Fracture of instruments used in rotary motion occurs in two different ways: fracture

Transcription

1 A Review of Cyclic Fatigue Testing of Nickel-Titanium Rotary Instruments Gianluca Plotino, DDS, PhD,* Nicola M. Grande, DDS, PhD,* Massimo Cordaro, MD, DDS, Luca Testarelli, DDS,* and Gianluca Gambarini, MD, DDS Abstract 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. (J Endod 2009;35: ) Key Words Cyclic fatigue, fracture, nickel-titanium, rotary instruments From *La Sapienza University, Rome, Italy and Catholic University of Sacred Heart, Rome, Italy. Address requests for reprints to Dr Gianluca Plotino, Via Eleonora Duse, Rome, Italy. address: gplotino@fastwebnet.it /$0 - see front matter Copyright ª 2009 American Association of Endodontists doi: /j.joen Fracture of instruments used in rotary motion occurs in two different ways: fracture caused by torsion and fracture caused by flexural fatigue (1 3). Torsional fracture occurs when an instrument tip or another part of the instrument is locked in a canal while the shank continues to rotate. When the elastic limit of the metal is exceeded by the torque exerted by the handpiece, fracture of the tip becomes inevitable (4). Instruments fractured because of torsional loads often carry specific signs such as plastic deformation (1). Fracture caused by fatigue through flexure occurs because of metal fatigue. The instrument does not bind in the canal, but it rotates freely in a curvature, generating tension/compression cycles at the point of maximum flexure until the fracture occurs (5, 6). As an instrument is held in a static position and continues to rotate, one half of the instrument shaft on the outside of the curve is in tension, whereas the half of the shaft on the inside of the curve is in compression. This repeated tension-compression cycle, caused by rotation within curved canals, increases cyclic fatigue of the instrument over time and may be an important factor in instrument fracture (5, 6). The impact of torsional fracture, metal fatigue, or fracture of nickel-titanium (NiTi) rotary instruments caused by a combination of torsional stress and accumulation of fatigue is still debated (6). Material fatigue appears to be an important reason for the separation of rotary instruments during clinical use. Peng et al (7) classified most of the fractured instrument analyzed as flexural failure, implying fatigue being the predominant mechanism for material failure. In a related study, Cheung et al (8) reported that the great majority (93%) of instruments appeared to have failed because of flexural fatigue. This might be explained as follows (8): first; fatigue-crack growth rates in NiTi alloys have been reported to be significantly greater than in other metals of similar strength (9). Thus, once a microcrack is initiated, it can quickly propagate to cause catastrophic failure. In comparison, Sattapan et al (1) reported that torsional fracture occurred in 55.7% of all fractured files, whereas flexural fatigue occurred in 44.3%. These results indicated that torsional failure, which may be caused by using too much apical force during instrumentation or by other contributing factors such as the pre-existing size of the canal, occurred more frequently than flexural fatigue, which may result from use in curved canals. Alapati et al (10) hypothesized that the clinical fracture of nickel-titanium rotary instruments was caused largely by a single overload incident (that causes ductile fracture) during instrumentation, rather than the result of significant alloy fatigue after a large number of loading cycles. Cheung et al (8) have questioned the macroscopic or lateral view examination of a separated file used in these studies, suggesting that more advanced fractography techniques are required in order to evaluate precisely the type of fracture. American National Standards Institute/American Dental Association (ANSI/ADA) specification. No. 28 (11) prescribes tests to measure strength under torsion and flexibility of stainless steel hand files. The same tests are adopted by ISO 3630/1 (12), which is designed for instruments having.02 ISO taper. To date, there is no specification or international standard to test cyclic fatigue resistance of endodontic rotary instruments, even if ISO and ADA are currently working to develop a new standard for NiTi endodontic instrument of greater taper. Such a new standard is required by manufacturers, scientists, and clinicians to define suitable mechanical properties of NiTi rotary instruments for a safe, efficient clinical use and to introduce universally accepted testing devices for experimental evaluation of products or prototypes. JOE Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1469

