Static and Dynamic Friction Comparison of Esthetic Orthodontic Archwires

BY Piotr Barysenka D.D.S., University of California, Los Angeles, 2012

THESIS Submitted as partial fulfillment of the requirements for the degree of Master of Science in Oral Sciences in the Graduate College of the University of Illinois at Chicago, 2015

Chicago, Illinois

Defense Committee: Budi Kusnoto, Chair and Advisor Maria Therese S. Galang-Boquiren Spiro Megremis, American Dental Association Grace Viana

AKNOWLEDGEMENTS I would like to express my sincere gratitude to the members of my committee for their time and expertise. It would be impossible to accomplish this project without the knowledge and help of Dr. Spiro Megremis. I would like to thank Dr. Budi Kusnoto for his time and guidance throughout my research. I am grateful to Dr. Maria Therese S. Galang-Boquiren for her encouragement and support. I would also like to express my appreciation to Ms. Maria Grace Costa Viana, who helped with statistical analysis and spent a generous amount of her time to assist in the completion of this project. Thank you to the American Dental Association and all members who helped me with this experiment. A special thank you to Victoria Ong, engineering research assistant, who helped to design and manufacture the testing device for this project. I appreciate your willingness to share your knowledge and guide me through laboratory equipment and procedures. I would like to thank Sarah Duchaj for her help with data preparation, organization, and for support throughout this whole project. I am also grateful for the generous donations from 3M Unitek and Dany MBT.

PB

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TABLE OF CONTENTS CHAPTER I.

II.

PAGE

INTRODUCTION .............................................................................................................. 1 A.

Background ............................................................................................................. 1

B.

Statement of the problem ........................................................................................ 3

C.

Purpose of the study ................................................................................................ 4

D.

Null hypotheses ....................................................................................................... 4

REVIEW OF LITERATURE ............................................................................................. 5 A.

Classic friction ........................................................................................................ 5

B.

Friction in orthodontics. .......................................................................................... 5

C.

Variables affecting friction: .................................................................................... 9 1.

2.

III.

Biologic variables

9

a.

Saliva............................................................................................... 9

b.

Oral forces ..................................................................................... 10

Physical variables

11

a.

Bracket material ............................................................................ 11

b.

Combination of wire and bracket material.................................... 14

c.

Slot size and bracket width ........................................................... 15

d.

Interbracket distance ..................................................................... 16

e.

Archwire size and shape ............................................................... 17

f.

Wire composition and stiffness ..................................................... 19

g.

Wire surface roughness ................................................................. 21

h.

Ligation ......................................................................................... 22

i.

Effect of coating on wire characteristics ....................................... 23

MATERIALS AND METHODS ...................................................................................... 26 A.

Friction-testing device .......................................................................................... 26

B.

Test brackets ......................................................................................................... 29

C.

Test wires .............................................................................................................. 32

D.

Randomization ...................................................................................................... 32

E.

Saliva..................................................................................................................... 33 iii

TABLE OF CONTENTS (continued) CHAPTER

IV.

IV.

VI.

PAGE

F.

Ligation ................................................................................................................. 33

G.

InstronTM testing machine ..................................................................................... 33

H.

Testing protocol .................................................................................................... 34

I.

Wire diameter measurement ................................................................................. 36

J.

Data analysis ......................................................................................................... 39

RESULTS .......................................................................................................................... 40 A.

Static and kinetic resistance to sliding .................................................................. 40

B.

Wire dimensions ................................................................................................... 45

C.

Association between stainless steel core diameter and friction values ................. 51

DISCUSSION ................................................................................................................... 53 A.

Friction-testing device and conditions .................................................................. 53

B.

Wire characteristics effect on movement pattern and kinetic friction .................. 56

C.

Effect of wire characteristics on static and kinetic friction ................................... 66 1.

Wire size and elastic properties……………………………………………………………….. 66

2.

Surface roughness……………………………………………………………………………………. 70

3.

Surface hardness……………………………………………………………………………………… 71

D.

Wire dimensions ................................................................................................... 72

E.

Limitations ............................................................................................................ 73

CONCLUSIONS. ............................................................................................................. 77

CITED LITERATURE ................................................................................................................. 79 VITA…………………………………………………………………………………………………………………………………….………84

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LIST OF TABLES

TABLE

PAGE

I: COMPARISON BETWEEN STATIC FRICTIONS (Fs) OF TESTED WIRES (grams) ........ 41 II: COMPARISON BETWEEN KINETIC FRICTIONS (Fk) OF TESTED WIRES (grams) .... 43 III: COMPARISON BETWEEN DIAMETERS AS RECEIVED (d) OF TESTED WIRES (inches) .......................................................................................................................................... 46 IV: COMPARISON BETWEEN SAMPLE MEASUREMENTS AND MANUFACTURERREPORTED WIRE SIZE ............................................................................................................. 47 V: COMPOSITION OF COATED WIRES (inches).................................................................... 49 VI: COMPARISON IN STAINLESS STEEL CORE DIAMETER AND COATING THICKNESS BETWEEN POLYMER- AND EPOXY-COATED WIRES (inches) .................. 49 VII: PEARSON CORRELATION TEST BETWEEN STAINLESS STEEL CORE DIAMETER AND FRICTION VALUES .......................................................................................................... 51

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LIST OF FIGURES

FIGURE

PAGE

1. Friction-testing device assembly. .............................................................................................. 26 2. Computer-aided drawing of a metal plate for mounting of brackets. ....................................... 28 3. Bracket positioning procedure. ................................................................................................. 30 4. Tip and torque elimination during bracket positioning. Top view. ........................................... 31 5. Recording of static and kinetic friction. .................................................................................... 36 6. Measurement of wire diameter using Nikon Profile Projector at 100x magnification. ............ 37 7. Comparison of static friction between tested wires. ................................................................. 42 8. Comparison of kinetic friction between tested wires. ............................................................... 44 9. Comparison of static and kinetic friction of tested wires. ........................................................ 45 10. A comparison between measured and manufacturer-listed wire size. .................................... 48 11. Composition of tested wires.................................................................................................... 50 12. Phases of tooth movement. ..................................................................................................... 54 13. Typical resistance graph for a stainless steel wire (wire #47). ............................................... 58 14. Scanning electron microscope image of a stainless steel wire end at 100x magnification..... 58 15. Scanning electron microscope image of a stainless steel wire at 220x magnification. .......... 59 16. Typical resistance graph for a rhodium-etched wire (wire #14). ............................................ 61 17. Scanning electron microscope image of a rhodium-etched wire end at 100x magnification. 62 18. Scanning electron microscope image of a rhodium-etched wire at 220x magnification. ....... 62 19. Typical resistance graph for an epoxy-coated wire (wire #29)............................................... 63 20. Scanning electron microscope image of an epoxy-coated wire end at 100x magnification. .. 64 vi

LIST OF FIGURES (continued)

FIGURE

PAGE

21. Scanning electron microscope image of an epoxy-coated wire at 220x magnification. ......... 64 22. Typical resistance graph for a polymer-coated wire (wire #39). ............................................ 65 23. Scanning electron microscope image of a polymer-coated wire end at 100x magnification. 65 24. Scanning electron microscope image of a polymer-coated wire at 220x magnification. ....... 66

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LIST OF ABBREVIATIONS

Fs

Static Friction

Fk

Kinetic Friction

Ni-Ti

Nickel-Titanium

TMA

Titanium Molybdenum Alloy

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SUMMARY

Increased interest in esthetic orthodontic appliances has led to the development of numerous alternatives to traditional stainless steel brackets. Tooth-colored wires were recently developed to further improve the appearance and confidence of orthodontic patients. Although different technological processes were established to create an esthetic orthodontic wire, their properties are not well known. Additional research is required to investigate the effects of different coatings on the clinical behavior of orthodontic wires. Friction is one of the most important factors that requires consideration by an orthodontist in order to anticipate predictable treatment mechanics and outcomes. Frictional properties of non-coated orthodontic wires of different alloys were extensively investigated and described in several studies over the decades. Limited knowledge about esthetic orthodontic wire characteristics, and their frictional behavior in particular, prevents practitioners from incorporating these wires in their everyday practice. Moreover, existing studies often fail to successfully reproduce in vivo conditions in their laboratory experiments, which ultimately produces conflicting results. This study aimed to compare static and kinetic frictional characteristics of different types of esthetic orthodontic wires that exist on the market today. The frictional behavior of polymercoated, epoxy-coated, and rhodium-etched wires was studied in laboratory conditions by simulating the intraoral environment. Values associated with static and kinetic friction of tested wires were compared to each other and to control, non-coated stainless steel wires. The dimensions of all wires and their components were also investigated since these factors were previously reported to have an effect on frictional characteristics. ix

SUMMARY (continued) Contrary to popular belief, the findings of this study showed that no correlation exists between wire diameter and the static and kinetic friction properties of that wire. Moreover, no correlation was found between frictional behavior and diameters of the stainless steel core in coated tooth-colored wires. It was determined that the actual size of all tested esthetic wires varied from the diameter reported by manufacturers. Significant deviations in wire size may affect clinical results and should be disclosed to and considered by orthodontists. Friction testing results revealed that polymer-coated wires showed the lowest values for static and kinetic friction, similar to the control stainless steel wires. Rhodium-etched and epoxy-coated wires produced more friction than the control non-coated wires, with the rhodium-etched group having the highest friction values. These results suggest that when lower friction is preferred in a clinical situation, polymer-coated wires are the best choice. The frictional characteristics of these wires are comparable to the most commonly used stainless steel wires of today.

x

I.

A.

INTRODUCTION

Background Since the beginning of orthodontics, the primary focus has been to improve the esthetics

and function of one’s smile. While the technologies and techniques may have changed and improved with time, the fundamental goal has always remained the same. In the past, children and adolescents included the majority of patients undergoing orthodontic treatment. Within the last two decades, however, more adult patients have been treated within this field to correct occlusal discrepancies and to improve their smile. As this patient population continues to steadily increase, the demand for esthetic orthodontic appliances is also continuing to grow. The traditional orthodontic appliance may be esthetically unappealing to some, which may prevent some patients from initiating treatment—especially within the adult population. Multiple alternatives to traditional braces, such as lingual appliances and clear aligners, exist on the market today. Many practitioners offer tooth-colored brackets as an esthetic alternative to metal appliances. A study conducted in 2009 by Rosvall and his colleagues demonstrated that ceramic appliances with esthetic wires were a significantly more acceptable treatment option for adults and their children as compared to traditional stainless steel brackets. Ultimately, the study revealed that respondents were more willing to pay for esthetic brackets and wires, even if the cost of treatment was considerably higher. (Rosvall et al., 2009). Achieving the ultimate goal of orthodontic treatment requires the movement of teeth to their most ideal and esthetic positions. In order to accomplish this, contemporary orthodontics typically utilizes the traditional straight wire appliance. The two basic elements of this appliance include the bracket, which is attached to the labial tooth surface, and a wire that is inserted into

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the bracket and delivers the force to move teeth. In recent years, lingual bracket placement has gained popularity in order to improve esthetics. This method, however, compromises patient comfort and is highly technique-sensitive, which prevents this modified appliance from becoming widely used. Therefore, traditional placement of brackets on the labial surface is the more acceptable method. Although the visibility is increased and the esthetic appeal is compromised, it still remains to be the most common treatment modality. Methods to address the visual appeal of the bracket-wire system are continuing to improve with new advancements in technology. Esthetic tooth-colored brackets were first developed in the 1970’s using acrylic (Russel et al., 2005). Further attempts to increase esthetics have led to the development of ceramic brackets, which is being increasingly utilized today. Additionally, recent advancements have also addressed the other component of the straight wire orthodontic appliance, leading to the development of a more esthetic of the orthodontic wire. Currently, several manufactures produce an esthetic orthodontic wire. However, while this is a more recent advancement within the field, the properties of these products are not well known and require more data on their frictional characteristics and effectiveness at delivering appropriate forces. Multiple studies were carried out to investigate the importance of friction in orthodontics. The majority of orthodontists seem to agree that friction slows down tooth movement. Careful planning of mechanics to overcome friction requires a full understanding of the cause and force of friction. The only way to reduce friction is to understand its source and how it affects tooth movement. In order to understand this from a clinical perspective, a laboratory experiment must effectively represent friction as it occurs during the movement of teeth in an oral environment. Numerous experiments were performed to study which components of the archwire-bracket

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system affect friction the most in a clinical setting. Unsuccessful attempts at reproducing in vivo conditions in most of the experiments led to inconsistent results between studies, however. Wire and bracket dimensions and composition were ultimately found to be among the most important factors that influence friction during tooth movement. Complex interactions between surfaces of two materials moving along each other create friction. While basic concepts of physics and variables that affect friction can be applied to the wire-bracket system (e.g., surface roughness and lubrication), conflicting results of multiple studies prove that there is a lack of clear understanding of the resultant friction between two materials during tooth movement. Additionally, the introduction of new materials to improve esthetics of brackets and wires make the presence and amount of friction even more challenging to predict. Further investigation of frictional forces between modified esthetic wires and toothcolored brackets is required to understand the amount of friction between these two surfaces. Additionally, it is important to devise an experimental model that closely represents the clinical conditions present during orthodontic tooth movement. With a thorough understanding of how much friction is present with new esthetic wires and brackets, practitioners can plan treatment mechanics accordingly in order to achieve set treatment goals. B.

