endo file separation discussion

Fracture

Friday, January 27, 12

Torsional


Cyclic


JOE 2009


An ideal model would involve instrumentation of curved canals innatural teeth. However, in such tests, a tooth can only be used once andthe shape of the root canal will change during instrumentation, makingit impossible to standardize experimental conditions. As a result, severaldevices and methods have been used to investigate in vitro cyclic fatiguefracture resistance of NiTi rotary endodontic instruments.

The aim of this review of the literature was to summarize andanalyze all the devices that have been used in endodontic literaturefor cyclic fatigue testing, explaining how differences in the devicesmay affect the fatigue behavior of rotary instruments and, consequently,the outcome of these studies.

Cyclic Fatigue Testing DevicesThe rotational bending is the test used in endodontic literature for

fatigue testing of NiTi rotary instruments. The devices used to test cyclicfatigue resistance of NiTi rotary instruments permit instruments to rotateuntil fracture using different geometric curvatures.

In 1986, Dederich and Zakariasen (13) underlined that a potentialproblem with the use of 360! rotary engine files in curved canals wasmetal fatigue and subsequent breakage. This study analyzed the effectof cyclical axial motion on engine-driven K-type stainless steel instru-ment failure. A thick-walled Pyrex capillary with a 1-mm diameterlumen was heated and bent to a curvature representative of a moderatelycurved root canal, without precise curvature parameters. The angle andradius of a circumference has been subsequently established by Pruettet al (14) as the benchmark parameters widely accepted to define thecharacteristics of a curvature. This study defined the parameters ofcanal curvature in a more exact manner then generally used inendodontic research. Canal curvature was historically defined usingthe method introduced by Schneider in 1971 (15). This methodused only a single parameter to define an angle in degrees. To determinethe degree of root curvature, Schneider drew a line parallel to the longaxis of the canal. A second line was drawn from the apical foramen tointersect with the first line at the point where the canal began to leave thelong axis of the canal. The acute angle formed was defined as the degreeof root curvature (Fig. 1). The shape of any root canal curvature wasmore accurately described by Pruett et al (14) using two parameters:

angle of curvature and radius of curvature (Fig. 2). To determine theseparameters, a straight line is drawn along the long axis of the coronalstraight portion of the canal. A second line is drawn along the longaxis of the apical straight portion of the canal. There is a point oneach of these lines at which the canal deviates to begin or end the canalcurvature. The curved portion of the canal is represented by a circlewith tangents at these two points. The angle of curvature is the numberof degrees on the arc of the circle between these two points. Angle ofcurvature can also be defined by the angle formed by perpendicularlines drawn from the points of deviation that intersect at the center ofthe circle. The length of these lines is the radius of the circle and definesthe radius of the canal curvature defined in millimeters. This parameterrepresents how abruptly a specific angle of curvature occurs as thecanal deviates from a straight line. The smaller is the radius of curvature,the more abrupt is the canal deviation. These two parameters are inde-pendent of each other.

Several studies have used artificial canals that were constructed bybending glass (16, 17) or metal (14, 18–25) cylindrical tubes withdifferent inner diameters and point of maximum curvature and usingdifferent radii and angles of curvature (Fig. 3). A glass tube of internaldiameter 1.2 mm was used by Anderson et al (16). It was curved byheating over a flame and curving over a metal cylinder, which gavea radius of curvature of 5 mm. They have used 45! and 90! angles ofcurvature, and the point of maximum curvature was 5 mm from thetip of the instrument. A small glass tube with an angle of 45! anda 5-mm curvature radius was used by Barbosa et (17) without speci-fying the inner diameter of the tube. The files were submitted to curva-ture between the third and the seventh millimeter from the tip.

Artificial canals used by Pruett et al (14) and Mize et al (18) werefabricated from 18-G, stainless steel needles having an internal diameterof 0.83 mm. A 2-mm and 5-mm radius of curvature measured to theinner aspect of the curve of the guide tubes. They have used 30!,45!, and 90! angles of curvature, and the point of maximum curvaturewas 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 radiusof curvature and where the point of maximum curvature was located. Toconstruct the artificial canals, Melo et al (21) used stainless steel nee-dles, with an external diameter of 1.6 mm and 40-mm long that werebent with the help of a gauge to provide a 5-mm curvature radiusand 45! curvature angle. The maximum curvature region was locatedat approximately 4.5 mm from the tip of the files. A 2-mm inner-diam-eter stainless steel tube was also used by Yao et al (22). They used anangle of 60! and a radius of curvature of 5 mm. The author used

Figure 1. The degree of root canal curvature obtained using the methoddescribed by Schneider for determining canal curvature using only one param-eter to define the angle. A has an angle of 43 and B has an agle of 52, eventhough both angles measured according to the method of Pruett et al. equaled60 degrees. Location of the curve along the canal will also change themeasured angle. (Reprinted with permission [14]).

Figure 2. The method described by Pruett et al (14) for describing canalgeometry using two parameters: radius of curvature and angle of curvature.(Reprinted with permission [14]).

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1470 Plotino et al. JOE — Volume 35, Number 11, November 2009


a dynamic model without specifying at what distance from the tip of theinstrument the point of maximum curvature was located. Stainless steeltubes with an inner diameter of 1.04 mm and a radius of curvature of 6mm were used by Lopes et al (23). The authors used two different arcslengths because in the curved canals with the same radius, it is possiblethat there are arcs (curved segments) with different lengths representedby angles with different degrees. They used an arc of 9.4 mm corre-sponding to an angle of 90! and an arc of 14.1 mm, correspondingto an angle of 135! when using a 6-mm radius of curvature. Theyused a straight coronal part of 10.6 mm and 5.9 mm, respectively, sothat the total length of the curved and straight parts was 20 mm. Buiet al (24) used artificial canals that were constructed by bendinga 16-mm gauge stainless steel Monojet blunt needle to a 5-mm radiuson curvature and angles of curvature of 30!, 45!, and 60!. The point ofmaximum curvature was 7 mm from the tip of the instrument.

Similar to Pruett et al (14), Kramkowski et al (25) constructedartificial canals by bending stainless steel tubing. Two canals werebent 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 canalwas 7 mm from the tip of the file. The artificial canals were insertedinto predrilled acrylic blocks for mounting in a fixed jig on the platformof the cyclic fatigue instrument. The jig was placed at the opposite end ofthe rotary handpiece fixed at a distance so that the files protrudedapproximately 2 mm out of the end of the tube. Instruments wererotated in the artificial canal with a consistent insertion and withdrawalof 8 mm. Silicone spray (CRC Industries Inc, Warminster, PA) as a lubri-cant 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 trajec-tory of greater radius and reduced angle, as it has been speculated inprevious articles that have used this type of methodology (19–21,26). Because of the inner diameter of the tubes (glass and metal) isgreater than that of the instruments, an instrument rotated in the tubewill follow a trajectory that is not predictable and without the parametersof radius and angle of curvature and point of maximum curvature thatwere 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 trajectoryin tubes that do not sufficiently constrain the shafts of the instruments,especially the smaller ones.

