Correlation between Temperature-dependent Fatigue Resistance and Differential Scanning Calorimetry Analysis for 2 Contemporary Rotary Instruments

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.

Effect of Temperatures on Cyclic Fatigue Resistance of 3 Different Ni-Ti Alloy Files

OBJECTIVE

The aim of this study was to investigate and compare the effect of temperature on the cyclic fatigue resistance of conventional (ProTaper Universal [PTU]), Gold-Wire (ProTaper Gold [PTG]), and Fire-Wire (EdgeTaper Platinum [ETP]) nickel-titanium alloy files.

METHOD

Twenty files from each system were tested for cyclic fatigue resistance in an artificial canal model. The experiments were performed at room temperature and body temperature in controlled temperature water. Magnified videos were recorded using a dental operating microscope integrated camera during testing to detect file fracture. The number of cycles to failure (NCF) was calculated. The type of failure was investigated macroscopically and microscopically with a dental operating microscope and scanning electron microscope, respectively.

RESULT

The NCF at room temperature was significantly higher compared with body temperature in each system (P < .001). Compared at the same temperature, the ETP group demonstrated the highest NCF, followed by the PTG and PTU groups (P < .001). All files demonstrated cyclic fatigue failure macroscopically and microscopically.

CONCLUSIONS

The 3 alloy files were affected by temperature. The cyclic fatigue resistance was reduced at the higher temperature and increased at the lower temperature. If the files are geometrically identical, files made of Fire-Wire are preferred compared with Gold-Wire and conventional nickel-titanium alloys based on cyclic fatigue resistance.

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.

Dynamic Cyclic Fatigue and Differential Scanning Calorimetry Analysis of R-Motion

INTRODUCTION

The aim of this study was to assess the dynamic cyclic fatigue resistance of an R-Motion file at simulated body temperature and document corresponding phase transformations compared to OneCurve and HyFlex EDM (HFEDM).

METHODS

R-Motion (25/.06), OneCurve (25/.06), and HFEDM (25/.06) files were selected and divided into 3 groups (n = 9) according to the file type. Dynamic cyclic fatigue testing was done with a custom-made artificial stainless-steel canal that had a 90° angle of curvature and a 5-mm radius of curvature. Files were operated continuously at body temperature until fracture in the artificial canal. The time to fracture was calculated. Statistical analysis was performed, and significance was set at 5%. Phase transformation temperatures for 2 instruments of each group were analysed by differential scanning calorimetry (DSC) analysis.

RESULTS

The highest mean time to fracture value was measured in the HFEDM group (277.84 ± 2.51), followed by the R-Motion group (115.09 ± 0.01), whilst the lowest value was found in the OneCurve group (44.28 ± 3.63). Post hoc pairwise comparisons were all statistically significant (P < .001). DSC heating curves show austinite start temperatures to be 33.94 °C and 43.32 °C and austinite finish temperatures to be 35.09 °C and 50 °C for R-Motion and HFEDM, respectively. DSC cooling curves show martensite start temperatures to be 27.54 °C and 44.52 °C and martensite finish temperatures to be 29.13 °C and 37.68 °C for R-Motion and HFEDM, respectively. DSC curves of OneCurve failed to demonstrate transformation temperatures within the tested heat range.

CONCLUSIONS

Crystalline arrangement of Ni and Ti atoms within the NiTi alloys greatly affects the dynamic cyclic fatigue resistance of the file.

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.

Mechanical Properties and Root Canal Shaping Ability of a Nickel-Titanium Rotary System for Minimally Invasive Endodontic Treatment: A Comparative In Vitro Study

Selection of an appropriate nickel−titanium (NiTi) rotary system is important for minimally invasive endodontic treatment, which aims to preserve as much root canal dentin as possible. This study aimed to evaluate selected mechanical properties and the root canal shaping ability of TruNatomy (TRN), a NiTi rotary system designed for minimally invasive endodontic shaping, in comparison with existing instruments: HyFlex EDM (HEDM), ProTaper Next (PTN), and WaveOne Gold (WOG). Load values measured with a cantilever bending test were ranked as TRN < HEDM < WOG < PTN (p < 0.05). A dynamic cyclic fatigue test revealed that the number of cycles to fracture was ranked as HEDM > WOG > TRN > PTN (p < 0.05). Torque and vertical force generated during instrumentation of J-shaped artificial resin canals were measured using an automated instrumentation device connected to a torque and vertical force measuring system; TRN exhibited smaller torque and vertical force values in most comparisons with the other instruments. The canal centering ratio for TRN was smaller than or comparable to that for the other instruments except for WOG at the apex level. Under the present experimental conditions, TRN showed higher flexibility and lower torque and vertical force values than the other instruments.

