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THE EFFECT OF EXTRANEOUS FORCES UPON THE FRICTIONAL CHARACTERISTICS OF SELF-LIGATING ORTHODONTIC BRACKETS AND NICKEL-TITANIUM ARCHWIRES UTILIZING A NOVEL IN VITRO MODEL David M. Bunkall, B.S., D.D.S. An Abstract Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2006 ABSTRACT In seeking a greater understanding of friction’s role in orthodontics, studies have been conducted to identify which aspects of the archwire, bracket, ligature, and/or oral environment are associated with frictional forces. Recent interest in self-ligating appliances that claim low or reduced frictional properties has added further incentive within the specialty to characterize friction as it relates to tooth movement. Few investigations have tested in the laboratory brackets and wires arranged in an actual archform as they exist in clinical reality. Additionally, few investigations have considered the effects of extraneous forces or perturbations upon such a bracket-archwire couple as would exist in the mouth during chewing or swallowing. The objectives of the present study were to 1) identify and evaluate the effects of extraneous forces or perturbations upon the frictional characteristics of metal self-ligating brackets in combination with nickeltitanium wires, 2) compare the frictional resistances of three different self-ligating brackets and three sizes of nickel-titanium wires, and 3) determine the effect of facial or palatal displacement of the brackets on frictional resistance values. To carry out this study, a 1 new experimental friction testing apparatus was designed which allowed for the placement of seven brackets (representing the seven teeth in a given dental quadrant—central incisor through second molar) into the desired hemi-archform configuration. Into this same apparatus, perturbations were introduced to simulate extraneous oral forces. The results of the investigation indicated that the bracket with the active clip generally produced greater frictional resistances with the larger two wires, but there were interactions. With misaligned brackets, frictional resistances were directly associated with the faciolingual dimension of the archwire. Misalignment of the brackets facially or lingually resulted in an average of a 10- to 13-fold increase in frictional resistance. The introduction of perturbations reduced frictional resistance in the brackets 35 to 70% compared to tests performed without perturbations. Additionally, any significant difference in frictional resistance across the three bracket types when the brackets were misaligned did not exist in the presence of the perturbations. 2 THE EFFECT OF EXTRANEOUS FORCES UPON THE FRICTIONAL CHARACTERISTICS OF SELF-LIGATING ORTHODONTIC BRACKETS AND NICKEL-TITANIUM ARCHWIRES UTILIZING A NOVEL IN VITRO MODEL David M. Bunkall, B.S., D.D.S. A Thesis Presented to the Faculty of the Graduate School of Saint Louis University in Partial Fulfillment of the Requirements for the Degree of Master of Science in Dentistry 2006 COMMITTEE IN CHARGE OF CANDIDACY: Professor Rolf G. Behrents Chairperson and Advisor Adjunct Professor Robert J. Nikolai Assistant Professor Donald R. Oliver i With a contained environment, there is the promise of friction. And that is where the drama comes from. --Bruce Greenwood ii DEDICATION This thesis is dedicated to my wife, Robyn, and our two wonderful sons, Cannon and Luke. May we always remember the Lord’s goodness and love toward our small family. iii ACKNOWLEDGEMENTS The author wishes to acknowledge all of the assistance rendered to him during the creation of this thesis. He offers his thanks to Dr. Rolf Behrents, Dr. Robert Nikolai, Dr. Donald Oliver, Mr. Joe Tricamo and associates at Saint Louis University’s experimental machine shop, Dr. Heidi Israel, Mr. Jeff Gardner, Dr. Steven Lemery, and Dr. Jeffrey Lingenbrink. The author also wants to acknowledge the support of GAC International, Ormco Corporation, and 3M/Unitek for supplying the brackets and wires for this study. iv TABLE OF CONTENTS List of Tables ........................................ vii List of Figures ........................................ ix CHAPTER I: INTRODUCTION ............................. 1 CHAPTER II: REVIEW OF THE LITERATURE Background ............................... 3 Friction in Orthodontics ................. 4 Variables Affecting Friction.............. 8 Biologic Variables .................. 8 Saliva ......................... 8 Oral Forces .................... 9 Physical Variables ..................12 Archwires ......................12 Wire Size .................12 Wire Stiffness and Shape ..12 Wire Alloy ................14 Wire Surface Texture ......15 Ligation .......................17 Ligation Force ............17 Ligation Type .............18 Brackets .......................18 Bracket Material ..........19 Slot Size .................20 Bracket Width .............21 Interbracket Distance .....22 Bracket Design ............23 Self-Ligating Brackets ...................25 Summary ..................................29 CHAPTER III: MATERIALS AND METHODS A Novel Friction Testing Device ..........32 Test Brackets and Archform Templates .....37 Test Wires ...............................40 Instron™ Testing Machine .................41 Perturbation Machine .....................42 Testing Protocol .........................45 Statistical Analysis .....................48 CHAPTER IV: RESULTS Static and Kinetic Frictional Resistance .50 Brackets .................................56 Wires ....................................76 v CHAPTER V: DISCUSSION Variables Affecting Friction .............85 Archwires ...........................85 Brackets ............................86 Bracket Width ..................86 Slot Size ......................88 Bracket Design .................91 Testing Environment .................93 Archform .......................93 Perturbations ..................94 Graphic Data/Stick and Slip ..............96 Limitations ..............................98 CHAPTER VI: SUMMARY AND CONCLUSION ..................103 References Cited .......................................106 Vita Auctoris ..........................................112 vi LIST OF TABLES Table 1: Descriptive statistics: Average kinetic frictional resistance in grams including data from all three wire sizes......................52 Table 2: Descriptive statistics: Maximum static frictional resistance in grams including data from all three wire sizes......................54 Table 3: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from all three wire sizes........57 Table 4: Results of Tukey HSD post hoc tests to clarify ranks across the three bracket types including contributions from all three wire sizes..........................................59 Table 5: The amount of average kinetic frictional resistance reduced by perturbations for each bracket type...................................62 Table 6: The amount of maximum static frictional resistance reduced by perturbations for each bracket type...................................62 Table 7: Descriptive statistics: Average kinetic frictional resistance in grams including data from both perturbation values..................63 Table 8: Descriptive statistics: Maximum static frictional resistance in grams including data from both perturbation values..................66 Table 9: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from both perturbation values....69 Table 10: Results of Tukey HSD post hoc tests to clarify ranks across the three bracket types including contributions from both perturbation values.........................................71 Table 11: The increase in average kinetic frictional resistance due to misalignment of brackets.....75 vii Table 12: The increase in maximum static frictional resistance due to misalignment of brackets.....75 Table 13: Descriptive statistics: Average kinetic frictional resistance in grams including data from all three bracket types...................77 Table 14: Descriptive statistics: Maximum static frictional resistance in grams including data from all three bracket types...................79 Table 15: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from all three bracket types.....81 Table 16: Results of Tukey HSD post hoc tests to clarify ranks across the three wire sizes......83 Table 17: Average bracket widths (in millimeters)........87 viii LIST OF FIGURES Figure 1: Contact angle ............................21 Figure 2: Damon 2™ .................................29 Figure 3: In-Ovation™ ..............................29 Figure 4: SmartClip™ ...............................29 Figure 5: The friction testing device ..............32 Figure 6: Diagram of an individual cylinder found in the experimental device ...............33 Figure 7: Diagram showing movements of stainless steel rod and brass ball within the cylinder..................................35 Figure 8: Diagram showing rotational movements of stainless steel rod and brass ball within the cylinder..............................36 Figure 9: Aligning brackets with the “zeroing” archform template ........................39 Figure 10: Templates for aligning and misaligning brackets .................................39 Figure 11: Device mounted in Instron™ machine .......42 Figure 12: Perturbation machine plunger .............43 Figure 13: Perturbation machine motor ...............43 Figure 14: Matrix showing all tested variable combinations .............................47 Figure 15: Example of a graph of frictional resistance values during a test with aligned brackets..........................51 Figure 16: Example of a graph of frictional resistance values during a test with misaligned brackets.......................51 ix Figure 17: Comparisons across three bracket types of average kinetic frictional resistances....53 Figure 18: Comparisons across three bracket types of maximum static frictional resistances.....55 Figure 19: Additional comparisons across three bracket types of average kinetic frictional resistances....................65 Figure 20: Additional comparisons across three bracket types of maximum static frictional resistances....................68 Figure 21: Comparisons across three wire sizes of average kinetic frictional resistances....78 Figure 22: Comparisons across three wire sizes of maximum static frictional resistances.....80 Figure 23: Occlusal view of anterior SmartClip™ brackets and a 0.016- x 0.022-inch NiTi™ wire in a misaligned set-up ..............90 Figure 24: Testing set-up showing bracket arrangement relative to Instron™ pulling arm ....93 Figure 25: Example of a graph of frictional resistance values showing the stick-slip phenomenon................................97 Figure 26: Close-up of Figure 25 showing the stickslip phenomenon...........................97 Figure 27: Close-up of a graph showing frictional resistance during one of the trials of Damon 2™ brackets with a 0.016-inch NiTi™ wire in the presence of perturbations ...100 x CHAPTER I – INTRODUCTION Despite a scarcity of literature on the subject through the first several decades of the twentieth century, orthodontists seemed to have accepted the hypothesis that friction within their treatment mechanics slow down tooth movement. In a personal communication to Halderson (1957), Johnson stated that “When force has been applied to these light wires, you must not bind or tie them tight in the brackets. You must give the wires a chance to move...” Studies have been carried out to understand friction’s role in orthodontics. Recent interest by the specialty in self-ligating appliances that claim low or reduced frictional properties has provided further incentive to more accurately characterize friction as it relates to tooth movement. Dozens of studies have sought to identify which aspects of the archwire, bracket, ligature, and/or oral environment are associated with frictional forces; however, few laboratory investigations have tested archwires within a series of brackets that simulates clinical reality -brackets placed in an archform as they would be found in the mouth. Additionally, few laboratory investigations have considered the effects of extraneous forces or 1 perturbations, as would exist during chewing and swallowing, upon such an archwire-bracket couple. To understand friction in orthodontics more fully it is critical that in vitro studies represent the clinical situation as closely as possible. It would be important to consider the effects of extraneous forces or perturbations upon frictional resistance levels of self-ligating brackets arranged in an archform as they would be found in the oral environment. Such an investigation would provide additional insight into the true nature of orthodontic friction. 