An Sem Investigation Into the Effects of Clinical Use on Heat-Treated Nickel-Titanium Rotary Endodontic Files

AN SEM INVESTIGATION INTO THE EFFECTS OF CLINICAL USE ON HEAT-TREATED NICKEL-TITANIUM ROTARY ENDODONTIC FILES

A Thesis

Presented in Partial Fulfillment of the Requirements for the Degree of Master of Science in the Graduate School of the Ohio State University

Thomas Gerard Burke Jr., D.D.S.

Graduate Program in Dentistry

The Ohio State University

2016

Master’s Examination Committee:

John Nusstein, D.D.S., M.S., Advisor

Melissa Drum, D.D.S., M.S.

Sara Fowler, D.M.D., M.S.

William Brantley, B.S., M.S., PhD

John Draper, PhD

Thomas Gerard Burke Jr., D.D.S.

2016

ABSTRACT

Introduction: The third generation of NiTi based rotary files involves a new proprietary thermomechanical process which improves fatigue resistance and flexibility. The purpose of this study was to evaluate the qualitative and quantitative wear/fatigue of three third-generation NiTi rotary endodontic file systems following use in extracted posterior human teeth in a simulated clinical model.

Materials and Methods: One hundred files (five packs of five files across four brands: ProFile Vortex™, Vortex Blue™, ProTaper Gold™, and Coltene HyFlex® CM™) were examined with an SEM at the same four positions along their length using a custom-fabricated sample jig before use and through three clinical simulations, and were then evaluated for wear and deformation. Files were categorized as usable, microscopically unacceptable and visually unacceptable.

Results: Twelve files visibly failed throughout this study (three separated and eight plastically deformed), and the majority of these failures occurred in the first use of the file. Many instruments were evaluated as microscopically unacceptable before use. A repeated-measures logistic regression analysis found no significant effects for brand, amount of use, and their interaction for files that remained visibly useful. Of those that rated microscopically useful, a significant effect was seen for number of uses (p=0.0127). Contrasts of uses showed significance for use 0 vs. 1, 0 vs. 2, and 0 vs. 3, indicating that unused files were more likely to be microscopically acceptable than used files. No significance was seen between contrasts of uses 1, 2, or 3.

Summary and Conclusions: No quantitative wear or distortion pattern specific to any brand was observed that limited multiple file uses. Poor surface condition observed microscopically is not necessarily a precursor to instrument failure. Third-generation controlled memory instruments tend to deform plastically and present minimal risk of clinical separation and canal obstruction through multiple uses.

DEDICATION

To Mom and Dad- for showing me what hard work looks like, how it pays off, and all the fun stuff you get to do when it does. To Kate and Pat- your baby brother is finally done with school. To Erin- I’ll tell you in person, every day.

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ACKNOWLEDGMENTS

Dr. Nusstein- I pity the fool that has not had the pleasure of working with you. Dr. Drum- Thanks for talking me into this. Your juggling act amazes me every day. Dr. Fowler- After long hours in predoc, thank you for keeping me calm and allowing me a lunch break every now and then. Dr. Brantley- With how often you appear in my reference section, I could have no better person supporting this project. Dr. Draper- Your willingness to understand dentistry encouraged me to understand statistics. Dr. Reader- You are one of the biggest reasons why I am proud to tell my colleagues where I learned endo. Dr. Beck- I cannot thank you enough for the hours you put into this project, it was a pleasure to have you on board. To Chase, Hannah, and Daniel- There are no other people I would have rather gone through this with. Thank you all!

VITA

September 8, 1987 ...... Born: Parma, Ohio

2006-2009 ...... The Ohio State University Fisher College of Business

2013...... Doctor of Dental Surgery, The Ohio State University Columbus, Ohio

2016...... Master of Science & Specialization in Endodontics Post-Doctoral Certificate, The Ohio State University, Columbus, Ohio

FIELD OF STUDY Major Field: Dentistry Specialization: Endodontics

TABLE OF CONTENTS Page Abstract ...... ii Dedication ...... iii Acknowledgements ...... iv Vita ...... v Table of Contents ...... vi List of Tables ...... vii List of Figures ...... viii Chapters: 1. Introduction ...... 1 2. Materials and Methods ...... 16 4. Results ...... 22 5. Discussion ...... 27 6. Summary and Conclusions ...... 42 Appendices: A. Tables ...... 45 B. Figures ...... 60 H. References ...... 81

LIST OF TABLES Table Page 1. Visual File Failures (Score of 5) by Brand ...... 46 2. Individual file failures (Visual failures)...... 47 3. File Brand by Canal Curvatures Experienced...... 48 4. ProFile Vortex™ Evaluation Summary ...... 49 5. ProTaper Gold™ Evaluation Summary ...... 50 6. Vortex Blue™ Evaluation Summary ...... 51 7. Coltene HyFlex® CM™ Evaluation Summary ...... 52 8. Visually Unacceptable (Score of 5) Files by Use ...... 53 9. Microscopically Unacceptable (Score of 4) Files by Use...... 54 10. Clinically Usable (Scores 1-3) Files by Use ...... 55 11. Regression Results for Visually Acceptable (Scores 1-4) Files ...... 56 12. Regression Results for Microscopically Acceptable (Scores 1-3) Files ...... 57 13. Microscopically Acceptable (Scores 1-3) Regression Contrasts ...... 58 14. Microscopically Acceptable (Scores 1-3) Odds Ratios ...... 59

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LIST OF FIGURES Table Page 1. Unit cells ...... 61 2. Schematic illustration of atom displacement ...... 62 3. SEM images of the cross-sectional geometry ...... 63 4. Austenite to martensite phase transformation ...... 64 5. Quanta 200 ...... 65 6. The custom-made aluminum jig (empty) ...... 66 7. The custom-made aluminum jig (with files) ...... 67 8. Evaluation summary by brand and use ...... 68 9. Visual file status by brand and number of uses ...... 69 10. Microscopic file status by brand and number of uses ...... 70 11. The D1 position of a Coltene® HyFlex™ file through each use ...... 71 12. A size 25.04 Vortex Blue™ instrument that separated during its first use ...... 72 13. A size 30.04 Coltene HyFlex® CM™ file that failed by plastic deformation ....73 14. A size S2 ProTaper Gold™ file that failed by plastic deformation ...... 74 15. A size 25.04 ProFile Vortex™ instrument that separated during its first use ....75 16. A size 30.04 Vortex Blue™ file experiencing progressive pitting ...... 76 17. The size 30.04 Vortex Blue™ after use 3 (surface pitting) ...... 77 18. An unused size 30.04 Vortex Blue™ file ...... 78 19. An unused size 30.04 ProFile Vortex™ file ...... 79 20. A size 40.04 Vortex Blue™ file (removal of edge rollover) ...... 80

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INTRODUCTION

The shape and degree of root canal curvature are significant obstacles that impose limitations on successful endodontic cleaning and shaping of root canals. The ability of a clinician to adequately negotiate the canal system is coincident with their ability to adequately clean the canal system (1). The introduction of nickel-titanium (NiTi) endodontic files has offered unseen flexibility when instrumenting curved canals while reducing the potential for file separation and canal transportation compared to their stainless steel predecessors (2).

Development of Nickel-Titanium for Dental Use

The near-equiatomic nickel-titanium alloy (NiTi) was first invented at the Naval

Ordnance Laboratory in Silver Spring, Maryland for use in missile manufacturing. The material proved effective as a missile nose cone, showing an improved ability to resist fatigue, heat, and impact (97). From its discovery and use in missile manufacturing, the advantages of NiTi for orthodontics were described initially in detail by Andreasen and

Morrow (3). When used as an orthodontic wire, Nitinol (Nickel-Titanium Naval

Ordinance Laboratory), as it is commonly known, can show remarkable “shape memory” characteristics. It can be manipulated and deformed but has the ability to return to its original shape when heated to a specific temperature. The originally marketed Nitinol orthodontic wire did not possess shape memory, but had the advantages of very low 1

elastic modulus and wide elastic working range. Clinically, these properties would correlate to fewer orthodontic archwire adjustments and shorter treatment time needed to accomplish tooth rotations and arch leveling (3,4).

The application of Nitinol to endodontics was first described in 1975 by Civjan et al. (5) and in 1988 by Walia et al (6). Unlike stainless steel files, which are normally manufactured by twisting a pre-milled wire to its desired design, Walia et al. (6) employed a unique manufacturing process of milling the desired file fluting from round

0.02 inch diameter orthodontic NiTi archwire blanks. The archwires were machined with a triangular cross-sectional geometry to match the size and geometry of #15 stainless steel K files, which were also fabricated by machining wire blanks. In this way, Walia and co-authors (6) could compare the performance characteristics and properties of stainless steel files with those of nickel-titanium files, using the same manufacturing technique. Limitations in material capabilities of stainless steel endodontic files have led to inadvertent procedural errors such as file separations, zipped canals (where the root canal constricts short of the natural apical foramen and then widens towards the apex due to instrument preparation), and canal perforations (7). Walia et al. (6) showed Nitinol files to have two to three times the elastic flexibility and superior torsional fracture resistance (the number of rotations needed to fracture a file when its tip is in a fixed position) compared to stainless steel files. In regard to torsional fracture resistance,

Nitinol files were able to complete 2.5 revolutions on average before fracturing, compared to 1.75 revolutions with stainless steel files. They speculated that these improved mechanical properties, particularly with the much lower elastic modulus of

Nitinol compared to stainless steel, would permit more accurate canal morphology 2

conformation (i.e., better maintenance of the natural canal position within the root) leading to fewer procedural errors (5,6).

Nickel-Titanium Material Characteristics

The first generation of the nickel-titanium alloy used in dentistry was composed of approximately 52% nickel, 45% titanium, and 3% cobalt (8). The atoms of this alloy are arranged in two principal crystalline structures, either martensite or austenite, dependent upon temperature and the mechanical stress the material is subject to (9). The austenitic crystalline structure is complex body-centered cubic, and exists at high temperatures (generally temperatures similar to those experienced intraorally- approximately 37C-or higher) and low stress (Figure 1). At low temperature and high stress, the monoclinic martensitic crystalline form predominates (9,10).

