Dynamic and Cyclic Fatigue of Engine-Driven Rotary Nickel-Titanium Endodontic Instruments

Transcription

1 JOURNAL OF ENDODONTICS rinted in U.S.A. Copyright 1999 by The American Association of Endodontists VOL. 25, NO. 6, JUNE 1999 Dynamic and Cyclic Fatigue of Engine-Driven Rotary Nickel-Titanium Endodontic Instruments Youssef Ha'ikel, DCD, DSO, hd, Ren~ Serfaty, DCD, Geoff Bateman, Bernard Senger, hd, and Claude Allemann, DCD, DSO The absence of adequate testing standards for engine-driven nickel-titanium (NiTi) instruments necessitates further study of these instruments in all areas. This study examined three groups of engine-driven rotary NiTi endodontic instruments (rofile, Hero, and Quantec) and assessed the times for dynamic fracture in relation to the radius of curvature to which the instruments were subjected during preparation, with the instrument diameter determined by size and taper and the mode by which the fracture occurred. Ten instruments were randomly selected representing each size and taper for each group and for each radius of curvature: 600 in total. The instruments were rotated at 350 rpm and introduced into a tempered steel curve that simulated a canal. Two radii of curvature of canals were used: 5 and 10 mm. Time at fracture was noted for all files, and the fracture faces of each file were analyzed with scanning electron microscopy. Radius of curvature was found to be the most significant factor in determining the fatigue resistance of the files. As radius of curvature decreased, fracture time decreased. of files was found to be significant in determining fracture time. As diameter increased, fracture time decreased. In all cases, fracture was found to be of a ductile nature, thus implicating cyclic fatigue as a major cause of failure and necessitating further analyses and setting of standards in this area. The preparation of curved canals represents a considerable probtern for the endodontic practitioner, tnadvertant errors, such as ledging and zip formation, occasionally occur, adversely altering root canal morphology and thus detrimentally affecting the prognosis for the subsequent root filling ~ l }. In addition, separation of traditional files and reamers in the canal can seriously jeopardize the therapy (2). As a consequence, new endodontic instruments and techniques have been introduced, which serve to minimize these risks. Of particular importance has been the development of nickeltitanium (NiTi) endodontic instruments. Walia et al. (3l initially investigated the torsional properties of an "experimental" endodontic file and reported two to three tirnes more flexibility in this file, compared with similar stainless-steel (SS) instruments. NiTi possesses the property of superelasticity (3). This property allows these instruments to flex far more than stainless steel before exceeding their elastic limits (3) and allows preparation of curved canals while minimizing transportation {4l. A number of studies have been conducted (5-10) evaluating NiTi files and their pertinence to international specifications as described by ISO (11) and ANSI/ADA {121. Rowan et al. (13) compared the torsional fracture of NiTi and SS files in clockwise and anticlockwise rotation, concluding that the force necessary to cause fracture (g cm/ was similar for both alloys and in both directions. In addition, efficiency of cutting has been evaluated by a number of methods (9, 14, 15). Despite an increased flexibility, failure is still, however, a concern because these instruments can undergo fracture within their elastic limit and without any visible signs of previous permanent deformation (16). The recent development of engine-driven rotary endodontic instruments of nonstandardized shape and variable size and taper necessitated further testing of cyclic fatigue. Rotation of endodontic instruments subjects them to both tensile and compressive forces in the area of curvature of the canal: with tensile forces on the outside of the canal curvature and compressive on the inside. This is of particular concern with respect to instrument failure: according to the authors" knowledge, only one study has been conducted in this area: that of ruett et al. (16). As indicated in their study, the continuous cycle of tensile and compressive forces to which engine-driven instruments are" subject produces a very destructive form of loading. It is the intention of this study to investigate the cyclic fatigue of three tapers of engine-driven rotary NiTi endodontic instruments: rofile, Hero, and Quantec. Investigation of fatigue will be made with respect to instrument size, taper, and radius of curvature to which the instrument is subjected in the preparation of curved canals. The diameters of each file were measured to assess accuracy of stated dimensions of taper, and an analysis was made with regard to the mode of fracture of the files. This study is based on a previous investigation by Hafkel et al. (17) and uses the same equipment for fatigue testing. 434

