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Effect of previous angular deformation on flexural fatigue resistance of controlled memory nickel-titanium… Aljazaeri, Bassim 2014

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Effect of previous angular deformation on flexural fatigue resistance ofcontrolled memory nickel-titanium endodontic instrumentsbyBassim AljazaeriBDS, King Saud University 1999AEGD, University of Southern California 2002A THESIS SUBMITTED IN PARTIAL FULFILLMENT OFTHE REQUIREMENTS FOR THE DEGREE OFMASTER OF SCIENCEinTHE FACULTY OF GRADUATE AND POSTDOCTORAL STUDIES(Craniofacial Science)THE UNIVERSITY OF BRITISH COLUMBIA(Vancouver)July 2014© Bassim Aljazaeri, 2014iiAbstractObjective: To evaluate the effect of torsional stress preloading angle on fatigue resistance ofTyphoon (TYP) CM instruments. Methodology: TYP NiTi 25/.04, TYP NiTi 40/.04, TYP CM25/.04 and TYP CM 40/.04 were rotated until fracture to obtain the mean angular deflectionaccording to the +-ISO 3630-1 standard. Files were pre-torqued to 25, 50, and 75% of theirelastic limit and then subjected to cyclic loading in a three-point binding device until fracture.The fatigue life was recorded for each file.The fracture surface of each fragment was examinedwith a scanning electron microscope. Results: The angle of rotation at fracture of TYP CM wassignificantly higher than that of TYP instruments (P < 0.05). However, there was no significantdifference between size 40 and size 25 in all types of files. The fatigue resistance of TYP CMwas significantly higher than that of TYP instruments (P < 0.05). Size 25/.04, TYP and TYP CMfiles in all three preloading groups had a significantly lower fatigue life than files with nopreloading (P < 0.05). Size 40/.04 TYP CM files in the 50% and 75% preloading groups had asignificantly lower fatigue life than files in the groups with no preloading (P < 0.05). Thefractured files in the preloading groups showed the typical pattern of fatigue failure.Conclusions: TYP CM files have a higher fatigue resistance than conventional TYP NiTi files,irrespective of the amount of previous torsional stress. Fatigue resistance of TYP CM and TYPinstruments was reduced after torsional stress preloading. Size 25/.04 file fatigue life wasaffected by preloading at lower distortion angles than was size 40/.04 fatigue life.iiiPrefaceThis thesis is an original, unpublished, and independent work by B. Aljazaeri. None ofthe text of the dissertation is taken directly from previously published or collaborative articles.This project was performed under the guidance and supervision of Dr. M Haapasalo andDr. Y. Shen. Bassim Aljazaeri was responsible for all parts of the research, including the fatiguetesting, torque testing, data gathered from such tests, and microscopic image photography. Therelative contribution of the collaborators in this project was: Dr. Bassim Aljazaeri 60%, Dr.Markus Haapasalo 10% and Dr. Ya Shen 30%. Manuscript editing was done by Dr. MarkusHaapasalo and Dr. Ya Shen.This research was supported in part by the Canadian Academy of Endodontics and theAmerican Association of Endodontists.ivTable of ContentsAbstract .........................................................................................................................................iiPreface ..........................................................................................................................................iiiTable of Contents ........................................................................................................................ ivList of Tables................................................................................................................................ viList of Figures ............................................................................................................................ viiList of Abbreviations.................................................................................................................... xAcknowledgements ..................................................................................................................... xiDedication.................................................................................................................................... xiiChapter 1. Introduction and review of the literature ............................................................... 11.1 History of nickel-titanium files…………………………………………………………...11.2 Relationship between the metallurgical properties and mechanical properties of CMNiTi endodontic instruments …………………………………………….....…………….41.3 Instrument fracture…………………...…………………………………………………..91.4 Study rationale ………………..……………………...………………………………….131.5 Aim ………………………………………………………………….……………………13Chapter 2. Methods ……………………………………………………………………………142.1 Sample preparation ……………………………………………………………………..142.2 Torque preloading ……………………………………………….…...……….……….. 142.3 Method to cyclically fatigue files ……………………………………………………….152.4 Fractographic examination ……………………………………………………….…….152.5 Statistical analysis ………………………………………………………………….……15vChapter 3: Results ……………………………………………………………………...………19Chapter 4: Discussion ………………………………………………...………...…………….. 48Chapter 5: Conclusion ……………………………………………………...…………..…….. 52References ………………………………………………………………………………………53viList of TablesTable 3.1 The distortion angles of files subjected to 25%, 50% and 75% of the averagemaximum angular deflection until the files broke ………………...…………..….21Table 3.2 The time of fatigue life (sec) of TYP CM and TYP files ………………………….33viiList of FiguresFigure 1.1 NiTi phase transformation …………………………………..………………………..3Figure 1.2 DSC curves of the TYP and TYP CM NiTi instruments ………………………….....7Figure 1.3 XRD patterns for NiTi TYP and TYP CM instruments at 25ºC ………………...…...7Figure 1.4 Typical DSC curves of CM and SE wires each with a diameter of 1.22 mm ………..8Figure 1.5 Various methods of fatigue testing for NiTi rotary instruments ………………...….12Figure 2.1 TYP CM NiTi instruments ……………………………………………………….