2.1. Study Design
In this in vitro study, we utilized 50 unused conventional NiTi wire alloy endodontic rotary instruments (RaCe
®, La Chaux-De-Fonds, Switzerland), 25 mm in length with a triangular cross-section. All NiTi endodontic rotary files were analyzed using scanning electron microscopy (SEM) (HITACHI S-4800, Fukuoka, Japan) in the Department of Mechanical, Energetic, and Materials Engineering of the School of Industrial Engineering of the University of Extremadura (Badajoz, Spain) under the following exposure parameters: acceleration voltage: 20 kV, magnification from 100× to 6500×, and a resolution between −1.0 nm at 15 kV and 2.0 nm at 1 kV, to confirm the taper (
Figure 1) and apical diameter (
Figure 2) values of the previously selected NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland).
None of the NiTi endodontic rotary files (RaCe®, La Chaux-De-Fonds, Switzerland) were discarded after analyzing for possible manufacture defects which could influence the cyclic fatigue resistance of the NiTi endodontic rotary files (RaCe®, La Chaux-De-Fonds, Switzerland). A controlled experimental trial was performed at the Dental Centre of Innovation and Advanced Specialties at Alfonso X El Sabio University (Madrid, Spain), between November and December 2020. The NiTi endodontic rotary files were selected and categorized into the following study groups: A: 250 µm apical diameter and 2% taper (n = 10) (25.02); B: 250 µm apical diameter and 4% taper (n = 10) (25.04); C: 250 µm apical diameter and 6% taper (n = 10) (25.06); D: 200 µm apical diameter and 6% taper (n = 10) (20.06); E: 300 µm apical diameter and 6% taper (n = 10) (30.06).
2.2. Experimental Cyclic Fatigue Model
The selected NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) were used in a custom-made device (utility model patent number ES1219520), designed by computer-aided design/computer-aided engineering (CAD/CAE) 2D/3D software (Midas FX+
®, Brunleys, Milton Keynes, UK) and created using 3D printing (ProJet
® 6000 3D Systems
©, Rock Hill, SC, USA) (
Figure 3A–D).
The endodontic rotary file (RaCe, La Chaux-De-Fonds, Switzerland) was submitted to micro computed tomography (Skyscan 1176, Bruker-MicroCT, Kontich, Belgium) with the following exposure parameters: 160.0 kilovolt peak, 56.0–58.0 microamperes, 500.0 ms, 720 projections, four frames, a tungsten target between 0.25 and 0.375 mm, a 3 µm resolution, and a pixel size of 0.127 µm, to obtain a standard tessellation language digital file that allowed the design of an accurate artificial root canal regarding the outer measurements of the selected endodontic rotary file (RaCe, La Chaux-De-Fonds, Switzerland), which ensured intimate contact.
Each one of the previously selected NiTi endodontic rotary files (RaCe, La Chaux-De-Fonds, Switzerland) were introduced in a custom-made artificial root canal designed with a 60° curvature according to Schneider’s measuring technique [
23] and 3 mm radius of curvature, based on the outer geometry (taper and cross-sections diameters) of each NiTi endodontic rotary file (RaCe, La Chaux-De-Fonds, Switzerland). The artificial root canals were designed using computer aided design/computer aided engineering (CAD/CAE) 2D/3D software (Midas FX+
®, Brunleys, Milton Keynes, UK) and manufactured by electrical discharge machining (EDM) molybdenum wire-cut technology (Cocchiola S.A., Buenos Aires, Argentina) from a stainless steel piece 2 mm in width.
The speed of the up-and-down movement of the artificial root canal were generated by the brushed DC gearmotor (Ref.: 1589, Pololu® Corporation, Las Vegas, NV, USA) regarding the signals emitted by the driver (Ref.: DRV8835, Pololu® Corporation, Las Vegas, NV, USA), which performed an H-bridge function that managed the speed of the up-and-down movement through Pulse Width Modulation (PWM) signals emitted by four switches modulated by transistors. The movement generated by the brushed DC gearmotor (Ref.: 1589, Pololu® Corporation, Las Vegas, NV, USA) was transferred to the artificial root canal support through a roller bearing system (Ref.: MR104ZZ, FAG, Schaeffler Herzogenaurach, Germany). The artificial root canal support moved in a pure axial motion through a lineal guide (Ref.: HGH35C 10249-1 001 MA, HIWIN Technologies Corp. Taichung, Taiwan).
The time to failure of the NiTi endodontic rotary files was detected by a light-dependent resistor (LDR) sensor (Ref.: C000025, Arduino LLC
®, Ivrea, Italy) located at the apex of the artificial root canal, which received the continuous light source emitted by a high-brightness white light-emitting diode (LED) (20,000 mcd) (Ref.: 12.675/5/b/c/20k, Batuled, Coslada, Spain), which was located opposite to the artificial root canal. The light signals emitted by the LED sensor were detected by the LDR (Ref.: C000025, Arduino LLC
®) sensor with a frequency of 50 ms to accurately identify the time of failure. The NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) were used with a 6:1 reduction handpiece (X-Smart Plus, Dentsply Maillefer) and torque-controlled motor with continuous rotation at 1000 rpm and 1.5 N/cm torque, according to the manufacturer’s instructions [
24].
The reduction handpiece (X-Smart Plus, Dentsply Maillefer, Ballaigues, Switzerland) was submitted to an industrial scan (3D Geomagic Capture Wrap, 3D Systems©, Rock Hill, SC, USA) to obtain an STL digital file, which allowed the design (Midas FX+
®, Brunleys) and manufacture (ProJet
® 6000. 3D Systems©, Rock Hill, SC, USA) of a custom-made support piece placed on the top of the cyclic fatigue testing device to prevent undesirable movements of the reduction handpiece (X-Smart Plus, Dentsply Maillefer) and, hence, the NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) inside the artificial root canal. (
Figure 4).
The NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) were used until fracture occurred to analyze the effect of the taper and apical diameter on the resistance of NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) to cyclic fatigue. All NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) were used in the dynamic cyclic fatigue device at a frequency of 60 pecking movements/min, according to a previous study [
25]. To reduce the friction between the reciprocating files and the artificial canal walls, special high-flow synthetic oil designed for the lubrication of mechanical parts (Singer All-Purpose Oil; Singer Corp., Barcelona, Spain) was applied. All NiTi endodontic rotary files (RaCe
®, La Chaux-De-Fonds, Switzerland) were used until fracture occurred. The time to failure, the number of cycles to failure, the number of cycles of in‑and‑out movements, and the length of the fractured file tip were measured and recorded.