2 CHAPTER II – REVIEW OF THE LITERATURE Background Friction was characterized as early as the fifteenth century by Leonardo Da Vinci. Among his illustrations on the subject, he proposed two basic laws of friction: 1) maximum frictional resistance is proportional to the pushing/normal force and 2) frictional resistance is independent of the contact area of the sliding interface. Later experimentation by Amontons in 1699 and Coulomb in 1781 formalized the friction laws recognized today as estimates of dry friction dynamics. (Roussouw, Kamelchuk, and Kusy, 2003) Friction can be defined as the force that opposes or resists the movement of two surfaces sliding over each other. Its direction of action is parallel and opposite to the direction of sliding. Its maximum magnitude is hypothesized to be proportional to a normal force; the constant is termed the coefficient of friction. The relationship is shown in mathematical form as F = N * _ where F is the frictional resistance, N is the normal force and _ is the coefficient of friction. (Roussow, 2003) The normal force (N) is the force perpendicular to the direction of sliding as well as the direction of frictional 3 force itself. It can be viewed as the force pushing the two surfaces together (Baker et al., 1987). For example, the normal force associated with pushing a heavy wooden box across a concrete floor equals the weight of the box or the force of gravity pushing the box down against the floor. The coefficient of friction (_) describes the relative difficulty of sliding specific surfaces past each other. Its value is a constant, determined by the combination of surface characteristics of the two materials involved. For each pair of contacting surfaces, there are two coefficients of friction. The first is the static coefficient of friction (_s) associated with the resistance to be overcome to initiate movement. The second is the kinetic coefficient of friction (_k) associated with the resistance continually overcome to keep a moving body moving at a constant speed. Based upon the classic laws of friction, the static coefficient of friction will be larger than the corresponding kinetic coefficient of friction. (Nanda and Ghosh, 1997; Rossouw et al., 2003) Friction in Orthodontics In 1997, Kusy and Whitley discussed the classical laws of friction in relation to orthodontic mechanics. They pointed out that certain principles remain true under the 4 parameters imposed by orthodontics while others do not. For example, the first law, states that friction is proportional to the normal force, the constant being the coefficient of friction. This law is followed without exception in orthodontic movement. The second law that notes the independence of friction from apparent contact area is usually obeyed in orthodontics. But the third law, “Coulomb’s Law” regarding the independence of friction magnitude from sliding velocity, most often never holds true when it comes to moving teeth with brackets and archwires. This departure from classical friction is in part due to the extremely slow velocity of orthodontic tooth movement as well as the fact that the velocity is ever changing. Rossouw’s group (2003) noted that at extremely low velocity it is impossible to distinguish static and kinetic frictional resistance into distinct phases and that the resultant instability in sliding motion may lead to cycles characterized by “sticking and slipping” variably involving a bracket (or tube) and wire. This inconsistent frictional force between orthodontic bracket and archwire is thus thought to play an important role in compromising tooth movement during various phases of active, orthodontic treatment. 5 According to Nanda and Ghosh (1997), when closing spaces between teeth in the mouth, orthodontists usually employ one of two methods. First, an orthodontist may use segmental mechanics utilizing closing loops bent into a continuous or sectional archwire. activation of these loops. technique. The space(s) close via This is a “frictionless” On the other hand, an orthodontist may choose to close the space(s) using a “sliding mechanics” technique in which brackets are moved or slid along an archwire, or the archwire is moved or slid through the brackets. Such a technique is strongly influenced by the friction arising at the interface of the bracket and the archwire (Nanda and Ghosh, 1997). Another instance in orthodontics when so-called “sliding mechanics” are commonly employed occurs at the beginning of treatment. Traditionally, orthodontists order their treatment sequence in such a way that they first prefer to level and align the crowded, spaced, or otherwise malposed teeth in each arch. Initial leveling and aligning, as it’s known, involves using “light,” highly flexible wires of relatively small cross-section. These wires are capable of engaging misaligned teeth and under elastic activation move teeth into alignment as the wire springs-back to its passive form (Waters, 1992). 6 As the light wire lies in all of the misaligned brackets, it possesses a set length from the distal end of the bracket on the most distal tooth on one side of the arch to the distal end of the bracket on the most distal tooth on the opposite side of the arch. As the teeth begin to move under the force of the elastically deformed wire, they begin to align themselves. Alignment of the teeth often results in a decrease in the total distance between brackets in the arch. This decrease is best visualized by the resulting excess archwire seen beyond the distal ends of the most distal brackets following alignment. While the teeth are moving toward their ideal positions as the appliance is de-activating, the excess archwire usually slides past the surfaces of the adjacent brackets. In this way, the leveling and aligning phase at the beginning of orthodontic treatment employs sliding mechanics and, therefore, is also subject to the influence of friction (Proffit, 2000). Orthodontics has sought greater understanding of the role of friction with various intra-oral bracket/archwire systems. Numerous variables have been identified that explain, in part, the roles that friction plays within the biomechanics of orthodontic treatment. 7 For simplicity, one can divide these variables into two categories: biological and physical. (Nanda and Ghosh, 1997) Variables Affecting Friction during Orthodontic Movement Biological Variables Saliva Under normal conditions in the oral cavity, an orthodontic appliance is consistently being bathed in saliva. As a lubricant for our mouths, saliva performs its role well; however, its role as a lubricant for archwires and brackets, despite focused research, is still in question. Thurow (1975) suggested that saliva acts as an excellent lubricant, but laboratory studies have shown no significant or consistent advantage of saliva in reducing friction (Andreasen and Quevedo, 1970; Ireland, Sherriff, and McDonald, 1991; Edwards, Davies, and Jones, 1995; Kusy and Whitley, 2000; Thorstenson and Kusy, 2001; Henao and Kusy, 2004). Still, it is possible that saliva may lubricate certain wire/bracket alloy couples while acting as an adhesive for couples of other alloys (Kusy, Whitley, and Prewitt, 1991). 8 Oral Forces The oral cavity is a changing environment. Whether influenced by the tongue, the jaw, or the peri-oral muscles, the mouth and its structures are subject to the effects of a variety of forces. These forces could impact the position of the teeth in their respective arches (Proffit, 2000). Forces produced during biting are transmitted through the teeth and eventually conveyed to cells and fibers in the periodontal ligament and adjacent bone. Often these forces are distributed and resisted by those tissues. On the other hand, if force exists for an adequate duration, these force-stimulated cells participate in a series of processes that might ultimately lead to the creation or destruction of surrounding connective tissue and/or bone. Such cellular activity, and its consequences, also permits orthodontic tooth movement within the alveolus. (Proffit, 2000) Occlusal forces can be characterized by their magnitude, duration, and frequency. Proffit (2000) noted that there is large variation in the magnitude of occlusal forces produced during function. Occlusal forces noted during swallowing varied from only a few kilograms to more than 30 kilograms, chewing forces measured between 10 to 30 9 kilograms, and maximum bite force ranged from 30 to as high as 127 kilograms (Gibbs et al., 1981; Proffit, Fields, and Nixon, 1983). Graf and Zander (1963) found that individuals show great variability in the rate of chewing as well as the frequency and duration of tooth contacts. The average length of tooth contact during mastication was measured by Gibbs and colleagues (1981) to be 194 msec while the average length of contact during swallowing was more than three times longer (683 msec). It is estimated that approximately 1800 chewing strokes occur during an average day and that swallowing occurs about 500 times during a 24hour period (Graf, 1969) for a total of 2300 potential daily tooth contacts. Some researchers have proposed that these oral forces and the resultant movement or “jiggling” of the teeth within the parameters of periodontal ligament tension and compression significantly reduces the frictional levels that might exist within an orthodontic appliance during tooth movement (Andreasen and Quevedo, 1970; Hixon et al., 1970; Thurow, 1975). Frank and Nikolai (1980) explained that occlusion and masticatory action alter the force levels between the bracket and wire so that any “friction locks” are broken and then reset over and over again. 10 Such ideas are not without precedent. In 1959, Fridman and Levesque described such a phenomenon using an industrial model consisting of a steel block sliding down an inclined plane. They noted that “the coefficient of static friction can be virtually reduced to zero as a result of increased [sonic] vibrations.” Using an orthodontic model, Braun and colleagues (1999) applied random perturbations to a test wire or bracket. Each perturbation produced a corresponding decline in frictional resistance with the resistance levels of more than 95% of the tests dropping completely to zero. When the same idea was tested within the oral cavity, Iwasaki’s group (2003) found that the vibrations introduced when the patient chewed gum did reduce static friction, but did not eliminate friction altogether. They concluded that masticatory forces do not consistently and predictably decrease friction. It is clear that teeth are subject to a wide array of light and heavy forces on a daily basis, but the effect of these forces upon orthodontic mechanics and tooth movement has yet to be clarified. 11 Physical Variables Archwires Parameters associated with the archwire that affect friction include the size, shape, and stiffness of the wire and its material composition and surface roughness. Wire Size As a wire increases in size, filling the slot of the bracket, the general trend is for the friction of the system to increase. Larger wire sizes decrease the movement permitted between wire and bracket slot (Andreasen and Quevedo, 1970) and increase the stiffness of the wire. Frank and Nikolai (1980) pointed out that for a given binding angulation, if the stiffness of the wire is increased, the normal component of contact force between the wire and bracket will also increase. Wire Stiffness and Shape The stiffness of an archwire is not only related to the dimensions of the wire, but also to its cross-sectional shape, and its material composition. It is also influenced by the length of wire between brackets as determined by interbracket distance and any addition of loops, bends, or helices to the length of wire. (Proffit, 2000) 12 Frank and Nikolai (1980) noted that, theoretically, at a given bracket slot angulation relative to the archwire, normal force (and by definition, the friction force) will increase with increasing wire stiffness. In contrast, their results showed increased friction levels for 0.020inch round wire compared to the stiffer 0.017- x 0.025- and 0.019- x 0.025-inch rectangular wires. This finding indicated the potential influence that a wire’s crosssectional shape can have on frictional resistance. They speculated that because the round wire, at higher angulations, makes only a point contact with the edge of the bracket-slot while the rectangular wires at the same angulations make broader line contacts, then the round wire experiences higher pressures per unit contact area and indentation (or notching) of the wire occurs. If correct, deformation of the wire would lead to increased friction. Kusy and Whitley (2000) verified the theory put forth by Frank and Nikolai (1980) that frictional force increases with increasing wire stiffness. They found that, as bracket slot angulation was increased relative to a straight length of archwire, wire alloys having greater material stiffness (such as cobalt-chromium or stainless steel wires) produced greater binding and greater resistance to sliding. Similar findings were obtained by 13 Thorstenson and Kusy (2002b) who compared rectangular stainless steel wires and nickel-titanium-alloy wires within bracket slots of increasing angulation. Wire Alloy The introduction of archwires composed of new metallics (nickel-titanium and beta-titanium alloys) over the last few decades has made a dramatic impact upon orthodontic treatment. These newer wire-alloys are associated with super-elasticity and with material stiffness values smaller than that of orthodontic stainless steel. As noted above, decreased stiffness values of these newer wires result in much smaller frictional resistances when significant archwire deflection occurs between brackets. Although tests (Angolkar et al., 1990; Downing, McCabe, and Gordon, 1994) have proven that a precise hierarchy of frictional resistances may never exist among different wire alloys, a common trend is observed with titanium alloy archwires often producing increased frictional levels in laboratory studies (Garner, Allai, and Moore, 1986; Drescher, Bourauel, and Schumacher, 1989; Tidy, 1989; Kapila et al., 1990; Kusy and Whitley, 1990a; 14 Kusy and Whitley, 2000). Drescher’s group (1989) determined that the wire material was an important factor in determining resistance to sliding. Their results showed TMA™ beta-titanium wires to be associated with the highest friction levels followed by Nitinol™, cobalt-chromium, and stainless steel. On this basis, several investigators have hypothesized that the reason for the greater frictional magnitudes from the titanium alloy wires may be due to sizable surface roughness (Frank and Nikolai, 1980; Garner et al., 1986). Wire Surface Texture Frank and Nikolai (1980), using a microscope at 100x magnification, found that Nitinol™ wires exhibited greater surface roughness than cobalt-chromium and stainless steel wires and rationalized that this finding may explain the increased frictional resistance associated with this alloy seen at smaller, non-binding bracket angulations. Garner and associates (1986) observed scanning electron micrographs of various wire surfaces in an attempt to explain why certain alloys showed larger frictional resistances than others. Their findings suggested that the roughness of the wire surface may account for the variance in frictional magnitudes seen with different wires. 