Cyclic fatigue of nickel-titanium rotary instruments after clinical use with low- and high-torque endodontic motors

An in vitro evaluation of the effect of deep dry cryotreatment on the cutting efficiency of three rotary nickel titanium instruments

CONTEXT

Cryogenic methods have been used to increase the strength of metals.

AIM

The aim of this study was to evaluate the effect of deep dry cryotherapy on the cyclic fatigue resistance of rotary nickel titanium instruments.

MATERIALS AND METHODS

Twenty K3, RaCe and Hero Shaper nickel titanium instruments, size 25, 0.06 taper, were taken for this study. Ten files were untreated (control group) and 10 files were deep dry cryogenically treated. Both the untreated and cryotreated files were subjected to cyclic fatigue evaluation. Cyclic fatigue was evaluated as the number of cycles it took for fracture of the instrument within a stainless steel shaping block of specific radius and angle of curvature.

STATISTICAL ANALYSIS

Mean values were compared between different study groups by using one-way analysis of variance (ANOVA) with P < 0.05 considered as the level of significance.

RESULTS

The results showed a significant increase in the resistance to cyclic fatigue of deep dry cryotreated NiTi files over untreated files.

CONCLUSIONS

It may thus be concluded that deep cryotherapy has improved the cyclic fatigue of NiTi rotary endodontic files.

Cyclic fatigue testing and metallographic analysis of nickel-titanium rotary instruments

INTRODUCTION

The aim of this study was to compare cyclic fatigue resistance of four nickel-titanium rotary systems and to evaluate their surface, fractographic, and matrix morphology.

METHODS

Four models of endodontic rotary files (EasyShape [Komet/Gebr. Brasseler, Lemgo, Germany], ProTaper [Dentsply Maillefer, Ballaigues, Switzerland], NRT [MANI Inc, Toshigi-Ken, Japan], and AlphaKite [Komet/Gebr. Brasseler]) were subjected to fatigue testing in artificial canals with angle of curvature of 45° and 60° and a radius of curvature of 5 mm until fracture occurred. Nickel-titanium (NiTi) alloy properties were investigated by light microscopy, environmental scanning electron microscopy (ESEM), and energy dispersive x-ray spectrophotometry (EDS). ESEM analysis was conducted on new files to examine surface characteristics and on fractured samples to identify the crack origin and the fractographic features.

RESULTS

Analysis of variance testing revealed significant differences (P < .001) among the groups. NRT files had the highest fatigue resistance followed by AlphaKite, EasyShape, and ProTaper. All the new files presented surface imperfections. Fractographic analysis found the crack initiation to originate at the level of surface irregularities. Optical microscope inspection of the NiTi alloy matrix disclosed different-sized nonmetallic inclusions among models. EDS analysis of these inclusions showed that they were composed of carbon and oxygen in addition to nickel and titanium. Under light microscopy, austenitic grains appeared larger near the handle and smaller near the tip in all instruments.

CONCLUSIONS

NRT files presented the longest fatigue life. All samples showed surface irregularities and nonmetallic inclusions. Austenitic grains were smaller near the tip than near the handle. The angle of curvature was confirmed to influence the fatigue life of NiTi instruments.

A numerical method for predicting the bending fatigue life of NiTi and stainless steel root canal instruments

AIM

To evaluate the bending fatigue lifetime of nickel-titanium alloy (NiTi) and stainless steel (SS) endodontic files using finite element analysis.

METHODOLOGY

The strain-life approach was adopted and two theoretical geometry profiles, the triangular (TR) and the square cross-sections, were considered. Both low-cycle fatigue (LCF) lifetime and high-cycle fatigue (HCF) lifetime were evaluated.

RESULTS

The bending fatigue behaviour was affected by the material property and the cross-sectional configuration of the instrument. Both the cross-section factor and material property had a substantial impact on fatigue lifetime. The NiTi material and TR geometry profiles were associated with better fatigue resistance than that of SS and square cross-sections.

CONCLUSIONS

Within the limitations of this study, finite element models were established for endodontic files to prejudge their fatigue lifetime, a tool that would be useful for dentist to prevent premature fatigue fracture of endodontic files.

A review of cyclic fatigue testing of nickel-titanium rotary instruments

INTRODUCTION

Fractured rotary nickel-titanium (NiTi) instruments have been classified into those that fail as a result of cyclic flexural fatigue or torsional failure or a combination of both. Clinically, NiTi rotary instruments are subjected to both torsional load and cyclic fatigue, and ongoing research aims to clarify the relative contributions of both factors to instrument separation.

