## 1 Introduction  
The most widely used endodontic irrigating solution is sodium hypochlorite (NaOCl), which acts as a bactericidal agent and dissolves the organic tissue present in the root canal system (RCS); however, it is not effective in endodontic smear layer removal. Chelating irrigants, such as ethylenediaminetetraaceticacid (EDTA), have shown their effectiveness in endodontic smear layer removal and improving RCS disinfection [1,2]. During root canal surgery, the application of endodontic instruments onto the dentinal root canal walls produces hard tissue debris deposition within the RCS, which may contain bacterial cells [1]. It can also interfere with the endodontic irrigation agents and root canal filling materials of the RCS, preventing RCS disinfection and sealing, especially in anatomically complex areas, such as the isthmus areas in mesial roots of lower molar teeth [2,3]. Isthmus areas are considered an important anatomical feature because they can contain smear layer remains of dental pulp tissue, necrotic tissues, and microorganisms, which can promote root canal re-infection after treatment [4]. [PERSON] et al. reported a prevalence of isthmus areas in the mesial root from the first lower molar teeth of 65.7% [5], whereas [PERSON] et al. reported a frequency range from 20% to 70% [6]. The isthmus, according to its anatomical characteristics, is classified into five categories: Type I--There are two separate root canals, but union does not appear. Type II--An isthmus between the two separate root canals is observed. Type III--There are three root canals connected by an isthmus. Type IV--There are two elongated root canals connected in the center. Type V--There is a single, very wide and elongated root canal [4,7].  
Traditionally, disinfection of the RCS has been performed by means of a passive irrigation procedure through endodontic irrigation syringes. However, this disinfection approach has demonstrated limited efficacy to clean the apical third of the RCS and the anatomical irregularities of the RCS, such as the isthmus. Therefore, new approaches have been developed to improve the cleaning capability of traditional disinfection procedures. The Endoactivator (Dentsply Maillefer, Ballaigues, Switzerland) endodontic irrigation system can promote the penetration of the irrigant agent within the dentinal tubules of the RCS by means of continuous sonic movement [8]. The EndoVac (Dentsply Maillefer, Ballaigues, Switzerland) endodontic irrigation system can also safely reduce the bacterial load present in the RCS, ensuring the presence of the irrigant agent in the apical third of the RCS and also preventing vapor lock formation, which avoids disinfection of the irrigant agent in the apical portion of the RCS [9,10]. The device allows for constantly renewed NaOCI irrigation throughout the root canal system by means of a negative apical pressure system. Two cannulas allow this flow of NaOCI: a macro cannula that introduces the NaOCI irrigation agent at the access cavity, and a micro cannula located in the apical constriction that aspirates the NaOCI irrigation agent and evacuates it from the RCS.  
The aim of the present study is to compare the efficacy of the EndoVac and Endoactivator endodontic irrigation systems for removing the endodontic smear layer present in the coronal, middle, and apical root third of the RCS and the isthmus from mesial roots from first lower molar teeth using micro-CT analysis, with a null hypothesis (H\({}_{0}\)). This hypothesis determines that there would exist no difference between the EndoVac and Endoactivator endodontic irrigation systems for removing the endodontic smear layer present in the coronal, middle, and apical root third of the RCS and the isthmus from mesial roots from first lower molar teeth using micro-CT analysis.