2 Figure 1. The degree of root canal curvature obtained using the method described by Schneider for determining canal curvature using only one parameter to define the angle. A has an angle of 43 and B has an agle of 52, even though both angles measured according to the method of Pruett et al. equaled 60 degrees. Location of the curve along the canal will also change the measured angle. (Reprinted with permission [14]). An ideal model would involve instrumentation of curved canals in natural teeth. However, in such tests, a tooth can only be used once and the shape of the root canal will change during instrumentation, making it impossible to standardize experimental conditions. As a result, several devices and methods have been used to investigate in vitro cyclic fatigue fracture resistance of NiTi rotary endodontic instruments. The aim of this review of the literature was to summarize and analyze all the devices that have been used in endodontic literature for cyclic fatigue testing, explaining how differences in the devices may affect the fatigue behavior of rotary instruments and, consequently, the outcome of these studies. Cyclic Fatigue Testing Devices The rotational bending is the test used in endodontic literature for fatigue testing of NiTi rotary instruments. The devices used to test cyclic fatigue resistance of NiTi rotary instruments permit instruments to rotate until fracture using different geometric curvatures. In 1986, Dederich and Zakariasen (13) underlined that a potential problem with the use of 360 rotary engine files in curved canals was metal fatigue and subsequent breakage. This study analyzed the effect of cyclical axial motion on engine-driven K-type stainless steel instrument failure. A thick-walled Pyrex capillary with a 1-mm diameter lumen was heated and bent to a curvature representative of a moderately curved root canal, without precise curvature parameters. The angle and radius of a circumference has been subsequently established by Pruett et al (14) as the benchmark parameters widely accepted to define the characteristics of a curvature. This study defined the parameters of canal curvature in a more exact manner then generally used in endodontic research. Canal curvature was historically defined using the method introduced by Schneider in 1971 (15). This method used only a single parameter to define an angle in degrees. To determine the degree of root curvature, Schneider drew a line parallel to the long axis of the canal. A second line was drawn from the apical foramen to intersect with the first line at the point where the canal began to leave the long axis of the canal. The acute angle formed was defined as the degree of root curvature (Fig. 1). The shape of any root canal curvature was more accurately described by Pruett et al (14) using two parameters: Figure 2. The method described by Pruett et al (14) for describing canal geometry using two parameters: radius of curvature and angle of curvature. (Reprinted with permission [14]). angle of curvature and radius of curvature (Fig. 2). To determine these parameters, a straight line is drawn along the long axis of the coronal straight portion of the canal. A second line is drawn along the long axis of the apical straight portion of the canal. There is a point on each of these lines at which the canal deviates to begin or end the canal curvature. The curved portion of the canal is represented by a circle with tangents at these two points. The angle of curvature is the number of degrees on the arc of the circle between these two points. Angle of curvature can also be defined by the angle formed by perpendicular lines drawn from the points of deviation that intersect at the center of the circle. The length of these lines is the radius of the circle and defines the radius of the canal curvature defined in millimeters. This parameter represents how abruptly a specific angle of curvature occurs as the canal deviates from a straight line. The smaller is the radius of curvature, the more abrupt is the canal deviation. These two parameters are independent of each other. Several studies have used artificial canals that were constructed by bending glass (16, 17) or metal (14, 18 25) cylindrical tubes with different inner diameters and point of maximum curvature and using different radii and angles of curvature (Fig. 3). A glass tube of internal diameter 1.2 mm was used by Anderson et al (16). It was curved by heating over a flame and curving over a metal cylinder, which gave a radius of curvature of 5 mm. They have used 45 and 90 angles of curvature, and the point of maximum curvature was 5 mm from the tip of the instrument. A small glass tube with an angle of 45 and a 5-mm curvature