If instruments of the same dimensions follow different trajectoriesin the test apparatus, a direct comparison between instruments ofdifferent brands may be difficult to establish and the results obtainedmay be unreliable and not consistent. Furthermore, it is unclear whatthe predictability of these parameters of radius and angle of curvatureand point of maximum curvature obtained by bending a straight metalor glass tube. Another problem with a loose-fitting canal is that the filemay ‘‘walk’’ or vibrate in that space, leading to a change in the magni-tude of stress and possibly leading to variations in the results.

Ounsi et al (27) have used a custom-designed stainless steel modelmimicking a 2-mm-wide artificial canal space. The constant diameter ofthe cavity that reproduced the curved canal presents the same problemsof the tube-like devices in lacking the reproducibility of the actualtrajectory followed by different files.

In other studies, the curvature of a rotary instrument was producedwhen worked against a sloped metal block using a groove machined intothe 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 ofan instrument. The different angles of curvature used in these studieswere determined according to Schneider’s method (15).

Li et al (28, 29) used a sloped carbon-steel block and calculatedonly the angle of curvature by Schneider’s method (15), without

Figure 3. A schematic drawing of curved glass or metal tubes used for fatiguetesting of NiTi rotary instruments.

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JOE — Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1471

considering the radius of curvature as done by Pruett et al (14). Theyused four angles of curvature: 37!, 40.5!, 45!, and 48!. In the study byKitchens et al (30), a 2-mm-wide groove was machined into the face ofa hardened 316 stainless steel block with polished chrome plating tokeep the file in place during testing. Three angles of curvature wereused and measured using Schneider’s method (14): 25!, 28!, and33.5!. Ray et al (31) used a highly polished area of a stainless steelblock with an incline of 15! to the horizontal plane, similar to Li etal (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 filecurvature according to Schneider’s method, which takes into consider-ation only the angle of curvature and not the more important radius ofcurvature. Furthermore, it is unclear if a 2-mm-wide groove is able toconstrain the tiny tip of an endodontic instrument, thus keeping the filein place during the test. Furthermore, using the previously describeddevice, the point in which the instrument begins to leave the establishedlong axis of the instrument is not predictable and depends greatly on thephysical and geometric properties of each instrument. It is really diffi-cult to establish this point precisely; if the instrument is not sufficientlyconstrained in a precise trajectory, the part of the instrument coronal to

the beginning of the curvature will gradually move away from the longaxis of the shaft. For this reason, the choice of this point may vary greatly,and the calculated angle may present great variability. Furthermore, it isnot possible to establish exactly the point of maximum curvaturebecause the physical and geometric features of the different instrumentsmay determine different bending properties, so that the point ofmaximum curvature may lie at different points and at a different distancefrom the tip of each file.

As mentioned earlier, bending properties of different files maydetermine a different trajectory if the file is not constrained in a precisetrajectory. If testing is completed for all different files at a given angle toensure consistency, the bending properties of the different files deter-mining different angles of curvature, thus biasing the results and thecomparisons.

To limit these problems, Cheung et al (32–36) constrained theinstrument into a curvature using three stainless steel pins (Fig. 5).They used three smooth cylindrical pins of 2-mm diameter froma high hardness stainless steel mounted in acrylic shims, which wereadjustable in the horizontal direction; the position of the pins deter-mines the curvature of the instrument. A small V-shaped grooveprepared on the lowest pin maintained the position of the tip of theinstrument during rotation. It has been reported in a three-pointbending test of NiTi wires that such constraints will produce a curvaturethat is circular (37). The authors affirmed that although this cannotactually be true, the approximation should be reasonable. Unfortu-nately, NiTi endodontic files are tapered and with different cross-sectional design. The different bending properties of the different files

Figure 5. A schematic drawing of the three stainless steel pins that con-strained 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 curvatureof a rotary instrument working against a sloped metal block. A groovemachined into the face of the block keeps the file in place during testing. (Re-printed with permission [28]).

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1472 Plotino et al. JOE — Volume 35, Number 11, November 2009


DiscussionTo date, several devices and methods have been used to investigate

in vitro the cyclic fatigue fracture resistance of NiTi rotary endodonticinstruments. In nearly all studies reported in the endodontic literature,the rotating instrument was either confined in a glass or metal tube, ina grooved block-and-rod assembly, or in a sloped metal block. Therehas 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 ofthe radius of curvature in those studies is likely to be overstated (ie, thefile was actually bent less severely than reported). Furthermore, eachdifferent instrument that fits loosely inside the device may followa more or less severe curvature depending on the stiffness of the file.That would explain the wide variation in the reported fatigue life. Thelarge scatter generally encountered in various forms of fatigue testswould add to the variation in the result.

These devices permit the instruments to rotate until fracture usingdifferent curvature. The resistance of rotary instruments to cyclic fatiguedecreases with increasing instrument diameters (5, 6), and it is specif-ically related to the metal mass of the instrument in the point ofmaximum stress (53). Moreover, the increased severity of the angleand radius of the curve, around which the instrument rotates, decreasesinstrument lifespan in vitro and clinically (5, 6, 53). If instruments ofthe same dimensions follow different trajectories in the test apparatus,a direct comparison between instruments of different brands may bedifficult to make, and the results obtained may be unreliable and incon-sistent.

It is evident from this review of the literature that an internationalstandard is needed to validate a device for cyclic fatigue tests of NiTirotary endodontic instruments. Ideally, such a device should allowtesting of all instruments with a precise trajectory in terms of radiusand angle of the curvature and point of the center of the curvature,allowing comparison of different instruments in different canals. Thisis a way of testing cyclic fatigue that could simulate more preciselythe clinical accumulation of flexural fatigue.

ConclusionsThis review analyzed several devices that have been used in

endodontic literature for cyclic fatigue testing and found that differencesin the methodology affected the fatigue behavior of rotary instrumentsand, consequently, the outcome of these studies.

Cyclic fatigue tests investigate the in vitro resistance to fracturecaused by the accumulation of metal fatigue, which is determined bythe tension/compression cycles at the point of maximum flexure. Theclinical relevance of the results of such tests is difficult to assess becausethis condition differs from intracanal instrumentation in which the frac-ture occurs because of several factors that act together at the same time,including torsional stress. This represents a pure mechanical test toextrapolate only one characteristic of the instruments (bending failure)in the same way as torsional resistance is defined and measured by aninternational standard.

Because NiTi rotary instruments are widely used, the need fora standardization of testing of their properties including cyclic fatigue

Figure 11. An example of three different artificial canals showing the different trajectory that the instrument follows.

