ORIGINAL ARTICLE

Friction between various self-ligating bracketsand archwire couples during sliding mechanics

Sennay Stefanos,a Antonino G. Secchi,b Guy Coby,c Nipul Tanna,a and Francis K. Manted

Philadelphia, Pa

Introduction: The aim of this study was to evaluate the frictional resistance between active and passive self-ligating brackets and 0.019 3 0.025-in stainless steel archwire during sliding mechanics by using anorthodontic sliding simulation device. Methods: Maxillary right first premolar active self-ligating bracketsIn-Ovation R, In-Ovation C (both, GAC International, Bohemia, NY), and SPEED (Strite Industries,Cambridge, Ontario, Canada), and passive self-ligating brackets SmartClip (3M Unitek, Monrovia, Calif),Synergy R (Rocky Mountain Orthodontics, Denver, Colo), and Damon 3mx (Ormco, Orange, Calif) with0.022-in slots were used. Frictional force was measured by using an orthodontic sliding simulation deviceattached to a universal testing machine. Each bracket-archwire combination was tested 30 times at0� angulation relative to the sliding direction. Statistical comparisons were performed with 1-way analysisof variance (ANOVA) followed by Dunn multiple comparisons. The level of statistical significance was set atP \0.05. Results: The Damon 3mx brackets had significantly the lowest mean static frictional force (8.6 g).The highest mean static frictional force was shown by the SPEED brackets (83.1 g). The other bracketswere ranked as follows, from highest to lowest, In-Ovation R, In-Ovation C, SmartClip, and Synergy R. Themean static frictional forces were all statistically different. The ranking of the kinetic frictional forces ofbracket-archwire combinations was the same as that for static frictional forces. All bracket-archwirecombinations showed significantly different kinetic frictional forces except SmartClip and In-Ovation C,which were not significantly different from each other. Conclusions: Passive self-ligating brackets havelower static and kinetic frictional resistance than do active self-ligating brackets with 0.019 3 0.025-instainless steel wire. (Am J Orthod Dentofacial Orthop 2010;138:463-7)

There has been increased use of self-ligatingbrackets in recent years, and several claimshave been made regarding their performance

compared with conventional edgewise brackets.1

Recent manufacturing modifications of brackets aimedat reducing friction between the archwire and thebracket slot are based on improved surface quality,slot geometry modifications, varied bracket width, andintegrated ligation systems.1 It has been reported thatfriction is determined largely by the nature of theligation.2,3 Therefore, various ligation systems havebeen introduced with the edgewise bracket to secure

From the School of Dental Medicine, University of Pennsylvania, Philadelphia.aPostgraduate student, Department of Orthodontics.bAssistant professor, Department of Orthodontics.cClinical assistant professor, Department of Orthodontics.dAssociate professor, Department of Restorative and Preventive Sciences.

The first author worked on this research while a resident at the Department of

Orthodontics of the University of Pennsylvania. The views expressed in this

article are those of the author and do not reflect the official policy or position

of the Department of the Navy, the Department of Defense, or the United States

government.

Reprint requests to: Antonino G. Secchi, 240 S 40th St, Evans Building, room

E-9, Philadelphia, PA 19104-6030; e-mail, antonino@dental.upenn.edu.

Submitted, August 2008; revised and accepted, November 2008.

0889-5406/$36.00

Copyright � 2010 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2008.11.029

the wire in the bracket. Some self-ligating bracketshave a spring clip that presses against the archwire,such as In-Ovation R and In-Ovation C (GAC Interna-tional, Bohemia, NY) and SPEED (Strite Industries,Cambridge, Ontario, Canada). These brackets havebeen described as active self-ligating because theremight be a ligation force during sliding mechanics.4

Other self-ligating brackets such as Damon 3mx(Ormco, Orange, Calif), SmartClip (3M Unitek,Monrovia, Calif),5,6 and Synergy R (Rocky MountainOrthodontics, Denver, Colo) have a clip-gate that doesnot press against the wire during sliding mechanics.7

