SHORT COMMUNICATION

Forces released during sliding mechanicswith passive self-ligating brackets ornonconventional elastomeric ligaturesLorenzo Franchi,a Tiziano Baccetti,a Matteo Camporesi,b and Ersilia Barbatoc

Florence and Rome, Italy

Introduction: The aim of this study was to evaluate the frictional forces generated by 4 types of passivestainless steel self-ligating brackets (SLBs) and by nonconventional elastomeric ligatures (NCEL) andconventional elastomeric ligatures (CEL) during sliding mechanics. Methods: An experimental modelconsisting of 5 aligned stainless steel 0.022-in brackets was used to assess frictional forces produced by theSLBs, NCEL, and CEL with a 0.019 � 0.025-in stainless steel wire. Results: Significantly smaller static andkinetic forces were generated by the SLBs and NCEL (�2 g) compared with the CEL (�500 g). No significantdifferences were found within the different types of SLBs, or between these and the NCEL. Conclusions:SLBs and NCEL are valid alternatives for low friction during sliding mechanics. (Am J Orthod Dentofacial

Orthop 2008;133:87-90)

Friction is determined mainly by the nature ofligation.1 In an in-vivo study, Iwasaki et al2

confirmed that, during sliding mechanics, 30%to 50% of the total friction force generated by apremolar bracket traveling along a .019 � .025-instainless steel archwire is due to the friction of theligation. Various methods, therefore, have been proposedto reduce the friction of ligation, such as loosely tiedstainless steel ligatures,3 self-ligating brackets (SLBs),4-7

and unconventional ligature systems.8-10 Stainless steelligatures produce variable ligation forces and are time-consuming to place.2

SLBs are ligatureless bracket systems that have amechanical device built into the bracket to close off theslot. From the patient’s perspective, SLBs are generallysmoother, more comfortable, and easier to clean be-cause of the absence of wire ligature; reduced chairtime is another significant advantage.11 In recent years,various SLBs have been developed: those that have aspring clip that presses against the archwire (“active”or “interactive” SLBs), such as SPEED (Strite Indus-

aAssistant professor, Department of Orthodontics, University of Florence, Flo-rence, Italy; Thomas M. Graber Visiting Scholar, Department of Orthodontics andPediatric Dentistry, School of Dentistry, University of Michigan, Ann Arbor.bResearch associate, Department of Orthodontics, University of Florence,Florence, Italy.cProfessor and head, Department of Orthodontics, University of Rome “LaSapienza,” Italy.Reprint requests to: Lorenzo Franchi, Dipartimento di Odontostomatologia,Università degli Studi di Firenze, Via del Ponte di Mezzo, 46-48, 50127,Firenze, Italy; e-mail, l.franchi@odonto.unifi.it.Submitted, June 2007; revised and accepted, August 2007.0889-5406/$34.00Copyright © 2008 by the American Association of Orthodontists.

doi:10.1016/j.ajodo.2007.08.011

tries, Cambridge, Ontario, Canada), In-Ovation (GACInternational, Bohemia, NY), Quick (Forestadent USA,St Louis, Mo), and Time2 brackets (American Orth-odontics, Sheboygan, Wis); and those in which theself-ligating clip does not press against the wire (“pas-sive” SLBs), such as Damon (SDS Ormco, Orange,Calif), SmartClip, (3M Unitek, Monrovia, Calif), Car-riere (Ortho Organizers, Carlsbad, Calif), and Opal(Ultradent Products, South Jordan, Utah). Passive SLBshave shown consistently less friction during slidingmechanics than active SLBs, with the exception ofundersized round archwires.4,5,7 Significant reductionin friction has been reported also for nonconventionalelastomeric ligatures (NCEL) (Slide; Leone Orthodon-tic Products, Sesto Fiorentino, Italy) on conventionalbrackets when compared with conventional elastomericligatures (CEL).10

The aim of this study was to evaluate the frictionalforces generated by 4 types of passive stainless steelSLBs and by NCEL when compared with CEL duringsliding mechanics.

