FRICTION PRODUCED BY ESTHETIC BRACKETS WITH VARYING LIGATION CHRISTINE KNOX ABENOJA

Transcription

1 FRICTION PRODUCED BY ESTHETIC BRACKETS WITH VARYING LIGATION by CHRISTINE KNOX ABENOJA ANDRE FERREIRA, COMMITTEE CHAIR LIONEL SADOWSKY JOHN BURGESS MARK LITAKER FIROZ RAHEMTULLA A THESIS Submitted to the graduate faculty of the University of Alabama at Birmingham, in partial fulfillment of the requirements for the degree of Master of Science BIRMINGHAM, ALABAMA 2008

2 FRICTION PRODUCED BY ESTHETIC BRACKETS WITH VARYING LIGATION CHRISTINE ABENOJA ORTHODONTICS ABSTRACT Controlling friction is an important aspect of orthodontics because it allows more predictable tooth movement with less anchorage loss. Friction at the bracket/archwire/ligation interface can reduce the force available for desired tooth movement. Initial stages of orthodontic treatment depend on the ability of the archwire to slide freely through the bracket, making friction an important aspect of all orthodontic brackets in the early phases of orthodontic treatment. The purpose of the present in vitro study was to measure the force to move esthetic brackets over an archwire using different ligation methods. A stainless steel model reproducing the maxillary right dentition was used to test brackets in combination with self-ligation, a new non-conventional elastomeric ligature or a conventional elastic ligature. Static and kinetic friction was measured as the force required pulling a inch superelastic nickel titanium archwire through the brackets. The test was first conducted with all the brackets aligned and then with the canine bracket moved apically 3mm. In the aligned configuration mean static friction for the Damon3 MX bracket (Ormco, Glendora, California) was significantly lower than that for the Inspire Ice bracket (Ormco, Glendora, California) with Slide ligation (Leone, Sesto Fiorentino, Firenze, Italy). There were no other significant differences. ii

3 In the misaligned configuration Clarity SL (3M Unitek, Monrovia, California), Clarity/Slide, and ICE/Slide each generated a modest amount of friction that was statistically similar and ranged from g. Each of these three combinations generated significantly less static friction than the Damon3 MX and the ICE/conventional elastic ligature in the misaligned configuration. Additionally, ICE/conventional elastic ligature in the misaligned configuration generated significantly more kinetic friction than the Damon3 MX. It can be concluded that a frictionless ligature can reduce the classical friction associated with esthetic brackets. iii

4 ACKNOWLEDGEMENTS The author thanks Drs. Deniz Cakir, Andre Ferreira, Lionel Sadowsky, John Burgess, Firoz Rahemtulla and Mark Litaker, Ms Sandre McNeal and the faculty members and residents of the Department of Orthodontics, University of Alabama School of Dentistry, Birmingham, Alabama. iv

5 TABLE OF CONTENTS Page ABSTRACT... ii ACKNOWLEDGEMENTS... iv LIST OF TABLES... vi LIST OF FIGURES... vii LITERATURE REVIEW...1 Elastic Modules...3 Stainless Steel Ligatures...7 Frictionless Ligatures...9 Self-ligation...13 Second-order Angulation...15 Ceramic Brackets...18 Archwire Alloy...20 Archwire Dimensions...21 Bracket Width...24 Surface Modifications...25 FRICTION PRODUCED BY ESTHETIC BRACKETS WITH VARYING LIGATION 27 CONCLUSIONS...47 GENERAL LIST OF REFERENCES...49 v

6 LIST OF TABLES Table Page 1 Aligned Static and Kinetic Friction Misaligned Static and Kinetic Friction...37 vi

7 LIST OF FIGURES Figure Page 1 Device...34 vii

8 LITERATURE REVIEW Friction is the resistance to motion when one object moves tangentially against another object. The normal force is the perpendicular component of the force acting on the contacting surfaces. The coefficient of friction for a given material surface is a constant and is dependent on the material s characteristics such as roughness, texture and hardness of the surface. The frictional force is the product of the coefficient of friction and the normal force. In order for one object to slide against another object the force applied has to overcome the frictional force created. 1,2,3 Static frictional force is the smallest force that is needed to begin movement of two contacting surfaces. Kinetic frictional force is the force needed to resist the sliding of one object over another. 4,5 Both static friction and kinetic friction are important to understand in orthodontics. Static friction is relevant because teeth move in a series of crown tipping/root up righting movements. 4 Since teeth move along an archwire in such a non-continuous manner static friction must be overcome each time the tooth begins to move. 6,7 Kinetic friction also referred to as dynamic friction - is also important because it is the friction that occurs between the orthodontic bracket and archwire once movement has been started. 4 There is insufficient evidence to indicate that optimal tooth movement is dependent on wire-bracket-ligation friction reduction. In theory, however, if the applied orthodontic force is sufficient to overcome friction in the wire-bracket-ligation interface, and still remain within a hypothetical ideal force range, then we should expect optimal 1

9 tooth movement. 8 Indeed, several authors agree that quantifying and controlling friction is an important aspect of orthodontics. It allows us to enact more predictable tooth movement and encounter less anchorage loss. 4,9 Friction always resists the sliding force applied and therefore reduces the effective force delivered to the tooth. 10 It has been estimated that friction can reduce available force by almost 40 percent and may be responsible for anchorage loss if the friction prevents movement of the tooth to which the bracket is attached. 11,12 If friction at the bracket-archwire interface is very high less than optimal forces may be achieved reducing the efficiency of the system and extending treatment time 5,13 ; this could produce no movement of the teeth at all or a loss of anchorage leading to compromised results. 4 Reducing friction between the bracket and archwire is desirable for active torque and sliding mechanics used in space closure and in leveling and aligning. 11,14 The initial stages of orthodontic treatment depend on the ability of the archwire to slide freely through the bracket, making friction an important aspect of every edgewise appliance, at least in the early phases of treatment. Reduced friction conserves anchorage by requiring lower net forces which correspondingly leads to lower reciprocal forces. 15 Minimizing friction in canine retraction allows the bulk of the force to be transferred to the teeth with less anchorage loss. 4 Also canines can be retracted separately along an archwire which reduces overall anchorage demands by reducing the root area of teeth being moved at any one time. 15 Clinically, friction is not routinely detrimental. Since friction is a force that delays or resists the relative motion of two objects there are some situations in which friction is desirable. For example, if the clinician were to use stainless steel brackets on posterior 2