15 Using laser spectroscopy, Kusy and colleagues (1988) quantitatively assessed the surface roughnesses of four orthodontic archwires composed of different alloys. Their results showed that nickel-titanium wires have the roughest surface followed by beta titanium and cobalt-chromium. The “least rough” wire was stainless steel. Unfortunately, Kusy and Whitley (1990b) noted that “a clear relationship does not always exist between surface roughness and the coefficients of friction.” Prososki and associates (1991) used a surface profilometer to measure surface roughness of twelve different wires, nine of which were composed of nickel-titanium alloys. No correlation could be found between surface roughness and frictional resistance between the different wire alloys. It is now believed that a major factor affecting the frictional characteristics of a wire is not its surface roughness but its surface chemistry (Proffit, 2000). The greater the titanium content of the wire alloy, the larger the wire’s reactivity or ability to form metal-metal bonds. For example, the surface of a beta-titanium wire has shown enough reactivity to “cold-weld” itself to a stainless steel bracket, making sliding impossible (Kusy and Whitley, 1990b). 16 Ligation The amount of force exerted by the ligature and the type of ligation used are considered important factors influencing friction of an orthodontic appliance. Ligation Force Soft stainless steel replaced brass as the ligature of choice among orthodontists in the mid twentieth century (Kusy, 2002). Like brass wire, stainless steel wire offers the orthodontist flexibility when ligating the archwire to the tooth attachments. Steel ligatures can be tightly tied or loosely tied, depending upon the reason for their application. The larger the ligating or applied force exerted on the wire-bracket apparatus, the greater the frictional force (Paulson, Speidel, and Isaacson, 1970, Frank and Nikolai, 1980; Stannard, Gau, and Hanna, 1986). This finding has been applied by other investigators to more recently developed elastomeric ligatures as well. Kapilla and colleagues (1990) proposed that the reason that wide brackets were associated with greater frictional levels was that the wider bracket tended to stretch its elastomeric ligature more, resulting in a larger normal force. 17 When comparing several ligation techniques, Edwards and colleagues (1995) noted that if the elastomerics were tied down to the bracket in a ‘figure 8’ pattern they produced a static frictional resistance that was significantly greater than a conventionally tied elastomeric or stainless steel ligature. Ligation Type The introduction of elastomeric ligatures into orthodontics has provided the orthodontist with certain advantages. For example, elastomerics are simple and quick to place, and a variety of color options are available and popular among younger patients. A definitive answer has not been found as to the relative frictional levels associated with this ligation type compared to stainless steel ligatures. Certain studies (Frank and Nikolai, 1980; Edwards et al., 1995, Braun et al., 1999) showed no difference between the two ligation methods, while others have demonstrated the superiority of stainless steel ligatures (Bednar, Gruendeman, and Sandrik, 1991). Brackets Studies indicate that bracket characteristics also appear to affect orthodontic frictional levels. 18 Specific characteristics include the bracket material, slot size, bracket width, interbracket distance, and bracket design. Bracket material Frictional studies have reported that brackets of differing compositions show a variance in frictional characteristics. Stainless steel brackets tend to generate less frictional resistance than ceramic brackets (Bednar et al., 1991; Kusy et al., 1991). Pratten and associates (1990) attributed greater friction in the ceramic brackets to greater surface roughness. Still, other studies indicate that ceramic brackets may show similar or even smaller friction magnitudes than stainless steel brackets (Kusy and Whitley, 1990a; Ireland et al., 1991; Omana, Moore, and Bagby, 1992). Omana’s group (1992) additionally discovered tremendous variation in the frictional levels produced by different types of ceramic brackets. Some of them showed much larger frictional values than the stainless steel bracket, while others showed comparable values. They attributed the reduced, ‘stainless steel- like’ frictional values of certain ceramic brackets to the ability of an injection-molding manufacturing process to produce very smooth ceramic surfaces. 19 Ireland and colleagues (1991) noted that, while single ceramic brackets showed lower friction values than their steel counterparts, when the brackets were placed in series, ceramic brackets tended to show an additive effect ---friction increased with increased numbers of brackets--while the stainless steel brackets showed no such trait. Slot Size With the testing of brackets in the laboratory, slot size did not play a significant role in determining the frictional resistance of a given bracket (Tidy, 1989), although it did influence the critical contact angle (Kusy and Whitley, 2000). Contact angle is the angle formed in the frontal view by the wire and a line parallel to the bracket slot. Figure 1. See The archwire is considered to lie passively within the slot as long as the contact angle is less than the critical contact angle which is attained when the wire begins to press against the slot wall. Kusy and Whitley (1999) pointed out that, when this critical contact angle is reached and surpassed, binding of the archwire and later notching of the wire begin to overwhelm classic friction, and resistance to sliding of the wire past the bracket increases dramatically. 20 Figure 1: Contact angle (modified from Proffit, 2000) Bracket Width The width of the bracket is the second factor that directly influences the critical contact angle of a given bracket. As illustrated in Figure 1, narrow brackets permit greater passive angulations prior to the archwire binding against the bracket, while the reverse is true for wide brackets. The effect of bracket width on friction has been studied by different investigators under different testing protocols. Frank and Nikolai (1980) studied wide-bracket, maximum-static-friction values of brackets with varied widths whose slots were set to specific angulations relative to the archwire. Their findings showed that increased bracket width resulted in increased friction. In contrast, Drescher and associates (1989) found that just the opposite was true. Their protocol permitted the 21 bracket to tip under a given retarding force; this design intended to simulate the effect of intrinsic biologic factors such as bone density, root configuration, and occlusion. In this situation, they found that narrower brackets were associated with increased friction. Although the findings of these two studies appear in conflict, they actually cannot be compared due to their differing testing protocols. In clinical practice it would appear that, when teeth and bracket slots are initially misaligned, greater relative friction and archwire binding would exist with wide brackets compared to narrow ones. On the other hand, after the teeth and the bracket slots are aligned and parallel, wide brackets would show less tipping during space closure than narrow brackets and, therefore, less binding of the archwire and lower friction. Interbracket distance Closely associated with bracket width is the distance between brackets. Greater interbracket distance results from narrower bracket widths and greater inter-bracket archwire length and, therefore, less archwire stiffness. Frank and Nikolai (1980) found that, although interbracket distance did not contribute substantially to the frictional resistance, when the distance was reduced and the bracket 22 slot was set at larger angulations, the frictional levels increased. Kusy and Whitley (2000) noted that interbracket distance significantly affected frictional resistance. Under their testing conditions, as interbracket distance was reduced from 18 mm to 8 mm, binding of the wire in the bracket increased about two-and-a-half times. Bracket design Many brackets available today are advertised as having minimal or low friction designs (Mendes and Roussouw, 2003). This contention seems to be particularly common among supporters of self-ligating brackets (Berger, 1990; Damon, 1998; Heiser, 1998). These claims of small frictional resistance have in many instances been validated (Shivapuja and Berger, 1994; Kapur, Sinha, and Nanda, 1998; Pizzoni, Ravnholt, and Melsen, 1998; Mendes and Rossouw, 2003) and in other instances been challenged (Bednar et al., 1991; Taylor and Ison, 1996). Shivapuja and Berger (1994) found that three types of self-ligating brackets all showed significantly less frictional resistance than conventional brackets tied with steel or elastomeric ligatures. Studies by Kapur’s group (1998) and Pizzoni’s group (1998) showed that self-ligating brackets offer clear advantages over conventional brackets. 23 Other studies comparing ligation methods have shown no significant difference in frictional resistances between self-ligating and conventional brackets (Bednar et al., 1991; Taylor and Ison, 1996; Iwasaki et al., 2003; Mendes and Rossouw, 2003). This seeming confusion may be explained, in part, on the basis of the great variety of brackets in combination with the great variety of ligatures and their various forms of application. Bednar’s group (1991) determined that the selfligating SPEED™ bracket (Strite Industries Ltd., Cambridge, Ontario, Canada) did not always produce the smallest frictional resistance. They noted much larger resistance values with the SPEED™ bracket when a 0.016- x 0.022-inch stainless steel wire was used. They reasoned that the bracket’s spring clip pressed the larger archwire against the base of the bracket-slot, increasing resistance. Henao and Kusy (2004; 2005) confirmed that, while self-ligating brackets produce less friction with smaller wires, there was little difference in resistance between them and the conventional brackets when larger wires were tested. Likely, the different designs of self-ligating brackets play a role in the frictional levels generated by them (Thorstenson and Kusy, 2002a). Some self-ligating brackets, such as the SPEED™ bracket noted above, have a 24 spring clip that closes over the bracket slot. With smaller wires, the spring clip merely acts as a gate, passively overlying the wire in the slot, but with larger wires, the spring clip actively presses the wire against the base or sides of the slot. Such an active ligation should theoretically result in larger normal forces and, thus, greater frictional forces. In contrast, other self- ligating brackets do not possess an active spring clip; rather, they utilize a simple gate to passively close their bracket slot. These brackets act like molar tubes, lacking ligation that seats the wire against the base or sides of the bracket slot. They are often referred to as being passive self-ligating brackets (Voudouris, 1997). Self-Ligating Brackets In 1935, Stolzenberg published an article in the International Journal of Orthodontics in which he explained some of the details and advantages of his new invention, the Russell Attachment. The Russell Attachment, an early self-ligating appliance of the modern era, was characterized by a 0.022- x 0.028-inch horizontal slot, a nut that was screwed down on the facial aspect of the attachment to close the slot, and a vertical channel through which soft-wire ligatures could be tied. 25 When closing the slot, the facial nut was adjusted with a “key,” or screwdriver, in such a way as to either passively lie over the archwire or to actively engage the wire. According to Stolzenberg (1946), the Russell Attachment had a distinct advantage over the more common appliances of the day in that it was simple to operate, it could be applied to a wide range of tooth movements, the attachments themselves could be reused, and the appliance saved time for both the orthodontist and the patient. Other early self-ligating appliances include the Boyd bracket and Ford lock which were both introduced in 1933 (Woodside, Berger, and Hanson, 2005). In 1971, the idea of a self-ligating appliance was reintroduced to orthodontics by Wildman (1972). Wildman’s Edge-lok™ bracket (Ormco Corp., Glendora, CA) was circular in shape and had a domed self-ligation cap. Using a special instrument, the operator could slide the domed cap occlusally to access the bracket slot. After the wire was in the bracket slot, the cap could be closed simply with finger pressure. When the cap was closed, the bracket became, in essence, a tube with a passive gate (Berger, 2000). A similar bracket, the Mobil-lock™ (Forestadent USA, St. Louis, MO) was introduced a couple of years later (Berger, 2000). 