2 Vol. 25, No. 6, June 1999 Cyclic Fatigue of NiTi Instruments 435 MATERIALS AND METHODS Endodontic Instruments Three types of NiTi endodontic instruments were used in this study: rofile (Maillefer SA, Ballaigues, Switzerland), Hero (Micromega, Besanqon, France), and Quantec (McSpadden, NT Co., Inc., Chattanooga, TN). The rofile group had six file sizes: numbers 15, 20, 25, 30, 35, and 40. In addition, the files each had two tapers of 4 and 6%. The instrument itself was a nonstandardized machined NiTi file. It had a noncutting point and a U-shape radial design in cross-section. The cross-section differed from the illustration given by the manufacturers and resembled a triangle with flattened points giving six cutting angles in all. The Hero group came in three file sizes of 20, 25, and 30, and in three tapers of 2, 4, and 6%. Files were found to be of a nonstandardized shape and were machined from NiTi. In crosssection, the files were found to be triangular, and again the tip of the instrument was noncutting. The Quantec group consisted of 10 files numbered 1 to 10 of variable sizes and tapers. These include size 25 at 6%; size 15 at 2%: size 20 at 2%; size 25 at 2, 3, 4, 5, and 6%; size 40 at 2%; and size 45 at 2%. Because the working length of number 8 and number 1 are identical, the instrument was only tested once. Again, this is a nonstandardized instrument machined from NiTi. The crosssection is difficult to determine and differs from the illustration given by the manufacturer. The file appears to have a very sophisticated shape with different cutting angles and a unique double curvature. The tip of the instrument cuts actively. For each group and for each of the two tested radii of curvature, 10 files were randomly selected representing each size and taper of their respective groups. Six hundred files were used in all. Measurement of the diameters of each file to determine taper was made with a micrometer. The testing device was composed of two parts: the test bed (Cary BMH, Meseltron SA, le Locle, Switzerland) and the axial sensor (Sylvac SA, Crissier, Switzerland), with a precision for the complete testing bed of _+2 /,m. Measurements were made of diameters at 3 mm (D3) and 13 mm (Dl3) from the tip, and calculations of taper were made taking mean and standard deviations for each instrument. Experimental Design A system (Fig. 1) was used that allowed fatigue testing while maintaining conditions as close as possible to the clinical situation. This comprised two main parts: a dental handpiece (WH 975 AE 500 Reduction 20/1: Dental Work, Burmoos, Austria) mounted in a support block and a shaping block for the endodontic files. The support block was independent of the rest of the device and was mounted on an electronically controlled table (SELEDATA SM 512C Selection; Lyss, Switzerland) to allow easy placement of the instrument. The shaping block was fixed and consisted of a concave-tempered steel radius and a convex-tempered steel cylinder that guaranteed curve of the instruments. The concave radius incorporated a notched V-form for guiding the instruments and came in two sizes: a radius of curvature of 5 mm and a radius of 10 mm (Figs. 1 to 3). The NiTi files were mounted in the lathe and were rotated at 350 rpm, which was recommended by the manufacturers, thus simulating the operating speed of a slow-speed handpiece. Files were FIG 1. Apparatus used for the experiment. (A) Dental handpiece that was independent from the rest of the device. (B) Tempered steel radius form of fixed radius. (C) Tempered steel cylinder to guarantee curvature of the instruments. FIG 2. Relationship of the tempered steel cylinder to the radius form. (,4) oint of the instrument at the end of the curve. (B) Cylinder to guarantee the curve of the instruments. (C) Instrument setting. cooled under a cold air spray during the experiment to prevent overheating and subsequent alteration of the NiTi alloy structure. The files were then guided into the V-notch of the radii and rotated until fracture occurred. Fracture was easily detectable because the tip of the instruments were visible at the end of the curve of the radii. A note of fatigue resistance time until fracture was measured with a 1/100 s chronometer. Analysis of the fracture face of each file was made with scanning electron microscopy (SEM) to determine mode of fracture. Statistical testing was then undertaken with the t test to identify significant differences between variables in the study. RESULTS Individual results for cyclic fatigue are displayed in Tables 1 to 3. Radius of Curvature Radius of curvature of the canal was found to be statistically significant in all cases, with one exception: Micromega Hero size

