## 1 Introduction  
Despite the continuous, steep evolution of dental technology, instruments and methods, a successful endodontic treatment is still a multifactorial and challenging procedure. A decreased or even non-existing field of vision to work, and limited access, especially in molar region and unique root canal anatomy, are some of the obstacles that a dentist should overcome during the treatment. The availability of 3D imaging in everyday practice has reduced the number of failures due to unexpected anatomical structures or pathological conditions [1, 2]. However, certain situations, even with the help of a 3D image, modern root canal treatment techniques and knowledge, cannot be solved from an orthograde direction [3]. Over the years, several different theories, instruments and materials have been developed. One of the most important steps for a successful root canal treatment is the chemo-mechanical preparation. It is carried out with a combination of mechanical instruments and thorough irrigation [4; 5]. The existence of an intracanal and periradicular biofilm is well known from the study of [PERSON] et al. [6]. Because of the presence of the bacterial biofilm and the complexity of root canal anatomy--especially in the apical third [2]--proper cleaning is a challenging goal for the endodontist. The preservation of the original root canal anatomy is essential; however, without shaping and smoothening the inner irregularities, the removal of the infected dentin and the success of irrigation are compromised. The aim is to avoid iatrogenic errors and provide sufficient disinfection [7]. In curved canals, the incidence of complications is higher, especially for apical transportation and divergence from the tooth's original axis [8; 9; 10]. These errors can lead to poorly cleaned or over-instrumented canals, resulting in insufficient cleaning and obturation [11]. Fortunately, most of these anatomical alterations or iatrogenic errors are not fatal due to the evolution of modern microsurgical endodontics; however, mastering orthograde root canal treatment and instruments is a cornerstone of prevention. Direct access to the root canal is a prerequisite of safe treatment. Up to a certain curvature, with the help of coronal preflaring, safe cleaning of the middle and apical third is provided [12]. Several methods were introduced over the last 60 years to determine the angle of the root curvature and classify teeth according to it [13; 14; 15; 16]. [PERSON]'s methodology is one of the most widely accepted for the determination of the curvature of canals. It is easy to carry out and of great accuracy in its results. For its determination, firstly, a line is drawn parallel to the long axis of the canal and a second line is then made from the apical foramen to intersect with the first one. The angle measured is the one that is formed between the intersection of these two lines. With this method, he categorized the teeth into straight root canals (<5\({}^{\circ}\)), slightly curved (10-20\({}^{\circ}\)), and severely curved ones (25-70\({}^{\circ}\)) [17]. In order to improve the quality and prognosis of a root canal treatment, in the 1900s, nickel-titanium (Ni-Ti) instruments were introduced to replace stainless-steel ones [18]. Endodontic files constructed from this alloy are more advantageous due to their better fracture resistance and elastic flexibility compared to their precedents [19]. Their mechanical properties and intraoral behavior are mainly influenced by their chemical composition and/or way of manufacturing [20; 21]. However, among others, their superelastic (SE) property and shape-memory effect (SME) can be challenging and can cause deformation of the original canal shape when not used properly [22; 23]. Techniques based on different moving patterns, such as reciprocating, centric rotary with reciprocating, eccentric rotary or transaxial motion and optimum glide path (OGP), aim for a more predictable clinical outcome [24; 25; 26; 27].