Mechanical Properties of Graded Thermodynamic Nickel Titanium Archwires in Bending and Torsion

Transcription

1 Mechanical Properties of Graded Thermodynamic Nickel Titanium Archwires in Bending and Torsion by Ouliana Oguienko A thesis submitted in conformity with the requirements for the degree of Master of Science Faculty of Dentistry University of Toronto Copyright by Ouliana Oguienko, 2017

2 The Mechanical Properties of Graded Thermodynamic Nickel Titanium Archwires in Bending and Torsion Abstract Ouliana Oguienko Master of Science Faculty of Dentistry University of Toronto 2017 Background: Graded thermodynamic nickel titanium (GT-NiTi) wires purportedly deliver differential force levels in different archwire regions. No studies evaluating the force delivery characteristics of such wires currently exist. Objective: To evaluate the mechanical properties of GT-NiTi wires in bending and torsion. Methods: Six samples of x and x of GT-NiTi wires were divided into incisor, premolar, and molar sections and each segment was tested in bending and torsion. The characteristics of the unloading superelastic plateau were evaluated and compared with similar segments of non-graded, control NiTi wires. Results: Statistically significant differences in mean force and moment levels were found between the three sections of GT-NiTi wires, but not in control wires. Small but statistically significant differences in length and slope were also found. Conclusion: GT-NiTi wires deliver different forces and moments in different archwire sections. Clinicians can use this knowledge to optimize force delivery for treatment. ii

3 Acknowledgments I would like to thank my supervisor, Dr. Sunjay Suri, for undertaking this project with me and his unwavering support and encouragement throughout. I would also like to thank my research committee members, Dr. Craig A. Simmons and Dr. Cari Whyne, for their input and involvement. Additionally, I would like to acknowledge Jian Wang for his extensive efforts in methods design and testing, Dr. Wendy Lou and Bingqing Shen for statistical analysis they provided, Edwin Wong for his help with error analysis, Alexander Josephson for editing, and Dr. John Daskalogiannakis for being a great sounding board. Special thanks to Dental Research Institute, Faculty of Dentistry, Alpha Omega Research Grant, the Woodside fund and the John Fasken fund for their financial contributions. iii

4 Table of Contents Acknowledgments... iii Table of Contents... iv List of Tables... vii List of Figures... ix List of Abbreviations... xii Chapter 1 Review of Literature Introduction Properties of Nickel Titanium Austenite, Martensite, Shape Memory and Superelasticity Temperature Transition Range, Heat- and Stress- Induced Martensite The Physics of Heat- and Stress- Induced Superelasticity and Shape Memory TTR and SIM Types of NiTi Archwires Stabilized Martensitic (work-hardened) Nitinol Superelastic-Active Austenitic NiTi Thermodynamic-Active Martensitic NiTi Graded Thermodynamic NiTi Comparing the Properties of Commercial NiTi Archwires Lab Bench Tests Summary Purpose of the Current Investigation Hypothesis...17 Chapter 2 Materials and Methods Wire Selection...18 iv

5 2.2 Preparation of Specimens Experimental Design Three-point bending test: Torsion Test Equipment Data Analysis...23 Chapter 3 Results Wires Dimensions Bending and Torsion Curves Repeatability of Superelastic Plateau Identification Error Analysis Bending Mean Force of the Unloading Superelastic Plateau Mean Length of the Unloading Superelastic Plateau Mean Slope of the Unloading Superelastic Plateau Torsion Mean Torsional Moment of the Unloading Superelastic Plateau Mean Length of the Unloading Superelastic Plateau Mean Slope of the Unloading Superelastic Plateau...46 Chapter 4 Discussion Cross-Sectional Dimensions Lab-Bench Tests Three-Point Bending Test Torsion Test Mean Force and Torsional Moment of the Unloading Superelastic Plateau Mean Length of the Unloading Superelastic Plateau in Bending and Torsion...56 v

6 4.5 Mean Slope of the Unloading Superelastic Plateau in Bending and Torsion Limitations of the study Clinical Application Suggestions for Future Research...61 Chapter 5 Conclusions...62 References...63 vi

7 List of Tables Table 1. Retail prices (CAD) for various NiTi carried by GAC (as of June 2017) Table 2. Orthodontic archwires tested. Conventional thermodynamic archwires NS were used as controls...18 Table 3. The height and width of the archwires tested. The results were the same for both BF and NS Table 4. Intraclass correlation coefficient (ICC) for identifying the starting and ending point of the unloading superelastic plateau for both bending and torsion tests.. 28 Table 5. Mean force (SD) of the unloading superelastic plateau in bending...31 Table 6. Mean increase (+) in force of the unloading superelastic plateau in bending expressed as percentage and grams of force difference from incisor to premolar (I à Pm), premolar to molar (Pm à M) and incisor to molar (I à M)..33 Table 7. Mean length (SD) of the unloading superelastic plateau in bending Table 8. Mean percent change (+ increase) in length of the unloading superelastic plateau in bending expressed as percentage and millimeter difference from incisor to premolar (I à Pm), premolar to molar (Pm à M) and incisor to molar (I à M) Table 9. Mean slope (SD) of the unloading superelastic plateau in bending Table 10. Mean increase (+) in the slope of the unloading superelastic plateau in bending expressed as percentage and grams per millimeter difference between incisor and premolar (I à Pm), premolar and molar (Pm à M), and incisor and molar (I à M)...39 Table 11. Mean moment (SD) of the unloading superelastic plateau in torsion.. 41 Table 12. Mean change (+ increase or decrease) in the torsional moment of the unloading superelastic plateau in bending expressed as percentage and gramsmillimeter difference between incisor to premolar (I à Pm), premolar to molar (Pm à M) and incisor to molar (I à M) vii

