## 1 Introduction  
The elimination of the bacterial population from the root canal system represents the most essential phase of endodontic treatment [1; 2]. The presence of persistent bacteria within the root canals after chemomechanical procedures is a key reason for endodontic failure [3; 4]. _Enterococcus faecalis_ is a Gram-positive, facultative anaerobic bacterium, commonly isolated from persistent root canal infections [5; 6]. The prevalence of _E. faecalis_ is usually high in cases of unsuccessful root canal treatment owing to the different virulence factors expressed by the bacterium. _E. faecalis_ can penetrate into dental tubules at different levels using these virulence factors and resist removal even after preparation [7]. Due to its highly virulent nature, the elimination of the bacterial population during root canal treatment is accomplished mostly via a combination of mechanical instrumentation, irrigation procedures, and intracanal medicaments [8]. Various improved nickel-titanium instrumentation systems with different characteristics have been developed to improve the procedure of intracanal bacterial reduction [9; 10]. The ProTaper Gold (Dentsply Maillefer, Ballaguues, Switzerland) file system is produced using M-Wire technology and employs a continuous rotational movement. Each file has a progressive taper shape and a convex triangular cross-sectional design that has been claimed to enhance its cutting efficiency [11]. Following this evolution, the TruNatomy (TRN; Dentsply Maillefer, Ballaigues, Switzerland) multi-file system was designed, featuring a small taper size for conservative preparations. This system has different taper sizes to protect the structural dentin in narrow endodontic access cavities. The manufacturer has suggested that TruNatomy files are more flexible and fatigue-resistant due to their unique design, which is provided by a special heat treatment [12; 13]. In 2017, the new 2 Shape file system was introduced by Micro-Mega using a Ni-Ti alloy named T-wire. The manufacturers produced this system with a unique cross-sectional design comprising a modified triple helix equipped with two main cutting edges for cutting efficiency, and an offset blade to enhance debris removal and increase the effectiveness of efficient selective cleaning [13]. In recent years, single file systems have also been developed. In these systems, the files perform a reciprocation motion, which requires special motors. Such systems include WaveOne Gold (Dentsply Maillefer, Ballagigues, Switzerland) and Reciproc Blue (RCP; VDW GmbH, Munich, Germany). The WaveOne Gold has been produced using a heat-treated gold metal alloy. It has a unique off-centered parallelogram cross-sectional shape with two cutting edges [9; 14], whilst the Reciproc Blue system has an S-shaped horizontal cross-sectional design, also with two cutting edges [15]. Selection of the appropriate access cavity preparation is important for efficient instrumentation and irrigation, and has an impact on the success of endodontic treatment [1]. The aim of the traditional endodontic access cavity (TEC) preparation is to achieve complete unroofing of the pulp chamber, to ensure that all the pulp horns are exposed, and to provide straight-line access to the root canals without undercuts. This cavity design can enable visualization of the pulp chamber and all of the root canal orifices, prevent errors during preparation, and increase the efficiency of irrigation. However, it also decreases the fracture resistance of the tooth due to the removal of a large amount of dentin tissue [16]. A minimally invasive endodontic access concept has been presented to preserve the roof of the pulp chamber and pericervical dentin as much as possible, and to decrease the fracture risk of teeth that have undergone endodontic treatment (1, 17). The mentioned concept has two types of access cavity design. The aim of the conservative endodontic access cavity (CEC) is to partially deroot the pulp chamber and preserve the pulp horns.