Influence of Different Angles of File Access on Cyclic Fatigue Resistance of Reciproc and Reciproc Blue Instruments

Influence of the brushing motions on the dynamic cyclic fatigue resistance of the Reciproc Blue instrument: In vitro study

INTRODUCTION

This study aimed to evaluate the dynamic cyclic fatigue resistance of Reciproc® Blue files subjected to brushing motion.

METHODS

Twenty Reciproc® Blue files (R25, VDW, Munich, Germany) were employed for chemical-mechanical preparation in root canals of 3D printed replicas of mandibular molars. The instruments were divided into two groups (n=10): Group BM, instrumentation with brushing motions; Group WBM, instrumentation without brushing motions. Subsequently, the same instruments underwent cyclic fatigue testing using artificial canals consisting of a stainless-steel plate and an apical segment with a 45° curvature. Data regarding the time to fracture, number of cycles to fracture (NCF), and length of the fractured fragment were collected and subjected to statistical analysis using the t-test or Mann-Whitney test, with a significance level of 5%. Additionally, the fractured fragments were examined under a scanning electron microscope.

RESULTS

No statistical differences were observed between the groups in terms of time to fracture (p<0.05), NCF (p<0.05), and length of the fractured fragment (p<0.05).

CONCLUSION

The brushing motion did not affect the dynamic cyclic fatigue resistance of Reciproc Blue files.

Canal-Centering and Apical Transportation Ability of Similar Cross-Section NiTi Instruments Working with Different Kinematics-Micro-CT-based In Vitro Analysis

Objective

Evaluating the canal-centering and apical transportation ability of endodontic file systems working with different kinematics but of similar cross section.

Materials and Methods

Sixty human maxillary first molar mesiobuccal (MB) roots were assigned to three experimental groups based on instrumentation techniques: Reciproc Blue (RB), Mtwo (M2), and OneShape (OS). Pre- and post-instrumentation micro-computed tomographic analysis was performed. Centering ability and apical transportation were analyzed at 3 mm, 6 mm, and 9 mm short of the apex. Statistical analysis was conducted using Mann-Whitney U test and Kruskal-Wallis test.

Results

OS showed better canal-centering ability than RB and M2 at 3 mm, 6 mm, and 9 mm. No significant difference among the tested groups was observed during the assessment of apical transportation (P < 0.05).

Conclusion

The systems evaluated safely prepared curved MB canals with minimal canal transportation. OS showed superior canal-centering ability compared to the other two groups.

Impact of pecking amplitude on cyclic fatigue of new nickel-titanium files

OBJECTIVES

The aim of this study is to determine the cyclic fatigue resistance of Mtwo Minimal in static and dynamic tests, with different amplitudes of pecking movements, at intracanal temperature.

MATERIALS AND METHODS

Two hundred new 25-mm Mtwo Minimal rotary files (#10/0.035, #17.5/0.045, #25/0.05, #40/0.03, #45/0.03) were tested in static and dynamic cyclic fatigue tests at 35°C (±1°C). An artificial stainless-steel canal was used. In the dynamic mode, axial movements were set at 1 and 3 mm. The number of cycles to fracture (NCF) was recorded and statistically analyzed by one-way analysis of variance (p < 0.05).

RESULTS

The 3-mm dynamic test showed significantly increased NCF than the other tests for the #10/0.035, #17.5/0.045, and #25/0.05 files (p < 0.05). The #40/0.03 and #45/0.03 files showed no significant differences in all the tests (p > 0.05).

CONCLUSION

Mtwo Minimal showed higher cyclic fatigue resistance in the dynamic test than the static test, except for the larger instruments. The 3-mm pecking amplitude increased the cyclic fatigue resistance of the smaller instruments.

