**5. Conclusions**

This numerical study confirms that the geometry of the root canal affects the fatigue life of rotary NiTi instruments. More curved root geometries, either by a higher inclination of the apical part of the canal with respect to the initial part at the entrance or by a lower radius of curvature, result in higher strain amplitudes in the file surface and to lower fatigue life. The radius of curvature in the curves of the root canal has a greater effect than the angle of curvature. Changes in the radius of curvature from 15 mm to 5 mm in the root canal can reduce fatigue life by factors close to ten. A change in the file pitch between 2 mm and 3 mm does not have an important effect on the fatigue life for the root canal geometries analysed, although the higher pitch exhibited a slightly lower life for root canals with low curvature. The degree of insertion of the file inside the root canal significantly affects the strain range obtained in the critical point of the file, and the strain range is reduced when the file is partially inserted within the file.

**Author Contributions:** Conceptualization, A.P.-G. and V.R.-C.; methodology, A.P.-G. and V.R.-C.; software, V.R.-C.; investigation, A.P.-G. and V.R.-C.; resources, Á.Z.-M. and V.F.-M.; writing—original draft preparation, A.P.-G.; writing—review and editing, V.R.-C.; supervision, Á.Z.-M. and V.F.-M.; project administration, Á.Z.-M. and V.F.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research received no external funding.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable. **Conflicts of Interest:** The authors declare no conflicts of interest.