3 436 Ha'ikel et al. Journal of Endodontics tapers for each size, statistical testing was not undertaken. It is, however, possible to see an almost linear relationship for file size 25, where an increase in taper from 2 to 6% produces a corresponding decrease in fracture time (Fig. 5). In general, an increase in taper of the files in each group may be expected to decrease the time until failure of the files in a situation of dynamic stress. Comparison Between the Three Instrument Groups FIG 3. Radius form independent from the rest of the device. (A) V-form for guiding the instruments. 30 at 4% (Tables 1 to 3). Thus, decreasing the radius of curvature from 10 mm to 5 mm to simulate a more acutely curved canal was found to shorten life expectancy significantly for the instrument (Figs. 4 to 6). Radius of curvature was found to be the most significant parameter in terms of prediction of instrument failure in this cyclic fatigue test. Instrument Size Differences were found for instruments with respect to the effect of size on dynamic fracture (Tables 1 to 3). In general, as instrument size increased, dynamic fracture times decreased--although not statistically analyzed--in general: a larger instrument was more likely to undergo fracture in less time under dynmnic stress than a smaller one. This, however, was not a uniform finding for any of the tested groups, although it would seem that this phenomenon became more pronounced as taper of the files increased and when the files were subjected to a more abrupt radius of curvature (Figs. 4 to 6). Instrument Measurement of the stated taper of each file in general showed quality control to be very good. In almost all cases, the files corresponded with the manufacturers stated dimensions of taper. Some concern, however, was expressed over the accuracy of the files in the rofile group sizes 30, 35, and 40 at a taper of 4% and Hero size 25 taper 4%, which were used for fracture testing at a radius of 5 ram, as well as size 20 at a taper of 4% used for fracture testing at a radius of 10 mm. Generally speaking, Quantec was the most precise group in relation to the given taper values, whereas Hero was the less precise group concerning the taper of 4 and 6% (Tables 1 to 3). Statistical testing indicated that significant differences existed with respect to the effect of increasing the taper and its influence on fracture time. Files in the Hero series were tested comparing tapers 2% with 4(}, 2% with 6%, and 4% with 6% at 5 and 10 mm radii (Table 4). Files in the rofile series were tested comparing tapers 4c~ with 6% at 5 and 10 mm radii (Table 5). As files in the Quantec series did not, in general, possess two Statistical analyses comparing groups of files were only made for size 25 and taper 4 and 6% instruments in the groups rofile/ Hero/Quantec, because these instruments alone shared similar size and taper for the two radii tested. Significant differences were found in all cases, with respect to fracture times, with the exception of that between rofile/hero at 4%, 5 mm radius (Figs. 7 and 8). Mode of Fracture In all cases, fracture was of a ductile nature, as evidenced by SEM with crater-like formations on the face of each instrument: rofile, Hero, and Quantec (Figs. 9 to I l/. DISCUSSION This study investigated the cyclic fatigue life of three groups of NiTi endodontic instruments. As described in ANSI/ADA specification no. 28 (12), instruments should be statically loaded until failure. The authors consider this to be an inadequate method of fracture resistance testing for engine-driven NiTi files, because by their nature they are intended to operate at a certain speed of rotational movement before insertion into the root canal, thus rendering static analyses irrelevant. Static analyses of torque at failure have shown NiTi instruments to perform satisfactorily, if not better than similar SS instruments (3, 5-7). Yet, this did not adequately address the clinical findings and the findings of other authors that NiTi files are liable to undergo sudden, unexpected fracture (16, 18). Cyclic fatigue plays an important part in this process, and there is a clear need for the development of a test protocol to assess and define standards of fracture resistance that may be acceptable clinically for engine-driven NiTi files. Instruments used in this study were operated at a constant speed of 350 rpm. In similar tests, ruett et al. (16) have shown alteration of operating speed in a range of 750 to 2000 rpm to have no significant effect on the number