Bertoncini, C.

Franchi, Lorenzo

Gandini, P.

Massironi, S.

Orsi, L.

English

Florence Research

Angle Orthodontist, Vol 78, No 5, 2008917DOI: 10.2319/090607-420.1Original ArticleIn Vitro Frictional Forces Generated by Three Different Ligation MethodsPaola Gandinia; Linda Orsib; Chiara Bertoncinic; Sarah Massironid; Lorenzo FranchieABSTRACTObjective: To test the hypothesis that there is no difference between the frictional forces producedby a passive self-ligating bracket (SLB) in vitro and a conventional bracket (CB) used with twotypes of elastomeric ligatures.Materials and Method: The brackets, wires and ligation methods used in vitro were a passiveSLB and a CB used with two types of elastomeric ligatures (conventional elastomeric ligature[CEL] and unconventional elastomeric ligatures [UEL]). The bracket ligation systems were testedwith two types of wires (0.014 super elastic nickel titanium wire and 0.019  0.025 stainlesssteel wire). Resistance to sliding of the bracket/wire/ligature systems was measured with an ex-perimental model mounted on the crosshead of an Instron testing machine with a 10 N load cell.Each sample was tested 10 consecutive times under a dry state.Results: Frictional forces close to 0 g were recorded in all tests with SLB and in all tests withUEL on CB with both wire types. Resistance to sliding increased significantly (87–177 g) (P .05) when CEL on CB was used with both wires.Conclusion: UELs may represent a valid alternative to passive SLBs for low-friction biomechan-ics.KEY WORDS: Friction; Self-ligating bracket; Low-friction biomechanics; Elastomeric ligaturesINTRODUCTIONWhen sliding biomechanics are used with fixed ap-pliances, the main force that contrasts tooth move-ment is the frictional force developed by the interactionof the bracket slot and the orthodontic wire. As theefficiency of fixed appliance therapy depends on thefraction of force delivered with respect to the force ap-plied,1 high frictional forces resulting from the interac-a Professor and Head, Department of Orthodontics, The Uni-versity of Pisa, Pisa, Italy.b Research Fellow, Department of Orthodontics, The Univer-sity of Pisa, Pisa, Italy.c Lecturer, Department of Orthodontics, The University ofPisa, Pisa, Italy.d Undergraduate student, School of Dentistry, The Universityof Pavia, Pavia, Italy.e Assistant Professor, Department of Orthodontics, The Uni-versity of Florence, Florence, Italy; Thomas M. Graber VisitingScholar, Department of Orthodontics and Pediatric Dentistry,School of Dentistry, The University of Michigan, Ann Arbor,Michigan.Corresponding author: Lorenzo Franchi, DDS, PhD, Diparti-mento di Odontostomatologia, Universita` degli Studi di Firenze,Via del Ponte di Mezzo, 46-48 50127, Firenze, Italy(e-mail: lorenzo.franchi@unifi.it)Accepted: October 2007. Submitted: September 2007. 2008 by The EH Angle Education and Research Foundation,Inc.tion between the bracket and the guiding archwire af-fect treatment outcomes and duration in a negativeway.2–6 During orthodontic treatment with fixed appli-ances, frictional forces should be kept to a minimumso that lower levels of force can be applied to obtainan optimal biological response for effective toothmovement.7,8Several factors can influence frictional resistance di-rectly or indirectly. Among these factors, features ofarchwire and bracket (in terms of size and material)have been investigated extensively in relation to fric-tion production,9–17 Methods and properties of archwireligation, which have an important role in generatingfriction, have received limited attention in litera-ture.6,18–24 Most investigations6,18–21 have concludedthat elastomeric modules significantly increase resis-tance to sliding compared with stainless steel liga-tures, especially when the latter are tied loosely.Since the 1980s, self-ligating brackets (SLBs) havebecome increasingly popular. These types of bracketsare characterized by the presence of a fourth mobilewall that converts the slot into a tube. SLBs areclaimed to reduce friction levels in a considerable waybecause they simply allow the wire to move freely intothe bracket slot. Several studies25–30 have demonstrat-ed a significant decrease in friction by using these918 GANDINI, ORSI, BERTONCINI, MASSIRONI, FRANCHIAngle Orthodontist, Vol 78, No 5, 2008Figure 1. Unconventional elastomeric ligature.Figure 2. Conventional elastomeric ligature.types of brackets with a reduction in the time neces-sary for single tooth movements.Recently, an innovative unconventional elastomericligature (Slide, Leone Orthodontic Products, SestoFiorentino, Firenze, Italy) has been introduced into themarket. Once applied on conventional brackets (CBs)this ligature is completely passive, like the labial coverof passive SLBs; thus, it guarantees the same freedomof sliding to the wire.31 Previous in vitro studies32,33have