Torsional Fracture Resistance of Niti Rotary Glide Path Files under Flexural Stress

This study aimed to evaluate the effects of flexural stress on the torsional fracture resistance of NiTi glide path files. PathFile #16/02, RaCe #15/04, RaCe Evo #15/04, HyFlex EDM #15/03, TruNatomy Glider #17/02, and V Taper 2H #17/04 were examined by scanning electron microscopy (SEM) (n = 3/brand) and subjected to differential scanning calorimetry (n = 2/brand). Torsional fracture resistance testing was performed in straight (ISO 3630-1) and flexural modes (n = 15/brand/mode). Flexural mode testing involved instruments rotating within a stainless-steel artificial double-curved canal. Ultimate strength and distortion angle until failure were recorded, and fractured instruments were examined by SEM. Statistical analyses involved independent sample t-test and one-way analysis of variance with Games–Howell pots hoc test. Austenitic transformation- finishing temperatures of heat-treated files were above body temperature. For RaCe Evo, HyFlex EDM, TruNatomy Glider, and V Taper 2H, the flexural mode resulted in a significantly higher distortion angle compared to the straight mode (p < 0.05). The maximum torque of RaCe Evo, HyFlex EDM increased with the flexural stress (p < 0.05). V taper 2H showed the highest distortion angle and ultimate strength. SEM showed typical patterns of torsional fracture for all tested files. The flexural stress positively affected distortion angle of heat-treated NiTi glide path files.

Present Status and Future Directions - Canal Shaping

This narrative review will focus on the evolution, present and future of engine‐driven root canal preparation. Root canal preparation changed drastically when Walia in 1988 introduced the use of nickel‐titanium (NiTi) alloys in Endodontics. In 2013, five generations of NiTi endodontic instruments had been established based on their metallurgical, mechanical properties and design features. Since then, manufacturers have been introducing further major changes in instrument design and characteristics that have not been translated in new recognized generations of instruments. In general, those changes have demonstrated enhanced instrument properties, but it is not clear yet if all those improvements are directly translated to an improvement in clinical success. This narrative review attempts to address the present status of engine‐driven instruments in terms of both evidence from laboratory‐based studies and clinical data, to identify potential further generations of instruments, and last to anticipate future directions for research and development.

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).

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.

Differential scanning calorimetric investigations of three rotary nickel-titanium instrument systems before and after simulated clinical uses

BACKGROUND

The transformation temperatures were important values, influencing the mechanical properties and clinical performance of nickel-titanium instruments. The aim of this study was to determine the transformation temperatures of three rotary nickel-titanium (NiTi) instruments: Reciproc, HyFlex CM Pro, and Neoniti before and after simulated clinical uses.

METHODS

Ninety new NiTi instruments of three single-file instruments: Reciproc, HyFlex CM Pro, and Neoniti were divided into three groups. Thirty instruments of each group were divided into 3 subgroups (10 instruments for each subgroup): new, one-time simulated clinical used and sterilised, and three times simulated clinical used and sterilized subgroups. The instruments were in the as-received condition for the new subgroups, one time used in the plastic endo-training blocks and sterilised for the one-time subgroups, and three times used in the plastic endo-training blocks and sterilised for the three times subgroups. Each instrument in subgroups was cut into four small segments of 4-5 mm. All segments of instruments were analysed using Differential Scanning Calorimetry (DSC). Data was collected and analysed using SPSS version 20.0 with ANOVA test or Kruskal-Wallis test at the significant level of 0.05.

RESULTS

There was not significant difference between before and after simulated clinical use with sterilised procedure in three NiTi instrument systems. The austenite-finish (A_f) temperatures of three instrument systems were higher than that of the human body (37 °C), of these, the A_f temperature of Neoniti was highest and that of HyFlex CM Pro was lowest.

CONCLUSIONS

The austenite-finish (A_f) temperatures of three NiTi instruments were higher than that of human body temperature, therefore, material was in the phase transformation from martensite to austenite, gives the instruments more flexibility when used in the clinical situation.

