ABSTRACT
Introduction: 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. Methods: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%.Results: Flat File 25.04 had lower cyclic fatigue in both kinematics than the Logic instruments (P < .05). Reciprocating motion improved the cyclic fatigue of the tested instruments (P < .05). Flat File 25.04 had similar torque to Logic2 25.05 (P < .05), and both were superior to Logic2 25.03 (P < .05). The angular deflection values ​​were different for the three tested instruments (P < .05), in the decreasing order: Logic2 25.03, 25.05, and Flat File 25.04. Conclusion: 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.
Keywords: Endodontics. Cyclic fatigue. Torsional resistance. NiTi instruments.
INTRODUCTION
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].
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].
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.
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].
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].
MATERIALS AND METHODS
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].
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.