Since its early use in orthodontics, NiTi wire has been categorized as superelastic or non-superelastic, with the non-superelastic type having a much lower springback than the superelastic type. Upon stress activation, such as flexure or torsion, superelastic NiTi will undergo a phase transformation from austenite to martensite (Figure 2). This phase transformation is reversed as the wire is deactivated, returning to its austenitic arrangement and original shape (10). The NiTi alloys typically used in the manufacturing of endodontic files shift from austenitic to martensitic configuration at temperatures below those encountered in a dental setting, thus its austenitic form predominates intraorally (13). Using differential scanning calorimetry (DSC), Leu et al. (14) showed that this superelastic transformation is more complicated than initially thought and involves an intermediate rhombohedral (R) crystalline structure known as R-phase. R-

3 phase is a temperature-dependent crystalline form, and this intermediate transition helps

NiTi absorb stresses and resist fatigue (13).

Studies have shown inadvertent procedural errors, including ledging of canal walls, zipping of the apical foramen, and instrument separation occur, when engine- driven rotary handpieces are used to instrument root canals with stainless steel files

(7,15). For these reasons, Esposito et al. (2) advised against instrumentation of curved canals to the apical foramen with rotary-driven stainless steel files. Thompson et al.

(16,17) suggested that perhaps the limitations were not in rotary-driven instrumentation but rather in the stainless steel files. The first nickel-titanium files were the NT nitinol files (NT Co., Chattanooga, TN), initially only marketed as hand files. Comparison of

NiTi files to stainless steel revealed their advantage in preparing the curved root canal, particularly in larger file sizes (35, 40, and 45). NiTi hand and rotary instruments allowed larger apical preparations while accurately maintaining the path of a curved canal (2).

Glossen et al. (18) found similar results regarding canal transportation, adding observations of improved canal dentin preservation and faster preparation with nickel- titanium rotary files. One of the first brands of engine-driven rotary NiTi files included the Profile Series 29 files (Dentsply; Tulsa Dental Specialties, Tulsa, OK). Early studies of these instruments demonstrated expedience of preparation and no canal blockages, instrument separations, or zipped canals (16,17).

The first generation of nickel-titanium endodontic instruments offered marked advantages over stainless steel, but not without recognizable limitations. Two fracture mechanisms have been found to be associated with NiTi file fracture: cyclic (flexural) fatigue and torsional overload (13). Cyclic fatigue occurs with the rotation of the 4 endodontic file within a curved root canal. Cyclic failure is due to work hardening and metal fatigue associated with repeated loading and unloading of the NiTi file, and is generally initiated by the introduction of a crack within the metal followed by propagation of the fracture and eventual instrument failure (19). Torsional load occurs as the file encounters frictional resistance as it rotates in the root canal. Torsional overload, or failure, develops when the file encounters friction and the force is transmitted to the file as torque. When the torque exceeds the torsional strength of the NiTi file, this leads to either plastic deformation (irreversible change to the original shape) of the file or fracture (20). Pruett et al. (19) concluded that these types of fractures occur with little-to- no observable warning or previous permanent deformation, making visual inspection of the file an unreliable tool in predicting potential file failure. Of the types of failures encountered during endodontic treatment, torsional overload seems to be the predominant occurrence. Using stereomicroscopy, Sattapan et al. (21) showed instruments favored torsional failure in 55.7% of file fractures compared to flexural failure, and could be observed as unwinding or rewinding at the fracture site. The flexural failures, however, occurred as clean breaks with no accompanying defects. The improved metallurgy of the first-generation nickel-titanium files could not completely eliminate file failure as a potential complication to endodontic treatment.

File design and cross-sectional geometry plays a major role in the performance and efficiency of endodontic instruments. Radial-landed instruments prepare the canal walls in a planing fashion with more passive engagement of dentin (Figure 3). The radial lands create more canal contact and, as a result, more friction, but serve to maintain canal position with few procedural errors (13). Non-landed instruments typically have 5 triangular cross sections and engage dentin in an active, cutting fashion (Figure 3). Active edges permit selective pressure on the rotary endodontic handpiece to advance the file into the canal and can produce changes in canal path. Radial-landed instruments are less likely to create an apical zip if extended beyond the apical constriction of the root canal, compared to non-landed instruments (13). Triangular cross-sections show less stress and deformation in curved canals than files with rectangular or S-shaped cross-sections, however, rectangular cross-sections show greater flexibility (23,24).

Material characteristics, as well as manufacturing methods, have been implicated in the failure of endodontic instruments. Manufacturers use unique and, typically, confidential methods to machine endodontic instruments. These methods employ ceramic or diamond-impregnated cutting wheels to mill NiTi wire blanks into files. The type and size of the cutting wheel used produces varying surface textures and often machining defects (25). Repeated cyclic fatigue, experienced as a file rotates around a canal curvature, may be the most important factor in file separation (26-28). A file rotating in a curved canal undergoes repeated tension and compression, with the tension occurring on the outside of the curvature and the greatest stress observed at the peak surface of the curve. Crack nucleation most often begins on the tension face of the file on the outside of the curve, and these surface defects present from manufacturing allow for propagation of the crack leading to instrument failure (29).

NiTi alloys have 3 stages of fatigue life characterized by (1) initiation of microcracks forming and growing along specific crystallographic planes or grain boundaries followed by (2) crack propagation, and ultimately (3) the crack reaching the

6 point where the remaining material is overstressed and the overload zone results (27).

Alapati et al. (30) reported on fracture patterns of clinically failed nickel-titanium endodontic instruments. They found ductile fracture, in which the fractured surface exhibits a dimpled rupture characteristic, to be the predominating fracture mechanism.

Some specimens exhibited transgranular fracture at alloy grain boundaries. The overload zone features a typical ductile failure. Nitinol alloys with increasing martensitic structure show remarkable fatigue-crack resistance compared to those of a primarily austenitic composition. The crack propagation rate of martensite is slower than that of austenite and its energy absorbing crystalline structure provides a damping effect. Upon examination with electron microscopy, the failed martensitic form reveals numerous branching fracture origins, while superelastic NiTi exhibits fewer crack nucleation sites (27,31).

Alapati et al. (32) described the role of dentin chips in the propagation of cracks. They concluded that during clinical use, dentin chips become wedged in manufacturing defects on the file and play a pivotal role in its ultimate failure. They also observed that these dentinal chips were not removed during ultrasonic cleaning and sterilization, leaving the files susceptible to failure in future clinical uses. Alapati et al. (33) discussed the use of electropolishing as a means to remove machining flaws that may potentiate file fracture.

Electropolishing is an electrochemical process designed to reduce and remove surface irregularities and milling marks, leaving a surface that is supposedly more resistant to fatigue and corrosion (13). The files are immersed in a highly ionic solution and put under an electric current which removes a thin layer of the alloy, essentially dissolving away the surface defects (34). Brands that use electropolishing in the manufacture of their

NiTi files include EndoSequence® files (Brasseler USA, Savannah, GA) and RaCe™ files (FKG Dentaire SA, La Chaux-de-Fonds, Switzerland). Other studies have presented concerns about how electropolishing could alter the performance and properties of the

NiTi. Bui et al. (35) showed that electropolishing actually reduced the cyclic fatigue, but found no reduction in cutting efficiency. Boessler et al. (36) concluded that electropolishing altered the cutting ability of NiTi files, resulting in an increase in torsional load experienced by electropolished files compared to untreated, machined files.

Herold et al. (25) showed that electropolishing did not inhibit the formation of microfractures in nickel-titanium files. Future modifications to the alloy would seek to address the shortcomings of first-generation NiTi.

Second-Generation NiTi Files

Recent changes in NiTi metallurgy (2007) have resulted in a NiTi alloy which is now more resistant to cyclic fatigue: M-wire™ (37). Using the most commercially pure form of Nitinol (508 Nitinol, composed of 55.8% nickel and the remainder titanium), the raw wire is drawn under tension and heat-treated at various temperatures, resulting in a phase shift in the metal with emphasis on the martensitic and pre-martensitic (R) phases while still maintaining its pseudoelasticity (38). The second generation of nickel-titanium endodontic instruments made with M-wire™ are reported to have more flexibility and resistance to cyclic fatigue as compared to traditional NiTi (38-40). Gao et al. (41) demonstrated this using M-wire™ Profile Vortex™ instruments (Dentsply Tulsa Dental

Specialties, Tulsa, OK). They machined files of Profile Vortex™ geometry using M-

8 wire™ and conventional NiTi. Using an artificially constructed, stainless steel canal with a 90-degree curvature, sample files were rotated until failure. The results showed significant improvements in cyclic fatigue for M-wire™ compared to conventional NiTi.

Another derivation in the second generation of NiTi endodontic files is the Twisted

File™ (Sybron Dental Specialties, Orange, CA). Following a similar manufacturing process to the stainless steel file predecessors, Twisted Files™ are not milled from a NiTi wire blank, but rather twisted into shape via a fluted NiTi blank, utilizing the deformable intermediate R-phase that forms during the nickel-titanium phase transition. Along with a proprietary surface treatment, the manufacturer claims a great increase in flexibility compared to other conventionally milled NiTi files, including ProTaper Next™

(Denstply, Tulsa Dental Specialties, Tulsa, OK) and RaCe™ files (42,43). A study conducted by Larsen et al. (39) showed some improvement in cyclic fatigue for M- wire™ and Twisted Files™ compared to traditional ground nickel-titanium files; however, a smaller tip size was used in the M-wire™ ProFile GT™ Series X (Denstply,

Tulsa Dental Specialties, Tulsa, OK) compared to the other file brands, which may account for the difference. Using differently branded files of the same tip size and taper,

Kim et al. (44) showed a significant improvement in cyclic fatigue for Twisted Files™ over several other NiTi files made from a grinding process.

