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The purpose of this paper is to assess the fatigue life of different sizes of HyFlex CM™ endodontic files when submitted to planar or to non-planar curvatures, and to two different rotational speeds, namely 500 rpm or 250 rpm. The influence of superimposing back and forth motion to rotational bending of endodontic files was also assessed.

In all, 64 HyFlex CM™ files of different sizes, namely ref. 0.04/20, 0.06/20, 0.04/35 and 0.06/35, were submitted to rotational bending tests at two different rotational speeds. The planar radius of curvature imposed to the endodontic files was about 4.5 mm, along an angle of 45°, in order to simulate an apical canal that would induce severe loading to the files during clinical treatment. Additionally, 11 Hyflex CM™ files ref. 0.04/20 were submitted to rotational bending tests at 500 rpm inside a mandible first molar manufactured through selective laser melting (SLM), aiming to simulate non-planar curvatures of a real tooth canal.

When considering planar curvature, the endodontic file ref. 0.06/20, tested at 250 rpm, has shown the highest fatigue resistance (4,185 revolutions, 1,004 seconds), while the lowest fatigue resistance was registered for instrument ref. 0.04/35 submitted to 500 rpm (747 revolutions, 89 seconds). Hence, depending on the rotational speed, surface finish and the size of the endodontic files tested (taper and tip’s diameter), significant differences in fatigue resistance were noticed. If non-planar curvatures were considered, the minimum fatigue resistance was equal to 107 seconds, and back and forth motion allowed extending the minimum fatigue lifetime to 140 seconds.

The fatigue resistance of endodontic files is frequently determined through in vitro fatigue tests carried out under single planar curvature or eventually under double planar curvatures. However, non-planar loadings are frequently induced on endodontic instruments when treating root canals with severe multiplanar curvatures. In the research herein presented, a mandible first molar was 3D printed by using the SLM technique in an AISI 316 L stainless steel and more realistic experimental fatigue tests were carried out.

The purpose of this paper is to improve the surface finish and, therefore, the fatigue resistance of some endodontic files by electrochemical polishing; the influence of applying an autoclave’s sterilising cycle to the endodontic files under study was also assessed during the research herein presented.

In total, 22 Ni-Ti endodontic instruments of two sizes, namely, ref. Hyflex 0.04/20 and 0.06/20 (taper: 0.04; 0.06 and tip diameter: 0.20 mm, respectively), were submitted to rotational bending tests at 500 rpm; the radius and the angle of curvature imposed to the endodontic instruments during the experimental tests were equal to 4.7 mm and 45°, respectively. In addition, 18 endodontic files (out of the 22) were subjected to electrochemical polishing before in vitro fatigue tests had been carried out. Several combinations of electrochemical parameters were considered during the preliminary experiments involving electrolyte, voltage, flow rate and duration. The endodontic files under study were weighted using a high-precision balance before and after polishing, and mass variation was determined. Also, a sterilising cycle was applied to six polished endodontic instruments, which were placed inside an autoclave at 134°C, for 30 min, and under a pressure of 2.16 bar.

The fatigue resistance of as-received endodontic files ref. 0.04/20 is much larger (+480 per cent) than the fatigue strength of endodontic files ref. 0.06/20. The weight variation due to electropolishing was comprehended between 0.53 mg and 1.01 mg, and the fatigue resistance of all polished endodontic instruments increased between 30 per cent (0.06/20) and 80 per cent (0.04/20) when compared with the fatigue resistance of the as-received files. The application of an autoclave’s sterilisation cycle had a negative impact on the fatigue resistance of instruments ref. 0.04/20 and a positive impact on the durability of instruments ref. 0.06/20.

The application of an electrochemical polishing process increased the fatigue resistance of the endodontic files under study significantly, and the values for the variables of the electrochemical process were never published elsewhere. The surface finish improvement was obtained by immersing the endodontic files in a Struers’ Electrolyte (ref. E2) for 3 s while applying 30 volts and a flow rate of 1.