Influence of NiTi Wire Diameter on Cyclic and Torsional Fatigue Resistance of Different Heat-Treated Endodontic Instruments

We compared the mechanical properties of 2Shape mini TS2 (Micro-Mega, Besançon, France) obtained from 1.0 diameter nickel-titanium (NiTi) wires and 2Shape TS2 from 1.2 diameter nickel-titanium (NiTi) wires differently thermally treated at room and body temperature. We used 120 NiTi TS2 1.0 and TS2 1.2 files made from controlled memory (CM) wire and T-wire (n = 10). Cyclic fatigue resistance was tested by recording the number of cycles to fracture (NCF) at room and body temperatures using a customized testing device. Maximum torque and angle of rotation at failure were recorded, according to ISO 3630-1. Data were analyzed by a two-way ANOVA (p < 0.05). The CM-wire files had significantly higher NCFs at both temperatures, independent of wire dimensions. Testing at body temperature negatively affected cyclic fatigue of all files. The 1.0-mm diameter T-wire instruments showed higher NCF than the 1.2-mm diameter, whereas no significant differences emerged between the two CM wires at either temperature. The maximum torque was not significantly different across files. The TS2 CM-wire files showed significantly higher angular rotation to fracture than T-wire files. The TS2 CM-wire prototypes showed higher cyclic fatigue resistance than T-wire prototypes, regardless of wire size, exhibiting suitable torsional properties. Torsional behavior appears to not be affected by NiTi wire size.

Impact of minimally invasive endodontics procedures on the development of dentinal microcracks

INTRODUCTION

To evaluate the impact of root canal preparation in teeth with ultraconservative access cavities (UltraAC) on the development of dentinal microcracks using a non-destructive longitudinal micro-CT imaging experimental model. Root canal preparation in teeth with traditional access cavities (TradAC) was used as control.

METHODS

Forty mandibular molars were scanned in a micro-CT device, anatomically matched, and distributed into 4 groups according to the type of access cavity and instrumentation system: traditional/Reciproc (TradAC/RC); traditional/XP-endo Shaper (TradAC/XP); ultraconservative/Reciproc (UltraAC/RC) and ultraconservative/XP-endo Shaper (UltraAC/XP). After root canal preparation, the teeth were rescanned. After reconstruction and co-registration procedures, the images were screened from the furcation level to the apex to identify the presence of dentinal microcracks.

RESULTS

From a total of 15340 cross-section images, 19.65% (3014 slices) had some dentinal microcrack. The qualitative analysis demonstrated the presence of some dentinal microcrack in 11%, 33%, 19%, and 15% of the images of cross-sections in TradAC/RC, TradAC/XP, UltraAC/RC, and UltraAC/XP groups, respectively. All dentinal microcracks observed after root canal preparation were already present in the corresponding images before canal instrumentation. Therefore, no new microcracks were detected, regardless of the access cavity and root canal instrumentation system.

CONCLUSIONS

Root canal preparation with Reciproc or XP-Endo Shaper under traditional or ultraconservative access cavities did not create dentinal microcracks in extracted mandibular molars.

Present status and future directions - Minimal endodontic access cavities

In the last decades, the move of Medicine towards minimally invasive treatments is notorious and scientifically grounded. As Dentistry naturally follows its footsteps, minimal access preparation also became a trend topic in the endodontic field. This procedure aims to maximize preservation of dentine tissue, backed up by the idea that this is an effective way to reduce the incidence of post-treatment tooth fracture. However, with the assessment of the body of evidence on this topic, it is possible to observe some key-points (a) the demand for nomenclature standardization, (b) the requirement of specific tools such as ultra-flexible instruments, visual magnification, superior illumination, and three-dimensional imaging technology, (c) minimally invasive treatment does not seem to affect orifice location and mechanical preparation when using adequate armamentarium, but it (d) may impair adequate canal cleaning, disinfection, and filling procedures, and also (e) it displays contradictory results regarding the ability to increase the tooth strengthen compared to the traditional access cavity. In spite of that, it is undeniable that methodological flaws of some benchtop studies using extracted teeth may be responsible for the conflicting data, thus triggering the need for more sophisticated devices/facilities and specifically designed research in an attempt to make it clear the role of the access size/design on long-term teeth survival. Moreover, it is inevitable that a clinical approach like minimal endodontic access cavities that demands complex tools and skilled and experienced operators bring to the fore doubts on its educational impact mainly when confronted with the conflicting scientific output, ultimately provoking a cost-benefit analysis of its implementation as a routine technique. In addition, this review discusses the ongoing scientific and clinical status of minimally invasive access cavities aiming to input an in-depth and unbiased view over the rationale behind them, uncovering not only the related conceptual and scientific flaws, but also outlining future directions for research and clinical practices. The conclusions attempt to skip from passionate disputes highlighting the current body of evidence as weak and incomplete to guide decision making, demanding the development of a close-to-in-situ laboratory model or a large and well-controlled clinical trial to solve this matter.