Statement of the problem New esthetic wires on the market have different types of coatings with different

thickness. Frictional characteristics of esthetic orthodontic wires are not well known. There are currently no studies that have evaluated friction of different tooth-colored orthodontic wires in esthetic ceramic brackets. Additionally, the dimensions of wire coatings are not clearly stated by manufacturers. The effect of esthetic coating on wire dimensions needs further investigation.

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C.

Purpose of the study The goal of this study is to compare static and kinetic frictional characteristics of

available esthetic orthodontic wires to each other and to non-coated stainless steel wires. The second goal is to compare actual wire dimensions and the wire size reported by manufacturers. The consistency of overall wire dimensions for each manufacturer as well as stainless steel core dimensions for coated wire will be evaluated. This study will investigate if there is an association between stainless steel core dimensions and the amount of static and kinetic friction for all wires tested. D.

Null hypotheses 1.

There is no mean difference in static and kinetic resistance to sliding between

tested wires. 2.

There is no mean difference between the wire diameter reported by manufacturer

and the actual wire dimensions for all tested wires. 3.

There is no association between stainless steel core dimensions and the amount of

static and kinetic resistance to sliding for all tested wires.

II.

A.

REVIEW OF LITERATURE

Classic friction Friction is defined as the force that resists relative movement of two objects sliding along

each other. Frictional force is tangential to the surfaces in contact and directed opposite to the direction of sliding (Nanda and Ghosh, 1997). The first law of friction states that maximum frictional resistance (F) is directly proportional to the normal force, where normal force (N) is perpendicular to the direction of sliding and the direction of friction. The force that pushes one surface against another is the normal force (N) (Baker et al., 1987). The relationship between friction and normal force can be described by the equation F=μN, where μ is a coefficient of friction. The second fundamental law states that frictional resistance is independent of the surface area of contacting objects. Every material has two different friction coefficients—static friction (μs) and kinetic (μk). The static coefficient of friction exists when an object is not moving. It is always greater than the kinetic coefficient, which exists when two objects are in constant motion relative to each other while in contact. A minimum amount of force required to initiate movement between two objects is called static frictional force (Fs). Kinetic frictional force (Fk) is the force that inhibits movement of one object over another at a constant speed (Kusy and Whitley, 1997). B.

Friction in orthodontics Orthodontic treatment involves the movement of teeth to a more ideal position. In

orthodontics, the force of friction results from the interaction between an archwire and the sides of the bracket, or the archwire and the ligation material (S. Burrow, 2009). The rate of tooth movement is dependent on the amount of force applied to the tooth. According to Quinu and

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Yoshikawa (1985), the speed of tooth movement increases proportionally with an increase in the applied force to a certain limit. After the limit has been reached, a significant increase in the amount of force does not produce a significant change in the rate of tooth movement. During the treatment phase involving bracket movement along the wire, friction acts in the opposite direction of the attempted movement. Therefore, friction may prevent the practitioner from achieving an optimal force needed to accomplish tooth movement. Orthodontists should anticipate and understand the amount of force needed to overcome friction in order to produce optimal tooth movement (Quinu and Yoshikawa., 1985). There are two common techniques described in orthodontics that are related to the force of friction. According to Nanda and Ghosh (1997), space between teeth can be closed utilizing two different methods. Using the method termed “frictionless mechanics,” teeth can be moved utilizing closing loops of different configurations. Teeth are tightly ligated to segments of the archwire. Movement of teeth toward each other occurs due to the change of configuration in the closing loop, which is placed between straight wire segments. The wire in the bracket slot is restricted from movement and, therefore, the activation force intended to move teeth is conserved due to absence of friction on the wire-bracket interface. The second technique is known as “sliding mechanics.” This method involves movement of the tooth-attached bracket along an archwire the movement of the archwire through a stationary bracket. In this process, part of the force applied to the bracket or wire, which creates the intended movement, is lost due to friction on the wire-bracket interface (Nanda and Ghosh, 1997). Sliding mechanics is also applicable to the early stages of orthodontic treatment. Specifically, when leveling and aligning severely misplaced teeth toward a more ideal position in all three planes of dimension. This is typically achieved by utilizing a flexible archwire that can

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be easily bent to engage misaligned brackets. While a flexible wire attempts to rebound to its original shape, it exerts light pressure on the teeth, which allows the practitioner to move dentition to a more ideal position. When the wire is engaged in misaligned brackets, additional wire length is required due to an increased distance between brackets. As teeth move to their desired positions, interbracket distance decreases and excess wire slides through the distal portions of terminal brackets in the arch. The sliding motion of the wire can be inhibited by friction, and therefore, the aligning force of the wire will be diminished by friction in this system (Proffit, 2007). According to Kusy and Whitley (1997), the first and second laws of friction are generally obeyed in orthodontics. Frictional force is proportional to the load applied to push moving surfaces together and the constant of that proportionality is independent of contact area. The third law of friction states that sliding velocity has no effect on the friction coefficient. However, this law is not applicable in orthodontics due to the characteristics of tooth movement. Reciprocating movement with negligible velocities occurs in the mouth when orthodontic forces are applied, which affects the friction coefficient. In a static state, a wire rests passively in the bracket slot and tooth movement does not occur. In this case, constant normal force (N) presses the wire against the bracket slot and minimal frictional force occurs. When a displacing force is applied to the tooth, movement is restricted at low levels of force. The static coefficient of friction is the minimum amount of force needed to initiate movement. To maintain movement, less force is required than the force needed to initiate movement, otherwise known as the kinetic coefficient of friction (Kusy and Whitley, 1997).

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In an ideal case, clearance exists between the wire and bracket as sliding movement occurs. The classic principles of friction are important to consider in this situation. In a clinical setting, however, a different condition occurs. Dentition is supported by the periodontal ligament, which suspends teeth in three dimensions. The periodontal ligament also provides resistance to tooth movement with a center of resistance apically located on the root, which is determined by anatomical factors. When orthodontic force is applied to a bracket that is attached to the tooth’s crown, it is impossible to apply force to the center of resistance. Therefore, tipping and rotation of the tooth occurs until the bracket edge begins to contact and progressively push on the wire, which steadily increases the friction (Drescher and Bourauel, 1989). Binding (BI) of the bracket edge and wire significantly increases friction (Kusy and Whitley, 1997). Wire that contacts the bracket progressively deforms and—due to elastic properties of the material—the wire pushes back on the bracket’s edge, providing anti-tip and anti-rotational movements of the tooth. Sliding mechanics produces “tipping and uprighting” type of tooth movement (Drescher and Bourauel, 1989). Another phenomenon that can increase friction is known as notching (NO). At high angulations and high torque, notching rapidly increases and stops sliding movement completely (Kusy and Whitley, 1997). Achieving the goal of orthodontic treatment requires appliances with high efficiency and reproducibility. According to studies, the efficiency of orthodontic movement varies between 40% and 88% (Kusy and Whitley, 1997). Complete elimination of friction is not possible. Therefore, orthodontists need to understand friction and variables affecting it in order to increase efficiency and achieve desired treatment results. Mechanical properties and characteristics of the appliance itself have a great effect on friction. At the same time, several biologic factors and

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conditions present in the mouth are known to have a significant impact on friction during tooth movement. C.

Variables affecting friction: 1.

Biologic variables a.

Saliva

Human saliva is constantly present in the mouth and performs numerous functions, including lubrication. Representing a mix of organic and inorganic substances, saliva interacts with all surfaces of the mouth and can potentially affect friction. Multiple studies focusing on friction were performed in a dry state and do not fully replicate clinical conditions of orthodontic treatment. Therefore, additional studies introduced either artificial or real human saliva in testing protocols in order to investigate the effect that saliva has on friction. Thurow (1975) suggested that saliva is one of the two factors that can reduce the force necessary for the orthodontic movement of teeth due its ability to act as a lubricant. In 1987, Baker and his colleagues supported this opinion and found that artificial saliva reduced frictional values by 15-19%—or about 28 grams—as compared to friction present in a dry state with similar wires and experimental design (Baker et al., 1987). However, other authors showed inconsistent results when saliva was introduced during friction testing (Henao and Kusy, 2004.). Kusy and Whitley (1991) compared friction coefficients of wires made from four alloys in stainless steel and polycrystalline brackets in dry and wet conditions. In this study, friction coefficients in ceramic brackets were often higher than in stainless steel brackets. It was found that wet conditions did not affect different alloys in a similar manner. The beta-titanium wire friction coefficient was reduced by 50% in a wet state versus a dry state, while the stainless steel

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wire friction coefficient increased in the presence of saliva. This suggests that there may be some adhesive behavior. Therefore, it is possible that saliva may act as a surface lubricant and reduce friction for some materials while the adhesive properties of saliva may increase friction for other materials (Kusy and Whitley, 1991). b.

Oral forces

The oral cavity is not a static environment. Structures of the oral cavity are responsible for a variety of functions including mastication, speech, and respiration. Muscle contractions and contact of oral structures with each other produce forces, which act on every object in the mouth. The periodontal ligament and surrounding alveolar bone is the supporting apparatus of dentition. Structures of the periodontium allow minute movements of teeth when pressure is applied in all three dimensions. This translates into minute movement of the orthodontic appliance and perturbations on the bracket/wire interface. Moreover, contact of the orthodontic appliance itself with oral structures and food particles produces minute movements of the appliance and its components (Braun et al., 1999). According to Thurow (1975), teeth in function provide a “walking effect” for the bracket along the archwire, and therefore, reduce friction. Teeth contact each other about 2,300 times throughout the day during chewing and swallowing (Graf, 1969). Occlusal forces vary greatly among individuals and can reach higher values of 176 newtons (Braun et al., 1996). According to Braun (1999), in 95.8% of his experiments, perturbation of the bracket-wire interface results in reduction of frictional resistance up to zero at the moment vibration occurred. Even when the wire size was large enough to fill the slot of the bracket with minimal clearance, a drop of 98% to 100% in frictional resistance was observed (Braun et al., 1999). Studies

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conducted by Bunkall (2006) showed that perturbations at a one per second rate reduced static and kinetic friction by 35-70%. In vivo studies by Iwasaki utilized chewing gum in order to reproduce vibrations in a patient’s mouth. Contrary to previous extra-oral studies, no significant and predictable reduction of friction was observed. Authors suggested that normal force produced by the ligation method had a greater effect on friction than perturbations that occur during mastication (Iwasaki et al., 2003). Multiple forces affect dentition and orthodontic appliances in the oral cavity. At this time no clear evidence exists on the extent and mechanism of oral forces effect on orthodontic treatment and more research is needed. 2.