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.

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JOE — Volume 35, Number 11, November 2009 Cyclic Fatigue of Rotary Instruments 1475


Effect of a Separated Instrument on BacterialPenetration of Obturated Root Canals

Jeffrey L. Saunders, DDS, Paul D. Eleazer, DDS, MS, Ping Zhang, PhD, DDS, andSusanne Michalek, PhD

The aim of this study was to determine the effect aseparated instrument has on the time required forbacterial penetration of obturated root canals.Twenty-six extracted human mandibular premo-lars with single canals were used in the study.Group 1 consisted of teeth that contained a sepa-rated size 40 Profile rotary file and were obturatedwith gutta-percha and zinc oxide eugenol sealer tothe level of the separated file. Group 2 consisted ofteeth that were similarly obturated, but without aseparated file. The negative control canals wereobturated and had the entire root surface sealedwith nail polish. The positive controls were obtu-rated without sealer. Streptococcus sanguis wasplaced in the access chamber daily, and penetra-tion was determined when turbidity was noted inthe culture broth. The results showed no signifi-cant difference between the two experimentalgroups.

Instrument separation is an unfortunate sequela of endodonticinstrumentation. Reasons include overuse of the instrument, im-proper use of the instrument, microcracks inherent in the newinstrument, and calcified or curved canals. When instrument sep-aration occurs, the clinician has the choice of leaving the instru-ment in the canal or attempting to remove it either surgically ornonsurgically. Several factors need to be weighed when determin-ing whether an attempt should be made to retrieve the separatedinstrument. Such factors include the position in the canal at whichseparation occurred, the amount of potential irritant remaining inthe canal, and the amount of damage that would be caused to theremaining tooth structure if instrument removal were attempted.Crump and Natkin (1) used statistical methods to show that sep-arated instruments did not adversely affect the success rate ofendodontic cases.Many experiments have been performed investigating penetra-

tion of root canal fillings using various dyes and isotopes (2–7).Torabinejad et al. (8) and Khayat et al. (9) have stated that bacterialpenetration may be more meaningful and clinically relevant inleakage studies. Torabinejad et al. (8) found complete penetration

of bacteria in more than 50% of the root canals in their study within19 days and 42 days when using Streptococcus epidermidis andProteus vulgaris, respectively. Khayat et al. (9) found completebacterial penetration in an average of 28.8 days and 25.4 days forroot canals filled using lateral condensation and vertical conden-sation, respectively, when human saliva was introduced into theaccess cavities. No study has been performed investigating theeffect of bacterial penetration of root canals with broken files plusgutta-percha and zinc oxide eugenol (ZOE) sealer.Because separation of instruments is a sequela of endodontic

treatment, it would be useful to know how such occurrences affectthe sealing ability of the obturation material. Knowing the answermay affect a clinician’s decision whether to attempt separatedinstrument retrieval. Through the endodontic literature, it is unclearwhether a fluted instrument separated in a root canal would allowquicker penetration of bacteria than the same length of gutta perchaand ZOE sealer. The purpose of this investigation was to determinethe effect a separated instrument has on the time of bacterialpenetration of canals obturated with gutta-percha and ZOE sealer.

MATERIALS AND METHODS

Twenty-six extracted mandibular premolars were used in thisstudy. All of the teeth were caries-free and contained either min-imal or no coronal restoration. The teeth all possessed fully formedapexes. The teeth had been stored in 10% formalin and were keptmoist throughout the experiment. The teeth were radiographedboth from the buccal and proximal direction to ensure one straightcanal. Standard endodontic access was achieved using a #2 roundbur and an Endo-Z bur. Canal length was determined by placing a#10 k-file through the canal space until it could be visualizedexiting the apical foramen. Working length was determined bysubtracting 1 mm from the canal length. All of the canals wereflared coronally with size #2 to #4 Gates Glidden drills, and theapical canal was instrumented to size 30 using hand files. The canalwas flushed with sodium hypochlorite between every instrument,and apical patency was maintained with a size 15 file throughoutthe instrumentation. The teeth were divided into four groups byrandom draw. The four groups consisted of a positive control; anegative control; experimental group 1, with a broken file; andexperimental group 2, with no broken file.After hand filing, 16 teeth were sequentially instrumented using

.04 Profile rotary instruments starting at a size 25 until a size 40

JOURNAL OF ENDODONTICS Printed in U.S.A.Copyright © 2004 by The American Association of Endodontists VOL. 30, NO. 3, MARCH 2004

177


would reach working length. Ten of these teeth were assigned toexperimental group 2. The remaining six teeth were divided intotwo control groups containing three teeth each. None of these teethcontained a broken file.For experimental group 1, 10 size 40 .04 Profile rotary instru-

ments were nicked with a #2 round bur 3 mm from the tip tofacilitate file separation at this point. The size 40 Profile rotatingat 300 rpm was then introduced with apical pressure into eachcanal until separation occurred. After instrument separation, eachof the 10 teeth in this group was again radiographed to ensure thatthe separation occurred in the apical third of the canal.After instrumentation, the roots of all 26 teeth were coated with

two applications of fingernail polish. The apical 2 mm of the rootswere not covered with polish, except for the three teeth in thenegative control group, which had their entire root coated. All 26teeth were then autoclaved.All teeth in the negative control group and experimental group

2 were obturated to the working length with gutta-percha and Roth811 sealer (Roth International, Chicago, IL) using the lateral con-densation technique. A sterile technique was used for obturation ofall teeth. A heated plugger was used to remove coronal gutta-percha until a standardized length of 10 mm of filling was left ineach canal. The same filling procedure was used in experimentalgroup 1 except that the canal was obturated to the separatedinstrument and only 7 mm of gutta-percha was left in the canal, sothat the total obturation length was 10 mm, including the 3 mm ofseparated instrument. The three teeth in the positive control groupwere obturated using the same lateral condensation technique,although without sealer.Culture medium, 5 ml, was placed in 26 individual test tubes.

Each of the 26 teeth was suspended using orthodontic ligature wireinto the test tube so that at least 2 mm of each tooth’s apex waswithin the broth. The chamber of each tooth was filled with asuspension of Streptococcus sanguis on day 0.A fresh bacterial suspension of S. sanguis, which was prepared

daily, was added to the access opening of each tooth until thechamber was nearly full. This procedure was performed every daythroughout the experiment. Penetration of the root canal was re-corded when turbidity was noted in the broth. Cultures werechecked daily until the final test system became positive at day 90.