10 teeth and ceramic brackets on the anterior teeth the difference in friction may allow faster movement of the posterior teeth and accomplish in certain cases - desirable anchorage loss. 11 Another example of using friction advantageously is provided by Rossouw. 4 By using figure eight elastomerics on incisors while simultaneously using reduced friction ligation on an adjacent high canine the clinician can minimize unwanted movement of the incisors. Knowledge of levels of friction generated between brackets and wires allows the clinician to make adjustments in the appliance to apply specific tooth movements. 9 There are many variables that affect friction. Broadly, these variables include the physical and mechanical properties of the materials involved including the brackets and archwires properties, the manner of bracket to archwire ligation as well as biological factors. 4 Many of the factors which influence friction have been investigated. The method of ligation used to engage the archwire into the bracket has a profound effect on the normal force generated and thus the amount of friction produced between the archwire and the bracket. A number of studies have investigated the method of ligation that produces the least amount of static and kinetic friction. Elastomeric Modules Elastomeric modules are convenient and easy to use. 6,16 Some elastomerics have been produced with a forty-five degree bend to allow very easy placement over the bracket tie wings. 1 Elastomerics are the preferred method of ligation for many clinicians and thus have been studied extensively. Edwards 17 reported in 1995 that in a study of stainless steel brackets and archwires that elastomerics tied in a figure eight pattern had 3

11 more fiction than both elastomerics conventionally tied and also more than stainless steel ligatures; while Teflon-coated ligatures had the lowest friction. Others have reported similar findings. 13 In 1998 Dowling et al 13 reported a study using an Instron universal testing machine in which a 0.018X0.025-inch stainless steel archwire was pulled through maxillary premolar brackets ligated with either grey, clear, orange or grey-fluorideimpregnated round modules or a grey rectangular elastomeric module. Clear elastomeric modules had frictional values that were significantly lower than the other types of modules. This study was repeated over a four week period during which time the specimens were stored in a simulated oral environment. They reported that the immersion used resulted in a reduction of ten to thirty-five percent of failure load strengths of all types of modules tested. The authors, therefore, recommended that elastomeric modules be replaced at each visit. Additional in vitro studies have shown that frictional forces generated by elastomerics decrease over three to four weeks with a concurrent decrease in failure load strength. 6 Taylor and Ison 18 found the latter to be true when they reported that frictional forces declined slowly over time following initial placement of an elastic module. In Taylor and Ison s 18 study they examined elastomerics and stainless steel ligatures that were either maximally tightened or loosely tied. Their study of friction using archwires pulled through three different brackets including the passive self-ligating Activa bracket with an Inston testing machine simulated extraction and non-extraction treatment. They found that loosely tied steel ties and stretched elastomeric modules reduced frictional forces. The technique of bracket ligation significantly affects friction according to Taylor 4

12 and Ison. 18 Minimal friction was recorded for the molar attachments when wires were not ligated indicating that frictional forces were attributed mainly to the ligation technique used in the premolars. One can reduce friction by stretching modules before placement, by loosely tying stainless steel ligatures, or by using a passive self-ligating bracket. They also noted that static friction doubled when two brackets were used indicating a linear increase in frictional forces with the number of brackets. This finding implies that in non-extraction treatment larger forces may be required to overcome friction. A 2004 study by Khambay, Millett and McHugh 1 evaluated methods of archwire ligation on frictional resistance. Using the Nene M3000 testing machine (Wellingborough, UK) a maxillary premolar stainless steel bracket was tested with X0.025-inch and 0.019X0.025-inch stainless steel and TMA archwires. The bracket and wire were ligated with purple, grey, Alastik (3M Unitek, Monrovia, California) or Super Slick (TP Orthodontic, La Porte, Indiana) elastomers or a.09 inch stainless steel ligature. Super Slick is a polymeric-coated ligature manufactured to reduce friction by becoming slippery in water or saliva. Thus this study was conducted in human saliva. However, they did not find that the polymeric-coated module produced the lowest friction. Instead Khambay, Millett and McHugh 1 discovered that the stainless steel ligatures produced the lowest friction. In their pilot study, however, they tested the Damon2 stainless steel self-ligating bracket (Ormco, Glendora, Califormina) which produced negligible mean frictional forces and therefore the authors concluded that selfligating brackets are the only current method of reducing friction. 5

13 In a similar study by Griffiths, et al 19 Super Slick was compared to round and rectangular modules using the stainless steel self-ligating Damon2 bracket and the monocrystalline Inspire bracket. The buccal segment model of one molar and two premolar brackets was arranged to allow inch and 0.019X0.025-inch stainless steel archwires to be pulled through using a testing machine. In all but two combinations round modules provided the least resistance to sliding with the rectangular modules the greatest and Super Slick in between. They also found that in general ceramic brackets had a greater resistance to sliding than the stainless steel brackets. They found the selfligating brackets to generate virtually zero friction. Griffiths, et al concluded that the self-ligating system is the best way to eliminate resistance to sliding by elastic ligatures. Chimenti, Franchi and Giuseppe 6 set out to evaluate the effect of the variation in the dimension of elastomeric ligautres on static fiction. Using the Instron testing machine they simulated the maxillary right buccal segment undergoing incisor retraction with sliding mechanics. Using a 0.019X0.025-inch stainless steel archwire and stainless steel brackets, three dimensions of elastomerics were tested; small, medium and large. They also tested two elastomerics lubricated with silicone. They found that the small and medium elastomerics produced significantly smaller static frictional forces than the large elastomerics. There was no statistically significant difference found between the small and medium elastomerics. The silicone-lubricated ligatures had statistically significant smaller friction than the small, medium or large non-lubricated elastomerics. They found that there was a significant positive correlation between the thickness of the modules and the frictional forces and between the outside diameter and the fictional forces. Since there was this significant positive correlation between the thickness of the 6

