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.