Objective To compare the effect of three techniques in root canal preparation on coronal microleakage of endodontically treated teeth. Methods Forty-eight human single-rooted teeth were divided into 4 groups randomly and subjected to different techniques on removal of smear layer. Group A, the control group, irrigation with
hydrogen peroxide（H2O2 ） and normal saline（NS）; group B, smear cleared with 5.25% sodium hypochlorite（NaClO） and ethylenediamine tetraacetis（EDTA） gel; group C, irrigation with Odontoson-M ultrasonics; group D, smear layer removed with Nd: YAG laser irradiation . Two specimens of every group were split longitudinally into equal segments and canal walls at coronal portion were examined under scanning electron microscope（SEM）. Then ten teeth of every group were filled with warm vertical condensation by Obtura Ⅱ thermoplasticized gutta-perch injection technique and sealed with zinc oxide eugenol（ZOE）. The teeth were immersed in 20 g·L-1 methylene blue for 7 days, then were demineralized, dehydrated and cleared. Linear dye penetration was determined under a steremicroscope. The data were statistically analyzed. Results In group A, there were smear layer and debris remained and dentinal tubules were covered. In
group B and group C, the smear layer of root canals were removed and dentinal tubules were open. In group D, the root canal dentinal tubules were sealed or semi-sealed and the smear layer were melted and ablated. The mean length of coronal microleakage for group A, B, C, D were （2.15±0.38）, （1.75±0.28）, （2.04±0.40）, （1.73±0.36）mm. The analysis showed that the linear in group A was longer than that in group B and group D clearly（PThe coronal microleakage following root canal treatment could be reduced by removing smear layer with different techniques in root canal preparation.
