Effects of Sodium Hypochlorite Concentration and Temperature on the Cyclic Fatigue Resistance of Heat-treated Nickel-titanium Rotary Instruments

Effect of phytic acid on chemical, structural, and mechanical characteristics of nickel-titanium endodontic files

This study investigated phytic acid (IP6) effect on chemical, structural, and mechanical characteristics of nickel-titanium (NiTi) files. The tested files were equally divided into groups according to the immersion protocol: sodium hypochlorite (NaOCl), ethylenediaminetetraacetic acid (EDTA), IP6, EDTA followed by NaOCl, and IP6 followed by NaOCl. These groups were then compared in terms of Ni, Ti, and chromium (Cr) ions release from the files. Microstructural changes using field emission scanning electron microscope (Fe-SEM) and energy dispersive X-ray spectroscopy (EDX) and surface roughness were analyzed. The mechanical characterization was conducted using cyclic fatigue resistance test. Fractured segments were scanned under SEM. Statistical analysis was performed using one-way ANOVA, Tukey test, Kruskal-Wallis test and Mann-Whitney U test. Results showed that NaOCl caused significant release of Cr, followed by IP6 and EDTA (P < 0.05). When files were pre-immersed in EDTA, NaOCl tended to induce less release of Ti and Cr. EDX evaluation revealed that the main surface elements were Ni, Ti, carbon, and oxygen. EDTA group contained the highest amount of carbon, while the control group showed the lowest. Surface roughness evaluation revealed no significant differences between groups despite the minor increases after immersion in certain groups. Black areas were observed in the NaOCl group which indicated corrosion. However, the cyclic fatigue test showed no significant differences between the groups.

Phase Transformation and Mechanical Behaviour of Different Heat-Treated Nickel-Titanium Rotary Instruments

OBJECTIVES

This study evaluated the phase composition, phase transformation behaviour, and mechanical properties of five heat-treated NiTi instruments.

METHODS

ProTaper NEXT (M-wire, PTN), ProTaper Gold (Gold-wire, PTG), One Curve (C-wire, OC), EdgeTaper Platinum (Fire-wire, ETP), NeoNiTi (electrical discharge machining-wire, NNA), and ProTaper Universal (conventional wire, PTU, control) with #25-tip size were tested (n = 12/group) for cyclic fatigue resistance (number of cycles to failure; NCF) and torsional resistance (angle of rotation to fracture and maximum torque at fracture [ultimate torsional strength]). The geometry and fracture surfaces of the tested instruments were examined by scanning electron microscopy. The phase transformation temperature and phase composition of the instruments were evaluated using differential scanning calorimetry and X-ray diffraction. Data were statistically analysed using one-way ANOVA and Tukey's post hoc test, with the significance level set at 5%.

RESULTS

PTG showed the highest NCF (P < .05) at 37°C, while ETP exhibited the highest angle of rotation to fracture, ultimate torsional strength, and stiffness (P < .05). Scanning electron microscopy demonstrated typical clusters of fatigue striations and numerous cracks after cyclic fatigue fracture, whereas there was a concentric abrasion pattern with a dimple and microvoids at the centre after torsional fracture. In differential scanning calorimetry curves, austenite-finishing temperatures of heat-treated instruments were higher than 37°C, whereas that of PTU was lower than 37°C. PTU showed strong peaks of austenite at 25 and 37°C, whereas ETP showed a strong peak of R-phase at 25°C, but mostly austenite phase at 37°C in X-ray diffraction.

CONCLUSIONS

Geometry, alloy type, and phase transformation temperatures of NiTi instruments affected their mechanical behaviour.

CLINICAL RELEVANCE

PTG showed the highest NCF, suitable for markedly curved canals. ETP had the highest torsional resistance, appropriate for narrow and constricted canals.

The effect of irrigation solution temperature and novel heat-treated rotary files on apical debris extrusion and canal preparation time

This study aimed to compare apical debris extrusion and canal preparation time with ProTaper Gold (PTG) and ProTaper Ultimate (PTU) files at different temperatures. Mesio-buccal roots of 60 mandibular first molars were distributed into six groups depending on the file type (PTG, PTU) and irrigation solution temperature (20, 37, 45°C). During instrumentation, extruded debris were collected and weighed to measure the mass in milligrams. The canal preparation time was recorded in seconds. ANOVA followed by Bonferroni post-hoc tests were used for analysis. The amount of debris extrusion was significantly higher in PTU, which was affected by the irrigation solution temperature (p < 0.05). The difference in canal preparation time was not significant between the two file systems, however, it was significant between the different temperatures (p = 0.001). Both file systems had shorter canal preparation times at 20°C. The irrigation solution temperature could influence the debris extrusion and time of canal preparation.