14 modules and the frictional forces, the decrease seen in the small and medium modules was be attributed to the smaller thickness of both compared to the large ligatures. Thus, of the three dimensions of a round elastomeric studied; inner diameter, outer diameter and thickness, the authors concluded that the decrease in friction could be attributed to smaller thickness. Stainless Steel Ligatures While one of the benefits of wire ligatures is full bracket engagement 16 most studies have reported that the increase in normal force from tight ligation will increase the frictional force 3. A range of ligating forces can be used when ligating with stainless steel ligatures 6,13. Therefore, the force used through stainless steel ligatures is subjective varying according to the orthodontist and it is unknown. 11,20 In a study by Iwasaki 21 it was found that there was not only variability of stainless steel ligation between operators but also within operators and they concluded that is difficult to study stainless steel ligatures since consistent ligation forces are difficult to attain 21. None the less, it is generally believed that reduced levels of friction can be obtained by loosely tied steel ties. 6,7,22 Teflon-coated stainless steel ligatures can be used as an esthetic ligature with ceramic brackets. In Defranco s 23 study, all mean static frictional forces were less with Teflon-coated stainless steel ligatures than with elastic ligatures. In fact, Teflon-coated stainless steel ligatures produced less friction than elastomerics regardless of bracket type (polycrystalline or monocrystalline), archwire type (stainless steel or nitinol) or 7

15 bracket/archwire angulation (5,10 or 15 degrees). Most likely the lower friction values were the result of the Teflon material possessing a lower coefficient of friction than the elastomerics. A study by Hain, Dhopathkar and Rock 7 in 2003 suggests that variation in friction is more likely due to the surface and material characteristics. In their study on the effects of ligation method on friction in sliding mechanics, they used an Instron machine to test the effect of friction by type of module, state of wetness, bracket type and tie configuration. They found that Slick modules reduced friction up to sixty percent especially when lubricated; SPEED self-ligating brackets (Strite Industries, Cambridge, Ontario, Canada) had less friction than other brackets using elastomeric modules; and stainless steel ligatures produced the least friction of all which was almost clinically negligible. According to Harradine 16 wire ligatures produce just percent of the friction of elastomerics. Furthermore, elastomerics at times do not exert enough force to fully engage the archwire and elastomeric degredation contributes loss of full engagement as the elastomeric permanently deforms over time. Stainless steel ligatures also accumulate less plaque than elastomerics. Yet, while it is clear that stainless steel ligatures may be superior to elastomerics in most respects there still is the issue of time 16. Shivapuja and Berger 24 have shown that using wire ligatures adds almost twelve minutes to the time needed to remove and replace two archwires. Many clinicians are unwilling to spend this additional time chairside. 8

16 Frictionless Ligatures Elastomeric ligatures hug the archwire on either side of the bracket s archwire slot. They do not allow the archwire freedom like a self-ligating bracket 25. A new lowfriction ligature has recently been introduced which forms a butterfly shape over the bracket and creates a tube-like structure in which the archwire can slide freely. Slide (Leone Orthdontic Products, Sesto Fiorentino, Firenze, Italy) is one such innovative ligature that has been manufactured by Leone Orthodontic Products. It is formed with a polyurethane mix by injection molding. Franchi and Baccetti have studied the Slide ligatures and reported their findings in the orthodontic literature. 13,22 They reproduced the right maxillary buccal segment with five inch stainless steel brackets from maxillary right central through the maxillary right second premolar. Frictional forces generated by a 0.019X0.025-inch stainless steel archwire were recorded by sliding the wire into the aligned brackets. Also friction produced by a inch super-elastic nickel-titanium archwire was evaluated both in the presence of aligned and misaligned brackets. The canine bracket was welded to a sliding bar to change its vertical position to create a 3mm apical canine offset. They compared the frictional forces generated by these non-conventional elastic ligatures (NCELs) to conventional elastic ligatures (CELs). Dry conditions were used and the test was run using an Instron testing machine. Static and kinetic friction forces were recorded while 15mm of wire was drawn through the brackets at 15mm per minute. Static friction was measured as the maximum initial rise on the Instron trace chart and kinetic friction measurements were carried out at 2,5 and 10 mm of extension and then averaged. 9

17 There were significant differences between the NCELs and CELs for both static and kinetic friction for all tested variables. Friction was minimal in the aligned brackets with the NCEL. In the misaligned brackets the NCEL group had less than half the amount of both static and kinetic friction than the CEL group. They found the data generated from the NCELs to be very similar to published data of self-ligating brackets and concluded that any type of conventional bracket system may be made to function with less friction like a self-ligating bracket by the use of the Slide ligature. 22 In a subsequent study Franchi and Baccetti used a similar set-up to simulate the leveling and aligning phase of orthodontic treatment. They studied the forces released by the archwires during alignment by allowing for four increasing amounts of upward displacement of the misaligned canine bracket. Franchi and Baccetti 14 tested three small round super-elastic nickel-titanium archwires. Again they found significant differences between the NCEL and CEL. The results showed that when a small amount of tooth alignment is needed (1.5mm) the effect of the NCEL is minimal but the differences between the NCEL and CEL become significant with 3.0, 4.5 and 6.0mm of misalignment. Building on the two studies just described, Campresi, Baccetti and Franchi 26 studied the forces released by ceramic brackets (Aqua {Leone Orthodontic Products, Sesto Fiorentino, Florence, Italy} and Mystique {GAC International, Bohemia, New York}) with Slide and another frictionless elastomeric by GAC called Neo-clips (GAC International, Bohemia, New York). Again, mimicking the leveling and aligning stage of orthodontic treatment, a inch super-elastic nickel-titanium wire was pulled through aligned and an increasingly misaligned canine bracket by the Instron universal testing 10

