## 4 Discussion  
Incomplete debridement of the root canals in the apical third presents a true challenge during disinfection. The null hypothesis to be tested in the current study, which assumed that there was no difference in irrigation solution penetration regardless of the used irrigation technique, was rejected, and the difference between the three methods of irrigation activation was clearly obvious and significantly detected. The current experimental in vitro study was planned to compare and evaluate the efficacy of root canal irrigating solution penetrability using three different irrigation activation techniques, namely: conventional needle irrigation (CN); side-vented endodontic needle irrigation (EN); and manual activation irrigation with gutta-percha cone (MA). A pilot study was conducted, and the sample size was calculated with the aid of G*Power (Version 3.1) ([PERSON] et al., 2009) [20]. The lowest mean values obtained in the pilot study (Group 1: 0.13, Group 2: 0.16, and Group 3: 0.06) with a SD of 0.11 were used to calculate the effect size, which was found to be 1.0023. Considering the actual power of 0.95 with an alpha of 0.05, there was a 97.2% chance of correctly rejecting the null hypothesis of no difference between the test scores for three groups with a total of 21 samples, or 7 samples per group. Accordingly, our study was performed on a total of sixty extracted human mandibular premolars, which were then subdivided into three groups, each of them composed of a total of twenty samples per group. Studies have shown that irrigation with the conventional needle method (CN) using a syringe and metal needle results in ineffective irrigation [8; 21]. To achieve a successful endodontic treatment, various irrigation techniques have been developed to facilitate the infiltration of the irrigation solution to reach the apical part of the canal. Several modifications to the needle-tip design have been proposed in recent years to improve irrigation effectiveness. Such modification encompasses the use of side-vented endodontic needles (EN) [20], as well as the use of apically fitting gutta-percha cones in an up-and-down motion at the working length (MA) [22; 23]. The use of a wide range of radiopaque substances to improve the contrast and visuality of a root canal system has long been advocated, with the goal of studying canal morphology and detailed anatomy. Contrast media are radiopaque solutions that can be introduced into various organs inside the human body to alter their contrast and to facilitate their radiographic visualization [24]. Similar principles of removing the pulp tissue and introducing radiopaque material with the aid of centrifuging or vacuum were limited in their use [25; 26; 27]. Diatrizoate sodium powder (Hypaque) has been used in some endodontic research. Hypaque can be dissolved in water with a similar viscosity and density to sodium hypochlorite. The use of iohexol (Omnipaque) as a contrast solution in this study is justified because it is a low osmolar agent, non-ionic, water-soluble and monomeric in structure. It has a viscosity and density similar to 5.25% NaOCI [28]. The results of this study revealed that irrigation with a conventional needle (CN) was ineffective in delivering irrigation to the apical third. Similar studies have shown that irrigation using traditional methods (CN) were insufficient for proper cleaning of the canal system to its full working length. Others have proven that using apical pressure to deliver irrigation solutions can result in effective irrigation inside the root canal system [29; 30; 31; 32; 33; 34; 35; 36; 37]. Many studies have shown the effectiveness of gutta-percha points in assisting the penetration of the irrigant into the canal. In the present study, the most effective penetration of the irrigant was observed in the manual activation group (MA). It was found that there