## 2 Materials and Methods  
### Specimens and Procedures  
For this in vitro experiment, fifty (n = 50) extracted human molars were selected, comprising thirty-five (35) maxillary and fifteen (15) mandibular teeth, all characterized by curved mesial or buccal canals. The teeth were sourced from a private dental center and the Department of Oral Surgery at the University of Szeged, Faculty of Dentistry. No information regarding patient age, gender, or general health conditions was collected for this study. The exclusion criteria for more detailed analysis included teeth with an indeterminate apex, those previously treated endodontically, teeth with root caries or resorption, fractures below the cemento-enamel junction (CEJ), and teeth with straight roots. All selected teeth were cleaned of calculus and debris, disinfected using a 2% sodium hypochlorite solution (NaOCI) for twenty minutes, and stored in a 10% formaldehyde solution (CH2O) for no more than six weeks. The teeth were positioned in colorful modeling clay (iclay IC 400 BK, AMOS Corporation, Dongtan, Hwaseong, Republic of Korea) formed into various shapes, each with at least one flat side. The specific shape was chosen to ensure stability during radiography, enabling each tooth to remain steady on a flat surface and ensuring that the X-ray tube was consistently perpendicular to both the film and the tooth plane. This approach ensured sample uniformity and measurement reliability. In greater detail, for the maxillary teeth, the palatal root faced the flat side of the base, while for the mandibular teeth, the lingual surface was aligned similarly. Additionally, three lead balls, each with a diameter of 1.5 mm, were embedded within the clay base to serve as reference points for the statistical analysis of the experiment. These markers were used at the end of the experiment to recalibrate the X-rays, ensuring there was no change in angle or shift in distance. The prepared bases were allowed to dry and harden for approximately 12-14 h.  
All teeth were digitally radiographed using the Soredex Digora Optime system (Dental Imaging Technologies Corporation, Hatfield, PA, USA) in the bucco-lingual plane, employing the standardized paralleling technique to minimize radiographic distortion. Periapical X-rays were captured using the DEXIS FOCUS(tm) X-ray unit (Dental Imaging Technologies Corporation, USA) with an exposure time of 0.114 s at 70 kV and 7 mA. Furthermore, all radiographs were taken at predetermined distances: the focus-to-sensor distance was 27 cm, and the object-to-sensor setup ensured each tooth lay on the radiographic film on a horizontal plane. In the subsequent steps, only the canals of interest were prepared: the mesial canals (mesiobuccal and mesiolingual) in mandibular molars and the buccal canals (mesiobuccal and distobuccal) in maxillary molars. In total, one hundred (100) canals were prepared (20 canals/group). All canals were pre-flared using Gates-Glidden drills sized 2 and 3. The working length of each canal was determined radiographically through a file control X-ray, assisted by an Initial Apical File. A chelating agent (Glyde File Prep RC Conditioner, Dentsply Sirona, Charlotte, NC, USA) was applied to lubricate the canals. The teeth were randomly divided into five groups, each containing ten teeth. Each group was treated with a different mechanical system, and only the curved canals of interest were prepared (mesiobuccal and distobuccal canals in maxillary teeth and mesiolingual canals in mandibular teeth). The chemical preparation followed [PERSON]'s protocol [29; 30]. The root canals in Group 1 were prepared using the Step-Back method [31; 32]. At the end of the procedure, a Hedstrom file of master apical file (MAF) size or one size smaller was used to refine the prepared steps along the canal walls [33].