Rotary Nickel-titanium Endodontic Instruments Research Articles

Deformation and fracture of K3 rotary nickel-titanium endodontic instruments after clinical use.

The aim was to evaluate the incidence and type of defects that occurred with K3 rotary nickel-titanium instruments during routine clinical use. A total of 2397K3 (G-PACKS, SybronEndo, West Collins, Orange, CA, USA) instruments were collected from a graduate endodontic clinic over 21months. All the instruments were limited to a maximum use of 30 canal preparations. The collected instruments were measured by a digital caliper to determine whether any fractures had occurred and then were visually inspected for deformation and fracture under a stereomicroscope. The surfaces of fractured instruments were further evaluated under a scanning electron microscope. Data were analysed using chi-square test and Kruskal-Wallis test. The incidence of instrument defect was 5.63%, consisting of 3.59% fractures and 2.05% deformations. The defect rates of 0.04 and 0.06 files were statistically higher than the other taper groups (P<0.003) except for 0.08 files (P>0.05). For the fractured instruments, 63.95% failed from flexural fatigue, whilst 36.05% failed from torsion. Flexural fracture was the major mode of fracture for instruments with larger taper. A routine check for instrument integrity particularly for 0.04 and 0.06 files at high magnification is recommended after each clinical use.

Influence of Heat Treatment of Nickel-Titanium Rotary Endodontic Instruments on Apical Preparation: A Micro–Computed Tomographic Study

IntroductionThe aim of this study was to make a 3-dimensional comparison of the canal transportation and changes in apical geometry using micro–computed tomographic imaging after canal preparation with K3 (SybronEndo, Orange, CA) and K3XF (SybronEndo) file systems. MethodsTwenty-eight mandibular molars were randomly divided into 2 groups according to the rotary system used in instrumentation: K3 or K3XF. The specimens were scanned by micro–computed tomographic imaging before and after instrumentation. Images before and after instrumentation from each group were compared with regard to canal volume, surface area, and structure model index (SMI) (paired t test, P < .05). After instrumentation, the canals from each group were compared regarding the changes in volume, surface area, SMI, and canal transportation in the last 4 apical mm (t test, P < .05). ResultsInstrumentation with the 2 rotary systems significantly changed the canal volume, surface area, and SMI (P < .05). There were no significant differences between instrument types concerning these parameters (P > .05). There were no significant differences between the 2 groups with regard to canal transportation in the last 4 apical mm (P > .05). ConclusionsBoth rotary systems showed adequate canal preparations with reduced values of canal transportation. Heat treatment did not influence changes in root canal geometry in the apical region.

Cyclic Fatigue Testing of Three Different Rotary Nickel Titanium Endodontic Instruments in Simulated Curved Canals - An in Vitro Sem Analysis

Instrument separation is a serious concern in endodontics. Stainless steel instruments usually deform before they separate whereas Nickel Titanium instruments do not show any sign of deformation. of the present study was to compare the cyclic rotations needed to fracture three different rotary nickel titanium endodontic instruments namely Profile, K3 Endo, RaCe in simulated curved canals having 2 different radii of curvature i.e., 5mm &10mm with a constant angle of curvature of 45°. The fractured surfaces were analysed using Scanning Electron Microscopy (SEM). Sixty rotary nickel titanium instruments were used for analysis in this study. The instruments were divided into three groups based on the different cross sections. Three groups were further divided into two sub groups based on radius of curvatures. Ten files were tested in each sub-group. Group A (Profile) showed increased number of cycles to failure i.e., low incidence of fracture at both the radii of curvatures when compared to the other two groups. SEM analysis showed that in Group A the fractured surface of the files tested were characterized by the presence of smooth area on their edges associated with nucleation and slow fatigue crack propagation when compared to Group B and C. Therefore Group A (Profile) demonstrated least cyclic fatigue when compared to the other two groups.

Ultrastructural analysis of the root canal walls after preparation with two rotary nickel-titanium endodontic instruments.

