## 4 Discussion
The torsional stress of these pre-stressed files was tested, and it was discovered that files with larger diameters were more torsional load resistant [44]. As torsional stress behavior is often affected by cross-sectional design configuration, [PERSON] et al. used a mathematical model to simulate torsional and bending forces in two separate cross-sectional designs [51]. This consideration was also highlighted in the present study, where the optimal form is found to be that of instrument Endostar, which has the closer to circular shape of the fractured section from all three types of investigated instruments (as shown in Figure 8(a3), with values provided in Figure 4b,c). In contrast, the shape of the Protaper instruments is the most disadvantage regarding torsional loadings, as it is almost rectangular in their cross-section after fracture (as shown in Figure 8c3, with values provided in Figure 4b,c). When comparing reciprocating NiTi instruments to continuous rotary motion, most studies assumed that reciprocating NiTi instruments were superior [52; 53]. While comparing movements was not an aim of the present study, the instruments of Group Reciproc (which perform a reciprocating movement) were found to be slightly superior to those of Group Protaper (which use a continuous rotational movement), therefore the above theory was confirmed by the present study, as well. The fatigue tolerance of endodontic files can be affected by metal modifications or procedures, as well as by other manufacturing methods. Prior to usage, [PERSON] et. al. examined surface characteristics and associated them with fracture strength. They discovered that files with a lot of machining grooves were more prone to cyclic fatigue fracture. Spiral milling cutting flutes into NiTi blanks is how most NiTi files are produced [54]. This was confirmed in the present study, as well. In the concluding part of the present work, the values of shear stresses were found to be quite similar for the three groups of instruments--but considering in the analysis the variations in fracture lengths of instruments within the same study group. This means that the disadvantage given by the shape per section for the Reciproc and Protaper instruments are compensated either by size or by the intrinsic properties of the material from which they are made. While this conclusion seems to level things for users, following this experimental study, technical recommendations can be extracted for current practice: if a blockage of the first 2 to 3 mm part of the tip in the root canal can be anticipated (by the excessive curving of the tip of the instrument), it is highly recommended that the handpiece is adjusted to torque values that do not exceed 1.5 to 2.5 N \(\cdot\) cm for Endostar instruments. However, in case of such a blockage of the first 2 to 3 mm of the tip in the root canal, it is recommended that the handpiece is set to torque values that do not exceed 1 to 2 N \(\cdot\) cm for Reciproc or Protaper instruments. From a clinical point of view, one has to stress that this can mean the difference between success and failure for the endodontic treatment, while failure means a fracture of the instrument inside the dental canal, therefore a loss of the tooth for the patient.