<div>ABSTRACT</div><div><b>Introduction:</b> This study aimed to evaluate the cyclic and
torsional fatigue resistance of a new nickel-titanium (Flat File 25.04)
instrument on continuous and reciprocating motion. <b>Methods:</b>Sixty instruments of the ProDesign Logic2 25.03 and 25.05 (Easy
Equipamentos Odontológicos, Belo Horizonte, Brazil), and MK Flat File
25.04 (n=20) (MK Life, Porto Alegre, Brazil) were used. For the cyclic
fatigue test, an artificial stainless steel simulated canal with an
angle of 60° and a radius of curvature of 5mm located 5mm from its tip
was used. Torque and angle of rotation at failure of instruments on
torsional fatigue test was based on the ISO 3630-1 protocol, in which
the 3mm tip of each instrument was fixed and connected to an electric
motor and a load cell. The fractured surface of each fragment was
examined by scanning electron microscopy. Data were analyzed using 1-way
analysis of variance and Tukey’s test with a significance level of 5%.<b>Results:</b> Flat File 25.04 had lower cyclic fatigue in both
kinematics than the Logic instruments (P &lt; .05). Reciprocating
motion improved the cyclic fatigue of the tested instruments (P
&lt; .05). Flat File 25.04 had similar torque to Logic2 25.05 (P
&lt; .05), and both were superior to Logic2 25.03 (P &lt;
.05). The angular deflection values ​​were different for the three
tested instruments (P &lt; .05), in the decreasing order: Logic2
25.03, 25.05, and Flat File 25.04. <b>Conclusion:</b> Flat File
presented acceptable resistance to cyclic and torsional fatigue
resistance. Reciprocating motion improved the cyclic fatigue resistance
of the instruments and can be considered when using programmable motors.</div><div>Keywords: Endodontics. Cyclic fatigue. Torsional resistance. NiTi
instruments.</div><div>INTRODUCTION</div><div>Conventional nickel-titanium (NiTi) alloy instruments present super
elasticity and memory control characteristics, predominantly in the
austenite phase. However, when induced by temperature drops or
mechanical stress application, they change to martensite, acquiring more
flexibility [1,2].</div><div>However, these instruments are subject to failure, for example,
torsional or cyclic fatigue [3]. Thus, technologies that prioritize
variations in kinematics, cross-sectional design, and surface treatments
have been proposed to improve their mechanical properties and,
consequently, their clinical performance and safety of use [4-8].</div><div>Recently, a new NiTi instrument was created with the trade name of Flat
File (MK Life, Porto Alegre, Brazil). This instrument has a Gold heat
treatment and is available in a sequence of three instruments: 20.04,
25.04, and 35.04. It has an innovative design, with a flat side, without
cutting blades, to allow more space for the irrigating solution, escape
for debris, and less pushing effect of the instrument inside the root
canal.</div><div>NiTi endodontic instruments are constantly subjected to tension and
compression forces during root canal preparation, especially in
accentuated curvatures. In addition, due to the simplification of the
preparation technique, often, when only one instrument is used, there is
a possibility that the tip of these instruments is in close contact with
the root canal walls and, therefore, subject to torsional fractures
[6].</div><div>Due to this, mechanical tests of cylic and torsional fatigue should be
performed, and the results compared with existing instruments before
their clinical use. Therefore, the objective of this study was to
compare a new NiTi instrument (Flat File) with another instrument with
similar dimensions (ProDesign Logic2) regarding its mechanical
properties of torsional and cyclic fatigue on the continuous and
reciprocating motion [7,8].</div><div>MATERIALS AND METHODS</div><div>Sample size determination was performed using G*Power 3.1 for Windows
(Heinrich Heine, University of Düsseldorf) and the Wilcoxon-Mann-Whitney
test from the ”t” test group. An alpha-type error of 0.05, a beta-type
power of 0.95, and an N2/N1 ratio of 1 were also adopted [8].</div><div>For the tests, a total of 60 NiTi instruments were selected: ProDesign
Logic2 25.03 (n=20) and 25.05 (n=20) (Easy Equipamentos Odontológicos,
Belo Horizonte, Brazil) and MK Flat File 25.04 (n=20) (MK Life, Porto
Alegre, Brazil). All instruments were inspected for possible defects or
deformations under a microscope (Alliance, São Paulo, Brazil) at 16x
magnification. Flat File instruments are sold in a sterile blister, with
no need for sterilization prior to testing. ProDesign Logic2 instruments
have been subjected to a sterilization process according to the
manufacturer’s standards to simulate a precondition of clinical use.</div>