8 Table 13. Mean length (SD) of the unloading superelastic plateau in torsion Table 14. Mean change (+ increase or decrease) in length of the unloading superelastic plateau in torsion expressed as percentage and degree difference between incisor to premolar (I à Pm), premolar to molar (Pm à M) and incisor to molar (I à M) Table 15. Mean slope (SD) of the unloading superelastic plateau in torsion...47 Table 16. Mean change (+ increase or - decrease) in the slope of the unloading superelastic plateau in bending expressed as percentage and gramsmillimeter per degree change from incisor to premolar (I à Pm), premolar to molar (Pm à M) and incisor to molar (I à M) viii

9 List of Figures Figure 1. Stress-strain curve showing superelastic behavior characteristic of some NiTi wires. This occurs at certain stress levels due to crystalline re-organization such that both martensitic and austenitic phases co-exist (modified from Ferreira et al. 1 ) Figure 2. Austenitic-Martensitic phase transformation vs. temperature graph. M martensite; A austenite, i initial, f final, d-deformation (temperature). As the temperature increases, the phase of the alloy is converted from completely martensite phase (<M f ) to completely austenite (>A f ). After M d, SIM cannot be induced (modified from Ferreira et al. 1 )....4 Figure 3. Stress-strain curve for different types of archwires, see text for description (adapted from Miura et al. 2 ) Figure 4. Schematic diagram illustrating the division of an archwire into three segments incisor (I), premolar (Pm) and molar (M)..19 Figure 5. Schematic diagram (left) and photographic (right) representation of the three-bending test set-up...20 Figure 6. Schematic diagram (left) and photographic (right) representation of the torsion test setup 22 Figure 7. Identification of the superelastic plateau on force-displacement curve acquired from the three-point bending test Figure 8. Identification of the superelastic plateau on torsional moment-angle curve acquired from the torsion test..24 Figure 9. Typical load-deflection curves of the incisor, premolar, molar segments of each type and dimension of wire tested (BF BioForce, C NeoSentalloy).. 26 Figure 10. Typical angle-torsion moment curves of the incisor, premolar, molar segments of each type and dimension of wire tested (BF BioForce, C NeoSentalloy) ix

10 Figure 11. Box plots representing the spread of the unloading superelastic plateau force values in each section of the archwire for each archwire type and dimension tested in bending (BF = BioForce, C = Control, NeoSentalloy)..32 Figure 12. Stacked columns representing the increase in the mean unloading superelastic plateau force values in bending for x BF (left) and x BF (right). Darker blue represents the additional force gain of its respective section relative to the adjacent, more anterior section (light blue). Percentage increase is expressed at the top (I = incisor; Pm = premolar; M = molar) Figure 13. Box plots representing the spread of the unloading superelastic plateau length values in each section of the archwire for each archwire type and dimension tested in bending (BF = BioForce, C = Control, NeoSentalloy).35 Figure 14. Stacked columns representing the increase in the mean unloading superelastic plateau length values in bending for x BF (left) and x BF (right). Darker green represents the additional length gain of its respective section relative to the adjacent, more anterior section (light green). Percentage increase is expressed at the top (I = incisor; Pm = premolar; M = molar)...36 Figure 15. Box plots representing the spread of the unloading superelastic plateau slope values in each section of the archwire for each archwire type and dimension tested in bending (BF = BioForce, C = Control, NeoSentalloy).38 Figure 16. Stacked columns representing the increase in the mean unloading superelastic plateau slope values in bending for x BF (left) and x BF (right). Darker orange represents the additional length gain of its respective section relative to the adjacent, more anterior section (light orange). Percentage increase is expressed at the top (I = incisor; Pm = premolar; M = molar) Figure 17. Stacked columns representing the increase in the mean unloading superelastic plateau slope values in bending for x C (left) and x C (right). Darker orange represents the additional length gain of its respective section relative to the adjacent, more anterior section (light orange). Percentage increase is expressed at the top(i = incisor; Pm = premolar; M = molar)...40 x

11 Figure 18. Box plots representing the spread of the unloading superelastic plateau torsional moment values in each section of the archwire for each archwire type and dimension tested in torsion (BF = BioForce, C = Control, NeoSentalloy)...41 Figure 19. Stacked columns representing the increase in the mean unloading superelastic plateau torsional moment values for x BF (left) and x BF (right). Darker blue represents the additional moment gain of its respective section relative to the adjacent, more anterior section (light blue). Percentage increase is expressed at the top (I = incisor; Pm = premolar; M = molar) Figure 20. Box plots representing the spread of the unloading superelastic plateau length values in each section of the archwire for each archwire type and dimension tested in torsion (BF = BioForce, C = Control, NeoSentalloy). 44 Figure 21. Stacked columns representing the decrease in the mean unloading superelastic plateau length values as changes from anterior to posterior sections in torsion for x BF (left) and x BF (right). Darker green represents the length decrease of its respective section relative to the adjacent, more anterior section (light green). Relative percent decrease is expressed at the bottom (I = incisor; Pm = premolar; M = molar).46 Figure 22. Box plots representing the spread of the unloading superelastic plateau slope values in each section of the archwire for each archwire type and dimension tested in torsion (BF = BioForce, C = Control, NeoSentalloy).47 Figure 23. Stacked columns representing the increase in the mean unloading superelastic plateau slope values in torsion for x BF (left) and x BF (right). Darker orange represents the additional length gain of its respective section relative to the adjacent, more anterior section (light orange). Percentage increase is expressed at the top (I = incisor; Pm = premolar; M = molar)...49 xi

12 List of Abbreviations ADA BF GT-NiTi ISO NiTi NGT-NiTi NS SD SIM SS TTR American Dental Association GAC s BioForce (Islandia, NY) Graded Thermodynamic Nickel Titanium International Organization for Standardization Nickel Titanium Non-Graded Thermodynamic Nickel Titanium GAC s NeoSentalloy (Islandia, NY) Standard Deviation Stress-Induced Martensite Stainless Steel Temperature Transition Range xii