Cyclic Fatigue Resistance of Blue Heat-Treated Instruments at Different Temperatures

The main aim is to evaluate the cyclic fatigue resistance of blue heat-treated instruments with different kinematics. Twenty-four endodontic instruments of the same brand were used for each of three experimental groups: VB (Vortex Blue 40/0.04), RB (RECIPROC Blue 40/0.06), and XB (X1 Blue 40/0.06). The instruments were randomly distributed and subjected to temperatures of 20°C and 37°C. The fatigue test was performed using a stainless steel device. Data were analysed using the Shapiro-Wilk test, Student's t-test, the F test, and Tukey's and Tamhane tests at significance level P=0.05. The instruments' cyclic fatigue resistance at both temperatures differed significantly for each instrument type (P < 0.001). The RB instruments displayed greater cyclic fatigue resistance at the tested temperatures compared with the VB and XB instruments (P < 0.001). Reciprocating kinematics positively influenced cyclic fatigue resistance. Blue heat-treated instruments showed decreased cyclic fatigue resistance as the temperature increased (P < 0.001).

Comparative evaluation of mechanical properties and shaping performance of heat-treated nickel titanium rotary instruments used in the single-length technique

This study aimed to evaluate the mechanical properties of contemporary heat-treated nickel-titanium (NiTi) rotary instruments used in the single-length technique [ProTaper Next (PTN), HyFlex EDM (EDM), and JIZAI (JZ)]. Bending loads, cyclic fatigue resistance, torque/force values and canal-centering ratios were evaluated for the three instruments and a non-heat-treated experimental NiTi instrument with the same geometry as JZ (nJZ). EDM and JZ exhibited significantly lower bending load and more cycles to failure compared with nJZ and PTN (p<0.05). PTN and JZ exhibited significantly better centering ability than nJZ and EDM (p<0.05). JZ and nJZ generated significantly smaller upward force and maximum torque than PTN and EDM (p<0.05). Under the present experimental condition, JZ exhibited flexibility and cyclic fatigue resistance comparable to EDM, better maintained the canal curvature than the other instruments, and generated smaller torque and screw-in force than PTN and EDM.

Cyclic Fatigue and Torsional Resistance of Four Martensite-Based Nickel Titanium Reciprocating Instruments

Objective:

To evaluate cyclic fatigue and torsional resistance of Reciproc Blue R25 (VDW, Munich, Germany), WaveOne Gold Primary (Dentsply Maillefer, Ballaigues, Switzerland), ProDesign R (Easy Equipamentos Odontológicos, Belo Horizonte, Brazil), and X1 Blue File (MK Life, Porto Alegre, Brazil) nickel titanium (NiTi) martensite reciprocating instruments.

Methods:

In each group, ten instruments were tested for cyclic fatigue resistance using a stainless-steel artificial canal (curvature angle of 80° and radius of 3 mm) and ten instruments for torsional failure according to ISO 3630-1 standard. The surface of the fractured instruments was examined under scanning electron microscopy (SEM) at ×250 magnification. The results were compared statistically with one-way ANOVA and post hoc Tukey tests and the alpha-type error was set at 5%.

Results:

X1 blue file and ProDesign Rinstruments showed highest time to fracture than Reciproc Blue and Wave One Gold (P<0.05). However, no differences were found between X1 Blue File and ProDesign R (P>0.05). In addition, Reciproc Blue demonstrated highest time to fracture than WaveOne Gold (P<0.05). The lowest torsional resistance (1.0±0.2 N.cm) and angle of rotation (412º±46) was observed in the ProDesign R group (P<0.05). SEM analyses of fractured surfaces showed a crack initiation area and overload fast fracture zone after cyclic fatigue test, and concentric abrasion marks with microvoids at the centre of rotation after torsional failure experiment.

Conclusion:

Overall, X1 Blue File and ProDesign R showed higher cyclic fatigue resistance than Reciproc Blue and WaveOne Gold instruments, while ProDesign R had the lowest torsional resistance and angular rotation values to fracture. SEM analysis of all instruments demonstrated typical failures features in both cyclic fatigue and torsional failure tests.

Effect of Curvature Location on Fatigue Resistance of Five Nickel-titanium Files Determined at Body Temperature

INTRODUCTION

The purpose of this study was to evaluate the effect of 5 different curvature locations on the fatigue resistance of thermomechanically treated nickel-titanium (NiTi) files and superelastic NiTi files at body temperature and to document the corresponding phase transformations.