shown that this unconventional elastomeric lig-ature (UEL) is able to reduce frictional forces with re-spect to conventional elastomeric ligatures (CEL) bothduring leveling and aligning and during sliding me-chanics.The aim of the present in vitro study was to comparethe frictional forces produced by a passive SLB andtwo types of elastomeric ligatures (UEL and CEL) ona CB used with two types of wires (0.014 super elasticnickel titanium [SE NiTi] wire and 0.019  0.025stainless steel [SS] wire) in the dry state.MATERIALS AND METHODSIn this in vitro study, two types of upper central in-cisor brackets were used, each incorporating 17torque and 4 angulation: a SS CB with 0.022 0.030 nominal slot dimensions (STEP, Leone Ortho-dontic Products) and a passive SLB with 0.022 0.0275 nominal slot dimension (SmartClip, 3M Unitek,Monrovia, Calif). Two types of orthodontic wires weretested: SE NiTi and SS wires with a nominal cross-section of 0.014 and 0.019  0.025, respectively(Leone Orthodontic Products). These wire dimensionswere chosen because round wires of small size arerecommended during the aligning and leveling phaseof orthodontic treatment while rectangular wires oflarger size are required during the final phase of treat-ment when a remarkable torque control is necessary.The wires were ligated into the slots of CBs usingeither CELs (silver medium mini modules with insidediameter of 1.3 mm, outside diameter of 3.1 mm, andthickness of 0.9 mm, Leone Orthodontic Products) orUELs (silver medium Slide ligatures, Leone Orthodon-tic Products) (Figures 1 through 3).Resistance to sliding produced by the differentbracket/wire/ligature combinations were measured us-ing a frictional testing device that was set on the cross-head of a testing machine (Instrom 4301, Canton, MA)with a load cell of 10 N. The experimental model con-sisted of— the bracket welded to a little steel bar;— the orthodontic wire, along which the bracket couldslide, clamped to a custom-made steel support;— the ligation method, consisting of CEL or UEL forthe CB and two lateral clips for the SLB.This apparatus was secured to a steel support es-pecially designed for this study, and the lower part ofthe support was locked to the lower fixed clamp of thetesting machine (Figure 4).Two little holes were present in the upper and lowerpart of the steel support. These holes allowed the wireto cross through and, once entered, it was held inplace by a simple system of screws. The base of thebracket was welded to a steel bar that was secured tothe upper movable clamp attached to the load cell.Care was taken to weld each bracket in a position sothat the slot was perfectly passive with respect to astraight section of 0.0215  0.025 SS wire mountedon the steel support.This device allowed the bracket to move along thewire as an axial tensile force was applied by the In-stron’s load cell with a crosshead speed of 6 mm/min.In the meantime, a computer connected to the testingmachine displayed a graph showing peak force vari-ation. Each of the six bracket/wire/ligation combina-tions was tested 10 times, with new elastomeric liga-tures on each trial, to minimize the influence of elasticdeformation. For every traction test over a distance of919FRICTION OF THREE LIGATION METHODSAngle Orthodontist, Vol 78, No 5, 2008Figure 3. Passive self-ligating bracket.Figure 4. Friction testing apparatus.12 mm at a speed of 6 mm/min the following frictionalforces were recorded: the maximum force needed tomove the bracket along the wire (static friction) andthe mean frictional force registered at 5 mm, at 7 mm,and at 9 mm of movement (kinetic friction). All mea-surements were performed under dry conditions atroom temperature of 20  2C.Statistical analysisData were tested for normal distribution (Shapiro-Wilks test). As normal distribution could not be as-sessed for all frictional forces recorded in the differentbracket/wire/ligation combinations, descriptive statis-tics for nonparametric tests were calculated. Differenc-es between frictional forces produced by the differentbracket/wire/ligation combinations were compared us-ing Kruskal-Wallis one-way analysis of variance onranks followed by Tukey’s post hoc test (P  .05)(SigmaStat 3.1, Systat Software, Point Richmond,Calif).RESULTSDescriptive statistics and statistical comparisons ofthe frictional forces recorded in the different bracket/wire/ligation combinations are reported in Table 1. Nostatistically significant difference was found betweenthe frictional forces produced by SLB and by UEL onCB when used with 0.014 SE NiTi wire and with0.019  0.025 SS wire. All of these values wereclose to 0 g (mean values ranging from 0.1 g to 1.2g). CEL on CB coupled with both types of wires gen-erated significantly greater static and kinetic frictionalforces with respect both to SLB and to UEL on CB(mean values ranging from 86.7 g to 