Physical variables

Physical variables affecting friction include the characteristics of the brackets and wires used, along with the ligation material that holds these two components together. Important bracket characteristics that affect friction are the bracket width, slot size, and its material composition. The distance between brackets as well as their positioning also play an important role in determining the overall friction of the system. Variables of the archwires that proved to have an effect on friction include the wire size, shape, material of the wire core determining wire stiffness, and surface characteristics. The type of ligation and force can also change the frictional characteristics of the system, which should also be considered. a. Bracket material Stainless steel brackets continue to be the most popular today, and have been commonly used for more than 50 years. With an increase in esthetic demands, however, manufacturers were

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pushed to look for different bracket materials that closely resemble the natural color of teeth. In 1970, the first esthetic bracket made from acrylic was introduced to the market. With further development in technology, polycarbonate was used instead of the acrylic material. Unfortunately, inherent problems with these materials lead to staining and structural instability, which results in loss of torque. Reinforcement of polycarbonate material with ceramics, fiberglass, and a metal slot improved bracket performance but did not solve the problem completely. Ceramic brackets, a more recently developed bracket material, address the disadvantages of polycarbonate brackets. Three different types of ceramic brackets exist today: polycrystalline alumina, monocrystalline, and zirconia. The manufacturing process is different for each type of ceramic and their properties vary. Studies showed that frictional forces are less when using traditional stainless steel brackets as compared to ceramic brackets of polycrystalline or monocrystalline composition. This observation was made for all wire types ligated with elastomeric ligatures or Teflon-coated stainless steel ligatures (De Franco et al., 1995). Similar findings were observed in a study conducted by Kusy and his colleagues, which showed that the performance of stainless steel brackets continuously proved to be superior as compared to any type of ceramics in dry and wet conditions—regardless of bracket or slot size (Kusy et al., 1991). These findings were confirmed in other studies, which also showed significantly less friction when using stainless steel brackets as compared to ceramic brackets (Loftus et al., 1999, Angolkar et al., 1990, Kusy and Whitley, 1997, Pillai et at., 2014). To reduce the friction coefficient of ceramic brackets, manufacturers introduced a bracket that incorporated a ceramic body and a slot lined with metal. In 2001, Kusy and Whitley compared frictional resistance of two traditional stainless steel brackets with two metal-lined slot

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polycrystalline ceramic brackets. They found that the resistance to sliding was universally similar in all brackets, and therefore, the addition of metal slots to ceramic brackets led to a similar performance results when compared to traditional stainless steel brackets (Kusy and Whitley, 2001). In 2003, Thorstenson and Kusy found that the addition of a stainless steel slot insert reduced the occurrence of ceramic bracket deformation under increased loads, but it did not reduce frictional resistance to the level of stainless steel brackets (Thorstenson and Kusy, 2003). Other studies showed that frictional resistance of ceramic brackets with stainless steel slots were lower than the traditional ceramic brackets with ceramic slots, but higher than stainless steel brackets (Nishio et al., 2004, Cacciafesta et al., 2003). The increase in friction of ceramic brackets with stainless steel slots as compared to traditional stainless steel brackets could be due to poor adaptation of stainless steel inserts to the ceramic slots and should be improved with manufacturing techniques (Nishio et al., 2004). Differences between types of ceramic brackets based on their composition, and therefore surface characteristics, were also studied. In 1994, Saunders and Kusy analyzed surface topography and frictional characteristics of polycrystalline and monocrystalline brackets. They showed that the surface of monocrystalline brackets is significantly smoother as compared to polycrystalline brackets. Despite the difference in surface morphology, both types of brackets showed similar results of frictional resistance when tested with similar wires. Authors suggested different mechanisms that effect friction, other than surface roughness. In rough polycrystalline brackets, the surface creates a rasp-like action when contacting the softer wire material. With time, however, detached particles combine with saliva and create a smear layer that covers the asperities, which ultimately reduces friction. In monocrystalline brackets, the surface was extremely smooth but the friction coefficient remained similar due to the sharp edges between

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bracket slots and the outside wall. Brackets with sapphire composition, and therefore increased surface hardness, peel off layers of material from the wire, increasing friction. For that reason, an increase of the angle between the wire and slot significantly increases friction when a monocrystalline bracket is used (Saunders and Kusy, 1994). The common opinion is that surface roughness has detrimental effects on friction, with rougher surfaces creating more friction than smooth surfaces. Some investigators attributed their findings of increased friction to the increased surface roughness of ceramic brackets, which they observed under an electron microscope (Pratten et al., 1990). Other studies showed that this is not always the case. When the friction coefficients were compared for different wire alloys against a stainless steel surface with 240 grid, 320 grid roughness (typical for orthodontic brackets) and a mirror-smooth polished surface, only a slight increase was observed in the friction coefficient with an increase in roughness (Kusy and Whitley, 1990). Surface hardness may play a significant role in determining the overall friction present. In 1994, Saunders and Kusy showed that when all other conditions are similar, harder materials presented with less friction than softer materials. When tested with ceramic brackets, different wire alloys showed that friction increased when the surface hardness decreased (Saunders and Kusy, 1994). b.

Combination of wire and bracket material

The combination of wire and bracket materials is also important to consider because it can have a more significant impact on friction than either material alone. In a passive system in which binding does not occur, the rules of conventional friction will apply and specific characteristics of brackets and wires can have predictable effects on the overall resistance to sliding.

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When brackets are not aligned perfectly and binding occurs on its edge, however, notching of the wire can increase friction by several times and is more significant than traditional friction. If the bracket material is much harder when compared to wire materials, severe notching occurs and friction increases. At the same time, if the bracket and wire are composed of composite and are also ligated with ligature of similar composition, frictional resistance appeared to be less than in any other combination. The superiority of frictional characteristics of the composite wire and bracket combination remained true when compared to the combination of stainless steel brackets and stainless steel wires, which was considered the gold standard due to the “least” amount of friction present between these two components (Kusy and Whitley, 1997). c.

Slot size and bracket width

The most commonly used slot sizes among orthodontists include 0.018 inches and 0.022 inches. The bracket slot size itself does not play a significant role in determining friction. In a study conducted by Tidy and Orth (1989), there was no difference in friction when two different slot sizes were compared, while all other conditions remained constant. (Tidy and Orth, 1989). The combination of bracket width, slot size, and wire size can influence friction. When wires passively rest in the slot, the classic laws of friction play the most significant role in overall frictional resistance. With an increase in bracket angulation, the angle between a bracket and wire reaches a critical angle. At this point, binding starts to influence resistance to sliding to a much greater extent. The critical angle is described as the minimum angle when a wire and bracket contact on the edge of the bracket. When the angulation between the couple increases, notching comes into play and becomes more important than other resistance to sliding components. The critical angle can be calculated if the bracket width, slot size, and wire dimensions are predetermined, and therefore, interaction of all three components influences

16

friction (Kusy and Whitley, 1997). Knowing bracket and wire dimensions, the critical angle can be calculated for all three dimensions and therefore, the first, second, and third order bracket displacement (Yeh et al., 2007). Earlier studies demonstrated that increased bracket width produced an increase in friction. These results were explained by an increase in contact surface area, which the second fundamental law of friction (Frank and Nikolai, 1980). Drescher constructed a simulated canine retraction model with three-dimensional freedom for a moving tooth and found that friction reduced with an increased bracket width. Authors attributed these results to an increased tipping movement, which was observed with narrow brackets (Drescher et al., 1989). In theory, when brackets are well aligned, narrow brackets increase critical angle values, providing more freedom between the wire and bracket. Therefore, narrow brackets should reduce frictional resistance (Kusy and Whitley, 1997). Although when tipping of the bracket was introduced to the system—in attempt of simulating in vivo tooth retraction with all forces affecting movement—narrow brackets showed significantly higher friction values when compared to wider brackets under similar conditions. This could be attributed to the increasing effect of wire stiffness when the contact angle is reached and binding occurs at the bracket edge, providing an anti-rotational moment (Husain and Kumar, 2011). In 1982, the effect of wire stiffness on friction rather than bracket width was also supported by Peterson and his group. They showed that bracket width had no effect on friction with stainless steel and Nitinol wires of different dimensions (Peterson et al., 1982). d.

Interbracket distance

The distance between brackets in the oral cavity can vary significantly. The size of teeth and their position relative to each other, as well as bracket width, can influence interbracket

17

distance. Whitley and Kusy (2007) conducted an experiment to determine how the interbracket width affects friction in active and passive configurations with archwires of different compositions. They found that when a wire passively fits in the bracket slot, interbracket distance has no effect on overall resistance to sliding and only classic friction between the slot wall and wire plays a role. Although, when brackets do not lie on a straight line and the critical angle is reached, friction is inversely proportional to interbracket distance. These findings were true and significant for all wire materials tested (Whitley and Kusy, 2007). An earlier study using a canine retraction model showed a very slight effect of interbracket distance on friction (Frank and Nikolai, 1980). e.

Archwire size and shape

As the wire size approaches the size of the slot, friction increases. Multiple studies have verified that smaller wires produce less friction due to increased clearance between the wire and slot walls. In addition, when comparing wires of the same composition, smaller wires have higher elasticity, which also contributes to a reduction in friction (Frank and Nikolai, 1980, Kapila et al., 1990, Kusy and Whitley, 1997). As described by Kusy and Whitley, when the wire passively fits in well-aligned brackets, only the contact between bracket walls and the wire exists. With the introduction of a bracket tip, and when the critical angle is reached, bracket edges will contact the wire and binding subsequently occurs. Binding has a greater effect on resistance to sliding when the critical angle is reached. A larger wire size has critical angle values lower than smaller wire sizes, and therefore, binding appears earlier with minimal bracket tip contributing to greater values of friction (Kusy and Whitley, 1997). Wire shape can also potentially influence the amount of friction produced. According to the classic law of friction, the size of the contact area should have no effect on the friction

18

coefficient. Therefore, rectangular and square wires should have similar friction coefficients to round wires, even though the contact area of a flat surface is larger than that of a round wire. This is true when brackets are well aligned and binding or notching does not occur. In an experiment conducted by Frank and Nikolai (1980), friction was measured for different wire sizes and shapes in brackets with varying angulations. Round 0.020-inch wires showed higher friction values than rectangular wires 0.017 x 0.025-inch and 0.019 x 0.025-inch. Authors explained their findings by a theory that at a higher angulation, when contact exists between archwire and bracket edges, round wires have less contact area with the bracket edge, and therefore, more force per area unit exists. This can lead to an increased tendency of wire deformation, notching of the wire, and a dramatic increase in friction. As a result, cessation of sliding can occur (Frank and Nikolai, 1980). In the simulated canine retraction model experiment, Drescher et al. concluded that the vertical dimension of the wire, rather than its shape, affects resistance to sliding. In this study, 0.016-inch round stainless steel wires showed similar frictional characteristics to 0.016 x 0.022inch stainless steel wires (Drescher et al., 1989). These results can be explained by the limitation of their experimental model. Only mesio-distal tipping of the tooth with the bracket was allowed, and therefore, only occlusal and gingival bracket edges were contacting the wire at certain angulations. If the tooth would be allowed to rotate around its long axis, contact, and therefore, binding would exist between the ligation material and wire—in addition to the base of the bracket edge and wire. The horizontal dimension of the wire would also play a significant role if first order rotations occur around the long axis of the tooth. Mathematical formulas to calculate the critical angle in all three planes were described by Yeh et al. in 2007. Depending on the plane of tooth displacement from an ideal straight line,

19

different wire and bracket slot dimensions influence the value of the critical angle when binding and a subsequent increase in friction occur. When first order rotation occurs, the width of the wire determines the critical angle. In second order mesio-distal tipping, vertical dimensions of the wire play a significant role in friction determination. Third order, or torque friction, depends on the wire height and width. Since the in vivo tooth moves in all three dimensions, all geometrical parameters of the wire will play a significant role in overall friction. An increase in wire size in any dimension will reduce the critical contact angle in corresponding dimensions and will produce binding earlier in the system, potentially increasing the overall resistance to sliding (Yeh et al., 2007). f.