Background:Root canal preparation may produce a large quantity of smear layer that covers canal walls.Aims:The aim of this study was to evaluate by Scanning Electron Microscope (SEM) the root canal dentine after instrumentation with nickel-titanium rotary files, in order to evaluate the presence/absence of smear layer and the presence/absence of open tubules on the root canal walls at the coronal, middle, and apical third of each sample.Materials and Methods:A total of 20 single-rooted freshly extracted teeth were selected and divided into two groups. For each group, root canals were shaped with Mtwo and Revo-S instruments under irrigation with Sodium hypochlorite and 17% ethylenediaminetetraacetic acid. Specimens were fractured longitudinally and SEM analyzed at standard magnification of ×1000 and ×5000. The presence/absence of smear layer and the presence/absence of open tubules at the coronal, middle, and apical third of each canal were evaluated using a three-step scale for scores.Statistical Analysis Used:Numeric data were analyzed using the Kruskall-Wallis and Mann-Whitney U-Statistical tests and significance was pre-determined at P < 0.05.Results and Conclusions:This study did not reveal differences among the two groups. Mtwo and Revo-S showed no significant difference between them and both presented very low smear layer scores and open tubules scores, with no significant difference among coronal, middle, and apical third. Mtwo and Revo-S rotary instruments seem to be effective in removing smear layer from canal walls.

Success of Ultrasonic Technique in Removing Fractured Rotary Nickel-Titanium Endodontic Instruments from Root Canals and Its Effect on the Required Force for Root Fracture

IntroductionFracture of rotary nickel-titanium files is a real concern among endodontists; it affects the long-term prognosis of treatment. Ultrasound is a favorable technique for removing broken files, although it may result in some complications. The aim of this study was to determine the success rate of ultrasonic technique in removing file fragments and to evaluate its influence on the force required to fracture a tooth root. MethodsAn in vitro study was carried out on 70 extracted maxillary premolars. In the experimental group a #30/.04 taper Hero file was guided into the canal to break and lodge therein. The lodged file was removed by ultrasonic vibration and use of a dental operating microscope. The canals were instrumented with Hero files by using crown-down technique and then obturated. All the teeth were prepared for mechanical testing in Testometric machine to measure the required force for root fracture. The Fisher exact test and independent two-sample t test were used to analyze data. ResultsUltrasonic technique exhibited a success rate of 80% in removing broken files. Success rate in the roots with file fracture before the curve was 11.5 times more than that of file fracture cases beyond the curve. Other factors such as bypass possibility did not affect success rate. The average time required for removing file fragments was 36.3 ± 7.15 minutes, which did not significantly differ in different file locations within the canal. According to mechanical test results, ultrasonic application did not significantly affect the required force for root fracture. The force required to fracture a root did not significantly differ in various file locations. Vertical fracture in the buccolingual direction (split tooth) was the most incident fracture pattern. Procedural errors observed in this study predominantly included transportation, perforation, and craze line. ConclusionsUltrasonic technique was successful in removing 80% of fractured rotary files and did not significantly affect the required force for root fracture.

Fatigue Life of Reciproc and Mtwo Instruments Subjected to Static and Dynamic Tests

IntroductionThis study evaluated the influence of flexibility and reciprocating movement on the fatigue life of endodontic instruments subjected to static and dynamic tests. MethodsThe rotary nickel-titanium instruments used in this study were Reciproc and Mtwo. The instruments were initially subjected to a cantilever-bending test and then to static and dynamic fatigue tests. Reciproc instruments were operated in reciprocating movement, whereas Mtwo instruments were worked in continuous rotation. ResultsThe means of bending resistance (maximum load in grams) of the instruments were 274.9 for Reciproc and 429 for Mtwo. The mean times (in seconds) to fracture of the instruments subjected to static and dynamic tests were 214.5 (static) and 286.3 (dynamic) for Reciproc and 38.9 (static) and 99 (dynamic) for Mtwo. The Student's t test revealed significant differences in all tests (P < .05). ConclusionsThe results of the present study showed longer fatigue life for instruments with higher flexibility, driven by reciprocating movement, and in the dynamic testing model. These findings reinforce the assumption that use of reciprocating movement is a means to prolong the fatigue life of rotary nickel-titanium endodontic instruments during instrumentation of curved canals.

