## 4 Discussion
The circumferential pattern is due to the Endo tip's special design that includes a flexibility zone (3 mm) with slits that allow the laser beam to be emitted laterally directly towards the walls of the root canal and generate rapid agitation of fluid [42]. The wavelength of the Er:YAG laser (2940 nm) is close to the absorption peak of water (3000 nm) [43], such that the energy of Er:YAG laser is maximally absorbed by the water-based irrigation solutions upon irradiation. This leads to explosive boiling irrigation, induction of cavitation bubbles in close proximity to canal walls during their collapses, generation of shear flows that are able to remove particles from the surface. These vapor bubbles create a volumetric expansion of 1600 times the original volume [44], which drives fluid along the root canal in a manner that improves smear-layer removal from the root-canal wall [45]. [PERSON] et al. [46] described and patented a procedure called Photon-Induced Photoacoustic Streaming (PIPS) for removing the smear layer and disinfecting root canals using an Er:YAG laser. Our previous study showed that irradiating closer to the biofilm induces a collateral bactericidal effect and reduces bacterial viability within the biofilm [17]. [PERSON] et al. [47] recently introduced shock wave-enhanced emission photoacoustic streaming (SWEEPS). [PERSON] et al. [48] described special laser tips placed only in the pulp chamber and advocated for cleaning while minimally shaping the root canal system. The results of the antimicrobial effect of the PIPS method in comparison to laser-activated irrigation are inconsistent [49]. The presents study was carried out on an _ex vivo_ model for evaluating microbial infection in the root dentin based on the model first introduced by [PERSON] and [PERSON] [50]. We found that combining laser activation with either 17% EDTA or 2.5% NaCl resulted in a statistically significant decrease in the number of bacterial colonies (\(p<0.01\)). The best antibacterial effect was achieved by combining the Er:YAG laser-Endo tip with 17% EDTA followed by 2.5% NaCl. This remaining CFU number using the Er:YAG laser-Endo tip was between 4% and 0.0004% when combined with EDTA alone or with both EDTA and NaCl, respectively. Moreover, the Er:YAG laser-Endo tip treatment significantly affected the CFU even when being used with only saline in the absence of any antiseptic solution (\(p<0.01\)). The influence of Er:YAG laser light absorbed by NaCl is very effective. The effect is likely caused by a physical mechanism. The Er:YAG laser light generates violent shock waves with greater liquid displacement. The most intense fluid motions are located near the fiber tip [51]. According to [PERSON] et al. [17], irradiating at closer proximity to the biofilm was found to significantly reduce bacterial viability within the biofilm (\(p<0.05\)). This observation explains the reason of using the novel Endo-tip in our research. Using fiber tip deeper inside the root canal resulted in better biofilm removal. The energy provided by the fiber tip is near the walls and along the entire root canal, generating shock waves that contribute to the efficacy of debridement and removal of the biofilm [52]. There are several safety limitations regarding the clinical use of laser energy with a Sapphire or optical fiber tip in intracanal treatment [53]. The energy from the tip is directed along the root canal and not necessarily laterally to the root canal walls [18]. Direct laser irradiation from the tip of the optical fiber in the vicinity of a tooth's apical foramen may result in the transmission of the laser beam beyond the foramen, which can be dangerous in proximity to the mental foramen or the mandibular nerve. [PERSON] [54] strongly recommended that the endodontic tip be improved to allow irradiation of all areas of the root canal walls. The rationale of a new Endo tip is to create ideal irradiation inside the root canal system, promote the efficacy of removing the smear layer and have a bacteriodic effect on bacteria inside the root canal.