Surface roughness and cyclic fatigue resistance of a novel shaping system: An in-vitro study

Recently developed Nickel-Titanium (NiTi) instruments with practical changes have resulted in safer instrumentation. In addition, topographical features on the file surface are a contributing factor to clinical durability. Therefore, this study aimed to investigate both the cyclic fatigue resistance and the roughness change of MTwo and Rotate instruments (VDW, Munich, Germany). Each instrument (n = 6/each group) was scanned with an atomic force microscopy prior to and after instrumentation. In addition, cyclic fatigue testing was conducted for each instrument (n = 11/each group) with stainless-steel blocks, including 45°-60°-90° degrees of curvature milled to the instruments' size. The roughness parameters increased for both systems after instrumentation (p<0.05). Both systems presented an increased roughness following instrumentation (p<0.05). The cyclic fatigue resistance was lowest at 90° for both systems (p<0.05), whereas the Rotate files presented a higher resistance than that of the Mtwo files (p<0.05). Compared to the Mtwo files, Rotate files presented better resistance, while the resistance decreased as the curvature increased.

Phase transformation behavior and mechanical properties of HyFlex EDM nickel-titanium endodontic rotary instrument: Evaluation at body temperature

Background/purpose

Temperature-dependent phase compositional changes influence the mechanical properties of heat-treated nickel-titanium (NiTi) rotary instruments. This study evaluated the phase composition, bending properties, and cyclic fatigue resistance of HyFlex EDM NiTi rotary instruments against differently heat-treated and non-heat-treated NiTi instruments at body temperature (BT).

Materials and methods

HyFlex EDM OneFile (EDM) instruments, heat-treated HyFlex CM (CM) and Twisted File (TF) instruments, and non-heat-treated K3 instruments (size #25/.08) were subjected to differential scanning calorimetry, and the martensitic, R-phase, and reverse transformation starting and finishing temperatures were determined. A cantilever bending test and a cyclic fatigue test were conducted at BT (37 °C ± 1.0 °C), and the bending load and number of cycles to failure (NCF) were recorded. Statistical analysis was performed using Kruskal-Wallis and Mann-Whitney U tests (α = 0.05).

Results

TF and K3 had reverse transformation finishing temperatures lower than BT, while those for EDM and CM were higher than BT. The bending loads at a 0.5 mm deflection were in the order of EDM < TF < CM < K3 (P < 0.05), and those at a 2.0 mm deflection were EDM < CM and TF < K3 (P < 0.05). EDM had the highest NCF among the four instruments (P < 0.05).

Conclusion

The EDM instrument had a reverse transformation finishing temperature higher than BT indicating its martensite/R-phase composition at BT. The EDM instrument had superior flexibility and greater resistance to cyclic fatigue than the CM, TF, and K3 instruments at BT.

Evaluation of cyclic fatigue behavior of RACE EVO rotary Files: In-vitro comparative study

Introduction

Comparative evaluation of the newly introduced; RACE EVO file system to the currently well-performing file systems like HyFlex EDM OneFile, Vortex Blue, ProTaper Gold (PTG) with regards to the cyclic fatigue resistance.

Materials

&Methods.The endodontic rotary files under evaluation were segregated into four groups as follows:; RACE EVO, PTG, Vortex Blue, and HyFlex EDM OneFile. A detailed physical analysis of the cyclic fatigue resistance was performed in 15 files of each type (n = 60). Under the test conditions of 37 °C the test samples were subjected to continuous motion at the manufacturer recommended speed within an artificial curved canal until it fractured. For all the test samples, with the recording of time of fracture, calculation was done for number of cycles to fracture (NCF). In addition, the length of the fractured segment was determined. The fractured surface was further evaluated with scanning electron microscopic (SEM) images.

Results

Highest significance for NCF was seen in HyFlex EDM OneFile followed by the Vortex Blue, PTG, and; RACE EVO (p < 0.05) by analyzing the data statistically. Significant differences among the lengths of the fractured segments were evident among tested files, ranging between 4.73 and 6.3 mm (p < 0.05) with HyFlex EDM OneFile uniquely differentiating from the others showing the fracture at 6.3 mm. The common features of cyclic fatigue failure was further substantiated by SEM images.

Conclusions

Compared to the HyFlex EDM OneFile, Vortex Blue, and PTG files; RACE EVO demonstrated significantly lower resistance to cyclic fatigue.

Effect of sodium hypochlorite temperature and concentration on the fracture resistance of root dentin

BACKGROUND

Sodium hypochlorite (NaOCl) is the most efficient root canal irrigant to date. The aim of this study was to compare the effect of NaOCl used at different temperatures and concentrations on the compressive strength of root dentin.

MATERIALS AND METHODS

Seventy-two extracted human single-canaled straight roots of comparable size and length were selected and randomly divided into six groups (n = 12): Group (A) served as a control with unprepared canals. The other groups were instrumented with rotary ProTaper Universal files up to size F3. Group (B) was irrigated with 1% NaOCl at room temperature, Group (C) with 1% NaOCl heated to 70 °C, Group (D) with 5.25% NaOCl at room temperature, and Group (E) with 5.25% NaOCl heated to 70 °C. Saline was used in Group (F). The roots were sectioned into 2-mm-thick disks that underwent compression testing using a universal testing machine. Data were analyzed using one-way ANOVA and post hoc Tukey tests. The significance level was set at p ≤ 0.05.