26 Designed by Hanson (1980), the SPEED™ bracket (an acronym for “Spring-loaded, Precision, Edgewise, Energy, and Delivery”) features a facial spring or clip which wraps around the body of the bracket. When the clip is closed over a larger archwire, it tends to press the wire against the base of the bracket slot. This results in a self- ligating appliance that is “active” in engaging the archwire rather than just passively covering the bracketslot. Hanson (1980) noted that the clip became activated when a wire wider than 0.017 inches was inserted into the 0.018- x 0.025-inch slot. Several years later in 1986, Pletcher created the Activa™ (Ormco/”A” Company, Orange, CA) bracket which had a rigid curved arm that rotated occluso-gingivally around the cylindrical body. This system attained little success in the marketplace in large part due to the ease at which patients could open the brackets themselves as well as its large bracket width and relatively small interbracket distance (Berger, 2000). The mid-to-late 1990s saw the development of three new self-ligating brackets. The Time™ bracket, created by Heiser and distributed by American Orthodontics (Sheboygan, WI), is similar in design to the SPEED™ bracket. Its stiff facial clip actively seats any wire larger than 0.018-inch 27 against the base of the slot. One advantage of having an active clip is that the clip will ensure full engagement of the wire and, thus, full torque control (Heiser, 1998). The other two brackets developed at this time were the TwinLock™ (Ormco/”A” Company, Orange, CA), by Wildman, and the Damon SL™ (Ormco/”A” Company, Orange, CA). These two brackets featured a sliding “passive” gate, similar to the Edgelok™ and Mobil-lock™ brackets of the 1970s. This gate does not exert a force upon a larger sized archwire to seat the wire against the base of the bracket as the clips of the SPEED™ and Time™ brackets do. Closure of the gate causes the bracket to become a tube that constrains the archwire within its lumen. Damon (1998) proposed that such “passive self-ligation” results in decreased friction levels between the wire and bracket. More recent versions of the Damon bracket include the Damon 2™ (Figure 2) and Damon 3™. In-Ovation™, a self-ligating bracket with a twin bracket configuration (Harridine, 2003) was introduced to the market by GAC International (Bohemia, NY) in 2000 (Figure 3). Like SPEED™ and Time™, In-Ovation™ and the newer model In-Ovation-R™ have a clip which is passive with smaller diameter archwires, but active with larger archwires (Harradine, 2003). 28 Figure 2: Damon 2™ Figure 3: In-Ovation™ Figure 4: SmartClip™ In 2004, 3M/Unitek (Monrovia, CA) introduced the most recent competitor in the self-ligating market, the SmartClip™ (Figure 4). SmartClip™ is a standard twin bracket with paired nickel-titanium clips lateral to the tie wings. Archwires are “snapped” to place and held in the bracket by the clips which are designed to release the wire when the force exerted by the wire exceeds a predetermined magnitude (Weinberger, 2005). Summary The literature strongly supports the idea that friction plays an important role in the interaction of the bracket and wire during orthodontic tooth movement. Regardless of the treatment philosophy, appliance, or technique, frictional force exists in orthodontics. As noted previously, there are numerous factors/ parameters associated with brackets, wires, ligatures, and 29 the oral environment that potentially influence frictional resistance. Acting separately and in concert, these variables produce a biomechanical system that is still not fully understood, predictable, nor controllable. For the practicing orthodontist it would be essential to know first of all: “Does friction matter?” We accept that it exists, but given the conditions of the mouth, does friction actually affect orthodontic mechanics. If the answer to that question is yes, then the next logical question would be: “How much does it matter?” If the answer to the question is that friction significantly matters, then the orthodontist is left to wonder how he or she can incorporate and use the knowledge and products that will best minimize undesired friction. For such a scenario, it would be important to study friction by testing a variety of bracket/wire combinations in differing “real-life” archform configurations under conditions that best simulate the oral environment. To this end the present study has been undertaken. How do self-ligating brackets differ in their frictional characteristics? If self-ligating brackets are misaligned facially or lingually in the arch with a first order discrepancy and the wire is forced to press against the given gate or clip, how is frictional resistance 30 affected? In addition it might be important to know if oral forces can be simulated in the laboratory. Do such extraneous forces impact frictional resistance levels? Utilizing a novel in vitro model designed at Saint Louis University, this study sought to answer these questions and shed further light upon the significance of friction during the initial alignment phase of orthodontic treatment. 31 CHAPTER III – MATERIALS AND METHODS A Novel Friction Testing Device A new experimental model, inspired by an earlier model created by Chimenti’s group (2005), was designed and manufactured in and by the orthodontic program and experimental machine shop of Saint Louis University. This device (Figure 5) attempts to reproduce an upper quadrant based upon the dimensions of the teeth and the average male maxillary archform according to Moyers and colleagues (1976). This device is intended to provide the researcher Figure 5: The friction testing device 32 with a platform upon which he or she can build any imaginable in vitro friction testing study. The device consists of a series of seven aluminum cylinders, representing the seven teeth in an upper quadrant from upper central incisor to upper second molar. Within each cylinder (Figure 6) is a brass ball that can be positioned vertically in the cylinder by the movement of upper and lower internal screws. A stainless steel rod passes through the center of the brass ball with a brass A B C E D F G H Figure 6: Diagram of an individual device (A = aluminum cylinder, B = ball, D = knuckle joint set screw, screw, G = lower internal screw, H cylinder found in the experimental upper internal screw, C = brass E = stainless steel rod F = set = brass face-plate). 33 face-plate attached to one end of the rod by a knuckle joint. Brackets and tubes are bonded to this face-plate for testing purposes. The rod is held tightly within the ball by two set screws. The cylinders are mounted to an aluminum block baseplate in a hemi-arch configuration. The various components of the device permit the positioning of the mounted bracket in each plane of space (Figures 7 and 8): vertically (up and down), mesiodistally (side-to-side), and faciolingually (in and out). Vertical position can be adjusted by moving the brass ball within the cylinder or moving the entire cylinder within its mounting to the baseplate. Side-to-side positioning (mesiodistal) requires either the turning of the ball and rod or turning of the entire cylinder in the desired direction as well as compensating for that movement with an adjustment of the face-plate knuckle joint. In-and-out positioning (faciolingual) occurs by moving the rod in-andout of the ball. Rotations of the bracket about the three axes of rotation are also permitted within this model. Rotations along the vertical axis can be accomplished by rotating the ball and rod within the cylinder or by rotating the entire cylinder from side to side. Rotations along the mesio- distal axis occur by moving the ball and rod up or down 34 D A up side to side in and out down C B Figure 7: Diagram showing movements of stainless steel rod and brass ball within the cylinder. Original drawing in center. A = right or mesial, B = down, C = out, and D = in. 35 A B vertical axis C faciolingual axis mesiodistal axis E D Figure 8: Diagram showing rotational movements of stainless steel rod and brass ball within the cylinder. Original drawing in middle on right. A = distal rotation about vertical axis, B = mesial rotation about vertical axis , C = twist or rotation about facio-lingual axis, D = up rotation about mesio-distal axis, and E = down rotation about mesiodistal axis. 36 within the cylinder while rotations along the facio-lingual axis take place by simply twisting the rod within the brass ball. This model is unique because of the following: 1) it is capable of placing an orthodontic bracket or tube precisely in any position in all three planes of space; 2) it emulates the six degrees of freedom that can be used to describe tooth position and tooth movement; 3) it permits a set of brackets and tubes to be placed in series in a hemi-archform configuration; 4) it is reusable; and 5) it operates conveniently with other friction testing equipment such as a mechanical testing machine. The same device was used in a parallel study by Lingenbrink (2006). Test Brackets and Archform Templates This new friction testing device was utilized to assess the frictional resistances of three different selfligating brackets. The three self-ligating brackets used were the Damon 2™ (Ormco Corp., Orange, CA), In-Ovation R™ (GAC International, Bohemia, NY), and SmartClip™ (3M/Unitek, Monrovia, CA) (Figures 2, 3, and 4). The choice of brackets was based upon the desire to test brackets that are popular and that represent different types of self-ligating mechanisms. 37 The Damon 2™ bracket has a facial gate that passively closes over the bracket slot, the In-Ovation R™ bracket has a facial clip that will actively push larger sized wires against the base of the slot, and the SmartClip™ bracket has clips on either side of the tie wings which passively contain the archwire within the slot, but may flex when the wire is pressed against them. Each quadrant set-up consisted of five self-ligating brackets, each with a 0.022-inch slot, spanning the upper left central incisor to upper left second premolar, a 0.022-inch first-molar tube, and a 0.022-inch second-molar tube. Brackets were leveled vertically and aligned into the desired archform utilizing a special “zeroing” archform template that attached to the baseplate of the device (Figure 9). The template effectively removed all torque and tip built into the individual brackets, eliminating any minor differences in bracket prescriptions across the bracket types. The archform template was made from a sheet of steel 0.021-inch in thickness. Brackets were positioned so that the edge of the archform template rested against the base of the slot. The molar tubes were positioned accordingly, using a 0.021- x 0.025-inch stainless steel wire that extended distally from the correctly positioned second premolar bracket. 38 Figure 9: Aligning brackets with the “zeroing” archform template For this study two different archform templates were used (Figure 10). The first was an archform based upon the average male maxillary arch described by Moyers’ group A B Figure 10: Templates for aligning (A) and misaligning (B) brackets 39 (1976). The second was a misaligned maxillary left archform with specific first-order discrepancies meant to represent a malposed arrangement of teeth common among individuals that present for orthodontic treatment. This archform was based upon a sampling of thirty-five pretreatment models of the primary investigator’s active patients. The models showed an average tendency for the upper left lateral to be lingual to the central and the canine to be facial to the lateral and first bicuspid. From this sampling, the second archform template was produced that inset the lateral one-mm lingually and offset the canine one-mm facially from the original “Moyers” archform. This set-up created a one-mm first-order discrepancy between the central and lateral, a two-mm discrepancy between the lateral and canine, and a one-mm discrepancy between canine and first bicuspid. Test Wires The wires to be tested during the friction study were of a nickel-titanium alloy (NiTi™, small broad maxillary archform, Ormco Corp., Orange, CA) in three different cross-sections: 0.016-inch round, 0.018-inch round, and 0.016 x 0.022-inch rectangular. Each preformed nickel- titanium archwire was cut in half at the midline. 