Study on bending behaviour of nickel–titanium rotary endodontic instruments by analytical and numerical analyses

AimTo develop analytical models and analyse the stress distribution and flexibility of nickel–titanium (NiTi) instruments subject to bending forces.MethodologyThe analytical method was used to analyse the behaviours of NiTi instruments under bending forces. Two NiTi instruments (RaCe and Mani NRT) with different cross-sections and geometries were considered. Analytical results were derived using Euler–Bernoulli nonlinear differential equations that took into account the screw pitch variation of these NiTi instruments. In addition, the nonlinear deformation analysis based on the analytical model and the finite element nonlinear analysis was carried out. Numerical results are obtained by carrying out a finite element method.ResultsAccording to analytical results, the maximum curvature of the instrument occurs near the instrument tip. Results of the finite element analysis revealed that the position of maximum von Mises stress was near the instrument tip. Therefore, the proposed analytical model can be used to predict the position of maximum curvature in the instrument where fracture may occur. Finally, results of analytical and numerical models were compatible.ConclusionThe proposed analytical model was validated by numerical results in analysing bending deformation of NiTi instruments. The analytical model is useful in the design and analysis of instruments. The proposed theoretical model is effective in studying the flexibility of NiTi instruments. Compared with the finite element method, the analytical model can deal conveniently and effectively with the subject of bending behaviour of rotary NiTi endodontic instruments.

Predicting in vivo failure of rotary nickel-titanium endodontic instruments under cyclic fatigue

The aim of this study was to examine the lifespan or number of cycles to failure of tapered rotary nickel-titanium (Ni-Ti) endodontic instruments. Simulated root canals with different curvatures were used to determine a relation between canal curvature and instrument lifespan. Using a novel mathematical model for the deformation of pseudoelastic Ni-Ti alloy, it was shown that maximum stress need not necessarily occur at the outer layer. On the basis of this observation, the Coffin-Manson relation was modified with parameters determined from this experiment. Results showed that the number of cycles to failure was influenced by the angle and radius of canal curvature and the size of instrument at the beginning of canal curvature. The resulting quantitative mathematical relation could be used to predict the lifespan of rotary Ni-Ti endodontic instruments under clinical conditions and thereby reduce the incidence of instrument failure in vivo.

Influence of Different Manufacturing Methods on the Cyclic Fatigue of Rotary Nickel-Titanium Endodontic Instruments

Introduction The aim of this study was to evaluate, by static and dynamic cyclic fatigue tests, the number of cycles to fracture (NCF) 2 types of rotary NiTi instruments: Twisted File (SybronEndo, Orange, CA), which is manufactured by a proprietary twisting process, and RaCe files (FKG Dentaire, La Chaux-de-Fonds, Switzerland), which are manufactured by grinding. Methods Twenty Twisted Files (TFs) and 20 RaCe files #25/.006 taper instruments were allowed to rotate freely in an artificial curved canal at 310 rpm in a static or a dynamic model until fracture occurred. Results Measurements of the fractured fragments showed that fracture occurred at the point of maximum flexure in the midpoint of the curved segment. The NCF was significantly lower for RaCe instruments compared with TFs. The NCF was also lower for instruments subjected to the static test compared with the dynamic model in both groups. Scanning electron microscopic analysis revealed ductile morphologic characteristics on the fractured surfaces of all instruments and no plastic deformation in their helical shafts. Conclusions Rotary NiTi endodontic instruments manufactured by twisting present greater resistance to cyclic fatigue compared with instruments manufactured by grinding. The fracture mode observed in all instruments was of the ductile type.