18 machine. The forces released by the archwire bracket - ligature combinations were recorded. Again they found that at a canine misalignment of 1.5 mm there was no significant difference between the NCELs and CELs. However, at 3.0 mm and 4.5 mm of apical canine misalignment, the esthetic low friction systems both released significantly greater amounts of force than the CELs. With less friction to overcome, this force is more readily available for actual tooth movement. In fact at 3.0 mm or more the CELs produced almost no amount of released force for alignment. Both low-friction systems behaved the same and there was no significant difference between them at any canine offsets. They reported again that when a small amount of alignment is needed the difference in performance of low-friction versus conventional elastic ligatures is minimal but it becomes significant at 3.0mm of misalignment and more. Low-friction ligature systems enable ceramic brackets to produce reduced friction during the initial stages of tooth movement. They proved that the NCELs are virtually frictionless and provide another very simple way to reduce friction during leveling and aligning without necessarily having to switch to passive self-ligating brackets. To prove that NCELs are a valid alternative to self-ligating brackets Franchi, Baccetti, Camporesi and Barbato 12 next studied the forces released during sliding mechanics with passive self-ligating brackets compared to stainless steel brackets with NCELs and CELs. Friction was measured during sliding mechanics on aligned brackets with a 0.019X0.025-inch stainless steel archwire. Significantly smaller static and kinetic forces were generated by the self-ligating brackes and the NCEL compared to the CEL. There was no significant difference between the self-ligating brackets and the NCEL. 11

19 Therefore NCEL may be an alternative to self-ligating brackets during sliding mechanics in addition to during leveling and aligning. In another study Franchi et al 27 analyzed the changes in the transverse dimension and maxillary arch perimeter developed clinically in association with using NCELs. Twenty consecutively treated non-extraction patients with mild maxillary crowding were treated with brackets and NCELs with0.014-inch and inch super-elastic nickeltitanium archwires. Dental casts of the maxillary arch were taken before treatment and at the end of the leveling and aligning phase which averaged six months. Their findings showed statistically significant increases in maxillary arch widths and perimeter with low-friction NCELs. Though they did not have a control group for direct comparison, the transverse and sagittal arch changes in untreated adolescents over a six month period is expected to be minimal. The significant increase in transverse widths of the maxillary arch caused a statistically significant increase in maxillary arch perimeter. Smaller pretreatment perimeters had greater increases in arch length. This result is a clinically favorable result for non-extraction treatment. However, the increases in arch width that used lingual points for measurement were consistently smaller than the increases recorded by using points located at cusps or occlusal fossae which indicate that the expansion of the maxillary arch was achieved with a component of buccal inclination of the posterior teeth. This was confirmed by the first molars also showing an increase in the angle of reciprocal inclination. The greatest transverse increases were found in the premolar and canine region which may have been due to the use of archwires with accentuated width in the canine-first premolar region for alignment. Franchi et al 27 concluded that the shape of the archwires contributed to the results. 12

20 Self-ligation To reduce friction generated by ligation one may choose loosely tied stainless steel ligatures, frictionless non-conventional elastic ligatures or self-ligating brackets. According to Harradine 16 the ideal properties of any ligation system would require that it be secure and robust, allow full bracket engagement of an archwire, have lower friction, be quick and easy for the clinician to employ and be comfortable for the patient and allow for good oral hygiene. Self-ligating brackets meet these criteria according to Harradine. 16 There are two types of self-ligating brackets. The first has a spring clip that pushes the wire into the bracket slot and thus is referred to as active. The second being completely passive does not have a clip to press on the archwire 1. Some active and passive self-ligating brackets have a fourth wall that in essence converts the bracket slot into a tube. Passive self-ligating brackets have been shown to produce negligible friction. 1,28,29 They have also been shown to produce less friction than active self-ligating brackets with the exception of using undersized round archwires. 12 Henao and Kusy 29 studied the friction associated with self-ligating brackets and reported that when archwire clearance was substantial that passive self-ligating brackets out-performed active selfligating brackets. They also concluded that as a malocclusion became worse and as the archwire size increased reducing clearance, the two self-ligating designs lost distinction because the frictional behavior then correlated more with the bending stiffness of the archwires. Smith, Rossouw and Watson 28 reported that ceramic brackets with and without a metal slot had the greatest friction followed in decreasing order by metal brackets, active self-ligating, variable self-ligating and passive self-ligating. 13

21 Self-ligating brackets generally produce lower levels of friction compared to conventional brackets. 13,16,28,30,31,32 Cacciafesta 32 studied the frictional forces generated by different bracket/archwire combinations. It was found that stainless steel self-ligating brackets generated significantly lower static and kinetic frictional forces than both conventional stainless steel and polycarbonate self-ligating brackets. Berger 25 studied the SPEED bracket by using an Instron machine to measure the force required to move the same archwire through the same distance in three different bracket systems with elastomerics, steel-ties, or self-ligation. He found the level of applied force necessary for movement to be considerably lower with the SPEED self-ligating bracket. The lower initial level of force was then followed by an almost constant low level of force. Berger concluded that with the SPEED self-ligating bracket there is less friction and archwire binding, thus, more continuous tooth movement with less hyalinization and more rapid tooth movement with the application of light force levels. In addition to producing less friction, self-ligating brackets have other advantages. They are smoother, more comfortable, and easier to clean and they reduce the time required chairside. 31 Harradine 16 argues that the combination of very low friction and very secure full archwire engagement is the most beneficial feature of self-ligating brackets. Full engagement of the archwire minimizes the need to regain control of teeth where full engagement was lost during treatment a challenge commonly encountered with conventional brackets and deformable elastomerics 16. Clinically, self-ligating brackets allow the wire to slide through the brackets of rotated and adjacent teeth to facilitate derotation 15. Thus, early alignment and resolution of rotations benefit from the low friction and full archwire engagement offered by self-ligating brackets. 16 Initially 14

