Effect of Irrigation with Zirconia Microparticles Combined with Different Activation Techniques on Smear Layer Removal: An Ex Vivo Study.

Background/Objective: Root canal debridement is limited by complex anatomy and the apical vapor lock phenomenon, reducing smear layer removal. This study evaluated the effect of a 5 µm zirconia microparticle suspension used with sonic, passive ultrasonic irrigation (PUI), or erbium, chromium-doped yttrium scandium gallium garnet (Er,Cr:YSGG) laser activation on smear layer removal across the root canal thirds. Methods: Sixty single-rooted human teeth were prepared to size F4 (40/0.06) using 5.25% NaOCl and randomly assigned to six groups (n = 10) combining activation modality (sonic, PUI, or Er,Cr:YSGG laser) and irrigant type (distilled water or 5 µm zirconia suspension) over a 5 min interrupted cycle. Smear layer removal was scored using scanning electron microscopy (SEM) and the Hülsmann grading system. Data were analyzed via Kruskal-Wallis and Friedman tests with post hoc pairwise comparisons (α = 0.05). Results: Inter-rater reliability was strong (Cohen's κ = 0.736-0.822; p < 0.001). Smear layer removal varied significantly across root thirds (p < 0.001). Ultrasonic + Zirconia demonstrated superior removal in the middle third compared with Sonic + Distilled Water (p = 0.005), and in the coronal third compared with Laser + Distilled Water (p = 0.003) and Sonic + Distilled Water (p = 0.026). Coronally, Ultrasonic + Distilled Water outperformed Laser + Distilled Water (p = 0.041). No significant differences occurred apically (p > 0.05). Conclusions: Incorporating 5 µm zirconia microparticles with passive ultrasonic activation improved smear layer removal, particularly in the coronal and middle thirds of the root canal under the conditions of this ex vivo study.