40 The “half-archwire” to be tested was placed into the series of self-ligating brackets and tubes, and an archwire lock was attached to its distal end. A new wire was used for each trial, and a new set of brackets was used to test each wire size. Instron™ Testing Machine After the wire was placed in the brackets, the entire apparatus was mounted onto the base of an Instron™ universal testing machine (Model 1011, Instron Corp., Canton, MA) with the distal end of the test wire pointing upward (Figure 11). The distal end of the wire with its archwire lock was aligned and connected to a customdesigned hook extending from a flexural load beam attached to the moveable crosshead of the testing machine. The wire was pulled through the brackets distally by the movable head of the machine at a velocity of one-mm per minute for a total test period of one-and-one-half minutes. Resistance levels encountered by the Instron™ testing machine, as it pulled the test wire through the brackets, were continuously recorded over the entire testing period. All testing was performed at room temperature under dry conditions. 41 Figure 11: Device mounted in Instron™ machine Perturbation Machine Based upon the hypothesis that forces and vibrations conveyed to the teeth during function reduce the magnitude of frictional resistance during orthodontic tooth movement, a machine was designed to introduce perturbations into the present friction testing study. The machine consisted of a motor connected to a plunger (Figures 12 and 13). 42 The Figure 12: Perturbation machine plunger. As viewed from above the friction testing device. The plunger was positioned to strike the baseplate from behind. Figure 13: Perturbation machine motor perturbator was activated periodically by a current (initiated by one of the switches) carried to an electromagnet that, in turn, “launched” the plunger (whose travel was limited by a stop). The speed of the rotating motor could be adjusted in order to control the frequency 43 of the plunger movement. The plunger was placed adjacent to the aluminum baseplate containing the seven cylinders and the brackets. Each time a switch was activated by the motor, the plunger struck the baseplate at a force of 200500 grams. During testing, the plunger hit the baseplate at a frequency of 1 hit per second or a total of 90 hits during the entire 90-second test period. While the frequency of perturbations used in this study (1x/second) far exceeds the frequency of daily tooth contacts in man (approximately 100x/hour; Graf, 1969), it must be remembered that the velocity at which the wire was being moved through the brackets (1 mm/minute) during the testing was as much as 43,000 times faster than the relative velocity of tooth or wire movement intra-orally that has been estimated to be 0.23 x 10-4 mm per minute or 1 mm per month (Braun et al., 1999). And so it seems reasonable to increase the frequency of perturbations introduced into the system when the wire is moving relatively quickly through the brackets. The magnitude of the plunger force hitting the baseplate is greatly reduced relative to average intra-oral forces. 200-500 grams is a very conservative force value compared to swallowing and chewing forces that can reach as much as 30 kilograms and 44 maximum bite forces that can exceed 100 kilograms (Gibbs et al., 1981; Proffit et al., 1983). Testing Protocol For each wire size, a new set of brackets was used. The seven brackets were affixed to the brass face-plates using cyano-acrylate adhesive. After the brackets were in place, the cylinders were mounted into the aluminum baseplate, and the brackets were lined up according to either the ideal “Moyers” archform alignment template or to the misaligned archform template with the specified firstorder discrepancies in the lateral and canine areas. The brackets and the archwire were wiped with 70% ethanol, and the wire was placed into the tubes and secured into the self-ligating brackets. An archwire lock was attached to the distal end of the test wire. The baseplate, with the brackets and test wire in place, was then mounted in the Instron™ testing machine. Each wire was pulled through the series of brackets at a velocity of one-mm per minute for a total test time of oneand-one-half minutes. At the end of the test the baseplate was removed from its support, the wire was removed from the brackets, and a new wire was placed in the brackets and tested in the 45 Instron™ machine following the same protocol as above. This process was repeated for a total of nine tests. After the nine tests, the perturbation machine was turned on, and the same process was again repeated for nine tests under the same testing conditions, but with the introduction of vibrations. After these eighteen tests (nine without perturbations and nine with perturbations) were complete, the brackets were repositioned based upon the shape of the second “zeroing” jig template. In this new configuration, the brackets and wires were tested using the same protocol described above for another set of eighteen tests (nine without perturbations and nine with perturbations. For each bracket type – wire size combination, 36 total tests were run. Of the 36 tests run, 9 tests were run under each of the following four conditions: 1) brackets aligned without perturbations; 2) brackets aligned with perturbations; 3) brackets misaligned without perturbations; and 4) brackets misaligned with perturbations. Nine possible unique bracket type – wire size combinations multiplied by the 36 tests per combination resulted in a grand total of 324 tests. Figure 14 contains a matrix showing each of the variables and how they were organized into the tests to be conducted. 46 016 NiTi 018 NiTi 016x022 NiTi 016 NiTi 018 NiTi 016x022 NiTi Damon 2™ Damon 2™ In-Ovation R™ In-Ovation R™ SmartClip™ SmartClip™ SmartClip™ brackets aligned brackets aligned 018 NiTi aligned SmartClip™ brackets misaligned 016 NiTi brackets misaligned misaligned with perturbations 3 bracket types x 3 wire sizes Each of the 36 unique combinations without perturbations x x 2 alignment configurations 9 repetition per combination x 2 perturbation options = = 36 unique combinations 324 total trials run while testing Figure 14: Matrix showing all tested variable combinations. Each of the 36 cubes represents one unique combination of bracket, wire, alignment configuration, and perturbation option. Statistical Analysis While pulling the test wire through the brackets, resistance levels encountered by the Instron™ machine were recorded by a printer connected to the testing machine. The recorded results of each test were manually evaluated. As a first step, the point on the graph corresponding to the value of maximum static resistance was identified. Starting from that point, values were measured at six second intervals along the graph for a total of ten values over sixty seconds. These ten points were averaged together to arrive at a kinetic resistance value for each individual test. There were three individual tests out of the total of 324 tests that did not continue for a minimum of sixty seconds following the point of maximum static resistance. These tests were not recognized by the software (SPSS version 13.0, SPSS Inc., Chicago, IL) used to analyze the data and were excluded from the analyses comparing average kinetic frictional resistances. All of the tests were divided into the following four groups: 1) aligned brackets and no perturbations present, 2) aligned brackets and perturbations present, 3) misaligned brackets and no perturbations present, and 48 4) misaligned brackets and perturbations present. Descriptive statistics were determined for the four groups and each group was analyzed using two separate one-way analyses of variance with the bracket type and the wire size serving as the independent variables. Any differences found between brackets and/or wires within each of the groups were further analyzed using the Tukey HSD post hoc test. These analyses intended to compare and contrast the effects of bracket alignment or misalignment and the absence or presence of perturbations on the frictional resistance of nickel-titanium archwires and self-ligating brackets in archforms simulating the early alignment phase of orthodontic treatment. Additional one-way analyses of variance and their corresponding post hoc tests were also performed to examine in more detail how the frictional resistance of each bracket type varied in relation to wire size and whether the brackets were aligned or misaligned. 49 CHAPTER IV – RESULTS Static and Kinetic Frictional Resistance Throughout the testing the values for maximum static and kinetic frictional resistance tended to follow the same pattern in relation to the various brackets and wires. Overall, as expected under the laws of classic friction, when the brackets were aligned, the maximum static resistance values tended to be greater than the kinetic resistance values (Tables 1 and 2 and Figures 15 through 18). On the other hand, when the brackets were misaligned, just the opposite was true – the kinetic resistance values tended to be greater than the maximum static resistance values. Please note the undersized subgroups in Table 1 for the Damon 2™ and In-Ovation R™ brackets under the column “Misaligned brackets, without perturbations, n.” As noted previously, smaller subgroup size was due to exclusion by the SPSS software of three trials that failed to run for a minimum of sixty seconds following the measured point of maximum static frictional resistance. Undersized groups can also be found in later tables that compare average kinetic frictional resistances. 50 maximum static resistance kinetic resistance range Figure 15: Example of a graph of frictional resistance values during a test with aligned brackets. Drawn from original tracing. Note the relative magnitude of maximum static resistance versus kinetic resistance. maximum static resistance kinetic resistance range Figure 16: Example of a graph of frictional resistance values during a test with misaligned brackets. Drawn from original tracing. Note the relative magnitude of kinetic resistance versus maximum static resistance. 51 Table 1: Descriptive statistics: Average kinetic frictional resistance in grams including data from all three wire sizes. Aligned brackets without perturbations Misaligned brackets with perturbations n mean(g) SD(g) n Damon 2™ 27 42.6 29.8 27 In-Ovation R™ 27 136.1 115.7 SmartClip™ 27 26.6 Total 81 68.4 mean(g) without perturbations with perturbations SD(g) n mean(g) SD(g) n mean(g) SD(g) 9.7 11.5 25 352.4 271.5 27 247.3 224.8 27 49.3 55.4 26 623.6 459.4 27 399.2 290.1 13.9 27 6.5 4.3 27 503.6 387.5 27 294.2 256.0 84.0 81 21.8 37.8 78 495.1 393.0 81 313.5 263.0 700.0 Resistance Force (grams) 600.0 500.0 400.0 Damon 2™ In-Ovation R™ SmartClip™ 300.0 200.0 100.0 0.0 Aligned without Perturbations Aligned with Perturbations Misaligned without Perturbations Misaligned with Perturbations Test Group Figure 17: Comparisons across three bracket types of average kinetic frictional resistances. Data include contributions from all three wire sizes. Table 2: Descriptive statistics: Maximum static frictional resistance in grams including data from all three wire sizes. Aligned brackets without perturbations Misaligned brackets with perturbations without perturbations with perturbations n mean(g) SD(g) n mean(g) SD(g) n mean(g) SD(g) n mean(g) SD(g) Damon 2™ 27 46.5 34.8 27 19.5 13.2 27 382.2 334.5 27 215.4 191.4 In-Ovation R™ 27 150.1 121.9 27 54.8 56.0 27 537.8 307.0 27 366.6 242.0 SmartClip™ 27 25.9 14.5 27 16.1 5.9 27 543.7 425.6 27 296.4 259.6 Total 81 74.2 91.0 81 30.1 37.4 81 487.9 362.7 81 292.8 238.1 600.0 Resistance Force (grams) 500.0 400.0 300.0 Damon 2™ In-Ovation R™ SmartClip™ 200.0 100.0 0.0 Aligned without Perturbations Aligned with Perturbations Misaligned without Perturbations Misaligned with Perturbations Test Group Figure 18: Comparisons across three bracket types of maximum static frictional resistances. Data include contributions from all three wire sizes. For convenience sake, from this point on, whenever static and kinetic frictional resistance values follow the same pattern for a given set of parameters, they will not be discussed separately; rather, they will be discussed generally as “frictional resistance.” Brackets The results of the one-way analyses of variance identified specific patterns with regard to the levels of frictional resistance associated with each of the three self-ligating brackets tested. During tests when all of the brackets were aligned, the Damon 2™ bracket and the SmartClip™ bracket both showed significantly less frictional resistance compared to the In-Ovation R™ bracket (Tables 3 and 4 and Figures 17 and 18). When the brackets were misaligned, there was overall no significant difference in maximum static frictional resistance among the brackets, but, in