13 1 Chapter 1 Review of Literature 1.1 Introduction Orthodontic archwires are an integral part of fixed orthodontic appliances. By engaging it into the brackets, the archwire exerts a force that generates orthodontic tooth movement. The magnitude of this force is important a force too high may lead to root resorption or delayed tooth movement, whereas a force too low may be insufficient to induce a biologic response. 3 In general, light continuous forces are thought to be more efficient in achieving the desired tooth movements compared to heavy forces. 4 In order to achieve light continuous forces, it is essential for the archwire to exhibit low stiffness and high range. The length of the free wire between brackets, the cross-sectional dimension of the wire, and the archwire alloy are three variables within the clinician s control that influence stiffness. 4 In the early days of orthodontics, when stainless steel (SS) was the primary alloy used, the only way to reduce stiffness for a particular application was to increase the wire length by incorporating loops, or to change the size of the wire. 4 With the advent of NiTi, the alloy type has become an important factor in archwire selection. 5 A plethora of NiTi archwires is available on the market today. The mechanical characteristics of any given NiTi wire are strongly influenced by manufacturer s processing and specifications. 6 8 Thus the force delivery capabilities of these archwires are much less intuitive. The clinician must rely on the information provided by the manufacturer, which often does not adequately describe the force values that can be expected from the archwire. 9 While a number of independent studies 6 8,10 14 have attempted to address this issue by comparing mechanical properties of various NiTi alloys in vitro under the same conditions, the selection of tested commercial NiTi wires is not comprehensive. The aim of the following literature review is to give the reader a deeper understanding of NiTi wires in the context of orthodontic practice. 1.2 Properties of Nickel Titanium The discovery of NiTi alloy can be traced back to the laboratory of W.J. Buehler, a metallurgist working for the U.S. Naval Ordnance Laboratory in the early 1960s as part of the space

14 2 program. 15 Using nickel and titanium in almost equal atomic proportions (55%Ni:45%Ti), he developed the first NiTi alloy which he named Nitinol, an acronym for Nickel Titanium Naval Ordnance Laboratory. Buehler observed that when the alloy was fixed in a specific shape, heattreated and subsequently cooled, it acquired special thermodynamic properties. In low temperatures, the alloy could be plastically deformed with ease but return to its original shape by merely increasing the temperature, a phenomenon not observed in most other metals. 15 Furthermore, NiTi was found to possess a number of other valuable characteristics. Compared to SS, NiTi is exceptionally resistant to permanent deformation, exhibits superb springback 16 and is times less stiff. 17 Good biocompatibility and corrosion resistance of this alloy make it an excellent choice to be used intraorally. 18 Before differences between the various types of NiTi archwires can be appreciated, it is essential to discuss the alloy s unique crystal structure and behavior including shape memory and superelasticity Austenite, Martensite, Shape Memory and Superelasticity NiTi exists in two main solid-state crystalline phases, austenite and martensite. Austenite is the stiffer form of the alloy that exhibits higher yield strength and is stable at higher temperatures. It has a body-centered (cubic or tetragonal) configuration. Martensite is the soft, pliable form of the alloy with lower strength and stiffness that is more stable at lower temperatures. It has a facecentered (hexagonal close packed) configuration. 1 Intermediate phase, rhombohedral R phase with simple hexagonal configuration also has been identified. 19 While the stiffness of the alloy in austenite phase is higher than in martensite phase, it is the lowest when these phases coexist or if the R phase is present. 20 In fact, it is when the alloy is in the solid-solid transitional phase that the so-called superelastic plateau is observed on a stressdeflection curve (Figure 1). Clinically, this means that the wire exerts the same amount of force over a range of deflections and alloys capable of producing such behavior are deemed superelastic. 1 Shape memory is another unique property arising from the alloy s ability to reversibly convert between the two solid phases. Clinically, this means that the wire will be able to return to its parent (austenitic) shape after an apparent permanent deformation while in the martensitic state. 2

15 3 The conversion from the austenitic to martensitic phase can occur either by lowering the ambient temperature or by applying stress. Thus, a distinction is often made when describing the superelastic and shape memory properties of NiTi alloys. Temperature-related superelasticity is referred to as thermoelastic behavior and stress-related superelasticity is called pseudoelastic behavior. 19 Similarly, shape memory can be either temperature- or stress-induced, respectively. The type of behavior that can be expected from any given NiTi wire is related to its temperature transition range (TTR) and is specified by the manufacturer. 19 Figure 1. Stress-strain curve showing superelastic behavior characteristic of some NiTi wires. This occurs at certain stress levels due to crystalline re-organization such that both martensitic and austenitic phases co-exist (modified from Ferreira et al. 1 ) Temperature Transition Range, Heat- and Stress- Induced Martensite Temperature transition range (TTR) describes the temperature range at which the martensitic phase changes to the austenitic phase and vice-versa. 21 For each NiTi archwire, four temperature points can be defined: M s, M f, A s, A f. M s and M f refer to martensite starting and finishing temperatures and A s, A f refer to austenite starting and finishing temperatures, respectively (Figure 2). Below M f, only martensite phase exists and above A f only austenite phase exists. Between these points, the alloy exhibits both phases in various proportions.

