The aim of this study was to compare the effect of 17% EDTA, 5% maleic acid, and Nd:YAG laser on smear layer removal by SEM. Eighty single-rooted teeth were divided into three groups of 25 according to the final procedure for smear layer removal: irrigation by 17% EDTA or 5% maleic acid or Nd:YAG laser irradiation. The other five teeth was used as control. Roots were sectioned into buccal and lingual parts, and smear layer presence was recorded in the coronal, middle, and apical thirds under SEM. Data were analyzed by Kruskal-Wallis, Mann-Whitney, Friedman, and Wilcoxon tests. There was no significant difference between smear layer removal of 17% EDTA and 5% maleic acid. Nd:YAG laser showed the least effect significantly. The coronal part of samples was significantly cleaner than the middle, and the middle was cleaner than the apical section. 17% EDTA and 5% maleic acid were more effective in smear layer removal compared to Nd:YAG laser. 1. Introduction The success of root canal treatment depends on cleaning and disinfection of the canal to perform adequate obturation. The smear layer is an amorphous irregular layer containing inorganic debris as well as organic materials like pulp tissue, odontoblastic process, necrotic debris, microorganisms, and their metabolic products. It appears only on instrumented root canal dentine [1, 2]. McComb and Smith were the initial investigators who found the smear layer on the instrumented root canal walls. They found it irregular, amorphous, and granular when viewed under the SEM [3]. Some investigators believed that the presence of the smear layer helps increase the success rate of endodontic treatment because this layer blocks dentinal tubules and prevents bacterial exchange by reducing dentine permeability. They reported that the presence of the smear layer could stop bacterial migration from dentinal tubules or bacterial invasion into the dentinal tubules [4, 5]. Instead, others focused on the smear layer removal. Br？nnstr？m and Pérez-Heredia et al. believed that the smear layer feeds microorganisms and helps them colonize [6, 7]. Some researchers have reported that the smear layer prevents or delays action of canal irrigation solutions for disinfection of the bacteria and microorganisms in dentine [8, 9]. Other investigators showed root canal sealers to have a better adhesion to the root canal wall after smear layer removal [10–12]. Different methods have been used to remove the smear layer. Sodium hypochlorite (NaOCl) is a common irrigation solution used in endodontic therapy because it has bactericidal
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