RESULTS

Turbidity was noted on day 1 for all three of the positive controlteeth. The culture medium of the three negative control teethshowed no turbidity over the 90-day course of the experiment.In experimental group 1, containing the separated files, all of the

samples showed turbidity by day 90. The amount of time it took forthe samples to show turbidity varied from 5 to 90 days, with anaverage of 43 days. (see Table 1).In experimental group 2, two of the samples were contaminated

during the initial setup. Teeth 6 and 8 of group 2 were fullysubmerged into the culture broth and were excluded from analysis.The amount of time it took for turbidity to occur in the group 2samples varied from 5 to 71 days, with an average of 44 days.A statistical analysis using a t test showed that the difference in

the two experimental groups was not statistically significant (t !"0.0857, with p ! 0.933).

DISCUSSION

In this study, the presence of turbidity indicated full bacterialpenetration through the root canals of the teeth. The results of thethree teeth in the positive control group showing turbidity by day1 confirm the results by Marshall and Massler (10), who found thatsealer is necessary to improve the apical sealing ability of obtura-tion material. Because none of the three negative control teethshowed turbidity after 90 days, it is apparent that the experimentalsetup provided a contamination-free environment.Although the average numbers of days for total bacterial pen-

etration (43 days for group 1 and 44 days for group 2) were verysimilar, there was a very large range for both groups. Other authorshave also reported large ranges for leakage of obturated rootcanals. Torabinejad et al. (8) and Khayat et al. (9) reported largeranges when examining bacterial penetration through root canalfillings. Swanson and Madison (4) also found a significant vari-ability when they examined the penetration of a dye throughobturated root canals.The large range of time for penetration of the experimental

groups can possibly be attributed to variable root canal anatomy,shape of canal preparation, and sealer type.The presence of 3 mm of separated instrument did not speed up

or slow down penetration of bacteria when compared with thenormally obturated experimental group. This result implies that theseparated file did not compromise obturation of the root canalspace. This fact is surprising, because root canal anatomy is quitevariable and is not perfectly round like the separated instrument.Also, the separated instrument has flutes, so it would be unlikelyto completely obturate the root canal space by itself. With sealerextruded into the flutes, the separated file may become the equiv-alent of any other obturation material.The results of this study indicate that the separated instrument

itself does not play a large role in the sealing ability of theobturation material. Perhaps more important to the success of theroot canal therapy is the coronal seal and absence of any residualirritant beyond the level of the separated instrument.

Dr. Saunders is an Endodontic Resident, School of Dentistry; Dr. Eleazeris Chairman, Department of Endodontics and Pulp Biology, School of Den-tistry; Dr. Zhang is a Research Associate, Department of Microbiology; and Dr.Michalek is Professor, Department of Microbiology, University of Alabama atBirmingham, Birmingham, Alabama.

TABLE 1. Effect of a 3-mm separated file on the bacterialpenetration in obturated single-rooted teeth*

Group 1(separated file)

Group 2(no separated file)

SampleNo. of daysfor turbidity

SampleNo. of daysfor turbidity

1 30 1 422 47 2 53 62 3 494 90 4 415 21 5 716 5 6 —7 33 7 318 52 8 —9 14 9 64

10 76 10 49Mean: 43 days Mean: 44 daysRange: 5–90 days Range: 5–71 days

* Rate of total bacterial penetration by S. sanguis in experimental group 1 (teethcontaining a separated instrument) and group 2 (no separated instrument). No statisticallysignificant difference (p ! 0.933) between group 1 and 2.

—, indicates sample lost due to contamination during initial setup.

178 Saunders et al. Journal of Endodontics


2 Studies included:Crump/Natkin 1970 - Spili et al 2005199 cases

Incidence of separation:Stainless Steel: 0.5-7.4 %Niti: 0.4-3.7%

Does retention of a separated instrument, compared w/ no retained instrument - result in a poorer clinical outcome?


Assessment: 1 year is the earliest possible foully time to determine whterh the lesion has healed

Influence of a preoperative RL80.7% of lesions healed when a periapical lesion was present92.4% remanning healthy when no lesion was present


Important finding - studies w/ no controls only used the presence of a preoperative periapical lesion to act as the main prognostic factor for successful mgmt. of these cases


Highest proportion of instrument fragments occurs in the apical third

Removal should be weighed against modest benefit


C L I N I C A L P R A C T I C E

196 JADA, Vol. 138 http://jada.ada.org February 2007

Background and Overview. With theincreased use of nickel-titanium (NiTi) rotary instru-ments for root canal preparation in endodontics,instrument fracture has become more prevalent.Extensive research has been conducted on the physicalproperties and mechanical characteristics of NiTi rotary instruments, aswell as the factors that can contribute to instrument failure. NiTi rotaryinstruments are subjected to torque and are susceptible to cyclic fatigue,which are the main causes of instrument fracture. However, with anunderstanding of how these instruments function in preparing rootcanals and by applying ways to reduce torque-generated metal fatigue,clinicians can use the instruments safely in clinical practice. Results. The author presents 12 measures that clinicians can take toprevent NiTi rotary instrument fracture and discusses them in detail.Clinical Implications. NiTi rotary instrument fracture complicatesthe progress, and compromises the prognosis of endodontic treatment.However, when clinicians take appropriate measures, rotary instrumentfractures can be prevented.Key Words. Nickel-titanium; rotary; instrument; fracture; prevention.JADA 2007;138(2):196-201.

D uring the past 15 years,nickel-titanium (NiTi)rotary instruments havebecome a part of thestandard armamen-

tarium in endodontics. They areused extensively by generalists andspecialists to facilitate the cleaningand shaping of root canals,1 and itappears that with the increasedapplication of these instruments incontemporary endodontic practice,fractures have become moreprevalent.2,3

Fractured instruments are a defi-nite hindrance to the goals ofcleaning, shaping and filling rootcanals,4,5 and they may adverselyaffect the outcome of endodontictreatment.2,6-8 Techniques forremoving fractured instrumentfragments from root canals havebeen described in the dental litera-ture.9,10 However, removal of frag-ments may be impossible or imprac-tical, especially when they are smalland located in the apical portion ofnarrow curved root canals or whenrepeated attempts at removal couldresult in excessive enlargement of

A B S T R A C T

ARTICLE4

Dr. Di Fiore is an associate professor of endodontics and the director, Predoctoral Endodontics, NewYork University, College of Dentistry, Department of Endodontics, 345 E. 24th St., New York, N.Y.10010. Address reprint requests to Dr. Di Fiore.

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A dozen ways to prevent nickel-titaniumrotary instrument fracture

Peter M. Di Fiore, DDS, MS

Copyright ©2007 American Dental Association. All rights reserved.


File Characteristics



JADA, Vol. 138 http://jada.ada.org February 2007 197

the canal or perforation of the root.7,8

Since instrument fracture complicates andcompromises endodontic treatment andprognosis,2,6-8 clinicians must be constantly awareof the possibility of rotary instrument fractureand take every precaution to avert this mishap.The purpose of this article is to present measuresthat clinical operators can take to reduce the riskof NiTi rotary instrument failure and preventfracture during root canal preparation.