Wire composition and stiffness

For many decades, stainless steel wires were the most commonly used wires in the field of orthodontics. A high stiffness of stainless steel alloy is the major disadvantage in some clinical situations and variety of alloys were developed in recent years to overcome this drawback. Currently, the most commonly used wires include stainless steel, cobalt chromium, Nitinol and its modifications, and titanium molybdenum alloys. All of these wires have different stiffness and surface characteristics, which can affect friction. Multiple laboratory studies were conducted to determine differences in the frictional characteristics between different wire alloys. When brackets are well aligned and rotation is not present, a wire lies passively in the slot and will only contact walls of the bracket, and potentially, the ligation material. In these conditions, surface characteristics of the wire, bracket, and ligation play the most significant role in overall resistance to sliding—as classic friction will only exist. In a passive configuration, TMA wires had higher friction values, followed by Ni-Ti wires. Stainless steel and cobalt chromium wires had comparable frictional characteristics and

20

lower friction coefficients than the other two alloys (Kusy and Whitley, 1990 (a), Kusy et al., 1991, Drescher et al., 1989). When a large deflection exists between brackets, and the binding between a wire and bracket edge plays a significant role, wire stiffness becomes more important than surface characteristics. With an increase in stiffness, normal force in the contact area between the wire and bracket increases, and therefore, an increase in friction occurs. In an experiment conducted by Frank and Nikolai in 1980, they showed that with a large enough bracket angulation for binding to occur, Nitinol exhibited significantly less friction when compared to stainless steel and Elgiloy wires of similar diameter. Authors explained this by Young’s modulus of elasticity, which is approximately six times lower for Nitinol than the other two materials (Frank and Nikolai, 1980). Peterson described similar results in 1982. He proved that, with a high angulation between the wire and brackets, Ni-Ti wires showed similar friction values to stainless steel. With an increase in wire dimension, however, Ni-Ti wires had a less increase in friction when compared to stainless steel. Additionally, with high angulations, larger Ni-Ti wires showed less friction than similar stainless steel wires (Peterson et al., 1982). Several other studies that allowed bracket tipping to reproduce binding on the wire-bracket interface showed results similar to straight wire experiments. In these studies, stainless steel wires showed the least amount of friction, followed by Nitinol and TMA (Tidy and Orth, 1989, Montasser et al., 2013). The importance of low stiffness on friction reduction is evident when brackets are not aligned and the critical angle is reached to produce binding. The combination of a particular wire size and its composition, along with other conditions, is important to consider. Low stiffness NiTi wires can show lower frictional characteristics when compared to stainless steel wires, even though steel wires have a smoother surface. In this comparison, low stiffness plays a more

21

important role in the reduction of friction than a rough surface to increase friction for Ni-Ti wires. At the same time, when comparing rough TMA with stainless steel, TMA wires resulted in higher frictional values, even with the lower stiffness characteristics of titanium alloy. Poor surface characteristics have a greater effect on friction in this case (Matarese et al, 2008). g.

Wire surface roughness

Different compositions of commonly used orthodontic wires allow a practitioner to choose the appropriate mechanical properties needed for various clinical situations. With differences in composition, the surface characteristics of wires vary greatly. A laser spectroscopy analysis of four basic alloy wires showed that stainless steel wires had the smoothest surface, followed by cobalt chromium and TMA. Nitinol wires had the roughest surface compared to all other compositions (Kusy et al., 1988). Later attempts to correlate friction and surface roughness showed that the least friction was produced for stainless steel and cobalt chromium wires. This supports the idea that smooth surfaces have less friction. Even though they appeared less rough than Nitinol, TMA wires showed increased friction. This proved that surface roughness is not the only factor that determines the resulting friction. After analyzing surfaces following this experiment, authors found that the adhesive behavior of betatitanium alloy was responsible for increased friction once it interacted with a stainless steel surface (Kusy and Whitley, 1990 (b)). Other authors showed the correlation between surface roughness and friction. In 2014, Meier compared friction of Ni-Ti and TMA wires with similar wires that underwent electrochemical surface refinement. Under an electron microscope, wires appeared significantly smoother after surface treatment. The loss of force due to friction was reduced from 36% to 26% for Ni-Ti wires and from 59% to 47% for TMA (Meier et al., 2014).

22

In different experiments, no correlation could be established between surface topography and frictional characteristics. After measuring surface roughness with profilometry and subsequently comparing results to frictional characteristics, it was found that wires with toothcolored coating—and also an increased surface roughness—did not always have a higher resistance to sliding than smoother surfaces (Rudge et al., 2014). h.

Ligation

In orthodontics, ligation describes the method of holding a wire in the bracket slot. Based on the ligation method, two major types of bracket designs exist. The traditional twin bracket has an open slot and external materials are required to provide ligation. Elastomeric ligatures of different composition, or stainless steel ligatures, are most commonly used for this purpose. Stainless steel ligatures are traditionally considered to be the gold standard of ligation. Less friction with stainless steel ligatures was reported for different wire-bracket configurations when compared to traditional elastomeric ligatures (Khambay et al., 2004, Bednar et al., 1991). Manufactures attempted to modify the composition and surface of elastomeric ligatures in order to provide appropriate ligation for various clinical situations. Ligatures with a modified surface to reduce friction are now present on the market and various results were reported on the effectiveness of these ligatures. Slick elastomeric modules from TP Orthodontics (La Porte, Ind) showed a significant reduction in friction when compared to traditional elastomeric ligatures (Hain et al., 2003). The effect of an intraoral environment on the ligation material can also have an impact on frictional characteristics of the system. When friction that occurs in the oral cavity between the wire and bracket ligated with elastomeric ligature was measured, and was subsequently compared to a similar system in the laboratory setting, intraoral friction was shown to be

23

significantly higher. This could be attributed to the decomposition of elastomeric ligatures in an intraoral environment (Iwasaki et al., 2003). Modified stainless steel ligatures with Teflon coating was developed by manufacturers in an attempt to provide a more esthetic ligation material while including properties of traditional stainless steel ligatures. In 1995, De Franco and Spiller showed that with different bracket-wire combinations and with angles varying from 0 to 15 degrees, frictional forces using Teflon-coated stainless steel ligatures were significantly less than with elastomeric ligatures (De Franco, 1995). The ligation force produced by the ligation method plays an important role in the overall resultant friction. With an increased ligation force, friction increases (Frank and Nikolai, 1980). Stainless steel ligatures can be used to ligate wires in a tight or loose manner, presumably changing the ligation force, and therefore, achieving the desired amount of friction. It has been shown that the ligation force varies not only between practitioners but also with the same operator on different occasions. Therefore, it is very difficult to reproduce ligation forces and predict the resultant friction using a stainless steel ligature (Iwasaki et al., 2003). However, a comparison of ligation forces between elastomeric and stainless steel ligatures showed no significant difference. The standard deviation increased with stainless steel ligatures, which proves that it is difficult to control ligation forces with stainless steel ties (Montasser et al., 2014). i.

Effect of coating on wire characteristics

Placing a white coating on the stainless steel, Ni-Ti, and TMA—or any other metal core material traditionally used in orthodontics—attains the esthetic color of orthodontic wires. The composition of the coating material currently used can be a modification of epoxy resin, polymer, or Teflon. Implantation of metal ions is also utilized to create tooth-colored wires. All

24

coatings have different properties that can have an effect on friction, as well as other wire characteristics. The appropriate coating thickness necessary for mechanical durability varies depending on the coating composition. To maintain constant wire dimensions, the core material size has to be reduced, and therefore, the mechanical properties of a coated wire will differ from a wire with the same dimensions without coating. The coating itself can also have an effect on wire characteristics. One of the effects that the coating has on resistance to sliding is through its surface morphology. Dayanne Lopes da Silva and Claudia Trindade Mattos (2013) evaluated five properties of coated orthodontic wires: inner wire dimensions, modulus of elasticity, modulus of resilience, maximum deflection force, and load deflection curve characteristics. They found that the wires with all surfaces coated had a significant reduction in their inner alloy core and showed reduced values for all parameters tested. Wires with only facial surface coating did not have a significantly reduced alloy core and showed similar mechanical behavior to the non-coated stainless steel group. Therefore, the reduction in size of the inner alloy core of a wire—in order to compensate for thickness of the coating material—can significantly modify the wire’s mechanical properties (Lopez et al., 2013). In 2010, Elayyan and Silikas compared mechanical properties of epoxy-coated wires in a 3-point bending test to uncoated superelastic NiTi wires. They found that coated wires showed significantly lower loading and unloading forces when compared to similar control groups. The coating thickness of wires tested was reported to be 0.002 inches (Elayyan et al., 2010). Similar results of the 3-point bending test were obtained in another experiment. It was also shown that polymer-coated wire surfaces had a rougher morphology and significantly reduced surface hardness. Gold and rhodium-coated wires showed

25

rougher morphology, but similar hardness. This was due to the presence of a very thin layer of coating (0.5 µm), which is required for gold and rhodium-coated wires (Iijima et al., 2012). Similar to traditional wire alloys, increased friction of coated wires was due to an increased surface roughness in some experiments. Polymer-coated and rhodium-coated wires showed similar or increased friction when compared to the control stainless steel wires, with a straight wire configuration including 5- and 10-degree bracket rotation. Rhodium-coated wires resulted in increased friction when compared to polymer-coated wires. After viewing the surface under an electron microscope, rhodium-coated wires were shown to be rougher than polymercoated and stainless steel wires (Kim et al., 2014). In 2012, Farronato and Maijer compared the resistance to sliding of Teflon-coated orthodontic wires and traditional, uncoated NiTi and stainless steel wires. They used self-ligating brackets from three different manufacturers arranged in eight different scenarios, from straight-line to different torque and displacement variables. The results showed significantly reduced friction coefficients with Teflon-coated wires for all bracket types and alignment scenarios (Farronato et al., 2012). Other authors showed no correlation between the surface roughness of coated esthetic wires and resistance to sliding (Rudge et al., 2014). Multiple factors affecting friction were described earlier. It is unclear at this time as to what extent the characteristics of each wire, bracket, or ligation material influence the overall resistance to sliding. The complex surface interactions produced between each wire-bracket pair under the unique conditions of the oral cavity should be investigated in order to determine frictional characteristics of the system. Experimental designs that closely reproduce in vivo conditions are required in order to compare different product characteristics until more definitive factors that influence friction are described in the field of orthodontics.

III. MATERIALS AND METHODS

A.

Friction-testing device A friction-testing device, inspired by the original experiment conducted by D.C. Tidy and

D.Orth in 1989, was designed and manufactured by the University of Illinois at Chicago Department of Orthodontics and American Dental Association Department of Research and Laboratories. This device (Fig 1.) attempts to simulate forces acting at the bracket-wire interface during canine retraction.

Figure 1. Friction-testing device assembly.

27

The device consists of a metal plate with four brackets mounted on the plate arranged in a straight line at 8mm intervals (Fig. 2). Horizontal and vertical reference lines for the brackets’ positioning were inscribed in the metal plate. The fifth bracket, located in the middle, is bonded to a plastic, upper left canine typodont tooth (Kilgore International Inc., Coldwater, MI) 5mm from the incisal edge and in the middle of the crown mesiodistally. The tooth is suspended within a 10mm space with a silk suture wrapped around the mesial surface of the bracket. The silk suture is located under the bracket wings underneath the ligation material and is attached to a load-measuring cell of the Instron™ Testing Machine. Tested wires are secured within bracket slots with a Teflon-coated stainless steel ligature. Tight ligation is applied to the mounted brackets. Once the suspended bracket is tightly ligated, one back turn is performed to allow loose ligation. At the center of the root portion of the typodont tooth, a metal hook is cemented, which was made from a 0.04-inch stainless steel round wire. The distance from the bracket slot to the hook is 50mm and a 20g weight is applied to the hook. The resultant moment of force at the bracket-wire interface will represent a single equivalent resistance force at the center of resistance of the root. This is similar to the forces that exist when a 100g force is applied to the center of resistance of the tooth, located about 10mm apically from the center of the bracket on a typical canine in a clinical setting. A two-point contact created at the bracket-wire interface produces a couple that counters the moment of that force.

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Figure 2. Computer-aided drawing of a metal plate for mounting of brackets.

29

A metal plate is mounted on the Instron™ Testing Machine. A crosshead attached to the suspended bracket with a silk suture moves upward at a speed of 5mm/min. To avoid interference of the suspension material between the plate and mounted brackets, the metal plate design allows for adjustment and fixation of the metal plate’s position in order to properly align it under the load cell. In each test, the suspended bracket was moved in relation to the mounted metal plate for a distance of no less than 2mm, and the load cell reading was recorded. The reading on the load cell represents the clinical retraction force applied to a canine tooth during retraction on the archwire. In a clinical setting, part of that force is lost in order to overcome the resistance force applied to the root surface by the periodontal ligament and surrounding bone. This retraction force is replicated in the experiment by a single equivalent force applied to the canine extension hook. The remainder of the force is lost due to friction on the wire-bracket interface. Therefore, in the experiment conducted, the difference between the load cell reading and the force created by the weight of the tooth with a 20g weight attached represents the produced friction. B.