22 small interbracket archwire deflections offer biocompatible tooth movement which combined with a wide twin bracket can give accurate rotation corrections. 30 Turnbull and Birnie 33 assessed the speed with which two archwires could be changed in self-ligating and conventional brackets with elastomerics. In their study the ligation of an archwire was twice as quick with the Damon2 self-ligating system. Opening a Damon slide was on average one second quicker per bracket than removing an elastomeric from the mini-twin brackets and closing a slide was 2 seconds faster per bracket. These differences in ligation time became more marked for larger wire sizes and they concluded that the average time saved was 1.3 minutes per visit. As with any system, self-ligating brackets also have disadvantages; one such disadvantage in the inability of the clinician to use partial slot archwire engagement. Additionally there is increased wire displacement with the use of self-ligation although this is easily corrected with v-shaped notches or crimpable stops. Perfect alignment may also be difficult to achieve because the archwire is not forced to the base of the slot. Likewise torque may not fully be expressed. These limitations can be handled, however, by using larger dimension wires to more fully express the values built into the bracket slot in all three dimensions. Also, TMA wires may be useful in finishing self-ligation cases to allow for localized torque in larger wires. 15 Second-orderAngulation Second order angulation has been found to be a critical factor in friction. 11,13 Resistance to sliding according to Thorstenson and Kusy 10,20 is not always dependent 15

23 wholly on what we think of as classical friction. The resistance to sliding of an archwirebracket combination is at times the combined effects of classical friction, elastic binding and physical notching. 10,20 In a passive system in which all teeth are aligned then only classical friction contributes to the resistance to sliding because binding and notching do not play a role. In active archwire configurations both classical friction and binding contribute to the resistance to sliding. The value for the component caused by friction is small compared to the more dominate role of binding and notching and it is dependent solely on the ligation force and not on the angle value applied to the bracket. A reduction in treatment time with self-ligation may be attributable to the absence of ligation which in turn lowers classical friction. 25 For each archwire-bracket combination a critical contact angle of second-order angualtion exists where classical friction gives way to binding. 20 The critical contact angle is the angle at which the archwire first contacts the edges of the slot walls. At greater angles the archwire may deform which in turn introduces the notching component to the equation. Thorstenson and Kusy 10 hypothesized that self-ligating brackets should have a lower resistance to sliding at all second-order angualtions than conventional brackets. They compared conventional stainless steel brackets ligated with stainless steel ligatures and self-ligating brackets. Using stainless steel archwires the resistance to sliding was measured at angles from negative nine to positive nine degrees. As the angulation increased all brackets exhibited increased resistance to sliding and at all angles the resistance to sliding of self-ligating brackets were lower than the conventional brackets. 16

24 The reason for this difference may be that the absence of ligation force leads to a lower magnitude of classical friction. Below the critical contact angle the conventional bracket had a constant value for the resistance to sliding that was dependent on the ligation force and the self-ligating bracket had little or no resistance to sliding below its critical contact angle. Beyond the critical contact angle the resistance to sliding increased as the angulation increased for both bracket types. The binding force of the self-ligating bracket increased a similar amount per degree as the conventional bracket. Therefore it was concluded that the self-ligating bracket allows more of the applied force to be used for sliding than the conventional bracket so that the self-ligating bracket can produce the same amount of sliding as the conventional couple at a lower applied force. In another study by Kusy with Henao 29 in 2005, it was found that below the critical contact angle self-ligating brackets with clips had higher frictional values than those with slides. DeFranco 23 also found that frictional resistances increased with increasing bracket-archwire angualtions. The friction contributed by the ligature had its most profound effect at low angulations. As the angulations increased the friction from binding at the bracket-archwire interface developed. He found that even at the higher angulations differences were still noted between the Teflon coated stainless steel ligatures and the conventional elastomeric ligatures that he was testing. Therefore, he concluded that the ligature effect is still significant regardless of the angulation. In Baccetti and Franchi s 22 work, they too recognized the importance of secondorder angulations on friction. They speculated that the misaligned canine in their study would increase the frictional forces regardless of the type of ligature because of the binding generated at the adjacent premolar and lateral incisor as well as both the mesial 17

25 and distal aspects of the canine bracket itself. As expected, the force due to binding resulted in additional frictional forces beyond that caused by ligation method. Ceramic Brackets Ceramic brackets have improved esthetics over stainless steel brackets but they have many disadvantages. Their brittleness leads to easily broken brackets and tie wings; they have a history of enamel breakage upon debonding; and they wear the enamel of opposing teeth 5. Ceramic brackets have higher friction than stainless steel brackets. 3,13,20,23,34,35 Kusy 20 states that stainless steel archwire-bracket combinations generate less classical friction than any combination involving ceramic brackets. In DeFranco s 23 study of ceramic brackets he found that less friction was generated by stainless steel brackets than ceramic brackets irrespective of arch wire size, composition or ligation method. There are two types of ceramics used in orthodontic brackets monocrystalline and polycrystalline. Monocrystalline brackets have a smoother surface than polycrystalline but the observed amount of friction between them is similar. 34 Angolkar 35 found frictional resistance significantly higher in ceramic brackets than in stainless steel brackets for most archwire sizes and alloys. He investigated the slot surfaces of stainless steel and ceramic brackets under scanning electron microscope. He noted that the ceramic slot showed more generalized small indentations and appeared less smooth than stainless steel. Angolkar 35 explained that monocrystalline brackets come from large single crystals of alumina which are milled into the desired shape and because 18