16 4 It can be said that between alloy s specific M f and A f the wire exhibits thermoelastic behavior. In other words, as the temperature is lowered below A f, the alloy undergoes heat-induced martensite transformation. At temperatures higher than A f, superelasticity can still be observed but with bending of the alloy. This is called pseudoelastic behavior and occurs due to stressinduced martensite (SIM) transformation. The temperature limit until which SIM can be induced is called M d. 19 For a typical NiTi alloy, TTR is broad and can range from -200 to +200 C. However, small additions of other elements such as copper or specific thermal processing can significantly narrow the range. 18 M d Figure 2. Austenitic-Martensitic phase transformation vs. temperature graph. M martensite; A austenite, i initial, f final, d - deformation (temperature). As the temperature increases, the phase of the alloy is converted from completely martensite phase (<M f ) to completely austenite (>A f ). After M d, SIM cannot be induced (modified from Ferreira et al. 1 ) The Physics of Heat- and Stress- Induced Superelasticity and Shape Memory When the temperature is lowered below A f, the atoms in austenite crystalline structure undergo a type of shearing to assume a new atomic arrangement. 15 This so-called twinned martensite

17 5 appears no different than the alloy in austenite phase on a macroscopic scale, however, its mechanical properties are different. This difference is easily appreciated when a force is applied. Compared to the alloy in the stiff austenitic state, the twinned martensite can be plastically deformed with relative ease, at which point it is referred to as de-twinned martensite. 18 Conversely, when the temperature increases, the atoms motion is also increased and the arrangement changes to a structure able to accommodate the increased motion. 15 At a macroscopic scale, the previously made bends are unraveled and the alloy assumes the original parent shape with its corresponding mechanical properties. 18 SIM transformation occurs because the introduced strain also leads to atomic shearing and hence de-twinned martensite state. 15 Up to 8% strain, the stress/strain rate reaches a plateau as the proportion of martensite phase increases. Strain above 8%, however, is beyond martensite shear and thus not recoverable. This is significant given that other metals such as SS are only able to regain full recovery up to a maximum of 1% strain TTR and SIM The TTR for NiTi alloys can be set sufficiently narrow to take advantage of these phase conversions in orthodontic treatments. For NiTi alloy with 1:1 nickel to titanium ratio, the TTR is around +100 C to below -50 C. 18 This range can be lowered by varying the ratio of nickel to titanium or by an addition of small amounts of other elements such as cobalt and iron. 15 For instance, some NiTi wires have A f set close to the body temperature. This indicates that the wire is in phase transition at room temperature and thus pliable and easy to engage into brackets. However, once it reaches oral temperature, the alloy turns completely austenitic. This allows it to regain its initial archform and apply the necessary orthodontic force to drive tooth movement. 18 As already mentioned, the highest temperature at which SIM can be induced is referred to as M d. 19 Fortunately this temperature limit is usually above A f and oral temperatures, 22 meaning that the martensite can be induced in local areas of severe crowding while the rest of the wire remains austenitic. This is beneficial as the force levels will be decreased rather than increased in the areas of most crowding, provided that A f is not much below the oral temperature. If A f is much too low, the wire will have a stronger tendency to remain austenitic and stiff even with deflections. 22

18 6 It is important to note the interdependent relationship between TTR and stress. Testing a few commercial thermodynamic NiTi wires in vitro, Santoro et al. 20 found that when the wire was engaged into brackets simulating clinical scenarios, the A f increased as a function of stress. That is, higher temperature was needed to turn a stressed wire to fully austenitic phase compared to a relaxed one. Conversely, the amount of stress required to induce SIM is directly related to temperature such that the higher the temperature, the more stress is needed to induce martensite. 8 Unstandardized temperature and wire states (stressed vs. unstressed) are among the many factors that complicate measuring force levels intraorally and are credible sources of variation in studies that attempt to characterize force delivery of NiTi wires in vitro Types of NiTi Archwires After Buehler s discovery of the NiTi alloy, it was George F. Andreasen who brought NiTi archwires into orthodontic use in Unitek Company first introduced NiTi wires commercially under its original name, Nitinol. Since then, a large diversity of NiTi wires has become available and the name NiTi serves as an umbrella term that describes the family of these archwires today. 18 Based on their mechanical properties and behavior, NiTi archwires can be grouped into four categories. Evans and Durning 21 proposed a chronological classification of orthodontic archwires to include the following five groups: Phase I: non-niti alloys such as stainless and gold alloys ( s) Phase II: stabilized martensitic alloys such as original Nitinol (the 1970 s); Phase III: superelastic-active austenitic alloys (eg. Chinese and Japanese, 1980 s) Phase IV: thermodynamic-active martensitic alloys (eg. CuNiTi 40 C, early 1990 s) Phase V: graded thermodynamic alloys (eg. GAC s BioForce (BF), late 1990 s). The following is a discussion of Phase II to Phase V NiTi archwires Stabilized Martensitic (work-hardened) Nitinol The original Nitinol wire did not exhibit phase transition characteristics because cold working inhibited this property and thus the wire was non-superelastic. 22 Although it did not demonstrate shape memory or superelasticity (hence the name, stabilized martensitic ), compared to the available archwires at the time, it still enjoyed great popularity due to its low stiffness (20% of

19 7 SS) and increased working range (2.5 times SS). 24 Clinicians appreciated its ability to be engaged into the most crowded cases without being permanently deformed Superelastic-Active Austenitic NiTi In the early 1980s, work at the General Research Institute for Non-ferrous Metals in Beijing and the Furukawa Electric Co Ltd in Japan lead to the development of the first superelastic wires. 19 Similar in composition, these wires came to be known as Chinese and Japanese NiTi wires, respectively. The TTR of these superelastic wires is set below oral temperature, therefore, they are austenitic at the habitual oral temperature but exhibit SIM under stress. 25 These wires have excellent springback properties (4-5 times SS) and provide constant force levels at increasing deflections (superelastic plateau). 2,26 Using tensile and three-point bending tests, Miura et al. 2 showed that compared to SS, cobaltchromium nickel (Co-Cr-Ni), and work-hardened NiTi (Nitinol), the superelastic NiTi archwires were the least likely to show permanent deformation after activation. It was also the only wire type to exhibit a superelastic plateau on a load-deflection curve (Figure 3). While the stress increases linearly with strain in other archwires, in superelastic NiTi archwires this is true up to a point (point a). After a certain level of strain is reached, the stress remains relatively constant despite increasing strain, demonstrating the superelastic plateau (a-b, and d-c). This occurs due to the phase transformation from austenite (point a) to martensite (point b) and reversely from martensite (point c) to austenite (point d). The difference in stress values between the loading and unloading curves is called hysteresis and can also be observed with the superelastic archwires. Hysteresis represents the difference in force needed to engage the wire into the brackets (loading) and the force exerted by the wire onto the dentition (unloading). 4 The unloading portion (c-d) is typically of interest to researchers and clinicians as it represents the working force driving orthodontic tooth movement. 9 In summary, due to SIM, superelastic NiTi wires are able to deliver low force levels in areas of crowding over large wire deflections. They were also thought to cause more physiologically sound tooth movement with minimal side effects. Needless to say, this archwire has become a popular choice for the aligning stage of orthodontic treatment. 21