of cycles to failure. As recommended by the manufacturers, however, instruments should be operated between 200 and 600 rpm to extend clinical life. Canals were simulated with a device that guaranteed fixed radii of curvature for the files. This radius of curvature is the radius of a circle that coincides with the path taken by the canal in the area of the most abrupt curvature. This allowed the reproducible simulation of clinical canal curvature. Values of radius were set at 5 mm to simulate an abruptly curved canal and 10 mm to simulate a canal with a more sweeping curvature. This variable of canal curvature was found to have a significant effect on all results for cyclic fatigue, with the exception of one instrument--micromega Hero size 30 at 4c~. The more abrupt the curvature, the less the time until premature instrument fracture. It would be reasonable to extrapolate from these results that the radius of curvature was directly proportional to the fatigue life of

4 Vol. 25, No. 6, June 1999 Cyclic Fatigue of NiTi Instruments 437 TABLE 1. rofile: instrument fatigue expressed in rotation time (seconds) until fracture (life span) and the taper of instruments measured between D3 and D13, (p) expressing the probability based on the t test of differences in fatigue time between 5 and 10 mm radii of curvature rofile 4% Radius 5 mm rofile 4% Radius 10 mm Mean + SD _ _ _ < _ _ _ < _ _ _ _ < _ ± < _ < _+ 0, _ _ <10 4 rofile 6% Radius 5 mm rofile 6% Radius 10 mm _ _ < _~ ± _ < _ _ _ < < _ < _ <10 4 TABLE 2. Hero: instrument fatigue expressed in rotation time (seconds) until fracture (life span) and the taper of instruments measured between D3 and D13, (p) expressing the probability based on the t test of differences in fatigue time between 5 and 10 mm radii of curvature Hero 2% Radius 5 mm Hero 2% Radius 10 mm Mean -~ SD Mean +_ SD _ _ _ < _ _ _ < _ _ <10 4 Hero 4% Radius 5 mm Hero 4% Radius 10 mm _ _ < _ _ < _ _ _ Hero 6% Radius 5 mm Hero 6% Radius 10 mm Mean SD Mean -- SD _ _ < _ _ < _ <10 4 the instrument. Thus, the clinical situation of severely curved or dilacerated roots should make the practitioner wary of this form of instrumentation. aradoxically, it was for these situations that NiTi flies were originally developed (3). In addition, as shown by this study, instruments that have been used in these situations of abrupt curvature will have suffered cyclic fatigue, and the clinician may be wise to discard these without another use. This is especially important given that it is often not possible to visually assess fatigue in a NiTi instrument in the same manner that one can see fatigue in a SS instrument (16). The diameter of the instrument as related to its taper, and in general to its size, has been shown by this study to be significant in determining fatigue life. This is in accordance with other studies (16, 17). An increased diameter at the point of maximum flexure in the canal, as indicated by these studies, will decrease fracture time. Thus, the dynamic time to fracture is indirectly proportional to the diameter of the instrument. An increase in instrument size and an increase in taper will theoretically reduce the life span of these instruments. This is in contrast with static torsion testing of torque at failure for both NiTi and stainless steel endodontic files that shows that an increase in diameter increases the resistance to fracture (14). Thus, it may be wise, clinically, to discard these larger instruments as regularly, if not more after, than those with a smaller size or taper. As evidenced by SEM, fracture in these instruments was solely of a ductile nature. The micrographs show a macroscopically smooth fracture face with crater-microvoid formations at higher magnification. As discussed by Lausten et al. (19), this is indica-

5 438 Hai'kel et al. Journal of Endodontics TABLE 3. Quantec: instrument fatigue expressed in rotation time (seconds) until fracture (life span) and the taper of instruments measured between D3 and D13, (p) expressing the probability based on the t test of differences in fatigue time between 5 and 10 mm radii of curvature Quantec Radius 5 mm Quantec Radius 10 mm No. Fatigue Mean SD 1 6% _ _ _ <10 '~ 2 2% _ _ _ <10 '~ 3 2% ± ± _ < % _ _ _ < % < % ± ± < % < % ± < % ± ± ± _ < % ± < ili so e i II Radius 5 rnm []Radius 10 mm t ROFILE 6 % FIG 4. Fatigue times of rofile group, taper 6% comparing radius 5 to 10 mm ~" i= t t T i T HERO 2% IIRadiui 5 mm I FIG 6. Fatigue times of Hero group, taper 2%, comparing radius 5 to 10 mm. T 3OOO o 1500 g I I 1 1,t I {~12~ I Radius 5 ram! i [] Radius t 0 mm I ~ 158 (25} (15} {20} (zs} 125) I25) I251 {251 {40) (45) 6% 2% 2% 2% 3% 4% 5% 6% 2% 2% QUANTEC FIG 5. Fatigue times of Quantec group, comparing radius 5 to 10 mm. tive