METHODS

EndoSequence (ES; Brasseler USA, Savannah, GA), EndoSequence CM (ESCM, Brasseler USA), K3 (SybronEndo, Orange, CA), K3XF (SybronEndo), and Vortex Blue (VB; Dentsply Tulsa Dental Specialties, Tulsa, OK) NiTi files (size 25/.04) were subjected to fatigue tests inside customized artificial canals containing a curvature of 60° and a 3-mm radius. There were 5 different canals based on the location of initial curvature; these included groups in which the distance between the canal orifice and the location of the curvature (DOC) was 5, 6, 8, 10, and 11 mm. The model canal was immersed in water at 37°C ± 1°C. The number of cycles to failure (N_f) was recorded, and the fracture surface of the fragments was examined by scanning electron microscopy. Two unused files of each brand were examined by differential scanning calorimetry.

RESULTS

The N_f was highest in the 10-mm and 11-mm DOC groups and lowest with the 5-mm and 6-mm DOC groups (P < .05). ESCM files had the highest fatigue resistance followed by the VB, K3XF, K3, and ES files (P < .05). ESCM files had the highest N_f (P < .05), and ES and K3 files had the lowest N_f (P < .05) depending on the curvature location. Two endothermic peaks were observed on the heating curve of the heat-treated files (ESCM, K3XF, and VB).

CONCLUSIONS

The location of the canal curvature had a significant effect on the fatigue resistance of both heat-treated and superelastic NiTi files. The fatigue life of files in the coronal curve was quite short.

Effect of canal curvature location on the cyclic fatigue resistance of reciprocating files

OBJECTIVES

To determine the effect of the location of the canal curvature on the fatigue resistance of WaveOne (WO), WaveOne Gold (WOG), Reciproc (Rec), and Reciproc Blue (RecB) files, and to examine the phase transformation behaviors of the reciprocating file systems.

MATERIAL AND METHODS

The instruments were subjected to fatigue testing in five artificial canals with a curvature of 60° angle and a 3-mm radius. The location of the curvature was unique for each canal. Each file was inserted 16 mm into the canal and operated until fracture occurred. The time to fracture was recorded and the length of the fragment was measured. Differential scanning calorimetry was used to characterize the thermal behavior of the files. The number of cycles to failure was analyzed using two-way analysis of variance and the post hoc Tukey test. The Kruskal-Wallis test was used to compare the mean fragment lengths between groups.

RESULTS

The instruments had significantly lower fatigue resistance in canals with curvatures in the middle and coronal canals compared with those with apical curvatures (p < 0.05). At all tested curvature locations, RecB had superior fatigue resistance compared with WO and Rec (p < 0.05). There were no significant differences between WOG and Rec in canals with curvatures in the middle and coronal canals. The DSC thermograms for RecB exhibit a single exothermic peak during cooling but double endothermic peaks during heating indicating that a two-step phase transformation from martensite to R-phase to austenite takes place.

CONCLUSIONS

The reciprocating instruments experience decreased cyclic fatigue resistance when operated in canals with coronal- and middle-third curvatures when compared with curvatures in the apical-third. Instrumenting coronally positioned curvatures with reciprocating files needs to be performed with caution.

CLINICAL RELEVANCE

The location of the root canal's curvature influences the fracture resistance of rotary files that are used with reciprocating movements. Therefore, caution needs to be exercised when using reciprocating instruments in canals with coronal or middle curvatures.

Effect of gamma-ray sterilization on phase transformation behavior and fatigue resistance of contemporary nickel-titanium instruments

OBJECTIVES

The aim of this study was to compare body temperature cyclic fatigue resistance (CF) of heat-treated NiTi rotary instruments submitted to gamma-irradiation or autoclave sterilization.

MATERIALS AND METHODS

A total of 176 instruments (88 Hyflex EDM (HEDM) and 88 TRUShape (TS)) were used. Forty-four of each were submitted to gamma-irradiation. Twenty-two in each group were further autoclaved, resulting in a total of eight groups (n = 22 each): TS-γ and HEDM-γ (gamma-irradiated)/TS-γ-ac and HEDM-γ-ac (gamma-irradiated and autoclaved)/TS-non-γ and HEDM-non-γ (non-sterilized controls, non-gamma-irradiated/non-autoclaved)/TS-non-γ-ac and HEDM-non-γ-ac (non-gamma-irradiated and autoclaved). Twenty in each were tested for CF. Weibull parameters were calculated and compared. The remainder was used for differential scanning calorimetry analysis.