METHODS

To date, there is no specification or international standard to test cyclic fatigue resistance of endodontic rotary instruments. As a consequence, several devices and methods have been used to investigate in vitro cyclic fatigue fracture resistance of NiTi rotary endodontic instruments. In nearly all studies reported in the endodontic literature, the rotating instrument was either confined in a glass or metal tube, in a grooved block-and-rod assembly, or in a sloped metal block.

RESULTS

There has been no mention of the "fit" of the instrument in the tube or groove. As the instrument is likely to be fitting loosely, the description of the radius of curvature in those studies is likely to be overstated (ie, the file was actually bent less severely than reported, adding a variability in the amount of flexural stress).

CONCLUSIONS

This review analyzed several devices that have been used in endodontic literature for cyclic fatigue testing and found that differences in the methodology affected the fatigue behavior of rotary instruments and, consequently, the outcome of these studies. An international standard for cyclic fatigue testing of NiTi rotary instruments is required to ensure uniformity of methodology and comparable results.

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Since the introduction of nickel-titanium alloy to endodontics, there have been many changes in instrument design, but no significant improvements in the raw material properties, or enhancements in the manufacturing process. Recently, a new method to produce nickel-titanium rotary (NTR) instruments has been developed, in an attempt to obtain instruments that are more flexible and resistant to fatigue. NTR instruments produced using the process of twisting (TF, SybronEndo, Orange, CA) were compared to NTR instruments from different manufacturers produced by a traditional grinding process. The aim of the study was to investigate whether cyclic fatigue resistance is increased for TF NTR files. Tests were performed with a cyclic fatigue device that evaluated cycles to failure of rotary instruments inside curved artificial canals. Results indicated that size 06-25 TF instruments showed a significant increase (P< .05). In the mean number of cycles to failurewhen compared to the other tested 06-25 NTR. Hence, it can be concluded that size 06-25 TF NTR instruments were found to be significantly more resistant to fatigue than those produced with the traditional grinding process.

Prevalence and management of instrument fracture in the postgraduate endodontic program at the Dental School of Athens: a five-year retrospective clinical study

The aim of the present clinical research was to investigate the prevalence and management of instrument fracture during root canal preparation by postgraduate students (Department of Endodontics, Dental School of Athens) and to determine the percentage of referred cases with fractured instruments managed by the same students of the program. A retrospective study was conducted by reviewing the dental notes of 1367 patients (2180 endodontic cases, 4897 root canals) treated between October 2001 and June 2006 by endodontic postgraduate students at the Dental School of Athens. Type of tooth and canal, type and length of fractured segments, level of instrument fracture, and management that followed were recorded. The overall prevalence of instrument fracture during root canal preparation by postgraduate students was 1.83%. The prevalence of endodontic cases with fractured instruments referred to the endodontic postgraduates was 7.41%. The prevalence of stainless steel hand and rotary nickel-titanium instrument fracture by postgraduate students were 0.55% and 1.33%, respectively. The prevalence of instruments fractured in the apical third (52.5%) was significantly higher when compared with coronal (12.5%) and middle (27.5%) thirds of the canals. The retrieval or bypass of fractured instruments was most successful in the coronal (100%) and middle (45.4%) thirds when compared with the apical third (37.5%) of the canals. The fracture frequency was higher in retreatment cases in relation to the respective rate of initial therapies. On the basis of the results of this study, the prevalence of endodontic instrument fracture by the postgraduate students was relatively low. The prevalence of fracture of nickel-titanium rotary instruments was more frequent than that of hand instruments. Retrieval or bypass of the fractured instruments in the apical third was less successful.

Cyclic fatigue of ProTaper rotary nickel-titanium instruments in artificial canals with 2 different radii of curvature

OBJECTIVE

This study compared the cyclic fatigue resistance of ProTaper rotary nickel-titanium (NiTi) files in artificial canals with 2 different radii of curvature.

STUDY DESIGN

One hundred ProTaper rotary instruments were used in this study. Twenty files of each size (S1, S2, F1, F2, and F3) were tested. Cyclic fatigue testing was performed using a device that allowed the instruments to rotate freely inside stainless steel artificial canals with radii of curvature of 5 and 10 mm. The number of cycles to fracture was recorded for each file and the data were analyzed using a t-test.

RESULTS

The 5-mm radius group had significantly fewer cycles to fracture than the 10-mm radius group for all file sizes. For sizes F2 and F3, the difference between the 5 and 10-mm radius groups was highly significant (P < .001). For the 10-mm group, the difference between S1 and F3 was significant (P < .05). For the more abrupt 5-mm radius group, the difference between S1-F2, S2-F2, and F1-F2 was statistically significant. The difference between S1-F3, S2-F3, and F1-F3 was highly significant (P < .001).