…16Figure 2.2 Torsional test for NiTi TYP and TYP CM instruments ………………………….....17Figure 2.3 Three-point bending apparatus for fatigue test ……………………………………...18Figure 3.1 Lateral-view scanning electron micrograph of size 25 TYP files with 25% preloadingof the maximum distortion angle ……………………………………..........................................22Figure 3.2 Lateral-view scanning electron micrograph of size 40 TYP files with 25% preloadingof the maximum distortion angle ………………….…………………………………………….23Figure 3.3 Lateral-view scanning electron micrograph of size 25 TYP CM files with 25%preloading of the maximum distortion angle ………………………………………………...….24Figure 3.4 Lateral-view scanning electron micrograph of size 25 TYP files with 50% preloadingof the maximum distortion angle ……………………………………………………………..…25Figure 3.5 Lateral-view scanning electron micrograph of size 25 TYP CM file with 50%preloading of the maximum distortion angle …………………………………………………....26Figure 3.6 Lateral-view scanning electron micrograph of size 40 TYP CM file with 50%preloading of the maximum distortion angle ……………………………………………………27Figure 3.7 Lateral-view scanning electron micrograph of size 25 TYP file with 75% preloadingof the maximum distortion angle ……………………………………………………………..…28Figure 3.8 Lateral-view scanning electron micrograph of size 40 TYP file with 75% preloadingof the maximum distortion angle ………………………………………………………………..29viiiFigure 3.9 Lateral-view scanning electron micrograph of size 25 TYP CM file with 75%preloading of the maximum distortion angle ……………………………………………………30Figure 3.10 Lateral-view scanning electron micrograph of size 25 TYP CM file with 75%preloading of the maximum distortion angle ……………………………………………………31Figure 3.11 Lateral-view scanning electron micrograph of size 40 TYP CM files with 75%preloading of the maximum distortion angle ………………………………………………....…32Figure 3.12 The time of fatigue life (sec) of TYP CM and TYP files ……………………….…34Figure 3.13 Fracture surfaces of size 25 TYP and TYP CM files after fatigue failure withouttorsional preloading ……………………………………………………………………………..35Figure 3.14 Fracture surfaces of size 25 TYP files after fatigue failure with 25% preloading ofthe maximum distortion angle ………………………………………………………………..…36Figure 3.15 Fracture surfaces of size 25 TYP CM files after fatigue failure with 25% preloadingof the maximum distortion angle …………………………………………………………..……37Figure 3.16 Fracture surfaces of size 40 TYP and TYP CM files after fatigue failure withoutpreloading ………………………………………………………………………………….……38Figure 3.17 Fracture surfaces of size 40 TYP and TYP CM files after fatigue failure with 25%preloading of the maximum distortion angle ……………………………………………………39Figure 3.18 Fracture surfaces of size 25 TYP and TYP CM files after fatigue failure with 50%preloading of the maximum distortion angle…………………………………………….………40Figure 3.19 Fracture surfaces of size 40 TYP and TYP CM files after fatigue failure with 50%preloading of the maximum distortion angle ……………………………………………………41Figure 3.20 Fracture surfaces of size 25 TYP and TYP CM files after fatigue failure with 75%preloading of the maximum distortion angle ……………………………………………………42Figure 3.21 Fracture surfaces of size 40 TYP and TYP CM files after fatigue failure with 75%preloading of the maximum distortion angle ……………………………………………………43Figure 3.22 Fracture surfaces of size 40 TYP and TYP CM instruments after separation byfatigue with the region of fatigue crack propagation and dimple area outlined , nopreloading……………………………………………………………………………………..…44ixFigure 3.23 Fracture surfaces of size 40 TYP  and TYP CM instruments after separation byfatigue with the region of fatigue crack propagation and dimple area outlined, 25% and 75%preloading …………………………………………………………………………………….…45Figure 3.24 Fracture surfaces of size 25 TYP (A) and TYP CM (D) instruments after separationby fatigue with the region of fatigue crack propagation and dimple area outlined, 25% and 75%preloading ………………………………………………………………………………….……46Figure 3.25 Fracture surface after instrument separation by torque of size 25 TYP and TYP CMfiles ………………………………………………………………………………………………47xList of AbbreviationsAf ………………………………………………………………….... Austenite finish temperatureAs …………………………………………………………………….. Austenite start temperatureCM …………………………………………………………………….…Controlled memory wireDSC ………………………………………………………….... Differential scanning calorimetryISO ………………………………………………. International Organization for StandardizationMf …………………………………………………………………. Martensite finish temperaturemNCF …………………………………………………………..Mean number of cycles to failureMs ………………………………………………………………....... Martensite start temperatureNiTi ………………………………………………………………………………. Nickel-titaniumSE ………………………………………………………………………………… Super-elasticityTYP …………………………………………………………….. Typhoon rotary endodontic filesTYP CM ………………………………....... Typhoon controlled-memory rotary endodontic filesXRD …………………………………………………………………………….. X-ray diffractionxiAcknowledgementsI would like to express my appreciation to the UBC Faculty of Dentistry clinical staff,administration, and my classmates for their support and efforts to create an enjoyable learningenvironment throughout the past three years.Thank you to both Dr. Haapasalo and Dr. Shen for your research ideas, mentoring andguidance in my research project. Thank you also to Dr. Ya Shen for helping with the statisticalanalysis. I’d like to thank Dr. Coil who is always there for us, sharing his knowledge andexperience.I would also like to thank Dr. Jolanta Aleksejūnienė for being on my committee and theinput. She had into making this work successful.My appreciation also goes out to Dr. Les Campell for his help in torsional testing and Dr.Tianfeng Du for her help in SEM imaging.xiiDedicationI dedicate this to my amazing family, parents, and my loving wife Alaa who have beensupporting me throughout my education. Without their support my dream would never cametrue. I also dedicate this to Dr. Marwan Abou-Rass who has mentored me since my first days inmy profession.Chapter 1 Introduction and review of the literature1.1 History of nickel-titanium files‘Nitinol’ (NiTi) is a nearly equiatomic intermetallic alloy of nickel and titanium. Its uniqueproperties were first revealed in 1962 at the Naval Ordnance Laboratory – indeed, the term‘Nitinol’ is the acronym for Nickel Titanium Naval Ordnance Laboratory (Wang et al. 1965).The commercial importance of Nitinol is due to its