RESULTS

A total of 255 disks were tested. The control group showed the highest compressive strength (p = 0.0112). However, this did not differ significantly from that of heated (p = 0.259) or unheated (p = 0.548) 1% NaOCl. There were no statistically significant differences between the groups of instrumented teeth.

CONCLUSION

Within the conditions of this study, irrigation with NaOCl at different concentrations and temperatures during root canal preparation did not affect the compressive strength of root dentin.

CLINICAL RELEVANCE

This study demonstrates that the use of NaOCl as a root canal irrigant is not associated with a clinically relevant decrease in root compressive strength, especially when compared to saline.

Effect of Temperature on the Cyclic Fatigue Resistance and Phase Transformation Behavior of Three Different NiTi Endodontic Instruments

Background This study aims to analyze the phase transformation behavior and evaluate the effect of different temperatures on the fracture strength of files. Methodology A total of 108 files were used, and cyclic fatigue tests were performed on HyFlex EDM, VDW.ROTATE, and TruNatomy at three different temperatures (+4°C, +35°C, and +60°C) The time to fracture of files was measured, and the number of cycles was calculated. In addition, the fractured fragment lengths were measured. The Kruskal-Wallis test was used to compare the data that were not normally distributed according to groups. The Mann-Whitney U test was performed for comparisons. P-values <0.05 were considered statistically significant. Results As the temperature increased for all files, the number of cycles to fracture (NCF) decreased. NCF was significantly higher for ROTATE at 4°C and 60°C and HyFlex EDM at 35°C. Conclusions The resistance to cyclic fatigue in all files decreased significantly as the temperature increased.

Influence of sodium hypochlorite on cyclic fatigue resistance of nickel-titanium instruments: A systematic review and meta-analysis of in vitro studies

OBJECTIVE

This systematic review/meta-analysis investigated the influence of NaOCl on cyclic fatigue resistance of endodontic NiTi instruments.

MATERIALS AND METHODS

A systematic search until July 2022 in PubMed/MEDLINE, Embase, Scopus, Web of Science, SciELO, Cochrane Library, and grey literature was conducted. According to the PECOS strategy, only in vitro studies evaluating the effects of NaOCl on the cyclic fatigue resistance of NiTi instruments were eligible. Cyclic fatigue resistance was the primary outcome. A modified Joanna Briggs Institute's Checklist was used for risk of bias assessment.

RESULTS

Of the 2,445 records screened, 37 studies were included. Most studies used simulated canals made of stainless-steel block with severe to moderate curvatures. NaOCl concentration varied from 1-6%, mainly at 37 °C. Regarding fatigue resistance, 23 studies using 1.2% to 6% NaOCl showed a reduction in the resistance compared to the control groups, especially when pre-heated. Four meta-analyses were performed according to the tested NiTi systems. The meta-analyses indicated that the PTU F2 files had higher reduction of fatigue resistance after exposure to 5.25% NaOCl; no differences between NaOCl and no immersion were observed for Reciproc R25, WaveOne 25.08, and WaveOne Gold Primary files. Included studies had low risk of bias.

CONCLUSION

NaOCl appears to reduce cyclic fatigue resistance of certain NiTi files, especially when they are pre-heated, particularly in conventional NiTi files compared to some heat-treated instruments. It is possible that the temperature of the solution may have a greater influence on resistance than NaOCl itself. Important to note that an overall tendency toward no significant influence was observed among various systems.

CLINICAL RELEVANCE

Precautions are necessary when a pre-heated high-concentration NaOCl is used to enhance its properties during root canal preparation, mainly using conventional wire.

Phase transformation and mechanical properties of heat-treated nickel-titanium rotary endodontic instruments at room and body temperatures

BACKGROUND

The aim of this study was to evaluate the phase composition, phase transformation temperatures, bending property, and cyclic fatigue resistance of different heat-treated nickel-titanium (NiTi) rotary instruments with the same tip diameter and taper at room (RT; 25 ± 1 °C) and body (BT; 37 ± 1 °C) temperatures.

METHODS

Five heat-treated NiTi rotary instruments, HyFlex EDM (EDM), HyFlex CM (CM), Vortex Blue (VB), RE file CT (RE) and JIZAI, and a non-heat-treated NiTi rotary instrument (Mtwo) with a size 40, 0.04 taper were investigated. Temperature-dependent phase transformation was examined with differential scanning calorimetry (DSC). The bending loads of the instruments at RT and BT were evaluated using a cantilever-bending test. Cyclic fatigue resistance at RT and BT was measured using a dynamic test, during which the instruments were rotated in combination with a 2-mm back-and-forth motion in an artificial curved canal, and the number of cycles to failure (NCF) was determined. The results were analyzed using two-way repeated measures analysis of variance, a simple main effect test, and the Bonferroni test (α = 0.05).

RESULTS

DSC results indicated that EDM and Mtwo were primarily composed of martensite/R-phase and austenite, respectively, while the other heat-treated instruments were composed of a mix of martensite/R-phase and austenite at the tested temperatures. Regardless of the temperature setting, the bending loads of heat-treated instruments were significantly lower than those of Mtwo (p < 0.05). EDM showed the lowest bending loads and highest NCF at both temperatures (p < 0.05). CM, VB, and JIZAI showed significantly higher bending loads at BT than at RT (p < 0.05). The NCF of all the heat-treated instruments, except VB, was lower at BT than at RT (p < 0.05). At BT, the NCF of CM, VB, RE, and JIZAI were not significantly higher than that of Mtwo (p > 0.05).