CONCLUSION

Cyclic fatigue resistance of ProTaper instruments was dependent on both instrument size and radius of curvature. Especially larger-size instruments should be used with great care in curved canals.

Influence of previous angular deformation on flexural fatigue resistance of K3 nickel-titanium rotary instruments

Several studies have evaluated the influence of torsion and bending loads on the mechanical resistance of nickel-titanium instruments. The aim of the present study was to analyze the influence of previous angular deformation on K3 files resistance to flexural fatigue. New files and files previously submitted to different angular deformation were tested for flexural fatigue, and the number of cycles to fracture was compared. The results indicated that as the angular deformation increases, the number of cycles attained under flexural fatigue condition decreases. A reduction of fatigue resistance was registered even when a significant plastic deformation was not imposed. From the obtained results, it may be suggested that the sequential loading, including torsion and flexural fatigue, can be responsible for instrument separation during clinical practice. Successive torsion overloads, occurring together with flexural fatigue, reduce the mechanical resistance of nickel-titanium files.

Fracture resistance of electropolished rotary nickel-titanium endodontic instruments

The purpose of this study was to investigate the effect of electropolishing on cyclic flexural fatigue and torsional strength of rotary nickel-titanium endodontic instruments. Electropolished and nonelectropolished ISO size 30 (0.04 taper) EndoWave (J Morita Corporation, Osaka, Japan), ProFile (Dentsply Maillefer, Ballaigues, Switzerland), and RaCe (FKG, La-Chaux De Fonds, Switzerland) instruments from the same manufacturing batches were investigated. The number of rotations to fracture and torque at fracture were determined and compared among the instruments tested. Instruments were viewed under a scanning electron microscope (SEM) to assess the degree and quality of electropolishing. Overall, electropolished instruments performed significantly better than nonelectropolished instruments in cyclic fatigue testing and, to a lesser extent, in static torsional loading. When viewing electropolished instruments with the SEM, milling grooves, cracks, pits, and areas of metal rollover were observed, although they were more evident in the nonelectropolished instruments. Electropolishing may have beneficial effects in prolonging the fatigue life of rotary NiTi endodontic instruments. The benefits of electropolishing are likely to be caused by a reduction in surface irregularities that serve as points for stress concentration and crack initiation.

Effect of Clinical Use on the Cyclic Fatigue Resistance of ProTaper Nickel-Titanium Rotary Instruments

The resistance of ProTaper (Dentsply Maillefer, Ballaigues, Switzerland) nickel-titanium rotary instruments to cyclic fatigue was examined after their initial use in straight or curved canals in vivo. These instruments were rotated freely inside a steel phantom until separation. The number of rotations before failure and the lengths of the separated fragments were compared with data derived from new instruments under the same experimental setup (n = 20). With the exception of F1 and F3, instruments previously used in curved canals were more susceptible to cyclic fatigue than those previously used in straight canals (p < 0.05). Separation occurred predominantly at the D10 to D12 level. For the F series, a negative correlation (p < 0.05) was observed between the number of rotations before failure and the file diameters at their separation levels. ProTaper F3 instruments are highly susceptible to cyclic fatigue failure and should be reused with caution irrespective of whether they are initially used for shaping straight or curved canals.

The principles of techniques for cleaning root canals

Chemomechanical preparation of the root canal includes both mechanical instrumentation and antibacterial irrigation, and is principally directed toward the elimination of micro-organisms from the root canal system. A variety of instruments and techniques have been developed and described for this critical stage of root canal treatment. Since their introduction in 1988, nickel-titanium (NiTi) rotary instruments have become a mainstay in clinical endodontics because of their exceptional ability to shape root canals with potentially fewer procedural complications. Safe clinical usage of NiTi instruments requires an understanding of basic metallurgy of the alloy including fracture mechanisms and their correlation to canal anatomy. This paper reviews the biologic principles of preparing root canals with an emphasis on correct use of current rotary NiTi instrumentation techniques and systems. The role and properties of contemporary root canal irrigants is also discussed.

A dozen ways to prevent nickel-titanium rotary instrument fracture

BACKGROUND

and Overview. With the increased use of nickel-titanium (NiTi) rotary instruments for root canal preparation in endodontics, instrument fracture has become more prevalent. Extensive research has been conducted on the physical properties and mechanical characteristics of NiTi rotary instruments, as well as the factors that can contribute to instrument failure. NiTi rotary instruments are subjected to torque and are susceptible to cyclic fatigue, which are the main causes of instrument fracture. However, with an understanding of how these instruments function in preparing root canals and by applying ways to reduce torque-generated metal fatigue, clinicians can use the instruments safely in clinical practice.

RESULTS

The author presents 12 measures that clinicians can take to prevent NiTi rotary instrument fracture and discusses them in detail.

