## 4 Discussion  
The smear layer hinders the successful disinfection and three-dimensional filling of the root canal system, as suggested by [PERSON] et al. (2018). Shaping root canals is challenging due to their anatomy even if new endodontic instruments are yearly launched on the market. In fact, more than 50% of the root canal surfaces remain untouched by mechanical instrumentation ([PERSON] et al., 2018). Moreover, packing debris into canal irregularities by reciprocating and/or rotary instrumentation has been described ([PERSON] et al., 2018). However, to the best of the authors' knowledge, this is the first study to evaluate the effects of electric current in AHTD removal. AHTD is produced during the chemo-mechanical shaping of root canal systems. To date, there is no clinical consensus that the removal of the smear layer improves the clinical treatment outcome of root canal treatment ([PERSON] et al., 2018). Nevertheless, removal of AHTD and smear layer provides better cleaning and disinfection of the root canal system, increasing  
Figure 5: EB group—3D evaluation of AHTD before (image **left**) and after (image **right**) irrigant activation. the adaptation of root filling materials [6; 32; 33]. The aims of the present study were to investigate the effectiveness of an electric current device and to compare it to endodontic sonic activation (EA) of irrigating solutions. Results showed the efficacy of electric fields in the activation of endodontic irrigants and AHTD removal in the final irrigation. Any comparison to the use of electric fields in endodontics cannot be exerted because no research has ever been published before. On the contrary, the use of electric current has been already applied in restorative dentistry. In fact, authors have shown that the application of electric current on etch-and-rinse and self-etching adhesive systems significantly increases resin impregnation and the hybridization of dentine [24; 25]. Moreover, [PERSON] et al. [26] have investigated the use of ElectroBond in simplified etch-and-rinse adhesives, showing that the application of electric current improves the shear bond strength (SBS) to dentin, reducing nanoleakage [8]. Endodontic irrigation and, more importantly, irrigant activation are crucial to further improve canal cleanliness and disinfection of the entire root canal system. Activation of endodontic irrigating solutions has been investigated during the last few decades, leading to the conclusion that it increases the removal of the smear layer [34]. Many authors have shown that EA is efficient in reducing the smear layer in the endodontic system. In fact, it has been shown that EA was more efficient in removing the smear layer in coronal and middle thirds than in the apical third [35], in accordance with the results of [PERSON] et al. [36] who evaluated the effectiveness of different activation techniques on debris and smear layer removal in curved root canals. Sonic agitation (EA and EDDY system) performed significantly better than syringe irrigation (SI), without a significant difference to PUI. This result is in agreement with the results of [PERSON] et al. [37]. MicroCT has been introduced in the last few decades in experimental procedures as a nondestructive method [38]. In endodontics, microCT has been widely used in the study of endodontic anatomy [39], shaping of root canal system [5; 10; 12; 27] and sealing ability of different materials [27; 40; 41; 42; 43; 44; 45]. Smear layer removal has been usually evaluated using scanning electron microscopy (SEM and FESEM). Unfortunately, this evaluation method has an important drawback: only a small portion of the endodontic surface can be analyzed and compared, leading to bias. Recently, microCT has been used to assess AHTD. [PERSON] et al. [46] evaluated the reduction in AHTD in isthmus-containing mesial root canals of mandibular molars by sonically and ultrasonically activated irrigation and compared results to manual irrigation using microCT imaging.