closely related properties of shape memoryand superelasticity (also referred to as pseudoelasticity). Both properties result from the phasetransformation between an austenitic (parent) phase [with a simple cubic B2 (CsCl) structure] toa martensite (daughter) phase (with a monoclinic B19´ structure) (Otsuka & Ren 2005). Sincethe first report of Walia et al. (1988), the superelastic equiatomic NiTi alloy has become the mostpopular material for the manufacturing of endodontic instruments. Walia et al. (1988) found thatsize 15 files which were made by superelastic nitinol orthodontic wire were shown to have twoto three times the elastic flexibility in bending and torsion, as well as superior resistance totorsional fracture when compared to similar stainless-steel instruments. Superelasticity isassociated with the occurrence of a phase transformation of the alloy upon application of stressabove a critical level, which takes place when the ambient temperature is above the so-calledaustenite-finish temperature of the material. This stress-induced martensitic transformationreverses spontaneously upon release of the stress such that the material returns to its originalshape and size (Saburi 1998). This special property manifests as an enhanced elasticity of theNiTi alloy, allowing the material to recover after large strains (or distortion). The superelasticityof NiTi allows deformations of as much as 8% strain to be fully recoverable, in comparison with2a maximum of less than 1% with other alloys such as stainless-steel (Thompson 20002)(Fig.1.1).NiTi endodontic instruments with superelasticity have gained extensive popularity amongstclinicians due to their higher flexibility and greater torsional resistance compared with traditionalinstruments made of stainless-steel (Walia et al. 1988, Thompson 2000). Many conventionalNiTi endodontic files of various geometric design are made of superelastic NiTi alloy, such asthe ProFile (Dentsply Maillefer, Ballaigues, Switzerland) and ProFile GT (Tulsa Dental Products,Tulsa, OK), Mtwo (VDW, Munich, Germany), Hero 642 (Micro-Mega, Geneva, Switzerland),K3 (SybronEndo, Orange, CA), LightSpeed (LightSpeed Technology, Inc., San Antonio, TX),and Quantec (Analytic, Orange, CA). All of these conventional rotary instruments are usedclinically in the austenitic state (at body temperature) (Diemer & Calas 2004, Hülsmann et al.2005, Shen et al. 2013a).While root canal treatment has benefitted from the introduction of rotary NiTi instruments, theirseparation during use has been a concern to clinicians (Pruett et al. 1997, Sattapan et al. 2000,Parashos et al. 2004, Shen et al. 2006, Cheung 2007). NiTi instruments have undergone a designrevolution such that instruments now can cut effectively while exhibiting resistance to fractureeven in the most challenging anatomical confines. The mechanical behavior of NiTi alloy isdetermined by the relative proportions and characteristics of the microstructural phases. Heattreatment (thermal processing) is one of the most fundamental approaches toward adjusting thetransition temperatures of NiTi alloys (Frick et al. 2005, Gutmann & Gao 2012) and affecting thefatigue resistance of NiTi endodontic files. Since 2007, several new thermomechanicalprocessing and manufacturing technologies have been developed to optimize the microstructureof NiTi alloys. Several new thermomechanically processed endodontic NiTi files such as the3HyFlex CM (HyFlex; Coltene Whaledent, Cuyahoga Falls, OH), TYPHOON™ Infinite FlexNiTi (TYP CM; Clinician’s Choice Dental Products, New Milford, CT), K3XF (SybronEndo,Orange, CA), ProFile GT Series X (GTX; Dentsply Tulsa Dental Specialties), ProFile Vortex(Vortex) and Vortex Blue (Dentsply Tulsa), and Twisted Files (TF; SybronEndo) have beenintroduced.Figure 1.1 NiTi phase transformation. (IEJ 2000:297-310)41.2 Relationship between the metallurgical properties and mechanical properties of CMNiTi endodontic instrumentsCM Wire (DS Dental, Johnson City, TN) is a novel NiTi alloy with flexible properties that wasintroduced in 2010. CM NiTi files have been manufactured using a special thermomechanicalprocess that controls the memory of the material, making the files extremely flexible. Thiscontrasts with what is found with conventional superelastic (SE) forms of NiTi. Both HyFlex andTYP are made from CM Wire (Shen et al. 2011b, Peters et al. 2012). They have a lower percentin weight of nickel (52 Ni %wt) than the common 54.5–57 Ni %wt of the great majority ofcommercially available SE NiTi rotary instruments (Zinelis et al. 2010).Shen et al. (2011b) examined the phase transformation behavior and microstructure of TYP CMNiTi instruments and conventional superelastic NiTi instruments. The differential scanningcalorimetry (DSC) results showed that the TYP CM and Vortex instruments had an Aftemperature exceeding 37ºC, whereas the NiTi instruments made from conventional superelasticNiTi wire (TYP) and the TF instruments had Af temperatures substantially below mouthtemperature (from 16ºC - 31ºC) (Fig.1.2). Af (Austenite finish temperature) is the temperature atwhich the transformation from a martensite structure to austenite structure is completed. Thehigher Af temperature of TYP CM instruments which was observed at room temperature with x-ray diffraction (XRD) was consistent with a mixture of austenite and martensite structures (Fig.1.3)5Zhou et al. (2012) used various mechanical methods (tensile, cyclic tensile, and cantileverbending tests) and metallurgical laboratory techniques to compare the mechanical properties andphase transformation behavior of raw CM wires with thermomechanical treatment and SE NiTiwires. The ultimate goal of their research was to provide deep insight into the connectionbetween the microstructural evolution and improved mechanical properties of CM wires. Theinvestigators found that the raw CM wires possessed a relatively higher Af than the SE wires (Fig.1.3). The critical plateau stress and ultimate tensile strength of the CM wires were lower thanthey were for the SE wires, but the maximum strain before fracture of the CM wires was morethan three times higher than it was for the SE wires. The maximum strain of the CM wires beforefracture (58.4% ± 7.5% to 