MATERIAL AND METHODS

An experimental model reproducing the right buc-cal segment of the maxillary arch was used to assess thefrictional forces produced by 4 types of passive SLBs(Damon 3 MX, SDS Ormco; SmartClip, 3M Unitek;Carriere, Ortho Organizers; and Opal-M, UltradentProducts), by NCEL (Slide; Leone Orthodontic Prod-ucts) on conventional stainless steel brackets (STEPbrackets; Leone Orthodontic Products), and by CEL

(silver mini modules; Leone Orthodontic Products) on

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88 Franchi et al

the same type of conventional stainless steel brackets.The buccal segment model consisted of 5 brackets forthe second premolar, the first premolar, the canine, thelateral incisor, and the central incisor. A section of0.0215 � 0.028-in stainless steel wire was used to alignthe brackets before fixing them with cyanoacrylate glueonto an acrylic block (Fig, A). The interbracket distancewas set at 8.5 mm.

An 18-cm-long 0.019 � 0.025-in stainless steelwire was tested. The wire was secured into the bracketsby using the self-ligating systems, the NCEL, or theCEL. The frictional forces generated by the 0.019 �0.025-in stainless steel wire with the 2 types of ligationsystems were recorded by sliding the wire into thealigned brackets. The friction generated by the testingunit consisting of wire, brackets, and ligation systemswas measured under dry conditions and at room tem-perature (20°C � 2°C) with a testing machine (model4301; Instron, Canton, Mass) with a load cell of 10 N

Fig. Experimental model and friction testing apparatus:A, in-vitro model of right buccal segment of maxillaryarch; B, the test wire was ligated into the experimentalmodel clamped to the machine’s crosshead.

(Fig, B). The testing machine had been calibrated by the

Instron Calibration Laboratory in terms of crossheaddisplacement/speed and load cell. The test wire wasinserted into the testing unit, and its bottom end wasclamped by a vise and mounted on the machine’scrosshead. The elastomeric ligatures were placed im-mediately before each test run to avoid ligature forcedecay. Frictional forces produced were tested 30 timeswith new wires on each occasion.

A total of 180 tests (30 tests for each of the 6 typesof ligation systems) were carried out. Static and kineticfriction forces were recorded while 15 mm of wire weredrawn through the brackets at a speed of 15 mm perminute. Static friction was defined as the force neededto start the wire moving through the bracket assembly.This force was measured as the maximal initial rise onthe machine’s chart trace. Recommendations for usagewith the testing machine strongly suggested that, intests measuring kinetic friction, this must be evaluatedas an average of the frictional forces appraised atsubsequent time periods during displacement.12 For thepurpose of this study, measurements of kinetic frictionwere performed at 2, 5, and 10 mm of displacement andthen averaged.

Statistical analysis

Descriptive statistics, including means, medians,standard deviations, and minimum and maximum val-ues were calculated for the static and kinetic frictionalforces produced by the various ligation systems withthe 0.019 � 0.025-in stainless steel wire. Becausenormal distribution of the data was not found (Shapiro-Wilks test), the comparisons between the ligation sys-tems were carried out with analysis of variance(ANOVA) on ranks with the Tukey post-hoc test(P �.05) (SigmaStat 3.1; Systat Software, Point Rich-mond, Calif).

RESULTS

The descriptive statistics and the comparisons ofstatic and kinetic frictional forces for the ligationsystems are shown in Tables I and II. The statisticaltests showed significantly smaller static and kineticforces generated by the SLBs and the NCEL whencompared with the CEL. No significant differenceswere found among the different SLBs, or between theseand the NCEL.

The average amount of both static and kineticfriction was minimal (�2 g) in the SLB and NCELgroups with aligned brackets with 0.019 � 0.025-instainless steel wire, whereas it was greater than 500 g

with CEL.