Third-Generation Niti Files

Advancements in metallurgy and innovation in endodontic instrument design are confined by the same limitations encountered by earlier systems. The ability to safely and accurately prepare increasingly more difficult canal systems without separating an 9 instrument or changing canal morphology is a driving goal. This influence has brought about the third, and most recent, generation of NiTi-based rotary instruments. Using a proprietary thermomechanical process, Controlled Memory NiTi Technology™ (D&S

Dental, Johnson City, TN) or CM Wire, as it has come to be known, shows even greater improvements in cyclic fatigue and flexibility, as well as increased martensitic composition at intraoral temperature compared to conventional nickel-titanium (45). The martensitic form shows great ease in deformation but will return to its original shape when heated above its austenite transformation temperature (known as austenite-finish temperature or Af) (27,28). Using DSC, Shen et al. (45) showed CM Wire and ProFile

Vortex™ files (M-wire™) to have Af temperatures above those observed intraorally

(50.4C for ProFile Vortex™ and 55.1C for CM Wire), correlating to an increase in martensitic structure at working (intraoral) temperatures.

Heat treatment of the alloy imparts the ability to raise the austenite finish temperature and maintain more of the desired martensitic form and characteristics at oral temperatures. Alapati et al. (47) showed that heat treating instruments for 15 minutes in a nitrogen atmosphere at 400°, 500° and 600C could raise the Af temperature between 45-

50C. Previous research on NiTi orthodontic wires had shown martensite to be less hard than austenite, so an endodontic file in its martensitic state could have decreased cutting efficiency (11). Heat treatment in a nitrogen atmosphere can result in surface nitrides that actually increase hardness and cutting efficiency (48)

Most notably, these files can be bent similarly to stainless steel and maintain a desired shape rather than returning to their original form, allowing better adaptation to the 10 natural curvature of the root canal and potentially less canal straightening (49). This characteristic could also facilitate canal access at difficult angles deep in the oral cavity, as well as negotiating a ledged canal. It is thought that these improved characteristics will allow the practitioner to visually identify the torsional deformation in a CM file before catastrophic failure occurs, leading to fewer instrument separations within the tooth.

Previously, it had been shown that file failure can occur with little-to-no warning, and the defects and irregularities that propagate file failure cannot be observed with the naked eye (19). Pereira et al. proposed visible distortion as a safety mechanism for multiple treatment use, using this identifiable torsional deformation as an indication for discarding used files (49).

The first of such controlled memory materials introduced in 2010 was CM Wire

(D&S Dental, Johnson City, TN). Instruments manufactured from CM Wire include

TYPHOON Infinite Flex instruments (Clinician’s Choice Dental Products, New Milford,

CT) and Coltene HyFlex® CM™ (Coltene Whaledent, Cuyahoga Falls, OH). Shen et al.

(50) reported on improved fatigue resistance and flexibility of this material. Pereira et al.

(49) confirmed these findings, but they also reported there was a decrease in torque strength when compared to M-wire™-based Profile Vortex™ instruments (Dentsply

Tulsa Dental Specialties, Tulsa, OK). It is thought that this decrease in torque strength could correlate to decreased cutting efficiency. Zhou et al. (51) examined raw CM wire and found it to have increased resistance to fracture, but decreased tensile strength compared to conventional superelastic NiTi. They also concluded that the heat-treated

CM wire had a significantly higher Af temperature compared to conventional NiTi, which correlates to an increase in the desired martensitic composition at intraoral temperatures. 11

Coltene HyFlex® CM™ files are machined from CM Wire, and it can be assumed that its metallurgic properties are consistent with other files made from CM Wire, namely

TYPHOON Infinite Flex Instruments. Peters et al. (52) presented findings showing superior fatigue resistance and lower torque when preparing a root canal compared to

® ™ conventional nickel-titanium. Shen et al. (53) found an Af of 47C for HyFlex CM files, which would allow the instruments to utilize the desired mechanics of the martensitic state at oral temperatures. The authors of this study thought file separation of

HyFlex® CM™ was an unlikely occurrence when the instruments are discarded after three clinical uses. However, multiple clinical uses caused microstructural changes including cracks along the length of machining grooves. The manufacturer claims that HyFlex®

CM™ instruments have the unique ability to deform under clinical stress but regain their shape after heat sterilization. Should a file not return to its manufactured shape after heat sterilization, it can be assumed that this file would not perform as the manufacturer intended and should not be used again. This serves as a safety mechanism to identify when a file should be discarded, a method to reduce cost by reusing instruments, and a means to fewer file separations through discarding those files no longer fit for clinical use

(54). Alazemi et al. (55) found that about one-third of HyFlex® CM™ instruments deformed during clinical use; however, two-thirds returned to their original shape after heat sterilization. More deformations were identified when magnification was used to examine instruments.

Vortex Blue™ (Dentsply Tulsa Dental Specialties, Tulsa, OK) is another controlled memory file made from 508 Nitinol that is machined and then undergoes a

12 heating and cooling treatment to gain shape memory characteristics. The process results in an instrument with increased martensitic structure at room temperature and improved fatigue resistance compared to traditional NiTi. The blue hue of the metal results from an oxide layer (titanium oxide – TiO2) manufactured onto the file. It is speculated that the hardness of the TiO2 layer may compensate for the loss of hardness of the underlying martensitic structure and may improve the cutting efficiency or wear resistance of the files (53). Gao et al. (56) reported Vortex Blue™ had superior cyclic fatigue and flexibility to M-wire™ and conventional NiTi. However, as postulated, Vortex Blue™ was shown to be inferior to M-wire™ in torsional strength and surface hardness.

Most recently, ProTaper Gold™ (Dentsply Tulsa Dental Specialties, Tulsa, OK) has been introduced, boasting improved metallurgy and flexibility over its predecessor due to a proprietary thermal treatment (57). ProTaper Gold™ exhibits a two-stage phase transformation with an intermediate R-phase and a high Af temperature, displaying more martensite at oral temperatures (58). Uygun et al. (59) demonstrated the superior cyclic fatigue of ProTaper Gold™ compared to superelastic ProTaper Universal™ and M- wire™-based ProTaper Next™ (Dentsply Tulsa Dental Specialties, Tulsa, OK). Elnaghy et al. (58) concluded similar results in cyclic fatigue; however, they also found ProTaper

Gold™ had inferior torsional strength and microhardness compared to superelastic

ProTaper Universal™. These findings are consistent with other controlled memory heat- treated instruments.

Clinical Implications

Clinically, the lifespan of an endodontic file typically begins with sterilization, followed by at least one clinical use, with ultrasonic cleaning and sterilization in between treatments. If files are to be used for multiple treatments they are, typically, visually inspected before use to detect defects. Numerous studies have explored the effects of sterilization and clinical use on endodontic instruments. Iverson et al. (60) originally explored the impact of various types of sterilization on stainless steel files. They concluded that 10 cycles of autoclave sterilization had no effect on torque strength of stainless steel. Morrison et al. (61) showed sterilization of stainless steel files had no impact on cutting efficiency and supported their findings with scanning electron microscopy. Knowing the effect of heat treatment of nickel-titanium on specific metallurgy and phase transformation, other studies explored the effects of sterilization on

NiTi-based files. Silvaggio and Hicks (62) found heat sterilization of Nitinol endodontic files up to 10 times did not increase the likelihood of file failure and in some cases increased torsional strength. Similar findings were shown by Casper et al. (63) comparing

M-wire™ Profile Vortex™, Twisted Files™, and 10 Series™ files (DS Dental, Johnson

City, TN) made with CM wire. None of the brands showed a significant decrease in performance or torsional strength after seven sterilization cycles. Alexandrou et al. (64) used scanning electron microscopy and observed a significant increase in surface roughness in nickel-titanium instruments that underwent up to 11 sterilization cycles.

Shen et al. (65-69) stated that currently there is no scientific method to evaluate the functional lifespan of a NiTi endodontic instrument. Some instruments will fail after their first clinical use. However, it has been identified that stresses encountered during use lead 14 to microcracks that are potential nucleation sites for future failure. The authors believe that instrument fracture would be as low as 0-0.26% if instruments were disposed of after a single treatment (65-69).

Some file systems are marketed as single-use (one treatment) systems, while others permit multiple uses following careful inspection for deformations after sterilization. WaveOne® (Dentsply Maillefer, Ballaigues, Switzerland) has a colored plastic sleeve that deforms after sterilization, preventing placement back in the rotary handpiece. Coltene HyFlex® CM™ instruments are meant to return to their original shape after sterilization, providing a mechanism for identifying a file that may be used again

(54). Both ProTaper Gold™ and Vortex Blue™ are marketed as single-use file systems

(Dentsply Tulsa Dental Specialties, Tulsa, OK). The improvements in cyclic fatigue of the third-generation systems present the question of multiple uses. Can these NiTi instruments be safely used more than once? Are there microscopic changes or wear patterns that can be observed over multiple file uses that would contraindicate this? Although some studies have looked at the wear of NiTi rotary files after multiple uses, none have looked at the actual degradation of the file after each use in an in-vitro situation using teeth. Also, the observable wear and fatigue characteristics of the third- generation, controlled memory files have not been evaluated in-vitro. Therefore, the purpose of this study is to evaluate the qualitative and quantitative wear/fatigue of three third-generation rotary endodontic file systems (Vortex Blue™, HyFlex® CM™, and

ProTaper Gold™) following use in extracted posterior human teeth in a simulated clinical model.

15 MATERIALS AND METHODS Five sets of files from four different endodontic rotary file systems were evaluated in this study. The file systems included: ProFile Vortex™ and Vortex Blue™ sizes 20-40,

0.04 taper, 25 mm length (Dentsply Tulsa Dental Specialties), Coltene Hyflex® CM™ sizes 25/.08 (19 mm), 20/.04, 25/.04, 20/.06, 30/.04, 40/.04, 25 mm length

(Coltene/Whaledent Inc.) and Protaper Gold™ Sx (19 mm), S1, S2, F1-3, 25 mm length

(Dentsply Tulsa Dental Specialties, Tulsa, OK).

Instruments were removed from the manufacturer packaging and soaked in an enzymatic cleaner for 20 minutes (Maxizyme Tabs, Henry Schein, Melville, NY) and placed in an ultrasonic bath containing the same solution for 10 minutes. After drip- drying the instruments, each set of instruments was packaged individually in labeled sterilization bags and placed in a sterilizer (Statim 5000, SciCan, Canonsburg, PA) at

134˚C for 15 minutes. This sterilization time included drying.

