Comparison of Conventional and New-generation Nickel-Titanium Files in Regard to Their Physical Properties

Electrochemical Properties of Nickel-Titanium Rotary Endodontic Instruments

INTRODUCTION

Nickel-titanium rotary endodontic files have been commercially available for decades, but more recent innovations have introduced heat-treated and surface-treated files. This study investigated the corrosion properties of various nickel-titanium files in normal saline and sodium hypochlorite (NaOCl).

METHODS

Ten different file brands of size 40 with a 0.04 taper were subjected to electrochemical testing in 0.9% NaCl (saline) and 5.25% NaOCl at room temperature. The Open Circuit Potential (OCP) was observed for 1 hour followed by a cyclic polarization test from -300 to 700 mV and back to -300 mV (vs OCP). Nonparametric ANOVA and a pairwise comparison (P < .05) were used for statistical analysis of the OCP at 1 hour and the corrosion current (Icorr) obtained via the cyclic polarization test.

RESULTS

Significant differences (P < .05) were found between files with respect to OCP and Icorr in both solutions. Nine files exhibited significantly greater (P < .05) Icorrs in NaOCl than in saline. Conversely, pitting corrosion was observed in the saline solution but not NaOCl. Weak and/or moderate correlations existed between OCP and Icorr measures in the 2 solutions.

CONCLUSION

Significant differences in electrochemical properties were observed among the 10 brands of files. Overall, there was not a clear trend between conventional, heat-treated, or surface-treated files among OCP or Icorr in either solution.

Comparing Cyclic Fatigue Resistance and Free Recovery Transformation Temperature of NiTi Endodontic Single-File Systems Using a Novel Testing Setup

The aim of this study was to assess the effect of body temperature (37 °C) on the cyclic fatigue resistance of three endodontic single-file systems using a new testing setup. One Shape® new generation (OS), WaveOne™ (WO) and WaveOne® GOLD (WOG), which are made from different NiTi alloys and operated in different motions (rotation/reciprocation), were evaluated. The study design included four groups. Each group comprised 30 files, 10 files of each of the three file systems, tested at 20 ± 2 °C (group 1 and 3) and at 37 ± 1 °C (group 2 and 4). All files were tested in a custom-made metal block with artificial canals of 60° angle, and a 5 mm and 3 mm radius of curvature, respectively. A heating element was attached to replicate a temperature of 37 °C. Files were introduced 18 mm into the canals and operated until failure. Transformation temperatures of five samples of each of the tested file systems were determined via the bend and free recovery (BFR) method. With the exception of WOG in canals with a 3 mm radius of curvature (p = 0.075), all the tested file systems showed statistically significantly less time needed to fracture when operated at 37 ± 1 °C compared to at 20 ± 2 °C in canals with a 5 mm and 3 mm radius of curvature using Mann-Whitney U test (p < 0.05). All file systems showed transformation temperatures below the body temperature. We concluded that body temperature directly affects the cyclic fatigue resistance of all tested file systems. Bend and free recovery can be suitable for the determination of austenite finish temperatures (Af) of endodontic instruments as it allows testing a longer portion of the instrument.

A review of the latest developments in rotary NiTi technology and root canal preparation

The introduction of nickel-titanium (NiTi) mechanical instruments dramatically changed clinical endodontics over the last few decades. Before NiTi, it was necessary to use more instruments to create an ideal root canal shape, and many approaches, sequences and techniques were developed over the years. Recently, NiTi endodontic instruments have undergone a series of changes brought about by modifications in design, surface treatments, and thermal treatments to improve their root canal preparation outcomes and reduce associated canal preparation risks during root canal treatment. Heat treatment is one of the most fundamental approaches to improving the fatigue resistance and flexibility of NiTi endodontic instruments. In addition, new kinematics have been developed to offer greater safety and efficiency. This narrative review describes the general properties and manufacturing of NiTi instruments, and the mechanical system evolution of NiTi instruments.

Mechanical and Metallurgical Characterization of Nickel-Titanium Wire Types for Rotary Endodontic Instrument Manufacture

This study aimed to evaluate and compare the effects of ambient temperature and post-manufacture heat-treatment on the mechanical behavior of nickel-titanium (NiTi) wires. Four types of commercial NiTi variants (Stock NiTi, heat treated "Blue", "Gold", "Superflex", all Dentsply Maillefer, Ballaigues, Switzerland) were stressed in a tensile testing machine in a temperature-controlled water bath at three different temperatures. Stress and strain values were extrapolated from the raw data, and 2-way ANOVA and Tukey's test for multiple comparisons were performed to compare the differences of the mechanical constants. Differential scanning calorimetry (DSC) tests established the martensitic transformation starting (M_s), finishing (M_f) and austenitic (reverse-martensitic) starting (A_s) and finishing (A_f) points. Austenitic modulus of elasticity and transformation stress values increased with temperature for all NiTi groups. The martensitic modulus of elasticity, maximum transformation strain and ultimate tensile stress were not significantly affected by temperature changes. Stock NiTi and Gold wire samples presented with clearly delineated austenitic and martensitic transformations in the DSC experiments. Differences in manufacturing/heat treatment conditions and ambient temperature affect the mechanical behavior of nickel-titanium and may have clinical implications. Further improvements to the experimental setup could be considered to provide more accurate measurements of strain.