radius was used by Barbosa et (17) without specifying the inner diameter of the tube. The files were submitted to curvature between the third and the seventh millimeter from the tip. Artificial canals used by Pruett et al (14) and Mize et al (18) were fabricated from 18-G, stainless steel needles having an internal diameter of 0.83 mm. A 2-mm and 5-mm radius of curvature measured to the inner aspect of the curve of the guide tubes. They have used 30, 45, and 90 angles of curvature, and the point of maximum curvature was 7 mm from the tip of the instrument. Yared et al (19, 20) used a 90 metal tube with an internal diameter of 2 mm without specifying radius of curvature and where the point of maximum curvature was located. To construct the artificial canals, Melo et al (21) used stainless steel needles, with an external diameter of 1.6 mm and 40-mm long that were bent with the help of a gauge to provide a 5-mm curvature radius and 45 curvature angle. The maximum curvature region was located at approximately 4.5 mm from the tip of the files. A 2-mm inner-diameter stainless steel tube was also used by Yao et al (22). They used an angle of 60 and a radius of curvature of 5 mm. The author used 1470 Plotino et al. JOE Volume 35, Number 11, November 2009

3 Figure 3. A schematic drawing of curved glass or metal tubes used for fatigue testing of NiTi rotary instruments. a dynamic model without specifying at what distance from the tip of the instrument the point of maximum curvature was located. Stainless steel tubes with an inner diameter of 1.04 mm and a radius of curvature of 6 mm were used by Lopes et al (23). The authors used two different arcs lengths because in the curved canals with the same radius, it is possible that there are arcs (curved segments) with different lengths represented by angles with different degrees. They used an arc of 9.4 mm corresponding to an angle of 90 and an arc of 14.1 mm, corresponding to an angle of 135 when using a 6-mm radius of curvature. They used a straight coronal part of 10.6 mm and 5.9 mm, respectively, so that the total length of the curved and straight parts was 20 mm. Bui et al (24) used artificial canals that were constructed by bending a 16-mm gauge stainless steel Monojet blunt needle to a 5-mm radius on curvature and angles of curvature of 30,45, and 60. The point of maximum curvature was 7 mm from the tip of the instrument. Similar to Pruett et al (14), Kramkowski et al (25) constructed artificial canals by bending stainless steel tubing. Two canals were bent to a 5-mm radius of curvature with angles of curvature of 45 and 60. The center of the radius in the curved section of the canal was 7 mm from the tip of the file. The artificial canals were inserted into predrilled acrylic blocks for mounting in a fixed jig on the platform of the cyclic fatigue instrument. The jig was placed at the opposite end of the rotary handpiece fixed at a distance so that the files protruded approximately 2 mm out of the end of the tube. Instruments were rotated in the artificial canal with a consistent insertion and withdrawal of 8 mm. Silicone spray (CRC Industries Inc, Warminster, PA) as a lubricant and a debris-clearing agent was applied between each file tested. Cylindrical tubes did not sufficiently restrict the instrument shaft, which spring back into its original straight shape, aligning into a trajectory of greater radius and reduced angle, as it has been speculated in previous articles that have used this type of methodology (19 21, 26). Because of the inner diameter of the tubes (glass and metal) is greater than that of the instruments, an instrument rotated in the tube will follow a trajectory that is not predictable and without the parameters of radius and angle of curvature and point of maximum curvature that were established when constructing the artificial canals. Furthermore, each instrument, depending on tip size, taper, design, pitch length, and morphologic and geometric features, will follow its own trajectory in tubes that do not sufficiently constrain the shafts of the instruments, especially the smaller ones. If instruments of the same dimensions follow different trajectories in the test apparatus, a direct comparison between instruments of different brands may be difficult to establish and the results obtained may be unreliable and not consistent. Furthermore, it is unclear what the predictability of these parameters of radius and angle of curvature and point of maximum curvature obtained by bending a straight metal or glass tube. Another problem with a loose-fitting canal is that the file may walk or vibrate in that space, leading to a change in the magnitude of stress and possibly leading to variations in the