26 alumina is the third-hardest material this procedure is difficult and may produce the granular and pitted surface of the ceramic brackets which may explain the increase in frictional force compared to stainless steel. A metal slot has been added to some ceramic brackets to combine the frictional characteristics of stainless steel with the esthetics of ceramics 5. In 2004 Nishio 11 compared the frictional force of three types of brackets - ceramic brackets, ceramic brackets with metal slots and stainless steel brackets. Ceramic brackets had the highest frictional force value with statistical significance followed in decreasing order by the ceramic bracket with a metal slot and the stainless steel bracket. Likewise, Cacciafesta 5 found that metal-insert brackets generated significantly lower friction than conventional ceramic brackets but more than stainless steel brackets. Nishio 11 looked at the brackets and archwires used in their study under an electron micrograph scan and found that roughness increased in a reciprocal manner; from stainless steel to ceramic with metal slot to all-ceramic. Ceramic brackets are hard and stiff and have a granular, pitted and porous surface which may contribute to a higher coefficient of friction. 3,5,11,35 Stainless steel allows for better polishing and therefore a smoother surface. They suggested that the difference between frictional force values for ceramics with a metal slot and stainless steel brackets may be due to the difficulty in fitting the metal to the ceramic due to their different expansion coefficients. Nishio s 11 results also showed that there was no difference in the frictional forces values with repeated use of the bracket-archwire combinations. The periodontal ligament is compressible and it allows teeth to tip until contacts are met between the archwire and diagonally opposite corners of bracket wings. Teeth 19

27 will also rotate until contacts are met between the archwire and the ligature or the labial cover of a self-ligating bracket. These events happen prior to teeth beginning to slide along an archwire. 3 To mimic this clinical scenario Loftus et al 3 created a dentoalveolar model that attempted to reproduce the elastic properties of the periodontal ligament. They tested four types of maxillary premolar brackets: stainless steel, stainless steel selfligating, all ceramic and ceramic with a stainless steel slot. They too found that ceramic brackets had higher frictional forces, but, they found no significant difference among the other three brackets. Archwire Alloy In the bracket-archwire-ligation triad, the archwire also contributes to friction. It is generally believed that stainless steel and nickel-titanium archwires generate relatively low amounts of friction compared to cobalt-chromium and beta-titanium archwires. Beta-titanium archwires are consistently the highest producer of friction. Nishio et al 11 evaluated the frictional force of ceramic and stainless steel brackets using stainless steel, nickel-titanium and beta-titanium archwires with angulations of zero and ten degrees between the bracket and wire. They found that stainless steel archwires had the lowest frictional force with statistical significance compared with nickel-titanium and beta-titanium archwires. Beta-titanium had greater friction values in all combinations and angulations than both stainless steel and nickel-titanium. Since stainless steel is less flexible than beta-titanium it appears that the elastic properties of the wire were secondary to the surface texture when evaluating frictional force

28 Loftus 3 used the dentoalveolar model previously described to test three different types of 0.019X0.025-inch wires; stainless steel, nickel-titanium and beta-titanium. Betatitanium wires produced higher friction with all four bracket types studied than nickeltitanium. They found a general trend that nickel-titanium archwires produced the least amount of friction followed by stainless steel and beta-titanium in increasing order. Likewise, Smith, Rossouw and Watson 28 in their evaluation of frictional resistance in simulated canine retraction found that stainless steel archwires had greater friction than nickel-titanium. In his study of various bracket-archwire combinations, Defranco 23 also found that stainless steel archwires generated greater friction than nickel-titanium archwires at five, ten, and fifteen degrees, though not at zero degrees. Cacciafesta 5 also compared the frictional forces produced by stainless steel, nickel-titanium and beta-titanium archwires. Using three types of maxillary canine brackets, and archwires of three sizes (0.016-inch, 0.017X0.025-inch, 0.019X0.025-inch), Cacciafesta found that beta-titanium had higher frictional resistance than stainless steel and nickel-titanium archwires. No difference was found between stainless steel and nickel-titanium archwires. All brackets showed higher static and kinetic frictional forces as the wire size increased. Archwire Dimensions As Cacciafesta 5 found, archwire dimension is another important contributor to friction. Friction increases with increased archwire size and rectangular archwires generate more friction than round wires and large diameter wires more than smaller 21

29 diameter. 3,13,18,23,28,29,31 More force is required as the archwire size is increased. This increase in required force is possibly explained by the large amount of friction that is likely created by the increase in area of contact between each wall of the archwire slot and the sides of the archwire. 25 Henao and Kusy 29 found that small and multistrand archwires gave lower frictional values and the values for rectangular wires increased as the height of the archwires increased. Angolkar et al 35 determined the frictional resistance of ceramic brackets compared to stainless steel brackets in combination with wires of different alloys and sizes. Using an Instron machine, he studied both inch and inch slot brackets and tested stainless steel, cobalt-chromium, beta-titanium and nickel-titanium archwires ligated with elastomerics. Friction in ceramic brackets increased as archwire size increased and rectangular archwires produced greater friction than round archwires. In their study, Angolkar et al 35 found that beta-titanium and nickel-titanium archwires were associated with higher friction than stainless steel or cobalt-chromium. This result followed the same general trend with the stainless steel brackets but testing with ceramic brackets resulted in significantly stronger frictional forces being recorded. Beta-titanium and nickel-titanium archwires have greater surface roughness than stainless steel and chromium-cobalt wires. This roughness may account for why there were higher frictional forces recorded with the ceramic brackets. The lower magnitude of friction observed with small cross-sectioned wires may be attributed to the stretch of the elastomerics leading to increased normal force when larger cross sections of wire are tied in. 7,35 22

