JOURNAL OF ENDODONTJCS
`Copyright © 2002 by The American Association of Endodontists
`
`Printed in U.S.A.
`VoL. 28, No. 11, NovEMBER 2002
`
`Fatigue Resistance of Engine-Driven Rotary Nickel(cid:173)
`Titanium Endodontic Instruments
`
`Marta Chaves Craveiro de Melo, MSc, Maria Guiomar de Azevedo Bahia, MSc, and
`Vicente Tadeu Lopes Buono, Dr Eng
`
`A comparative study of the fatigue resistance of
`engine-driven nickel-titanium endodontic instru(cid:173)
`ments was performed, aiming to access the influ(cid:173)
`ence of the cutting flute design and of the size of
`the files that reach the working length in curved
`canal shaping. Geometrical conditions similar to
`those found in practice were used. Series 29 #5
`Profile, together with #6 and #8 Quantec instru(cid:173)
`ments, were tested in artificial canals with a 45-
`degree angle of curvature and 5-mm radius of cur(cid:173)
`It was observed that the size of the
`vature.
`instrument, which determines the maximum strain
`amplitude during cyclic deformation, is the most
`important factor controlling fatigue resistance. The
`effect of heat sterilization on the fatigue resistance
`of the instruments was also examined. The results
`obtained indicate that the application of five ster(cid:173)
`ilization procedures in dry heat increases the av(cid:173)
`erage number of cycles to failure of unused instru(cid:173)
`ments by approximately 70%.
`
`The concepts of cleaning and shaping of the root canal system
`(RCS) established by Schilder (1) make up, together with tridi(cid:173)
`mensional obturation, the basis of endodontic therapy. For straight
`root canals, the stages of cleaning and shaping are relatively simple
`procedures, but the preparation of curved root canals may lead to
`ledging, perforation, or even instrument separation. Generally,
`these procedure failures are caused by the trend of the endodontic
`instrument to return to its original straight form when inserted into
`a curved root canal, due to the rigidity of the materials used for its
`manufacturing. Previously the preparation of root canals was car(cid:173)
`ried out with carbon steel reamers, which in addition to their low
`flexibility had low resistance to corrosion. The corrosion problems
`were solved with the use of stainless steel instruments (2), but the
`elasticity modulus of these materials is still relatively high, and the
`occurrence of failures during instrumentation of curved root canals
`continued to depend exclusively on the expertise of the endodontist
`(3). In 1988, Walia et al. (4) introduced a new material for the
`manufacturing of endodontic instruments: the nickel-titanium orth(cid:173)
`odontic wire. The nickel-titanium alloys, with an approximately
`
`equiatomic composition, have in addition to high resistance to
`corrosion and excellent biocompatibility some special features: the
`shape memory effect (SME) and superelasticity (SE). The SME
`occurs in specific conditions in which the metal is defonned at a
`certain temperature in an apparently permanent way, but it recov(cid:173)
`ers its original form when moderately heated. The SE is a particular
`case of the SME in which the shape recovery temperature is lower
`than the temperature of defonnation. This means that shape recov(cid:173)
`ery happens immediately after defonnation interruption and load
`withdrawal. The tenn superelasticity is related to the fact that the
`recoverable strain obtained is much higher than that which may
`develop in the elastic strain regimen of metals. Both the SME and
`the SE are associated with the occurrence of a phase transformation
`in the solid state that has special characteristics: the martensitic
`transformation, which may be induced by the application of stress
`and reversed by moderate heating of the material (5).
`The manufacturing of endodontic instruments using superelastic
`nickel-titanium (Ni-Ti) alloys has provided an important develop(cid:173)
`ment in the techniques of cleaning and shaping of the RCS. The
`high flexibility associated with the SE allows the use of this
`material in engine-driven rotary instruments for the preparation of
`curved root canals. The main advantage of this new technique is
`the increased efficacy for endodontic treatment. However, despite
`the evident advantages of the new technique, Ni-Ti rotary instru(cid:173)
`ments may undergo failure by fatigue when used in curved canals
`due to the tension/compression cycles to which they are subjected
`when flexed in the region of maximum curvature of the canals (6).
