Antimicrobial effectiveness of passive ultrasonic irrigation following preparation with single-file and multi-file NiTi instrumentation systems in flattened root canals

Effective disinfection of the root canal system (RCS) depends on the interaction between mechanical preparation and irrigation protocols, particularly in anatomically complex root canals. The influence of passive ultrasonic irrigation (PUI) on antimicrobial outcomes following preparation with single-file nickel–titanium reciprocating file systems (SNTRFSs) or multi-file nickel–titanium rotary file systems (MNTRFSs) in flattened root canals remains unclear. The objective of this ex vivo study was to evaluate the antimicrobial effectiveness of PUI in flattened root canals previously prepared with an MNTRFS—ProTaper Universal (Dentsply-Maillefer, Ballaigues, Switzerland)—or an SNTRFS—Reciproc (VDW GmbH, Munich, Germany)—and contaminated with Enterococcus faecalis . Initially, 60 mandibular central incisors with flattened root canals were randomly allocated to two instrumentation groups ( n = 30 each). After 21 days of bacterial contamination, specimens from each instrumentation group were randomly assigned to one of three final irrigation protocols (FIPs) ( n = 10): (1) distilled water activated by PUI, (2) 2.5% sodium hypochlorite (NaOCl) activated by PUI, or (3) conventional syringe irrigation (CSI) using 2.5% NaOCl. Bacterial metabolic activity before and after irrigation was assessed using an MTT assay, and the percentage reduction in bacterial metabolic activity was calculated from changes in optical density. Data were analyzed using one-way ANOVA and Tukey’s post hoc test (α = 0.05). Groups treated with PUI generally exhibited greater reductions in bacterial metabolic activity than those treated with CSI ( P < 0.05). The combination of the MNTRFS with 2.5% NaOCl and PUI yielded the greatest mean reduction in bacterial metabolic activity, whereas the lowest antimicrobial effect was observed in specimens instrumented with the SNTRFS and treated with 2.5% NaOCl using CSI. Although all the FIPs reduced bacterial metabolic activity, residual activity remained detectable in all groups. Within the limitations of this ex vivo study, PUI was associated with greater reductions in bacterial metabolic activity under standardized laboratory conditions, particularly after preparation with the evaluated MNTRFS. However, complete bacterial inactivation was not achieved with any of the evaluated FIPs.