## 4 Discussion  
It is well established that endodontic treatment aims to shape and clean the root canal system to heal or prevent periapical disease [26]. This goal can be mainly achieved by significantly decreasing the bacterial inoculum inside the root canal system and preventing re-infection [27]. Although primary endodontic infection is mainly caused by a polymicrobial infection that is predominantly anaerobic, secondary endodontic infection is characterized by fewer strains and is especially facultative [28]. _E. faecalis_ is the most common isolated bacteria strain from failed endodontic cases [28,29]. Therefore, numerous studies have used _E. faecalis_ as a gold standard for comparing residual infection after different treatment techniques and materials [7,30]. Yet, the main limitation of these studies is that they cannot be fully standardized due to variations in canal size and microanatomy. Furthermore, _E. faecalis_ infection can be very challenging to eliminate due to its high resistance to medicaments used during root canal treatment and its survival capability [29,31]. Clinically, various recommended solutions and techniques have been offered to disinfect the root canal system [26]. Among them, the most commonly used solutions are NaCl and EDTA. All agents used in this study are in clinical use for endodontic treatments. Long term use of NaOCl in the root canal can lead to adverse effects. However, in our study it was used for only 60 s, which does not cause any damage to the tooth [32]. EDTA may also be cytotoxic if applied for long periods, while its short application for 60 s as done in our study, does not have any significant side effects on the tooth [33]. A conventional syringe is the basic way to deliver the irrigation solution inside the root canal [34]. The disadvantage of this method is that it has little effect on the apical area [35] or inside the dentinal tubules [36]. As a result, several techniques have been tested to improve the efficiency of those solutions [37]. However, in most cases, it was found that microorganisms inside the complex root canal system can escape the disinfection effect of these solutions due to their ability to penetrate the dentinal tubules. Furthermore, it was also found that biofilm is highly resistant to different antibacterial agents. In the last decade, many studies have been conducted to investigate the effect of the Er:YAG laser with different solutions and techniques to disinfect the root canal system [38,39]. It was observed that using an Er,Cr:YSGG laser irradiation with 3% NaOCl inhibited the growth of _E. faecalis_ and pro  
Figure 5: The number of colonies after treatment on _E. faecalis_-infected teeth with Er:YAG laser-Endo tip. Group 1: The CFU of _E. faecalis_ in control infected samples without treatment (black). Group 7: The CFU of _E. faecalis_ after saline irrigation (blue). Group 5: The CFU of _E. faecalis_ after using an Er:YAG laser-Endo tip and saline irrigation (red). N = 8 in each group. * \(p\) < 0.01 for treated samples compared to control. vided complete elimination of the bacteria in all canals. Our previous study found that using an Er:YAG laser with 17% EDTA produced the best results for removing the smear layer [24]. This approach allowed the irrigation solution to reach the open dental tubule, enhancing its killing effect on the _E. faecalis_ biofilm. Laser-activated irrigation, with its new side-firing spiral and Endo tip, significantly enhances the effectiveness of root canal disinfection by cavitation effect [40]. The new Endo Tip was specially designed to fit the shape and volume of root canals after they were prepared. This design allows the Er:YAG laser beam to be delivered to the canal walls and its effectiveness can also be improved by adding a final rinse with NaCl [38; 41].