`The fatigue failure occurs unexpectedly, without any sign of pre(cid:173)
`vious pennanent defonnation; and visual inspection, therefore, is
`not an adequate method for evaluation of the useful life of these
`instruments. In fact, there are no test protocols that establish
`minimum standards regarding their fatigue resistance, which could
`guide endodontists on their use in clinical practice. Additional
`studies are necessary to improve the understanding of the criteria
`that may be used to evaluate the reuse conditions of Ni-Ti engine(cid:173)
`driven rotary instruments. Moreover, sterilization of endodontic
`instruments either for new files or for their reuse involves repeated
`exposure to heating/cooling cycles. The effect of these thermal
`cycles on the properties of Ni-Ti instruments has been investigated
`by several authors (7-10), but results are contradictory.
`The objective of this study was to evaluate the influence on
`fatigue resistance of the design of the cutting flutes and the size of
`Ni-Ti engine-driven rotary endodontic instruments by using geo(cid:173)
`metrical conditions similar to those found in curved canal shaping.
`
`765
`
`GOLD STANDARD EXHIBIT 2045
`US ENDODONTICS v. GOLD STANDARD
`CASE PGR2015-00019
`
`

`

`766
`
`Chaves Craveiro de Melo et al.
`
`Journal of Endodontics
`
`TABLE 1. NCF of nonsterilized instruments
`
`Instrument
`
`#5 ProFile
`#6 Quantec
`#8 Quantec
`
`Minimum
`
`2.060
`2.158
`1.104
`
`Median
`
`NCF Descriptive Measurements
`P25
`P75
`2.251
`2.390
`2.238
`2.434
`1.168
`1.246
`
`2.321
`2.294
`1.194
`
`Maximum
`
`2.528
`2.561
`1.300
`
`p
`
`Conclusion
`
`⬍0.001
`
`P5⫽Q6⬎Q8
`
`The probability significance (p) refers to the Kruskal-Wallis test (11).
`P5 ⫽ #5 ProFile; Q6 ⫽ #6 Quantec; Q8 ⫽ #8 Quantec.
`
`Three types of files from two different manufacturers were se-
`lected, having as criteria the geometrical characteristics mentioned
`above, and the fact that in the methodologies proposed by the
`manufacturers these three instruments are the first to reach the total
`working length, at which time they are subjected to cyclic defor-
`mation during the preparation of curved canals. The effect of
`multiple sterilizations by dry heat on fatigue resistance of these
`instruments, in the same simulated working conditions, was also
`analyzed.
`
`MATERIALS AND METHODS
`
`The instruments evaluated were #5 ProFile Series 29 0.04 taper
`(Tulsa Dental Products, Tulsa, OK), #6 Quantec 0.04 taper and #8
`Quantec .06 taper files (Tycon Inc., Chattanooga, TN), whose
`differences in the cutting flutes design are well known. The #5
`ProFile 0.04 taper and #8 Quantec 0.06 taper files were chosen
`because they have different sizes and constitute the instruments of
`greatest size proposed by the respective techniques to be used to
`working length. Because these two files have different tapers, the
`#6 Quantec 0.04 taper was also selected so that a comparison could
`be made among files of different cutting flutes design and same
`taper. All instruments used in this work were purchased from the
`usual suppliers, withdrawn from sealed boxes, randomly selected,
`and then sequentially numbered for reference. The instruments
`used to evaluate the sterilization effect were selected in the same
`way, packed in metal boxes, and placed into a sterilizer previously
`heated at 170°C, where they were kept for 1 h. After the sterilizer
`was turned off, the instruments remained inside it for approxi-
`mately 1 h until they reached room temperature. This procedure
`was applied once for #5 ProFile and five times for #5 ProFile, #6
`and #8 Quantec files.
`The files were subjected to fatigue tests (10 files for each condition)
`using a fixation system for the driving equipment and an artificial
`canal similar to that used by Pruett et al. (6), with the instrument
`rotating freely inside the artificial canal. The handpiece was fixed on
`a steel stand with the help of two brass brackets, built in such way as
`to house the two driving systems used, Tulsa for ProFile files and
`Tycon for Quantec files. A steel bracket was made for fixation and
`alignment of the artificial canal. To construct the artificial canals,
`stainless steel needles, with 1.6 mm of external diameter and 40-mm
`long, were bent with the help of a gauge to provide the desired radius
`and angle of curvature. After being bent, the needles were cut to keep
`straight sections of approximately 4.0 mm before and 1.5 mm after the
`curvature. Measurements of the radius and the angle of curvature of
`the artificial canals, according to the method proposed by Pruett et al.
`(6), as well as the length of the straight sections were made with a
`profile projector (Mitutoyo, Japan).