CLINICAL IMPLICATIONS

NiTi rotary instrument fracture complicates the progress, and compromises the prognosis of endodontic treatment. However, when clinicians take appropriate measures, rotary instrument fractures can be prevented.

The corrosion of nickel?titanium rotary endodontic instruments in sodium hypochlorite

AIM

To evaluate the corrosion resistance of nickel-titanium (NiTi) endodontic rotary instruments immersed in 5.25% sodium hypochlorite (NaOCl) solution.

METHODOLOGY

The corrosion performance of NiTi instruments (S1 25 mm, ProTaper Dentsplay Maillefer, Ballaigues, Switzerland) was evaluated using commercial 5.25% NaOCl solution (pH = 12.3), and the same solution partially neutralized adding H2SO4 to reach pH = 10.1. Electrochemical measurements were carried out using a potentiostat equipped with a five-channel zero resistance ammeter (ZRA) for galvanic current measurements. The instruments were sectioned into three parts (cutting part, noncutting part and shank) and degreased with acetone and rinsing with demineralized water prior to being immersed in NaOCl solution for testing. Each set of the three parts constituted one 'virtual' instrument through the ZRA, giving access to the galvanic currents that circulate between the three parts. Nine instruments were employed to check the reproducibility of the electrochemical measurements.

RESULTS

The corrosion potential (E(corr)) of the NiTi alloy reached the passive domain in approximately 20 s of immersion in the solution having a pH 10.1. After this initial period the potential remained steady, indicating that stable passivation was achieved. However, at pH 12.3 no stationary state was achieved even after 6000 s of immersion time. Thus, the alloy was not stable in this medium from a corrosion point of view.

CONCLUSIONS

The corrosion resistance of NiTi alloy was enhanced by lowering the pH of NaOCl solution to 10.1, which allows the system to reach the stability domain of the passivating species TiO2 and NiO2.

Rotary NiTi Instrument Fracture and its Consequences

The fracture of endodontic instruments is a procedural problem creating a major obstacle to normally routine therapy. With the advent of rotary nickel-titanium (NiTi) instruments this issue seems to have assumed such prominence as to be a considerable hindrance to the adoption of this major technical advancement. Considerable research has been undertaken to understand the mechanisms of failure of NiTi alloy to minimize its occurrence. This has led to changes in instrument design, instrumentation protocols, and manufacturing methods. In addition, factors related to clinician experience, technique, and competence have been shown to be influential. From an assessment of the literature presented, we derive clinical recommendations concerning prevention and management of this complication.

A retrospective clinical study of incidence of root canal instrument separation in an endodontics graduate program: a PennEndo database study

This study investigated the incidence of hand and rotary instrument separation (IS) in the endodontics graduate program at the University of Pennsylvania between 2000 and 2004. In 4,865 endodontic resident cases the incidence of hand and rotary IS was 0.25% and 1.68%, respectively. The odds for rotary IS were seven times more than for hand IS. The probability of separating a file in apical third was 33, and 6 times more likely when compared to coronal and middle thirds of the canals. The highest percentage of IS occurred in mandibular (55.5%) and maxillary (33.3%) molars. Furthermore, the odds of separating a file in molars were 2.9 times greater than premolars. Among the ProFile series 29 rotary instruments, the .06 taper # 5 and # 6 files separated the most. There was no significant difference in IS between the use of torque controlled versus nontorque controlled handpieces, nor between first and second year residency.

A comparison of cyclic fatigue between used and new Mtwo Ni–Ti rotary instruments

AIM

To evaluate the cyclic fatigue of Mtwo Ni-Ti rotary instruments after controlled clinical use in molar teeth.

METHODOLOGY

Twenty Mtwo instruments of each size were selected and divided into two groups: group A consisted of 10 new instruments (control group); group B consisted of 10 used instruments. Each instrument in group B was used to clean and shape 10 root canals of molar teeth in patients. Cyclic fatigue testing of instruments was performed in tapered artificial canals with a 5-mm radius of curvature and a 60 degrees angle of curvature. In all 140, instruments were rotated until fracture and the number of cycles to failure was recorded. Data were analysed using one-way anova, Tukey's HSD test and independent sample t-test to determine any statistical difference; the significance was determined at the 95% confidence level.

RESULTS

A reduction of cycles to failure between new (group A) and used (group B) instruments was apparent. A statistically significant difference (P < 0.05) was noted between instruments of groups A and B in all sizes with the exception of size 40, 0.04 taper.

CONCLUSIONS

Clinical use significantly reduced cyclic fatigue resistance of Mtwo rotary instruments when compared with an unused control group. However, all the instruments had minimal instrument fatigue when discarded after controlled clinical use.