ROTARY INSTRUMENT ASSESSMENT

NiTi. NiTi is a superelastic shape-memorymetallic alloy that, when flexed, undergoes amartensitic transformation from its originalaustenitic structure and, if stressed beyond itselastic limit, will rupture. Duringroot canal preparation, NiTi rotaryinstruments are subjected to cyclicfatigue, which can lead to distortionand fracture, especially when theinstruments are flexed severely.11

Extensive investigations of thephysical properties of NiTi rotaryfiles under dynamic testing pro-cedures have shown that torsionalstress and cyclic fatigue are themain causes of instrumentfracture.11-14 Observations of thefractured surfaces of NiTi rotary instrumentsunder scanning electron microscopic (SEM)examination have revealed the presence ofperipheral serrations, dimples and craters thatare characteristic of ductile-type fractures.13-15

Therefore, NiTi rotary instruments should not besubjected to excessive torsional and bendingstress during operation.

Instrument design. NiTi rotary instrumentsare available in a variety of types, with differentfunctional features that affect the manner inwhich they engage and cut dentin. The structuralcharacteristics and mechanical designs of theseinstruments have a definite influence on theirsusceptibility to fracture.15-19 In particular, thesize, taper and cutting flute depths are importantfactors that affect the torsional and bending prop-erties of rotary instruments.15-19

Size. A comparative study of the fatigue resis-tance of NiTi rotary instruments of different sizesand flute designs revealed that large instrumentswere highly susceptible to fatigue failure.15

Research has demonstrated that as an instru-ment’s cross-sectional diameter increases, it

becomes less resistant to cyclic fatigue.18 Afterperforming dynamic stress tests on various NiTirotary instruments, investigators found that asan instrument’s size and taper increase, thetorque generated during rotation increases andthe fracture time decreases.12,13

Taper. In cyclic fatigue-to-fracture tests of dif-ferent NiTi rotary instruments, researchers foundthat 0.06 taper instruments had less resistance tofracture than did 0.04 taper instruments.19 Shenand colleagues20 compared the types of failuresthat occurred with NiTi rotary instruments ofvarious geometric designs and reported that avery high percentage (21 percent) of the instru-ments that fractured had progressively largertapers and a much lower percentage (7 percent)

had consistently even tapers. Theyalso noted that failures for progres-sively tapered instruments tendedto be fractures, whereas for evenlytapered instruments, failurestended to be unwinding deforma-tions.20 In this regard, instrumentsthat show unwinding as a failurecharacteristic may be safer for usethan those that fracture sponta-neously. Additionally, Guilford andcolleagues,21 in comparing the

torque required to fracture different types ofrotary instruments, found that progressivelytapered instruments failed rapidly with littlerotation.

Cutting flute depth. Instruments with deep cut-ting flutes and progressively larger variabletapers have rapidly changing cross-sectionaldiameters along the entire length of their shafts.These instruments develop high torque levelsthat make them more prone to metal fatigue andfracture. However, instruments that have shallowcutting flutes, evenly tapered shafts and consis-tently shaped cross-sectional areas are moreresistant to fracture. This is because the torsionaland bending stresses that develop during use aredistributed uniformly along these instruments’entire length.19-21 Therefore, practitioners shouldbe completely familiar with the mechanical fea-tures and working limitations of rotary instru-ments and select those that are less prone tofracture.

Size, taper and cutting flute depths

are important factorsthat affect the

torsional and bendingproperties of rotary

instruments.

ABBREVIATION KEY. NiTi: Nickel-titanium. SEM: Scanning electron microscopic.





















instruments.























instruments.




CANAL CURVATURE ASSESSMENT



Instrument use. Microstructure and surfaceanalysis of unused NiTi rotary instrumentsrevealed that there were distortions in the latticestructure of the alloy, variations in the microhard-ness of the metal, and machining and millingmarks as well as metal strips and microcracks ontheir surfaces.16,22,23 It is important for clinicians torealize that these pre-existing conditions asso-ciated with the manufacturing process may con-tribute to the propagation of instrument fracturesduring use.24,25 Cyclic fatigue and torsional testingprocedures that measured rotation time andtorque level at fracture have demonstrated thatused rotary instruments are significantly moresusceptible to fracture than are new ones.26,27

These findings are further supported by SEMobservations of used instruments that revealedsigns of deterioration, including surface cracksthat can progress to fractures withfurther use.22-25,28 Sotokawa29 foundthat by applying a systematicschedule for the disposal ofendodontic instruments, the inci-dence of fracture can be reduced.

Therefore, it is advisable and pru-dent to dispose of all instrumentsafter they have been used for a spe-cific number of clinical cases, ratherthan wait for deformations and dis-tortions to appear. Manufacturersrecommend that rotary instruments be discardedafter they have been used for one clinical case.


The fracture potential of an instrument rotatingin a curved canal becomes greater as the angle ofcurvature increases and the radius of curvaturedecreases.11,13,14 Zelada and colleagues30 andMartin and colleagues31 reported that during thepreparation of root canals in extracted molarteeth, all instrument fractures occurred inseverely curved canals with angles of curvaturegreater than 30 degrees. A careful preoperativeradiographic examination with fine hand instru-ments in the canals will reveal the presence andacuity of root canal curvatures. Therefore, whencurvatures are present, the operator should bewary of the possibility of a fracture and proceedcautiously during root canal preparation.

ACCESS PREPARATION

In the internal configuration of an adequateendodontic access preparation, the entrances to

the root canal orifices are not obstructed by thepresence of excessive dentinal bulk or restorativematerial, and there is an unimpeded directionfrom which instruments can approach the apicalportion of the root canal or the point of initial rootcanal curvature.32 When instruments can nego-tiate root canals easily, bending and flexingstresses are lessened and the potential for frac-ture is reduced. Endodontic access becomes evenmore crucial for avoiding an instrument fracturewhen teeth are difficult to reach because of lim-ited mouth opening.33 Therefore, practitionersshould make adequate access preparation a pri-ority, as an important first step in avoiding rotaryinstrument fracture.

CANAL ORIFICE ENLARGEMENT

The enlargement of root canal orifices facilitatesthe negotiation and instrumentationof the apical part of root canals,especially in curved canals of multi-rooted teeth.34,35 Leeb34 used maxil-lary and mandibular extractedmolars with curved roots to demon-strate that after the canal orificeswere enlarged, instruments moreeasily penetrated the canals. Thecanal orifice can be enlarged effec-tively by the sequential use of nos. 4and 2 low-speed, long-shank round

burs followed by nos. 4 and 3 Gates Gliddendrills.34,35 These instruments, used carefully, canefficiently create a 2- to 4-millimeter oval-shapedfunnel that serves as an accessible unobstructedentrance and guides rotary instruments into theroot canal without causing excessive bending orbinding, which could lead to metal fatigue. There-fore, practitioners should enlarge orifices beforeintroducing NiTi rotary instruments into the rootcanal.