20 8 Figure 3. Stress-strain curve for different types of archwires, see text for description (adapted from Miura et al. 2 ) Thermodynamic-Active Martensitic NiTi As the manufacturing processes improved, it became possible to narrow the TTR and produce alloys that would also exhibit heat-induced shape memory effect and thermoelasticity in contrast to pseudoelasticity characteristic of the previous generation NiTi. 21 While active-austenitic NiTi wires are fully austenitic at and above room temperatures, the A f of thermodynamic active martensitic NiTi wires is above room temperatures. This implies that they only turn rigid (austenitic) when inside the mouth but are relatively pliable and are easily engaged into most malpositioned brackets at room temperature. 12 How rigid the wire becomes intraorally depends on the intraoral temperature and the A f temperature set by the manufacturer. If the predominant average intraoral temperature is considered to be around 35.5 C, 27 thermodynamic NiTi wires with A f of 27 C would generally deliver higher forces than a wire with A f of 40 C which would

21 9 only achieve full austenitic phase and the highest force expression after occasional sips of hot food/liquid. 8 The ability to more precisely control TTR has allowed the manufacturers to develop wires of the same dimension but different force levels. Sentalloy and NeoSentalloy (NS) archwires by GAC are examples of thermodynamic archwires and are available in heavy, medium, and light force variety as dictated by their individually set A f. Copper NiTi (CuNiTi) wires are also classified as thermodynamic. They were developed after discovering that by incorporating small amounts of copper into NiTi alloys, it increased their sensitivity to oral temperatures. 21 They are similarly available in three types, CuNiTi 27 C, CuNiTi 35 C, and CuNiTi 40 C, the names of which reflect their A f temperatures and correspond to heavy, medium, and light force delivery, respectively. 28 The mechanical properties of thermodynamic NiTi have been considered to have several advantages over previous generations. These archwires generally deliver 25-30% of the force of active austenitic archwires at the same temperatures. 21 Since the manufacturing of such wires allows relatively large rectangular dimension wires to deliver very light forces, they can potentially be used in the very beginning of treatment to gain three dimension control. 21 Exceptionally low forces may be especially beneficial in treatment of patients with reduced periodontium. 22 Some evidence also suggests that dynamic loading may be more favorable for bone remodeling than static loading. 29 Since the force levels are likely to fluctuate throughout the day with the intake of food/drinks with thermodynamic archwires, these archwires may be more biologically favorable. 21 On the downside, thermodynamic wires are more expensive and very sensitive to manufacturing processes. They may provide no or very little force in the unloading curve if not made correctly or used in small diameters, and may thus be inefficient. 8 It is also not possible to feel how much force the wire may be delivering intraorally as with conventional non-thermal wires. 4 It is important to note that the manufacturers of thermoelastic wires typically report force values delivered at 35 C to 37 C, considered representative of oral temperature for an average individual. 27 However, in mouth-breathers, the intraoral temperature is lower, typically in the ranges of C, and thus thermodynamic wires with lower A f may need to be used to achieve the desired force levels. Indeed, Sakima et al. 8 showed that very light rectangular thermal NiTi

22 10 wires such as Ormco s CuNiTi40 C and NS F200g exerted no force in temperatures below 35 C in a bending test and thus may not be appropriate for mouth breathers Graded Thermodynamic NiTi It is generally accepted that to achieve uniform stress that optimizes the rate of tooth movement, the force magnitude should correspond to the surface area of the tooth. 4 More specifically, much lower forces are needed to efficiently move anterior teeth than posterior teeth. Graded thermodynamic NiTi (GT-NiTi) archwires were developed based around this concept and claim to deliver progressively heavier forces from anterior to posterior sections of the archwire of uniform dimension. 21 The development of these archwires became possible when direct electric resistance heat treatment (DERHT) was introduced. It uses electric pliers connected to a power supply to isolate archwire segments and deliver electric current to generate heat thereby modifying the TTR. 5 DERHT allows the same archwire to have different TTRs and thus different force levels within the same archwire rather than between individual archwires as in non-graded thermodynamic (NGT-NiTi) described above. 21 It is critical that annealing is well controlled as excessive heat treatment is known to obliterate superelastic properties of NiTi wires. 2,24 Many brands offer their own version of GT-NiTi archwires today including BF (GAC), Titanol Triple Force (Forestadent), Tri-Force Thermal (Ortho Organizers), and Tritanium Memory Wire (American Ortho). BF archwires were the first phase V NiTi archwire to become available on the market. 21 They are currently available in square or rectangular cross sections. For x archwire, the manufacturer claims to deliver approximately 80 grams of force at the incisor section, 180g at premolar level, and 280g at the molars in a three-point bending test deflected to 2mm over a 14mm span at body temperature. 30 The force level is claimed to gradually increase from anterior to posterior sections in increments. These archwires are labeled as body heat-activated, superelastic and exhibit shape memory effect. 30 One may also obtain BF archwires with IonGuard finish. Through ionization implanting process, these archwires purportedly have less friction than non-coated wires with no effect on its superelastic properties. 30 Compared to other NiTi archwires carried by the same company, BF archwires are at the top of the price range (Table 1).