Nickel-titanium rotary instrument fracture: a clinical practice assessment

AIM

To prospectively determine the incidence of nickel-titanium rotary instrument fracture in an endodontic clinical practice setting.

METHODOLOGY

Eleven second year endodontic residents, using four nickel-titanium rotary instrument systems (ProFile, ProTaper, GTRotary and K3Endo) according to the recommendations of the manufacturers, instrumented 3181 canals in 1403 teeth of 1235 patients, in a dental school post-graduate endodontic clinic, in 1 year. The incidence of instrument fracture was determined based on the number of instruments used. When fracture occurred, data were collected concerning the type, size, taper and prior use of the fractured instruments, the length and location of the fragment within the root canal and the curvature of the canal.

RESULTS

The overall incidence of instrument fracture was 0.39%. The incidence of fracture for ProFile, ProTaper, GTRotary and K3Endo files was 0.28%, 0.41%, 0.39% and 0.52%, respectively. There was no statistically significant difference between instrument systems. The percentage of teeth in which instruments fractured was 1.9% (0.28% for anterior teeth, 1.56% for pre-molars and 2.74% for molars). A total of 26 instruments fractured, of which 23 had tapers of 0.06 or greater. Most of the fragments were located in the apical third of the root canal, and both the median and mode amongst the fragment lengths were 2 mm.

CONCLUSIONS

The low incidence of nickel-titanium rotary instrument fracture supports the continued use of these instruments in root canal treatment.

Fracture of ProFile nickel-titanium rotary instruments: a laboratory simulation assessment

AIM

To determine the incidence of ProFile nickel-titanium rotary instrument fracture in an endodontic laboratory simulation.

METHODOLOGY

Three hundred and sixty dental students used 2880 ProFile nickel-titanium rotary instruments to prepare 1440 simulated root canals in 720 plastic teeth, and another 2880 ProFile nickel-titanium rotary instruments to prepare 1440 natural root canals in 720 extracted teeth. A standardized crown-down rotary instrumentation technique was used, which included measures to prevent fracture. Rotary instrument fracture was monitored during and after completion of the laboratory simulation exercises to determine the incidence of fracture. When fracture occurred, data were collected concerning the size of the instrument, the length of the instrument fragment, the location of the fragment in the canal and the curvature of the canal in which the instrument fractured.

RESULTS

The incidence of instrument fracture was 0.41% in plastic simulated canals and 0.31% in natural root canals. The overall incidence of instrument fracture was 0.36%. Of the instruments that fractured 67% were size 25, 0.04 taper; and 81% of the fragments were located in the apical third of the canal. The mean, median and mode of the fragment lengths were all 3 mm.

CONCLUSIONS

The low overall incidence of fracture in this study suggests that ProFile rotary instruments are safe for use by dental students in laboratory simulations and that if preventive measures are taken the incidence of instrument fracture can be minimized.

Defects in ProTaper S1 instruments after clinical use: fractographic examination

AIM

To investigate the mode of failure of a brand of nickel-titanium instruments separated during clinical use, by detailed examination of the fracture surface.

METHODOLOGY

A total of 122 ProTaper S1 instruments were discarded from an endodontic clinic at a stomatological school in China over a period of 17 months; 28 had fractured. These fractured instruments were ultrasonically cleaned, autoclaved and then examined under a scanning electron microscope. From the lateral view the fracture was classified into 'torsional' or 'flexural'. The specimens were then re-mounted and the presence of characteristics of shear failure and fatigue striations was recorded under high-power view of the fracture surface. The difference in the mean lengths of fractured segment between the shear and fatigue groups was compared using Student's t-test.

RESULTS

Twenty-seven separated instruments were available for analysis. Under low-power magnification, only two fell into the category of 'torsional' failure when examined laterally; the others appeared to be 'flexural'. Close examination of the fracture surface revealed the presence of fatigue striations in 18 specimens. Nine instruments (including the two putative 'torsional' failures above) fell into the shear fracture group, in which fatigue striations were absent or characteristics of shear failure of the material were found. The mean length of fractured segments resulting from fatigue failure (4.3+/-1.9 mm) was significantly greater than that for shear failure (2.5+/-0.8 mm) (P<0.001, two-sample t-test).

CONCLUSIONS

Examination of the fracture surface at high magnification is essential to reveal features that may indicate the possible origin of cracks and the mode of material failure. Macroscopic or lateral examination of separated instruments would fail to reveal the true mechanism of failure. Fatigue seems to be an important reason for the separation of rotary instruments during clinical use.

Influence of different types of automated devices on the shaping ability of rotary nickel-titanium FlexMaster instruments

AIM

To compare the shaping ability of rotary FlexMaster nickel-titanium instruments in simulated curved canals and in curved canals of extracted teeth when set into permanent rotation with three different torque-limited automated devices.