Heat treatment of nickel-titanium rotary endodontic instruments: effects on bending properties and shaping abilities

To evaluate the bending properties and shaping abilities of nickel-titanium endodontic instruments processed by heat treatment. K3 files were heated for 30 min at 400 °C (group 400), 450 °C (group 450) and 500 °C (group 500). Files that were not heat treated served as controls. A cantilever-bending test was used to evaluate changes in specimen flexibility caused by heat treatment. Curved root canal models were prepared. The times required for preparation, deformation and fracture were recorded. Pre- and postoperative images were superimposed. The amounts of resin removed from both the inner and the outer sides of the curvature in the apical 6 mm were determined. In the cantilever-bending test, load values of the control group and group 500 were higher than those of groups 400 and 450 at the elastic range (P<0.05). At the superelastic range, the bending load of the control group was the highest amongst all groups (P<0.05). Regarding shaping ability, in the control group, root canals at the apex were transported more to the outer side of the curvature compared with those of all heat-treated groups (P<0.05). Root canals of group 400 at 3 mm from the apex were transported less compared with those of other groups (P<0.05). No significant difference was found in working time amongst the groups. In group 450, there was no plastic deformation or fracture of the file. Heat treatment of files might improve their flexibility, making them more effective for preparation of curved canals.

Effects of Electropolishing Surface Treatment on the Cyclic Fatigue Resistance of BioRace Nickel-Titanium Rotary Instruments

Introduction This study evaluated the influence of electropolishing surface treatment on the number of cycles to fracture of BioRace rotary nickel-titanium endodontic instruments. Methods BioRace size BR5C instruments with or without electropolishing surface treatment were used in an artificial curved canal under rotational speed of 300 rpm until fracture. Fractured surfaces and the helical shafts of fractured instruments were analyzed by scanning electron microscopy (SEM). Results Polished instruments displayed a significantly higher number of cycles to fracture when compared with nonpolished instruments ( P < .001). Actually, the number of cycles to fracture of a polished BR5C instrument was 124% higher than that of a nonpolished instrument. SEM analysis showed that the fractured surface of both polished and nonpolished BR5C instruments had ductile morphologic characteristics. Evaluation of the separated fragments after cyclic fatigue testing showed the presence of microcracks near the fracture surface. Polished instruments exhibited fine cracks that assumed an irregular path (zigzag crack pattern), whereas nonpolished instruments showed cracks running along the machining grooves. Conclusions Electropolishing surface treatment of BioRace endodontic instruments significantly increased the cyclic fatigue resistance.

A Comparative Computational Analysis of the Mechanical Behavior of Two Nickel-Titanium Rotary Endodontic Instruments

Introduction A finite element model of two nickel-titanium (Ni-Ti) rotary endodontic instruments (ProTaper and SystemGT; Dentsply-Maillefer, Ballaigues, Switzerland) was developed to investigate the mechanical behavior of these devices and to identify the benefits/limitations of different geometries during instrumentation in various root canals. Methods Instrument shape, curved root canal geometry, and Ni-Ti alloy pseudo-elastic behavior were investigated in this study using computational techniques. Two different operating conditions were simulated: (1) the file insertion-removal cycle which resembles the standard working condition and (2) the file subjected to a torque in the counter-clockwise direction, which mimics the auto-reverse movement of the instrument when the tip is locked in the canal wall. Results The simulations of standard and auto-reverse conditions produced bending and torsion loading conditions in the files, respectively. In the standard situation in which different canal shapes were considered, the strains in the SystemGT were generally lower than the strains in the ProTaper and always in the pseudo-elastic range; in only 1 case did the ProTaper overcame the pseudo-elastic range limit. In the auto-reverse situation, a better behavior of the ProTaper was detected. Conclusions The two simulated conditions highlighted the different mechanical properties of the files; the SystemGT showed slightly better performances under flexural solicitation, whereas the Protaper presented better behavior under torsion solicitations.