84.7% ± 6.8%) was much higher than it was for that of the SE wires(16.7% ± 3.8% to 27.5% ± 5.4%).NiTi alloy has three distinct microstructural phases: austenite, martensite, and R-phase. |Themartensite phase's crystal structure (known as a monoclinic, or B19' structure) has the uniqueability to undergo limited deformation in some ways without breaking atomic bonds. This typeof deformation is known as twinning, which consists of the rearrangement of atomic planeswithout causing slip, or permanent deformation. During this deformation, the maternsite is ableto undergo about 6–8% strain. When martensite is reverted to austenite by heating, the originalaustenitic structure is restored, regardless of whether the martensite phase was deformed. Thusthe name ‘shape memory’ refers to the fact that the shape of the high temperature austenite phaseis ‘remembered’, even though the alloy is severely deformed at a lower temperature. Traditionalsuperelastic NiTi files are in austenite phase at body temperature. However, CM NiTi files are ina mixture of austenite and martensite phases at body temperature. The martensitic phase of NiTi6has some unique properties that have made it an ideal material for many applications (Davis2000). The martensitic phase transformation has excellent damping characteristics because of theenergy absorption characteristics of its twinned phase structure. Compared with austenite, themartensite favors reducing the risk of file fracture under high stress because it can be plasticallydeformed rather than broken. On the other hand, the martensitic form of NiTi has remarkablefatigue resistance. The instruments of martensite phase can be easily deformed, yet they willrecover their shape on heating above the transformation temperatures. Hence, it is not surprisingthat NiTi instruments made from CM wires were significantly more resistant to fatigue failurethan instruments made from conventional NiTi wire: Instruments made from CM Wire (TYPCM and DSSS0250425NEYY CM [NEYY CM]) were nearly 300%–900% more resistant tofatigue failure in a 3-point bending device than instruments made from conventional NiTi wirewith the same design in a dry enviorment (Shen et al. 2011a) as well as under various otherconditions (Shen et al. 2012a) in a 3-point bending device.Peters et al. (2012) evaluated torsional and fatigue limits, as well as torque during canalpreparation of HyFlex (CM wire) instruments. They found that HyFlex rotary instruments arebendable and flexible and have similar torsional resistance compared to instruments made ofconventional NiTi. Fatigue resistance is much higher, and torque during preparation is less,compared to other rotary instruments tested previously under similar conditions.7Figure 1.2 DSC curves of the TYP and TYP CM NiTi instruments. Heating (upper) and cooling(lower) curves are shown. (JOE 2011:1566-71)Figure 1.3 XRD patterns for NiTi TYP and TYP CM instruments at 25ºC. (JOE 2011:1566-71)8Figure 1.4 (A) DSC curves of raw and heat-treated (HT) SE and CM wires with diameter of 1.22mm. (B) DSC curves of raw SE wires with diameter of 1.22 mm and 0.64 mm. (C) Tensilestress-strain curves of raw CM and SE wires with diameter of 1.22 mm. Test was conducted atroom temperature and oral temperature (37ºC). (D) Tensile stress-strain response of SE and CMwires during loading-unloading process performed at room temperature. (E) Flexural load-deflection curves of raw SE wires. (F) Flexural load-deflection curves of raw CM wires. Testswere conducted at room temperature (RT, 23ºC ± 2ºC), oral temperature (37ºC), and 60ºC. (JOE2012; 1535-40)91.3 Instrument fractureIt has been reported that fracture of a NiTi instrument may occur from either torsional or flexuralfatigue or a combination of the two (Sattapan et al. 2000; Cheung et al. 2005; Wei et al. 2007).The incidence of conventional superelastic NiTi instrument fracture in clinical practice for filesused multiple times has varied from 3% to 21% (Sattapan et al. 2000, Parashos et al. 2004, Penget al. 2005, Alapati et al. 2005, Shen et al. 2006). Although fractured instruments may notcompromise the outcome if the treatment is performed to a high standard (Spili et al. 2005), theretained file fragments may impede microbial control beyond the obstruction. Moreover,excessive removal of the tooth structure in an attempt to retrieve an instrument fragment may beassociated with root perforation and reduced root strength (Souter & Messer 2005).Torsional fracture occurs when the torque resulting from the contact between the instrument andcanal wall exceeds the torsional strength of the instrument or when the instrument tip is locked ina canal while the rest continues to rotate. Fracture caused by flexural fatigue occurs when arotary endodontic instrument that has already been weakened by metal fatigue is placed understress. A rotary instrument not only needs to be resistant to cyclic fatigue by having sufficientflexibility to rotate in curved root canals, but also possess sufficient shear strength to resisttorsional failure. A variety of experiments with different methods have been developed forstudying the properties of NiTi instruments (Plotino et al. 2009, Shen & Cheung 2013). However,the results of these various studies can hardly be compared with one another because there weremany uncontrolled variables in these experiments. To date, there is no specification or10international standard for testing the cyclic fatigue or torsional resistance of endodontic rotaryinstruments (Shen et al. 2013a).All of the previous studies attempted to simulate an instrument rotating with a curvature and thendetermine how long such an instrument would last before fatigue fracture occurred. Theoretically,the device for fatigue tests should confine the rotary file into a precise trajectory, in terms of theradius and angle of the curvature and the location of the maximum curvature. Four methods ofimposing a curvature on a rotating NiTi instrument have been described: (i) curved metal tube(or a hypodermic needle); (ii) grooved block-and-rod assembly; (iii) rotation against an inclinedplane; and (iv) three-point bend of a rotating instrument (Cheung 2009) (Fig.1.4). A three-pointbending device is a rather