Strength properties of rotating nickel-titanium endodontic instruments in terms of resistance to cyclic fatigue: A nonrandomized experimental study

Background. Clinicians frequently encounter difficulties when selecting nickel-titanium instruments for specific clinical cases, primarily due to a lack of knowledge of what should be considered. Despite all the information available in the literature, the algorithm for making a decision is difficult to find. However, the bending angle and curvature radius of the root canal are known as essential factors to consider when diagnosing the anatomical and topographical features of tooth root canals during endodontic treatment.

Objective. To evaluate the strength properties of rotating nickel-titanium endodontic instruments by conducting a comparative experimental study on their resistance to cyclic fatigue.

Methods. A nonrandomized experimental study was conducted using two systems of rotating nickel-titanium instruments, the RM file (Rusmed, Russia) and ProTaper Universal (Dentsply Sirona, USA). Fusion 360 (Autodesk, USA) 3D modeling software was used to design root canal phantoms, which are blocks with a series of 5 channels for each instrument size, simulating a 45° root canal angle, a curvature radius of 5 mm, with a change in taper for each specific instrument, taking into account a magnification factor of 1.05. The following values were taken as the tip diameters for each instrument: S1 = 0.17 mm, S2 = 0.2 mm, F1 = 0.2 mm, F2 = 0.25 mm, F3 = 0.3 mm. Blocks with individual instrument sizes were made from metal using selective laser sintering. The blocks were fixed to a rigid canvas, and then the endodontic instruments under investigation were inserted through the canal openings along their entire length. Rotation was performed by an endomotor at a constant speed of 350 rpm, with a torque of 2.0 N/cm. Number of instrument cycles before failure was measured, recording the time of instrument failure. Next, surface wear of instrument fragments was studied using a JEOL JSM-7500F scanning electron microscope (Tokyo Boeki, Japan). The primary data base was created in Excel 2016 (Microsoft Corporation, USA). Hypothesis testing and sample comparison were performed using a JASP 0.19.1. open source software (JASP, Netherlands). Parametric criteria (Student’s t-test for independent samples) were used in the statistical data processing, and statistically significant differences were accepted at a level of p ≤ 0.05.

Results. For the RM file system, S1 instrument showed an average number of rotation cycles of 292.00 ± 38.45, with a minimum value of 227 cycles and a maximum value of 340 cycles. S2 instrument demonstrated higher performance with an average value of 345.00 ± 62.72 cycles, ranging from 266 to 438 cycles. F1 instrument exhibited an average resistance of 256.00 ± 31.92 cycles, ranging from 188 to 291 cycles. The lowest values among the RM file instruments were recorded for F2 (111.00 ± 48.34 cycles, range 40–184) and F3 (82.00 ± 26.55 cycles, range 43–130). For the ProTaper Universal system, S1 instrument showed the highest performance with a mean of 395.00 ± 38.12 cycles, with a minimum of 306 cycles and a maximum of 450 cycles. S2 instrument of this system had an average durability of 170.00 ± 21.55 cycles with a range from 144 to 202 cycles. ProTaper Universal F-series instruments demonstrated relatively low performance: F1 had an average of 72.00 ± 10.75 cycles (58–91), F2 with a 86.00 ± 22.06 cycle (60–120), and F3 with a 79.00 ± 26.33 cycle (38–116).

Conclusion. The data obtained demonstrate the advantage of the RM file system over ProTaper Universal in terms of resistance to cyclic fatigue. This system can be successfully used for treating complex root canals, including cases of pronounced curvature. Key factors in this case include using instruments within the manufacturer’s specified service life, maintaining optimal rotation speed, ensuring adequate irrigation, and controlling the force applied.

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