METHODOLOGY

Root canal instrumentation was performed with three different torque-limited automated devices (ENDOadvance, SIRONiTi and Endo IT motor) by FlexMaster nickel-titanium instruments up to size 35. Simulated canals: 28 degree and 35 degree curved canals in resin blocks (n = 20 canals in each group) were prepared. Pre- and post-instrumentation images were recorded and assessment of canal shape was completed with a computer image analysis program. Extracted teeth: A total of 60 curved root canals were divided into three groups, which were balanced with respect to the angle and the radius of canal curvature. Straightening of the canal curvatures was determined with a computer image analysis program. Incidence of canal aberrations, preparation time, changes of working length and instrument failures were recorded both in simulated and real canals.

RESULTS

In simulated and real canals, instrumentation with Endo IT was significantly faster than with SIRONiTi (P < 0.05). With respect to canal aberrations in simulated canals, there were no significant differences between the devices (P > 0.05), even though more aberrations were created with ENDOadvance and SIRONiTi. In real canals, the Endo IT motor resulted in significantly less straightening during instrumentation (P < 0.05) than SIRONiTi. A total of three instruments separated (one file in each group) during the enlargement of 35 degree curved simulated canals. All systems maintained working distance well.

CONCLUSIONS

All systems respected original root canal curvature well and were safe, indicating that torque-limited rotation handpieces are suitable for preparing curved root canals.

Decrease in the fatigue resistance of nickel-titanium rotary instruments after clinical use in curved root canals

OBJECTIVE

The changes in fatigue resistance of nickel-titanium rotary ProFile instruments after clinical use for shaping 10 curved molar root canals were evaluated in this study.

STUDY DESIGN

Twenty-five sets of files #20, #25, and #30 and tapers .04 and .06 were divided into 2 groups, one with 10 sets of new files that were tested in a fatigue test bench device as a control. The other, experimental group, with 15 sets of clinically used files, was tested in the same device. The Student t test was employed to compare mean values of the measured parameters.

RESULTS

A statistically significant decrease in the number of cycles to failure was determined for the clinically used files, as compared with the new ones. The fracture point was the same for all files tested.

CONCLUSIONS

The clinical use of ProFile instruments for shaping curved canals reduces their fatigue resistance.

Defects in ProTaper S1 instruments after clinical use: longitudinal examination

AIM

To evaluate defects in ProTaper shaping instrument S1 after a defined schedule of clinical use.

METHODOLOGY

Among all ProTaper files discarded from an endodontic clinic at a stomatological school in China over a period of 17 months, 122 S1 instruments were collected. They were ultrasonically cleaned, autoclaved and then examined in the laboratory. Any instrument separation was noted; the average length involved was determined. The 0.5 mm region on either side of this length of discarded but intact instruments was examined circumferentially by scanning electron microscope. The region adjacent to the broken end of the fractured instruments was also examined in the same way.

RESULTS

One specimen was lost during processing. Of the remaining 121 instruments, 27 were separated with a mean fracture length of 3.67 mm from the tip. Of these, two files showed macroscopically torsional fracture and the others showed signs of flexural fatigue. Only one file that had not fractured showed visible unwinding defects. Examination of the 3.1-4.1 mm region of other unfractured instruments revealed the presence of microcracks, surface debris, pitting and/or wear of their cutting edges. Some debris particles seemed to have been trapped in crack-like structures.

CONCLUSIONS

Multiple use of ProTaper S1 pre-disposed the instrument to microcrack formation and wear of the cutting edges. There was a low prevalence of plastic deformation and most ProTaper S1 instruments failed without discernible sign of unwinding of the flutes. Further studies should address the mode of failure and the role of debris particles in the fracture mechanism.

Bending Fatigue Study of Nickel-Titanium Gates Glidden Drills

ProFile nickel-titanium Gates Glidden drills were tested in bending fatigue to simulate clinical conditions. Ten samples each in sizes #1 through #6 were placed in a device that deflected the drill head 4 mm from the axis. The drill head was placed inside a ball bearing fixture, which allowed it to run free at 4000 rpm, and the total number of revolutions was recorded until failure. Fracture surfaces were examined with a scanning electron microscope to determine the initiation site and nature of the failure process. Mean +/- SD for the number of revolutions to failure for the drill sizes were: #1: 1826.3 +/- 542.5; #2: 5395.7 +/- 2581.5; #3: 694.4 +/- 516.8; #4: 261.0 +/- 138.0; #5: 49.6 +/- 14.9; #6: 195.9 +/- 78.5. All drills failed in a ductile mode, and fracture initiation sites appeared to be coincident with machining grooves or other flaws, suggesting the need for improved manufacturing procedures.