Micro-XRD and temperature-modulated DSC investigation of nickel–titanium rotary endodontic instruments

Objectives Employ Micro-X-ray diffraction and temperature-modulated differential scanning calorimetry to investigate microstructural phases, phase transformations, and effects of heat treatment for rotary nickel–titanium instruments. Methods Representative as-received and clinically used ProFile GT and ProTaper instruments were principally studied. Micro-XRD analyses (Cu Kα X-rays) were performed at 25 °C on areas of approximately 50 μm diameter near the tip and up to 9 mm from the tip. TMDSC analyses were performed from −80 to 100 °C and back to −80 °C on segments cut from instruments, using a linear heating and cooling rate of 2 °C/min, sinusoidal oscillation of 0.318 °C, and period of 60 s. Instruments were also heat treated 15 min in a nitrogen atmosphere at 400, 500, 600 and 850 °C, and analyzed. Results At all Micro-XRD analysis regions the strongest peak occurred near 42°, indicating that instruments were mostly austenite, with perhaps some R-phase and martensite. Tip and adjacent regions had smallest peak intensities, indicative of greater work hardening, and the intensity at other sites depended on the instrument. TMDSC heating and cooling curves had single peaks for transformations between martensite and austenite. Austenite-finish ( A f) temperatures and enthalpy changes were similar for as-received and used instruments. Heat treatments at 400, 500 and 600 °C raised the A f temperature to 45–50 °C, and heat treatment at 850 °C caused drastic changes in transformation behavior. Significance Micro-XRD provides novel information about NiTi phases at different positions on instruments. TMDSC indicates that heat treatment might yield instruments with substantial martensite and improved clinical performance.

Evaluating Two Techniques on Removal of Fractured Rotary Nickel-Titanium Endodontic Instruments from Root Canals: An In Vitro Study

This study evaluated the ultrasonic technique (US) and instrument removal system (iRS) in removing fractured rotary nickel-titanium (NiTi) from root canals. A total of 30 extracted human molars with closed apices were collected. Profile .06 taper rotary endodontic instruments (Dentsply Maillefer, Ballaigues, Switzerland) 25-mm long and ISO size 25 were driven into the mesial canal to be fractured in the curve. Angles and radii of curvature were measured for all the canals with the fractured fragments, and then canals were classified into slight, moderate, and severe subgroups according to their curvature angles. Within each subgroup, teeth were randomly distributed into US or iRS according to measured angles and radii; a total of 15 canals were assigned to both US and iRS techniques. The overall success rate was 70% ( n = 21). Less curved canals and longer radii of curvature showed more success. The median time used for retrieval was 40 minutes using the US technique, whereas it was 55 minutes for iRS. The removal of fractured rotary NiTi endodontic instruments was more successful with less curved canals and longer radius of curvature above 4.4 mm.

Quantitative and Qualitative Elemental Analysis of Different Nickel–Titanium Rotary Instruments by Using Scanning Electron Microscopy and Energy Dispersive Spectroscopy

This study was designed to determine if the alloy composition shares an influence with the geometric design on the physical behavior of nickel-titanium rotary endodontic instruments. ProTaper, HERO, and K3 files were selected. After sterilized and cleaning with alcohol, surface analysis was performed using energy dispersive spectroscopy. Measurements were performed on the active part and on the shank. SEM images of fractured instruments were also obtained and assessed. All three types of instruments were composed mainly of Nickel (54.3%, SD ± 0.8) and Titanium (45.2%, SD ± 0.9). SEM images revealed similar aspect with the presence of Kirkendall voids regularly distributed in the alloy. The results indicate that the difference in properties and behavior of these three endodontic rotary shaping instruments is solely related to the respective geometric characteristics of the instrument design.

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.