Test brackets The friction produced between ceramic brackets and esthetic-coated archwires was

measured in this experiment. Polycrystalline alumina Clarity Advanced TM (3M/Unitek, Monrovia, CA) brackets were used. These brackets were chosen to represent ceramic brackets, which are esthetic and popular amongst practitioners today. The experimental set-up consisted of five brackets to represent the upper right quadrant, from the upper right central incisor to the second premolar. The 0.018-inch slot size was chosen, as it is one of the most commonly used slots in a clinical situation. The central incisor, lateral incisor, and first and second premolar brackets were positioned for bonding by utilizing horizontal and vertical reference lines on the metal plate. The leveling and alignment of brackets

30

were confirmed and adjusted utilizing a straight, 0.017 x 0.025-inch stainless steel wire that was fully inserted into the bracket slots prior to composite curing (Fig. 3). Retractable pins incorporated in the metal plate design were located on both ends of the plate. The pins served to align and hold the straight wire in a repeatable position each time new brackets were bonded.

Figure 3. Bracket positioning procedure.

Utilization of the described bracket positioning and bonding techniques effectively eliminated the prescription tip and torque built into the bracket and provided linear alignment in a passive configuration, without binding of the wire at the bracket slot corners. Excess composite was used to eliminate the bracket base anatomy and allowed positioning of the brackets on a straight line (Fig. 4).

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Figure 4. Tip and torque elimination during bracket positioning. Top view.

The canine bracket was bonded to the facial surface of the typodont tooth, utilizing vertical and mesiodistal reference lines 5mm from the incisal edge and in the middle of the crown mesiodistally. The horizontal reference line was placed 5mm from the incisal edge, measured with a bracket-positioning gage (Orthopli, Philadelphia, PA). The vertical reference line was placed connecting the tip of the incisal edge to the middle of the cervical curvature. The bonding procedure consisted of following steps: 1) Application of one layer of Asure Universal Bonding ResinTM (Reliance Orthodontic Products, Itasca, IL) for 5 seconds, 2) Gentle air drying for 10 seconds, 3) Application of Pad Lock® adhesive (Reliance Orthodontic Products, Itasca, IL) to the bracket pad, 4) Bracket placement and thorough adhesive removal with an explorer, and 5) Adhesive curing with a VALOTM curing light (Opal Orthodontics, South Jordan, UT) for 10 seconds. One skilled operator with 2.5x loop magnification performed all bonding procedures. After bonding the first bracket, a PVS positioning jig was made with polyvinylsiloxane impression material. All consequent canine brackets were bonded utilizing the initial positioning jig. Bonded brackets were used for a maximum of ten test runs with the same type of wires. The tested brackets were then replaced with a new set. A bracket-removing plier

32

(Orthopli, Philadelphia, PA) was used to remove brackets when needed. The remaining composite was removed with sharp scaler. C.

Test wires Three types of esthetic wires were included in this study: polymer-coated (Dany BMT

Co., South Korea), rhodium-etched (DB Orthodontics Limited, Ryefi eld Way, Silsden, West Yorkshire, BD20 0EF, United Kingdom), and epoxy-coated (G&H Orthodontics, Franklin, IN). These wires were chosen to represent different types of esthetic coating materials available on the market today. All tested wires had a cross section diameter of 0.016 inches, as indicated by the manufacturers. Rhodium-etched wires (DB Orthodontics Limited, Ryefi eld Way, Silsden, West Yorkshire, BD20 0EF, United Kingdom), according to manufacturer description, included a stainless steel core of 0.016 inches in diameter and a thin, 0.002µm rhodium coating. Manufacturers described the polymer- and epoxy-coated wires as comprising a 0.014-inch stainless steel core and a 0.002-inch esthetic coating, resulting in a diameter that measured 0.016 inches. Pre-formed stainless steel Accu-Form 0.016-inch Upper Arch Medium Arch (Dentsply International, York, PA) was included as the control stainless steel wire, which had a 0.016-inch diameter and lacked an outer coating. Each pre-formed wire was cut in the middle and straightened according to a straight-line template. Non-coated ends of the wire were then removed. A length of 50mm from one end of the wire was measured with an electronic caliper and then subsequently cut. A 90-degree bend was created for reference purposes, as measured 2mm from the end of the wire. D.

Randomization Due to the distinct characteristics of each wire type, randomization of the wires was not

possible in this experiment. Pre-packaged wires were opened and each wire was subsequently placed in individual plastic bags and assigned a number from 10 to 49. Numbers corresponding

33

to wire type were recorded and matched with the final results obtained after the end of the experiment. E.

Saliva To represent conditions close to an in vivo intraoral environment, artificial saliva was

applied to the canine bracket before each test run. One drop, or approximately 0.05ml of artificial saliva, was applied directly to the brackets with a medical syringe 30 seconds before the test was performed. Artificial saliva was prepared at a laboratory of the University of Illinois at Chicago College of Dentistry using the following formula: 20 mmol of Hepes (4-(2-hydroxyethyl)-1piperazineethanesulfonic acid), 0.538 mmol of Calcium Chloride Dihydrate (CaCl2.2H20), 0.451 mmol of Potassium Phosphate (KH2PO4), 43.330 mmol of Potassium Chloride (KCl), and 4M Sodium Hydroxide (NaOH) to adjust the pH to 6.75. F.

Ligation Ligation with a tooth-colored pre-cut 0.012-inch diameter Teflon-coated stainless steel

ligature ties (Modern Arch Orthodontic Supplies, Wyomissing, PA) was performed to secure the wire to the brackets. One skilled operator tightly ligated four brackets bonded to the stainless steel plate. The suspended canine bracket was ligated loosely in order to minimize friction between the wire and bracket surfaces during movement. To ensure consistent ligation force, the canine bracket was initially ligated tightly to the feel of the operator and 1.5 back turns were subsequently performed to create a loose ligation. A test run was performed without force applied to the canine metal hook after each ligation procedure in order to detect if excessive ligation forces were present. G.

InstronTM testing machine The MTS InstronTM (Noorwod, MA) universal testing machine model #5582, serial

#04415 with Testworks 4 Version 4.12 D Build 1012, was used for the tests performed. All

34

experiments were conducted at the American Dental Association’s facilities in Chicago and the equipment was properly calibrated prior to testing. H.

Testing protocol For each type of wire, a new set of brackets was used. Four brackets were bonded to a

steel plate and the middle, the canine bracket was bonded to a plastic typodont tooth by utilizing a bracket-positioning jig. The metal plate was placed and secured in the InstronTM testing machine. Tight ligation was applied to the stationary brackets. To ensure loose ligation to the movable bracket, the ligature was fully tightened initially and 1.5 reverse turns of the ligature was subsequently performed to allow free sliding. The movable bracket was suspended to the load cell with a silk suture, as previously described. Passive suspension with silk suture that did not touch any other components of the test set-up was verified, and the position of the metal plate was adjusted accordingly. To ensure appropriate ligation with minimal friction before each test, an unloaded trial run was performed. The load was not applied to the plastic tooth extension hook and the resultant friction was measured. A crosshead movement of no less than 0.5mm was recorded for each trial run. If the friction created by ligation was greater than 0.1 newtons, all brackets and ligatures were discarded and a new set of brackets was bonded and ligated. When proper ligation was verified, the crosshead was returned to the initial position where the canine tooth was suspended 1.5mm above the bottom edge of the metal plate niche, without touching the plate. Passive alignment of the silk suture was verified at this time and adjusted appropriately. A 20g weight was then suspended at the canine extension hook. An artificial saliva drop was applied to the canine bracket 30 seconds before testing. A crosshead movement of no less than 2mm at 5mm/min speed was recorded for each wire. The force required to move the suspended tooth was recorded in grams at 0.1-second intervals. Ligature

35

wires were cut and removed after each testing and the archwire was returned to its assigned plastic bag. The force required to move the suspended tooth with the attached load was recorded in grams for each wire. The force recording was performed for a minimum of 25 seconds, or 2mm of crosshead movement. An InstronTM testing machine recorded the results. A graph was then constructed, which represents the load recorded by the loading cell at 0.1-second intervals for a 2mm movement (Fig. 5). The highest load recorded by the loading cell immediately before bracket movement began was recorded as maximum static friction. After movement initiated the force required to continue tooth movement for the next 10 seconds was recorded at 0.1-second intervals. The mean value for the force recorded for 10 seconds following initiation of tooth movement was calculated. This recording represents the kinetic resistance to sliding for each wire.

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Figure 5. Recording of static and kinetic friction.

I.

Wire diameter measurement Wire diameter measurements were performed two times using a Nikon Profile Projector

(Nippon Kogaku, Tokyo, Japan). A 100x magnification was used to measure the wires at a 0.001mm resolution (Fig. 6).

37

Figure 6. Measurement of wire diameter using Nikon Profile Projector at 100x magnification.

The first diameter measurement was performed prior to friction testing. Each wire was measured in three points. The wire was placed on the projector’s glass table with the end

38

containing a 90-degree bend on the left, with the bent portion facing up and away from the operator. The bent portion was designated as a reference point in order to recreate the position and orientation of the wire for future measurements. The reference end of the wire was centered and the focus was adjusted appropriately. The projector’s x-axis zero line was aligned with the distal end of the wire’s bent portion, and this was set as the zero position on the x-axis. Diameter measurements were performed at 10mm, 20mm, and 30mm distances from the zero position of the x-axis, according to the projector’s readings. When the appropriate position on the x-axis was located, the focus was readjusted to achieve the best visualization of the border created by the wire’s shadow. The y-axis zero line on the projector screen was aligned with the upper border of the shadow and the y-axis measurement was then set as zero. The projector screen’s zero line for the y-axis was subsequently aligned with the lower border of the wire’s shadow without changing the position of the x-axis. At this position, the measurement on the y-axis was recorded, representing the diameter of the wire. Measurements were repeated and recorded for all three points on each wire. After the friction experiment was performed and the wire was removed from the frictiontesting device, all polymer-coated and epoxy-coated wires were put through a coating removal procedure. Each wire was placed in a plastic container with an appropriate chemical solution to remove its coating. A solution of methyl ethyl ketone was used to dissolve the esthetic coating. After coating removal, the diameter was measured again in three locations for each wire following the protocol for the measurements described earlier. A 10-count sample size was used for each wire tested. The diameter measurements for all wires were recorded before friction testing. For epoxy-coated (G&H Orthodontics, Franklin, IN) and polymer-coated (Dany BMT Co., South Korea) wires, the second diameter measurement was

39

recorded after friction testing was completed and the coating was removed. The mean values for the wire diameter were calculated for each individual wire using measurements at three reference points. The mean value for each type of wire was calculated. The second diameter measurement for polymer-coated wires, after coating removal, represents the diameter of the inner stainless steel core and consistency of its dimensions. For each polymer-coated wire, the thickness and consistency of its coating was calculated by subtracting the dimensions of the inner stainless steel core from the corresponding diameter measurement of this wire with coating in each measuring point of reference. The mean value of the coating thickness for each individual wire and for polymer-coated and epoxy-coated wire groups was calculated. J.

Data analysis One-way ANOVA and Pearson correlation tests were performed to compare the mean

differences of static and kinetic resistance to sliding of tested wires among manufacturers, and also, to test if there was an association between stainless steel core dimensions and frictional characteristics. An independent Student T-test was performed to compare the mean difference of the stainless steel core diameter and coating thickness between two manufacturers. A one sample T-test was used to compare measured wire sizes to the manufacturers’ reported dimensions. The data was analyzed using SPSS version 22.0 (Chicago, IL). The statistical significance was set at p < 0.05.

IV. RESULTS

A.

Static and kinetic resistance to sliding A one-way ANOVA test was performed to evaluate mean differences in static friction

(Fs) and kinetic friction (Fk) between tested wires. Statistically significant mean differences were found among the four types of tested wires in static resistance to sliding and kinetic resistance to sliding, F (3.39) = 14.473 and 10.651, p < 0.001. Post hoc Bonferonni test was performed to evaluate if statistically significant differences exist in static friction (Fs) between tested wires. Table I summarizes these results:

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TABLE I COMPARISON BETWEEN STATIC FRICTIONS (Fs) OF TESTED WIRES (grams) Comparison

Wire type

N

Mean ± S.D.