regard to kinetic resistance, the Damon 2™ bracket showed significantly less resistance than the In-Ovation R™ bracket. Overall, the introduction of perturbations into the system had no effect upon the rank of average kinetic frictional differences across the three brackets when they were aligned, but when misaligned, the introduction of 56 Table 3: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from all three wire sizes. aligned brackets and no perturbations aligned brackets and perturbations Sum of Squares df mean square F Sig. Rk between groups within groups total 188639.5 376340.7 564980.2 2 78 80 94319.728 4824.881 19.549 .000** Rs between groups within groups total 239445.4 423022.8 662468.2 2 78 80 119722.704 5423.369 22.075 .000** Rk between groups within groups total 30735.6 83611.8 114347.4 2 78 80 15367.811 1071.946 14.336 .000** Rs between groups within groups total 24848.1 87030.7 111878.8 2 78 80 12424.049 1115.778 11.135 .000** Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 Table 3 continued Sum of Squares misaligned brackets and no perturbations misaligned brackets and perturbations df mean square F Sig. Rk between groups within groups total 940495.7 10950767.0 11891262.7 2 75 77 470247.861 146010.223 3.221 .046** Rs between groups within groups total 452891.9 10069289.0 10522180.9 2 78 80 226445.938 129093.443 1.754 .180 Rk between groups within groups total 326823.3 5206097.5 5532920.8 2 78 80 163411.630 66744.839 2.448 .093 Rs between groups within groups total 309088.1 4226651.5 4535739.6 2 78 80 154544.049 54187.840 2.852 .064 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 Table 4: Results of Tukey HSD post hoc tests to clarify ranks across the three bracket types including contributions from all three wire sizes. Means (in grams) in adjacent columns are statistically unequal. alpha = .05 n Rk aligned brackets and no perturbations Rs Rk aligned brackets and perturbations Rs 1 Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 42.6 Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 46.5 Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 9.7 Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 19.5 2 3 136.1 26.6 150.1 25.9 49.3 6.5 54.8 16.1 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance Table 4 continued alpha = .05 n Rk misaligned brackets and no perturbations misaligned brackets and perturbations 1 Damon 2™ In-Ovation R™ SmartClip™ 25 26 27 352.4 Rs Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 382.2 537.8 543.7 Rk Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 247.3 399.2 294.2 Rs Damon 2™ In-Ovation R™ SmartClip™ 27 27 27 215.4 366.6 296.4 503.6 2 3 623.6 503.6 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance perturbations caused the differences in kinetic frictional resistance among the brackets to disappear. Perturbations reduced the magnitude of kinetic frictional resistance of the brackets by an average of 72.2% and the static frictional resistance of the brackets by 53.1%. When the brackets were misaligned, perturbations reduced the kinetic frictional resistance by an average of 35.8% and the static frictional resistance by 40.3% (Tables 5 and 6). Additional one-way analyses of variance in which frictional resistances could be compared across brackets were performed for each wire size individually (Tables 7 through 10 and Figures 19 and 20). These analyses point out that in regard to kinetic frictional resistance, when the brackets were aligned, there was no significant difference among the brackets when a 0.016-inch NiTi wire was tested, yet when a 0.018-inch or a 0.016- x 0.022-inch wire was used, the Damon 2™ and the SmartClip™ brackets both showed significantly less kinetic frictional resistance than the In-Ovation R™ bracket. Maximum static friction resistance values showed a similar pattern with the larger wires, but with the 0.016-inch wire, the SmartClip™ bracket exhibited significantly less resistance than the In-Ovation R™ bracket. 61 Table 5: The amount of average kinetic frictional resistance reduced by perturbations for each bracket type. Damon 2™ In-Ovation R™ SmartClip™ grams reduced 32.9 86.8 20.1 Average % reduced: Aligned percentage reduced 77.2% 63.8% 75.6% grams reduced 105.1 224.4 209.4 72.2% Misaligned percentage reduced 29.8% 36.0% 41.6% 35.8% Table 6: The amount of maximum static frictional resistance reduced by perturbations for each bracket type. Damon 2™ In-Ovation R™ SmartClip™ grams reduced 27.0 95.3 9.8 Average % reduced: Aligned percentage reduced 58.1% 63.5% 37.8% 53.1% grams reduced 166.8 171.1 247.3 Misaligned percentage reduced 43.6% 31.8% 45.5% 40.3% Table 7: Descriptive statistics: Average kinetic frictional resistances in grams including data from both perturbation values. 016 NiTi aligned brackets n mean(g) SD(g) 018 NiTi misaligned brackets aligned brackets misaligned brackets n mean(g) SD(g) n mean(g) SD(g) n mean(g) SD(g) Damon 2™ 18 9.4 8.3 18 88.1 48.1 18 17.3 13.0 18 202.5 64.3 In-Ovation R™ 18 11.8 9.1 18 148.9 62.0 18 63.2 42.4 17 381.5 71.4 SmartClip™ 18 8.4 3.4 18 123.5 53.3 18 17.4 17.7 18 239.4 82.1 Total 54 9.9 7.4 54 120.2 59.3 54 32.7 34.7 53 272.5 105.3 Table 7 continued 016x022 NiTi aligned brackets misaligned brackets n mean(g) SD(g) n mean(g) SD(g) Damon 2™ 18 51.8 33.2 16 641.0 136.1 In-Ovation R™ 18 203.1 92.3 18 990.4 266.1 SmartClip™ 18 23.8 13.9 18 833.7 218.7 Total 54 92.9 97.3 52 828.6 255.5 1200.0 Resistance Force (grams) 1000.0 800.0 600.0 Damon 2™ In-Ovation R™ SmartClip™ 400.0 200.0 0.0 Alignment Misalignment Alignment Misalignment Alignment Misalignment with 016 NiTi with 016 NiTi with 018 NiTi with 018 NiTi with 016x022 with 016x022 NiTi NiTi Test Group Figure 19: Additional comparisons across three bracket types of average kinetic frictional resistances. Data include contributions from both perturbation values. Table 8: Descriptive statistics: Maximum static frictional resistances in grams including data from both perturbation values. 016 NiTi aligned brackets n mean(g) Damon 2™ 18 14.7 In-Ovation R™ 18 SmartClip™ Total SD(g) 018 NiTi misaligned brackets aligned brackets n mean(g) SD(g) n mean(g) 7.8 18 89.9 54.5 18 23.3 17.1 7.0 18 147.6 67.6 18 18 10.1 2.2 18 136.2 62.3 54 14.0 6.8 54 124.6 65.5 SD(g) misaligned brackets n mean(g) SD(g) 7.5 18 180.4 62.1 76.2 53.8 18 429.2 117.2 18 25.0 11.8 18 261.8 103.6 54 41.5 40.1 54 290.5 141.5 Table 8 continued 016x022 NiTi aligned brackets misaligned brackets n mean(g) SD(g) n mean(g) SD(g) Damon 2™ 18 61.0 36.1 18 626.2 260.6 In-Ovation R™ 18 214.1 100.7 18 779.8 161.9 SmartClip™ 18 27.8 10.8 18 862.2 308.7 Total 54 101.0 102.1 54 756.0 265.5 1000.0 900.0 Resistance Force (grams) 800.0 700.0 600.0 500.0 Damon 2™ In-Ovation R™ SmartClip™ 400.0 300.0 200.0 100.0 0.0 Alignment with 016 NiTi Misalignment with 016 NiTi Alignment with 018 NiTi Misalignment Alignment Misalignment with 018 with 016x022 with 016x022 NiTi NiTi NiTi Test Group Figure 20: Additional comparisons across three bracket types of maximum static frictional resistances. Data include contributions from both perturbation values. Table 9: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from both perturbation values. 016 NiTi and aligned brackets 018 NiTi and aligned brackets 016x022 NiTi and aligned brackets Sum of Squares df mean square F Sig. Rk between groups within groups total 108.2 2768.9 2877.1 2 51 53 54.090 54.293 .996 Rs between groups within groups total 455.8 1963.2 2418.0 2 51 53 227.907 34.493 5.921 .005** Rk between groups within groups total 25196.2 38772.0 63968.2 2 51 53 12598.124 760.235 16.571 .000** Rs between groups within groups total 32578.8 52462.7 85041.5 2 51 53 16289.389 1028.681 15.835 .000** Rk between groups within groups total 334876.0 166739.6 501615.6 2 51 53 167437.995 3269.404 51.214 .000** Rs between groups within groups total 355454.7 196502.3 551957.0 2 51 53 177727.352 3852.986 46.127 .000** Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 .376 Table 9 continued 016 NiTi and misaligned brackets 018 NiTi and misaligned brackets 016x022 NiTi and misaligned brackets Sum of Squares df mean square F Sig. Rk between groups within groups total 33546.4 153068.0 186614.4 2 51 53 16773.204 3001.334 5.589 .006** Rs between groups within groups total 33580.7 194044.3 227625.0 2 51 53 16790.352 3804.791 4.413 .017** Rk between groups within groups total 310011.7 266546.6 576558.3 2 50 52 155005.824 5330.932 29.077 .000** Rs between groups within groups total 579421.6 481813.9 1061235.5 2 51 53 289710.796 9447.331 30.666 .000** Rk between groups within groups total 1034723.5 2294792.3 3329515.8 2 49 51 517361.757 46832.496 11.047 .000** Rs between groups within groups total 516481.8 3220258.1 3736739.9 2 51 53 258240.907 63142.316 4.090 .023** Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 Table 10: Results of Tukey HSD post hoc tests to clarify ranks across the three bracket types including contributions from both perturbation values. Means (in grams) in adjacent columns are statistically unequal. alpha = .05 n 016 NiTi and aligned brackets Rk Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 9.4 11.8 8.4 Rs Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 14.7 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 88.1 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 Rk 016 NiTi and misaligned brackets 1 Rs 2 3 14.7 17.1 10.1 123.5 148.9 123.5 89.9 136.2 147.6 136.2 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance Table 10 continued alpha = .05 n Rk 018 NiTi and aligned brackets Rs Rk 018 NiTi and misaligned brackets Rs 1 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 17.3 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 23.3 Damon 2™ In-Ovation R™ SmartClip™ 18 17 18 202.5 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 180.4 2 3 63.2 17.4 76.2 25.0 381.5 239.4 429.2 261.8 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance Table 10 continued alpha = .05 n Rk 016x022 NiTi and aligned brackets Rs Rk 016x022 NiTi and misaligned brackets Rs 1 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 51.8 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 61.0 Damon 2™ In-Ovation R™ SmartClip™ 16 18 18 641.0 Damon 2™ In-Ovation R™ SmartClip™ 18 18 18 626.2 779.8 2 3 203.1 23.8 214.1 27.8 990.4 833.7 779.8 862.2 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance When the brackets were misaligned and a 0.016-inch wire was tested, the Damon 2™ bracket showed significantly less frictional resistance than the In-Ovation R™ bracket. With the 0.016-inch wire, the SmartClip™ showed no significant difference from the other two brackets. The testing of a 0.018-inch wire with the brackets misaligned showed that the Damon 2™ and the SmartClip™ brackets showed no significant difference between each other, but both produced significantly less kinetic resistance than the InOvation R™ bracket. The maximum static resistance level of the Damon 2™ bracket was significantly less than that of the SmartClip™ which, in turn, was significantly less than that of the In-Ovation R™. Finally, misaligned brackets along with a 0.016- x 0.022-inch wire resulted in the Damon 2™ bracket showing significantly less kinetic frictional resistance than either the SmartClip™ or the In-Ovation R™ bracket and less maximum static frictional resistance than the SmartClip™ bracket. The effect of misalignment upon frictional resistance levels was to increase kinetic resistance by 13 times and static resistance by over 10 times (Tables 11 and 12). 