`To carry out the fatigue tests, the instruments were positioned in
`such a way that they kept approximately 1 mm of their final length
`outside the artificial canal, and therefore were visible to the oper-
`
`ator. In this way, the maximum curvature region was located at
`approximately 4.5 mm from the tip of the files. The testing time
`was registered with a digital chronometer, started at the beginning
`of the test and stopped at the moment the operator detected instru-
`ment separation by observing the displacement of the tip protrud-
`ing from the artificial canal. The rotation speeds were selected
`considering, within the range recommended by the manufacturers,
`those available for the driving systems nearest to each other: 315
`rpm for #5 ProFile files and 340 rpm for #6 and #8 Quantec files.
`The friction of the files with the canal walls was minimized with
`the use of RC-Prep (Premier, Norristown, PA) as lubricant.
`The geometry of the artificial canal was chosen based on pre-
`liminary tests, in which the use of a canal having a 5 mm-curvature
`radius and 45-degree curvature angle gave the best results in terms
`of repeatability. The parameter used in comparing the behavior of
`file fatigue was the number of cycles to failure (NCF) determined
`as the product of the rotation speed used and the test time duration.
`The instruments fractured in the fatigue tests were inspected
`under a stereomicroscope (Wild M8, Germany) in magnifications
`up to ⫻50 using a fixation device that allowed them to rotate
`around their main axis, either in a horizontal position or inclined at
`60 degrees. The fracture surfaces were analyzed by scanning
`electron microscopy (SEM) (Jeol JSM 5410, Japan) with the pur-
`pose of determining the characteristics of the fracture process for
`the test conditions. Three fractured instruments from each group,
`randomly selected, were prepared for Vickers microhardness tests,
`involving the following stages: cold mounting, fine grinding, and
`diamond polishing. The specimens were then submitted to micro-
`hardness testing (10 measurements per group of 3 instruments) in
`a Durimet 2 (Leitz, Germany) apparatus, using a load of 1.961 N.
`
`RESULTS
`
`The results related to fatigue resistance of the investigated
`instruments, expressed as the NCF are shown in Table 1. The
`influence of the design of the cutting flutes and the instrument size
`was evaluated by means of the Kruskal-Wallis test (11), applied to
`the NCF values determined in the fatigue testing of nonsterilized
`files. It can be observed that the mean number of cycles to failure
`of #5 ProFile files does not significantly differ from #6 Quantec
`files, with both presenting considerably higher values (almost
`twice) than those observed for #8 Quantec files.
`The effect of sterilization, applied once to #5 ProFile instru-
`ments and five times to #5 Profile and #6 and #8 Quantec instru-
`ments, was analyzed according to the same statistical technique.
`The results are shown in Table 2. Figure 1 summarizes the results
`obtained for nonsterilized and sterilized files, in terms of the
`average NCF values. It shows that there was no appreciable vari-
`ation in the mean number of cycles to failure for #5 ProFile
`instruments sterilized in only one procedure. However, when these
`instruments were subjected to five sterilization procedures, there
`
`

`

`Vol. 28, No. 11, November 2002
`
`Fatigue of Ni-Ti Instruments
`
`767
`
`TABLE 2. NCF of sterilized and nonsterilized instruments
`
`Instrument
`
`Sterilization
`
`#5 ProFile
`
`#6 Quantec
`
`#8 Quantec
`
`None
`1 time
`5 times
`None
`5 times
`None
`5 times
`
`Minimum
`
`2.060
`2.225
`3.612
`2.158
`3.693
`1.104
`1.966
`
`The probability significance (p) refers to the Kruskal-Wallis test (11).
`S0 ⫽ nonsterilized; S1 ⫽ sterilized 1 time; S5 ⫽ sterilized 5 times.
`
`Median
`
`NCF descriptive measurements
`P25
`P75
`2.251
`2.390
`2.293
`2.465
`3.818
`4.120
`2.238
`2.434
`3.784
`4.090
`1.168
`1.246
`2.030
`2.205
`
`2.321
`2.379
`3.937
`2.294
`3.856
`1.196
`2.101
`
`Maximum
`
`2.528
`2.671
`4.227
`2.561
`4.139
`1.300
`2.302
`
`p
`
`Conclusion
`
`⬍0.001
`
`S0⫽S1⬍S5
`
`⬍0.001
`
`⬍0.001
`
`S0⬍S5
`
`S0⬍S5
`
`FIG. 1. Average values of the number of cycles to failure of nonster-
`ilized and sterilized instruments.