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.

Influence of access cavity preparation on the dentine thickness of mesial canals of mandibular molars prepared with reciprocating instruments

AIM

To evaluate the influence of traditional and conservative access cavity preparations on the remaining dentine thickness in the coronal third of mesial canals of extracted mandibular molars prepared with reciprocating instruments using micro-computed tomography as the analytical tool.

METHODOLOGY

Seventy extracted mandibular molars were scanned at a pixel size of 19 μm. From this initial sample, 20 teeth were selected, pair-matched and distributed into two groups (n = 10) according to the access cavity preparation: traditional (TradAC) or conservative (ConsAC). The root canals were sequentially enlarged with Reciproc Blue R25 (size 25, 0.08v taper) and R40 (size 40, 0.06v taper) instruments. A new scan was performed and the postoperative stacks were coregistered with their respective preoperative datasets. A colour-coded cross sections of the roots were created and used to identify and measure the smallest dentine thickness related to both MB and ML canals at 1.0-mm intervals from the furcation level of up to 5 mm in the apical direction, in both mesial and distal aspects of the roots, before and after preparation. The statistical analyses were performed with paired-samples t-test, independent-samples Student T-test and Chi-Square test with a significance level of 5%.

RESULTS

At all levels of both groups, dentine thickness before preparation was greater than after preparation (p < .05). No difference in the percentage of dentine reduction was observed between TradAC and ConsAC groups (p > .05), but a significantly greater reduction was observed to the distal aspect of the roots (p < .05). After root canal preparation, dentine thinner than 0.5 mm was observed mostly along the distal aspect of the root (10% to 15%) of the MB and ML canals, with no influence of the access cavity type on its incidence to either mesial (X²  = 1.66; p = .2) or distal (X²  = 0.40; p = .5) directions. In the TradAC group, dentine thickness in most slices was greater than 1.0 mm after preparation (n = 124) whilst, in the ConsAC, it ranged from 0.5 to 1.0 mm (n = 136).

CONCLUSION

Traditional or conservative access cavity preparation in extracted mandibular molars did not influence the remaining dentine thickness in the coronal third of mesial canals enlarged with thermomechanically treated nickel-titanium reciprocating instruments.

Influence of conservative endodontic access cavities on instrumentation of oval-shaped straight root canals

AIM

To evaluate the influence of the design of endodontic access cavities on the percentage of unprepared areas of canal walls and flexural fatigue of instruments activated by reciprocating movement in oval-shaped straight root canals of extracted teeth.

METHODOLOGY

Forty-two mandibular incisors with oval canals were scanned by a microcomputed tomography (micro-CT) device for homogeneous selection and distribution of the samples. Then, the teeth were divided into two groups (n = 21) according to the design of access cavity being tested: ultraconservative endodontic access cavity (UltraAC) and traditional access cavity. The canals were accessed with the aid of a surgical microscope, instrumented with the WaveOne Gold Medium system and irrigated with 2.5% NaOCl and 17% EDTA. The unprepared areas of the canal wall were analysed by overlaying images before and after instrumentation and expressed as percentages. micro-CT data were analysed using t-test, Mann-Whitney and Wilcoxon tests. The endodontic instruments used during instrumentation were subjected to static flexural fatigue testing using an artificial stainless steel canal with a 60° angle of curvature and a radius of 5 mm, located 5 mm from the tip of the instrument. The instruments were activated until fracture occurred, and the time in seconds for the fracture was recorded using a digital timer. The number of cycles to fracture was calculated and analysed statistically. For flexural fatigue data, an anova test complemented by a Tukey range test was used. The significance level of 5% was used for all analyses.