A single-file endodontic treatment: A promising endodontic concept

nema

Testing Cyclic Fatigue Resistance of Nickel Titanium Rotary Endodontic Instruments: A Validation Study for a Minimum Quality Criterion in a Standardized Environment

Introduction: Cyclic fatigue resistance of rotary endodontic instruments has been extensively studied in the last two decades, yet with no standardization. While new low-cost instruments are now manufactured, a standard is lacking to guarantee a minimum quality. This study aimed to validate a new model for CF testing through a fixture proposed for ISO Specification 3630-1 and to establish a minimum quality standard based on testing conditions and material characteristics.

Materials and methods: Size 25/0.06 instruments of ProFile Vortex (PF) and Vortex Blue (VB) were run until failure in curved metallic fixtures (5 or 7.5 mm radius) built according to a proposal for an additional test for the ISO 3630-1 standard. High resolution videos were recorded, number of cycles to failure (NCF) registered and apical fragments measured with a digital caliper. Surface strain was determined from test dimensions and fragment lengths. Mean life, β and η parameters were calculated with Weibull analysis. NCF data were compared using student's t-tests and referenced to a minimum required cycles at fracture (Cmin) based on austenite finish temperatures, testing temperature and deformation.

Results: VB instruments were statistically more resistant than PF in both 7.5 mm radius curve (p = 0.001) and 5 mm radius curve (p = 0.002) simulated canals. Weibull probability plots showed higher mean life for VB than PV. NCF for both instruments were higher than Cmin.

Conclusions: The NCF results in this study support the findings of previous studies where VB and PF were compared. The novel test design appears a suitable addition to ISO 3630-1.

Impact of Peracetic Acid on the Dynamic Cyclic Fatigue of Heat-Treated Nickel-Titanium Rotary Endodontic Instrument

Peracetic acid (PAA) is widely used as a sterilizing/disinfecting agent, and, in endodontics, it has been introduced as a promising irrigant in root canal treatment. It has been used at different concentrations to achieve various functions. However, endodontic instruments in contact with PAA of a certain concentration may affect their fatigue resistance. Therefore, the aim of this study was to investigate the impact of PAA on the cyclic fatigue resistance of three commercial heat-treated nickel-titanium (NiTi) rotary files. Three types of heat-treated NiTi rotary files were selected: One Curve (OC), ProTaper Gold (PTG), and Wave One Gold (WOG). Each type was divided into three subgroups (n = 6 for each file type): (1) untreated instruments; (2) files immersed in 0.002% PAA; and (3) files immersed in 0.35% PAA. The performance of each file type was tested in a simulated canal. The number of cycles to fracture (NCF) was determined to assess cyclic fatigue resistance of the files. Independent sample t-test was applied to compare each treated file within a subgroup with its respective control group, and one-way ANOVA was used for comparison among the main groups. All types of tested files revealed a significant decline in the cyclic fatigue resistance after exposure to 0.002% PAA except the PTG (P=0.209). After exposure of the files to a higher concentration (0.35% PAA), a dramatic reduction was demonstrated by all the groups. Before and after exposure of the files to PAA, PTG displayed the highest cyclic fatigue resistance, followed by the WOG, while the OC showed the lowest resistance. Exposure of heated-treated NiTi files to PAA in a relatively high or low concentration adversely affects the cyclic fatigue resistance. The PTG files demonstrated the best performance among the tested types and can be disinfected with 0.002% PAA for clinical purpose.

Phase transformation behaviors and mechanical properties of NiTi endodontic files after gold heat treatment and blue heat treatment

WaveOne Gold, ProTaper Gold, Reciproc Blue, ProTaper Next, WaveOne and ProTaper files were selected to compare the phase transformation behaviors and mechanical properties of nickel-titanium (NiTi) rotary files after gold heat and blue heat treatments. The reverse transformation finishing point temperatures of WaveOne Gold, ProTaper Gold, ProTaper Next and WaveOne were higher than those of the other two instruments investigated. At a deflection of 0.5 mm, the loads were significantly varied except for ProTaper Next and ProTaper Gold. At a deflection of 3.0 mm, the loads of Reciproc Blue and WaveOne Gold were significantly varied compared to WaveOne and ProTaper. Cycles to failure were reduced in the order of WaveOne Gold, Reciproc Blue, ProTaper Gold, ProTaper Next, WaveOne and ProTaper. NiTi instruments after gold heat and blue heat treatments exhibited significantly higher bending properties and cyclic fatigue resistances, representing an improved performance over traditional and M-wire instruments.