Influence of Deep Dry Cryogenic Treatment on Cutting Efficiency and Wear Resistance of Nickel–Titanium Rotary Endodontic Instruments

This study tested the effect of cryogenic treatment on nickel-titanium (NiTi) endodontic instruments. Ten rotary NiTi instruments (ProFile 30, 0.06; Dentsply Maillefer Instruments SA, Baillegues, Switzerland)) were subjected to deep dry cryogenic treatment at liquid nitrogen temperature (-185 degrees C) (group A) and compared with that of nontreated instruments (group B). Root canals of 30 extracted permanent mandibular premolars were prepared to size 25 using ProFile 0.04 taper rotary NiTi instruments. Each instrument was used in 4 root canals for a total of 240 seconds. Cutting efficiency in rotary motion was assessed from weight loss of tooth samples after instrumentation. Wear resistance was assessed from surface defects on cutting heads pre- and postinstrumentation using scanning electron microscope. The weight loss was significantly higher in group A (p < 0.05, Student t test). The presence of surface defects after use was higher in group B. Deep dry cryogenic treatment increases the cutting efficiency significantly but not the wear resistance.

Phase transformation behaviour and bending properties of hybrid nickel–titanium rotary endodontic instruments

To investigate the bending properties of hybrid rotary nickel-titanium endodontic instruments in relation to their transformation behaviour. Four types of nickel-titanium rotary endodontic instruments with different cross-sectional shapes (triangular-based and rectangular-based) and different heat treatment conditions (super-elastic type and hybrid type with shape memory effect) were selected to investigate bending properties and phase transformation behaviour. Bending load of the instruments was measured in a cantilever-bending test at 37 degrees C with the maximum deflection of 3.0 mm. A commercial rotary instrument, ProFile (PF; Dentsply Maillefer, Ballaigues, Switzerland) was used as a reference for the bending test. Phase transformation temperatures were calculated from the diagrams obtained from differential scanning calorimetry. Data were analysed by anova and Scheffe's test. The bending load values of the hybrid type that had undergone additional heat treatment at the tip were significantly lower (P < 0.05) than those of the super-elastic type with no additional heat treatment. The bending load values of rectangular-based cross-sectional shaped instruments were significantly lower (P < 0.05) than those of triangular-based cross-sectional shaped instruments. Phase transformation temperatures (M(s) and A(f) points) of the hybrid type were significantly higher (P < 0.05) than the super-elastic type. The M(f) and A(s) points of the tip part were significantly higher (P < 0.05) than those of the whole part of the hybrid instrument. Additional heat treatment of hybrid nickel-titanium instruments may be effective in increasing the flexibility of nickel-titanium rotary instruments.

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

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

Influence of sterilization on mechanical properties and fatigue resistance of nickel–titanium rotary endodontic instruments

To evaluate the effect of repeated sterilization cycles in dry oven or autoclave, on the mechanical behaviour and fatigue resistance of rotary endodontic Ni-Ti instruments. New Ni-Ti instruments were subjected to five consecutive sterilization cycles in a dry oven or steam autoclave. Microhardness was measured in the nonmachined parts of the shanks of instruments using a Vickers indenter. Specimens of Ni-Ti wires were submitted to the same sterilization protocol and tensile tested until rupture. A group of instruments were fatigued to one half of their average fatigue life and then sterilized. New and sterilized instruments were fatigue tested until rupture. anova tests at alpha = 0.05 were used for statistical analysis. Sterilization procedures resulted in no significant changes in Vickers microhardness, nor in the parameters describing the mechanical behaviour of the wires. However, the number of cycles to failure was statistically higher for all instruments after dry heat or autoclave sterilization cycles. In the instruments previously fatigued to one half of their fatigue life, autoclave sterilization gave rise to an increase of 39% in the remaining number of cycles to failure. Changes in the mechanical properties of Ni-Ti endodontic instruments after five cycles of commonly used sterilization procedures were insignificant. The sterilization procedures are safe as they produced a significant increase in the fatigue resistance of the instruments.