30 Henao and Kusy 31 evaluated frictional resistance of conventional and self-ligating brackets. Typodont models were created that replicated a patient s oral cavity with misaligned teeth. Four manufactures mounted their own self-ligating and conventional brackets onto the models. The four quadrants were ranked relative to the degree of malocclusion. Nickel-titanium wires were ligated with elastic modules to the conventional brackets. The Instron machine was used in a multi-bracket study to draw inch, 0.016X0.022-inch and 0.019X0.025-inch archwires through the misaligned brackets. For the conventional bracket, the drawing force increased as the archwire size and malocclusion increased. For the self-ligating brackets the drawing force values also increased as the archwire size increased. However, in general, there was a significant difference between conventional and self-ligating brackets. More specifically, for the inch archwire there was a statistically significant difference between the conventional and self-ligating brackets. The presence of rotations, angulations and torque increased the drawing force values as did the inherently smaller interbracket distances of the lower arch. Similar trends were observed from quadrant to quadrant for both the conventional and self-ligating brackets and yet the force values of all selfligating brackets were lower than the force values of the conventional brackets for each inch nickel-titanium wire in every quadrant. This finding confirms that the selfligating brackets were better with the smaller wires that are used early in treatment. For the 0.016X.022-inch and the 0.019X0.025-inch archwires there was no significant difference between the bracket types. Thus Henao and Kusy 31 claim that the bracket types become more comparable with larger wire sizes. The validity of this statement is questionable however, because unlike in these authors study, one would not clinically 23

31 initiate large rectangular wires in a dentition that was misaligned. It can be concluded from this study of simulated in vivo conditions that self-ligating brackets outperformed conventional brackets when used with small archwires and the increase in drawing force corresponded with the increase in malocclusion and this increase is directly related to the combined effects of decreasing archwire clearance and interbracket distances. In a follow-up study Henao and Kusy 29 used three archwires that were recommended by each manufacturer for their corresponding self-ligating brackets. The archwires chosen were meant to represent three stages of orthodontic treatment. They conducted a cross-comparison study that evaluated archwires from their first experiment and coupled them with other self-ligating brackets and the conventional bracket that was ligated with elastomeric ligatures. Predictably, none of the third-stage archwires could engage the brackets in any of the quadrants. But, when the brackets and their suggested archwires were combined two trends emerged. The smallest wires gave the lowest friction and values for the rectangular wires increased as the height of the archwires increased. Also, the results for Stage I and Stage II archwires were significantly different. The cross-comparison experiments suggested that the height of the wires influenced the force. Bracket Width Tests of bracket width produce inconsistent results regarding friction. 13 Some have found narrow brackets to increase friction while others have found the width of the 24

32 bracket to be insignificant and inter-bracket distance to have little effect on frictional resistance. 36 Henao and Kusy 31 found that interbracket distance is inversely related to resistance to sliding. Larger brackets have more friction which may be due to the decreased interbracket distance as suggested by Henao and Kusy 31 or it may be due to the greater normal force exerted by the modules as a result of additional stretching required for engagement as reported by Hain 7. Meling et al 37 suggests that if the clinician wishes to use a wide bracket for mesio-distal control a flexible archwire for leveling and aligning is required. The conclusion reached from their experiment was that even small deflections of thin wires can lead to second-order couples of large magnitudes so that good leveling with the elimination of mesio-distal axial inclinations is important before moving up in archwire size. Elimination or the mesio-distal axial inclinations will reduce large unwanted forces from second-order bending. Surface Modifications Surface modifications can be applied to archwires and brackets. Ion implantation is useful because just about any ion can be implanted without changing the overall dimensions of an appliance. Manufacturers can therefore maintain properties and improve the surface characteristics such as corrosion resistance, coloration and friction. 20 As the titanium content of an alloy increases, there tends to be a greater resistance to sliding. This difference can be seen in the fact that beta-titanium archwires which contain about 80% titanium have a higher coefficient of friction than nickel-titanium 25

33 archwires that are composed of roughly 50% titanium. If the titanium wires could be implanted with ions the surface chemistry could be altered and the coefficient of friction could be reduced

34 FRICTION PRODUCED BY ESTHETIC BRACKETS WITH VARYING LIGATION by CHRISTINE K. ABENOJA, ANDRE FERREIRA, P. LIONEL SADOWSKY, JOHN O. BURGESS AND MARK S. LITAKER In preparation for The Angle Orthodontist Format adapted for thesis 27

35 ABSTRACT Controlling friction is an important aspect of orthodontics because it allows more predictable tooth movement with less anchorage loss. Friction at the bracket/archwire/ligation interface can reduce the force available for desired tooth movement. Initial stages of orthodontic treatment depend on the ability of the archwire to slide freely through the bracket, making friction an important aspect of all orthodontic brackets in the early phases of orthodontic treatment. The purpose of the present in vitro study was to measure the force to move esthetic brackets over an archwire using different ligation methods. A stainless steel model reproducing the maxillary right dentition was used to test brackets in combination with self-ligation, a new non-conventional elastomeric ligature or a conventional elastic ligature. Static and kinetic friction was measured as the force required pulling a inch superelastic nickel titanium archwire through the brackets. The test was first conducted with all the brackets aligned and then with the canine bracket moved apically 3mm. In the aligned configuration mean static friction for the Damon3 MX bracket (Ormco, Glendora, California) was significantly lower than that for the Inspire Ice bracket (Ormco, Glendora, California) with Slide ligation (Leone, Sesto Fiorentino, Firenze, Italy). There were no other significant differences. In the misaligned configuration Clarity SL (3M Unitek, Monrovia, California), Clarity/Slide, and ICE/Slide each generated a modest amount of friction that was 28

36 statistically similar and ranged from g. Each of these three combinations generated significantly less static friction than the Damon3 MX and the ICE/conventional elastic ligature in the misaligned configuration. Additionally, ICE/conventional elastic ligature in the misaligned configuration generated significantly more kinetic friction than the Damon3 MX. It can be concluded that a frictionless ligature can reduce the classical friction associated with esthetic brackets. 29