RESULTS

TS instruments lasted significantly longer if not submitted to gamma-irradiation (TS-non-γ mean-life = 101.5 s, 95 CI% 91.7-112.3; TS-γ mean-life = 83.2 s, 95 CI% 76-91.1); further autoclaving did not significantly affect CF. Gamma-irradiation did not affect HEDM (HEDM-γ mean-life = 717.9 s, 95 CI% 636.8-809.3; HEDM-non-γ: mean-life = 678.8 s, 95 CI% 595.1-744.2); further sterilization significantly decreased fatigue resistance (HEDM-γ-ac mean-life = 524.1 s, 95 CI% 476.1-576.8; HEDM-non-γ-ac mean-life = 570.6, 95 CI% 512.3-635.5). Gamma irradiation benefited HEDM conversion to martensite state and its maintenance, while further autoclave sterilization reverted the benefit.

CONCLUSIONS

Gamma-irradiation and autoclaving differentially affected fatigue life span and transformation temperatures of contemporary martensitic rotary instruments manufactured with different methods.

CLINICAL RELEVANCE

Gamma-ray processing is increasingly utilized by manufacturers for clinician to use fully sterile packaged instruments; however, the sterilization method of choice should vary depending on the effects on the behavior of contemporary martensitic rotary instruments manufactured with different methods.

Bending resistance and cyclic fatigue resistance of WaveOne Gold, Reciproc Blue, and HyFlex EDM instruments

Background/purpose

Several single-file systems manufactured using different heat treatment and operated by different kinematics have been released. This study compared the bending resistance and cyclic fatigue resistance of three NiTi files, and examined their phase-transformation behavior.

Materials and methods

WaveOne Gold Primary (WOG), Reciproc Blue R25 (RPB), and HyFlex EDM OneFile (HDM) were tested (N = 40/instrument). A vertical load was applied to a point 3 mm from the tip, and the stress was measured until a displacement of 3 mm. Tests were conducted at either room temperature (RT: 22 °C) or body temperature (BT: 37 °C) (n = 10). Cyclic fatigue resistance tests were performed in an artificial canal, which had a curvature angle of 40° and a 5-mm radius. Tests were conducted at either RT or BT (n = 10). Instruments were operated according to the manufacturers' instructions. Test results were analyzed using the Kruskal–Wallis and the Mann–Whitney tests. Additional three instruments of each brand were subjected to differential scanning calorimetry (DSC).

Results

At RT the bending resistance of three files were not significantly different. However, at BT the bending resistance of RPB was highest, followed by WOG, and HDM (P < 0.05). At RT, RPB demonstrated the longest fracture time, followed by HDM, and WOG (P < 0.05). At BT, HDM had the longest fracture time, followed by RPB, and WOG (P < 0.05). The WOG, RPB consisted of austenite in a considerable proportion, whereas HDM was mainly martensite state at BT.

Conclusion

HDM presented superior flexibility and cyclic fatigue resistance at BT.

Effect of autoclave sterilization on cyclic fatigue and torsional fracture resistance of NiTi rotary instruments

The purpose of this study is to assess the effect of autoclave sterilization on the cyclic fatigue and torsional fracture resistance of ProTaper Universal (PTU), K3XF, HyFlex EDM (EDM), and TF adaptive (TFA). Sixty instruments from each file type were divided into two categories for cyclic fatigue group (CGr) and torsional fracture group (TGr). CGr and TGr were divided into three subgroups, respectively, consisting of ten instruments from each file type. Cyclic fatigue fracture test was performed using artificial canal made of stainless steel, and the mean number of cycles to failure (NCF) were determined. CGr1, the files were tested to establish baseline for NCF; CGr2, the files were tested cyclic fatigue after 10 cycles of autoclave; CGr3, instruments were autoclaved after being cycled to 25, 50, and 75% of corresponding NCF determined in CGr1, followed by cyclic fatigue test. The torsional fracture test was performed without autoclave (TGr1), after 3-cycle autoclave (TGr2), and 7-cycle autoclave (TGr3), respectively, which evaluated maximum torque and angular deflection. NCF, maximum torque and angular deflection were compared using one-way ANOVA with Bonferroni test. Two-way ANOVA was performed to determine the interaction between 'autoclave treatment' and 'type of NiTi file'. EDM showed highest NCF within the same autoclave treatment. TFA presented the lowest maximum torque and the highest angular deflection, and PTU presented the lowest angular deflection. Within the same NiTi file systems, most of NCF, maximum torque and angular deflection of tested files were not significantly influenced by autoclave condition.