These are described as passive self-ligating brackets.Self-ligating brackets have been reported to have

lower frictional forces during sliding mechanics andrequire less chair-side assistance.4,8 They are generallysmoother, more comfortable for the patient, and easierto clean.9 According to Thorstenson and Kusy,10,11 thelower friction of self-ligating brackets might be partlyexplained by a greater critical contact angle with thewire. There are conflicting claims of superior perfor-mance regarding friction for both passive and activeself-ligating brackets.8,12,13 This conflict might be dueto the wide variations in data reported in previousstudies. Some studies show wide ranges between

463

Fig 1. The orthodontic sliding simulation device used inthis study.

Fig 2. Force vs displacement plot for static and kineticfriction.

464 Stefanos et al American Journal of Orthodontics and Dentofacial Orthopedics

October 2010

minimum and maximum values,2,12,14 and others havereported mean frictional force values with highstandard deviations, making it difficult to determinewhether there are significant differences betweenbrackets.9,15,16

In orthodontics, research efforts to understand thefactors that influence frictional resistance when consid-ering sliding mechanics have been focused on bracketwidth, archwire material,17-20 archwire size, second-orderangulation,18,20-23 ligation type and technique,2,6-8,13-17

effect of saliva,23,24 and interbracket distance. Thesefactors are critical when considering the clinicalapplication of sliding mechanics.15

According to Pizzoni et al,25 experimental setups todetermine the effect of the above-mentioned factors onfriction can be divided into 4 main groups: (1) archwiressliding through contact flats; (2) archwires sliding throughbrackets parallel to the brackets slot; (3) archwires slidingthrough brackets with different second- and third-orderangulations; and (4) brackets submitted to a force witha certain degree of tipping allowed. The experimentaldesign of this study provided for archwires slidingthrough brackets parallel to the bracket slot; it falls intothe second group.

Our hypothesis was that there is no significantdifference in the resistance to sliding of a 0.019 3

0.025-in stainless steel archwire between active andpassive self-ligating brackets. The purpose of this studywas to compare the static and kinetic frictional forcesgenerated between active and passive self-ligatingbrackets by using a sliding simulator coupled witha 10-N load cell.

MATERIAL AND METHODS

An orthodontic sliding simulation device modifiedfrom that reported by Articolo and Kusy19 and

Articolo26 was used to simulate the clinical use of ortho-dontic brackets (Fig 1). The simulation device consistedof a special fixture mounted to the base of a mechanicaltesting machine (model 4206, Instron, Canton, Mass).The fixture held a bracket slot that allowed for reproduc-ible bracket positioning and was attached to an angula-tion dial. A test archwire was suspended from a colletconnected to the force transducer and the transversebeam of the testing machine. The orthodontic devicewas modified for self-ligating brackets by removing0.010 in of ligature wire attached to a piston assemblyto transmit a normal force. The weight of the colletholding the archwire was reduced to minimize noisein data collection.

The brackets were cemented onto the simulation de-vice bracket slot and initially set to be passive (0�) in thesecond order via the adjustable angulation dial. Alltested brackets had –7� of torque and 0� of second-order angulations, with the exception of Damon 3mx,which has 2� of distal offset. The 2� distal offset ofthe Damon 3mx bracket was compensated for by usingthe angulation dial. All angulations and torque values ofthe brackets remained fixed during data collection.During testing, the transverse beam with the colletholding a 0.019 3 0.025-in stainless steel archwirewas lifted up to draw the archwire through the bracket.The drawing force was monitored by the machine’s loadcell (10 N) and transmitted to computer software(version 2.0, Measure, National Instruments, Austin,Tex) for plotting drawing force vs distance charts.