MANUAL INSTRUMENTATION

Hand instruments can create a smooth, open pas-sageway for rotary instruments to follow as theyprogress to the apical terminus. Three studieshave demonstrated that manual root canal instru-mentation with fine stainless steel hand instru-ments, used in a step-back manner before rotaryinstruments were used, significantly reduced theincidence of rotary instrument fracture during thepreparation of curved canals.36-38 Roland and col-leagues36 and Patino and colleagues37 used finehand instruments to enlarge curved root canals in

Practitioners shouldenlarge root canal

orifices before introducing nickel-

titanium rotary instruments into

the canal.






















instruments.




Instrument Use








ACCESS PREPARATION











the canal.



ACCESS PREPARATION


ROTATIONAL SPEED AND TORQUECONTROL



extracted molars manually, and Berutti and col-leagues38 manually enlarged standardized curvedcanals in resin blocks to a size 20 hand instru-ment, creating a glide path for rotary instru-ments. All of these studies showed that manualenlargement of root canals with fine hand instru-ments significantly reduced the failure rate ofrotary instruments.36-38 Therefore, practitionersshould use rotary instruments only after rootcanals have been negotiated and enlarged withfine hand instruments.


Electric motors have been developed to controlboth rotational speed and torque during rootcanal instrumentation so that when the torque onan instrument, rotating at a constant speed,reaches a preset level, the motor automaticallyreverses its rotational direction and allows thefile to be withdrawn before it locks and fracturesin the root canal.39 Gabel and colleagues40 investi-gated the influence of rotational speed on thefailure of NiTi rotary instruments for the prepa-ration of root canals in extracted molar teeth andfound that instrument distortion and fracturewere four times more likely to occur at higherrotational speeds (333 rotations per minute) thanat lower rotational speeds (167 rpm). Gambarini41

found that instruments used in low-torque motors(< 1 Newton per centimeter) were more resistantto fracture than those used in high-torque motors(> 3 N/cm). Therefore, practitioners should useelectric motors set at low rotational speeds andlow torque levels during root canal preparation.

CROWN-DOWN TECHNIQUE

In the crown-down technique, larger instrumentsare used first in the coronal aspect of the canal,followed sequentially by smaller instruments inthe apical aspect of the canal. The advantages ofthis technique are that it removes infectedcoronal dentin and obstructions before apicalpreparation and enlarges canals incrementally. Ina study by Blum and colleagues42 in which theroot canals of mandibular incisors were preparedwith NiTi rotary instruments using either a step-back or crown-down technique, the researchersfound that less vertical force and torque were cre-ated with the crown-down technique and thatinstrument tips had less contact with dentin andless stress during the early phases of instrumen-tation. This is important, since test fractures per-

formed on NiTi rotary instruments have demon-strated that fractures tend to occur close to thetip.43 Studies of the failure of NiTi rotary instru-ments used to prepare root canals in extractedmolar teeth found that a greater number of frac-tures and distortions occurred with sizes 20 and25, and that most instruments fractured within 1 to 3 mm from the tip.30,40,44 Additionally, in twoseparate investigations of failure of rotary instru-ments that were used with the crown-down tech-nique to prepare a total of 210 curved root canalsin extracted teeth, researchers found that no frac-tures occurred and only two instruments becamedeformed.45,46 Therefore, practitioners shouldapply the crown-down technique as a standardoperational procedure with rotary instruments.

IRRIGATION AND LUBRICATION

Irrigation and lubrication are essential for accom-plishing adequate débridement of root canals.SEM studies of the efficacy of root canal cleaninghave demonstrated that dentinal debris gener-ated during instrumentation becomes packed inroot canals and that irrigation is necessary for itsremoval.47-49 A preparation of urea peroxide andethylenediamine tetraacetic acid as a lubricant50

is a combination commonly used for root canalpreparation.1 Irrigants and lubricants reduce rootcanal clogging, frictional resistance and mechan-ical overloading, thereby decreasing the torsionalstresses placed on rotating instruments. There-fore, during root canal preparation, practitionersshould lubricate instruments generously and irri-gate canals copiously.

ROTARY INSTRUMENT MANIPULATION

The manner in which NiTi rotary files are manip-ulated for preparing root canals is extremelyimportant. It has been shown that a cyclic axialmotion applied to rotary instruments during oper-ation was significant in preventing prematurefracture.51 Also, a pecking or pumping motion,which lowers apical forces during root canalpreparation, has been advocated by researchersas an important way to prevent instrumentbinding and torque-generated cyclic fatigue.12-14

Li and colleagues14 tested the cyclic fatigue ofNiTi rotary instruments under static anddynamic pecking motion conditions and foundthat as the pecking distance increased, the frac-ture time increased, suggesting that this type ofinstrument manipulation is critical for preventingrotary instrument fracture. Yared and col-



CROWN-DOWN TECHNIQUe


OPERATOR PROFICIENCY



leagues45 found that when a slight apical pumpingmotion was used to reduce the development ofexcessive torque during the root canal preparationof extracted molar teeth, no instruments frac-tured. Therefore, one may conclude that practi-tioners should use a pecking or pumping move-ment when manipulating rotary instruments.


Studies have demonstrated that higher rates ofNiTi rotary instrument fracture occur with inex-perienced operators than with experiencedones.52,53 Rotary instruments tend to thread andscrew into root canals, which subjects them tohigh levels of torque as they bind and lock in thecanal.39,52,53 In addition, instrument locking may beenhanced when the root canal preparation beginsto acquire the shape and taper of larger instru-ments as they extend deeper into the canal, cre-ating a taper-lock effect.39,52,53 This is a valid con-cern; Schrader and Peters54 found that using NiTirotary instruments with different tapers reducedcanal contact areas and instrument fatigue–related failures during root canal preparation inextracted teeth. The operator’s ability to senseand resist these binding and locking tendencies isa skill that can be obtained only with experience.Yared and colleagues,39,52,53 in several extensiveinvestigations, showed that preclinical training inthe use of NiTi rotary instruments for the prepa-ration of root canals in extracted molar teeth wascrucial for avoiding instrument fracture. There-fore, inexperienced operators should engage inpreclinical training exercises as learning experi-ences before using these instruments on patients,then proceed carefully in clinical practice as theygain experience.

SUMMARY AND CONCLUSION

There are several measures that practitioners cantake to prevent NiTi rotary instrument fractureduring root canal preparation:davoid subjecting NiTi rotary instruments toexcessive stress;duse instruments that are less prone to fracture;dfollow an instrument use protocol;dassess root canal curvatures radiographicallyand instrument them carefully;densure that the endodontic access preparationis adequate;dopen orifices before negotiating canals;denlarge root canals with fine hand instruments;

dset rotational speed and torque at low levels;duse the crown-down technique;dirrigate and lubricate root canals during preparation;dmanipulate rotary instruments with a peckingor pumping motion;dif inexperienced, engage in preclinical trainingin the use of rotary instruments.