`
`was an increase of approximately 70% of this parameter. The same
`was true for #6 and #8 Quantec files, which had increases of 68%
`and 76%, respectively, in the mean number of cycles to failure
`after five sterilization procedures.
`The examination of the fatigue-tested instruments showed that
`all files fractured without geometric distortions that could be
`associated with fracture by torsional overloading. The fracture
`surfaces of the instruments of different design and size were
`similar. In fact, no significant differences were found between the
`nonsterilized instruments and those subjected to one or five ster-
`ilization procedures. The appearance of the fracture surfaces, as-
`sessed by SEM, indicates that the breakage of the analyzed instru-
`ments was due to fatigue. The main characteristics were presence
`of small areas of nucleation and slow crack propagation, which are
`called smooth regions, peripherally to the cross section, and large
`central fibrous areas, associated to final ductile breakage (Fig. 2).
`Some details of the fracture surfaces, like the nucleation of several
`cracks and the presence of fatigue striations are shown in Figs. 3
`and 4.
`The statistical analysis of the results obtained in the Vickers
`microhardness measurements are shown in Table 3. It can be
`observed that according to the Mann-Whitney U test (11) there is
`little variation in the mean values of this parameter for the files due
`to their design and size and an increase of approximately 8% in the
`mean values after five sterilization procedures.
`
`DISCUSSION
`
`The fatigue behavior of nonsterilized #5 ProFile and #6 and #8
`Quantec files observed in this study is in accordance with the
`
`FIG. 2. Fracture surface of a #8 Quantec file, showing smooth re-
`gions of nucleation and slow crack propagation at the periphery and
`a large central fibrous area (original magnification ⫻150).
`
`FIG. 3. Region of nucleation and slow crack propagation in the
`fractured surface of a #8 Quantec file showing multiple cracks
`(original magnification ⫻3500).
`
`pattern reported by Pruett et al. (6), Serene et al. (7), and Haikel et
`al. (12), in which the largest instruments were more susceptible to
`fatigue failure. The results also show that the design of the cutting
`flutes does not influence the fatigue resistance of instruments of the
`same size, #5 ProFile and #6 Quantec files, which have the same
`behavior. On the other hand, the instrument size has a strong
`
`

`

`768
`
`Chaves Craveiro de Melo et al.
`
`Journal of Endodontics
`
`nent deformation, i.e. the material yield point, which determines
`the ease of the nucleation of fatigue cracks. Therefore, there is no
`way to correlate the results mentioned above with the fatigue
`resistance measurements carried out in this work.
`The fact that #5 ProFile and #6 Quantec files fracture after a
`number of cycles significantly higher than that of #8 Quantec files
`may be related to two factors: the first, and the most important, is
`that the tensile stress on the external surface of the instrument
`increases in the maximum curvature region, for the same radius of
`curvature, as the size of the instrument increases. Another factor to
`be considered, specific to the methodology used, is the greater
`relaxation of the smaller instruments inside the artificial canal,
`resulting in their less effective curvature during the tests. These
`two factors make the larger instruments subjected to greater tensile
`stress per cycle, which results in their breakage in a fewer number
`of cycles.
`The fatigue behavior of the files subjected to five sterilization
`procedures, which points to a substantial improvement of fatigue
`resistance, taken together with the results of the Vickers micro-
`hardness tests presented in Table 3, supports the suggestion by
`Serene et al. (7) that sterilization increases Ni-Ti rotary instruments
`fatigue life through the increase of hardness and torsional resis-
`tance of the material. This correlation is not clearly established in
`the works of other authors (8, 9, 15). However, considering that the
`difficulty of nucleation and propagation of fatigue cracks is di-
`rectly associated with the mechanical strength of the material, the
`increase of instrument fatigue resistance found in this work after
`five sterilization procedures may be a direct consequence of the
`observed increase in hardness.
`Because Ni-Ti engine-driven instruments need activation to a
`predetermined speed before their insertion into the root canal, the
`test model used, which permits the instrument to rotate freely
`without its tip being fixed, is more appropriate than the ADA
`specification no. 28. However, the test used in this work departs
`from clinical practice by at least two aspects, which deserve
`consideration. First, in the experiments performed, the files rotated
`statically in the artificial canal, without incorporating the in and out
`movement proposed for RCS preparation with these instruments.