23 11 Archwire type (GAC) Price per 1 pack (10 wires) Lowland $22.57 Sentalloy $62.77 Copperloy $84.50 NeoSentalloy $90.67 BioForce $ Table 1. Retail prices (CAD) for various NiTi carried by GAC (as of June 2017). 1.4 Comparing the Properties of Commercial NiTi Archwires Slight variations in composition and manufacturing processes strongly influence the mechanical properties of NiTi archwires. The sensitivity of NiTi to these factors is so significant that controlling the chemistry and processing have presented a continuous challenge for the makers of NiTi archwires and is reflected in the evolution of their types. 31 To illustrate, an increase in nickel content from 50% to 51% will result in a TTR change of 100 C. In order to achieve a TTR within ± 5 C accuracy, the NiTi composition must be controlled within ± 0.05%. Such compositional control is significantly more stringent than SS and requires more precise chemical-analysis methods than the standard to confirm the alloy formulation. 31 For the above reason, large variation in mechanical properties of NiTi wires is observed between different manufacturers. Even within the same batch of some NiTi brands, Tonner and Waters 26 found 9-10% variation in the initial slopes and plateau force values of the load-deflection curves. Although companies use terms such as superelastic and thermally active to label their products, these terms can be quite misleading as the actual behavior of the archwires can be quite varied. Some brands provide more information about their product, such as TTR and force levels at specific deflections, which can be helpful. However, the testing conditions used to derive these values are not always appropriate or representative of the clinical situation and must be kept in mind. 22 To verify manufacturers promotional claims and meaningfully compare commercially available NiTi archwires, a handful of researchers have surveyed their mechanical properties

24 12 using standardized testing. Even though the results obtained from these in vitro tests cannot be directly applied to clinical scenarios, the test provides good reproducibility and a common platform to evaluate mechanical properties of the existing archwires. 10, Lab Bench Tests Orthodontic archwires have been studied using tension, 32 bending, 6 8,10,12,33 35 torsion, 13,14,36 38 or cantilever tests. 24,39 Traditionally, cantilever test has been the test of choice. However, for archwires with good springback properties such as NiTi, this test can erroneously suggest the existence of superelastic-like characteristics. 2,24 Thus for NiTi archwires, bending and torsion are thought to be the most clinically relevant tests and have been employed in more recent literature Three-Point Bending Historically, three-point bending test draws from structural engineering via simple beam theory. It assumes a passive straight wire segment, small deflections, and no friction between wire and supports (brackets). The relationship between force (F) and deflection (δ) in simply supported three-point bending test for linearly elastic material can be summarized as follows: 40 F = 48EIδ L * where L is the distance between supports, I is the moment of inertia of the cross-section, and E is the modulus of elasticity. This equation is useful when comparing archwires of different alloys and cross-sectional configurations in the same three-point bending set up. 34 Extensive work by Kusy and colleagues 41,42 have utilized theoretical calculations to characterize orthodontic archwire materials by stiffness, strength, and range in easy-to-use nomograms to aid clinicians with archwire selection. Schaus and Nikolai, 43 however, criticized the application of the bending theory to orthodontic applications on the grounds that many theoretical assumptions are violated while some important clinical influences are unaccounted for. Studying flexural stiffness of conventional (non-elastic) preformed archwires, they found that the effect of elastic moduli and inter-bracket distance is less significant than the theory suggests. What is more, parameters such as preformed curvature of the wire at the activation site, malalignment direction with respect to the curvature, bracket-wire friction, and pre-activation fit to the dentition also

25 13 modified the regional flexural stiffness of the archwire. Moreover, the theory is only valid for deflections much smaller than what can be encountered clinically (2-3mm). 12 Applying the simple beam theory to shape memory alloys, such as superelastic NiTi, is even more problematic. This is because the alloy does not follow the linear stress-strain relationship described by Hooke s law that characterizes conventional alloys. 16 Moreover, flexural rigidity (EI) in superelastic NiTi wires decreases with increased deflection range, unlike their SS counterparts where it remains constant. 16,44 Thus, simple beam formulas to determine flexural rigidity can only be applied to the performance of these archwires during their austenitic states. They cannot be applied during SIM states which are most relevant to clinical applications. Thus, these wires cannot be categorized by their mathematically derived elastic modulus. Although stress-strain curves cannot be easily generated for NiTi wires, the three-point bending test is, nonetheless, an important tool as it provides a common platform to compare the performance of these wires in a clinically meaningful way. 10 To standardize the results and compare data across studies, the American Dental Association recommends employing the ISO three-point bending protocol for testing archwires used in fixed and removable orthodontic appliances. Many studies have followed this recommendation. 6,10 12,17,46 49 Since achieving low continuous forces is of utmost interest to a clinician, evaluating the parameters of the unloading superelastic plateau, such as force levels, has been the focus. To better emulate the clinical scenario, other studies have tested the wires in deflection secured to brackets in various configurations ranging from three-brackets 9 to phantom head jigs with a full arch bracket set. 48 Therefore, when comparing the values between studies, one must be cognizant of the type of test used to derive the data as the values may vary considerably depending on the method of testing. To illustrate, for a given archwire, Oltjen et al. 17 found the stiffness to be times higher at 2 mm deflections and up to 40 times higher at 3 mm deflections in a three-bracket bending test compared to a three-point bending test Superelasticity When testing commercially available NiTi archwires, it has been shown that work-hardened wires typically exhibit larger differences in loads between increments of deflection compared to active austenitic and active martensitic wires. This implies that the wire shows a more constant