new method in the endodontic literature although the principle haslong been used in an engineering context (Eggeler et al. 2004). It has been used to impose acurvature on the rotating instrument (Cheung & Darvell 2007, Cheung et al. 2007a, Zinelis et al.2007); with this method, the strain amplitude on the surface of the instrument may be estimatedfor each and every specimen. The strain-life approach simulates the clinical situation; therefore,it is considered an appropriate means for examining the fatigue behavior of NiTi rotary files(Cheung & Darvell 2007, Cheung et al. 2007a & b, Shen et al. 2011a & 2012a). This test methodallows the experiment to be carried out in various environments (Shen et al. 2012a & 2014).The measurement of the torsional strength of root canal instruments is typically performed in atorsiometer according to the procedure described by the American Dental Associationspecification #28 (ANSI/ADA Specification No. 28, 2002). The ISO/ANSI specifications have11prescribed a test method for (stainless-steel) root canal reamers and files in which 3mm of the tipof the instrument is rigidly fixed and subjected to twisting in a clockwise or a counterclockwisedirection A wide variety of rotary NiTi instruments have been tested in this manner. However,torsional failure due to such a monotonic condition rarely occurs clinically. Both the ADAmethod (ANSI/ADA Specification No. 28, 2002) and ISO protocol #3630-1 (InternationalOrganization for Standardization 2008) only simulate torsional bending in straight canals, as thetorque is measured in relation to the rotation axis. However, rotary instruments are subject tovarying loads in actual clinical situations, with fractures likely to be the result of a combinationof repetitive flexural and torsional stresses.12Figure 1.5 Various methods of fatigue testing for NiTi rotary instruments reported in theliterature and the estimation of surface strain in each case: (a) curved metal tube; (b) groovedblock-and-rod; (c) inclined plane; and (d) rotation with a three-point bend. (Endod Topics2009:1-26)131.4 Study rationaleIt is known that rotary instruments experience both cyclic fatigue and torsional stresssimultaneously when actively cutting dentin in curved canals. Cyclic fatigue of NiTi rotaryinstruments has been studied extensively under simulated conditions (Plotino et al. 2009, Cheung2009, Shen & Cheung 2013). The fatigue resistance of TYP CM files has been evaluated as anisolated process (Shen et al. 2011a, 2012a, Peters et al. 2012). Limited information about thetorsional resistance of thermomechanically treated TYP CM files is available. Recently, thetorsional property of another CM instrument (HyFlex CM) manufactured from a similar alloyhas been reported (Peters et al. 2012). Currently, Campbell et al. (2014) evaluated the effect ofcyclic fatigue on torsional failure of TYP CM instruments. However, how previous torsionalloading may affect the properties of TYPCM instruments is not known.The null hypothesis tested was that fatigue resistance of TYP CM instruments is not affected bytorsional stress preloading.1.5 AimTo evaluate the effect of torsional stress preloading angle on fatigue resistance of Typhoon (TYP)CM instruments.14Chapter 2: Methods2.1 Sample preparationTyphoon™ (Clinician’s Choice Dental Products, New Milford CT) rotary endodontic files wereselected for this study because the same cross sectional design was found in both the NiTi andCM files (Fig. 1.2).To evaluate the effect that torsional preloading may have on cyclic fatigue, torsional preloadingwas done on the files under three conditions. Twelve (12) unused instruments in each group wereexposed to either 25%, 50%, or 75% of their respective mean distortion angle at torsionalfracture (Table 3.1). After preloading to various extents, fatigue tests were performed to measurethe fatigue life.2.2 Torque preloadingThe torsion tests were performed based on International Organization for Standardization ISO3630-1 (International Organization for Standardization, 2008) using a torsion machine: 3 mm ofthe instrument tip was secured firmly in a specifically designed soft brass holder. The apparatuswas composed of a torque sensor (Futek Model TFF 400, Futek, CA, USA) and a low-speedrotating motor (Fig. 2.2). The instrument's shank was then rotated at 2 rpm until fracturesoccurred. Before testing, each instrument handle was removed at the point where the handle isattached to the shaft. The end of the shaft was clamped into a chuck connected to a reversiblegeared motor. The torsional load and distortion angular were recorded until the instrument broke(Ullmann & Peters 2005; Gao et al. 2012; Lopes et al. 2013, Campbell et al. 2014).152.3 Method to Cyclically Fatigue FilesFor the determination of resistance to cyclic fatigue, unused size 25/.04 and size 40/.04 TYP andTYP CM instruments (n = 12 in each group) were placed in a 3-point bending apparatus with a14 mm radius and 45° curve in deionized water (Shen et al. 2011a & 2012a) (Fig. 2.3). Briefly,each NiTi instrument was constrained to a curve by 3 rigid, stainless-steel pins; a calibrateddigital photograph of the curvature was taken. Only a 16-mm length from the tip of theinstrument was immersed in deionized water at the temperature of 23 ± 2 ºC. The instrumentswere then rotated at 500 rpm until fracture occurred to determine baseline scores (Table 3.2).The fatigue life, or the total seconds to failure, was recorded.2.4 Fractographic ExaminationThe fracture surfaces of all fragments were examined under a scanning electron microscope(SEM; Stereoscan 260; Cambridge Instruments, Cambridge, UK). In instruments that failed dueto fatigue only, the number of crack origin(s) for each specimen was recorded. The region inwhich the dimple area could be found was outlined on the photomicrograph for fatigue failuregroups, and measured with ImageJ 1.4 g software (National Institutes of Health, Bethesda, MD)on each photomicrograph (Shen et al. 2011a & 2012a).2.5 Statistical analysisThe results were analyzed with a two-way ANOVA and post hoc analysis using software (SPSSfor Windows 11.0, SPSS, Chicago, IL) at a significance level of P < 0.05.16Figure 2.1 TYP CM NiTi instruments. The working