Instrument fracture is a serious iatrogenicmishap that can complicate and compromiseendodontic treatment. It therefore is imperativethat clinicians using these instruments in prac-tice apply all appropriate measures to reduce therisk of fracture. Recently published laboratoryand clinical assessment studies have shown thatwhen operators are aware of the possibility ofinstrument fractures and take measures to avoidthem, the incidence of fracture can be as low asfour per 1,000.55,56

■

1. Averbach RE, Kleier DJ. Endodontics in the 21st century: therotary revolution. Compend Contin Educ Dent 2001;22(1):27-34.

2. Spili P, Parashos P, Messer HH. The impact of instrument fractureon outcome of endodontic treatment. J Endod 2005;31(12):845-50.

3. Parashos P, Gordon I, Messer HH. Factors influencing defects ofrotary nickel-titanium endodontic instruments after clinical use. JEndod 2004;30(10):722-5.

4. Schilder H. Cleaning and shaping the root canal. Dent Clin NorthAm 1974;18(2):269-96.

5. Schilder H. Filling root canals in three dimensions. Dent ClinNorth Am 1967;11:723-44.

6. Crump MC, Natkin E. Relationship of broken root canal instru-ments to endodontic case prognosis: a clinical investigation. JADA1970;80(6):1341-7.

7. Hulsmann M, Schinkel I. Influence of several factors on the suc-cess or failure of removal of fractured instruments from root canals.Endod Dent Traumatol 1999;15(6):252-8.

8. Souter NJ, Messer HH. Complications associated with fracturedfile removal using an ultrasonic technique. J Endod 2005;31(6):450-2.

9. Hulsmann M. Removal of fractured instruments using a combinedautomated/ultrasonic technique. J Endod 1994;20(3):144-6.

10. Ward JR, Parashos P, Messer HH. Evaluation of an ultrasonictechnique to remove fractured rotary nickel-titanium endodonticinstruments from root canals: an experimental study. J Endod2003;29(11):756-63.

11. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing ofnickel-titanium endodontic instruments. J Endod 1997;23(2):77-85.

12. Sattapan B, Palamara JE, Messer HH. Torque during canalinstrumentation using rotary nickel-titanium files. J Endod 200026(3):156-60.

13. Haikel Y, Serfaty R, Bateman G, Singer B, Allemann C. Dynamicand cyclic fatigue of engine driven rotary nickel-titanium endodonticinstruments. J Endod 1999;25(6):434-40.

14. Li UM, Lee BS, Shih CT, Lan WH, Lin CP. Cyclic fatigue onendodontic nickel titanium rotary instruments: static and dynamictests. J Endod 2002;28(6):448-51.

15. Chaves Craveiro de Melo M, Guiomar de Azevedo Bahia M, LopesBuono VT. Fatigue resistance of engine-driven rotary nickel-titaniumendodontic instruments. J Endod 2002;28(11):765-9.

16. Kuhn G, Tavernier B, Jordan L. Influence of structure on nickel-titanium endodontic instruments failure. J Endod 2001;27(8):516-20.

17. Xu X, Eng M, Zheng Y, Eng D. Comparative study of torsionaland bending properties for six models of nickel-titanium root canalinstruments with different cross-sections. J Endod 2006;32(4):372-5.

18. Ullmann CJ, Peters OA. Effects of cyclic fatigue on static fractureloads in ProTaper nickel-titanium rotary instruments. J Endod2005;31(3):183-6.

19. Yao JH, Schwartz SA, Beeson TJ. Cyclic fatigue of three types ofrotary nickel-titanium files in a dynamic model. J Endod 2006;32(1):55-7.






extracted molars manually, and Berutti and col-leagues38 manually enlarged standardized curvedcanals in resin blocks to a size 20 hand instru-ment, creating a glide path for rotary instru-ments. All of these studies showed that manualenlargement of root canals with fine hand instru-ments significantly reduced the failure rate ofrotary instruments.36-38 Therefore, practitionersshould use rotary instruments only after rootcanals have been negotiated and enlarged withfine hand instruments.


Electric motors have been developed to controlboth rotational speed and torque during rootcanal instrumentation so that when the torque onan instrument, rotating at a constant speed,reaches a preset level, the motor automaticallyreverses its rotational direction and allows thefile to be withdrawn before it locks and fracturesin the root canal.39 Gabel and colleagues40 investi-gated the influence of rotational speed on thefailure of NiTi rotary instruments for the prepa-ration of root canals in extracted molar teeth andfound that instrument distortion and fracturewere four times more likely to occur at higherrotational speeds (333 rotations per minute) thanat lower rotational speeds (167 rpm). Gambarini41

found that instruments used in low-torque motors(< 1 Newton per centimeter) were more resistantto fracture than those used in high-torque motors(> 3 N/cm). Therefore, practitioners should useelectric motors set at low rotational speeds andlow torque levels during root canal preparation.

CROWN-DOWN TECHNIQUE

In the crown-down technique, larger instrumentsare used first in the coronal aspect of the canal,followed sequentially by smaller instruments inthe apical aspect of the canal. The advantages ofthis technique are that it removes infectedcoronal dentin and obstructions before apicalpreparation and enlarges canals incrementally. Ina study by Blum and colleagues42 in which theroot canals of mandibular incisors were preparedwith NiTi rotary instruments using either a step-back or crown-down technique, the researchersfound that less vertical force and torque were cre-ated with the crown-down technique and thatinstrument tips had less contact with dentin andless stress during the early phases of instrumen-tation. This is important, since test fractures per-

formed on NiTi rotary instruments have demon-strated that fractures tend to occur close to thetip.43 Studies of the failure of NiTi rotary instru-ments used to prepare root canals in extractedmolar teeth found that a greater number of frac-tures and distortions occurred with sizes 20 and25, and that most instruments fractured within 1 to 3 mm from the tip.30,40,44 Additionally, in twoseparate investigations of failure of rotary instru-ments that were used with the crown-down tech-nique to prepare a total of 210 curved root canalsin extracted teeth, researchers found that no frac-tures occurred and only two instruments becamedeformed.45,46 Therefore, practitioners shouldapply the crown-down technique as a standardoperational procedure with rotary instruments.


