## 4 Discussion  
The results presented in this study reject the null hypothesis (H\({}_{0}\)) that determines that there would be no differences between the EndoVac and Endoactivator endodontic irrigation systems for removing the endodontic smear layer present in the coronal, middle, and apical third of the RCS and the isthmus. This study analyzed the endodontic smear layer removal capacity of two endodontic irrigation systems in the RCS and the isthmus from mesial roots of first lower molar teeth by means of micro-CT analysis [9]. The endodontic smear layer removal has been previously analyzed by Scanning Electron Microscope [11; 12] and Confocal Laser Scanning Microscopy [13]; however, these measurement procedures require the longitudinal section of the dental root, only a limited region of the RCS can be analyzed, and does not allow for the measurement of the endodontic smear layer volume [14]. Micro-CT analysis allows an accurate 3D measurement technology to quantify the endodontic smear layer volume in the RCS; however, micro-CT cannot detect the remaining soft tissue and requires tooth extraction as well as the previously described measurement procedures [15; 16]. Few articles have reported the use of micro-CT technology to analyze the endodontic smear layer removal in the RCS of mesial roots from lower molar teeth and especially containing isthmus [11; 17]. It has been reported that the smear layer can contain inorganic dentin debris and organic cells, such as parts of the odontoblastic process, bacterial cells, and necrotic pulp tissue; therefore, it is very important to analyze the ability to remove smear layer from irrigation systems to prevent secondary endodontic infections and facilitate sealing of root canal systems. [18]. It has been reported that 130 \(\upmu\)m of NaOCl can be introduced into the dental tubules [19], while _Enterococcus faecalis_ can penetrate up to a depth of 151 \(\upmu\)m and attach to collagen fibers present [20], leaving bacteria harbored in deeper layers, isthmus, accessory canals, anastomoses, and fins [21; 22]. Furthermore, endodontic smear layer formation during RCS surgery can cause blockage of dental tubules and isthmus, avoiding the effect of NaOCl inside the RCS [23]. [PERSON] et al. reported that negative microbiological cultures obtained from the RCS show a root canal treatment success rate close to 94%. However, positive microbiological cultures reduce endodontic success rate to 68% [24]. Hence, the elimination of the pathogenic microflora from the RCS is a determinant factor in the outcome of the endodontic therapy. Chelating solutions have created soluble calcium chelates in contact with calcium ions from root dentin to remove the endodontic smear layer, improving the efficacy of NaOCl inside the RCS [25]. Furthermore, supplemental irrigation procedures after root canal preparation with chelating agents, passive ultrasonic irrigation [13; 26], sonic irrigation [27], and apical negative-pressure procedures [13] have been shown to improve the endodontic smear layer removal of the canal walls and also increase endodontic sealer penetration into the dental tubules and isthmus. However, few reports have been performed comparing sonic irrigation and apical negative pressure. Overall, in this study the mean endodontic smear layer removal of the EA and EV study groups was reduced from the coronal to the apical root thirds, perhaps because the dental tubules reduce its number and diameter toward the apical root third [11] and due to the sclerotic dentin present at the apical root third, which shows a higher resistance to irrigant solutions to remove the smear layer and also to endodontic sealer penetration [9]. This agrees with previous studies, which showed that irrigating solutions are less effective in the apical root third [13].