Cleaned instruments were then analyzed with a scanning electron microscope

(SEM) (Quanta 200, FEI, Hillsboro, Oregon) prior to cleaning or sterilization (Figure 5).

A custom-made aluminum jig (Ted Pella, Inc., Redding, CA) was fabricated in the Ohio

State University Department of Chemistry and Biochemistry Machine Shop so that the orientation of the files could be maintained throughout the study and the same surface and cutting edges could be repeatedly examined following each procedure (Figure 6 and

Figure 7). Specimens were imaged using an Everhart-Thornley Detector (ETD) and an

accelerating voltage of 20 kV. Care was taken to keep the image working distance slightly greater than 10 mm to avoid damaging the unit, with slight variance allowed for proper image focus. Photomicrographs (secondary electron images) were taken of the leading edge of the cutting blades at the tip of the file (D0), 1 mm from the tip (D1), 3 mm from the tip (D3), and 5 mm from the tip (D5) at a standard magnification of 200x

(70,71). The images were recorded digitally and labeled according to the file pack, file brand, stage of examination, file size, and position on the file (example:

1VORTEX_NEW_40_D0 denoted the 40.04 file of the first ProFile Vortex™ package imaged new out of the package at the D0 position). After initial imaging, the files were then ready to be used on selected, de-identified, extracted posterior teeth.

Sixty extracted, de-identified posterior teeth (mandibular and maxillary molars and premolars) were collected and sorted from a collection of over 1000 teeth. Selected teeth were rinsed in a 3% NaOCl solution (Clorox®, Oakland, CA), and exterior tissue was removed from the roots with a periodontal scaler. Inclusion criteria for the test teeth were: intact roots with closed apices, patent canals (as determined with a #8 K-file after access opening), individual canals (established radiographically–no Type 2 or 4 canals), no signs of internal or external root resorption or root fractures (evaluated under 2.5x magnification using loupes (Designs for Vision, Ronkonkoma, NY)). Radiographs were taken of all teeth to calculate root canal curvature as described by Pruett et al. (19).

Canals were divided into classifications of; <30˚, 30˚–45˚, 45˚–60˚, and >60˚ curvature. Teeth were categorized as premolar, mandibular molar, and maxillary molar, and stored in sealed, opaque jars. Teeth were randomly selected for each set of

endodontic files just prior to endodontic treatment with that set of files. Each file set treated one of each type of tooth. The number of canals, curvature of each canal, and whether the file was used in the clinical simulation were recorded.

The crown of each tooth was removed at the CEJ using a diamond-impregnated blade (Vari/Cut VC-50, Leco, St. Joseph, MI). Access opening was further enlarged with a #4 round bur (Komet USA, Rock Hill, SC) in a high-speed handpiece. Canals were located and patency was determined with a #8 K-file. The working length of each canal was determined by working a #10 K-File (Kerr, Gilbert, AZ) out the apex of the canal until it could just be visualized, and then the file was retracted 1 mm. Working lengths were measured using Endoring® II (Jordco Inc., Beaverton, OR). All instrumentation was done by a single operator (TB). Initial canal path was prepared using a #15 K-File.

Orifice shapers (ProFile Vortex™ 25/0.12 and 40/0.10) were utilized to achieve straight line access to the canals for the ProFile Vortex™ and Vortex Blue™ file groups. Orifice modification and shaping for the HyFlex® CM™ and the ProTaper Gold™ groups were achieved using the Hyflex® CM™ sizes 25/.08 (19 mm) and the Protaper Gold™ Sx (19 mm), respectively. EDTA gel (Glyde, Dentsply Maillefer, Tulsa, OK) was placed on each rotary file tip before use. Root canals were irrigated with approximately 1 mL 3%

NaOCl solution per canal with a 27-gauge needle (BD PrecisionGlide™, Becton

Dickinson and Co., Franklin Lakes, NJ), placed passively into each canal, after every two instruments used. Between the uses of each rotary instrument, a #10 K-File was placed in the root canal to working length to maintain patency of the canal. Files were cleaned with a 4x4 cotton sponge soaked in 70% isopropyl alcohol after each use and were stored in

EndoFoam™ (Jordco Inc., Beaverton, OR) during instrumentation of the tooth. The same set of files was used to instrument all of the canals for that tooth. Files that separated or showed catastrophic distortion were replaced with a similar file of the same brand and size to allow complete instrumentation of the canals. The old file was removed, labeled, and examined with the SEM. The replacement file was noted in the treatment history of that particular set of files, along with information regarding whether the new file was used during that particular clinical simulation and in which canal the separation occurred.

Rotary instrumentation followed the directions of each manufacturer in terms of rotational speed and torque settings. An 8:1 reduction handpiece (TUL-8M, Dentsply

Tulsa Dental Specialties, Tulsa, OK) was utilized on a torque-control motor (Aseptico®

DTC®, Dentsply Tulsa Dental Specialties, Tulsa, OK). Instrumentation of canals continued to a minimal apical size of 25 or maximum size of 40 (with taper corresponding to the file system used), depending on the anatomy of the individual canal. This was meant to mimic most random clinical situations. Following preparation of all the canals in the tooth, the rotary files were cleaned in an ultrasonic bath (as previously described) and autoclaved (as previously described). The instruments were then re-examined under the SEM at 200x (at the same locations previously described, using the custom jig to confirm proper orientation of the files) and digital images were prepared, labeled (as previously described) and stored for evaluation. The SX file of

ProTaper Gold™ and the 25/.08 file of Coltene HyFlex® CM™ were used but not examined under SEM in this study because they are meant for orifice modification and are rarely used in the apical third of the canal. This process was repeated two more times,

resulting in the set of files being utilized for three different teeth. A final SEM evaluation was conducted on the cleaned and sterilized files after the third clinical simulation.

Classification/Grading Of Instruments:

Two examiners analyzed the randomized digital images on a computer screen and scored the files for the presence of irregular cutting edges, grooves, microcracks/cavities, burs and distortions of the flutes. The evaluators then rescored a newly randomized set of the same images four weeks later to confirm intra-rater reliability. This rating process included all image sets following the completion of three simulated clinical uses. The scores were recorded on a data sheet beside their corresponding randomized number, and later reassigned to the original image label.

Each evaluator was calibrated to the grading criteria prior to both grading periods, using sample images that most accurately represent each category. Evaluators analyzed the randomized images of the files from each stage of this study, collectively, with no knowledge to which stage the image they were evaluating belonged. The following criteria were utilized to evaluate the NiTi files: 1) long axis of file was undistorted – no stretching or compression of flutes and no wear on file edges; 2) 1-3 defective/worn areas on cutting edges; 3) long axis of the file was mildly distorted – there was stretching/compression of the flutes and/or 4-5 worn areas on cutting edges; 4) long axis was moderately distorted – severe wear on flutes with more than 5 wear areas on cutting edges; 5) breakage of the file or visible deformation after sterilization (71,72). Any files that showed breakage or visible deformation after use and sterilization received a score of

5 for all images to be taken on that file for that clinical simulation and were discontinued for future use/evaluation.

For each series (use 0 through 3) of the study, evaluators assigned a score to four positions on a file (D0, D1, D3, D5). The file was then assigned the maximum score from all four positions for that series and placed into one of three categories. (1) Usable: files that scored 1-3 having wear/defects/deformation that would not deter the examiner from continuing to use that instrument. (2) Microscopically unacceptable: files that scored 4 having wear/defects/deformation that would deter the examiner from continuing to use that instrument but could not be observed without the aid of scanning electron microscopy. (3) Visually unacceptable: files that scored 5 were deformed or separated after sterilization and could be identified as such by visual human inspection. When interobserver disagreement existed between what one observer thought was usable and the other thought was microscopically unacceptable, the two observers discussed their evaluation and came to a consensus.

Any file added to the study to replace one that had failed was not included in the final statistical analysis of wear and deformation. This was done to ensure that each file examined had an equal chance at being used through three clinical simulations.

Kappa statistics for intra-rater and inter-rater reliability were calculated. Fisher’s exact test was used to evaluate the curvatures experienced by each file brand. Instrument grading results were tabulated and analyzed using repeated-measures logistic regression for visual use and microscopic use. Odds ratios between brands and uses were calculated.

Statistical significance was set at α=0.05.

21 RESULTS For each instrument use in the study, evaluators assigned a score to four positions on the file (D0, D1, D3, and D5). The file was then assigned the maximum score from all four positions for that file use and placed into one of three categories: (1) Usable: files that scored 1-3 having wear/defects/deformation that would not deter the examiner from continuing to use that instrument, (2) Microscopically unacceptable: files that scored 4 having wear/defects/deformation that would deter the examiner from continuing to use that instrument, but could not be observed without the aid of scanning electron microscopy, and (3) Visually unacceptable: files that scored 5 that were deformed or separated after sterilization and could be identified as such by visual human inspection.

When interobserver disagreement existed between what one observer thought was usable and the other thought was microscopically unacceptable, the two observers discussed their evaluation and came to a consensus. Statistical analysis was performed using these three categories.

Prior to grouping files into the three categories, weighted Kappa statistics were calculated based upon the original grading scale (of 1-5). Intra-rater reliability values for each evaluator were 0.6809 and 0.7608. Inter-rater reliability values for the first and second round of grading were 0.5176 and 0.4613, respectively.

Table 1 shows the overall file failures by brand. Vortex Blue™ files and Coltene

HyFlex® CM™ files had the most overall failures with four each. The overall failure rate was 12.0%.

Table 2 outlines the details of each visual file failure. Only three of the twelve failures were actual file separations; the remaining failures involved plastic deformation that remained after sterilization of the files. All separations occurred approximately at the

D1 level (1 mm from the tip of the file). The overall separation rate was 3.0%. The majority of the failures occurred in teeth in which the most severe canal curvature of any treated root was 30-45°. The majority of visual failures occurred during the first use of the files, and no predilection was seen for tooth type.

Table 3 details the number of each type of root curvatures (<30˚, 30-45˚, 45-60˚, and >60˚) experienced by file sets of each brand over three uses. A significant relationship exists between file brand and the number of canal curvature types experienced (p=0.0426, Fisher’s Exact Test), suggesting that one brand may have experienced more or less difficult canal curvatures over three uses compared to another.