CONCLUSIONS

Heat-treated NiTi instruments exhibited lower bending loads and higher NCF values than Mtwo. However, this tendency was less pronounced at BT than at RT, especially in the NCF values of instruments with a mixture of martensite/R-phase and austenite phases at the tested temperatures.

Impact of cooling on shaping ability of thermally treated files in canal models with double curvature

Background

This study compared the ability of thermally treated files in shaping simulated canals with double curvature. Fifty-six canals were enlarged to a final size of 25 with ProTaper Next (PTN) or ZenFlex (ZF).

Materials

Half of the samples were shaped with cooled files (n = 14 each). The amount of removed resin was measured and canal deviation was determined at eight levels. Shaping time and maximum shaping torque values were also recorded. Data were statistically analyzed using analysis of variance and LSD, Kruskal-Wallis, and chi-square tests at a 0.05 significance level.

Results

Compared to PTN and cooled PTN, ZF and cooled ZF required lesser time to shape the canals. The maximum torques were found comparable between the groups. All the groups generated negligible deviations at every canal level evaluated and maintained the canal geometry. Although not significant, the cooled PTN and ZF files exhibited lesser canal deviations than their counterparts.

Conclusion

All groups demonstrated similar shaping ability whilst maintaining the original curvature of the canal in simulated canals with double curvature. However, ZF groups were able to shape the canals faster than PTN groups. There was a trend that cooled files made lesser canal deviations compared to their counterparts.

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.

Influence of Sodium Hypochlorite and Chlorhexidine on the Dynamic Cyclic Fatigue Resistance of XP Endo Shaper Instruments

OBJECTIVE

 This study evaluated the dynamic cyclic fatigue resistance of the XP-Endo Shaper (XPS), associated with chlorhexidine digluconate (CHX) or sodium hypochlorite (NaOCl) in two different formulations: gel (G) or liquid (L).

MATERIALS AND METHODS

 Sixty XPS were used in an artificial stainless-steel canal, and the files were fully immersed in the irrigating solution throughout the experiment until the fracture. The files were divided into six groups (n = 10) based on the irrigation solution used: NaOCl(L), NaOCl(G), CHX(L), CHX(G), natrosol gel (NAT) (control), and lubricating oil (LO) (control). The artificial canal was manufactured 1.5 mm wide, 20 mm long, and, 3.5 mm deep with a straight cervical segment measuring 14.29 mm; an apical segment of 4.71 mm with 3 mm radius; and 90 degrees of curvature apical 1 mm long straight segment. Resistance to cyclic fatigue was determined by recording the number of cycles to fracture (NCF).

RESULTS

 The CHX(G), CHX(L), and OIL (LO) groups showed no significant difference between them and presented longer time to fracture (p > 0.05). NaOCl(L) shows the lowest NCF without significant differences between NaOCl(G) and NAT. The NCF of the NaOCl(G) was statistically similar to the CHX(L) and statistically lower than the CHX(G) and OIL groups. NAT did not present a statistical difference of the NaOCl(L), NaOCl(G), and presented a significantly lower NCF than the CHX(G) (p < 0.01).

CONCLUSION

 The use of CHX(G) resulted in increased cyclic fatigue resistance of the XPS instruments compared to NaOCl or LO.

Effect of NaOCl and EDTA irrigating solutions on the cyclic fatigue resistance of EdgeTaper Platinum instruments

BACKGROUND

To compare the solution on the cyclic fatigue resistance of EdgeTaper Platinum (ETP) instruments with that of ProTaper Gold (PTG) in the presence of ethylenediaminetetraacetic acid (EDTA) or sodium hypochlorite (NaOCl) irrigating solutions.

METHODS

Sixty PTG and 60 ETP instruments with the same size (#25) and taper (8%) were tested under 17% EDTA, 5.25% NaOCl, or distilled water (n = 20 each). Cyclic fatigue was tested using artificial canals that were milled in stainless steel blocks using a laser micromachining technique. The canals had a curvature angle of 60 and a curvature radius of 5 mm. The center of maximum curvature was set at 5 mm from the instrument tip. The block of artificial canals was stabilized inside a container that was filled with one of the tested solutions. The temperature was fixed at 37 °C with a tolerance limit of 1 °C. The number of cycles to fracture (NCF) was calculated and the fractured surfaces were examined using a scanning electron microscope. All statistical analyses were performed using SPSS software Version 20 (IBM-SPSS Inc., Chicago, IL) at a confidence level of 95%.

RESULTS

ETP showed higher NCF than PTG in any of the tested solutions (P < 0.05). Within each group, NaOCl drastically decreased the NCF compared to water and EDTA (P < 0.05) and changing the solution from distilled water to EDTA did not affect the fatigue resistance (P > 0.05).