A comparison of torque required to fracture rotary files with tips bound in simulated curved canal

The purpose of this study was to compare torque force and rotation needed to fracture three types of nickel titanium alloy rotary instruments in a simulated curved root canal space that were bound at the file tip. Files of similar size tips were studied. The files studied were ProFiles with 0.04 taper diameters of 15, 20, 25, 30, 35, 40, 45; 0.04 ProFile GT sizes 20, 30, 40; and ProTaper files sizes S1, S2, F1, F2, and F3. All files were 25 mm in length. Unwinding was defined as the rotation in degrees it took for a file to fracture after the first evidence of permanent deformation. All files exhibited permanent deformation before breaking, with the ProFile GT files demonstrating the greatest unwinding. The #45 0.04 ProFile withstood the most force while the #20 ProFile GT required the least amount of force before beginning to exhibit permanent deformation. The S1 and S2 ProTaper files fractured with so little rotation that no extended data were recorded. Generally, as the file diameter increased, the force needed to begin unwinding also increased. Also, as the file diameter increased, the force needed to fracture also increased.

SEM observations of nickel-titanium rotary endodontic instruments that fractured during clinical Use

Numerous discarded ProFile GT, ProFile, and ProTaper nickel-titanium rotary instruments obtained from two graduate endodontic clinics were examined by scanning electron microscopy. These instruments had an unknown history of clinical use and had fractured or experienced considerable permanent torsional deformation without complete separation. The failure processes generally exhibited substantial ductile character, evidenced by a dimpled rupture fracture surface. Crack propagation at grain boundaries and cleavage surfaces indicative of transgranular fracture were observed for some specimens. It appeared that oxide particles from the manufacturing process served as nucleating sites for the microvoids, leading to dimpled rupture. A previously unreported fracture mode also was observed, in which crack propagation, approximately parallel to the local flute orientation, connected pitted regions on the surface. Combining present and previous scanning electron microscopy observations of clinically failed instruments, suggestions are offered for improving their fracture resistance.

Effect of cyclic fatigue on static fracture loads in ProTaper nickel-titanium rotary instruments

The aim of the present study was to evaluate static fracture loads of ProTaper Nickel-Titanium instruments that had been subjected to various degrees of cyclic fatigue. Torque and angle at failure of new instruments and instruments that had been stressed to 30, 60, or 90% of their cyclic fatigue rotations in a simulated canal (90 degrees and 5 mm radius) were tested according to ISO 3630-1. With unused ProTaper instruments, resistance to cyclic fatigue decreased with diameter increase and ranged from 158 to 450 rotations. Torque at failure ranged from 0.5 to 2.1 Ncm and showed a strong linear relationship to instrument diameter (r = 0.9) while angle at failure was weakly related to diameter (r = 0.46). Cyclic prestressing significantly reduced torsional resistance in finishing files, while shaping files were largely unaffected. In conclusion, build-up of tension within NiTi rotary instruments depends on instrument diameter. Clinically, larger instruments that have been subjected to some cyclic fatigue should be used with great care or discarded.

Susceptibility to localized corrosion of stainless steel and NiTi endodontic instruments in irrigating solutions

AIM

To evaluate the pitting and crevice corrosion characteristics of stainless steel (SS) and NiTi endodontic files in R-EDTA and NaOCl irrigating solutions.

METHODOLOGY

The corrosion behaviour of two H-files produced from different SS alloys (Mani, AISI 303 SS, Dentsply Maillefer, AISI 304 SS) and one file produced from NiTi alloy (Maillefer) was determined in R-EDTA and NaOCl irrigating solutions by the cyclic potentiodynamic polarization method. The cutting flutes of 12 files of each material were embedded in an epoxy resin, polished, exposed to the irrigating solutions and used as an electrode. An Ag/AgCl electrode was used as a reference, a platinum plate was used as a counter electrode and polarization curves were obtained for all files in R-EDTA and NaOCl irrigating solutions in 37 degrees C with a potential scan rate of 5 mV min(-1). Corrosion potential (Ecorr), Corrosion current density (Icorr) and Pitting potential (Epit) were calculated from each curve. The results were statistically analysed with two-way anova and Student-Newman-Keuls (SNK) multiple comparison test with materials and irrigating solutions serving as discriminating variables (a = 0.05).

RESULTS

Cyclic polarization curves presented negative hysteresis implying that pitting or crevice corrosion are not likely to occur for all the materials examined in both irrigating solutions. In NaOCl all materials showed significantly higher Ecorr (P = 0.011) as well as lower Icorr compared with R-EDTA reagent. Moreover, all materials demonstrated equal Epit in NaOCl, which was to be found significantly lower (P = 0.009) than the value of Epit in R-EDTA.

