Aim: This ex-vivo study evaluated the efficiency of an Er, Cr: YSGG laser (2780 nm) at different power settings to remove smear layer from the apical third of mesial roots of first lower molars by using SEM. Materials & Methods: 42 mesial root canals of first mandibular molars (type II Vertucci) were divided into 4 groups of 10 teeth each. The rest 2 teeth were used as control groups. After coronal access, all teeth were instrumented by Protaper Gold rotary files up to size F2 (25/0.08), followed an irrigation protocol with saline. Then, the experimental groups were irradiated by Er, Cr: YSGG laser. Four different output powers were tested, namely 1.25 W, 1.5 W, 2 W, 2.5 W was used to irradiate the roots in Group 1 (G1), Group 2 (G2), Group 3 (G3) and Group 4 (G4) respectively. Control group (n=2) was instrumented as experimental groups but not irradiated. Teeth were sectioned longitudinally and observed under SEM. Results were statistically analyzed with the Kruskal-Wallis Test and Mann-Whitney Test. Results: None of the tested groups succeeded in removing completely the smear layer from apical dentinal walls. The results showed no statistically significant differences between laser groups in removing smear layer from apical third of lower molars. Group 3 (2 W) showed better outcomes but it was not statistically significant. Conclusion:The outcome of the present study showed that laser-assisted smear layer removal with an Er, Cr: YSGG laser with the tested parameters is not predicable for the apical third of root canals. Clinical significance: The presence of chelating factor may play an important role in the mechanism of laser-assisted removal of smear layer in the apical third of root canals. Further research needs to be performed in order to find the optimal irradiation protocol.

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