results. Ounsi et al (27) have used a custom-designed stainless steel model mimicking a 2-mm-wide artificial canal space. The constant diameter of the cavity that reproduced the curved canal presents the same problems of the tube-like devices in lacking the reproducibility of the actual trajectory followed by different files. In other studies, the curvature of a rotary instrument was produced when worked against a sloped metal block using a groove machined into the face of the block to keep the file in place during testing (28 31) (Fig. 4). The block had sufficient hardness to resist the operation of an instrument. The different angles of curvature used in these studies were determined according to Schneider s method (15). Li et al (28, 29) used a sloped carbon-steel block and calculated only the angle of curvature by Schneider s method (15), without JOE Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1471

4 Figure 5. A schematic drawing of the three stainless steel pins that constrained the instrument into the curvature in the studies by Cheung et al (32 36). Figure 4. The inclined plane used in some studies to produce the curvature of a rotary instrument working against a sloped metal block. A groove machined into the face of the block keeps the file in place during testing. (Reprinted with permission [28]). considering the radius of curvature as done by Pruett et al (14). They used four angles of curvature: 37, 40.5,45, and 48. In the study by Kitchens et al (30), a 2-mm-wide groove was machined into the face of a hardened 316 stainless steel block with polished chrome plating to keep the file in place during testing. Three angles of curvature were used and measured using Schneider s method (14): 25,28, and Ray et al (31) used a highly polished area of a stainless steel block with an incline of 15 to the horizontal plane, similar to Li et al (28, 29). At a maximum flexure, all files produced an angle of 28 determined according to Schneider s method (15). Despite the radius of curvature having been recognized as the most important factor influencing cyclic fatigue, these studies measured file curvature according to Schneider s method, which takes into consideration only the angle of curvature and not the more important radius of curvature. Furthermore, it is unclear if a 2-mm-wide groove is able to constrain the tiny tip of an endodontic instrument, thus keeping the file in place during the test. Furthermore, using the previously described device, the point in which the instrument begins to leave the established long axis of the instrument is not predictable and depends greatly on the physical and geometric properties of each instrument. It is really difficult to establish this point precisely; if the instrument is not sufficiently constrained in a precise trajectory, the part of the instrument coronal to the beginning of the curvature will gradually move away from the long axis of the shaft. For this reason, the choice of this point may vary greatly, and the calculated angle may present great variability. Furthermore, it is not possible to establish exactly the point of maximum curvature because the physical and geometric features of the different instruments may determine different bending properties, so that the point of maximum curvature may lie at different points and at a different distance from the tip of each file. As mentioned earlier, bending properties of different files may determine a different trajectory if the file is not constrained in a precise trajectory. If testing is completed for all different files at a given angle to ensure consistency, the bending properties of the different files determining different angles of curvature, thus biasing the results and the comparisons. To limit these problems, Cheung et al (32 36) constrained the instrument into a curvature using three stainless steel pins (Fig. 5). They used three smooth cylindrical pins of 2-mm diameter from a high hardness stainless steel mounted in acrylic shims, which were adjustable in the horizontal direction; the position of the pins determines the curvature of the instrument. A small V-shaped groove prepared on the lowest pin maintained the position of the tip of the instrument during rotation. It has been reported in a three-point bending test of NiTi wires that such constraints will produce a curvature that is circular (37). The authors affirmed that although this cannot actually be true, the approximation should be reasonable. Unfortunately, NiTi endodontic files are tapered and with different crosssectional design. The different bending properties of the different files 1472 Plotino et al. JOE Volume 35, Number 11, November 2009