RESULTS

There was no significant difference between the groups related to unprepared areas by the instrument during canal preparation (p > .05). The difference in flexural fatigue resistance between the groups was not significant.

CONCLUSION

The use of UltraAC did not interfere with the canal instrumentation of extracted mandibular incisors with straight and oval canals. There was no difference in the flexural fatigue resistance of the instruments in relation to access cavity design.

Ten years of minimally invasive access cavities in Endodontics: a bibliometric analysis of the 25 most-cited studies

Objectives

This study aimed to analyze the main features of the 25 most-cited articles in minimally invasive access cavities.

Materials and Methods

An electronic search was conducted on the Clarivate Analytics' Web of Science ‘All Databases’ to identify the most-cited articles related to this topic. Citation counts were cross-matched with data from Elsevier's Scopus and Google Scholar. Information about authors, contributing institutions and countries, year and journal of publication, study design and topic, access cavity, and keywords were analyzed.

Results

The top 25 most-cited articles received a total of 572 (Web of Science), 1,160 (Google Scholar) and 631 (Scopus) citations. It was observed a positive significant association between the number of citations and age of publication (r = 0.6907, p < 0.0001); however, there was no significant association regarding citation density and age of publication (r = −0.2631, p = 0.2038). The Journal of Endodontics made the highest contribution (n = 15, 60%). The United States had the largest number of publications (n = 7) followed by Brazil (n = 4), with the most contributions from the University of Tennessee and Grande Rio University (n = 3), respectively. The highest number of most-cited articles were ex vivo studies (n = 16), and ‘fracture resistance’ was the major topic studied (n = 10).

Conclusions

This study revealed a growing interest for researchers in the field of minimally invasive access cavities. Future trends are focused on the expansion of collaborative networks and the conduction of laboratory studies on under-investigated parameters.

Influence of surrounding temperature and angle of file access on cyclic fatigue resistance of two single file nickel-titanium instruments

The purpose of this study was to evaluate the influence of surrounding temperature and angle of file access on cyclic fatigue resistance of F6 SkyTaper (F6ST) and One Curve (OC). 120 new files #25.06 were tested at two insertion angles (0° and 20°) at room (20°C ± 1°C) and body (35°C ± 1°C) temperatures in a 16-mm stainless steel artificial canal (60° curvature and 5-mm radius), using a customised device. Cyclic fatigue resistance was expressed as time to fracture (TtF) in seconds. Data were analysed statistically (P < 0.05). All instruments exhibited lower TtF at 35°C (P < 0.05). An access of 20° did not significantly influence the TtF of tested instruments, independently from the temperature. OC exhibited higher TtF of F6ST at 20°C with a 20° inclination (P < 0.05). Under the present conditions, F6ST and OC showed a significant reduction of cyclic fatigue resistance at body temperature. A file inclined insertion did not affect the cyclic fatigue resistance of tested instruments at both temperatures.

Cyclic fatigue and torsional resistance evaluation of Reciproc R25 instruments after simulated clinical use

BACKGROUND

To assess the influence of multiple uses on the cyclic flexural fatigue and torsional resistance of Reciproc R25 (REC) instruments.

METHODS

256 canals (2 for each mandibular molar) were prepared using Reciproc R25 instruments. A total of 96 25-mm-long Reciproc R25 instruments were divided in six groups (n=8) on the basis of different number of canal shaped: new instruments (control group), 1, 2, 3, 4 or 6 canals shaped during simulated clinical use for other groups respectively. Sixteen files were used for each of the six groups (8 instruments for cyclic fatigue test and 8 ones for torsional test). Times to fracture (TtF) for cyclic fatigue and ultimate torsional strength and angle of rotation to fracture for torsional resistance were recorded. Data were statistically evaluated by the analysis of variance (ANOVA) with P<.05.