74 Table 11: The increase in average kinetic frictional resistance due to misalignment of brackets. Comparing bracket types by wire size. grams Damon 2™ 78.7 In-Ovation R™ 137.1 SmartClip™ 115.1 Average % increase 016 NiTi percentage 937.2% 1261.9% 1470.2% 1223.1% grams 185.2 318.3 222.0 018 NiTi percentage 1170.5% 603.6% 1375.9% 1050.0% 016x022 NiTi grams percentage 589.2 1237.5% 787.3 487.6% 809.9 3502.9% 1742.7% Overall average % increase 1338.6% Table 12: The increase in maximum static frictional resistance due to misalignment of brackets. Comparing bracket types by wire size. grams Damon 2™ 75.2 In-Ovation R™ 130.5 SmartClip™ 126.1 Average % increase 016 NiTi percentage 611.6% 863.2% 1348.5% 941.1% grams 157.1 353.0 236.8 018 NiTi percentage 774.2% 563.3% 1047.2% 794.9% 016x022 NiTi grams percentage 565.2 1026.6% 565.7 364.2% 834.4 3101.4% 1497.4% Overall average % increase 1077.8% Wires One-way analyses of variance were conducted to assess frictional resistance associated with each wire size. The analyses revealed that average kinetic and maximum static resistances followed precisely the same patterns. When the brackets were aligned, both the 0.016-inch and the 0.018inch round NiTi wires showed significantly less frictional resistance than the rectangular 0.016- x 0.022-inch NiTi wire (Tables 13 through 16 and Figures 21 and 22). With misaligned brackets there were significant differences across all of the three wires. The 0.016- x 0.022-inch wires showed greater frictional resistance than the 0.018inch wires that, in turn, showed greater frictional resistance than the 0.016-inch wires. The introduction of perturbations to these testing parameters did not change the rank of frictional-resistance magnitudes across the wires, whether the brackets were aligned or not. 76 Table 13: Descriptive statistics: Average kinetic frictional resistance in grams including data from all three bracket types. Aligned brackets without perturbations SD(g) Misaligned brackets with perturbations n mean(g) n mean(g) 016 NiTi 27 16.1 5.0 27 3.7 018 NiTi 27 55.2 36.1 27 016x022 NiTi 27 134.1 113.6 Total 81 68.4 84.0 SD(g) without perturbations with perturbations n mean(g) SD(g) n mean(g) SD(g) 2.5 27 166.0 44.3 27 74.3 29.2 10.1 10.2 25 330.8 89.5 27 216.3 88.2 27 51.7 53.6 26 1021.5 212.7 27 650.0 131.8 81 21.8 37.8 78 393.0 81 313.5 263.0 495.1 1200.0 Resistance Force (grams) 1000.0 800.0 600.0 016 NiTi 018 NiTi 016x022 NiTi 400.0 200.0 0.0 Aligned without Perturbations Aligned with Perturbations Misaligned without Perturbations Misaligned with Perturbations Test Group Figure 21: Comparisons across three wire sizes of average kinetic frictional resistances. Data include contributions from all three bracket types. Table 14: Descriptive statistics: Maximum static frictional resistance in grams including data from all three bracket types. Aligned brackets without perturbations n mean(g) 016 NiTi 27 17.7 018 NiTi 27 016x022 NiTi Total SD(g) Misaligned brackets with perturbations n mean(g) 7.6 27 10.3 62.1 48.4 27 27 142.8 121.4 81 74.2 91.0 SD(g) without perturbations with perturbations n mean(g) SD(g) n mean(g) SD(g) 2.5 27 174.9 47.9 27 74.3 34.7 20.9 7.0 27 357.9 143.4 27 223.1 104.0 27 59.2 53.6 27 931.0 242.3 27 581.1 146.6 81 30.1 37.4 81 487.9 362.7 81 293.8 238.1 1000.0 900.0 Resistance Force (grams) 800.0 700.0 600.0 500.0 016 NiTi 018 NiTi 016x022 NiTi 400.0 300.0 200.0 100.0 0.0 Aligned without Perturbations Aligned with Perturbations Misaligned without Perturbations Misaligned with Perturbations Test Group Figure 22: Comparisons across three wire sizes of maximum static frictional resistances. Data include contributions from all three bracket types. Table 15: Summary of one-way ANOVAs of average kinetic and maximum static frictional forces including contributions from all three bracket types. aligned and no perturbations aligned and perturbations Sum of Squares df mean square F Sig. Rk between groups within groups total 195097.0 369883.2 564980.2 2 78 80 97548.508 4742.092 20.571 .000** Rs between groups within groups total 217128.1 445340.1 662468.2 2 78 80 108564.037 5709.489 19.015 .000** Rk between groups within groups total 36753.4 77594.0 114347.4 2 78 80 18376.680 994.795 18.473 .000** Rs between groups within groups total 35741.7 76137.1 111878.8 2 78 80 17870.827 976.117 18.308 .000** Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 Table 15 continued Sum of Squares misaligned and no perturbations misaligned and perturbations df mean square F Sig. Rk between groups within groups total 10553552.0 1337710.8 11891262.8 2 75 77 5276775.802 17836.144 295.847 .000** Rs between groups within groups total 8401958.1 2120222.3 10522180.4 2 78 80 4200979.049 27182.337 154.548 .000** Rk between groups within groups total 4856802.0 676118.7 5532920.7 2 78 80 2428401.021 8668.188 280.151 .000** Rs between groups within groups total 3664679.4 871060.2 4535739.6 2 78 80 1832339.716 11167.438 164.079 .000** Rk = kinetic frictional resistance, Rs = maximum static frictional resistance **P<0.05 Table 16: Results of Tukey HSD post hoc tests to clarify ranks across the three wire sizes. Data include contributions from all three bracket types. Means (in grams) in adjacent columns are statistically unequal. alpha = .05 n Rk aligned and no perturbations Rs Rk aligned and perturbations Rs 1 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 16.1 55.2 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 17.7 62.1 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 3.7 10.1 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 10.3 20.9 2 3 134.1 142.8 51.7 59.2 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance Table 16 continued alpha = .05 n Rk misaligned and no perturbations Rs Rk misaligned and perturbations Rs 1 016 NiTi 018 NiTi 016x022 NiTi 27 26 25 166.0 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 174.9 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 74.3 016 NiTi 018 NiTi 016x022 NiTi 27 27 27 74.3 2 3 330.8 1021.5 357.9 931.0 216.3 650.0 223.1 581.1 Rk = kinetic frictional resistance, Rs = maximum static frictional resistance CHAPTER V – DISCUSSION The results of this study support the idea that friction plays an important role in the interaction of bracket and wire surfaces when the two are sliding past each other. Regardless of the type of self-ligating bracket used and regardless of the dimensions of nickeltitanium wire used, friction was present during testing. As noted previously, there are numerous factors associated with archwires, brackets, and the oral or testing environment that potentially influence frictional resistance. Many of these factors or variables can be evaluated, based upon the results of the present study. Variables Affecting Friction Archwires Following the trend found in previous friction studies (Andreasen and Quevedo, 1970; Frank and Nikolai, 1980; Drescher et al., 1989), the results of this study showed a general tendency for the level of frictional resistance to increase with increasing wire size. This was especially clear when the brackets were misaligned. Under misaligned conditions, the 0.016 x 0.022-inch wire showed more than six times the average kinetic frictional resistance and 85 more than five times the average maximum static frictional resistance compared to the 0.016-inch wire. Compared to the 0.018-inch wire under the same conditions, the 0.016 x 0.022-inch wire had over three times the average kinetic resistance and two-and-a-half times the average maximum static resistance. This bigger, stiffer wire produced a greater normal component of contact force between itself and the bracket-slot base and/or the self-ligating clip/gate, resulting in much larger frictional resistance magnitudes. Even when the brackets were aligned, the means showed a progression of increasing frictional resistance from 0.016- to 0.018- to 0.016- x 0.022-inch wires, although there was no statistical difference between the 0.016- and 0.018-inch wires. Brackets Bracket Width Three samples of each of the different brackets (central incisor through bicuspid – all bracket systems tested use the same bracket for both 1st and 2nd bicuspids) and the two molar tubes of the three tested bracket types were measured using a digital caliper (Chicago Brand Industrial Incorporated, Fremont, CA) and averaged for 86 Table 17: Average bracket widths (in millimeters). Central Lateral Canine Bicuspids Damon 2™ 2.66 2.67 2.65 2.66 In-Ovation R™ 2.87 2.68 2.89 2.95 SmartClip™ 3.63 3.10 3.38 3.35 Mean bracket width 2.66 2.87 3.36 1st molar tube 2nd molar tube 4.07 3.88 3.97 1.94 3.94 2.46 width. Width was defined as the distance between the lateral aspect of the tie wings for the Damon 2™ and InOvation R™ brackets and the distance between the lateral aspect of the self-ligating clips for the SmartClip™ bracket. Measurements varied across the bracket types 17). (Table The Damon 2™ was the narrowest of the brackets (mean bracket width of 2.66 mm) followed by In-Ovation R™ (2.87 mm) and SmartClip™ (3.36 mm). SmartClip™ showed the greatest range of bracket widths. The true extent to which bracket width influenced the results of the present study is unclear because of the number of other variables associated with the brackets such as slot size and selfligation mechanism. Still, given the inverse relationship between bracket width and interbracket distance, the direct relationship between interbracket distance and wire flexibility, and the inverse relationship between wire 87 flexibility and contact force, one can assume that wider brackets that are out of alignment experience larger forces between wire and bracket. Such forces can, in theory, result in a relative increase in frictional resistance experienced by the wire/bracket complex. If one assumed that bracket widths did affect the level of frictional resistance, then one would conclude that in this study, the SmartClip™ had a larger resistance magnitude and the Damon 2™ and In-Ovation R™ had smaller resistance magnitudes than they would have shown if each bracket was the same standard width. Slot Size All brackets used in the study had a slot size of 0.022-inch. The depth of the slot for each bracket is slightly different. The SmartClip™ bracket has a depth of 0.028-inch (personal communication with 3M/Unitek, 2005); the depth of a Damon 2™ and an In-Ovation R™ bracket are both 0.027 inches (Harradine, 2003). The presence of an active clip, though, causes the true “passive” slot depth of the In-Ovation R™ bracket to be approximately 0.018 inches. For practical purposes, it can be assumed that the slot depths of the Damon 2™ and SmartClip™ brackets are 88 approximately equal and both are larger than the “passive” slot depth of the In-Ovation R™ bracket. With a 0.016-inch wire, the three brackets showed no statistical difference in kinetic frictional resistance when they were aligned (though there was a statistically significant difference between the maximum static resistance of the SmartClip™ and In-Ovation R™ brackets). After the wire size was increased to 0.018-inch or 0.016- x 0.022-inch, although the brackets were still aligned, the In-Ovation R™ bracket tended to show significantly greater frictional resistance than the other two brackets. When considering slot size, slot depth, and bracket width, one must consider the effect of these upon contact angle (Figure 1). In regard to this study, it is not so much the facial view of contact angle in which we are interested, but rather, it is the contact angle seen from the occlusal view that is most interesting (Figure 23). During part of this study, the self-ligating brackets of the lateral and canine were respectively displaced one mm lingually and one mm facially. In theory, such first-order discrepancies would minimize the “reduced friction” properties of self-ligating brackets by pressing the archwire against the bracket’s gate or clip. 89 Figure 23: Occlusal view of anterior SmartClip™ brackets and a 0.016- x 0.022-inch NiTi™ wire in misaligned set-up Hypothetically, under such circumstances, a narrow bracket with a large slot depth should permit a greater contact angle (as viewed from the occlusal) prior to archwire binding. The possibility of a greater contact angle prior to binding would, therefore, lead to less frictional resistance compared to other brackets that cannot accommodate larger contact angles. Interestingly, the bracket that best fits these criteria (narrow width and large slot depth) is the Damon 2™ bracket. As expected per this hypothesis, the Damon 2™ bracket showed the smallest frictional resistance levels under all scenarios when the brackets were misaligned. 90 Bracket Design Confirming the work of previous studies (Thomas, Sherriff, and Birnie, 1998; Thorstenson and Kusy, 2002a; Henao and Kusy, 2005), the present study found that selfligating brackets with clips exhibited greater frictional resistance than self-ligating brackets with slides or gates. According to Harradine (2003), any wire with a diameter greater than 0.018-inch will activate the InOvation R™ clip resulting in increased normal force upon the wire. As reported earlier, kinetic frictional resistance with a 0.016-inch wire when the brackets were aligned was statistically the same across the three bracket types, but, after the wire size was increased to 0.018-inch or 0.016- x 0.022-inch, though all the brackets were still aligned, the In-Ovation R™ bracket showed significantly greater frictional resistance than the other two brackets. Under testing circumstances when the brackets were aligned, the SmartClip™ bracket consistently showed kinetic frictional resistance levels that were equal to or less than the kinetic frictional resistance of the Damon 2™. Yet when the lateral and canine brackets were misaligned, the SmartClip™ showed a larger than normal (relative to the other two brackets) increase in the level of kinetic and 91 static resistance (Tables 11 and 12). This greater increase may be related to the bracket’s width (and relatively reduced interbracket distance), or it may be associated with the mechanism of the bracket’s selfligating lateral clips. Weinberger (2005) points out that the clips are meant to flex and release archwires that exert forces exceeding a predetermined level. It’s likely that under this testing protocol, when the SmartClip™ brackets were misaligned lingually and facially and the wire was pressed against the clips of the lateral and first bicuspid brackets, that the clips were forced to flex. It is possible that such flexure of the clips exerted an additional force upon the wire thus explaining the significant increase in frictional resistance seen during misalignment of the SmartClip™ bracket. Additionally, the larger increase in frictional resistance by the SmartClip™ bracket may be associated with the fact that the lateral clips are composed of a nickel-titanium alloy. The interaction of these nickel-titanium clips with the nickeltitanium wires could help to explain the increased frictional resistance values associated with the SmartClip™ bracket when the brackets were misaligned. 