CONCLUSIONS

ETP showed improved cyclic fatigue performance compared to PTG in all tested irrigating solutions. EDTA can be used in combination with NiTi instruments during canal instrumentation without compromising the cyclic fatigue resistance of PTG and ETP instruments. However, NaOCl drastically decreased the NCF.

The Efficacy of Selected Sodium Hypochlorite Heating Methods for Increasing and Maintaining Its Intracanal Temperature—An Ex Vivo Study

Background: Enhancement of the temperature of sodium hypochlorite (NaOCl) solution would increase its cleaning potential and decontamination of the root canal system. Therefore, the aim of the present in vitro investigation was to compare the efficacy of different methods of NaOCl heating by evaluating the temperature profiles developed at different levels of the root canal system. Methods: Five thermocouples were applied at different levels of the root canal system of extracted human premolars. NaOCl solution was heated according to two methods: extraoral heating (50 °C, 60 °C, and 70 °C) using a magnetic hotplate heater and intracanal heating by F-06, XF-30/04, and ML-12 pluggers at 100 °C, 150 °C, and 180 °C. Results: The extraoral heating method was ineffective to produce a significant temperature increase at the root apex. Comparable results were obtained using the intracanal heating method through the ML-12 plugger that showed slightly better results only when set at 180 °C. On the other hand, negligible differences were observed in terms of temperature maintenance at several levels of the root between the F-06 and XF-30/04 pluggers, even though the time intervals were higher in case of XF-30/04. Conclusions: The intracanal heating method provided a better temperature persistence in the middle third of the root canal system. Conversely, extraoral heating was ineffective to produce a significant temperature increase at the apex of the root. Comparable results were obtained even using the ML-12 plugger.

Effect of body temperature on the cyclic fatigue resistance of the nickel”titanium endodontic instruments: A systematic review and meta-analysis of in vitro studies

Aim:

The aim of this systematic review was to compare the effect of body temperature (I) on the cyclic fatigue resistance (O) of nickel–titanium (NiTi) endodontic instruments (P) to that of room temperature (C).

Methods:

The study was registered in the PROSPERO database (CRD42020204286). A systematic search in PubMed, Scopus, Web of Science, Google Scholar, and OpenGrey was conducted in English until December 31, 2021. In vitro studies comparing the cyclic fatigue resistance of NiTi instruments at the body (35°C ± 2°C) and room temperature (20°C–25°C) were included. Eligible studies were evaluated for risk of bias and meta-analyzed to estimate the effects.

Results:

Twenty-one studies out of 347 met the criteria for inclusion. The meta-analysis included six studies (n = 215) with comparative study parameters. The overall effect sizes (5.49; 95% confidence interval [CI]: 4.04–6.93) were significantly different (P < 0.001), indicating that the mean values at room temperature were significantly (P < 0.001) higher. The effect sizes for full rotary motion (standardized mean difference [SMD]: 4.80; 95% CI: 3.04–6.56) and reciprocating motion (SMD: 6.37; 95% CI: 3.63–9.11) were not significantly different (P = 0.346). Heterogeneity was high (I² = 94%). Sensitivity analysis revealed that the SMD values were not significantly different (P > 0.05) from the overall effect size, indicating that none of the studies had an effect on the overall effect size.

Conclusions:

Within the limitation of the study, the cyclic fatigue resistance of heat-treated NiTi endodontic files is significantly reduced at body temperature when compared to room temperature. Cyclic fatigue testing should be conducted at simulated body temperature.

A critical analysis of research methods and experimental models to study the physical properties of NiTi instruments and their fracture characteristics

The aim of this review is to provide a critical overview of the physical properties (surface hardness, cutting efficiency, bending properties, flexibility and cyclic fatigue resistance) of NiTi instruments. Frequently used experimental models regarding these aspects will be presented and discussed with regard to their strengths and weaknesses. For all these aspects, a plethora of experimental models have been described. Based on a critical appraisal and especially taking the appropriate translation of experimental findings to clinical endodontics into account, suggestions for future research based on clearly defined and valid experimental methodologies will be provided. Up to now, very few attempts have been made to assess which particular physical properties of NiTi instruments exert an impact on the clinical outcome of root canal treatment. Departure from merely focusing on physical properties and fracture characteristics towards more biological aspects in terms of treatment outcome is essential.

Evidence for reduced torsional resistance of rotary files under curved position

Objectives

The aim of this in vitro study was to investigate the torsional fatigue resistance of different rotary files in straight and curved positions at a simulated intracanal temperature.

Methods

Forty-eight size #25 files from ProTaper Universal (PTU), ProTaper Next (PTN) and ProTaper Gold (PTG) were tested for torsional resistance in both straight and curved (90 degrees) positions (n = 8 each). The experiment was conducted at a simulated intracanal temperature of 35 ± 1 °C. The torque gauge was reset before every use, and uniform torsional stress was applied by rotating the file clockwise at a speed of 40 rpm until fracture. Torque failure values were recorded, and fractured surfaces were tested using a scanning electron microscopy (SEM). Statistical analysis of the data was completed using one-way ANOVA and post-hoc test (LSD) in order to compare between the tested systems in each file position. Student’s t-test was also performed to compare between the two positions in each rotary system, and significance level was set at 5%.