Influence of autoclave sterilization procedures on the cyclic fatigue resistance of heat-treated nickel-titanium instruments: a systematic review

Objectives

This systematic review evaluated the influence of autoclave sterilization procedures on the cyclic fatigue resistance of heat-treated nickel-titanium (NiTi) instruments.

Materials and Methods

A systematic search without restrictions was conducted in the following electronic databases: PubMed, Scopus, Web of Science, ScienceDirect, Cochrane, and Open Grey. The hand search was also performed in the main endodontic journals. The eligible studies were submitted to the methodological assessment and data extraction.

Results

From 203 abstracts, a total of 10 articles matched the eligible criteria. After reading the full articles, 2 were excluded because of the absence of the heat-treated instruments in the experimental design and 3 due to the lack of a control group using heat-treated instruments without autoclave sterilization. From the 5 included studies, 1 presented a low risk of bias, 3 presented moderate and 1 high risk. It was observed heterogeneous findings in the included studies, with autoclave sterilization cycles increasing, decreasing or not affecting the cyclic fatigue life of heat-treated NiTi instruments. However, the retrieved studies evaluating the cyclic fatigue resistance of endodontic instruments presented different protocols and assessing outcomes, this variability makes the findings less comparable within and also between groups and preclude the establishment of an unbiased scientific evidence base.

Conclusions

Considering the little scientific evidence and considerable risk of bias, it is still possible to conclude that autoclave sterilization procedures appear to influence the cyclic fatigue resistance of heat-treated NiTi instruments.

A critical appraisal of studies on cyclic fatigue resistance of engine-driven endodontic instruments

The endodontic literature contains a plethora of studies on static and dynamic cyclic fatigue resistance tests performed on a large array of rotary or reciprocating nickel-titanium endodontic instruments. It was the aim of this review to summarize the currently available evidence to point out the different outcomes from static versus dynamic tests and to assess whether cyclic fatigue tests provide useful data and information for clinical practice. An electronic literature research in the database PubMed was performed using appropriate search terms, and the titles and abstract were screened for relevance. Language was restricted to English. The review reveals marked differences between the results obtained in static and dynamic tests, and also, the results for the same instruments assessed either under static or dynamic experimental conditions are widely inconsistent. Between the lowest and the highest value for one and the same pathfinding instrument was a factor of about 123 when cyclic fatigue was assessed either under static or dynamic experimental conditions. Moreover, standard deviations of up to 30% have been reported. Environmental temperature has a 500% impact on the lifetime of instruments. In conclusion, fatigue resistance tests conducted under room temperature should be regarded as having little meaning and the scientific and clinical benefits of fatigue resistance tests are very limited. These data should be provided by the manufacturer of the instruments.

Cyclic fatigue of Reciproc Blue and Reciproc instruments exposed to intracanal temperature in simulated severe apical curvature

OBJECTIVE

The aim of this study was to evaluate the cyclic fatigue resistance of Reciproc and Reciproc Blue by testing in a severe apical curvature at intracanal temperature.

MATERIALS AND METHODS

Eighteen Reciproc R25 (25.08) and Reciproc Blue (25.08) instruments were tested in a cyclic fatigue device at body temperature (37 °C) using a stainless steel block with an artificial canal with a curvature angle of 90° and radius of curvature of 2 mm. The number of cycles to fracture (NCF) was calculated. The instruments were examined by differential scanning calorimetry (DSC). The data were analyzed using both Student's t tests and Weibull analysis.

RESULTS

The NCF values of Reciproc R25 were significantly lower than Reciproc Blue R25 (P < 0.05). There was no significant difference between the instruments regarding the length of fractured fragments (P > 0.05).

CONCLUSIONS

Reciproc Blue R25 instruments displayed significantly higher NCF than Reciproc R25.

CLINICAL RELEVANCE

This study reported that novel reciprocating blue wire instruments exhibited higher cyclic fatigue resistance than its precedence M-wire instrument when tested in severely curvatured canals.

Cyclic fatigue resistance of new reciprocating files (Reciproc Blue, WaveOne Gold, and SmartTrack) in two different curved canals

AIM

In the present study, we compared the cyclic fatigue resistance (CFR) of Reciproc Blue, WaveOne Gold, and SmartTrack files in curved artificial canals.

METHODS

Ninety new Reciproc Blue R25, WaveOne Gold Primary, and SmartTrack X1 files were tested in artificial canals with 45° and 60° angles of curvature. CFR was determined by recording the time to fracture in the artificial canals. Two-way analysis of variance was used to analyze the data.