RESULTS

As for cyclic fatigue resistance, there was no significant difference between new REC and REC used in one, two and three canals respectively (P>.05). REC used in four and six canals showed significant lower times to fracture when compared with other groups (P<.05) with REC used in six canals showing the significant lowest times to fracture (P<.05). Considering torsional resistance parameters, no significant difference was observed between the new and used instruments (P>.05).

CONCLUSIONS

REC25 files showed a significant decrease in cyclic fatigue resistance only when used in four or more molar canals while their torsional behavior was not affected by multiple uses.

Evaluation of the Cyclic Fatigue of Two Single Files at Body and Room Temperature with Different Radii of Curvature

Background: To compare the influence of different temperatures and curvature radii on the cyclic fatigue resistance of F6 SkyTaper (F6ST) and One Curve (OC) single file nickel-titanium rotary instruments. Methods: A total of 120 instruments of F6ST and OC #25.06 were evaluated in 5 mm and 3 mm curvature radii at two temperatures (20 °C ± 1 °C and 37 °C ± 1 °C) in 16 mm stainless steel artificial canals associated with a curvature of 60°. The cyclic fatigue of tested files was assessed by employing a customized testing apparatus and expressed as times to fracture (TtF). A statistical analysis was performed with the significance level set at 95%. Results: All instruments decreased their TtF at 37 °C except for OC in the 3 mm radius, in which no significant difference was detected between 20 °C and 37 °C. A 3 mm curvature radius negatively affected TtF of all tested instruments, except for F6ST at 20 °C. F6ST had higher TtF than OC in the 3 mm radius at 20 °C, with no significant difference between them in the other tested conditions. Conclusions: Under the limits of the present in vitro study, body temperature impaired cyclic fatigue resistance of all files, except for OC in the 3 mm curvature radius. All instruments exhibited lower times to fracture in the 3 mm radius, excluding F6ST at 20 °C.

Does the type of endodontic access influence in the cyclic fatigue resistance of reciprocating instruments?

INTRODUCTION

The aim of the present study was to compare the cyclic fatigue resistance of Reciproc R25 (R25) and Reciproc Blue R25 (R25B) instruments, after simulated clinical use in traditional (TradAC) and ultraconservative (UltraAC) endodontic access cavities.

METHODS

Forty mandibular molars were randomly assigned into the following groups, according to the type of access and instrument to be used: TradAC and R25, TradAC and R25B, UltraAC and R25, and UltraAC and R25B. Teeth were accessed accordingly, and the root canals were prepared using "RECIPROC ALL" kinematics. The cyclic fatigue resistance of the forty used instruments was obtained measuring the time to fracture in an artificial stainless-steel canal. Ten brand new R25 and R25B were used as control groups. The fracture surfaces and the side cutting edges of the instruments were examined with a scanning electron microscope. Data were statistically analyzed using one-way ANOVA and post hoc Tukey tests with a significance level of P < 0.05.

RESULTS

R25B instruments showed significantly higher cyclic fatigue resistance than R25, regardless of the access cavity type (P < 0.05). No differences were observed in the cyclic fatigue resistance between instruments without simulated clinical use and used in TradAC (P > 0.05). R25 and R25B used in UltraAC showed significantly lower cyclic fatigue resistance compared with the instruments used in TradAC and without simulated clinical use (P < 0.05).

CONCLUSIONS

R25B files showed improved cyclic fatigue resistance than R25. The use of R25B and R25 files in mandibular molars with UltraACs decreased their cyclic fatigue resistance, compared with TradAC.

CLINICAL RELEVANCE

The use of Reciproc and Reciproc Blue files in mandibular molars with ultra-conservative endodontic access cavities reduced their cyclic fatigue resistance. Clinicians should be aware about the reduced cyclic fatigue resistance of these files when used in mandibular molars with UltraAC, due to the synergistic effect of access angulation and severe curvature induced in the endodontic files.