Results

In the straight position, torsional resistance of PTN was significantly lower than that of PTU and PTG (p < 0.001), while the PTU and PTG had comparable results (p > 0.05). In the curved position, torsional resistance was found to be comparable among all systems (p > 0.05). Within each system, the curved position resulted in a significantly lower torsional resistance when compared to the straight position (p < 0.001). SEM examination revealed similar torsional failure behavior for all files tested under both positions.

Conclusions

Our study identified that placing files in a curved position was associated with a significant decrease in torsional resistance of all systems, and this necessitates reconsidering the torque settings proposed by the manufacturers. Whether these effects can be translated into in vivo studies remains to be investigated, and thus future studies are essential to provide confirming evidence.

Effect of Core Mass and Alloy on Cyclic Fatigue Resistance of Different Nickel-Titanium Endodontic Instruments in Matching Artificial Canals

Instrument failure during root canal preparation is still a concern among endodontists. However, it remains unclear whether the use of more martensitic alloys or the cross-sectional design parameters (i.e., core mass) significantly improve fracture resistance. The aim of the study was to evaluate the impact of core mass and alloy on dynamic cyclic fatigue resistance of nickel-titanium endodontic instruments in matching artificial canals at body temperature. Two groups were tested. (A) taper 0.04: F360 (Komet, Lemgo, Germany), Twisted file (Sybron Endo, Glendora, CA, USA) (=TF), JIZAI (Mani, Tochigi, Japan) (=J_04) (all size #25) and the variable tapered TruNatomy (Dentsply, Ballaigues, Switzerland) (size #26) (=TN). (B) size #25; taper 0.06: (Mtwo (VDW, Munich, Germany), JIZAI (Mani) (=J_06), and variable tapered Hyflex EDM OneFile (Coltene Whaledent, Altstätten, Switzerland) (=HF). Time, number of cycles to fracture (NCF), and number and length of fractured fragments were recorded and statistically analysed using ANOVA Student-Newman-Keuls, Kruskal–Wallis or Chi-square test (significance level = 0.05). (A) TN showed the significantly shortest time until fracture, followed by TF, F360 and J_04 which also differed significantly, while NCF showed the following order: F360 < TN < TF < J_04 (p < 0.05). Only one J_04 but all instruments of the other groups fractured within the test-limit of 10 min. (B) Mtwo was significantly inferior concerning time until fracture and NCF, compared to J_06 and HF (p < 0.05), which did not differ significantly (p > 0.05). While all Mtwo instruments fractured, only four instruments failed in the other groups (p < 0.05). Within the limitations of this study, alloy and cross-sectional design (i.e., core mass) were critical factors regarding instrument failure, but none of these factors could be determined as a main parameter for increased or decreased time, and cycles to fracture. Rather, it seemed to be the interaction of multiple factors (e.g., longitudinal and cross-sectional design, alloy, and rotational speed) that was responsible for differences in the time and cycles to fracture. Nonetheless, all instruments had lifetimes that allow safe clinical use. However, the superiority or inferiority of an instrument with regard to cyclic fatigue based on laboratory results—even when identical trajectories are guaranteed—may be considered questionable, as the characteristics and design parameters of the instruments vary considerably, and the experimental setups lack additional clinical parameters and thus clinical relevance.

Body temperature fatigue behaviour of reciprocating and rotary glide path instruments in sodium hypochlorite solutions alone or combined with etidronate

The aim of the study was to compare cyclic fatigue (CF) resistance of reciprocating and rotary glide path instruments in the presence of irrigation solutions at body temperature. CF resistance of 100 instruments (50 ProGlider and 50 WaveOne Gold Glider) was tested in sodium hypochlorite solutions alone (NaOCl) or in combination with etidronate (NaOCl/HEBP). The CF platform with a simulated canal (curvature = 60°/radius = 3 mm) was submerged in a temperature-controlled bath with the solutions kept at 37 ± 1°C controlled by a precision mercury glass thermometer. Instruments were actioned following manufacturer guidelines, and time to failure was recorded and compared among groups with Weibull analysis. Reciprocating glide path preparation instruments were more resistant to CF than rotary instruments in both irrigation solutions. The irrigation solution did not affect the mean life of ProGlider, but reciprocating instruments lasted significantly longer used in presence of NaOCl compared to NaOCl/HEBP.

Effect of immersion in sodium hypochlorite on the cyclic fatigue resistance of three rotary instruments

Context:

It is essential to know if the immersion in sodium hypochlorite (NaOCl) influences the cyclic fatigue resistance of nickel-titaniumm (NiTi) files.

Aim:

This study was aimed to evaluate the resistance to the cyclic fatigue of three NiTi systems, as well as the influence of 5.25% NaOCl on their resistance.