5 Figure 7. (A) A grooved block-and-rod showing the relationship of the tempered steel cylinder to the radius form to guarantee the curvature of the instruments. (B) The radius form with the V-groove for guiding the instrument. (Reprinted with permission [39]). Figure 6. A schematic drawing of the two hard-steel pins that constrained the instrument in the trajectory chosen in the study by Zinelis et al (37). and the different bending properties between the coronal and apical portion of the same file may determine a different trajectory between the pins if the file is not constrained precisely. Zinelis et al (38) tested instruments for fatigue in a similar device that allowed the instruments to be tested in a rotating bending position inside a guide that had the form of an artificial root canal engraved on the surface of only two hard-steel pins to constrain the instruments in the trajectory chosen (Fig. 6). The instruments were rotated inside the artificial canal with a 5-mm bending arc of curvature at a constant speed of 200 rpm without specifying the angle and radius of the curvature. Other studies (3, 26, 39 50) have used an artificial canal that consisted of a grooved concave tempered steel block and a convextempered steel cylinder, which when held and fixed together, guaranteed the curve of the instruments (Fig. 7A). The concave radius block incorporated a notched V-form for guiding the instruments and was constructed with different radii and angles of curvature (Fig. 7B). The concave grooved block of the assembly used in these studies represents the portion of the artificial canal that guides the instrument. This trajectory is given to the external surface of the instrument and not to its axis, resulting in a different actual trajectory for instruments with different taper. Moreover, there has been no mention of the fit of the instrument in the block-and-rod assembly. The convex cylinder contacts the tapered instrument in a nonpredictable way, so that the instrument may fit loosely and radius and angle of curvature may not be repeatable. Furthermore, it is difficult to control exactly the depth of the instrument in these devices so that the point of the instrument, which lies in the center of the curvature, may vary consistently. Larsen et al (51) tested instruments in a metal block with a simulated canal with a 60 angle of curvature and a 3-mm radius of curvature to the center of the 1.5-mm wide canal. The block was made from 300 series stainless steel. A swivelling acrylic top face cover allowed for JOE Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1473

6 Figure 9. Artificial root canals constructed with different type of curvatures permit to test instruments in different conditions to obtain a complete picture of the fatigue behavior of an instrument. Figure 8. The experimental model used by Larsen et al (51). (Reprinted with permission [51]). visualization of the files rotating in the canal and the removal of broken instruments between tests. A marker of permanent red ink was placed at 19 mm on the metal block to standardize instrument placement. From the figure published in the article (Fig. 8), it is clear that the cylindrical canal grooved in the metal block was not able to constraint the instrument in a precise trajectory and that to insert the instruments at the same depth may be difficult. Other studies (52 55) tried to overcome this problem by manufacturing an artificial canal that reproduced instrument size and taper, thus providing the instrument with a suitable trajectory. To ensure the accuracy of the size of each canal, a copper duplicate of each instrument was milled increasing the original size of the instrument by 0.1 mm using a computer numeric control machining bench (Bridgeport VMC 760XP3; Hardinge Machine Tools Ltd, Leicester, UK). The copper duplicates were constructed according to the curvature parameters that were chosen for the study. With these negative molds, the artificial canals were made using a die-sinking electrical-discharge machining process (Agietron Hyperspark 3; AGIE Sa, Losone, Switzerland) in a stainlesssteel block. The blocks were hardened through annealing. The depth of each artificial canal was machined to the maximum diameter of the instrument +0.1 mm, allowing the instrument to rotate freely inside the artificial canal. A simulated root canal with a precise angle and radius of curvature can be constructed on the dimensions of every instrument to be tested. They are milled in stainless-steel blocks with a precision milling machine, and then the block is tempered. All the parameters (ie, angle and radius of the curvature and position of the center of the curvature) may be chosen by the researcher before to construction of the artificial canal so