## 4 Discussion  
Shaping and cleaning of the root canal represents an important step in endodontics to obtain bacterial load reduction and ensure long-term healing. Several studies demonstrated that mechanical instrumentation alone is unsuccessful in completely removing the remaining bacteria and smear layer [2; 5; 8]. Irrigation plays a crucial role during endodontic treatment as it removes the smear layer and bacteria and decontaminates the surfaces of the root canal system, even when involving anatomical complexities [23; 39]. Several protocols and systems have been introduced that combine irrigating solutions and activation methods in order to achieve satisfactory disinfection. They decrease the side effects of irrigants on dentine structure and increase the capability of endodontic and restorative materials in sealing and filling the root canal system [40]. This work aimed to investigate the efficacy of four different irrigation protocols for removing the smear layer. Since some groups present statistically significant differences, the null hypothesis is partially rejected. Indeed, Group A demonstrates the capability of the conventional needle irrigation to obtain good results in the coronal root third with a Score #1; however, in the remaining two-thirds, the removal of the smear layer appears insufficient. Indeed, scanning electron micrographs show a massive presence of a smear layer which completely covers the wall of the apical third, as indicated by Score #5 according to the criteria of [PERSON] et al. [38]. This finding is in agreement with several research studies, which indicate the insufficient ability of the conventional needle system to remove the smear layer in the apical third [41; 42; 43]. Furthermore, the obtained results demonstrate that the amount of smear layer remaining in the coronal and middle levels is less than that of the apical third in all four groups, thus proving that less irrigation is delivered to the apical region due to the difficulty in reaching a narrower space. In addition, the diminished performance of the conventional irrigation system in the apical region can be attributed to the formation of the vapor lock or, rather, an air bubble or voids formation inside the canal, which avoids the proper irrigant delivery to the apical third [44]. Contrarily, the other three tested protocols show the capability to eliminate the vapor lock, allowing the access of irrigants to the apical area, as displayed in Figure 3. Furthermore, it is noteworthy that the IriFlex propylene tip tested in Group B allows the irrigant solution to reach and clean the apical area, even in anatomically complex areas, due to the flexibility and 30G needle shape. Although it is not evident in this study, the literature claims that there is a need to use other protocols and instruments to activate the irrigation solution and obtain adequate disinfection and cleaning of the entire endodontic space. The ultrasonic irrigation is probably one of the most generally used strategies to optimize disinfection [2]. Indeed, the activation system tested in Group C displays promising results, although a uniform smear layer still covers the root canal surface in the apical third. Many investigations underlined that ultrasonic activation methods of irrigants are necessary to produce clinical effectiveness [45; 46; 47; 48]. These systems exploit the cavitation effect, which consists of the process in which millions of bubbles are formed during canal irrigation [10]. These bubbles grow in size, move around, and burst, significantly amplifying the cleaning effect of the system. The movement of the tip produces a special effect called acoustic turbulence, which accelerates the movement of the liquid inside the root canal [49]. Ultrasonic agitation suspends tissue remnants and dentin debris, transporting them coronally and making their removal easier [50]. It also helps irrigants to reach the entire anatomy of the canal, including isthmuses, lateral canals, apical ramifications, etc.