Methods:

Files from the Sequence Rotary File (MK life, Porto Alegre, Brazil), ProTaper Next (PTN) (Dentsply, Maillefer, Baillagues, Switzerland), and HyFlex EDM (Coltène Whaledent, Allstatten, Switzerland) systems were tested and separated into Group 1)-without immersion in NaOCl solution and Group 2 and Group 3-immersed in 5.25% NaOCl solution, for 1 min, and 5 min, respectively. All instruments were submitted to the cyclic fatigue test. Images of the instruments were observed by scanning electron microscopy, to evaluate the type of defect presented on the fractured surface. The test performed for the analysis of two independent factors was a two-way analysis of variance.

Results:

HyFlex EDM presented a greater number of cycles until fracture when compared to others (P < 0.001). The control group showed a greater number of cycles until fracture than the same instruments submitted to NaOCl (P < 0.001). Sequence instruments showed the lowest values of cycles until fracture (P < 0.001). There was no statistically significant difference between treatments (P = 0.998). PTN, showed intermediate values of fracture resistance when compared to the HyFlex EDM and sequence instruments (P < 0.001), but without intragroup differences (P = 0.437).

Conclusions:

Hyflex EDM instruments were the most resistant to cyclic fatigue.

Resistance to Cyclic Fatigue of Nickel-Titanium Files Immersed in Sodium Hypochlorite at Body Temperature

Materials and Methods

Forty-five files from each brand were randomly assigned to three groups (n = 15) and subjected to the following: no immersion (control), 1-minute immersion, and 5-minute immersion in 2.5% NaOCl at 37°C. CF for all the files was tested within a well-lubricated stainless-steel artificial canal in a water bath at 37°C simulating body temperature. The procedure was video recorded, and the number of cycles to fracture (NCF) was calculated by multiplying the time taken to fracture, with the number of rotations per second. The data were analyzed for normality, and thereafter, one-way ANOVA with multiple Bonferroni was used as the post hoc test.

Results

The resistance to CF of OC files was significantly higher than PTG files in all groups. In groups immersed in NaOCl for one minute, OC files showed significant drop in the CF resistance; PTG files showed a drop as well but not significantly. Both files demonstrated a significant drop in resistance to CF after immersion in NaOCl for five minutes compared to the control group.

Conclusion

OC files were significantly more resistant to CF compared to PTG in all groups. Immersion in 2.5% NaOCl for 5 minutes significantly reduced the resistance to CF for all the files.

Effects of Simultaneous Liquid or Gel Sodium Hypochlorite Irrigation on the Cyclic Fatigue of Two Single-File Nickel-Titanium Instruments

To evaluate the effect of simultaneous liquid or gel sodium hypochlorite (NaOCl) irrigation on cyclic fatigue of F6 SkyTaper (F6ST) and OneCurve (OC) single files, 180 new 25/0.06 F6ST and OC files were divided into 6 groups (n = 15) for each brand. Groups 1 and 4 included new instruments not exposed to NaOCl at 20 °C and 37 °C, respectively. Groups 2 and 5 included files activated with liquid NaOCl at 20 °C and 37 °C, respectively. Groups 3 and 6 consisted of instruments tested with NaOCl gel at 20 °C and 37 °C, respectively. Instruments were subjected to a fatigue test using a novel customized device. Data were expressed as time to fracture (TtF) and statistically analyzed (p < 0.05) after checking their normality through the Shapiro–Wilk test. Because they were normally distributed, 2-way analyses of variance (ANOVA) and the Tukey multiple comparison post-hoc test were used. Time to fracture of all tested instruments decreased at 37 °C (p < 0.05). At 20 °C, NaOCl improved TtF of F6ST and OC (p < 0.05). NaOCl liquid increased TtF of F6ST (p < 0.05) in comparison with gel, while there was no difference between the two formulations for OC. At 37 °C, both NaOCl formulations had no significant influence on TtF for F6ST, while they increased TtF of OC (p < 0.05). NaOCl improved the cyclic fatigue resistance of OC, independently of the temperature, while for F6ST the negative impact of higher temperature reduced the irrigant benefits.

Comparison of the cyclic fatigue resistance of VDW.ROTATE, TruNatomy, 2Shape, and HyFlex CM nickel-titanium rotary files at body temperature

Objectives

This study aims to compare the cyclic fatigue resistance of VDW.ROTATE, TruNatomy, 2Shape, and HyFlex CM nickel-titanium (NiTi) rotary files at body temperature.

Materials and Methods

In total, 80 VDW.ROTATE (25/0.04), TruNatomy (26/0.04), 2Shape (25/0.04), and HyFlex CM (25/0.04) NiTi rotary files (n = 20 in each group) were subjected to static cyclic fatigue testing at body temperature (37°C) in stainless-steel artificial canals prepared according to the size and taper of the instruments until fracture occurred. The number of cycles to fracture (NCF) was calculated, and the lengths of the fractured fragments were measured. The data were statistically analyzed using a 1-way analysis of variance and post hoc Tamhane tests at the 5% significance level (p < 0.05).

Results

There were significant differences in the cyclic fatigue resistance among the groups (p < 0.05), with the highest to lowest NCF values of the files as follows: VDW.ROTATE, HyFlex CM, 2Shape, and TruNatomy. There was no significant difference in the lengths of the fractured fragments among the groups. The scanning electron microscope images of the files revealed typical characteristics of fracture due to cyclic fatigue.