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Franchi et al 89

DISCUSSION

Clinical evidence of the beneficial effects of low-friction archwire ligatures on the biomechanical char-acteristics of orthodontic treatment can be derivedfrom either passive SLBs4,5 o r NCEL on conventionalbrackets,10 in which the archwire is not compressedinto the slot by a ligation structure. Our aim in thisstudy was to compare the friction generated by theseinnovative ligation systems (SLBs and NCEL) with thefriction produced by CEL. The research was tailored totest the friction during the fundamental therapeuticphase of sliding mechanics on aligned brackets with arectangular archwire. We used a device specificallydesigned and manufactured to simulate the clinicalconditions of a dental arch section to study static andkinetic attritions.10

Both the SLBs and the NCEL showed levels offriction that were significantly lower than those pro-duced by CEL during sliding mechanics with a 0.019 �0.025-in rectangular wire. The amounts of static andkinetic frictional forces exerted by the SLBs and theNCEL were minimal (�2 g) when compared with theCEL that exhibited more than 500 g of force on averagefor both frictional force types. The significant differ-ences between SLBs and NCEL vs CEL are similar tothose reported by Pizzoni et al4 and Thomas et al.5

However, both studies used a single-bracket experi-

Table I. Descriptive statistics and statistical comparison

Mean Medi

Damon 3 MX (1) 1.2 1.SmartClip (2) 1.8 1.Opal-M (3) 1.3 1.Carriere (4) 1.3 1.STEP w/slide (5) 1.3 1.STEP w/conventional elastastic ligature (6) 590.7 587.

*P �.05.

Table II. Descriptive statistics and statistical compariso

Mean Medi

Damon 3 MX (1) 0.6 0.SmartClip (2) 1.0 0.Opal-M (3) 0.9 0.Carriere (4) 0.6 0.STEP w/slide (5) 0.7 0.STEP w/conventional elastastic ligature (6) 541.6 538.

*P �.05.

mental model, whereas we attempted to reproduce the

clinical condition of 5 aligned brackets in a dental archsegment.

The data concerning the elastomeric ligatures, how-ever, should be considered with caution. Each test with themachine was performed with new elastomeric ligatures.We made no attempt to evaluate the effects of time andoral environment on the amount of force released withdifferent types of elastomeric ligatures.13 Frictional resis-tance is reduced after presoaking elastomeric modules insaliva for 1 week.14

This investigation demonstrated clearly that mini-mal amounts of friction are generated with 4 types ofpassive SLBs that are commercially available. Theliterature reports values of frictional forces for activeSLBs that are 5 times greater than passive SLBs.10

Based on the results of this study, NCEL also producesignificantly lower levels of frictional forces than CEL,so that NCEL might be a valid alternative to passiveSLBs during sliding mechanics.

CONCLUSIONS

In this study, we found that both passive SLBs andNCEL produce significantly smaller frictional forces(�2 g) than CEL (�500 g).

We thank 3M Unitek, Ortho Organizers, LeoneOrthodontic Products, and Ultradent Products for sup-

tatic frictional forces (g)

scriptive statisticsSignificant statistical

comparisons*SD Minimum Maximum

0.4 0.5 3.00.8 0.6 3.3 1 vs 60.3 0.7 1.9 2 vs 60.7 0.4 2.8 3 vs 60.7 0.6 3.3 4 vs 6

36.7 529.1 656.2 5 vs 6

kinetic frictional forces (g)

scriptive statisticsSignificant statistical

comparisons*SD Minimum Maximum

0.3 0.1 1.20.6 0.2 2.3 1 vs 60.3 0.3 1.5 2 vs 60.4 0.1 1.6 3 vs 60.3 0.1 1.5 4 vs 6

40.2 491.4 631.6 5 vs 6

s of s

De

an

174303

ns of

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699576

plying the test materials.

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90 Franchi et al

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