of a ductile fracture wherein the tensile stresses on the outer surface of the instrument play a dominant role in fracture initiation. This results in a uniform topography of microvoid coalescence on the fracture face. In contrast, a brittle fracture shows no such topography (19). This observation of ductile fracture again emphasizes the importance of cyclic fatigue in NiTi instrument failure and adds weight to the need for the development of a test protocol to define and assess cyclic fatigue values for these instruments. Statistical comparison between instrument groups was only possible with instruments of 3 different designs: Quantec, Hero, and rofile, size 25 and tapers 4 and 6%, because these alone had comparable diameters. With these instruments, rofile performed significantly better in terms of fracture resistance times at tapers 4 and 6% and radius 10 mm. and at taper 6% and radius 5 ram. The most challenging situation in terms of dynamic stresses was the radius of 5 mm and the taper of 6%. At a radius of 5 mm and a taper of 4%, Quantec performed significantly better. The highest values of resistance to fracture in this study were found for instruments in the 2% taper for the Quantec series. It was noteworthy though that these instruments were of small diameter (sizes 15, 20, and 25) and had a lesser taper at 2%, which was also the case for Hero 2% at 10 mm radius, where the same result was obtained, even though its design is very different from that of Quantec. The tapers stated by the manufacturers were found, in general. to be accurate when measured with the testing device (Radial Sensor and Cary). There was some concern over the inconsistencies of the rofile series (Table 1) sizes 30 (taper 3.3%), 35, 40 (taper 4.5%) at taper stated 4% and Hero series (Table 2) size 25 at taper stated 4% (taper 3.7%), which were used in fracture testing in the canal of radius 5 mm, as well as for Hero size 20 at taper stated 4% (taper 3.5c}) used in fracture testing in the canal of radius 10 ram. Because no protocol exists that stipulates consistency of taper for tiles of taper 4 and 6%, it is difficult to quantify the significance of these results. ISO stipulations for accuracy of diameter of the file (1 I t indicate that diameters for sizes 8 to 60 measured at specific points should not deviate by more than _+0.02 mm. It would seem that these NiTi files would be irrelevant to the accuracy demanded for those of tapers 4 and 6% Hero, and within the accuracy demanded for taper 2q- Hero and Quantec (Table 3). This is an area that requires careful examination and setting of standards, because this study has shown the taper of files to have an indirect relationship with respect to dynalnic

6 Voi. 25, No. 6, June 1999 Cyclic Fatigue of NiTi Instruments 439 TABLE 4. robability (p} based on the t test of differences in fatigue time between different tapers of the Hero group Radius 5 mm Radius 10 mm Hero No. 2%-4% 2%-6% 4%-6% 2%-4% 2%-6% 4%-6% () () () () () () " < " <10-4 <10 4 < " <10 4 <10 4 <10 4 < " 30 <10 2 <10 4 <10 4 <10 4 <10 4 <10 3 * Not statistjca/ly significant. TABLE 5. robability (p) based on the t test of differences in fatigue time between 4% and 6% tapers of the rofile group Radius 5 mm Radius 10 mm rofile No. 4%-6% 4%-6% () 15 0,19" 0.78* 20 <0.05 < <10-4 < " < < * 40 <10 4 <10-4 * Not statistically significant () 7O0 600 Radius 5 mm ; [] Radius 10 mm 500 = g 300 I FiG 9. SEM of fracture face of rofile OO 100 I ROFILE QUANTEC HERO Size 25 4 % FIG 7. Fatigue time comparisons between rofile, Quantec, and Hero files size 25, taper 4% for the two tested radii 5 and 10 mm. All of the differences were significant (p < 0.05), except for rofile/hero at 4% 5 mm radius O ~u, ~mm~ u ~_dlu, 10 m~ g 25O! ~ 200 ~- 150 FIG 10. SEM of fracture face of Hero. x200. ROFILE QUAHTEC HERO Size 25 6% FIG 8. Fatigue time comparisons between rofile, Quantec, and Hero files size 25, taper 6% for the two tested radii 5 and 10 mm. All of the differences were significant (p < 0.05). fracture times. As fracture time decreases with an increasing taper, it would seem logical that standards should be more exact for a file more likely to undergo fracture. Even though it is difficult to quantify the clinical significance of these results, given the lack of previous studies of this nature and the absence of any defined standards for these instruments, it might however be reasonable to discuss some general clinical principles. This study shows engine-driven instrument failure to be of concern due to cyclic fatigue. Thus, the practitioner may be advised to discard these instruments after a certain period of time, because it is recommended for SS instruments. The clinician should be wary of canals with an abrupt curvature where the NiTi files are most likely to undergo fracture. A tactile appreciation of instrument weakness, absent with the handpiece, may be more valuable in preventing instrument failure in this case. After instrumentation of particularly curved canals, it might be reasonable for the clinician to discard the instrument, which will have been weakened by cyclic fatigue. In addition, the clinician should not assume that a