37 INTRODUCTION Friction is the resistance to motion when one object moves tangentially against another object. The normal force is the perpendicular component of the force acting on the contacting surfaces. The coefficient of friction for a given material surface is a constant and is dependent on the material s characteristics such as texture and hardness of the surface. The frictional force is the product of the coefficient of friction and the normal force. In order for one object to slide against another object the force applied has to overcome the frictional force created. 1,2,3 Static frictional force is the smallest force that is needed to begin movement of two contacting surfaces. Kinetic frictional force is the force needed to resist the sliding of one object over another. 4,5 Static friction is relevant because teeth move in a series of crown tipping/root up righting movement. 4 Because teeth move along an archwire in such a non-continuous manner, static friction must be overcome each time the tooth begins to move. 6,7 Kinetic friction is also important since it is the friction that occurs between the orthodontic bracket and archwire once movement has been started. 4 In theory, if the applied orthodontic force is sufficient to overcome friction in the wire-bracket-ligation interface, and still remain within a hypothetical ideal orthodontic force range, then one should expect optimal tooth movement. 8 Controlling friction is an important aspect of orthodontics because it allows more predictable tooth movement with less anchorage loss. 4,9 It has been estimated that friction can reduce available force by 30

38 almost 40 percent. 10,11 This reduction in available force may be responsible for compromised treatment efficiency and extended treatment times. 4,5,12 Reducing friction between the bracket and archwire is desirable in leveling and aligning, during sliding mechanics used in space closure and in torquing. 10,13 Initial stages of orthodontic treatment depend on the ability of the archwire to slide freely through the bracket, making friction an important aspect of all orthodontic brackets in the early phases of treatment. Reduced friction conserves anchorage by requiring lower net forces which correspondingly leads to lower reciprocal forces. 14 Clinically, friction is not always undesirable and it can be used by the clinician to their advantage. Knowledge of levels of friction generated between brackets and wires will allow the clinician to make adjustments in the appliance choice to apply specific tooth movements. 9 There are many variables that affect friction. These variables include the physical and mechanical properties of the materials involved including the properties of the brackets and archwires, the manner of bracket to archwire ligation, as well as biological factors. 4 The method of ligation used to engage the archwire into the bracket has a profound effect on the force generated and thus the amount of friction produced between the archwire and the bracket. Elastomeric ligatures hug the archwire on either side of the bracket s archwire slot. They do not allow the archwire freedom like a self-ligating bracket. 15 A low-friction ligature has recently been introduced which forms a butterfly shape over the bracket and creates a tube-like structure in which the archwire can slide freely. Slide (Leone 31

39 Orthodontic Products, Sesto Fiorentino, Firenze, Italy) is one such innovative ligature. It is formed with a polyurethane mix by injection molding. The purpose of the present in vitro study was to measure the force to move esthetic brackets over an archwire using various ligation methods. Clinically relevant modes of ligation were compared including self-ligation, frictionless and conventional elastic ligatures. An all-ceramic bracket was compared with a ceramic bracket fitted with a stainless steel slot. A desired outcome was to determine which combination of esthetic bracket and ligature generates the least amount of friction in the simulated initial stage of leveling during orthodontic treatment. MATERIALS AND METHODS A model reproducing the maxillary right dentition was fabricated by Leone Orthodontic Products and modified by the University of Alabama at Birmingham machinists (Figure 1). Five inch pre-adjusted brackets (from central incisor through the second premolar) were aligned using a X0.028 stainless steel archwire (Ormco, Glenora, California). Four brackets were selected for testing; Damon3 MX (Ormco), Clarity SL (3M Unitek, Monrovia, Califormia), Clarity (3M Unitek), and Inspire ICE (Ormco). Damon3 MX and Clarity SL are both self-ligating brackets. Clarity and Inspire ICE were both tested with the frictionless Slide ligature ( Aqua size medium, Leone Orthodontic Products, Sesto Fiorentino, Firenze, Italy) and the Inspire ICE bracket was also tested with a conventional elastic ligature (3M Unitek). There were five groups: 1) Damon3 MX SL, 2) Clarity SL, 3) Clarity/Slide, 4) ICE/Slide, 5) ICE/Conventional elastic ligature. It was intended that the Damon3 MX would act as a 32

40 low-friction standard and the all-ceramic monocrystalline Inspire ICE bracket ligated with a conventional elastic ligature would act as a high-friction control. The brackets were set apart at a clinically relevant 3.7mm 16,17 and bonded to an acrylic block with Plastic Conditioner (Reliance Orthodontic Products, Itasca, Illinois), Assure Bonding Resin (Reliance) and Pad Lock Paste (Reliance). The canine bracket was bonded to a sliding metal bar that was adjusted to emulate an erupting canine with a 3mm gingival offset. A 10.5cm straight inch superelastic nickel titanium wire (Ormco) was held in the brackets by one of three methods: self-ligation, the Slide ligature or a conventional elastic ligature. All ligation was done immediately prior to testing to avoid the force decay of elastomerics. New wires and ligatures were used for each test. Under dry conditions, the Instron universal testing machine measured the friction generated as the wire was pulled through the brackets 15mm at a rate of 15mm/minute. The Instron 5565 (Instron Corp, Canton, Mass) was calibrated and used at room temperature with a load cell of 500N. The test was conducted with all of the brackets aligned initially and then the canine bracket was moved apically 3mm and each test re-done. Each of the 5 groups was tested 20 times for a total of 200 tests. Both static and kinetic friction was measured. Static friction was identified as the maximum initial rise in force and the kinetic friction was determined from averaging the amount of force generated at 2mm, 5mm, and 10mm of wire extension. 33

41 Figure 1. Statistical Analysis The primary statistical analysis method used to compare mean force measurements among the groups was analysis of variance (ANOVA) using bracket/ligation combination as the independent variable of interest, and individual bracket type as a block variable to account for multiple observations made on each bracket. Measurements made under aligned and misaligned conditions differed by orders of magnitude, so separate analyses were conducted for these conditions. The highfriction control group was not included in the ANOVA for the aligned condition for the 34