Torsional Resistance of Heat-Treated Nickel-Titanium Instruments under Different Temperature Conditions

Advancing Nitinol: From heat treatment to surface functionalization for nickel–titanium (NiTi) instruments in endodontics

Nickel-titanium (NiTi) alloy has been extensively researched in endodontics, particularly in cleaning and shaping the root canal system. Research advances have primarily focused on the design, shape, and geometry of the NiTi files as well as metallurgy and mechanical properties. So far, extensive investigations have been made surrounding surface and thermomechanical treatments, however, limited work has been done in the realm of surface functionalization to augment its performance in endodontics. This review summarizes the unique characteristics, current use, and latest developments in thermomechanically treated NiTi endodontic files. It discusses recent improvements in nano-engineering and the possibility of customizing the NiTi file surface for added functionalization. Whilst clinical translation of this technology has yet to be fully realized, future research direction will lie in the use of nanotechnology.

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Nitinol (Nickel Titanium alloy) is widely used to clean/shape root canal system in endodontics.

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To enhance its performance, various thermo-mechanical and nano-engineering modifications have been performed.

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This comprehensive review summarizes the latest advances and future trends relating to functionalized NiTi endodontic files.

Evaluation of design, mechanical properties, and torque/force generation of heat-treated NiTi glide path instruments

BACKGROUND

Recently, various kinds of heat-treated nickel-titanium (NiTi) glide path instruments have been manufactured. This study aimed to investigate design, phase transformation behavior, mechanical properties of TruNatomy Glider (#17/02), V Taper 2H (#14/03), and HyFlex EDM (#15/03) and compare torque/force generated during simulated glide path preparation with them.

METHODS

The designs and phase-transformation behaviors of the instruments were examined via scanning electron microscopy (n = 3) and differential scanning calorimetry (n = 2). Their bending (n = 15), torsional (n = 15), and cyclic fatigue resistances (n = 15) were tested. The ultimate strength and distortion angle were obtained from torsional resistance test. The number of cycles to failure (NCF) was calculated from cyclic fatigue resistance test. The preparation of the glide path was simulated using a double-curved artificial canal (n = 15), and the maximum torque and screw-in forces were measured. Data except NCF was compared between brands with one-way ANOVA with Tukey's honestly significant difference test. NCF was analyzed via Kruskal-Wallis and Mann-Whitney U tests.

RESULTS

TruNatomy Glider had the greatest number of threads. TruNatomy Glider showed progressive taper, while V Taper 2H and HyFlex EDM had constant taper. The austenitic transformation-finish temperatures of all the instruments were above body temperature. V Taper 2H demonstrated significantly lower ultimate strength, higher distortion angle, and a higher number of cycles to failure compared with HyFlex EDM and TruNatomy Glider (p < 0.05). The maximum torque generated during preparing glide path was lowest for V Taper 2H, and the maximum screw-in force was lowest for HyFlex EDM (p < 0.05). TruNatomy Glider generated the highest torque and screw-in force during the apical preparation.

CONCLUSIONS

V Taper 2H #14/03 showed superior cyclic fatigue resistance and lower ultimate strength. TruNatomy Glider generated greater clockwise torque and screw-in force during apical preparation. The mechanical properties, torque, and screw-force was affected by design of heat-treated glide path instruments. Cervical pre-flaring prior to glide path instrument is recommended.

Torsional, Static and Dynamic Cyclic Fatigue Resistance of Reciprocating and Continuous Rotating NiTi Instruments

INTRODUCTION

To evaluate torsional, dynamic and static cyclic fatigue resistance of the reciprocating One Reci (OR, Micromega, Besançon, France), WaveOne Gold (WOG, Dentsply Maillefer, Ballaigues, Switzerland), rotary One Curve (OC, MicroMega, Besançon, France) and ProTaper Next X2 (PTN X2, Dentsply Sirona, Ballaigues, Switzerland) instruments.

METHODS

A total of 120 NiTi instruments (n = 30) OR, WOG, OC and PTN X2 were used. Torque and rotation angle until failure under static torsion loading were measured according to the ISO 3630-1. Static and dynamic fatigue resistance was measured as the time to fracture (TTF) in an artificial stainless-steel canal with a 60° angle and 5-mm radius of curvature at intracanal temperature. The results were analyzed with one-way ANOVA and post-hoc Tukey tests. The alpha-type error was set at 5%. Fracture instruments from torsion and fatigue tests were examined with scanning electron microscope.

RESULTS

The OR showed higher static fatigue resistance and rotation angle at fracture than the WOG, OC and PTN X2 (P < .05). The WOG exhibited higher torsional resistance than the others (P < .05). Dynamic cyclic fatigue resistance tests had highest TTF than static for the PTN X2 and the WOG groups (P < .05). In the dynamic tests, the OR and WOG showed higher TTF than the OC and PTN X2 (P < .05).