Irrigation and lubrication are essential for accom-plishing adequate débridement of root canals.SEM studies of the efficacy of root canal cleaninghave demonstrated that dentinal debris gener-ated during instrumentation becomes packed inroot canals and that irrigation is necessary for itsremoval.47-49 A preparation of urea peroxide andethylenediamine tetraacetic acid as a lubricant50

is a combination commonly used for root canalpreparation.1 Irrigants and lubricants reduce rootcanal clogging, frictional resistance and mechan-ical overloading, thereby decreasing the torsionalstresses placed on rotating instruments. There-fore, during root canal preparation, practitionersshould lubricate instruments generously and irri-gate canals copiously.


The manner in which NiTi rotary files are manip-ulated for preparing root canals is extremelyimportant. It has been shown that a cyclic axialmotion applied to rotary instruments during oper-ation was significant in preventing prematurefracture.51 Also, a pecking or pumping motion,which lowers apical forces during root canalpreparation, has been advocated by researchersas an important way to prevent instrumentbinding and torque-generated cyclic fatigue.12-14

Li and colleagues14 tested the cyclic fatigue ofNiTi rotary instruments under static anddynamic pecking motion conditions and foundthat as the pecking distance increased, the frac-ture time increased, suggesting that this type ofinstrument manipulation is critical for preventingrotary instrument fracture. Yared and col-





leagues45 found that when a slight apical pumpingmotion was used to reduce the development ofexcessive torque during the root canal preparationof extracted molar teeth, no instruments frac-tured. Therefore, one may conclude that practi-tioners should use a pecking or pumping move-ment when manipulating rotary instruments.


Studies have demonstrated that higher rates ofNiTi rotary instrument fracture occur with inex-perienced operators than with experiencedones.52,53 Rotary instruments tend to thread andscrew into root canals, which subjects them tohigh levels of torque as they bind and lock in thecanal.39,52,53 In addition, instrument locking may beenhanced when the root canal preparation beginsto acquire the shape and taper of larger instru-ments as they extend deeper into the canal, cre-ating a taper-lock effect.39,52,53 This is a valid con-cern; Schrader and Peters54 found that using NiTirotary instruments with different tapers reducedcanal contact areas and instrument fatigue–related failures during root canal preparation inextracted teeth. The operator’s ability to senseand resist these binding and locking tendencies isa skill that can be obtained only with experience.Yared and colleagues,39,52,53 in several extensiveinvestigations, showed that preclinical training inthe use of NiTi rotary instruments for the prepa-ration of root canals in extracted molar teeth wascrucial for avoiding instrument fracture. There-fore, inexperienced operators should engage inpreclinical training exercises as learning experi-ences before using these instruments on patients,then proceed carefully in clinical practice as theygain experience.

SUMMARY AND CONCLUSION

There are several measures that practitioners cantake to prevent NiTi rotary instrument fractureduring root canal preparation:davoid subjecting NiTi rotary instruments toexcessive stress;duse instruments that are less prone to fracture;dfollow an instrument use protocol;dassess root canal curvatures radiographicallyand instrument them carefully;densure that the endodontic access preparationis adequate;dopen orifices before negotiating canals;denlarge root canals with fine hand instruments;

dset rotational speed and torque at low levels;duse the crown-down technique;dirrigate and lubricate root canals during preparation;dmanipulate rotary instruments with a peckingor pumping motion;dif inexperienced, engage in preclinical trainingin the use of rotary instruments.

Instrument fracture is a serious iatrogenicmishap that can complicate and compromiseendodontic treatment. It therefore is imperativethat clinicians using these instruments in prac-tice apply all appropriate measures to reduce therisk of fracture. Recently published laboratoryand clinical assessment studies have shown thatwhen operators are aware of the possibility ofinstrument fractures and take measures to avoidthem, the incidence of fracture can be as low asfour per 1,000.55,56

■

1. Averbach RE, Kleier DJ. Endodontics in the 21st century: therotary revolution. Compend Contin Educ Dent 2001;22(1):27-34.

2. Spili P, Parashos P, Messer HH. The impact of instrument fractureon outcome of endodontic treatment. J Endod 2005;31(12):845-50.

3. Parashos P, Gordon I, Messer HH. Factors influencing defects ofrotary nickel-titanium endodontic instruments after clinical use. JEndod 2004;30(10):722-5.

4. Schilder H. Cleaning and shaping the root canal. Dent Clin NorthAm 1974;18(2):269-96.

5. Schilder H. Filling root canals in three dimensions. Dent ClinNorth Am 1967;11:723-44.

6. Crump MC, Natkin E. Relationship of broken root canal instru-ments to endodontic case prognosis: a clinical investigation. JADA1970;80(6):1341-7.

7. Hulsmann M, Schinkel I. Influence of several factors on the suc-cess or failure of removal of fractured instruments from root canals.Endod Dent Traumatol 1999;15(6):252-8.

8. Souter NJ, Messer HH. Complications associated with fracturedfile removal using an ultrasonic technique. J Endod 2005;31(6):450-2.

9. Hulsmann M. Removal of fractured instruments using a combinedautomated/ultrasonic technique. J Endod 1994;20(3):144-6.

10. Ward JR, Parashos P, Messer HH. Evaluation of an ultrasonictechnique to remove fractured rotary nickel-titanium endodonticinstruments from root canals: an experimental study. J Endod2003;29(11):756-63.

11. Pruett JP, Clement DJ, Carnes DL Jr. Cyclic fatigue testing ofnickel-titanium endodontic instruments. J Endod 1997;23(2):77-85.

12. Sattapan B, Palamara JE, Messer HH. Torque during canalinstrumentation using rotary nickel-titanium files. J Endod 200026(3):156-60.

13. Haikel Y, Serfaty R, Bateman G, Singer B, Allemann C. Dynamicand cyclic fatigue of engine driven rotary nickel-titanium endodonticinstruments. J Endod 1999;25(6):434-40.

14. Li UM, Lee BS, Shih CT, Lan WH, Lin CP. Cyclic fatigue onendodontic nickel titanium rotary instruments: static and dynamictests. J Endod 2002;28(6):448-51.

15. Chaves Craveiro de Melo M, Guiomar de Azevedo Bahia M, LopesBuono VT. Fatigue resistance of engine-driven rotary nickel-titaniumendodontic instruments. J Endod 2002;28(11):765-9.

16. Kuhn G, Tavernier B, Jordan L. Influence of structure on nickel-titanium endodontic instruments failure. J Endod 2001;27(8):516-20.

17. Xu X, Eng M, Zheng Y, Eng D. Comparative study of torsionaland bending properties for six models of nickel-titanium root canalinstruments with different cross-sections. J Endod 2006;32(4):372-5.

18. Ullmann CJ, Peters OA. Effects of cyclic fatigue on static fractureloads in ProTaper nickel-titanium rotary instruments. J Endod2005;31(3):183-6.

19. Yao JH, Schwartz SA, Beeson TJ. Cyclic fatigue of three types ofrotary nickel-titanium files in a dynamic model. J Endod 2006;32(1):55-7.



endo file separation discussion

Business