RESULTS

In the canal with a 45° angle of curvature, no significant differences were observed amongst Reciproc Blue, WaveOne Gold, and SmartTrack (P > .05). In the canal with a 60° angle of curvature, Reciproc Blue and SmartTrack had a greater CFR than WaveOne Gold (P < .05); there was no significant difference between the Reciproc Blue and SmartTrack files (P > .05).

CONCLUSION

The results of the present study showed that Reciproc Blue and SmartTrack files exhibited greater CFR than WaveOne Gold only in canals with a 60° angle of curvature.

Evaluation of remaining dentin thickness and fracture resistance of conventional and conservative access and biomechanical preparation in molars using cone-beam computed tomography: An in vitro study

Aim:

The present study aimed to compare the remaining dentin thickness (RDT) and fracture resistance of conventional and conservative access and biomechanical preparation in molars using cone-beam computed tomography (CBCT).

Methodology:

A total of 60 freshly extracted human molars were selected and were randomly divided into two groups of conventional and conservative access preparation group (n = 30). Samples were subjected to pre-CBCT scan at the pericervical region for the measurement of total dentin thickness. For the conventional group, samples were accessed and biomechanical preparation was done using K3 XF file. For conservative group, samples were accessed using CK microendodontic burs using a dental operating microscope and biomechanical preparation was done using self-adjusting file. After obturation and postobturation with nanohybrid composite restoration, samples of both groups were subjected to post-CBCT scan at pericervical region for the measurement of RDT. The samples were then loaded to fracture in the Instron Universal Testing Machine, and the data were analyzed using paired sample t-test and independent sample t-test.

Results:

The mean RDT was less in conventional group than conservative group. Pericervical dentin was preserved more in conservative group. The statistical difference among both the experimental group was highly significant (<0.001). The mean load at fracture was less in conventional group than conservative group (<0.001).

Conclusion:

Coronal dentin was conserved in molars when accessed through conservative than through conventional. The dentin conservation afforded an increased resistance to fracture in conservative group which is doubled the fracture resistance in conventional group.

Effect of surface treatment on the mechanical properties of nickel-titanium files with a similar cross-section

OBJECTIVES

The aim of this study was to compare the mechanical properties of various nickel-titanium (NiTi) files with similar tapers and cross-sectional areas depending on whether they were surface-treated.

MATERIALS AND METHODS

Three NiTi file systems with a similar convex triangular cross-section and the same ISO #25 tip size were selected for this study: G6 (G6), ProTaper Universal (PTU), and Dia-PT (DPT). To test torsional resistance, 5 mm of the straightened file's tip was fixed between polycarbonate blocks (n = 15/group) and continuous clockwise rotation until fracture was conducted using a customized device. To evaluate cyclic fatigue resistance, files were rotated in an artificial curved canal until fracture in a dynamic mode (n = 15/group). The torsional data were analyzed using 1-way analysis of variance and the Tukey post-hoc comparison test, while the cyclic fatigue data were analyzed using the Mann-Whitney U test at a significance level of 95%.

RESULTS

PTU showed significantly greater toughness, followed by DPT and G6 (p < 0.05). G6 showed the lowest resistance in ultimate torsional strength, while it showed a higher fracture angle than the other files (p < 0.05). In the cyclic fatigue test, DPT showed a significantly higher number of cycles to failure than PTU or G6 (p < 0.05).

CONCLUSIONS

Within the limitations of this study, it can be concluded that the torsional resistance of NiTi files was affected by the cross-sectional area, while the cyclic fatigue resistance of NiTi files was influenced by the surface treatment.

Fracture Resistance of K3 Nickel-Titanium Files Made from Different Thermal Treatments

The purpose of this study was to compare fracture resistances of K3 nickel-titanium files made from different thermal treatments. K3 (SybronEndo, Orange, CA), K3XF (SybronEndo), and experimentally heat treated K3 (K3H) were used. For the cyclic fatigue test, the samples were rotated with up-and-down motion in the artificial canal with the curvature of 60 degrees until the fracture occurred. The number of cycles to fracture (NCF) was measured. For the torsional fracture test, the samples were tightly bound and rotated until the fracture occurred. Elastic modulus (EM), ultimate torsional strength (UTS), and angle of rotation to fracture (ARF) were measured. The results were statistically analyzed by one-way ANOVA. The NCF of K3H was higher than those of K3 and K3XF (P < 0.05). The EM of K3XF and K3H was lower than that of K3 (P < 0.05). There was no significant difference in UTS. The ARF of K3XF was higher than that of K3 (P < 0.05). K3XF and K3H showed more flexibility than K3. The maximum torsional angle of K3XF was higher than that of K3, but there was no significant difference on the UTS in all three groups.