## 2 Materials and Methods
### Mechanical Tests  
Ten specimens of each NiTi file type were subjected to the following four mechanical tests: bending, buckling, cyclic fatigue, and torsional resistance. All mechanical tests and canal shaping were performed using a universal testing machine (UTM; Universal Mechanics Analyzer, IB Systems, Seoul, Republic of Korea).  
The cantilever bending test was conducted based on ISO 3630-1:2019 [17; 18], using the UTM to evaluate the bending resistance of the instruments. An apical 3 mm length from the tip of the NiTi file was tightly clamped into a chuck connected to the load cell of the UTM. The file tip was bent clockwise at 2 rpm. The bending resistance (Ncm) was recorded as the torque value when the instrument was rotated at 45\({}^{\circ}\).  
The buckling resistance test was performed using a lithium disilicate block (IPS e.max CAD; Ivoclar Vivadent, Schaan, Liechtenstein) having a small dimple. The shaft of the NiTi file handle was fixed to the UTM, and the instrument tip was in contact with a cube block. The small dimple helped restrain the file tip on the block. The file was moved downward at a crosshead speed of 1.2 mm/s. The buckling resistance (gf) was recorded as the maximum load during a 1-mm axial movement of the file.  
The cyclic fatigue resistance was evaluated using a customized jig with a curved canal made of stainless steel, which was connected to the load cell of the UTM. The artificial canal had a 1.5-mm diameter, and the curvature had a radius of 2 mm and an angle of 45\({}^{\circ}\), according to [PERSON]'s method [19]. A file was inserted into the artificial curved canal of the jig to a depth of 19 mm. The instruments were operated using an X-Smart motor (Dentsply Sirona), and the handpiece of the motor was firmly fixed to the UTM. The instrument was rotated according to the manufacturers' recommended rotational speed: 500, 800, 200, and 200 rpm for TruNatomy Prime, XP-endo Shaper, Spring Endo, and Spring H instruments, respectively. The time at which a file separated was recorded. The number of cycles to fracture (NCF) was calculated by multiplying the speed of each instrument in rpm with the fracture time.  
The torsional resistance test was performed based on ISO 3630-1:2019 [17; 18]. An apical 3 mm length from the tip of the NiTi file was tightly clamped into a chuck connected to the load cell of the UTM. The shaft of the instrument was fastened to an opposing chuck, which was rotated clockwise at 2 rpm until the file fractured. The ultimate torsional strength (UTS) and angle of rotation to fracture (ARF) were recorded as the clockwise torque and angle of rotation, respectively, when the instrument fractured. The clockwise torque and angle of rotation recorded in real time were plotted. In the graph, the slope of the elastic deformation section which occurred at the beginning of the torsional resistance test was determined as the elastic modulus (EM).  
After the cyclic fatigue and torsional resistance tests, the fractured instruments were cleaned with 70% alcohol and subjected to fractographic analysis using field-emission scanning electron microscopy (FE-SEM) (JSM-7800F Prime; JEOL Ltd., Akishima, Tokyo, Japan). Different magnifications (\(\times\)180, \(\times\)200, \(\times\)250, and \(\times\)1000) were used for FE-SEM analysis.  
The data of the mechanical test results (bending and buckling resistances, NCF, UTS, ARF, and EM) were normally distributed according to the Shapiro-Wilk test, while the homogeneity of the variances was not satisfied by [PERSON]'s test. One-way ANOVA was performed to compare the mechanical test results between different file types; intergroup comparison was performed with the Games-Howell test. [PERSON]'s correlation analysis was conducted among the six variables of the mechanical test results, i.e., bending and buckling resistances, NCF, UTS, ARF, and EM. Statistical analysis was performed using SPSS 25 (IBM, Armonk, NY, USA) at a 95% significance level.