that every type of trajectory may be used to test the instruments (Fig. 9). In fact, it should be of somewhat importance to test instruments at different curvatures to obtain a complete picture of the fatigue behavior for an instrument. The artificial canal was milled on a stainless-steel block that was connected to a main frame to which a mobile plastic support for the handpiece was connected. The dental handpiece was mounted on a mobile device that allowed for precise and simple placement of each instrument inside the artificial canal, ensuring three-dimensional alignment and positioning of the instruments to the same depth (Fig. 10). The artificial canal was covered with a tempered glass to prevent the instruments from slipping out and to allow for observation of the rotating instrument. Fracture was easily detectable because the instruments were visible through the glass window. To reduce the friction of the file as it contacted the artificial canal walls, a special high-flow synthetic oil designed for lubrication of mechanical parts (Super Oil; Singer Co Ltd, Elizabethport, NJ) was applied. Because environmental conditions have been shown to significantly affect the fatigue behavior of NiTi rotary instruments and fatigue testing has been suggested to be in a service-like environment (36), sodium hypochlorite may be used as lubricant in such tests. The present device sought to overcome the limitations of other experimental designs in terms of the models used for testing. Each artificial canal was specifically designed for each instrument in terms of size and taper, giving it a precise trajectory. If the artificial canal is not identical (in shape and size) to the instrument, its trajectory will not respond to the established parameters, thus having a reduced curvature during the test. This can influence the results of cyclic fatigue tests. A study submitted for publication has shown that if the artificial canal does not sufficiently restrict the instrument shaft, it would tend to spring back into its original straight shape, aligning into a trajectory of greater radius and reduced angle. The study reported that the less the adaptation of the instrument to the artificial canal is, the more the variation in the parameters of the curvature investigated is (Fig. 11). An artificial canal manufactured as described previously to reproduce instrument size and taper seems to guarantee that different NiTi rotary instruments may follow a precise and repeatable trajectory in terms of radius and angle of curvature (56) (Fig. 12). This may ensure comparable results between different instruments tested for fatigue life. The results of another study submitted for publication have shown that even small variations in the parameters of the curvature followed by an instrument subjected to flexural fatigue resulted in a significant influence on the results of the cyclic fatigue test. Figure 10. A dental handpiece mounted on the mobile block with the instrument inserted in an artificial root canal Plotino et al. JOE Volume 35, Number 11, November 2009

7 Figure 11. An example of three different artificial canals showing the different trajectory that the instrument follows. Discussion To date, several devices and methods have been used to investigate in vitro the 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. There has been no mention of the fit of the instrument in the tube or groove. Because 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). Furthermore, each different instrument that fits loosely inside the device may follow a more or less severe curvature depending on the stiffness of the file. That would explain the wide variation in the reported fatigue life. The large scatter generally encountered in various forms of fatigue tests would add to the variation in the result. These devices permit the instruments to rotate until fracture using different curvature. The resistance of rotary instruments to cyclic fatigue decreases with increasing instrument diameters (5, 6), and it is specifically related to the metal mass of the instrument in the point of maximum stress (53). Moreover, the increased severity of the angle and radius of the curve, around which the instrument rotates, decreases instrument lifespan in vitro and clinically (5, 6, 53). If instruments of the same dimensions follow different trajectories in the test apparatus, a direct comparison between instruments of different brands may be difficult to make, and the results obtained may be unreliable and inconsistent. It is evident from this review of the literature that an international standard is needed to validate a device for cyclic fatigue tests of NiTi rotary endodontic instruments. Ideally, such a device should allow testing of all instruments with a precise trajectory in terms of radius and angle of the curvature and point of the center of the curvature, allowing comparison of different instruments in different canals. This is a way of testing cyclic fatigue that could simulate more precisely the clinical accumulation of flexural fatigue. 