parts of the files have been bent intospecific curved positions, which is not possible with conventional superelastic NiTi files.17Figure 2.2 Torsional test for NiTi TYP and TYP CM instruments. (Courtesy Y. Shen)18Figure 2.3 Three-point bending apparatus for fatigue tests of NiTi TYP and TYP CMinstruments.19Chapter 3: ResultsThe angle of rotation at fracture of TYP CM was significantly higher than that of TYPinstruments (two-way ANOVA; P < 0.05) (Table 3.1). However, there was no significantdifference between size 40 and size 25. The fatigue resistance of TYP CM was also significantlyhigher than that of TYP instruments (two-way ANOVA; P < 0.05)(Table 3.2). Size #25 files hada higher fatigue resistance than size #40 files (post hoc analysis; P < 0.05).TYP and TYP CM size 25 files preloaded with torsional stress had a significantly lower fatiguelife than files in the groups without preloading (post hoc analysis; P < 0.05) (Table 3.2), evenwith a small amount torsional preloading. There was no significant difference in fatigueresistance among the 25%, 50% and 75% preloading groups. In size 40/.04, TYP CM files in the50% preloading group had a significantly lower fatigue life than files without preloading (posthoc analysis; P < 0.05). However, there was no significant difference in fatigue resistance ofconventional TYP file size 40 with or without preloading.There was little difference in the longitudinal or lateral view between new and 25% torsionallypreloaded TYP files (Fig. 3.1 & 3.2): the lateral aspect of the preloaded file did not show anyspecific topographic features. In TYP CM size 25 and 40 only some files had a slight plasticdeformation after 25% torsional preloading (Fig. 3.3). In the 50% torsionally preloaded group,plastic deformation occurred in size 25 and 40 TYP CM files as well as in TYP size 25 files (Fig.3.4 - 3.6). After 75% torsional preloading, all files had plastic deformation 2-4 mm away fromthe instrument tip in the lateral view (Fig. 3.7 - 3.11). There were numerous microcracks on the20plastic deformation area (Fig. 3.7 - 3.11). Of particular interest, the microcracks did not seem tofollow the machining grooves on the instrument surface, but rather ran irregularly (Fig. 3.7 -3.11). The length of the fractured piece ranged from 2.7 - 3.3 mm.Fractographically, in instruments failed by fatigue only or fatigue after torsional loading, thecrack origins and areas showing microscopic fatigue-striations and dimple rupture could beidentified on all fracture surfaces (Fig. 3.13 - 3.24). Most TYP NiTi instruments (10/12 for size25 and 9/12 for size 40) had a single crack origin, while TYP CM files had a higher number ofmultiple crack origins than TYP files. The areas occupied by the dimple region of the totalsurface area of the fractured cross-sections in size 40 files were significantly larger in TYPinstruments than in TYP CM (Fig. 3.23) files without preloading (70.7 ± 12.1 for TYP vs. 24.6 ±9.8 for TYP CM) and files preloaded with torsional stress (71.6 ± 10.7 for TYP vs. 22.5 ± 11.2for TYP CM) (P < 0.05). In TYP files size 25, the areas occupied by the dimple region of thetotal surface area of the fractured cross-sections files in without preloading were slightly larger(35.2 ± 9.9) than in TYP CM files (18.9 ± 8.2) (Fig. 3.24). There was no significant differencebetween TYP files with (30.7 ± 8.2) and without preloading (35.2 ± 9.9). The fractographycorresponded to the torsional failure and showed the torsional fracture pattern with circularabrasion marks and skewed dimples near the center of rotation (Fig. 3.25).21Table 3.1 The distortion angles (in degrees) of files (mean ± S.D.) subjected to 25%, 50% and75% of the average maximum angular deflection until the file broken. Different superscriptsletters indicate a statistically significant difference at P < 0.05 (post hoc analysis).22Figure 3.1 Lateral-view scanning electron micrograph of size 25 TYP files with 25% preloadingof the maximum distortion angle (A - C).23Figure 3.2 Lateral-view scanning electron micrograph of size 40 TYP files with 25% preloadingof the maximum distortion angle (A); (B) high magnification view of the bottom file seen in (A);(C) high magnification view of (B).24Figure 3.3 Lateral-view scanning electron micrograph of size 25 TYP CM files with 25%preloading of the maximum distortion angle (A); (B) high magnification view of (A); size 40TYP CM files with 25% preloading of the maximum distortion angle (C); (D) high magnificationview of the bottom file seen in (C).25Figure 3.4 Lateral-view scanning electron micrograph of size 25 TYP files with 50% preloadingof the maximum distortion angle (A); (B) high magnification view of the top file seen in (A);size 40 TYP file with 50% preloading of the maximum distortion angle (C); (D) highmagnification view of (C).26Figure 3.5 Lateral-view scanning electron micrograph of a size 25 TYP CM file with 50%preloading of the maximum distortion angle (A); (B-D) high magnification view of (A).27Figure 3.6 Lateral-view scanning electron micrograph of a size 40 TYP CM file with 50%preloading of the maximum distortion angle (A); (B) high magnification view of the bottom fileshown in (A); (C) size 40 TYP CM file with 50% preloading of the maximum distortion angle;(D) high magnification view of (C).28Figure 3.7 Lateral-view scanning electron micrograph of a size 25 TYP file with 75%preloading of the maximum distortion angle (A); (B-D) high magnification view of (A).29Figure 3.8 Lateral-view scanning electron micrograph of a size 40 TYP file with 75%preloading of the maximum distortion angle (A); (B-D) high magnification view of (A).30Figure 3.9 Lateral-view scanning electron micrograph of a size 25 TYP CM file with 75%preloading of the maximum distortion angle (A); (B) high magnification view of the top file seenin (A); (C & D) high magnification view of (B).31Figure 3.10 Lateral-view scanning electron micrograph of a size 25 TYP CM file with 75%preloading of the maximum distortion angle (A); (B) high magnification view of the bottom fileseen in (A); (C & D) high magnification view of (B).32Figure 3.11 Lateral-view scanning electron micrograph of size 40 TYP CM files with 75%preloading of the maximum distortion angle (A); (B) high magnification view of the top file seenin (A); (C) high magnification view of the bottom file seen in (A); (D) high magnification viewof (C).33Table 3.2 The time of fatigue life (in seconds) of TYP CM and TYP files (mean ± S. D.) untilfracture at a curvature of 40º with a 14 mm radius in water conditions after being exposed totorsional stress at 25%, 50% and 75% preloading of the maximum distortion angle beforefracture. Different superscripts letters indicate a statistically significant difference at P < 0.05(post hoc analysis).Files Preloading of distortion angles0 25% 50% 75%TYP 25/.04 852 ± 212a 317 ± 115b 483 ± 131bc 354 ± 93bTYP CM 25/.04 3043 ± 579b 796 ± 226a 814 ± 173a 474 ± 120bcTYP 40/.04 184 ± 57b 235 ± 65b 254 ± 26b 184 ± 45TYP CM 40/.04 901 ± 206a 1070 ± 198 524 ± 68c 429 ± 120c(sec) (sec) (sec) (sec)(sec) (sec) (sec) (sec)(sec) (sec) (sec) (sec)(sec) (sec) (sec) (sec)34Figure 3.12 The time of fatigue life (sec) of TYP CM and TYP files until fracture at a curvatureof 40º with a 14 mm radius in water conditions after being exposed to torsional stress at 25%, 50%and 75% preloading of the maximum distortion angle before fracture. Different superscriptsindicate statistically significant difference (P < 0.05).Seconds35Figure 3.13 Fracture surfaces of size 25 TYP (A & B) and TYP CM (C & D) files after fatiguefailure without torsional preloading.36Figure 3.14 Fracture surfaces of size 25 TYP files after fatigue failure with 25% preloading ofthe maximum distortion angle.37Figure 3.15 Fracture surfaces of size 25 TYP CM files after fatigue failure with 25% preloadingof the maximum distortion angle.38Figure 3.16 Fracture surfaces of size 40 TYP (A & B) and TYP CM (C & D) files after fatiguefailure without preloading.39Figure 3.17 Fracture surfaces of size 40 TYP (A & B) and TYP CM (C & D) files after fatiguefailure with 25% preloading of the maximum distortion angle.40Figure 3.18 Fracture surfaces of size 25 TYP (A & B) and TYP CM (C & D) files after fatiguefailure with 50% preloading of the maximum distortion angle.41Figure 3.19 Fracture surfaces of size 40 TYP (A & B) and TYP CM (C & D) files after fatiguefailure with 50% preloading of the maximum distortion angle.42Figure 3.20 Fracture surfaces of size 25 TYP (A & B) and TYP CM (C & D) files after fatiguefailure with 75% preloading of the maximum distortion angle.43Figure 3.21 Fracture surfaces of size 40 TYP (A & B) and TYP CM (C & D) files after fatiguefailure with 75% preloading of the maximum distortion angle.44Figure 3.22 Fracture surfaces of size 40 TYP (A) and TYP CM (B) instruments after separationby fatigue with the region of fatigue crack propagation and dimple area outlined (dotted line).45Figure 3.23 Fracture surfaces of size 40 TYP (A) and TYP CM (D) instruments after separationdue to fatigue with the region of fatigue crack propagation and dimple area outlined (dotted line);TYP after fatigue failure with 25% (B) and 75% (C) preloading of the maximum distortion angle;TYP CM after fatigue failure with 25% (E) and 75% (F) preloading of the maximum distortionangle.46Figure 3.24 Fracture surfaces of size 25 TYP (A) and TYP CM (D) instruments after separationdue to fatigue with the region of fatigue crack propagation and dimple area outlined (dotted line);TYP after fatigue failure with 25% (B) and 75% (C) preloading of the maximum distortion angle;TYP CM after fatigue failure with 25% (E) and 75% (F) preloading of the maximum distortionangle.47Figure 3.25 Fracture surface after instrument separation due to torque of size 25 TYP (A) andTYP CM (B) files.48Chapter 4: DiscussionRotational bending and torsional stress will both develop in a rotary instrument during clinicalsituations. Kim et al. (2012) showed that cyclic fatigue had a significant effect on torsionalfracture resistance on conventional superelastic ProFile (Dentsply Tulsa Dental) and ProTaper(Dentsply Tulsa Dental) instruments. Later, Cheung et al. (2013) studied the effect of torsionalpreloading on the cyclic fatigue life of ProFile size 25/.06 and ProTaper F1 rotary instruments.Recently, Campbell et al. (2014) evaluated the effect of cyclic fatigue on torsional failure ofthermomechanically treated TYP CM instruments. The aim of this study was to evaluate theeffect of torsional stress preloading angle on fatigue resistance of Typhoon (TYP) CMinstruments and TYP instruments of two sizes (25/.04 and 40/.04) in water conditions.Typhoon CM rotary instruments were recently characterized by a high austenitic finishtemperature (Af) of approximately 55ºC, indicating that at body temperature, the instrumentwould contain a significant proportion of martensitic alloy (Shen et al. 2011b, Zhou et al. 2012).The martensitic form of NiTi has high resistance to fatigue. Therefore, it was not surprising thatthe TYP CM instruments were more resistant to cyclic fatigue than the TYP instruments both inair and in water. An instrument should be resistant to cyclic fatigue and have sufficient flexibilityto permit the preparation of curved systems but also sufficient torque strength so that instrumentseparation does not occur. As a general rule, while being resistant to cyclic fatigue, flexibleinstruments have been assumed to be less resistant to torsional load than stiff instruments.Wycoff and Berzins (2012) found that the post-twisting Twisted Files displayed the least amount49of torsional stress resistance and the highest angular deflection at fracture compared withtraditionally manufactured NiTi files of a similar cross-sectional design. Their findings supportthe results of the present study where TYP CM files had a higher maximum angular deflection attorsional fracture than instruments made of superelastic NiTi (TYP files).The influence of previous torsional angular deformation on the flexural fatigue life onconventional NiTi files has also been studied (Galvão et al. 2007, Bahia et al. 2008). Unused K3instruments were submitted to a pre-defined rotation before the flexural fatigue test. The resultsindicated that as the prior angular deformation increases, the number of cycles attained underflexural fatigue condition decreases (Galvão et al. 2007). A reduction in the fatigue resistancewas registered even with prior