# Torsional Fracture Resistance of Niti Rotary Glide Path Files under Flexural Stress
## 4 Discussion  
In this study, the effect of flexural stress on the torsional resistance of NiTi glide path files was material dependent. PathFile and RaCe, which are manufactured without heat treatment, were not affected by flexural stress. In contrast, heat-treated NiTi files showed increased torsional resistance during flexural stress. RaCe Evo and HyFlex EDM showed significantly higher distortion angle and ultimate strength in flexural than straight modes. Similarly, TruNatomy Glider and V Taper 2H showed higher distortion angle under flexure. Although there was no significant difference, the mean ultimate strength of V Taper 2H was higher at straight mode compared to that of flexural mode (Table 2). The smooth, flawless surface of V Taper 2H might favorably act upon the ultimate strength in straight mode, yet further study is needed. Larger distortion angles under flexural stress in heat-treated instruments can be attributed to their phase composition. While PathFile and RaCe consist of austenite, HyFlex EDM contain austenite and martensite (Figure 3A,B,D). RaCe Evo, TruNatotomy Glider, and V Taper 2H are a mixed state of austenite, martensite, and R-phase (Figure 3C,E,F). When torsional fracture resistance testing was performed in flexural mode, V Taper 2H, RaCe Evo, and TruNatomy Glider, which possess some martensite and R-phase at room temperature, showed larger distortion angles than other files. The elastic modulus of R-phase is lower than martensite, which is lower than that of austenite [10]. Therefore, instruments containing martensite and R-phase can deform more readily under flexural stress. Indeed, charting of torque with distortion angle (Figure 4) showed that increased distortion angles from flexural stress involved plastic deformation, which are a warning sign for torsional fracture of the instrument. The mean distortion angle of RaCe was higher for straight mode compared to that of flexural mode, although the difference was not statistically significant (Table 3). The smooth, electropolished surface and alternating pitch length of RaCe might have affected the distortion angle in straight mode, yet further research is needed. In this study V Taper 2H showed the highest distortion angle and ultimate strength. This instrument's large diameter at 3 mm from the tip (D3), relatively short pitch length, and metallurgical properties contributed to the highest torsional fracture resistance. Estimated D3 size was largest for V Taper 2H (0.29 mm), followed by RaCe and RaCe Evo (0.27 mm), HyFlex EDM (0.24 mm), TruNatomy Glider (0.23 mm), and PathFile (0.22 mm). PathFile and RaCe, manufactured from conventional NiTi alloy, showed lower ultimate strength than those of heat-treated instruments, which agrees with previous reports that manufacturing method and metal mass affect torsional strength [14; 15]. The ultimate strength of TruNatomy Glider was higher than PathFile, and comparable to HyFlex EDM or RaCe Evo, which have larger D3 than TruNatomy glider. However, TruNatomy Glider had the shortest pitch length (Figure 2E), and thereby a greater number of threads than other instruments. More threads as well as square cross-sectional design ensured that TruNatomy Glider withstood torsional load despite its small diameter. This agrees with previous reports that NiTi files with more threads [16; 17; 18; 19], and instruments with larger core mass and square cross sections had higher torsional resistance [15; 20]. In contrast to flexural mode, for torsional resistance in straight mode, there were no significant differences in the distortion angles between PathFile, made from conventional NiTi alloy, and HyFlex EDM or TruNatomy Glider that are heat-treated instruments. PathFile exhibited superior flexibility in the 3-6 mm portion of the instrument due to smaller taper (2%), and more threads in the middle portion (Figure 2). Indeed, multiple factors including diameter, pitch length, and heat treatment affect distortion angles of NiTi glide path files [14; 15; 21; 22].