7 440 Hai~el et al. Journal of Endodontics prints to Dr. Youssef HaTkel, INSERM U-424, U.F.R. Odontologie, Universite Louis asteur, 1, lace de I'hopital, Strasbourg, France References,: : i,, 4 >, )- ~ ~ J,,;1 :::%:,',~ W FIG 11. SEM of fracture face of Quantec. x200. larger instrument will have a longer life span. It has been shown by this study not to be the case. This study did not address the effect of axial movement during preparation of the canals, which would be expected to increase the clinical life span of these instruments, not concentrating the stress in a particular area (20). It should be emphasized to the clinician that these instruments are not designed to be used in a manner similar to this study. Lingering in the canal at a certain depth will encourage premature fracture (16). Cyclic fatigue has been shown to be of concern in engine-driven NiTi instruments by this study, and the endodontist may be wise to treat engine-driven NiTi instruments with caution until such time as adequate testing regimes are instituted, and adequate values for fracture resistance and accuracy of taper have been determined. The authors would like to acknowledge NT Co. (Tycom), Maillefer, and Micromega. Drs. Hdlkel, Serfaty, and Allemann are affiliated with the Faculty of Dentistry, Department of Endodontics, Universit6 Louis asteur, Strasbourg, France. Drs. Hdrkel and Senger are affiliated with the National Institute of Health and Medical Research (INSERM U-424), Strasbourg, France. Mr. Bateman is affiliated with the ERASMUS Exchange rogramme Faculty of Dentistry, University of Glasgow, Glasgow, Scotland. Address requests for re- 1. Weine FS. Endodontic therapy. 4th ed. St. Louis: CV Mosby, Grossman LI. Guidelines for the prevention of fracture of root canal instruments Oral Surg Oral Med Oral atho11969;28: Walia H, Brantley WA, Gerstein H. An initial investigation of the bending and torsional properties of Nitinol root canal files J Endodon 1988;14: Glossan CR, Hailer RH, Dove SB, del Rio CE. A comparison of root canal preparations using NiTi hand, NiTi engine driven and K-Flex endodontic instruments J Endodon 1995;21: Camps J J, ertot WJ. Torsional and stiffness properties of Canal Master U stainless steel and nickel titanium instruments. J Endodon 1994;20: Camps J J, ertot WJ. Torsional and stiffness properties of NiTi K-files. Int Endod J 1995;28: Tepel J, Schafer E, Hoppe W. roperties of endodontic hand instruments used in rotary motion art 3. Resistance to bending and fracture J Endodon 1997;23: Marsicovetere ES, Burges JO, Clement D J, del Rio CE. Torsional testing of the Lightspeed nickel titanium system J Endodon 1997;23: Hdl'kel Y, Serfaty R, Wilson, Speisser JM, Allemann C. Cutting efficiency of nickel-titanium endodontic instruments and the effect of sodium hypochlorite treatment. J Endodon 1998;24: Hdrkel Y, Serfaty R, Wilson, Speisser JM, AIlemann C. Mechanical properties of nickel-titanium endodontic instruments and the effect of sodium hypochlorite treatment J Endodon 1998;24: International Standards Organization. ISO , Dental root canal instruments art 1. Geneva, Switzerland: ISO, 1992: ANSl/ADA. ANSI/ADA Revised American National Standards Institute/ American Dental Association specification no. 28 for root canal files and reamers types K and H. New York: ANSI/ADA, Rowan MB, Nicholls J, Steiner J. Torsional properties of stainless steel and nickel titanium endodontic files J Endodon 1996;22: Camps J J, ertot WJ. Machining efficiency of NiTi K-type fries in a linear motion. Int Endod J 1995;28: Kazemi RB, Stenman E, Spangberg LSW. Machining efficiency and wear resistance of NiTi endodonhc files. Oral Surg 1996;81: ruett J, Clement D J, Carnes DL Jr. Cyclic fatigue testing of NiTi endodontic instruments. J Endodon 1997;23: Hdl!<el Y, Gasser, Allemann C. Dynamic fracture of hybrid endodontic hand instruments compared with traditional files. J Endodon 1991 ;17: Zuolo ML, Walton RE. Instrument deterioration with usage: nickel titanium versus stainless steel. Quint Int 1997;28: Lausten LL, Leubke NH, Brantley WA. Bending and metallurgical properties of rotary endodontic instruments. IV. Gates Glidden and eeso drills. J Endodon 1993;19: Dederich DN, Zakariasen KL The effects of cyclical axial motion on rotary endodontic instrument fatigue. Oral Surg 1986;61:192-6.