Maxillary right first premolar brackets with 0.022-inslots and prescriptions as described above were used.Each archwire-bracket couple was cleaned with 95%ethanol and compressed air just before evaluation. Alltesting was done in the dry state in prevailing air at21�C. Each test consisted of 1 bracket and 1 archwireat 0� angulation. Two examiners (N.T. and S.S.) verifiedproper mounting of the brackets under 10-times magni-fication. The static frictional force was measured as theinitial rise or peak force required to initiate movement

Table I. Descriptive statistics and statistical compari-sons of static frictional forces

BracketMean

(g) SDMedian

(g)Minimum

(g)Maximum

(g)

Damon 3mx 8.6 0.4 8.6 7.9 9.3

In-Ovation R 38.1 1.6 38.2 34.2 41.6

In-Ovation C 33.4 1.2 33.3 30.3 35.3

SmartClip 30.3 2.3 30.3 26.8 36.0

SPEED 83.1 2.5 82.6 79.3 89.3

Synergy R 23.8 1.5 23.8 21.4 27.8

All mean static friction values are significantly different from each

other (P \0.05).

Table II. Descriptive statistics and statistical compari-sons of kinetic frictional forces

BracketMean

(g) SDMedian

(g)Minimum

(g)Maximum

(g)

Damon 3mx 6.0 0.9 5.8 5.1 7.4

In-Ovation R 34.1 2.1 34.1 31.4 36.4

In-Ovation C 28.8a 1.4 28.8 26.6 30.1

SmartClip 30.1a 1.3 30.1 28.5 31.6

SPEED 81.7 2.9 81.7 77.8 85.5

Synergy R 21.8 3.2 21.8 16.9 25.2

Mean kinetic friction values with the same superscript letter are not

significantly different (P \0.05).

American Journal of Orthodontics and Dentofacial Orthopedics Stefanos et al 465Volume 138, Number 4

of the wire through the bracket (Fig 2). The peak forcewas halved and defined as static frictional force.27 Anew bracket-archwire combination was used for eachtest. Each test was performed 30 times.

The drawing force required to maintain movementbeyond the point of initial displacement was averagedand then halved and recorded as the kinetic frictionalforce. The archwire was drawn through the bracketa distance of 20 mm at a speed of 1 cm per minute fordetermination of kinetic friction. Data were obtainedat a rate of 5 scans per second for 2 minutes.

The 1-way analysis of variance (ANOVA) and theDunn multiple comparison tests were performed withstatistical software (version 3.5, SigmaStat, Systat Soft-ware, Point Richmond, Calif). The level of statisticalsignificance was set at P \0.05.

RESULTS

A plot of force vs displacement obtained duringfriction testing is shown in Figure 2. The region fordetermining static friction was designated ‘‘staticfriction,’’ and regions for determination of kineticfriction were called ‘‘kinetic friction.’’ Table I showsthe results of the mean static frictional forces for thebrackets investigated. Statistical analysis showed thatpassive self-ligating brackets ranked as follows fromlowest to highest mean static friction: Damon3mx(8.6 g), Synergy R (23.8 g), and SmartClip (30.3 g).The static friction of the passive self-ligating bracketswas significantly different from each other. Amongthe active self-ligating brackets, the ranking from lowestto highest static friction was In-Ovation C (33.4 g),In-Ovation R (38.1 g), and SPEED (83.1g). The activeself-ligating brackets showed significantly higher staticfrictional forces than the passive self-ligating brackets.

The kinetic frictional force values are shown inTable II. The ranking of the kinetic frictional forces ofpassive and active self-ligating brackets to archwire com-binations was the same as that for the static frictional

forces. Kinetic friction values for all bracket-archwirecombinations were significantly different, except forSmartClip and In-Ovation C.

DISCUSSION

These results show significant differences in bothstatic and kinetic frictional forces among passive andactive self-ligating brackets with a 0.019 3 0.025-instainless steel archwire. The exception to this findingwas that kinetic frictional forces associated with Smart-Clip, a passive self-ligating bracket, were not statisti-cally different from those of In-Ovation C, an activeself-ligating bracket. Among the passive self-ligatingbrackets investigated, Damon3mx had the lowest andSmartClip the highest static and kinetic frictionalforces. The results further showed significant differ-ences between active self-ligating brackets, withSPEED having the highest static and kinetic frictionalforces. Our findings agree with those of previousstudies, that passive self-ligating brackets generatelower static and kinetic frictional forces than do activeself-ligating brackets.9,25,28