177.4 g).DISCUSSIONThe present study compared the friction generatedby a passive SLB with the frictional forces producedby an innovative type of UEL on CB and by CEL onCB. The results of the present investigation indicatedthat both SLB and UEL on CB produced significantlylower frictional forces compared with CEL on CB whencoupled both with .014 SE NiTi wire and with 0.019 0.025 SS wire. These results fully agree with thoseof previous studies22,24–28,30 that found that passiveSLBs generated smaller frictional forces than conven-tional ligatures on CBs. The significant differences be-tween SLB and CEL on CB in the current study arevery similar to those reported by Thomas et al26 andHain et al24 who used a single-bracket experimentalmodel.Recently, an innovative UEL, manufactured with aspecial polyurethane mix by injection molding (Slide),was introduced. Once the ligature is applied on thebracket it simulates the labial cover of a passive self-ligating bracket, thus transforming the slot into a tubethat allows the archwire to slide freely. The results ofthe present study confirm previous findings by Baccettiand Franchi33 who reported significantly lower levels offriction for CB with UEL compared with CB with CEL920 GANDINI, ORSI, BERTONCINI, MASSIRONI, FRANCHIAngle Orthodontist, Vol 78, No 5, 2008Table 1. Descriptive Statistics and Comparisons (ANOVA on ranks with Tukey’s post hoc test) of Frictional Forces (g)SLB (1)Mean Median SD Range Min MaxSF 0.014 NiTi 0.4 0.4 0.1 0.3 0.2 0.5KF 0.014 NiTi 0.1 0.1 0.1 0.2 0.0 0.2SF 0.019  0.025 SS 1.0 0.9 0.4 1.2 0.6 1.8KF 0.019  0.025 SS 0.7 0.6 0.2 0.6 0.4 1.0LFEL on CB (2)Mean Median SD Range Min MaxSF 0.014 NiTi 0.5 0.5 0.1 0.4 0.3 0.7KF 0.014 NiTi 0.1 0.1 0.1 0.1 0.0 0.1SF 0.019  0.025 SS 1.2 0.9 0.9 3.0 0.6 3.6KF 0.019  0.025 SS 0.5 0.5 0.1 0.5 0.2 0.7CEL on CB (3)Mean Median SD Range Min MaxSF 0.014 NiTi 119.2 92.8 81.6 280.7 63.5 344.3KF 0.014 NiTi 86.7 85.3 24.1 75.6 57.6 133.2SF 0.019  0.025 SS 177.4 171.9 17.4 55.4 161.1 216.5KF 0.019  0.025 SS 155.6 153.3 18.6 57.1 135.5 192.6Statistical Comparisons1 vs 2q Sig1 vs 3q Sig2 vs 3q SigSF 0.014 NiTi 1.132 NS 7.035 * 5.903 *KF 0.014 NiTi 0.462 NS 6.102 * 6.564 *SF 0.019  0.025 SS 0.063 NS 4.427 * 4.364 *KF 0.019  0.025 SS 0.733 NS 4.165 * 4.898 *SLB indicates self-ligating bracket; UEL, unconventional elastomeric ligature; CB, conventional bracket CEL, conventional elastomeric liga-ture; SF, static friction; KF, kinetic friction; q, studentized range statistic; Sig, significance; NS, not significant.* P  .05.during sliding mechanics with 0.014 SE NiTi wire and0.019  0.025 SS wire.Based on the results of the present study, UELs areable to produce significantly lower levels of frictionalforces than CEL when applied on CB; thus, UELs mayrepresent a valid alternative to passive self-ligatingbrackets for low-friction biomechanics. One of the clin-ical advantages that arises from the use of UELs isthat they can be placed on every type of CBs withconsiderable cost reduction compared with SLBs. An-other advantage is that the clinician can apply frictionand low-friction mechanics simultaneously on thesame archwire by using CEL and UEL only in partic-ular segments. For example, during en masse spaceclosure on a rectangular stainless steel archwire,UELs can be used in the posterior segments to reducefriction, while CELs are used in the anterior segmentto maximize torque expression and control.The clinical interpretation of these experimentaldata, however, requires further considerations thatmodulate the findings. It should be stressed that invitro studies cannot reproduce exactly what occurs invivo in the oral cavity during orthodontic tooth move-ment. Minimal adjustments at the bracket/wire/ligaturesystem may significantly change frictional resistancebecause of physiologic oral functions as well as theoral tissues or food contacting the orthodontic appli-ance. UELs may represent a valid alternative to pas-sive SLBs for low-friction biomechanics.CONCLUSIONS• SLB and UEL on CB are able to produce significantlylower frictional forces compared with CEL on CBwhen coupled with .014 SE NiTi wire and with0.019  0.025 SS wire.REFERENCES1. Kusy RP, Whitley JQ. Friction between different wire-brack-et configurations and materials. Semin Orthod. 1997;3:166–177.2. Rossouw PE. Friction: an overview. Semin Orthod. 2003;9:218–222.3. Drescher D, Bourauel C, Schumacher HA. Frictional forcesbetween bracket and arch wire. Am J Orthod DentofacialOrthop. 1989;96:397–404.4. Kapila S, Angolkar PD, Duncanson MG Jr, Nanda RS. 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In vitro frictional forces generated by three different ligation methods

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In vitro frictional forces generated by three different ligation methods

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