Cutting efficiency of heat-treated nickel-titanium single-file systems at different incidence angles

Cutting efficiency of Reciproc R25 (REC) and Reciproc blue R25 (REB) at different inclinations was evaluated. Sixty new files were tested at 90°, 70° and 45° of inclination in relation to the sample (n = 10), using a customised machine. All files were activated in reciprocation against standardised gypsum blocks for 120 s. Cutting efficiency was determined by measuring the block weight loss with an analytical balance and measuring the length of the block surface cut using a digital calliper. Data were statistically analysed (two-way ANOVA, Bonferroni t-test) with the significance level set at P < 0.05. There was no difference for REC among the tested angles. REB had no statistical difference between 90° and 70°; however, its cutting efficiency significantly increased at 45°. There was a significant difference between REC and REB at 45° only. Under these conditions, increased file inclination to 45° and blue heat treatment improved cutting efficiency of reciprocating files.

Cyclic fatigue resistance of VDW.ROTATE and Reciproc Blue nickel-titanium files at root canal temperature

Background. This study aimed to compare the VDW.ROTATE instruments with the Reciproc Blue instruments in different kinematics in terms of the cyclic fatigue resistance.

Methods. Sixty instruments, 40 VDW.ROTATE and 20 Reciproc Blue instruments, were divided into three groups (n=20): VDW.ROTATE was used in both continuous rotation and reciprocation, and Reciproc Blue was used in reciprocation only. The cyclic fatigue resistance test was carried out in an artificial canal (60°, r=3 mm) at an intracanal temperature of 35±2°C until fracture, and the time to fracture was recorded in seconds. The data were analyzed statistically using Kruskal–Wallis and Tamhane’s T2 tests (P<0.05).

Results. DAll the reciprocating motion groups resulted in a longer mean duration to failure than the continuous rotation motion group (P<0.05).

Conclusion. It was observed that the Reciproc Blue instruments had higher cyclic fatigue resistance than VDW.ROTATE instruments (P<0.05). Recent studies have shown that reciprocal movement increases cyclic fatigue resistance compared to rotational movement. The VDW.ROTATE instrument, which has a similar size, design, and alloy as the Reciproc Blue instrument, can also be used by clinicians in reciprocating motion with endo motors capable of reciprocating in different directions. However, even if the cyclic fatigue resistance increases by using VDW.ROTATE instruments in reciprocation, the cyclic fatigue resistance is lower than Reciproc Blue instruments.

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.

Cyclic Fatigue Resistance of Nickel-titanium Rotary Instruments according to the Angle of File Access and Radius of Root Canal

INTRODUCTION

The aim of this study was to compare the influences from different access angles and curvature radii on cyclic fatigue resistance of nickel-titanium rotary files.

METHODS

Two file systems (2Shape [TS; MicroMega, Besançon, France] and HyFlex CM [HCM; Coltène/Whaledent, Allstätten, Switzerland]) were used. A total of 192 instruments of TS #25/.04 (TS1), TS #25/.06 (TS2), HCM #25/.04, and HCM #25/.06 were evaluated at 3 insertion angles (0°, 10°, and 20°) and 2 radii (5 mm and 3 mm) in 16-mm stainless steel artificial canals with a 60° curvature. Cyclic fatigue resistance was determined by the number of cycles to failure (NCF) using a customized testing device. Data were analyzed statistically with the significance level established at 95%.

RESULTS

In the 3-mm radius canal, the instruments showed lower cyclic fatigue resistance than in the 5-mm radius canal (P < .05). HCM #25/.06 and all .04 taper instruments had a significant NCF reduction at 20° and 10° in the 3-mm radius canal (P < .05), whereas TS2 showed no significant differences. In the 5-mm radius of curvature, although .06 taper instruments had no significant NCF reduction for each angle tested, .04 taper files exhibited significant NCF reduction when tested at 20° (P < .05). Comparing the same size instruments, HCM had higher NCF than TS (P < .05). Instruments with a .04 taper exhibited higher NCF than the .06 ones with the same heat treatment (P < .05).

CONCLUSIONS

An inclined insertion into the canals decreased cyclic fatigue resistance of thermal-treated instruments with a .04 taper at all radii of curvature tested. The synergistic effect of a small radius of curvature and access angulation of heat-treated instruments decreases their fatigue resistance.

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.