Effects of Sodium Hypochlorite Concentration and Final Irrigation Protocol on Residual Filling Material in Retreated Oval Root Canals Obturated with a Calcium Silicate-Based Sealer: A Scanning Electron Microscopy Study

Background and Objectives: Residual filling material may persist on the walls of oval root canals after mechanical retreatment, and the influence of sodium hypochlorite (NaOCl) concentration and supplementary irrigation procedures on its removal remains unclear. This laboratory study used scanning electron microscopy (SEM) to evaluate the effects of three NaOCl concentrations and five final irrigation protocols on residual filling material in retreated oval canals of mandibular premolars. The protocols tested were conventional needle irrigation (CNI), EDDY, passive ultrasonic irrigation (PUI), XP-endo Finisher R (XPFR), and Shock Wave Enhanced Emission Photoacoustic Streaming (SWEEPS). Materials and Methods: One hundred twenty extracted mandibular premolars with a single root and an oval canal were instrumented with Reciproc R25 and filled with CeraSeal and a matching gutta-percha cone using the single-cone technique. Root canal retreatment was performed using ProTaper Universal Retreatment instruments followed by Reciproc R40. The teeth were then distributed among 15 experimental subgroups according to the final irrigation protocol and NaOCl concentration used: 2.5%, 5.25%, or 8.25% (n = 8). After irrigation, each root was divided longitudinally, and one undamaged canal half was randomly chosen for SEM evaluation. Residual filling material was scored in the coronal, middle, and apical regions using an ordinal scale ranging from 1 to 5. Examiner agreement was determined with quadratically weighted Cohen’s kappa, while treatment effects were examined using a cumulative link mixed model (CLMM). Results: Residual filling material scores differed significantly according to the final irrigation protocol (p < 0.001) and root canal region (p = 0.007), but not according to NaOCl concentration (p = 0.236). The interaction between protocol and canal region was also significant (p = 0.036). In the overall comparisons, CNI produced the highest scores, whereas SWEEPS produced the lowest, and EDDY, PUI, and XPFR did not differ significantly from one another. Apical scores were higher than coronal and middle scores in the EDDY, PUI, and SWEEPS protocols, while XPFR showed no significant variation among canal regions. Conclusions: Residual filling material scores after retreatment were influenced by the final irrigation protocol and root canal region, whereas NaOCl concentration had no significant main effect. SWEEPS resulted in the lowest overall residual filling material scores and CNI in the highest, while EDDY, PUI, and XPFR showed comparable overall performance, although protocol performance varied across root canal regions.