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.

Effect of Temperature of Sodium Hypochlorite on Cyclic Fatigue Resistance of Heat-treated Reciprocating Files

INTRODUCTION

The aim of this study was to evaluate the effect of different temperatures of sodium hypochlorite (NaOCl) on the cyclic fatigue resistance of the heat-treated file systems.

METHODS

The cyclic fatigue resistance of Reciproc 25 (VDW Dental, Munich, Germany), Reciproc Blue R25 (VDW), WaveOne Primary (Dentsply Maillefer, Ballaigues, Switzerland), WaveOne Gold Primary (Dentsply Maillefer), and One Shape (25.06; Micro Mega, Besançon, France) was tested in the following conditions: control (no immersion), immersion in distilled water (DW) at 37°C ± 1°C, immersion in DW at 60°C ± 1°C, immersion in NaOCl at 37°C ± 1°C, and immersion in NaOCl at 60°C ± 1°C. The immersion period was 5 minutes for all conditions. A stainless steel artificial canal with a curvature of 60° and a 5-mm radius was used. The time to failure and fragment lengths were recorded for each instrument, and data were subjected to statistical analysis.

RESULTS

Reciproc Blue showed greater cyclic fatigue resistance in all conditions compared with the other systems (P < .05). Immersion in NaOCl at 60 °C negatively affected all instruments' cyclic fatigue resistance except Reciproc Blue. Immersion in DW at 60 °C increased the cyclic fatigue resistance of Reciproc Blue. Heat-treated files presented higher fatigue resistance than traditional files.

CONCLUSIONS

A high temperature of NaOCl affected the cyclic fatigue life of the file systems. Instruments produced with heat treatment presented a longer fatigue life.

Impact of a modified motion on the fatigue life of NiTi reciprocating instruments: a Weibull analysis

OBJECTIVES

To evaluate the impact of a modified motion on the fatigue life of four brands of nickel-titanium (NiTi) reciprocating instruments.

MATERIALS AND METHODS

Cyclic fatigue (CF) resistance of 160 instruments was evaluated in an artificial stainless-steel canal (90° angle, 5-mm radius of curvature). WaveOne and WaveOne Gold (Denstply Maillefer, Baillagues, Switzerland) and Reciproc and Reciproc Blue (VDW, Munich, Germany) were tested with two different motions: (1) X-Smart Plus (Denstply Maillefer) endodontic motor and (2) a 4:1 contra-angle with an experimental motion (EVO) with different rotation angles and based on a sinusoidal acceleration. Motions with X-Smart Plus and EVO were recorded and analyzed at a reduced speed with VLC Media Player software for a more accurate analysis. Mean half-life, beta, and eta Weibull parameters were determined and compared.

RESULTS

Reciproc Blue resulted the most resistant instruments either with EVO or X-Smart. WaveOne Gold lasted significantly longer than WaveOne with EVO (probability of 91%) while no significant differences were found with X-Smart. Considering NCF, Reciproc, WaveOne Gold, and Reciproc Blue lasted significantly longer with EVO (probabilities of 66%, 80%, and 89% respectively). WaveOne Gold showed the highest beta parameter.

CONCLUSIONS

The experimental motion was found to have a positive impact on fatigue lifetime of reciprocating instruments.

CLINICAL RELEVANCE

Current findings provide insight for future improvements in the clinical use of reciprocating files. Experimental motions may be considered when searching for additional strategies in order to increase the safer use of NiTi files during endodontic procedures.

Nickel-titanium instruments in endodontics: a concise review of the state of the art

The introduction of automated instrumentation in endodontics represented a major advance in progress for this specialty, with improvements in the quality and predictability of root canal preparation and a significant reduction in procedural errors. In recent years, endodontic instruments have undergone a series of changes brought about by modifications in design, surface treatments, and thermal treatments. In addition, new movements have also been incorporated to offer greater safety and efficiency, optimizing the properties of the NiTi alloy, especially through eccentric rotary motion. An understanding of the mechanical properties of these new NiTi instruments and their effect on the clinical performance of root canal preparation is essential if dental practitioners are to select the instruments that provide optimal clinical outcomes, especially in curved or flattened canals. The objective of this literature review is to present and discuss the characteristics of the NiTi alloys used in the major instrumentation systems available in the market, as well as the influence of the metallurgical and mechanical properties of NiTi instruments and the movements that drive them, to enable more accurate and predictable planning of root canal preparation.