CONCLUSIONS

Under these experimental condition, One Reci exhibited suitable mechanical properties with the highest cyclic fatigue resistance and angle of rotation among the tested instruments.

What meaningful information are the instruments mechanical testing giving us? A comprehensive review

Instruments mechanical strength and flexibility are traditionally tested by running cyclic fatigue, torsional, bending, buckling and microhardness tests. Several cyclic fatigue test models have been used in endodontics, all capable of providing a curved trajectory for the instrument to rotate. The cyclic fatigue testing allowed to identify conditions that may affect the fatigue strength outcomes, such as canal radius and degree of curvature, handpiece static vs dynamic motions, test temperature, kinematics, instrument previously wear and sterilization cycles, or instrument's size and metal alloy features. Due to the international test specifications for both torsional and bending tests, the variations of their models are not as many as for cyclic fatigue. These tests have also identified conditions capable of affecting the outcomes, such as kinematics, instrument's preloading, cross-sectional diameters, or alloy heat treatments. Buckling and microhardness are less common, with the metal alloy being considered to have a major influence on the results. Instruments mechanical testing, having all these individual conditions as independent variables, allowed to understand them and moulded the way the technical procedures are performed clinically. Even though the artificiality and simplicity of these tests will hardly mimic real working situations, and independently of being capable of producing cornerstone knowledge, these tests are also associated with inconsistency, lack of reproducibility and low external validity. Several attempts have been made to increase the generalizability of the outcomes by adding test settings that intend to mimic the clinical condition. Although pertinent, these settings may also add variabilities inherent to their concepts and practical applications in the laboratory environment. Although the actual studies should be seen as laboratory mechanical tests that measure very specific parameters under very particular conditions and that by far do not mimic the clinical condition, the lower validity drawback seems to be possible to be minimized when achieving a comprehensive understanding of the instrument behaviour. A Finite Elements Method and/or a multimethod research approach may lead to superior data collection, analysis, and results' interpretation, which when associated with a reliable confounding factors control and proper study designs may be helpful tools and strategies in order to increase the reliability of the outcomes.

Experimental Analysis on the Impact of Current on the Strength and Lifespan of a Ni-Ti Element

Intelligent materials, especially materials with shape memory, are an important discovery, with technical applications in the medical and aerospace field, among others, which led to the development of systems and applications with multiple advantages and disadvantages due to ignorance about their functionality. This paper presents an application developed in the research laboratory for determining and monitoring the behavior of a material element with Ni-Ti shape memory, and its lifespan. The application allows the stress level of the Ni-Ti element subjected to numerous repeated cycles of deformation to be determined by supplying it to a constant electric current. Thus, the results show the variation of the Ni-Ti element force, in the form of a spring, at the ambient temperature variations as well as force variations at different numbers of attempts. The Ni-Ti alloy has both shape retention and superelasticity properties, being the most common in the fields of applicability. Due to its unique properties, it can be used in the most demanding applications in the medical field, usually involving difficult conditions of resistance to fatigue.

Metallurgical Tests in Endodontics: A Narrative Review

Since there are no reviews of the literature on this theme, the aim of this narrative review is to summarize the metallurgical tests used in endodontics, pointing out their functional use and their pros and cons and giving readers a user-friendly guide to serve as an orientation aid in the plethora of metallurgical tests. With this purpose, a literature search for articles published between January 2001 and December 2021 was conducted, using the electronic database PubMed to collect all published articles regarding the metallurgical tests used in endodontics for the evaluation of NiTi rotary instruments. The search was conducted using the following keywords: “metallurgy”, “differential scanning calorimetry” (DSC), “X-ray diffraction” (XRD), “atomic force microscopy” (AFM), “energy-dispersive X-ray spectroscopy” (EDS), “focused ion beam analysis” (FIB) and “Auger electron spectroscopy” (AES) combined with the term “endodontics” or “NiTi rotary instruments”. Considering the inclusion and exclusion criteria, of the 248 articles found, only 81 were included in the narrative review. According to the results, more than 50% of the selected articles were published in one of the two most relevant journals in endodontics: International Endodontic Journal (22.2%) and Journal of Endodontics (29.6%). The most popular metallurgical test was DSC, with 43 related articles, followed by EDS (33 articles), AFM (22 articles) and XRD (21 articles). Few studies were conducted using other tests such as FIB (2 articles), micro-Raman spectroscopy (4 articles), metallographic analysis (7 articles) and Auger electron spectroscopy (2 articles).