Conclusions

The VDW.ROTATE files had the highest cyclic fatigue resistance, and the TruNatomy and 2Shape files had the lowest cyclic fatigue resistance in artificial canals at body temperature.

Mechanical Properties and Metallurgical Features of New Green NiTi Reciprocating Instruments

To evaluate the properties of two nickel-titanium (NiTi) reciprocating endodontic instruments (commercially known as Procodile and Reziflow), a total of 40 size 25 and 0.06 taper new Procodile and Reziflow instruments (n = 20) were subjected to cyclic fatigue tests (60° angle of curvature, 5-mm radius) at 20 °C and 37 °C and a torsional test based on ISO 3630-1. The fracture surface of each fragment was examined. The morphological, mechanical, chemical, thermal, and phase composition characteristics of the files were investigated by field-emission gun scanning electron microscopy (FEG-SEM) equipped with an energy-dispersive X-ray (EDX) detector, focused ion beam analysis (FIB), micro-Raman spectroscopy, X-ray diffraction (XRD), differential scanning calorimetry (DSC), and Auger electron spectroscopy (AES). Reziflow showed higher cyclic fatigue resistance than Procodile at 37 °C (p < 0.05). The maximum torsional strength of Procodile was lower than that of Reziflow (p < 0.05). No difference was found between their angular rotations to fracture (p > 0.05). SEM, FIB, Micro-Raman, and AES analyses revealed the presence of an Nb/Nb2O5 coating on the Procodile surface. DSC and XRD analysis confirmed that both files consist of an almost austenitic phase structure at 37 °C. The cyclic fatigue resistance of Procodile and Reziflow significantly decreases upon exposure to body temperature.

Factors influencing incidents of complications while using nickel-titanium rotary instruments for root canal treatment

Background

To investigate the complications associated with the use of nickel titanium rotary instruments (NiTi-RIs) for root canal treatments (RCTs), in Saudi Arabia dental practice, and to explore the influencing factors.

Methods

After obtaining an ethical approval, two pilot studies were conducted to formulate the final questionnaire. The sample size was measured taking into consideration 60% expected response rates and confidence level of 99.9%. The questionnaire was emailed to 600 general dentists (GDs) randomly selected from the dental register and all of the endodontists (175). The email’s introduction clarified objectives of the study and guaranteed that all of the collected information would remain confidential. A reminder was sent after 10 weeks. The data were collected and analyzed using the chi-squared test at a 0.05 significance level.

Results

With a 51% overall response rate, 71.9% off the respondents used NiTi-RIs. The majority (83.1%) experienced complications while using NiTi-RIs; with the instruments’ fracture being significantly the most common complication (52.7%) (p < 0.001). The majority (87.7%) experienced NiTi-RIs’ fractureat least once; with more endodontists (94.3%) than GDs (83.3%) (p < 0.001). The greater the number of weekly performed RCTs and participants’ experiences, the more NiTi-RIs fractures and the greater the number of fracture incidents (p < 0.001). While 60% of those who performed 1–3 RCTs per week experienced NiTi-RIs fractures, 100% of those who performed more than 12 RCTs per week did so. The highest percentage of those who experienced more than 10 fractured NiTi-RIs (60%) was within the group who performed more than 12 RCTs per week. Although fracture incidents decreased with a smaller number of reuses, there was no significant correlation between the number of fractured instruments and NiTi-RIs discard strategy (p ≥ 0.05).

Conclusion

Fracture incidence was the most common complication while using NiTi-RIs, regardless of the clinicians’ experiences and skills. While the single use may reduce NiTi-RIs fractures, to some extent, the greater number of RCTs performed per week was the most influential factor.

Fatigue resistance of ProTaper gold exposed to high-concentration sodium hypochlorite in double curvature artificial canal

This study aimed to evaluate and compare the fatigue resistance of ProTaper Gold (PTG) and ProTaper Universal (PTU) in artificial single and double curvature canals in 5% sodium hypochlorite (NaOCl) at body temperature (37 °C). PTG and PTU files (size F1) were subjected to fatigue tests in two different artificial ceramic canals. The single curvature model had a 60° curvature angle with a 5 mm radius. The double curvature model had a 60° curvature angle with a 5 mm radius and a second 30° curvature with a 2 mm radius. A file segment was introduced into the artificial canal and immersed in water or 5% NaOCl at 37 °C. The total number of cycles to fracture (NCF) was recorded. Data were analyzed using t-test and linear regression analysis. The NCF of all files was significantly influenced by the type of NiTi metal alloy (P < .01), canal curvatures (P < .01), and the environmental conditions (P < .05). PTG had higher fatigue resistance than PTU files in both single and double curvature canals (P < .05). The NCF of PTU files in 5% NaOCl was shorter than that in water (P < .05). The mean length of broken PTG was significantly shorter than those of PTU files in both single and double curvature canals (P < .01). The fatigue performance of PTG is better than that of PTU in both single and double curvature. Environmental conditions may affect the fatigue behavior of PTU files with single curvature.

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.