RhodiumRhodium-etched 10 180.327796±25.01506 etched vs. PolymerPolymer-coated 10 107.782903±23.82217 coated. Rhodium- Rhodium-etched 10 180.327796±25.01506 etched vs. Stainless Stainless steel 10 118.461613±18.83872 steel RhodiumRhodium-etched 10 180.327796±25.01506 etched vs. EpoxyEpoxy-coated 10 159.994655±41.13524 coated EpoxyEpoxy-coated 10 159.994655±41.13524 coated vs. PolymerPolymer-coated 10 107.782903±23.82217 coated EpoxyEpoxy-coated 10 159.994655±41.13524 coated vs. Stainless Stainless steel 10 118.461613±18.83872 steel PolymerPolymer-coated 10 107.782903±23.82217 coated vs. Stainless Stainless steel 10 118.461613±18.83872 steel *p-values statistically significant at p ≤ .05.

Mean Difference p-value* 72.5448933*

≤ 0.001

61.8661826*

≤ 0.001

20.3331413

0.713

52.2117520*

0.001

41.5330414*

0.015

-10.6787106

1.000

42

Static Friction Fs (grams)

250 200 150 100 50 0 180.327796

159.994655

107.782903

118.461613

Rhodium-etched

Epoxy-coated

Polymer-coated

Stainless steel

Wire Type

Figure 7. Comparison of static friction between tested wires.

Rhodium-etched wires differed significantly from polymer-coated wires and stainless steel wires, p-value < 0.001. Epoxy-coated wires differed significantly from polymer-coated wires and stainless steel wires, p-values equal 0.001 and 0.015, respectively. No statistically significant difference was found between rhodium-etched and epoxy-coated wires, and between stainless steel and polymer-coated wires. A post hoc Bonferonni test was performed to evaluate if statistically significant differences exist in kinetic friction (Fk) between tested wires. Table II summarizes these results:

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TABLE II COMPARISON BETWEEN KINETIC FRICTIONS (Fk) OF TESTED WIRES (grams) Comparison Wire type. N Mean ± S.D. Mean Difference RhodiumRhodium-etched 10 152.436841 ± 13.88606 etched vs. 52.1741407* PolymerPolymer-coated 10 100.262701 ± 19.43943 coated. RhodiumRhodium-etched 10 152.436841 ± 13.88606 etched vs. 39.0969292* Stainless Stainless steel 10 113.339912 ± 19.41868 steel RhodiumRhodium-etched 10 152.436841 ± 13.88606 etched vs. 1.0854425 EpoxyEpoxy-coated 10 151.351399 ± 41.29580 coated EpoxyEpoxy-coated 10 151.351399 ± 41.29580 coated vs. 51.0886982* PolymerPolymer-coated 10 100.262701 ± 19.43943 coated EpoxyEpoxy-coated 10 151.351399 ± 41.29580 coated vs. 38.0114867* Stainless Stainless steel 10 113.339912 ± 19.41868 steel PolymerPolymer-coated 10 100.262701 ± 19.43943 coated vs. -13.0772114 Stainless Stainless steel 10 113.339912 ± 19.41868 steel *p-values statistically significant at α ≤ 0.05.

p-value* ≤ 0.001

0.010

1.000

≤ 0.001

0.013

1.000

44

250

Kinetic Friction Fs (grams)

200

150

100

50

0 Series1

Rhodium-etched

Epoxy-coated

Polymer-coated

Stainless-steel

152.436841

151.351399

100.262701

113.339912

Wire Type

Figure 8. Comparison of kinetic friction between tested wires.

Rhodium-etched wires differed significantly from polymer-coated wires and stainless steel wires, with a p-value of < 0.001 and a p-value = 0.010, respectively. Epoxy-coated wires differed significantly from polymer-coated wires and stainless steel wires, with p-values < 0.001 and p-value = 0.013, respectively. No statistically significant difference was found between rhodium-etched and epoxy-coated wires, and between polymer-coated and stainless steel wires.

45

250

Friction (grams)

200

150 Static Friction Fs 100

Kinetic Friction Fk

50

0 Rhodium-etched

Epoxy-coated

Polymer-coated

Stainless steel

Groups of wires

Error bars 95% Cl

Figure 9. Comparison of static and kinetic friction of tested wires.

B.

Wire dimensions One-way ANOVA test was performed to evaluate mean differences in diameter as

received between tested wires. Statistically significant mean differences were found among the four types of tested wires in diameter, F (3.39) = 150.520, p < 0.001. Post hoc Bonferonni test results are summarized in Table III:

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TABLE III COMPARISON BETWEEN DIAMETERS AS RECEIVED (d) OF TESTED WIRES (inches) Comparison Rhodiumetched vs. Polymercoated. Rhodiumetched vs. Stainless steel Rhodiumetched vs. Epoxycoated Epoxycoated vs. Polymercoated Epoxycoated vs. Stainless steel

Wire type

N

Mean ± S.D.

Rhodium-etched

10

0.016197 ± 0.0000811

Polymer-coated

10

0.017196 ± 0.0003901

Rhodium-etched

10

0.016197 ± 0.0000811

Stainless steel

10

0.015936 ± 0.0001320

Rhodium-etched

10

0.016197 ± 0.0000811

Epoxy-coated

10

0.015199 ± 0.0000694

Epoxy-coated

10

0.015199 ± 0.0000694

Polymer-coated

10

0.017196 ± 0.0003901

Epoxy-coated

10

0.015199 ± 0.0000694

Stainless steel

10

0.015936 ± 0.0001320

PolymerPolymer-coated 10 0.017196 ± 0.0003901 coated vs. Stainless Stainless steel 10 0.015936 ± 0.0001320 steel *p-values statistically significant at α ≤ .05.

Mean Difference

p-value*

-0.0009987*

≤ 0.001

0.0002612

0.056

0.0009974*

≤ 0.001

-0.0019961*

≤ 0.001

-0.0007362*

≤ 0.001

0.0012598*

≤ 0.001

A statistically significant difference was found between rhodium-etched and polymercoated wires, rhodium-etched and epoxy-coated wires, epoxy-coated and polymer-coated wires, epoxy-coated and stainless-steel wires, and polymer-coated and stainless-steel wires, p ≤ 0.001 for all pairs. No statistically significant difference was found between the pair of polymer-coated and stainless steel groups, p = 0.056.

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A one sample T-test was performed to assess mean differences between actual dimensions of the wires measured in the experiment and sizes reported by the manufacturers’. Table IV summarizes these findings:

TABLE IV COMPARISON BETWEEN SAMPLE MEASUREMENTS AND MANUFACTURERREPORTED WIRE SIZE Manufacturerlisted wire size (inches)

N

Rhodium-etched

0.016

10

Polymer-coated

0.016

Epoxy-coated Stainless steel

Wire type

Measured wire size (inches)

Mean Difference

p-value*

0.016197 ± 0.0000811

0.0001969*

≤ 0.001

10

0.017196 ± 0.0003901

0.0011955*

≤ 0.001

0.016

10

0.015199 ± 0.0000694

-0.0008005*

≤ 0.001

0.016

10

0.015936 ± 0.0001320

-0.0000643

0.158

Mean ± S.D.

*p-values statistically significant at p ≤ 0.05.

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Figure 10. A comparison between measured and manufacturer-listed wire size.

For all wires, manufacturers reported the wire diameter to be at 0.016 inches. A statistically significant difference was found between manufacturer-listed and the measured wire diameter for all coated wires. The diameters of rhodium-etched, polymer-coated, and epoxycoated wires differed significantly from the manufacturer-reported 0.016 inches, p < 0.001 for all wires. The non-coated stainless steel wire diameter was not statistically different from the size listed by the manufacturer. Mean wire dimensions for polymer-coated and epoxy-coated groups, size of stainless steel core, and coating thickness is summarized in Table V:

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TABLE V COMPOSITION OF COATED WIRES (inches) Wire Type

Total wire diameter

Stainless steel core diameter

Coating thickness

Polymer-coated

0.017196 ± 0.0003901

0.015700 ± 0.0003192

0.001494 ± 0.0002149

Epoxy-coated

0.015199 ± 0.0000694

0.013812 ± 0.0000814

0.001387 ± 0.0000852

Independent samples T-test was performed to evaluate the mean differences in stainless steel core dimensions and coating thickness between polymer-coated and epoxy-coated wires. Table VI summarizes these findings:

TABLE VI COMPARISON IN STAINLESS STEEL CORE DIAMETER AND COATING THICKNESS BETWEEN POLYMER- AND EPOXY-COATED WIRES (inches) Comparing characteristics Stainless steel core diameter

Esthetic coating thickness

SS core diameter

Comparing wire types

N

Epoxy-coated

10

0.013812 ± 0.0000814

Polymer-coated

10

0.015701 ± 0.0003193

Epoxy-coated

10

0.001387 ± 0.0000853

Polymer-coated

10

*p-values statistically significant at p ≤ .05.

Mean ± S.D.

0.001495 ± 0.000215

Mean Difference (grams)

p-value*

-0.0018885*

≤ 0.001

-0.0001076

0.167

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A statistically significant mean difference was found in the stainless steel core diameter between epoxy-coated wires and polymer-coated wires, p-value < 0.001. No statistically significant difference was found in the esthetic coating thickness between these two wires, pvalue = 0.167.

0.02

Wire diameter (inches)

0.018 0.016 0.014 0.012 Total diameter

0.01 0.008

SS core diameter

0.006

Coating thickness

0.004 0.002 0 Rhodium-etched

Epoxy-coated

Polymer-coated

Wire type

Figure 11. Composition of tested wires.

Stainless-steel

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C.

Association between stainless steel core diameter and friction values In order to investigate if there was a statistically significant association among static

resistance to sliding (in grams), kinetic resistance to sliding (in grams), and stainless steel core diameter (in inches) of all tested wires, correlations were computed. No statistically significant correlations were found between the reduction in stainless steel core dimensions and the amount of static and kinetic resistance to sliding for all tested wires. Results are summarized in Table VII:

TABLE VII PEARSON CORRELATION TEST BETWEEN STAINLESS STEEL CORE DIAMETER AND FRICTION VALUES Wire type

N

Rhodium-etched Polymer-coated Epoxy-coated Stainless steel

10 10 10 10

Correlation with static friction Coefficient of Sig. (2-tailed) correlation (r)

-0.134 -0.01 -0.066 0.015

0.712 0.977 0.856 0.967

Correlation with kinetic friction Coefficient of Sig. (2-tailed) correlation (r)

0.272 -0.09 -0.082 -0.085

0.448 0.806 0.822 0.816

A statistically significant correlation, p-value < 0.05, was found between static resistance to sliding and kinetic resistance to sliding for rhodium-etched, epoxy-coated, and stainless steel groups. The Pearson correlation for groups 1 (rhodium-etched wires), 2 (epoxy-coated wires), and 4 (stainless steel wires [non-coated]) only for variables of static resistance to sliding and kinetic resistance to sliding were approximately, r (10) ranging from > 0.70 to 0.89, p-value < 0.05. The r² indicates that approximately from 50% to 79% of the variance in static resistance to

52

sliding can be accounted from kinetic resistance to sliding in the following groups: groups 1 (rhodium-etched wires), 2 (epoxy-coated wires), and 4 (stainless steel wires [non-coated]).

IV. DISCUSSION

The importance of friction in orthodontics was supported by the results of this study. A significant amount of resistance to sliding was observed for all wire types tested. Although the pattern of friction was similar for all wires, the values and characteristics of resistance to sliding varied among specimens. The test results provide data for a better understanding of the frictional behavior of esthetic orthodontic wires. The effects of varying composition and surface characteristics on static and kinetic friction were ultimately investigated. A.

Friction-testing device and conditions The experiment design was chosen from various options described in the literature. This

model allows the reproduction of in vivo conditions in the laboratory setting and accounts for a large number of variables that affect friction during orthodontic tooth movement (Tidy D.C., 1989). While utilizing sliding mechanics, retraction of the canine involves application of the force to the canine bracket, which is ligated to the archwire (Fig. 12a). The periodontal ligament of the tooth resists the retraction force and acts in the direction opposite to tooth movement. The retarding tooth movement biologic force has its center of resistance located down the root of the tooth. With an increasing retraction force, the tooth rotates in two directions: the sagittal plane, or mesio-distally (Fig. 12b), and in the occlusal plane (Fig. 12c). When the critical angle is achieved, the bracket wall and ligation material contact the wire. Therefore, friction exists in two locations: between bracket walls and the wire (Fig. 12b), and between ligation material and the wire (Fig. 12c). The retraction force applied to the tooth is continuous and eventually leads to elastic deformation of the wire. The load at the point of contact between the bracket, ligation

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material, and the wire increases. As a result, friction also increases. An anti-rotation force is produced by the archwire resisting deformation. This force acts to upright the root (Fig. 12d). When equilibrium is achieved among forces acting on the tooth and forces created on the interface between the bracket and archwire, the tooth slides along the archwire in a series of tipping and uprighting strokes (Drescher and Bourauel, 1989).