ProTaper Gold™ files experienced the most <30˚ canals and no canal curvatures greater than 45˚.

Tables 4 through 7 summarize the overall evaluation of each brand. ProFile

Vortex™ had the most files (11) graded as microscopically unacceptable at eleven files

(Table 4), followed by Coltene HyFlex® CM™ having 10 files microscopically unacceptable through three clinical simulations (Table 7). Table 4 shows that no size

40.04 ProFile Vortex™ instrument was ever deemed microscopically or visually unacceptable. The same result was found for size 20.04 Vortex Blue™ instruments

(Table 6). All files (F1, F2, and F3) of ProTaper Gold™ remained usable throughout the entire study (Table 5). Size 20.06 of Coltene HyFlex® CM™ files was most commonly graded as microscopically unacceptable in all uses of this study. Figure 8 shows the overall evaluation summary by brand and use.

It is important to note that microscopically unacceptable grades (score of 4) in

Tables 4-7 and Table 10 do not directly correlate to individual files. For example, a total of 10 Coltene HyFlex® CM™ files were graded “microscopically unacceptable” from unused and through three uses (Table 7). This could represent 10 different files with all receiving the same score, but it could also represent the same files receiving multiple

“microscopically unacceptable” scores throughout multiple uses. Also, a file graded as microscopically unacceptable after use 1 may have been graded as usable in later uses.

Table 8 shows the number of files (8 total) graded as visually unacceptable after their first use, meaning the most instrument failures occurred in the first use. Fewer instrument failures were seen in uses 2 and 3.

Table 9 includes multiple instruments of each brand being graded as microscopically unacceptable before and after use in any tooth. This indicates that the evaluators observed microscopic surface conditions of unused files that they thought were unacceptable for clinical use. The total number of files graded as microscopically

unacceptable at each use of the study stayed relatively similar, ranging from 8.0% in the clean (unused) file group to 11.0% in tooth (use) 3.

Table 10 displays the instruments of each brand that were graded as usable in each use, ranging from 92.0% in the clean (unused) file group to 86.0% in tooth (use) 3.

Repeated-measures logistic regression was performed for instruments that were visually useful (scores 1-4) and for those evaluated as microscopically useful (acceptable under SEM or scores 1-3). In analyzing these data, assumptions were made for microscopically unacceptable files; once an instrument was graded as microscopically unacceptable, it remained so throughout continued uses for this analysis. Table 11 indicates that there were no significant effects for brand, amount of use, and their interaction in terms of visual grade. Figure 9 depicts the percentage of files that remained visually useful (scores 1-4) for each use by brand, based on the logistic regression (Table

11).

Table 12 demonstrates the impact of use on file graded microscopically. No significance between brands, or the interaction of use and brand, was found. However, the number of uses did have a role in the scores (p=0.0127). Contrasts of uses showed significance for use 0 vs. 1, 0 vs. 2, and 0 vs. 3, indicating that unused files were more likely to be microscopically acceptable than used files. Contrasts of uses 1, 2, and 3 showed no significance, indicating that microscopically acceptable files were similarly common after any amount of use (Table 13). Figure 10 represents the percentage of files

that remained microscopically useful (scores 1-3) for each use by brand (based on Table

12).

Odds ratios analysis shows at use 0 (unused) a file of any brand was 3.287 times more likely to be microscopically acceptable (acceptable under SEM examination) than at use 1, 4.309 times at use 2, and 5.403 times at use 3. No brand was significantly more likely to be microscopically acceptable than any other brand (Table 14).

DISCUSSION

This study aimed to approximate as accurate a clinical model as possible in vitro.

One of the goals of the author was to examine the clinical lifespan of four brands of endodontic rotary files as would be observed in practice. Previous research has examined some of the brands of NiTi endodontic files included in this study after undergoing three clinical treatments (73). To the best of the author’s knowledge, no other study has examined these brands of third-generation, heat-treated endodontic instruments after each of three clinical simulations, using real teeth. ProFile Vortex™ instruments, second- generation, M-wire™-based files, were included in the examination to allow comparison of the third-generation brands to their predecessor.

Materials and Methods

Extracted, de-identified teeth were pooled and then randomly selected for use.

Each pack of rotary files was used to instrument one premolar, one maxillary molar, and one mandibular molar. These teeth were chosen to best represent the patient population of an endodontist and the substrate on which the instruments operate. Previous studies have shown maxillary and mandibular molars, followed by premolars, to be the most commonly endodontically treated teeth and also the teeth most commonly referred to an endodontist for treatment (74).

Several studies have observed discrepancies between the preparation of simulated canals and real teeth. It has been shown that simulated canals (in resin blocks) produce higher torque values during instrumentation compared to dentin (75). Also, more rotations of the file were required to complete the preparation of simulated canals (75).

Schafer and Schlingemann (76) and Schafer and Florek (77) observed lower deformation and fracture of instruments in real teeth compared to simulated canals.

Only teeth with closed, individual, canals and no signs of internal or external resorption were included in the study. Clinicians often treat teeth with open apices and external or internal resorption differently, with more emphasis on hand instrumentation or reliance on irrigating solutions. As the present study examined rotary endondontic instruments, these teeth were excluded from use. Teeth with type 2 and type 4 canals were excluded as well, as these canals can curve in multiple directions in multiple planes making it difficult to measure root curvature.

Aydin et al. (78) examined changes in dentin hardness after exposure to various storage solutions used in preserving teeth, concluding that teeth stored for 2 months in

0.1% NaOCl showed similar dentin microhardness as that observed in vivo. At 12 months, however, a significant decrease in dentin microhardness was observed. In the present study teeth were stored in a 3% NaOCl solution for at least 2 months, some teeth as long as 6 months. Due to the use of more concentrated NaOCl solution and the length of time stored, the teeth instrumented in this study may have had decreased dentin microhardness compared to the in vivo environment. However, microhardness testing would need to be conducted to confirm this. The files used in this study may have

encountered slightly softer dentin than that in the canals of a patient’s tooth, which would result in less resistance, and thus less stress, as the file turned in the canal.

Root curvature was calculated for each root as described by Pruett et al. (19).

Teeth were separated by tooth type and stored in opaque jars. Teeth were selected at random from these jars to ensure that no bias in terms of number of canals, canal diameter, or canal curvature was imparted to any file brand. Five packs of each brand of files were used to allow a balance of easy and difficult teeth to instrument. Despite efforts to maintain an equality of treatment methods across brands, there was a significant relationship (p=0.0426, Fisher’s Exact Test) between file brand and the number of canal curvature types experienced (Table 3). ProTaper Gold™ files experienced the most <30˚ canals and fewer of the higher degree of curvature canals compared to the other brands.

This may have translated to less overall stress experienced by these files in this study

(Table 3).

Previous studies have examined different brands of files with corresponding tip size and taper. These studies typically examined only one size of file from each pack of files. File size 25 has been commonly examined across brands in similar SEM models to compare instrument wear. This file was chosen because it is typically used to the full length of the canal to prepare the apical third, subjecting the instrument to both torsional and flexural stress (72,79,80). This file size is also quite common across multiple brands, although file tapers may vary, making it easy to compare the brands. An apical enlargement of 0.25 mm has been cited as yielding the minimum apical size to allow debridement and microorganism removal from the apical region of the canal (81). Other studies have encouraged larger apical preparations than previously indicated to allow

irrigant penetration into the apical third of the canal (82-84). The author of the present study chose to examine the entire pack of files in each brand because the manufacturer instructions encourage the pack to be used as a set. Varying canal shapes and apical sizes permit larger files to instrument into the apical region, and this study model allowed observation of the effects of this instrumentation on all file sizes in the file set. This model also allowed the examiners to observe which file sizes demonstrated more wear and degradation than others.

Canals were prepared according to the instrument manufacturer instructions as would be done in a clinical setting. This included the order of file use, irrigation and recapitulation instructions. Canal lubricant was also used on every rotary file (Glyde,

Densply Maillefer, Tulsa, OK). As previously stated, failed files (separated files and unwound files) that occurred during a clinical simulation were replaced with a new identical file in order to complete treatment and subsequent clinical simulations, as would be done in a clinical setting. Coltene HyFlex® CM™ files permit continued use when a certain degree of unwinding is observed during clinical use. These files were used to complete a clinical simulation only when they had unwound but not rewound in the opposite direction (54). If the file was still unwound after it was subsequently sterilized, it was considered a failure (score of 5). It must be noted that the replacement files were subject to fewer uses and they were not part of the study from the first simulation. These replacement instruments were not included in the statistical analysis for this reason.

Sattapan et al. (21) showed that the fracture of Quantec Series 2000 (Tycom Corp,

Irvine, CA) nickel-titanium instruments tend to occur close to the tip. Zelada et al. (85) supported these findings in ProFile instruments, observing fractures most often within 5

mm of the tip of the file, specifically in the 1-3 mm range. Based on these findings, this study chose to observe wear and deformation occurring within this range: at the tip (D0),

1 mm from the tip (D1), 3 mm from the tip (D3), and 5 mm from the tip (D5). A magnification of 200x was chosen because it permitted the examination of the desired regions of the files, while still allowing the examiners to observe wear in great detail.

Previous studies attempted to observe similar detail using similar levels of magnification

(70,71).

The custom-made aluminum jig allowed the author to maintain the orientation of the files and examine the same surface and cutting edges repeatedly with the SEM following each procedure throughout the study. The design of the jig and the set screws allowed an entire pack of files to be examined at once in the exact same orientation after each use, greatly expediting the imaging phase of this study.

The 1-5 grading scale for wear and deformation was adapted from the scale used in previous studies on degradation of nickel-titanium rotary files through multiple uses

(71,72). These studies used the inter-class correlation coefficient (ICC) as their measure of evaluator reliability. The investigators in these previous studies believed the rating scale to be a valid tool of measurement and supported this with ICC statistics of intra- rater and inter-rater reliability ranging from 0.68 to 0.92 (71,72). This 1-5 scale, however, produces categorical data and the ICC is better suited for evaluating continuous data, making the ICC an inappropriate method to evaluate inter-rater and intra-rater reliability of this grading scale. The Kappa statistic is better suited to evaluate reliability of categorical data, so the true reliability of these studies is unknown.

