Fatigue Resistance of Rotary Endodontic Files Submitted to Axial Motion in Multiplanar Canals Manufactured by 3D Printing

The objective of the research herein presented was to assess the time to fracture of Hyflex CM files subjected to rotational bending tests, with or without back and forth motion, in an apical canal similar to those existent in a real tooth with multiplanar curvatures. Therefore, a lower first molar was modelled and manufactured in an AISI 316L stainless steel using selective laser melting technology, featuring the required multiplanar curvatures, which were confirmed by radiography. Thereafter, endodontic files were submitted to either rotational bending tests without back and forth motion (Group A) or rotational bending tests with back and forth motion (Group B) inside an artificial root canal with primary, secondary and tertiary curvatures. Time to fracture was recorded and fragments of the endodontic files tested were observed by optical microscopy. Instruments of Group A showed an average time to fracture of 119 seconds and an average fractured tip length of 4.9 mm, while instruments of group B displayed an average time to fracture of 194.1 seconds (+63%) and an average fractured tip length equal to 4.13 mm. All instruments fractured due to fatigue crack propagation. Hence, it was possible to conclude that back and forth motion extended the fatigue lifetime of the files tested once allowed diminishing the number of cycles with higher stress range applied, and spread the induced stresses by an enlarged area of the instrument. Additionally, almost plane surfaces were observed at fractured cross sections of the instruments tested, allowing to infer a very low influence of torsional loading when compared with bending.