CONCLUSIONS

None of the tested materials is susceptible to pitting or crevice corrosion in R-EDTA and NaOCl solutions and from this standpoint are appropriate for the production of endodontic files.

Effect of Pitch Length on the Behavior of Rotary Triple Helix Root Canal Instruments

The behavior of nickel-titanium instruments depends largely on the cross-section of the working part. The effect of pitch length was evaluated using two instruments with the same cross-section (triple helix; 0.6% taper). One had a short pitch (0.5 mm at the tip to 0.9 mm at 16 mm at the end of the working part). The other had a long pitch (1.18 mm/2 mm). A dynamometer recorded tensional stress (MHz) and the tendency to screw in (Fz) during simulated canal preparations. Rotational speed was kept constant, and a continuous pecking movement was used. At the end of the preparations, the average Fz was 5.02 N for the short-pitch instrument and 1.47 N for the long-pitch instrument (p < 0.001). The average Mz varied from 0.88 N (short pitch) to 0.35 N (long pitch). Increasing the pitch decreased torsional load sharing and the tendency to screw in.

Proposed Role of Embedded Dentin Chips for the Clinical Failure of Nickel-Titanium Rotary Instruments

Discarded ProFile and ProTaper nickel-titanium rotary instruments, with unknown history of clinical use, were obtained from graduate endodontic clinics at Ohio State University and University of Texas Health Science Center at Houston Dental Branch. These discarded instruments and as-received instruments of both types were examined with a scanning electron microscope to investigate effects of clinical use and causes of failure. For used ProTaper instruments, dentinal debris was wedged mostly in narrow, radial, land-type regions and less on convex flute surfaces. For used ProFile instruments, dentinal debris was wedged mostly in the metal rollover and on concave flute surfaces. Used instruments of both types exhibited widened machining grooves, and elongated and stretched roll-over. Dentin chips were wedged in surface micro-cracks that appeared to propagate from original machining flaws and widen during in vivo root canal preparation. From our observational study, wedged dentinal deposits seem to play a pivotal role for clinical failure of these instruments.

Scanning electron microscope observations of new and used nickel-titanium rotary files

The appearances of the tip sections of ProFile 0.04 taper and Lightspeed 25-mm long, ISO size 25, nickel-titanium rotary instruments were compared with a scanning electron microscope in the as-received condition and after one, three, and six simulated clinical uses to prepare mesial canals of extracted mandibular molars. For the used ProFile instruments, there was some flattening of the characteristic material rollover and minor apparent wear at the edges of the flutes, but there was little change in the tip regions of the used Lightspeed instruments. Deposits on the surfaces of the instruments were attributed to the manufacturing processes and the in vitro preparation of root canals in the extracted teeth. The simulated clinical use did not cause substantial changes in the regions of these two brands of rotary instruments that are involved in the clinical preparation of root canals.

Influence of torque control motors and the operator's proficiency on ProTaper failures

OBJECTIVE

The purpose of this study was to evaluate the influence of 2 electric torque control motors and operator experience with a specific nickel-titanium rotary instrumentation technique on the incidence of deformation and separation of instruments.

STUDY DESIGN

ProTaper (PT) nickel-titanium rotary instruments were used at 300 rpm. In the first part of the study, electric high torque control (group 1) and low torque control (group 2) motors were compared. In the second part of the study, 3 operators with varying experience (groups 3, 4, and 5) were also compared. Twenty sets of PT instruments and 100 canals of extracted human molars were used in each group. Each set of PT instruments was used in up to 5 canals and sterilized before each case. For irrigation, 2.5% NaOCl was used. The number of deformed and separated instruments among the groups (within each part of the study) was statistically analyzed for significance with pair-wise comparisons by using the Fisher exact test (alpha =.05).

RESULTS

In part 1, instrument deformation and separation did not occur in groups 1 and 2. In part 2, 25 and 12 instruments were deformed and separated, respectively, with the least experienced operator. Instrument deformation and separation did not occur with the most experienced operator. The Fisher exact test revealed a significant difference between groups 3 and 4 with respect to instrument deformation (P =.0296). In addition, the Fisher exact test revealed that the incidence of instrument deformation was statistically different between groups 3 and 5 (P <.0001) and groups 4 and 5 (P =.0018). The incidence of instrument separation was significantly higher in group 5 than in groups 3 and 4 (P =.001).

CONCLUSIONS

Preclinical training in the use of the PT technique at 300 rpm is crucial to prevent instrument separation and reduce the incidence of instrument deformation. The use of an electric high torque control motor is safe with the experienced operator.