Initially, four clinicians examined and scored the file images for deformation and wear. Initial Kappa statistics showed poor reliability between these four examiners. The author then decided that two clinicians with greater experience examining SEM photomicrographs would be more appropriate to evaluate the images for this study. Even with two, instead of four, experienced evaluators, the Kappa statistics of both intra-rater and inter-rater reliability of the present study were less than desired based on the 1-5 scale (0.6809 and 0.7608 for intra-rater reliability and 0.5176 for inter-rater reliability).

Arantes et al. (71) considered Kappa statistics of 0.68 to be satisfactory and 0.87 to be excellent. With repeated calibration between grading periods, the evaluators expressed difficulty discerning areas of surface wear from machining defects, as well as identifying torsional file changes without seeing the original condition of the file for comparison.

Also, the poor reliability may have been related to the amount of detail exposed at 200x magnification. Observing the files at such a powerful magnification exposes many microscopic irregularities on the file surface that would most likely not be seen at much lower magnification. (Compare Figure 12 at 200x to Figure 13 at 25x.) Files with a larger pitch (the distance from the crest of one flute to the next) can be imaged at an angle as to appear unwound even though they are not. This proved to be an area of discord between the evaluators.

The poor reliability of the initial evaluation resulted in changes in the way the data were analyzed. The maximum score (1-4) from all four evaluated locations on a file

(D0, D1, D3, and D5) was used as the final file grade for each use. Because previous studies have suggested that file failure can be initiated from any defect on the file surface

(29,30,53), the author decided the maximum score should reflect the overall condition of

the file after that particular use. To help with the data analysis, the grades were re- categorized to identify conditions and changes of the endodontic instruments that could potentially make them unfit for continued use and that could not be observed by visual inspection. The evaluators agreed that scores of 1-3 represented surface conditions that would not deter them from using the instrument (usable) and a score of 4 represented an instrument judged potentially unfit for use as seen at 200x magnification

(microscopically unacceptable). When interobserver disagreement existed between what one observer thought was usable and the other thought was microscopically unacceptable, the two observers discussed their evaluation and came to a consensus. Previous studies have employed a similar agreement between evaluators (86,87).

Results

Vortex Blue™ and Coltene HyFlex® CM™ had the most visual file failures with four failures for each brand, with all but one of these visual failures being by plastic deformation (Tables 1, 2, and 8). Both failures of ProFile Vortex™ files were separations, and all three separations (two separations for ProFile Vortex™ and one for

Vortex Blue™) occurred at approximately the D1 position. Only one separation occurred in the third-generation file brands (Vortex Blue™) and that failure occurred in the first use of the instrument (Table 2 and Figure 12). Shen et al. (73) also reported Vortex

Blue™ instruments fractured in first use. The overall rate of file separation was similar to those reported in previous studies on file fracture (88-90)

All other visual file failures (score of 5) of third-generation brands involved plastic deformation (files appearing permanently unwound after use and subsequent

sterilization). There appeared to be a tendency for Vortex Blue™, ProTaper Gold™, and

Coltene HyFlex® CM™ files to plastically deform rather than separate in failure.

ProTaper Gold™ and Coltene HyFlex® CM™ files only showed this plastic deformation type of failure (Table 2 and Figures 13 and 14). This characteristic may allow the clinician to identify a file no longer fit for use before potential separation and blockage of the canal occurs. This is marketed as a feature of the Coltene HyFlex® CM™ files (54).

In both ProFile Vortex™ and Vortex Blue™ files, visual failures (score of 5) occurred in sizes 25.04, 30.04, and 35.04, with no visual failures occurring in the smallest and largest file sizes 20.04 and 40.04, respectively (Tables 2, 4, and 6). Sizes 20.04 through 35.04 ProFile Vortex™ instruments received microscopically unacceptable grades (scores of 4) at some point during this study, three of which occurred before any use; however, size 40.04 did not present this problem (Table 4). At no point in this study did sizes 20.04, 25.04, and 35.04 Vortex Blue™ files receive a grade of microscopically unacceptable (Table 6). ProFile Vortex™ and Vortex Blue™ files have the same cross- sectional geometry, tip sizes, and taper, with the only difference being their intrinsic metallurgy. ProFile Vortex™ files are constructed of M-wire™, while Vortex Blue™ files are heat-treated NiTi with a titanium oxide coating to increase cyclic fatigue and surface hardness. This difference in metallurgy may account for difference in visual failure features (separation vs. plastic deformation) observed in this study.

The present study observed a unique finding among Vortex Blue™ files. Multiple

Vortex Blue™ instruments displayed characteristic surface defects that had the appearance of extensive pitting (Figures 16 and 17). This condition appeared to worsen

with continued use, as if the surface of the instrument was chipping off. Examiners frequently scored this appearance as severe wear (score of 4) of the instrument. As previously mentioned, a titanium oxide coating is meant to compensate for the surface hardness lost in the heat treatment process (53). This wear pattern appears as voids in the surface layer which is progressively being removed through continued use. This observation suggests that repeated use of Vortex Blue™ files can potentially result in significant defects in the file surface that are not detectable by visual inspection, causing reduction in hardness and overall cutting efficiency, which may lend support to the manufacturer recommendation for single use. The author did not observe any Vortex

Blue™ files fracturing or visibly deforming as a result of this type of defect, however. To the best of the author’s knowledge, no previous study has reported these findings. Future studies could include Vickers microhardness or nanoindenter measurements of the defect area compared to the unaffected file surface to confirm a difference in surface hardness.

A change in cutting efficiency of these files could be explored using the method described by Peters et al. (96).

Only sizes S1 and S2 ProTaper Gold™ files were ever graded as visually or microscopically unacceptable (scores 4 and 5) throughout this study, suggesting that these files experienced the most stress and wear of any ProTaper Gold™ files (Table 5).

Only one ProTaper Gold™ file was graded as microscopically unacceptable before use.

Previous studies have shown the F3 size of ProTaper® Universal files to have the lowest number of cycles to fracture and the highest incidence of separation of any ProTaper®

Universal file (57,90,91). ProTaper Gold™ files use the same files sizes and geometry as

ProTaper® Universal; however, these findings were not seen in the present study. This is

most likely attributable to the increase in file flexibility gained in the heat treatment process used in the manufacturing of ProTaper Gold™. Table 5 shows that all size F3 files were rated usable in each series of this study.

Table 7 shows that Coltene HyFlex® CM™ file size 20.06 was most often rated as microscopically unacceptable (score of 4) and only the smallest two Coltene HyFlex®

CM™ file sizes, 20.04 and 20.06, were microscopically unacceptable at any time in this study. Three 20.06 files were graded “microscopically unacceptable” before use in any tooth. Visible failures occurred in all sizes of Coltene HyFlex® CM™ files, except 40.04, at some point in this study. Alazemi et al. (55) found that size 30.04 Coltene HyFlex®

CM™ file failed to return to its original shape after sterilization most often, an observation not seen in the present study.

Hieawy et al. (57) and Elnaghy et al. (58) showed that ProTaper Gold™ had

® higher Af temperature and improved flexibility compared to its predecessor, ProTaper

Universal. An increased martensitic composition at oral temperatures was shown to allow greater amounts of deformation compared to a predominantly austenitic NiTi alloy by

® Park et al. (92). ProTaper Gold™ and Coltene HyFlex CM™ have similarly high Af temperatures and thus similar martensitic content at oral temperatures (45,57). With similar metallurgic characteristics, it can be assumed that ProTaper Gold™ instruments are also meant to visibly deform without separating, similar to Coltene HyFlex® CM™ files, which is supported by the findings of this study (Table 2 and Figures 13 and 14).

® ProFile Vortex™ files have a similar Af temperature to Coltene HyFlex CM™ files

(approximately 50°C) while Vortex Blue™ files have lower Af temperature (56,73). Shen et al. (73) suggested that the improved fatigue and flexibility characteristics of the third-

generation brands may also be due, in part, to their unique phase transformation involving the intermediate R-phase. This could explain how Vortex Blue™ files can have

® lower Af temperature than ProTaper Gold™ and Coltene HyFlex CM™ and still behave like a controlled memory file. Contradictory to findings by Alapati et al. (47), a differential scanning calorimetry (DSC) analysis of ProFile Vortex™ instruments by

Shen et al. (50) failed to show an intermediate R-phase between the martensitic and austenitic phases, which could also correlate to decreased deformation in these files.

However, it is important to note the temperature-modulated DSC provides much greater resolution of NiTi transformations, compared to conventional DSC. Bradley et al. (9) suggested these discrepancies in the NiTi phase proportions are related to variability in the complex heat-treatment process. In the present study, ProFile Vortex™ instruments only showed separation failure (Figure 15); however, previous research is contradictory about whether this is related to the presence or absence of an intermediate R-phase.

We expected the observable condition of the files to degrade with repeated use, as most materials under repeated stress do. Tables 4-7 do not show an overwhelming increase in the number of microscopically unacceptable (score of 4) files through three uses. Table 9 includes 8% of files graded microscopically unacceptable as unused, which increased to 11% by use 3, suggesting a slight observable increase in microscopic degradation through multiple use. Future studies could explore if this trend continues beyond three uses. It did not seem that the microscopically unacceptable files progressed to visual failures (score of 5) in later uses. This could indicate that microscopic file condition is not a valuable predictor of visual file failure. It was noticed in this study that some brands failed to show any quantifiable change, and in some instances were

observed to improve through repeated use (decrease from a score of 4 to a score of 3 with use). This could be directly related to the condition in which the files are received from the manufacturer and the quality of the milling process. The examiners observed varying degrees of surface defects and irregularities in many unused instruments. The most common surface defect was edge rollover, an elongated accumulation of metal flashing along the cutting edges of the files that results from the milling process (Figure 18) (93).

Through repeated use, rollover can be worn off the edges of the file, making the surface condition appear smoother and free of defects (Figure 20).