`In other words, maximum deformation always occurred in the
`same region of the instrument, at the segment located at the
`maximum curvature of the artificial canal. The in and out move-
`ment used in clinical practice makes the segment of the file
`subjected to maximum fatigue vary continuously, which may in-
`crease the useful life of the instruments relative to the results
`presented in this work. Another important aspect to consider is the
`use of sodium hypochlorite as an irrigant during RCS preparation,
`because some studies have shown that Ni-Ti alloys exhibit a
`tendency to corrosion under simulated clinical conditions in the
`
`FIG. 4. Fatigue striations in the smooth region of the fractured
`surface of a #8 Quantec file (original magnification ⫻7500).
`
`influence on its performance, because the average NCF of the files
`of smaller size, #5 ProFile and #6 Quantec, is almost twice that of
`#8 Quantec files. This leads us to the manufacturer’s recommen-
`dation of using #8 Quantec file up to the working length. When
`opting for the use of the recommended technique, the difference of
`behavior between #6 and #8 Quantec in terms of NCF should be
`taken into consideration. If the two files were to be used at the
`same length and #8 Quantec file had half of the NCF compared
`with #6 file, then the largest size instrument should not be used the
`same number of times as the smaller size but rather be discarded
`earlier to prevent fatigue failure.
`The ADA specification no. 28 evaluates instrument resistance
`through the determination of the values for maximum torque and
`maximum angular deflection to fracture. Maximum angular de-
`flection values stipulated by this specification decrease with the
`increase of instrument gauge. Several authors have compared frac-
`ture resistance of Ni-Ti instruments using as a parameter the
`maximum angular deflection measurements stipulated by this
`specification. The reported results are, however, controversial;
`some works show an increase of the maximum angular deflection
`with the increase of the size of the instruments (13, 14), whereas
`other studies had both an increase and decrease of this measure-
`ment, reaching no conclusive results (15–18). It is important to
`observe that the maximum torque and the maximum angular de-
`flection do not reflect adequately the mechanical behavior of the
`material, because they are parameters dependent on the instrument
`dimensions. On the other hand, the fatigue resistance is under the
`direct influence of the stress required for the beginning of perma-
`
`TABLE 3. Vickers microhardness (MHV) of the analyzed instruments
`
`Instrument
`
`Sterilization
`
`Minimum
`
`#5 ProFile
`
`#6 Quantec
`
`#8 Quantec
`
`None
`1 time
`5 times
`None
`5 times
`None
`5 times
`
`319
`330
`353
`323
`334
`317
`343
`
`The probability significance (p) refers to the Mann-Whitney U test (11).
`S0 ⫽ nonsterilized; S1 ⫽ sterilized 1 time; S5 ⫽ sterilized 5 times.
`
`Median
`
`MHV descriptive measurements
`P25
`P75
`341
`352
`332
`347
`366
`377
`329
`357
`354
`368
`330
`352
`358
`380
`
`348
`338
`367
`340
`362
`342
`371
`
`Maximum
`
`p
`
`Conclusion
`
`360
`352
`391
`362
`371
`371
`388
`
`⬍0.001
`
`S0⫽S1⬍S5
`
`0.009
`
`0.002
`
`S0⬍S5
`
`S0⬍S5
`
`

`

`Vol. 28, No. 11, November 2002
`
`Fatigue of Ni-Ti Instruments
`
`769
`
`presence of this compound (19, 20). Corrosion may cause the loss
`of material mass, favoring the fracture of endodontic instruments.
`Therefore, the number of cycles to failure found in this study, in
`which the instruments did not come in contact with sodium hypo-
`chlorite, may be reduced in clinical practice due to the corrosive
`effect of this compound.
`
`This work was partially supported by Fundacao de Amparo a Pesquisa do
`Estado de Minas Gerais, FAPEMIG, Belo Horizonte, MG, Brazil.
`
`Drs. Chaves Craveiro de Melo and Guiomar de Azevedo Bahia are affiliated
`with the Faculty of Dentistry, and Dr. Lopes Buono is affiliated with the
`Department of Metallurgical and Materials Engineering, Universidade Federal
`de Minas Gerais, Belo Horizonte, MG, Brazil. Address requests for reprints to
`Prof. Dr. Vicente T.L. Buono, Departamento de Engenharia Metalurgica e de
`Materiais, Escola de Engenharia da UFMG, Rua Espirito Santo, 35/206,
`30160-030 Belo Horizonte, MG, Brazil
`
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`Copyright© by the American Association of Endodontists. Unauthorized reproduction of this article is prohibited.
`
`