The similarity between the kinetic frictional forcesgenerated by SmartClip and In-Ovation C is an interest-ing result. SmartClip has been described as a pro-grammed nickel-titanium clip that releases the wire ifthe force of ligation needed to keep the wire in theslot exceeds a certain limit.5,29 This phenomenon wasnot observed in this study. The classification ofSmartClip as a passive self-ligating bracket mighthold true when the archwire size is smaller than 0.0193 0.025 in.5 At higher archwire sizes, some ligationforce might be exerted.12 Based on these results, it isquestionable to classify SmartClip as a passive self-ligating bracket for all archwire sizes. The relativelylow static and kinetic frictional force values observedfor In-Ovation C (an active self-ligating bracket) mightbe explained by the rhodium coating on the clip, accord-ing to the manufacturer’s information. However, the

466 Stefanos et al American Journal of Orthodontics and Dentofacial Orthopedics

October 2010

stability of the coating on In-Ovation C brackets duringtreatment is unknown.

The simulation device coupled with a 10-N load cellprovided friction values with relatively low standard de-viations and a low spread of values between minimumand maximum friction (Tables I and II) comparedwith other studies.2,9,12,14,16

The data obtained in this laboratory study do not de-scribe the complete complex clinical situation. Since theligation force of self-ligating brackets is predictable andindependent of force decay, the comparative dataobtained are useful for guiding the selection of wire-bracket combinations for sliding mechanics.1 TheDamon3mx bracket showed significantly the lowestfrictional force compared with the other brackets tested.This bracket will be expected to show less friction insliding mechanics with rectangular 0.019 3 0.025-instainless steel wire. Conversely, it might be difficult tofully express the bracket’s prescription if used with0.019 3 0.025-in stainless steel as a finishing wire.The SPEED bracket generated the highest static andkinetic frictional forces. SPEED brackets could morefavorably express the bracket prescription if used with0.019 3 0.025-in stainless steel wire. On the contrary,SPEED brackets might not favor sliding mechanics.

An important aspect to consider when evaluatingbracket design is the normal (perpendicular) force ofligation.2,3 In most studies, the frictional force decreasesas the normal ligation force is minimized.11,14,15

Different methods of ligation that have beenintroduced with edgewise brackets have resulted invarying normal forces and their correspondingfrictional forces. Frictional forces are important tostudy because a large, variable percentage of the forceapplied by the orthodontist is lost to overcome frictioninstead of moving teeth.26,30 Information about thefriction of orthodontic brackets and archwire systemsis important for improving the effectiveness oforthodontic treatment.

The selection of brackets should be based on the de-sired clinical outcome. Low frictional forces might bedesired during leveling and aligning but could be inap-propriate for expressing the torque in the bracket orachieving other objectives of finishing and detailing.Likewise, high frictional forces might be desired for ex-pressing torque in the bracket or finishing and detailingbut be inappropriate for the leveling and aligning stagesof treatment.

CONCLUSIONS

This study confirmed that passive self-ligatingbrackets have lower static and kinetic frictional forces

compared with active self-ligating brackets when cou-pled with 0.019 3 0.025-in stainless steel wire. TheDamon3mx bracket has significantly the lowest staticand kinetic frictional forces, and SPEED has the highestfrictional force of the brackets investigated. The simula-tion device and low threshold load cell provided datawith lower measurement variations when comparingdifferences in friction of sliding.

We thank 3M Unitek, GAC International, StriteIndustries, Ormco, and Rocky Mountain Orthodonticsfor providing the materials tested in this study; RobertL. Vanarsdall, Jr, Department of Orthodontics, Schoolof Dental Medicine, University of Pennsylvania, for sup-port and advice in designing the research; Alex Radin,Department of Materials Science and Engineering,School of Engineering, University of Pennsylvania, andLaurence Articolo, orthodontist of Blackwood, NJ fortheir technical expertise in the construction of the slidingsimulation device.

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