Table 9 shows that 8% of files were graded as microscopically unacceptable before any use. No consistent type of defect was observed on these microscopically unacceptable unused files. Defects observed included large amounts of edge rollover, rounded edges, edges with large chips in them, and pitting observed on the files surfaces

(Figures 16-19). These findings suggest a need for improved or refined machining processes. Electropolishing has been suggested as a way to improve the surface condition of endodontic files, in which files are immersed in a highly ionic solution under current to remove irregularities and milling marks (34). However, it has not been definitively shown that this process improves clinical performance and efficiency (13,25,33-36).

Table 9 also summarizes the number of files at each series that were evaluated to be microscopically unacceptable after use. The total was 9% of files after Tooth 1 and dropped to 8% after Tooth 2. Though the data allude to the files improving through use, there are other explanations for this occurrence. If surface condition of one of the files worsened, the file might be rated as visually unacceptable in the next series of evaluations of the study. Also, if repeated use removed edge rollover, the visual condition

of the file might improve (Figure 20). This finding was also observed by Troian et al.

(72) and Eggert et al. (94). Although edges of the files appear less irregular, this does not mean the instruments have improved clinical ability. Surface abrasion from repeated use would most likely make the file less efficient, requiring more rotations within the canal space and greater potential cyclic fatigue.

Valois et al. (95) showed that stainless-steel, hand NiTi, and rotary NiTi all contain topographical irregularities resulting from the machining process and all show significant differences between the surfaces of files produced by the same industrial process. Although these defects are not produced by clinical use, Kuhn et al. (29) and

Alapati et al. (32,93) discussed how these machining defects can act as nucleation sites for crack formation. This could provide a mechanism for early endodontic instrument failure, as described by Alapati et al. (30).

Both Table 2 and Table 8 demonstrate that, alarmingly, the most visual file failures (grade of 5) occurred during the first clinical simulation. Also, in all but one instrument, files that failed were rated as usable in all previous evaluations. This would indicate that under the present study criteria, that there were no observable indications that the instrument would fail in all but one file. This is consistent with findings by Pruett et al. (19), who showed that file fracture and plastic deformation occur with little-to-no observable warning or prior wear. The regression results for visually acceptable files

(grades 1-4) indicated that all brands tested were just as likely to be visually acceptable at any number of uses, meaning file brand and number of uses did not influence visual file failure (Table 11).

Regression analysis of microscopically acceptable files (grades 1-3) revealed significance for the number of uses of a file (p=0.0127) but no significance for file brand or the interaction of brand and use number, meaning that no brand significantly showed more wear than another (Table 12). Contrasts of individual uses only proved significant when no use was compared to use 1, 2, and 3. This suggests that levels of wear could not be distinguished between files used 1, 2, and 3 times (Table 13). These results demonstrate that used files do indeed show microscopic wear compared to unused files.

This logistic regression analysis did, however, assume that once a file was graded as microscopically unacceptable its use was discontinued, which was not the case with the methods of this study. Files were often used multiple times after their first

“microscopically unacceptable” grading and may have been given a subsequent “usable” score as previously described. This may have confounded significance for the number of uses seen in the regression analysis.

The odds ratios show that at use 0 (unused) a file of any brand was 3.287 times more likely to be microscopically acceptable (grades 1-3) than at use 1, 4.309 times at use 2, and 5.403 times at use 3, suggesting there is a greater difference in appearance of unused and used files at 3 uses compared to 1 use (Table 14). The assumptions made for the logistic regression also applied to the odds ratios, however.

As previously mentioned in the Materials and Methods section, to avoid bias, examiners were not permitted to know an image brand, position, or stage of treatment, nor were they permitted to see images in series through the three treatment simulations.

This did present some difficulty in identifying torsional deformation of the files. After the observational data from the examiners were collected, the author observed the images in

order of use from the first image at no use to the last image after the third clinical simulation. Greater torsional deformation could be seen in this manner of observation.

One unique observation was that the greatest torsional change seemed to occur commonly after the first use of the file, with minimal change occurring into the third use

(Figure 11). This could be explained by a greater amount of work hardening during the initial use compared to later uses of the same instrument, in which repeated use of the instrument leads to an increase in the proportion of martensite in the alloy and allows less deformation. DSC plots for Coltene HyFlex® CM™ files presented by Shen et al. (53) show only small changes in the Af temperatures for segments of used, deformed and unused files. Further DSC analysis of these brands through multiple uses is recommended.

SUMMARY AND CONCLUSIONS

The purpose of this study was to evaluate the qualitative and quantitative wear/fatigue of three third-generation rotary endodontic file systems (Vortex Blue™,

HyFlex® CM™, and ProTaper Gold™), as well as one second-generation rotary endodontic file system (ProFile Vortex™), following use in extracted posterior human teeth in a simulated clinical model.

One hundred files (five packs of five files across four brands: ProFile Vortex™,

Vortex Blue™, ProTaper Gold™, and Coltene HyFlex® CM™) were examined with an

SEM at the same four positions along their length using a custom-fabricated jig through three clinical simulations. Each pack of files was used to instrument one premolar, one maxillary molar, and one mandibular molar, and simulations were designed to mimic a clinical setting as closely as possible. Two evaluators examined the SEM images and graded them for detection of wear and deformation. Files were categorized as usable, microscopically unacceptable, and visually unacceptable before use and through three simulated uses.

The initial 1-5 grading scale used to evaluate wear and distortion proved to be an unreliable instrument of evaluation. Files from each brand were graded microscopically unacceptable before being used, suggesting a need for improved manufacturing processes. A repeated-measures logistic regression analysis found no significant effects

for brand, amount of use, and their interaction in terms of visual grade, meaning that no file brand was consistently more visibly acceptable (Scores 1-4) and files in their first, second, or third use were no more visually acceptable than another. When analyzing the wear of files that remained microscopically useful (scores 1-3), a significant effect was seen for number of uses (p=0.0127). Contrasts of uses showed significance for use 0 vs. 1, 0 vs. 2, and 0 vs. 3, indicating that unused files were more likely to be microscopically acceptable than used files. No significance was seen for file brand, meaning that no brand was more likely to be graded as microscopically useful than another.

The present study failed to observe a quantitative wear or distortion pattern specific to any brand that limited multiple uses, nor did any one brand show significantly more or less wear/fatigue than the others. Files evaluated as microscopically unacceptable rarely (once) progressed to visually unacceptable (grade of 5) in subsequent uses, suggesting that poor surface condition is not necessarily a precursor to instrument failure.

The ability to predict when and how a file will fail appears to be multimodal, including examination of the instrument for wear, deformation, changes in metallurgic phase composition, surface hardness, and crystalline structure (65-69). Besides visual inspection, most of these examinations cannot be performed in an endodontist’s office.

Previous studies (25,90) have supported using instruments for multiple treatments, since no significant increase in the incidence of separation was observed. This opinion is supported by the findings of this study. Files should be used and disposed of at the operator’s discretion based upon his or her best understanding of the stresses

experienced by the instrument. Third-generation, controlled memory instruments tend to deform plastically and should be discarded when this is observed. These files, however, present minimal risk of clinical separation and canal obstruction, and their use in multiple treatments is supported by the findings and observations of this study.

APPENDIX A TABLES

Table 1 Visual File Failures (Score of 5) by Brand

File Brand N File Failures Failure Rate (%) PROFILE VORTEX™ 25 2 8.0 VORTEX BLUE™ 25 4 16.0 PROTAPER GOLD™ 25 2 8.0 COLTENE HYFLEX®CM™ 25 4 16.0 TOTAL 100 12 12.0

Table 2 Individual file failures (Visual failures)

Use Most Severe Level of File Of Separation or Canal Separation(measured Brand Size/Taper Separation Deformation Tooth Type Curvature from the file tip) PROFILE 25.04 Tooth 1 Separation Premolar 45-60 1 mm VORTEX™ 30.04 Tooth 2 Separation Max. Molar 30-45 1 mm 25.04 Tooth 3 Deformation Max. Molar <30 N/A VORTEX 35.04 Tooth 1 Deformation Mand. Molar 30-45 N/A BLUE™ 30.04 Tooth 1 Deformation Mand. Molar 30-45 N/A 25.04 Tooth 1 Separation Mand. Molar 30-45 1 mm PROTAPER S2 Tooth 1 Deformation Max. Molar 30-45 N/A GOLD™ S1 Tooth 1 Deformation Max. Molar 30-45 N/A

30.04 Tooth 1 Deformation Premolar 30-45 N/A 47

COLTENE 20.06 Tooth 3 Deformation Premolar <30 N/A ® HYFLEX CM™ 25.04 Tooth 3 Deformation Premoalr <30 N/A 20.04 Tooth 1 Deformation Mand. Molar <30 N/A

Table 3 File Brand by Canal Curvatures Experienced

COUNT OF CANAL BRAND CURVATURES p- <30˚ 30-45˚ 45-60˚ >60˚ Total value† VORTEX BLUE™ 27 5 5 0 37 PROTAPER GOLD™ 34 2 0 0 36 COLTENE HYFLEX®CM™ 25 7 4 2 38 PROFILE VORTEX™ 29 9 3 0 41 TOTAL 115 23 12 2 152 0.0426

†Fisher’s Exact Test

Table 4 ProFile Vortex™ Evaluation Summary

Visually Microscopically Unacceptable Unacceptable Usable TOTAL UNUSED 0 3 22 25 20.04 0 0 5 5 25.04 0 2 3 5 30.04 0 1 4 5 35.04 0 0 5 5 40.04 0 0 5 5 TOOTH 1 1 3 21 25 20.04 0 0 5 5 25.04 1 1 3 5 30.04 0 1 4 5 35.04 0 1 4 5 40.04 0 0 5 5 TOOTH 2 1 2 21 24 20.04 0 0 5 5 25.04 0 1 3 4 30.04 1 1 3 5 35.04 0 0 5 5 40.04 0 0 5 5 TOOTH 3 0 3 20 23 20.04 0 1 4 5 25.04 0 1 3 4 30.04 0 1 3 4 35.04 0 0 5 5 40.04 0 0 5 5 TOTAL 2 11 84 97