26 14 force exertion (superelastic plateau) over an increasing range of activations. 11 However, considerable variation in the superelastic properties between brands of different superelastic NiTi wires as measured on a load-deflection curve has been reported. Segner and Ibe 9 and Bartzela et al. 10 found that a significant fraction of NiTi archwires that claimed superelastic properties showed either no or only weak superelastic behavior. Since one of the fundamental appeals of using NiTi is their ability to deliver constant forces over a range of activation (ie. superelastic plateau), some studies examined the deflection needed to induce superelasticity. Of truly superelastic archwires, Segner and Ibe 9 showed that a deflection of at least 0.8 mm was needed for pseudoelastic property to be expressed in a three-bracket bending test. 9 Using a three-point bending test, Tonner and Waters 47 showed that a deflection of 2 mm was needed. Thus, in routine orthodontics, where deflections of 1 mm are most common, the archwire may not reach the threshold for SIM transformation to take advantage of its superelastic behavior. 9 Therefore, in cases of minor crowding, the force levels would be proportional to the extent of deflection as with a standard archwire and superelastic NiTi may not render an advantage over conventional NiTi or multistranded SS Force Levels In general, the literature tends to support the statement that work-hardened NiTi wires deliver the highest force levels followed by superelastic (austenitic-active) and thermal (martensitic-active) archwires at oral temperatures. Using a three-point bending test at 37 C, Nakano et al. 11 compared the mechanical properties of and x wires of 42 brands from nine different manufacturers. For wires at 1.5 mm deflection, a difference of 136 g in load was observed between the heaviest work-hardened archwire (Aline) and the lightest NiTi wire (CuNiTi35 C). This difference was approximately double for x gauge wires. Using a three-bracket bending test, Sakima et al. 8 compared a number of rectangular x thermodynamic and active-austenitic NiTi archwires. At 37 C, the study found that the force at the unloading plateau ranged from g for the selected thermal archwires and g for the austenitic archwires. CuNiTi 40 C (Ormco) and NS F200 delivered the lowest forces followed by CuNiTi 35 C (Ormco) and Thermal NiTi (Wonder Wire Corp.). Lombardo et al. 7 also corroborated these results and found that thermodynamic archwires delivered 24% lighter forces over a 13% greater range of deflections compared to their austenitic counterparts.

27 15 They concluded that thermodynamic archwires deliver lighter, more continuous forces over a greater deflection range. While certain generalizations can be made, it is important to keep in mind the large variations in force-deflection characteristics that exist between different manufacturers. This is so even for wires that are labeled with the same descriptive key words such as superelastic and thermallyactive. In fact, Tonner and Waters 26 found that superelastic NiTi wires of the same size but different manufacturer varied by as much as 600% in force levels at oral temperature. 26 Besides the type and brand of NiTi, archwire size and the extent of deflection also have an influence on the force levels. Gatto et al. 6 recently conducted a study that evaluates these factors. Using a three-point bending test at 37 C, the authors compared the performance of and superelastic NiTi archwires with their thermal counterparts from four popular manufacturers. As expected, archwires that were bigger in diameter, deflected to a smaller extent, and those that were austenitic active were associated with higher force levels. Another interesting finding was that the superelastic archwires from 3M and GAC tended to produce lower force levels than the archwires from American Orthodontics and Rocky Mountain Orthodontics. The effect of deflection on force levels is important one to keep in mind in clinical practice. In this study as well as in one by Tonner and Waters 26 it has been shown that the higher the degree of activation, the lower the force tends to be. This means that simply retying the wire back into the brackets can increase its active force. 16 To summarize, the NiTi archwires in order of decreasing force levels are work-hardened, superelastic (austenitic active), and thermal (martensitic active). Thermal archwires also exhibit superelasticity at smaller deflections and are active over a longer range of deflection compared to the other archwires. Currently, only one study mentions testing phase V or graded thermodynamic NiTi wires. Mullins et al. 50 have looked at x BF wires and found the force of the superelastic plateau in the anterior region to be comparable to NS F100. No published studies that confirm region-specific force levels claimed by the manufacturer exist.

28 Torsion While the superelastic effect in bending has been explored abundantly, 2,7 12,14,26,33,47,48,50 fewer studies evaluated it in torsion. 13,14,36,38,51,52 The behaviors of an alloy under bending stress is different when under torsional stress and must be distinguished. 51 In orthodontics, third order movements depend on the torsional load created by twisting of an edgewise wire within the bracket slot. Unlike in bending, no ADA-recommended protocols for torsion exist and various methods have been employed in the literature. Torsional moment values at various degrees of twist are most commonly reported. Current evidence suggests NiTi wires may not be an ideal choice for torque expression compared to SS. A recent study 13 compared torsional behavior of ten commercially available x NiTi and multistranded SS wires in slot brackets. The group found that most NiTi wires did not exhibit superelasticity in torsion as described in bending. Moreover, multistranded D-Rect SS rectangular archwire showed a more favorable torsional stiffness than the majority of NiTi archwires at 35 C. Among the NiTi wires, the expression of the superelastic effect also appears to have a torque angle threshold. Partowi et al. 36 studied the mechanical properties of SS and superelastic NiTi in torsion and found that the mean starting point of the load-deflection plateau for superelastic NiTi wires was 20 at oral temperatures, on average. Moreover, the torquing moment produced at the plateau of some, especially smaller, rectangular superelastic NiTi wires were found to be below the moment threshold needed to induce torque movement in incisors. A large manufacturerdependent variation in torsional behavior and load has been reported 14,36 such that predicting torque delivery based on NiTi archwire size is not feasible. 36 The effect of temperature on the mechanical properties of NiTi wires in torsion has also been explored. Bolender et al. 13 found that the expression of the superelastic plateau was temperature dependent with greater number of the tested NiTi wires showing superelastic plateau at 20 C compared to 35 C and no plateau at 50 C. Meling and Ødegaar 53 found that exposure of the thermodynamic superelastic rectangular NiTi wires to cold water (10 C), reduced the torsional stiffness up to 85% compared to the torsional stiffness at body temperature, and remained up to 50% less stiff after two hours at oral temperatures. This is in contrast to exposure to hot water (80 C), which increased the torsional stiffness of the wires. This effect disappeared quickly