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. Cyclic fatigue tests investigate the in vitro resistance to fracture caused by the accumulation of metal fatigue, which is determined by the tension/compression cycles at the point of maximum flexure. The clinical relevance of the results of such tests is difficult to assess because this condition differs from intracanal instrumentation in which the fracture occurs because of several factors that act together at the same time, including torsional stress. This represents a pure mechanical test to extrapolate only one characteristic of the instruments (bending failure) in the same way as torsional resistance is defined and measured by an international standard. Because NiTi rotary instruments are widely used, the need for a standardization of testing of their properties including cyclic fatigue Figure 12. An example of several instruments that follows the same trajectory in the artificial canal constructed to test cyclic fatigue. From the left: (A) ProTaper F2, (B) ProFile 25/.06, (C) Race 25/.06, (D) EndoSequence 25/.06, (E) Twisted File 25/.06, (F) V Taper 25/.08, (G) Mtwo 25/.06. JOE Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1475

8 is required to ensure the uniformity of methodology and comparable results for a safer, efficient clinical use. This is not easy and explains why after more than 15 years since the introduction of nickel-titanium rotary instruments, no specification or standard have yet been published. Acknowledgments We are very grateful to Maria Carola Rossini and Giulia Rossini for the drawings of Figures 3, 5, and 6. References 1. Sattapan B, Nervo GJ, Palamara JEA, et al. Defects in rotary nickel-titanium files after clinical use. J Endod 2000;26: Serene TP, Adams JD, Saxena A. Nickel-titanium instruments. Applications in endodontics. St. Louis, MO: Ishiyaku EuroAmerica, Inc.; Ullmann CJ, Peters OA. Effect of cyclic fatigue on static fracture loads in ProTaper nickel-titanium rotary instruments. J Endod 2005;31: Martín B, Zelada G, Varela P, et al. Factors influencing the fracture of nickeltitanium rotary instruments. Int Endod J 2003;36: Peters OA. Current challenges and concepts in the preparation of root canal systems: a review. J Endod 2004;30: Parashos P, Gordon I, Messer HH. Factors influencing defects of rotary nickel titanium endodontic instruments after clinical use. J Endod 2004;30: Peng B, Shen Y, Cheung GSP, et al. Defects in ProTaper S1 instruments after clinical use: longitudinal examination. Int Endod J 2005;38: Cheung GS, Peng B, Bian Z, et al. Defects in ProTaper S1 instruments after clinical use: fractographic examination. Int Endod J 2005;38: Dauskardt RH, Duerig TW, Ritchie RO. Effect of in situ phase transformation on fatigue-crack propagation in Ti-Ni shape memory alloy. In: Proceedings of Materials Research Society International Meeting on Advanced Materials, Vol. 9. Pittsburgh, PA: Materials Research Society; 1989: Alapati SB, Brantley WA, Svec TA, et al. SEM observations of nickel-titanium rotary endodontic instruments that fractured during clinical Use. J Endod 2005;31: ANSI/ADA Specification N Root canal files and reamers, type K for hand use. Chicago, IL: American Dental Association. 12. ISO :2008. Dentistry Root canal instruments Part 1: General requirements and test methods. 13. Dederich DN, Zakariasen KL. The effects of cyclical axial motion on rotary endodontic instrument fatigue. Oral Surg Oral Med Oral Pathol 1986;61: Pruett JP, Clement DJ, Carnes DL. Cyclic fatigue testing of nickel titanium endodontic instruments. J Endod 1997;23: Schneider SW. A comparison of canal preparations in straight and curved root canals. Oral Surg Oral Med Oral Pathol 1971;32: Anderson ME, Price JWH, Parashos P. Fracture resistance of electropolished rotary nickel-titanium endodontic instruments. J Endod 2007;33: Barbosa FOG, Gomes JA, de Araujo MCP. Influence of previous angular deformation on flexural fatigue resistance of K3 nickel-titanium rotary instruments. J Endod 2007;33: Mize SB, Clement DJ, Pruett JP, et al. 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Measurement of the trajectory of different NiTi rotary instruments in an artificial canal specifically designed for cyclic fatigue tests. Oral Surg Oral Med Oral Pathol Oral Radiol Endod 2009 Sep;108(3):e Plotino et al. JOE Volume 35, Number 11, November 2009