compared to non-activated irrigation in a laboratory study.
Two hundred and fourty extracted human single-rooted maxillary anterior teeth were divided into two main groups (G, n = 120) according to the initial preparation size of the root canal (G1: size 25, .06 taper, G2: size 40, .06 taper). Root canals were inoculated with E. faecalis and S. oralis. After 5 days, G1 received combined instrumentation (up to size 40, .06 taper) and irrigation/activation, whereas G2 received solely irrigation/activation protocols. In both groups, irrigation was performed with sodium hypochlorite (NaOCl 1 %) or physiological saline (NaCl 0.9 %), using non-activated syringe irrigation, sonic activation (2 x 30 s), or ultrasonic activation (2 x 30 s). Logarithmic reduction factors (LRFs) of colony-forming units were analyzed separately for dentine-adherent and planktonic bacteria immediately after irrigation/activation protocols (timepoint 1) or after 5 days of further incubation (timepoint 2) by analysis of variance (ANOVA) and post-hoc tests (Tukey HSD, t-Test). The significance level was set at 0.05.
In G1 subgroups (combined instrumentation with irrigation/activation), LRFs were significantly affected by the applied irrigation solution (P 0.05; ANOVA). In G2 subgroups (solely irrigation/activation), both, irrigant solution and activation, significantly affected LRFs (P < 0.0001, ANOVA). Sonic activation resulted in significantly higher LRFs than ultrasonic activation (P < 0.0001) which had significantly greater reductions than non-activated irrigation (P < 0.05; Tukey HSD). At T2, strong bacterial regrowth was observed in all groups; however, a significant bacterial reduction was detected for factors instrumentation, irrigant solution and activation (P < 0.0001; ANOVA). Similar LRFs were found for dentine-adherent and planktonic bacterial cells in all groups (r = 0.91 at T1, r = 0.8 at T2).
In this laboratory study on extracted maxillary anterior teeth high-frequency sonic activation resulted in a greater bacterial reduction compared to ultrasonic activation in groups receiving solely irrigation/activation protocols, however, irrigation using NaOCl and ultrasonic activation also contributed significantly to bacterial reduction compared to the control groups.

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