92 Testing Environment Archform This study was different from the majority of past friction studies in that a seven-tooth dental segment in a half-arch shape was simulated. Placing brackets in a curvilinear arrangement likely caused normal forces to be continuously changing as the archwire was pulled through the brackets. Considering that in the template archform, the central bracket was turned nearly perpendicular to the direction of pull (Figure 24), one can assume, that as the wire was pulled distally, the portion of the wire within Figure 24: Testing set-up showing bracket arrangement relative to Instron™ pulling arm 93 the central bracket was being pressed more and more against the base of the slot. Taylor and Ison (1996) referenced one of their preliminary studies that “indicated that any residual curve in the wire influenced friction.” Because their bracket set-up was in a straight line, they made sure to test only straight lengths of wire. Clearly, differences between the archform of the nickel-titanium wires and the archform of the brackets could result in increased frictional resistance. The frictional resistance values from the present study were consistent with those values noted by Henao and Kusy (2005), who also tested nickel-titanium wires and self-ligating brackets in a simulated archform. They also concluded that self-ligating brackets with active clips tended to exhibit greater frictional values than selfligating brackets with slides or gates. Perturbations Though not as dramatic as the reductions reported by Braun and colleagues (1999), the vibrations introduced into the present system by the perturbation machine caused a notable decrease in the frictional resistance levels of each bracket type tested. The reason for the difference in 94 reduction in the current study is most likely due to the difference in protocol. Braun’s group (1999) tested a single bracket compared to the present seven-bracket design. They noted in their conclusions that “Frictional resistance may...be an important consideration when an arch wire must simultaneously move through several in-line brackets...We believe that it is unlikely that perturbations would result in simultaneous, synchronous relative motion in all of the related bracket/arch wire interfaces” (Braun et al., 1999). Their prediction was shown to be true in this study. Of the 162 tests conducted in the presence of perturbations, only 7 of them (4.3%) had kinetic resistance values of 0 grams. The vast majority of the tests showed some level of frictional resistance at all times even in the presence of perturbations. In general, when the brackets were aligned the decrease in kinetic resistance caused by the perturbations averaged over 70% while the decrease in maximum static resistance averaged over 50%. Perturbations caused the misaligned brackets to experience an average decrease in kinetic resistance of 35% and an average decrease in maximum static resistance of 40%. In light of the increased binding and friction that was likely occurring from the faciolingual bending activation of the 95 wires and the creation of greater normal forces, the difference in resistance reduction between aligned brackets and misaligned brackets is understandable. Interestingly, this sizable decrease in frictional resistance occurred using forces that are significantly smaller (200 – 500 grams) than those found in the oral environment which range from a few kilograms to over one hundred kilograms. Future studies utilizing different force levels may show similar or even greater reductions in frictional resistance. Graphic Data/Stick and Slip While the test wire was being pulled through the series of test brackets, an adjacent printer recorded the resistance encountered by the Instron™ machine in the form of a graph. Some of the graphs had a relatively smooth contour (Figure 16). This was the result of classic friction from the bracket and wire sliding past one another. Other graphs, such as the one seen in Figure 25, showed a distinct “peak and valley” pattern. Most often these occurred with larger wires when the brackets were misaligned. The cause of such peaks is the “stick-slip phenomenon” discussed by Rossouw and associates in 2003. 96 Figure 25: Example of a graph of frictional resistance values showing the stick-slip phenomenon. Drawn from original tracing. Such a phenomenon occurs when the static frictional resistance builds to a maximum level and then is followed by a sudden release or slip of the wire (Figure 26). Peak Valley Figure 26: Close-up of Figure 25 showing the stick (dark arrow) slip (white arrow) phenomenon. Peak = maximum static resistance prior to movement. Valley = end of displacement, beginning of new static phase. 97 The slip corresponds to rapid displacement of the wire. This displacement ends after the “valley” is reached and a new “peak” begins to form, starting the process all over again. This type of bracket-wire interaction is, therefore, a series of starts and stops or a repetition of archwire binding and releasing, binding and releasing. When the “slip” occurred during the testing, an audible “click” could be heard. According to Rossouw and colleagues (2003), there is evidence of this kind of stick-slip activity in the orthodontics. They explained that this phenomenon “may be inferred from scanning electron micrographs that reveal permanent deformation of archwires subjected to intermittent binding and sliding at bracket surfaces.” Considering stick-slip and the very slow velocities associated with tooth movements, it appears that orthodontics is more influenced by maximum static frictional resistance than kinetic frictional resistance (Omana et al., 1992). Limitations There are many reasons why one must be cautious in correlating the results of laboratory in vitro friction studies with in vivo orthodontic friction because 98 experimental conditions do not always correctly represent the clinical situation (Rossouw, 2003). The biggest limitation to bench-top testing is the inability to replicate the oral environment and especially to replicate the extremely slow speeds and irregular nature of tooth movement. Slow velocities may produce dynamics that do not correspond to the classic understanding of friction (Rossouw et al., 2003). In vitro studies tend to test at constant speeds at magnitudes thousands of times greater than that found in the mouth. In their study, Drescher’s group (1989) tried to account for the biologic resistance that a tooth feels in opposition to its tendency to tip from orthodontic forces. The incorporation or simulation of investing tissues associated with a tooth would greatly enhance the replication of the oral environment. Unfortunately, the design of the new friction testing machine did not include an “artificial” periodontal ligament (PDL). Because the connective tissue surrounding the tooth and maintaining the tooth within its socket absorbs and distributes a significant portion of the forces encountered by the tooth, the addition of a simulated PDL to the experimental model might permit a more realistic testing environment. 99 Maximum static friction measurements, when frictional resistance levels were relatively low, proved to be a challenge due to the testing protocol. According to the chosen protocol, the primary investigator did not turn on the perturbation machine until all clearance between the archwire lock and the hook had been eliminated by the Instron™ machine and the Instron™ began to experience resistance. This choice was based upon the desire to be able to determine the beginning of each active testing period. Often while testing the smaller wires when the brackets were aligned, the frictional resistance levels were small - approximately 20 grams or less (Figure 27). “maximum static resistance” Figure 27: Close-up of a graph showing frictional resistance during one of the trials of Damon 2™ brackets with a 0.016-inch NiTi™ wire in the presence of perturbations. The point of “maximum static resistance” corresponds to the precise moment the plunger from the perturbation machine first struck the baseplate and immediately dropped the frictional resistance to zero. 100 Under such circumstances, if perturbations were introduced, the resistance level would immediate drop to zero or near zero precisely at the same time that the plunger struck the testing baseplate for the first time. Resistance values would remain near zero for the entire testing period. The “maximum” static frictional resistance value measured under these situations was directly related to the speed at which the operator would notice the rise in initial static resistance and then activate the perturbation machine. In other words, in theory, the longer it took to turn on the perturbation machine, the greater the recorded “maximum” static resistance on the graph. If the protocol had been different and the perturbation machine was activated prior to the start of the static resistance period of the test, it may have been shown that for a portion of the tests conducted during this study the maximum static resistance value was actually significantly lower (possibly even zero grams) compared to the measured value. For this reason, static frictional resistance levels for the smaller round nickel-titanium wires in this study should be considered with some uncertainty. 101 Finally, the present study was limited to selfligating brackets in combination with nickel-titanium wires arranged in two set archforms: 1) aligned and 2) misaligned with first-order discrepancies at the lateral and canine. The flexibility of the new testing machine will permit multiple future studies incorporating a variety of different bracket types, wire types, and archform set-ups. 102 CHAPTER VI – SUMMARY AND CONCLUSIONS The primary purpose of this study was to answer the question: Do perturbations or vibrations significantly affect the frictional resistance of self-ligating brackets and nickel-titanium wires when introduced into an in vitro model simulating the initial alignment phase of orthodontic treatment? The results of this study clearly show that, when perturbations were introduced into the in vitro model, both the average kinetic and the maximum static frictional resistance was reduced by 35 – 70%. Other questions to be answered include: “How do selfligating brackets differ in their frictional characteristics?” Resistance values from this study provide additional evidence that self-ligating brackets with slides or gates, otherwise known as “passive selfligating brackets,” contribute less to frictional resistance than self-ligating brackets with active clips. The SmartClip™ bracket and its unique self-ligating system were influenced more than the other two brackets by the alignment or misalignment of the brackets. When the SmartClip™ brackets were aligned, the clips sat passively over the wire, and the bracket’s frictional resistance appeared to be at least as small as that of the Damon 2™ 103 bracket. Yet, when the brackets were misaligned and the wire was pressed against the slot or the clips, the SmartClip™ bracket showed a considerable increase in resistance relative to other self-ligating brackets. This increase may be explained by the larger width of the bracket, added friction or binding associated with the potential flexure of the clips, and/or the nickel-titanium composition of the clips. If self-ligating brackets are affixed to teeth that are misaligned facially or lingually in the arch and the wire is forced to press against the given gate or clip, how is frictional resistance affected? Foremost, frictional resistance increases dramatically. Results from this study showed that, on the average, kinetic frictional resistance increased thirteen times and maximum static resistance increased over ten times. Under some circumstances with the 0.016- x 0.022-inch wire, those values were as high as thirty-five times greater than “pre-misaligned” levels. 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Br J Weinberger GL. Utilizing the SmartClip Self-Ligating Appliance. Orthodontic Perspectives: The System Approach. 2005, Vol. XII, No. 1, p.3-7. 3M-Unitek Publication, Monrovia, CA. Wildman AJ, Hice TL, Lang HM, Lee IF, Strauch EC Jr. Round Table: The Edgelock bracket. J Clin Orthod. 1972;6:61323. Woodside DG, Berger JL, Hanson GH. Self-ligation orthodontics with the SPEED appliance. In: Graber TM, Vanarsdall RL, Vig KWL (ed). Orthodontics: current principles and techniques. St. Louis, MO, Elsevier Mosby, 2005; pp 718. 111 VITA AUCTORIS David Michael Bunkall was born on March 19, 1974, in Salt Lake City, Utah, to Larry and Carolyn Bunkall. He graduated Magna Cum Laude from Brigham Young University in Provo, Utah, in 1999 with a Bachelor of Science degree in Zoology. From 1999 to 2003, he attended dental school at the University of Washington in Seattle, Washington and graduated with honors. Surgery degree in 2003. He was awarded a Doctor of Dental It is anticipated that in January of 2006, David will graduate from Saint Louis University with a Master of Science in Dentistry with an emphasis in Orthodontics. In May of 1997, David married Robyn Dover. the parents of two sons, Cannon and Luke. 112 They are