## 1 Introduction  
Creating an access cavity to reach the complex root canal anatomy is the first important step in non-surgical endodontic procedures. Obtaining information about the complex composition of the root canal system is imperative to comply with the principles to be followed during traditional and conservative access cavity preparation and to find solutions to the problems encountered [1]. The main goal of endodontic procedures is to adequately shape, clean, and obturate the root canal system to prevent apical periodontitis [2]. Recently, different designs of minimally invasive root canal access cavities have been proposed to remove as little tissue as possible to achieve the goal of root canal treatment. The goal of this concept is to increase the resistance of the tooth to cracks and fractures after root canal treatment by maximally preserving the healthy structure of the tooth during the preparation of the access cavity [3]. However, the smaller the access cavity, the more difficult the visibility and disinfection of the root canal system become, and the more difficult it can be to find, shape, clean, and obturate the root canals. At the same time, poor visibility because of the small access cavity may increase the risk of iatrogenic complications, which may affect treatment outcomes [4]. Lately, technological developments have brought innovations in the field of endodontics, such as new-generation rotary systems, magnification and lighting systems, and cone beam computed tomography (CBCT), that provide clinicians convenience during treatment. Using these technologies allows endodontic procedures to preserve healthy dental tissues more and prevent the loss of teeth [5]. Traditional access cavity (TAC) preparation aims to provide direct access to the root canal system. Direct access helps to increase biomechanical efficiency and prohibits errors that may occur in various treatment stages [6]. In contrast, dental tissue removed during TAC preparation can reduce the resistance of teeth to fracture under occlusal forces [7; 8]. Current developments recommend choosing the access cavity design that preserves more dentin and brings the endodontic-restorative relationship to an ideal state in terms of the preservation of tooth structure [9]. One of the most common causes of root canal treatment failure is reported to be the significant loss of dental tissue due to untreated caries and fractures of endodontically treated teeth [10]; this is also directly related to the amount of tissue lost and specific cavity configurations [11]. Conservative access cavity (CAC) preparation requires a dynamic approach that follows a conservative dental tissue removal procedure from the beginning of the procedure, which is expanded until adequate vision is achieved, rather than a predetermined access cavity form. This approach involves partial roof removal of the pulp chamber to protect the pulp horns, accompanied by cavity walls that are slightly concave and beveled towards the occlusal surface. This design aims to visualize the floor of the pulp chamber and all root canal openings from different angles [12], which means that clinicians can visualize the pulp chamber and floor by changing the angle of the mirror, though not at the same angle [13]. The minimally invasive endodontic concept, in addition to the differences in the access cavity, uses endodontic files with smaller tapers and innovative designs that aim to minimize the removal of tooth tissue [14]. Two different Ni-Ti instrument systems were used in this study. The first is One Reci (OR) (MicroMega, Besancon, France). This file is a recently released single-file system that works with a reciprocating motion. OR files undergo a heat treatment (C-wire) aimed at increasing the instrument's flexibility and ability of the file to stay at the center of the canal.