torsional loads below the elastic limit of the material (as low as90° angular deformation). However, Cheung et al. (2013) found that the torsional preloadswithin the superelastic limit of the material may improve the cyclic fatigue resistance ofconventional NiTi instruments. The 50% and 75% torsionally preloaded ProFile and all ProTaperfiles had a higher number of cycles to failure than files without preloading. Comparisonsbetween studies, however, cannot be directly made because different instruments, materials,sample sizes and methodologies were used.Shen et al. (2014) evaluated the effect of and torsional preloading on the cyclic fatigue life ofheat-treated size 25/.06 K3XF NiTi instruments. They found that a slight torsional pre-loadingreduced the flexural fatigue resistance of K3 and K3XF files. This is in agreement with thepresent study: the fatigue life of TYP CM and TYP files of size 25/.04 in torsionally preloaded50files was lower than in files without preloading, even with as little as 25% torsional preloading.This behavior could be associated with the generation of surface defects during torsional pre-loading, which can act as crack nucleation sites during flexural fatigue. Any residual stress afterthe torsional preloading may interact with the propagating fatigue crack and manifest asbranching of cracks from the fracture site. It seems that torsional overloads would act in tandemwith flexural fatigue to reduce the resistance of NiTi files to failure in clinical situations.However, in large size files (40/.04), despite the previous history of torsional stress application,the fatigue resistance was not affected by the magnitude of the torsional preloads in conventionalTYP NiTi instruments; the fatigue resistance of TYP CM files was reduced only after 50% and75% torsional preloading. In Campbell et al. (2014) study, cyclic fatigue was not detrimental tothe file’s ability to withstand the torsional load of TYP and TYP CM files of size 25/.04.However, in the larger size (40/.04), the 75% preloading TYP instruments had reduced torsionalstrength; precycling of TYP CM instruments showed slight reduction in the instrument’sdistortion angle, but there was no correlation with the amount of preloading.The fatigue life can be expressed as the number of loading cycles required to initiate a fatiguecrack and to propagate the crack to a critical size. With continued cyclic loading, the growth ofthe dominant crack or cracks will continue until the remaining uncracked section of thecomponent no longer can support the load (Cheung et al. 2005). At this point, the fracturetoughness is exceeded, and the remaining cross-section of the material experiences rapid fracture.This rapid overload fracture is the third stage of fatigue failure, which manifests as the dimpleregion. The area occupied by the crack growth region (or dimple region) has been previouslyexamined quantitatively (Shen et al. 2011a, 2012a, 2014). Theoretically, the continuing51reduction in the net area of the remaining intact section because of the progressive propagationof a fatigue crack would lower the load-bearing capacity of the part to such an extent that itfractures in the next load cycle as a result of simple overload. In the present study, the relativesizes of the fractioned areas occupied by the dimple region in size 40 TYP CM instruments withand without preloading were significantly smaller than in TYP instruments. One explanation isthat the fatigue resistance of size 40 TYP CM files is significantly higher than that of TYPinstruments. Interestingly, the relative size of the dimple area was smaller also in size 25 TYPCM files without preloading than in files with preloading because the fatigue resistance of TYPCM files was 3 - 4 times higher than that of conventional superelastic TYP NiTi files.52Chapter 5: ConclusionThe fatigue resistance of TYP CM files was significantly higher than that of TYP files.  The size25 files had a higher fatigue resistance than size 40 files. The angle of rotation at fracture of TYPCM files was significantly higher than that of TYP files. However, while the difference was clearbetween the two groups (CM and conventional NiTi), there was no significant differencebetween size 40 and size 25 files within each group. Fatigue resistance of TYP CM and TYPinstruments seems to be affected by preloading of distortion angles in the smaller size 25/.04files, even with a small amount (25%) torsional preloading. However, there was no significantdifference in fatigue resistance of size 40 TYP files with and without torsional preloading. TYPCM files of size 40/.04 in the 50% preloading group had a significantly lower fatigue life thanthe files in the groups with no preloading. The relative size of the fractured area occupied by thedimples was significantly smaller in size 40 TYP CM instruments than in TYP instrumentsirrespective of preloading. The size of the dimple area was smaller also in size 25 TYP CM fileswithout preloading than in files with preloading.53ReferencesAlapati SB, Brantley WA, Svec TA, Powers JM, Nusstein JM, Daehn GS (2005) SEMobservations of nickel-titanium rotary endodontic instruments that fractured during clinical use.J Endod 31: 40-3.ANSI/ADA Specification No. 28. Root canal files and reamers, type K for hand use. Chicago, IL:American Dental Association, 2002.Bahia MG, Melo MC, Buono VT (2008) Influence of cyclic torsional loading on the fatigueresistance of K3 instruments. Int Endod J 10: 883-91.Cheung GS, Peng B, Bian Z, Shen Y, Darvell BW (2005) Defects in ProTaper S1 instrumentsafter clinical use: fractographic examination. Int Endod J 38: 802-9.Cheung GS, Darvell BW (2007) Low-cycle fatigue of NiTi rotary instruments of various cross-sectional shapes. Int Endod J 40: 626-32.Cheung GS, Shen Y, Darvell BW (2007a) Does electropolishing improve the low-cycle fatiguebehavior of a nickel-titanium rotary instrument in hypochlorite? J Endod 33: 1217-21.Cheung GS, Shen Y, Darvell BW (2007b) Effect of environment on low-cycle fatigue of anickel-titanium instrument. J Endod 33: 1433-7.Cheung GSP (2009) Instrument fracture: mechanisms, removal of fragments, and clinicaloutcomes. 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