29 17 thereafter, however. This study suggests the expression of torque when using some thermodynamic NiTi wires may be reduced after ingestion of cold liquids or foods. 1.5 Summary NiTi wires have revolutionized the practice of orthodontics. Due to their unique properties of shape memory and superelasticity, these wires are able to deliver low and continuous forces most favorable for orthodontic tooth movement. Their manufacturing, however, is a highly sophisticated process requiring extreme precision in chemical composition, processing, and heattreatment. Proprietary variations in manufacturing have led to the production of a large diversity of NiTi wires each displaying unique behavior that may or may not optimize their superelastic/shape memory potentials. Discrepancies in claimed and actual properties of NiTi wires have been shown through standardized testing of different NiTi wires by independent investigators. Such studies are helpful in guiding clinicians in archwire selection. The properties of the latest generation NiTi wire, GT-NiTi, have not been explored in the literature. 1.6 Purpose of the Current Investigation To characterize the mechanical properties of GT-NiTi in different regions of the wire and compare them to those of NGT-NiTi in bending and torsion. 1.7 Hypothesis There is no difference in the mean force, length, and slope of the superelastic plateau during deactivation between anterior, middle, and posterior sections of BF archwire as determined by the three-point bending test. There is no difference in the mean moment, length, and slope of the superelastic plateau during deactivation between anterior, middle, and posterior sections of BF archwire as determined by the torsion test.

30 18 Chapter 2 Materials and Methods 2.1 Wire Selection Two cross sectional sizes of BF wires (0.016 x and x ) were selected for this experiment. These archwire dimensions were chosen to represent the ends of the range of archwires used most commonly. BF brand was selected to represent GT-NiTi wires as it was the first of its kind to be developed and released on the market. 30 Medium maxillary BF AccuForm archwires were tested and medium maxillary AccuForm NS archwires of the same dimensions (F240g x and F300g x ) were used as controls (Table 2). The dimensions of archwires were verified with a digital caliper with ±0.005 mm sensitivity (6 Mastercraft Digital Caliper). The wires were obtained from the manufacturer and wires from the same lot number were used. Code Wire Description LOT # Size (inch) Type of NiTi BF 16 x 22 BioForce, GAC K2Y x Graded Thermodynamic C 16 x 22 NeoSentalloy (F240), GAC K x Thermodynamic BF 18 x 25 BioForce, GAC K3Y x Graded Thermodynamic C 18 x 25 NeoSentalloy (F300), GAC K2X x Thermodynamic Table 2. Orthodontic archwires tested. Conventional thermodynamic archwires NS were used as controls.

31 Preparation of Specimens For the three-point bending test, each of the wire segments was prepared as follows: one side of the archwire was divided into three consecutive 30 mm long segments representing incisor, premolar, and molar sections (Figure 4). Fifteen millimeters on either side of the machined midline of the archwire was marked with a permanent marker (0.5mm) to define the incisor section and one of the remaining segments was divided into two segments of equal length (30 mm) corresponding to the premolar and molar regions. The mid-point of these latter segments was also marked on the wire to guide the testing position. One archwire yielded one sample of each region and the rest of the wire was discarded as shown. The length was measured by adapting a 30mm long cord along the archwire and cuts were made using an orthodontic ligature cutter. In compliance with ISO (2014) 45 recommendation, six specimens of a single product from one batch were used for each test. Thus, each segment was loaded and unloaded once and the test was repeated on six different samples of the same region of wires to obtain an average. The same specimens (n=6) reduced to 15 mm in length were used for torsion testing. A total of 72 specimens were tested. Specimen preparations and mounting were done by one investigator. Pm M I 30 mm Figure 4. Schematic diagram illustrating the division of an archwire into three segments incisor (I), premolar (Pm) and molar (M). 2.3 Experimental Design Three-point bending test: The testing specifications closely follow ISO standards for orthodontic wire testing for type 2 wires (wires that display nonlinear elastic behavior), however, after developing the methodology through a pilot investigation, some modifications were made in order to allow testing of the curved sections of the archwire. The following protocol was employed (Figure 5):

32 20 The crosshead rate was 1mm/min The wire was subjected to a three-point bend test A span of 10 mm between supports was utilized The wire was tested in the bucco-lingual direction of the wire with indenter loading on the lingual side The wire was lightly held by custom-built slotted frames to minimize wire twisting during deflection Deflections were carried out with centrally-placed indenter The wire was deflected to 3.1 mm Preloading was ~0.2 N The radius of indenter and supports was 0.2 mm The test was performed at a temperature of 36±1 C to simulate oral temperature using a water bath. The temperature was monitored continuously using a laboratory thermometer (±0.5 C) by the same operator performing the testing. Figure 5. Schematic diagram (left) and photographic (right) representation of the three-bending test set-up.

33 Torsion Test The specimen was held at one end by a metal clamp and secured to a pulley using dental composite (Dentsply, Spectrum TPH3, Konstanz, Germany) on the other. The pulley was suspended on a SS wire via a cord (PowerPro Spectra microfilament braided fishing line, 50lb, Springfield, MO) to allow free rotation. The translational movement of the pulley was restricted by a custom-built metal frame to safeguard pure rotational movement. The distance between attachments (the length of free wire) was 5 mm. Each sample was loaded in torsion by vertical displacement of lever arm (SS wire) to a maximum of 4.6 mm, yielding ~45 of rotational displacement of the pulley and unloaded to zero with a speed of 2.5 mm/min. A 50 N load cell was used at 2.5 N scale with ~0.1 N preloading, and testing was done at 36±1 C temperature using a water bath (Figure 6). The temperature was monitored continuously using a laboratory thermometer (±0.5 C) by the same operator performing the testing. Rotational angle was registered with standardized photographs of the pulley taken at every 1 mm displacement (Figure 6, right bottom) during activation and deactivation. In every photo, the angle of rotation was measured using a protractor. The average linear displacement in millimeters was converted to average rotation displacement in degrees using linear regression. The torsional moment was calculated as force times the radius of the pulley and the converted data was used to generate torsional moment-angle curves.

34 22 Load Cell Load Cell Stainless steel wire 4.6 mm 45 Cord 5 mm 10 mm Specimen Pulley Metal frame Figure 6. Schematic diagram (left) and photographic (right) representation of the torsion test set-up.