A

C

B

D

Figure 12. Phases of tooth movement.

The test model used in our experiment allowed us to reproduce the mechanics of tooth movement that occur in vivo. The canine tooth is suspended with silk suture wrapped around wings of the bracket. The silk suture is attached to the loading cell that moves upwards during

55

the experiment, replicating the retraction force applied to the canine with a c-chain or a coil spring. The site of application of this retraction force, in relation to the center of resistance, is identical to the intraoral canine retraction scenario. The biologic force that resists tooth movement by acting through the tooth’s center of resistance is imitated by the load, which is suspended from the hook attached to the plastic tooth. The applied load creates a moment of force at the contact between the bracket and archwire. The moment of force created in the experiment was designed to be equal to the moment of force created by a biologic resistance of 100 grams applied at the canine’s center of resistance, located 15mm down the root from the cusp tip. Since other forces are not acting on the canine in this experiment, tooth rotations, wire deformation, and the resulting friction are similar to the in vivo canine retraction conditions as described earlier. The friction measurement utilizing the canine retraction model, similar to the one used in our experiment, is commonly described in orthodontic literature. Different methods were utilized for simulating the biologic force acting on the root surface to resist tooth movement. Simulation of the periodontal ligament is possible by utilizing light body viscosity PVS impression material, as described by Loftus et al. (1999), in order to replicate the biologic force of resistance. In 1989, Drescher et al. used a tooth mounted in elastic rubber foam to evaluate orthodontic frictional forces. Another example of simulating the biologic force in a laboratory setting includes utilizing a mixture of utility and base plate paraffin waxes in order to replicate the periodontal ligament (Tanne et al., 1991). Although each method to replicate forces created by the periodontal ligament has their individual advantages, the load application described by Tidy D.C. (1989) is more consistent and easier to reproduce.

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It is nearly impossible to fully replicate in vivo conditions in a laboratory setting. Due to variable conditions in the oral cavity, absolute friction values obtained in this study may be different from the actual values produced during canine retraction in a live patient. However, the friction-testing device and methods designed for this research allowed us to closely replicate characteristics of canine movement during retraction. Testing conditions were replicated from one wire to another. Therefore, the results obtained in this study allow us to compare friction characteristics between tested wires and also commonly used stainless steel wires. This knowledge will allow practitioners to plan their mechanics with better accuracy and to obtain more predictable treatment results. B.

Wire characteristics effect on movement pattern and kinetic friction Throughout the experiment, all wires showed similar patterns of static and kinetic

resistance to siding. The classic laws of friction were followed by all groups of wires and static resistance to sliding was generally lower than kinetic resistance to sliding. These results correlate with the data reported by Kusy and Whitley (1997). Typical resistance graphs recorded by the testing device for each wire are represented in figures 13, 16, 19, and 22. The classic pattern of static and kinetic friction is demonstrated in each graph. Before the maximum resistance to siding is reached, suspended tooth movement will not be observed. The amount of load recorded by the testing device’s loading cell steadily increases. When the maximum static resistance to sliding value for each wire is reached, a sudden drop in the reading of the load cell appears on the graph. At that time, the suspended bracket will begin moving for the first time. From this point, movement of the tooth is considered to be continuous and resistance to sliding can be called kinetic resistance. Rapid movement of the bracket will finish shortly after. Pressure will then begin to increase until a new

57

peak is reached, in which case, the bracket will slip into a rapid movement. Unlike two smooth surfaces that slide in a smooth motion against each other, movement of the brackets can be described as the “stick and slip” phenomenon. A bracket that moves along the archwire at a slow velocity actually experiences a series of rapid movements followed by no movement at all. In 2003, Rossouw and his colleagues suggested the existence of such movement in orthodontics. After friction testing was finished, one wire of each type was randomly selected to obtain scanning electron microscope images of the wire surface. Parts of the wire that were not used for friction testing were examined to evaluate surface topography. Two images were obtained for each wire: an image of the wire end at 100x magnification and an image of the wire surface adjacent to the end at 220x magnification. Although some similarities between groups exist in the graphs’ shape for static and kinetic friction, there are some distinguished characteristics for each individual wire type. Stainless steel wires produced a relatively smooth kinetic friction line (figure 13). Peak heights are small and uniform along the entire movement path. This can be explained by the smooth surface of the stainless steel wire. Few defects and artifacts as a result of manufacturing imperfections and handling may be responsible for some “bumps” along the recorded resistance to sliding graph. Stainless steel wire images confirm that surface topography is relatively smooth with small, scattered defects and artifacts (figure 14, 15).

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Figure 13. Typical resistance graph for a stainless steel wire (wire #47).

Figure 14. Scanning electron microscope image of a stainless steel wire end at 100x magnification.

59

Figure 15. Scanning electron microscope image of a stainless steel wire at 220x magnification.

Rhodium-etched wires (DB Orthodontics Limited, Ryefi eld Way, Silsden, West Yorkshire, BD20 0EF, United Kingdom) produced very similar results as compared to the stainless steel wire static and kinetics friction graphs (Fig. 16). The kinetic resistance graph exhibits uniform peaks but the height of these peaks is significantly larger than stainless steel graphs, and even of any of the other wire types tested. Scanning electron microscope images of the wire reveal a very rough surface (Fig. 17, 18.). An increase in surface roughness can explain higher dimensions of the peaks seen on the resistance to sliding graph. The kinetic resistance to sliding for epoxy-coated wires (G&H Orthodontics, Franklin, IN) had significant fluctuations in value (Fig. 19). As opposed to rapid spikes in resistance observed with other wires, an increase in load required for continuous movement along epoxy-

60

coated wires appeared as smooth waves on the graph at 2.5-3 second intervals. On the scanning electron microscope images, the surface of the wire appears smooth with scattered defects and imperfections (Fig. 20, 21). It is not possible to explain the kinetic resistance to sliding pattern of this wire from the information available regarding its surface roughness. Other factors might be responsible for the kinetic friction pattern of this wire, such as surface hardness or the specific interactions between epoxy coating, ceramic bracket, and ligation material. The polymer-coated wires (Dany BMT Co., South Korea) pattern of recorded kinetic resistance to sliding has the combined characteristics of stainless steel and epoxy-coated wires (Fig. 22). Uniform small peaks exist along the kinetic friction line, resembling stainless steel wire behavior. Smooth and slow fluctuations in the kinetic friction level exist, but intervals between fluctuations are much larger and, therefore, the motion appears to be smoother when compared to epoxy-coated wires. Scanning electron microscope images reveal a smooth surface with scattered defects and artifacts—similar to the epoxy-coated wire surface (Fig. 23, 24). Differences in the movement pattern of tested wires can be easily identified on the resistance to sliding graphs that represent the results obtained in our experiment. While some characteristics can be explained by surface roughness variations observed on the scanning electron microscope images of tested wires, more information is required to explain the frictional behavior of each wire type. Wire dimensions and its consistency, stiffness, absolute surface hardness of the wire, and differences in surface hardness between the wire, bracket, and ligature are only several characteristics that could alter the motion pattern of tested wires.

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Figure 16. Typical resistance graph for a rhodium-etched wire (wire #14).

62

Figure 17. Scanning electron microscope image of a rhodium-etched wire end at 100x magnification.

Figure 18. Scanning electron microscope image of a rhodium-etched wire at 220x magnification.

63

Figure 19. Typical resistance graph for an epoxy-coated wire (wire #29).

64

Figure 20. Scanning electron microscope image of an epoxy-coated wire end at 100x magnification.

Figure 21. Scanning electron microscope image of an epoxy-coated wire at 220x magnification.

65

Figure 22. Typical resistance graph for a polymer-coated wire (wire #39).

Figure 23. Scanning electron microscope image of a polymer-coated wire end at 100x magnification.

66

Figure 24. Scanning electron microscope image of a polymer-coated wire at 220x magnification.

C.

Effect of wire characteristics on static and kinetic friction The same brand of polycrystalline alumina brackets were used with all wires tested in our

experiment. The bracket positioning and experiment design allowed us to consider the wire type as the only variable when conducting consecutive tests. Therefore, differences in friction values should be a result of varying properties of tested wires. Archwire characteristics that proved to affect friction in earlier studies include the wire shape, size, material of the core determining wire stiffness, and wire surface characteristics (Drescher et al., 1989, Kusy and Whitley, 1997). 1. Wire size and elastic properties Rhodium-etched and epoxy-coated wires showed the highest amount of static and kinetic friction. Stainless steel and polymer-coated wires did not significantly differ from each other and friction values were significantly lower when compared to the former two wires. For every wire

67

type tested in this experiment, only round wires with a 0.016-inch diameter were utilized. Therefore, the wire shape was not a factor that could affect friction dissimilarities. Each wire diameter was measured before the experiment and the analysis showed that actual wire sizes were not consistent with the diameter reported by manufacturers for all coated wires. The mean diameter for epoxy-coated wires was smaller than 0.016 inches and was measured at 0.015199 ± 0.0000694 inches. Rhodium-coated and polymer-coated wires were significantly larger than measurements reported by manufacturers, with mean diameters measuring at 0.016197 ± 0.0000811 inches and 0.017196 ± 0.0003901 inches, respectively. The stainless steel wire diameter was not significantly different from 0.016 inches, however. Multiple studies concluded that an increase in wire size leads to higher amounts of friction produced (Frank and Nikolai, 1980, Kapila et al., 1990, Kusy and Whitley, 1997, Frank and Nikolai, 1980, Yeh et al., 2007). Results from our experiment do not follow that established rule. Polymer-coated wires had the largest diameter and showed the lowest static and kinetic friction values. At the same time, epoxy-coated wires were among the pair with the highestrecorded friction value, even though these wires had a smaller diameter overall. Therefore, for tested esthetic wires in our experimental model, wire size may be not the most important factor affecting the resultant static and kinetic friction. In friction testing studies, when brackets are aligned or misaligned at a fixed degree of misalignment, larger wires reach the critical angle earlier. This is one of the factors that contribute to increased friction. Stiffness also plays a role in this scenario. Stiffer wires produce a higher force on the contact point between the bracket edge and wire—or between the wire and ligature. Therefore, classic friction increases. Moreover, notching also increases due to increased

68

force at the contact point. All of this information leads to the conclusion that wires of larger size and higher stiffness produce more friction (Frank and Nikolai, 1980, Kapila et al., 1990, Kusy and Whitley, 1997, Frank and Nikolai, 1980, Yeh et al., 2007). The experimental model of orthodontic tooth movement simulating the periodontal ligament shows that the resultant friction is affected differently by the size and stiffness of the wire. The amount of force at the contact point between the bracket and wire is dependent on the equilibrium between two factors: the moment of force at the bracket of a sliding tooth created by the biologic resistance force, and the stiffness of the wire. Consider the following two scenarios when examining a smaller wire with less stiffness and a larger wire with increased stiffness. For the smaller wire, the contact angle is larger. Therefore, after the force is applied to the tooth, the moment when the bracket edge first contacts the wire will be reached later in time. This means that before the contact occurs, the tooth rotates around its long axis and tips mesio-distally without actually sliding along the wire. After the contact is reached, the wire progressively deforms under the retraction force until a balance is reached between the amount of friction and retraction force. The point of balance between all forces is when movement starts. This balance will be reached later in time, since smaller, less stiff wires will deform further before equilibrium of all forces is reached and movement begins. Lastly, with a thicker and stiffer wire, the bracket will contact the wire earlier in time and wire plastic deformation will subsequently begin. Due to increased stiffness, the same amount of force at the wire-bracket interface will be reached at an earlier time. At this point, less deformation of the wire—or less rotation of the tooth—will be present.