Table 5 ProTaper Gold™ Evaluation Summary

Visually Microscopically Unacceptable Unacceptable Usable TOTAL UNUSED 0 1 24 25 S1 0 1 4 5 S2 0 0 5 5 F1 0 0 5 5 F2 0 0 5 5 F3 0 0 5 5 TOOTH 1 2 2 21 25 S1 1 1 3 5 S2 1 1 3 5 F1 0 0 5 5 F2 0 0 5 5 F3 0 0 5 5 TOOTH 2 0 1 22 23 S1 0 1 3 4 S2 0 0 4 4 F1 0 0 5 5 F2 0 0 5 5 F3 0 0 5 5 TOOTH 3 0 2 21 23 S1 0 1 3 4 S2 0 1 3 4 F1 0 0 5 5 F2 0 0 5 5 F3 0 0 5 5 TOTAL 2 6 88 96

Table 6 Vortex Blue™ Evaluation Summary

Visually Microscopically Unacceptable Unacceptable Usable TOTAL UNUSED 0 1 24 25 20.04 0 0 5 5 25.04 0 0 5 5 30.04 0 0 5 5 35.04 0 0 5 5 40.04 0 1 4 5 TOOTH 1 3 3 19 25 20.04 0 0 5 5 25.04 1 0 4 5 30.04 1 2 2 5 35.04 1 0 4 5 40.04 0 1 4 5 TOOTH 2 0 1 21 22 20.04 0 0 5 5 25.04 0 0 4 4 30.04 0 1 3 4 35.04 0 0 4 4 40.04 0 0 5 5 TOOTH 3 1 2 19 22 20.04 0 0 5 5 25.04 1 0 3 4 30.04 0 1 3 4 35.04 0 0 4 4 40.04 0 1 4 5 TOTAL 4 7 83 94

Table 7 Coltene HyFlex® CM™ Evaluation Summary

Visually Microscopically Unacceptable Unacceptable Usable TOTAL UNUSED 0 3 22 25 20.04 0 0 5 5 20.06 0 3 2 5 25.04 0 0 5 5 30.04 0 0 5 5 40.04 0 0 5 5 TOOTH 1 2 1 22 25 20.04 1 0 4 5 20.06 0 1 4 5 25.04 0 0 5 5 30.04 1 0 4 5 40.04 0 0 5 5 TOOTH 2 0 3 20 23 20.04 0 1 3 4 20.06 0 2 3 5 25.04 0 0 5 5 30.04 0 0 4 4 40.04 0 0 5 5 TOOTH 3 2 3 18 23 20.04 0 1 3 4 20.06 1 2 2 5 25.04 1 0 4 5 30.04 0 0 4 4 40.04 0 0 5 5 TOTAL 4 10 82 96

Table 8 Visually Unacceptable (Score of 5) Files by Use

VORTEX PROTAPER COLTENE PROFILE BLUE™ GOLD™ HYFLEX®CM™ VORTEX™ TOTAL N % UNUSED 0 0 0 0 0 100 0.0 TOOTH 1 3 2 2 1 8 100 8.0 TOOTH 2 0 0 0 1 1 92 1.0 TOOTH 3 1 0 2 0 3 91 3.0 TOTAL 4 2 4 2 12 383 3.0

Table 9 Microscopically Unacceptable (Score of 4) Files by Use

VORTEX PROTAPER COLTENE PROFILE BLUE™ GOLD™ HYFLEX®CM™ VORTEX™ TOTAL N % UNUSED 1 1 3 3 8 100 8.00 TOOTH 1 3 2 1 3 9 100 9.00 TOOTH 2 1 1 3 2 7 92 8.00 TOOTH 3 2 2 3 3 10 91 11.0 TOTAL 7 6 10 11 34 383 9.00

Table 10 Clinically Usable (Scores 1-3) Files by Use

VORTEX PROTAPER COLTENE PROFILE BLUE™ GOLD™ HYFLEX®CM™ VORTEX™ TOTAL N % UNUSED 24 24 22 22 92 100 92.0 TOOTH 1 19 21 22 21 83 100 83.0 TOOTH 2 21 22 20 21 84 92 91.0 TOOTH 3 19 21 18 20 78 91 86.0 TOTAL 83 88 82 84 337 383 88.0

Table 11 Regression Results for Visually Acceptable (Scores 1-4) Files

Num er. Denom. Chi- Pr > Effect DF* DF* Square Chi-Sq BRAND 3 284 2.59 0.4589 USE NUMBER 2 284 1.03 0.5971 USE NO.*BRAND 6 284 0.910 0.9888

* DF= degrees of freedom

Table 12 Regression Results for Microscopically Acceptable (Scores 1-3) Files

Num er. Denom. Chi- Pr > Effect DF* DF* Square Chi-Sq BRAND 3 380 2.640 0.4510 USE NUMBER 3 380 10.83 0.01270 USENO.*BRAND 9 380 2.220 0.9875

* DF= degrees of freedom

Table 13 Microscopically Acceptable (Scores 1-3) Regression Contrasts

Num er. Denom. F Pr > F USE DF* DF* Value 0 VS 1 1 380 4.880 0.02770 0 VS 2 1 380 7.610 0.006100 0 VS 3 1 380 10.33 0.001400 1 VS 2 1 380 0.4800 0.4900 1 VS 3 1 380 1.670 0.1976 2 VS 3 1 380 0.3700 0.5422

* DF= degrees of freedom

Table 14 Microscopically Acceptable (Scores 1-3) Odds Ratios

95% Confidence Comparison Estimate DF* Limits

VORTEX BLUE™ vs 0.7220 380 0.2640 1.9760 PROTAPER GOLD™ VORTEX BLUE™ vs 1.501 380 0.6200 3.6380 COLTENE HYFLEX®CM™ VORTEX BLUE™ vs 1.049 380 0.4240 2.5990 PROFILE VORTEX™ PROTAPER GOLD™ vs 2.079 380 0.8370 5.1670 COLTENE HYFLEX®CM™ PROTAPER GOLD™ vs 1.453 380 0.5720 3.6870 PROFILE VORTEX™ ® COLTENE HYFLEX CM™ vs 0.6990 380 0.3140 1.5540 PROFILE VORTEX™ USE NO. 0 vs 1 3.287 380 1.140 9.4790

USE NO. 0 vs 2 4.309 380 1.522 12.202

USE NO. 0 vs 3 5.403 380 1.925 15.164

USE NO. 1 vs 2 1.311 380 0.6070 2.8320

USE NO. 1 vs 3 1.644 380 0.7710 3.5040

USE NO. 2 vs 3 1.254 380 0.6050 2.6000

* DF= degrees of freedom

APPENDIX B FIGURES

Figure 1. Unit cells for the simple cubic (A), body- centered cubic (B), face-centered cubic (C), and hexagonal close-packed (D) structures (11).

Figure 2. Schematic illustration of atom displacement for twinning transformation from austenite NiTi to martensite NiTi (12).

Figure 3. SEM images of the cross-sectional geometry of a radial-landed instrument (left) and a non-landed, triangular instrument (right) (22).

Figure 4. Austenite to martensite phase transformation, showing shape memory behavior. The martensitic phase of NiTi can be deformed and will return to its original shape when heated above the austenite-finish temperature (46).

Figure 5. Quanta 200 (FEI, Hillsboro, Oregon) SEM used to examine the instruments.

Figure 6. The custom-made aluminum jig (Ted Pella, Inc., Redding, CA) was fabricated in the Ohio State University Department of Chemistry and Biochemistry Machine Shop so that the orientation of the files could be maintained throughout the study and the same surface and cutting edges could be repeatedly examined following each procedure.

Figure 7. The custom-made aluminum jig (Ted Pella, Inc., Redding, CA) with Vortex Blue™ files inserted and maintaining the same file orientation.

10 BLUE GOLD 5 HYFLEX

0 VORTEX

Usable Usable Usable Usable

Unacceptable-EM Unacceptable-EM Unacceptable-EM Unacceptable-EM

Unacceptable-Visual Unacceptable-Visual Unacceptable-Visual CLEAN TOOTH 1 TOOTH 2 TOOTH 3

Figure 8. Evaluation summary by brand and use.

Visual File Status by Brand and Number of Uses 100

90 4)

- 80 70 60 50 40 30 20 % USEFUL (Scores 1 (Scores USEFUL % 10 0 VortexVORTEX BLUE™Blue PROTAPERGOLD GOLD™ COLTENEHYFLEX HYFLEX ®CM™ PROFILEVORTEX VORTEX™ USE 1 88 92 92 96 USE 2 88 92 92 92 USE 3 84 92 84 92

Figure 9. Visual file status by brand and number of uses.

Microscopic File Status by Brand and Number of Uses 100

3) 80 - 70 60 50 40 30

20 % USEFUL (Scores 1 (Scores USEFUL % 10 0 ® VORTEXBLUE BLUE™ PROTAPERGOLD GOLD™ COLTENEHYFLEX HYFLEX CM™ PROFILEVORTEX VORTEX™ USE 0 96 96 88 88

USE 1 76 84 80 84 USE 2 76 80 72 80 USE 3 72 80 64 76

Figure 10. Microscopic file status by brand and number of uses.

CLEAN TOOTH 1

TOOTH 2 TOOTH 3

Figure 11. The D1 position of a Coltene® HyFlex™ file through each use.

Figure 12. A size 25.04 Vortex Blue™ instrument that separated during its first use.

Figure 13. A size 30.04 Coltene HyFlex® CM™ file that failed by plastic deformation in its first use, observed at 25X magnification.

Figure 14. A size S2 ProTaper Gold™ file that failed by plastic deformation in its first use, observed at 50X magnification.

Figure 15. A size 25.04 ProFile Vortex™ instrument that separated during its first use.

CLEAN TOOTH 1

TOOTH 2 TOOTH 3

Figure 16. A size 30.04 Vortex Blue™ file experiencing progressive pitting of the file surface through each use.

Figure 17. The size 30.04 Vortex Blue™ after use 3 from the previous image, showing the severity of the surface pitting.

Figure 18. An unused size 30.04 Vortex Blue™ file, showing edge rollover and debris accumulation.

Figure 19. An unused size 30.04 ProFile Vortex™ file, showing edge rollover and edge defects.

CLEAN TOOTH 1

TOOTH 2 TOOTH 3

Figure 20. A size 40.04 Vortex Blue™ file, showing removal of edge rollover through continued use.

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