## 1 Introduction  
Endodontic treatment aims to reduce the amount of microorganisms inside the root canal system in order to heal or prevent lesions of endodontic origin. Endodontic procedures involve chemo-mechanical treatment of the root canal system with irrigations to improve debridement and disinfection. The complex anatomy and microstructure of the root canals hinder the mechanical cleaning to reach all the canal spaces, resulting in microbes left behind [1]. Thus, the common cleaning procedures are unable to remove all the biofilm-covered surfaces and all the infected regions [2, 3, 4]. Biofilms are organized, surface-attached microbial communities embedded in an extracellular polymeric substance (EPS), which protects the microbes from external stress stimuli [5]. Mixed microbial biofilms can appear within the dental tubules and root canal spaces, which require interventions [6]. The Gram-positive _Enterococcus faecalis_ bacterium has been related to the failure of root canal treatment since the bacteria may survive the endodontic procedures and penetrate the dental tubules up to 800 microns [6]. Biomechanical preparation of the tooth root is accomplished by irrigation solution. The most commonly used solution in endodontic treatment is sodium hypochlorite (NaOCI) which is well-known to be highly potent at killing bacteria [7] and causes organic tissue dissolution [8]. However, NaOCI has unwanted side effects that can result in ecchymosis, tissue necrosis and paresthesia [9]. Another commonly used irrigation solution contains EDTA that chelates calcium ions, resulting in the removal of the mineralized portion of the smear layer [10]. Previous studies have shown that alternate irrigation with EDTA and NaOCI improve the removal of the infected region [11]. Since the conventional irrigation techniques have the limitation of not reaching all parts of the root canal as a result of its complex anatomy, new strategies with improved physiochemical properties have been developed. Already in 1994, [PERSON] [12] suggested that it would be preferable to develop a local treatment technique with antimicrobial activities that can reach the dental tubules, rather than using the systemic antibiotic approach. Laser technology presents a safe and efficient treatment for eradicating bacteria and removing the infected regions from root canals [13; 14]. The Er:YAG laser technique has become popular especially after its usage on teeth has been approval by the United States Food and Drug Administration (FDA) [13]. Importantly, the Er:YAG laser has a bactericidal activity against several bacterial species [15; 16]. According to [PERSON] et al. [17], improved anti-bacterial and anti-biofilm effects can be reached by increasing the Er:YAG laser energy-effective power (W), and by using a larger size tip, or by being in closer proximity to the biofilm. In this study, the Sapphire tip was used; however, this tip has several limitations. Using this technique, the emission of the laser beam is directed along the root canal and not necessarily laterally along the root canal walls. Thus, it is almost impossible to obtain a uniform 360-degree coverage of the internal aspect of the root canals [18]. In the present study, we propose to use a new side-firing spiral Endo tip to increase the coverage. The Endo tip is specially designed for cleaning and disinfecting the root canals during endodontic treatment and endodontic retreatment (Figure 1). The tip has a hollow, flexible, conical, and round cross-section with circumferential spiral slits throughout its length which fits the shape and volume of the processed root canals. The treatment is based on the delivery of Er:YAG laser light which is highly absorptive in water at approximately 1-3 \(\upmu\)m penetration depth [19].