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At different points in time, the balance of friction and retraction forces allowing tooth movement for two described wires may be achieved. However, the amount of force created by the deformed wire, which pushes back on the bracket edges and ligation material, will potentially be the same for both smaller and larger wires. As a result, balancing friction forces should be similar for these two wires. Notching and binding may add to the picture by increasing or decreasing friction. For a particular wire, it is difficult to predict how the amount of deflection will affect notching. The bracket edge design and differences between bracket and wire surface hardness will most likely affect notching and binding—not wire diameter and stiffness. Friction studies that simulate the periodontal retarding force often show different results than previously described studies, which included aligned or fixed misaligned brackets. In the study conducted by Loftus et al., the periodontal ligament force was simulated with polyvinylsiloxane impression material. They found no difference between friction values for stainless steel and nickel titanium wires and between stainless steel and beta-titanium wires of the same size (Loftus et al., 1999). Another study utilized a tooth mounted in elastic rubber foam in order to evaluate orthodontic frictional forces. The results proved that “the following factors affected friction in a decreasing order: retarding force (biologic resistance), surface roughness of the wire, wire size (vertical dimension), bracket width, and elastic properties of the wire” (Drescher et al., 1989). Again, wire size and stiffness are at the end of the list of factors affecting friction. Similar results were obtained in a study that simulated the retarding force using a weight applied to the root of the sliding tooth. This study was done by Tidy in 1989, and we consequently designed our experimental model after the work of this author. He demonstrated no difference in frictional values for wires of similar composition that varied in size.

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In our experiment, no statistically significant correlations were found between stainless steel core dimensions and the amount of static and kinetic resistance to sliding for all tested wires. These results are in agreement with previously discussed studies (Drescher et al., 1989, Tidy et al., 1989, Loftus et al., 1999). Stainless steel core diameter—and therefore stiffness—is not among the most important factors that affect friction in our study. 2. Surface roughness Surface roughness was not directly measured in this study. Scanning electron microscope images were obtained to visually assess the surface of tested wires. After reviewing the scanning electron microscope images, it is clear that rhodium-etched wires have a significantly rougher surface when compared to stainless steel, epoxy-coated, and polymer-coated wires. The visually rough surface of rhodium-coated wires is one of the elements that may have contributed to the highest friction values for this wire. An increased surface roughness of rhodium-coated wires was also previously reported by Kim et al. in 2014 and Iijima et al. in 2012. Polymer-coated wires were also previously reported to have a rougher surface than stainless steel (Iijima et al., 2012). We made similar observations when examining the scanning electron microscope images. Despite its rougher surface as compared to stainless steel wires, similar values of static and kinetic resistance to sliding for polymer-coated wires may be the result of other contributing factors. In other experiments, no correlation could be established between surface topography and frictional characteristics. In 2014, it was determined that wires with tooth-colored coating— and also increased surface roughness—did not always have a higher resistance to sliding than smoother surfaces (Rudge et al., 2014).

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Results from our study relating to static and kinetic friction analysis and its association with surface roughness coincide with the results of other coated esthetic archwires studies. In 2014, Yunmi Kim et al. showed higher static and kinetic friction for rhodium-coated wires when compared to uncoated stainless steel wires and polymer-coated wires. These differences in friction values were observed when angulation between the wire and bracket was introduced to the experimental model. The absolute values for friction between two experiments cannot be compared due to different wire sizes used and variations in the experiment design. 3. Surface hardness Surface hardness was not measured in this experiment. Earlier studies suggest that surface hardness has a direct effect on frictional characteristics of the material. Materials with increased surface hardness showed lower resistance to sliding when compared to softer materials (Saunders and Kusy, 1994). When the bracket material is harder than the archwire material, notching and wire damage increases. The ceramic bracket surface hardness is much larger than in metals and wire damage easily occurs (Kusy and Whitley, 1997). Polymer-coated wires showed even lower surface hardness when compared to stainless steel, while gold and rhodiumcoated wires have a similar surface hardness to stainless steel wires (Iijima et al., 2012). The softer surface of epoxy-coated wires can help us understand the higher friction values recorded for this wire. However, the high friction of rhodium-etched wires, while its surface hardness is similar to low friction stainless steel wires, indicates surface hardness does not always play the most important role in overall friction. Low friction of polymer-coated wires also may have resulted from other wire properties rather than surface hardness.

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A difference in surface hardness between two particular surfaces—or the bracket and wire—can also influence friction through notching and surface damage. The specific interaction between surface of the ceramic bracket and wire can have an effect on the friction values. It was previously shown that the cold welding effect creates more friction when beta titanium wires are used in metal brackets (Kusy and Whitley, 1990b). Wires made from composite material used in composite brackets showed lower friction than stainless steel wires in stainless steel brackets (Kusy and Whitley, 1997). It is possible that the specific surface interaction between ceramics and wire coating can increase or decrease friction. Further studies are required to investigate interactions between bracket materials and esthetic wires. D.

Wire dimensions Wire sizes for all tested esthetically coated wires were significantly different from the

dimensions reported by manufacturers. Epoxy-coated wires were smaller than the reported 0.016 inches. Polymer-coated wires and rhodium-etched wires were significantly larger than the reported 0.016 inches. A statistically significant difference in diameter of rhodium-etched wires from the reported 0.016 inches may not be clinically significant. The mean wire diameter was measured to be 0.016197 ± 0.0000811 inches. An increase of 0.0002 inches should not affect the physical properties of the wire to a clinically significant level. Epoxy-coated wires were measured to be significantly smaller than 0.016 inches, with a mean diameter measured at 0.015199 ± 0.0000694 inches. This could lead to delivery of less than expected torque and inaccurate tooth movements during the finishing stages of orthodontic treatment. According to Dellinger (1978), 0.017 x 0.025-inch wires have a deviation angle (or

73

“play”) in the 0.018-inch slot bracket of approximately 3.39 degrees. Therefore, deviation of the epoxy-coated wire size by approximately 0.0008 inches can have a negative effect on the clinician’s ability to apply predictable forces during orthodontic treatment. To allow for an appropriate esthetic coating thickness, the stainless steel core of these wires was further reduced to 0.013812 ± 0.0000814 inches. This reduction can lead to clinically significant differences in wire properties. Reduced dimensions of the inner core of esthetic wires was proved to result in reduced values for modulus of elasticity, modulus of resilience, maximum deflection force, and load deflection curve characteristics (da Silva et al., 2012, Washington et al., 2015). The mean diameter of polymer-coated wires was measured to be 0.017196 ± 0.0003901 inches. This is a statistically and clinically significant deviation in size from the manufacturerreported 0.016 inches. The stainless steel core diameter was reduced to 0.015700 ± 0.0003192 inches. One can expect this wire to have properties similar to the 0.016-inch stainless steel archwires since the inner core dimensions are similar to that measurement. However, the effect of surface coating needs to be further investigated. Due to the significantly increased wire diameter, it can be difficult, or sometimes impossible, to insert the wire in the bracket slot when working with larger wires. Moreover, torque delivery is difficult to predict with these wires. E.

Limitations Results of any laboratory study attempting to replicate in vivo conditions should be

interpreted with caution and this study is no exception. Biologic conditions in the oral cavity are unique and constantly changing, which makes it nearly impossible to mimic these conditions in the laboratory.

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Presence of saliva in the oral cavity affects friction during orthodontic tooth movement and can act as a lubricant or adhesive—depending on the saliva composition and surface characteristics of objects generating friction. Even one individual can have fluctuations in saliva composition throughout the day. Slight changes of salivary components can potentially alter its effect on friction. According to Preetha and Banerjee (2005), there are two common types of saliva substitutes that exist on the market: one group based on carboxymethylcellulose and another group based on xanthan gum. The existing substitutes do not resemble human saliva in multiple aspects. Biophysical criteria are not fully reproduced and the resulting effect on friction can potentially be different. In this study, we used one of the common formulations for a saliva substitute. The results may be different if another formulation is used or if natural human saliva is utilized for the experiment (Preetha and Banerjee, 2005). Chewing force or perturbations created when chewing or clenching teeth together was not reproduced in this experiment. As previously described, some authors who tested friction with vibrations introduced in their experimental model suggest that chewing force can reduce friction from 30-70% (Bunkall et al., 2006) to 98%-100% (Braun et al., 1999). In vivo studies by Iwasaki (2003) showed no significant and predictable friction reduction in patients who were instructed to use chewing gum in order to reproduce vibrations (Iwasaki et al., 2003). Therefore, while introduction of vibrations in our experimental model could potentially change the friction values recorded, it is not clear if this would provide a better representation of in vivo conditions. Characteristics of orthodontic tooth movement in the oral cavity are unique due to extremely low velocity and the irregular nature of tooth movement. Inability to fully reproduce biologic tooth movement in a laboratory setting may be one of the major limitations of any in vitro friction experiment. Constant movement of the crosshead in this experiment is a poor

75

reproduction of the “stick and slip phenomenon” that occurs in the oral cavity. An attempt to emulate resistance of the periodontal ligament and its effect on tooth movement was made by introducing a lateral force applied at the lever arm of the suspended tooth. Three-dimensional freedom of the suspended tooth also created a more realistic model of the interaction between the sliding bracket and archwire and possibly recreated some characteristics of orthodontic tooth movement. However, due to the inability to completely reconstruct the periodontium and its effect on tooth movement in a laboratory setting, we cannot be sure that experimental conditions fully replicate in vivo orthodontic tooth movement. The velocity of intraoral tooth movement is approximated at around 1mm per month, or 0.23×10-4mm per minute. The crosshead movement in this experiment was set at 5mm per minute, which is significantly greater and could alter the values obtained for frictional resistance. When velocity values are that low, static frictional characteristics could be more important for overall tooth movement than kinetic frictional characteristics. The main goal of this experiment was to compare the friction created by different esthetic wires to each other and to stainless steel wires with well-known characteristics. The testing conditions had to be replicated very precisely each time, with the wire type being the only changing variable. If this condition is consistent—even though absolute values obtained for friction could be different from what exists intraorally—it would still be possible to evaluate friction properties of tested wires and compare them to stainless steel, and also between different types of esthetic coatings. Constructed exclusively for this experiment, the friction-testing device allowed us to reproduce testing conditions very accurately each time. The one variable that was poorly

76

controlled was the ligation force. Research showed that an increased ligation force increases frictional forces (Frank and Nikolai, 1980). One skilled operator performed ligation in the described experiment, but the ligation force was not measured. When a stainless steel ligature was used, the ligation force was difficult to control and varied between operators and even with the same operator on different occasions (Iwasaki et al., 2003, Montasser et al., 2014). Results from this experiment showed a high standard deviation of mean static and kinetic friction for some wire groups. An inconsistency in ligation force is one of the factors that may have affected the resultant friction differences between wires in each coating group. This could have an effect on reported variations in frictional characteristics of different types of archwires tested.

VI. CONCLUSIONS

The conclusions of this study can be summarized as follows: 1. There is a statistically significant mean difference in static and kinetic resistance to sliding between rhodium-etched, polymer-coated, and stainless steel non-coated wires. Epoxy-coated wires differed significantly from polymer-coated and stainless steel wires. Rhodium-coated and epoxy-coated wires showed similar results of high static and kinetic friction mean values with no statistically significant difference. Polymer-coated wires showed the lowest mean friction values, comparable to stainless steel wires. 2. A statistically significant mean difference was observed between the measured wire diameter and dimensions reported by manufacturers for all coated wires. The mean diameter of polymer-coated and rhodium-etched wires was significantly larger and the diameter of epoxy-coated wires was significantly smaller than manufacturerreported sizes. No significant mean difference was found between our measurements and the reported diameter of non-coated stainless steel wires. Differences in the diameters of rhodium-coated wires, while statistically significant, are minor and may be not clinically significant. An increased size of polymer-coated wires and a decreased size of epoxy-coated wires are significant enough to cause a large discrepancy when used clinically. 3. This experiment showed no statistically significant correlation between the stainless steel core diameter of tested wires and their static and kinetic resistance to sliding. An

78

influence of other factors may have resulted in significant differences in frictional characteristics of studied wires.

79

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VITA

NAME

Piotr Barysenka

EDUCATION

Doctor of Dentistry, Belarussian State Medical University, Minsk, Belarus, 2008 D.D.S. University of California Los Angeles School of Dentistry, Los Angeles, California, 2012 M.S., Oral Sciences, University of Illinois at Chicago, Chicago, Illinois, 2015 Certificate, Orthodontics, University of Illinois at Chicago, Chicago, Illinois, 2015

PROFFESIONAL MEMBERSHIP

American Association of Orthodontists American Dental Association Chicago Dental Society Illinois Society of Orthodontists