## 4 Discussion  
In contemporary endodontics, a variety of irrigation delivery and activating devices are available to improve the disinfection and debridement of the root canal system during treatment. A survey among members of the American Association of Endodontists reported that nearly half of them used these devices, with 48% using ultrasonic activation and 34% opting for sonic activation, to improve irrigation efficacy [23]. While published investigations have addressed and compared the effectiveness of ultrasonic and sonic irrigant activation against conventional manual needle irrigation, these studies focused on distinct experimental settings, neglecting the impact of the apical canal preparation size on the irrigating outcomes [7; 13; 14; 15; 16]. In this context, the present study addresses, through multiple clinically relevant parameters, the efficacy of different irrigation procedures on two apical sizes, i.e., 35/.06 and 50/.06, by conducting an _ex vivo_ investigation in human teeth. One of the most significant risks associated with the irrigation procedure remains the extrusion of debris and irrigant solution into the periapical region, which may cause post-operative undesirable outcomes, as periapical inflammation, postoperative pain, and, ultimately, compromise the success of root canal treatment [24; 25; 26]. Although all irrigation procedures have an associated risk of apical extrusion, the extension of this extrusion may differ according to the instrumentation techniques and devices employed [27; 28]. In the present study, the extrusion risk with conventional needle irrigation did not appear to be influenced by the apical size. On the contrary, both ultrasonic and sonic activation procedures exhibited a higher risk of extrusion, involving both irrigant and debris, particularly within the larger apical size, i.e., 50/.06. Notably, these values tended to be higher in the ultrasonic group. These results are consistent with a previous study [27], which similarly observed a reduction in the frequency of extrusion in teeth with a lower apical preparation size (i.e., 35/.06, compared to 50/.06). Similarly, another study argues that larger apical preparations might increase the risk of canal transportation and perforation, which raises concerns in considering larger canals size preparations [18]. On the other hand, when root canal preparation was increased to a size 50/.06, a significant reduction in residual debris and smear layer was observed, in contrast to a size 35/.06, particularly after activation of the irrigant. Similarly, it has been reported that a basic preparation to a size 25/.06 produced significantly less clean root canal walls than a size 40/.04 [29]. An enlargement of the apical preparation has been advanced for improved cleaning, through better acoustic streaming and penetration of irrigants, which is a critical aspect considering that the remaining tissue and debris can negatively impact endodontic treatment. Such debris can interfere with the adhesion of root-filling materials and provide a niche and nutritional source for microorganisms, potentially contributing to the development of secondary infection [30; 31]. Additionally, both sonic and ultrasonic activation procedures significantly enhanced the removal of dentin debris and smear layer, compared to conventional manual needle irrigation, regardless of the apical size. These results are in accordance with most of the literature, which advocates that irrigant activation exhibits enhanced canal debridement efficacy over the use of needle irrigation alone [7; 16; 32]. Comparing both activation methods, ultrasonic activation demonstrated superior debris and smear layer removal than sonic activation. The outperformance of ultrasonic activation may be justified by the higher driving frequency of ultrasound compared to that of the sonic device [9].