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SHORT COMMUNICATION
Forces released during sliding mechanics
with passive self-ligating brackets or
nonconventional elastomeric ligatures
Lorenzo 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 passive
stainless steel self-ligating brackets (SLBs) and by nonconventional elastomeric ligatures (NCEL) and
conventional elastomeric ligatures (CEL) during sliding mechanics. Methods: An experimental model
consisting of 5 aligned stainless steel 0.022-in brackets was used to assess frictional forces produced by the
SLBs, NCEL, and CEL with a 0.019 ⫻ 0.025-in stainless steel wire. Results: Significantly smaller static and
kinetic forces were generated by the SLBs and NCEL (⬍2 g) compared with the CEL (⬎500 g). No significant
differences 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)
F
riction is determined mainly by the nature of
ligation.1 In an in-vivo study, Iwasaki et al2
confirmed that, during sliding mechanics, 30%
to 50% of the total friction force generated by a
premolar bracket traveling along a .019 ⫻ .025-in
stainless steel archwire is due to the friction of the
ligation. Various methods, therefore, have been proposed
to reduce the friction of ligation, such as loosely tied
stainless steel ligatures,3 self-ligating brackets (SLBs),4-7
and unconventional ligature systems.8-10 Stainless steel
ligatures produce variable ligation forces and are timeconsuming to place.2
SLBs are ligatureless bracket systems that have a
mechanical device built into the bracket to close off the
slot. From the patient’s perspective, SLBs are generally
smoother, more comfortable, and easier to clean because of the absence of wire ligature; reduced chair
time is another significant advantage.11 In recent years,
various SLBs have been developed: those that have a
spring clip that presses against the archwire (“active”
or “interactive” SLBs), such as SPEED (Strite Indus-
tries, Cambridge, Ontario, Canada), In-Ovation (GAC
International, Bohemia, NY), Quick (Forestadent USA,
St Louis, Mo), and Time2 brackets (American Orthodontics, Sheboygan, Wis); and those in which the
self-ligating clip does not press against the wire (“passive” SLBs), such as Damon (SDS Ormco, Orange,
Calif), SmartClip, (3M Unitek, Monrovia, Calif), Carriere (Ortho Organizers, Carlsbad, Calif), and Opal
(Ultradent Products, South Jordan, Utah). Passive SLBs
have shown consistently less friction during sliding
mechanics than active SLBs, with the exception of
undersized round archwires.4,5,7 Significant reduction
in friction has been reported also for nonconventional
elastomeric ligatures (NCEL) (Slide; Leone Orthodontic Products, Sesto Fiorentino, Italy) on conventional
brackets when compared with conventional elastomeric
ligatures (CEL).10
The aim of this study was to evaluate the frictional
forces generated by 4 types of passive stainless steel
SLBs and by NCEL when compared with CEL during
sliding mechanics.
a
Assistant professor, Department of Orthodontics, University of Florence, Florence, Italy; Thomas M. Graber Visiting Scholar, Department of Orthodontics and
Pediatric Dentistry, School of Dentistry, University of Michigan, Ann Arbor.
b
Research associate, Department of Orthodontics, University of Florence,
Florence, Italy.
c
Professor and head, Department of Orthodontics, University of Rome “La
Sapienza,” 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, [email protected].
Submitted, June 2007; revised and accepted, August 2007.
0889-5406/$34.00
Copyright © 2008 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2007.08.011
MATERIAL AND METHODS
An experimental model reproducing the right buccal segment of the maxillary arch was used to assess the
frictional forces produced by 4 types of passive SLBs
(Damon 3 MX, SDS Ormco; SmartClip, 3M Unitek;
Carriere, Ortho Organizers; and Opal-M, Ultradent
Products), by NCEL (Slide; Leone Orthodontic Products) on conventional stainless steel brackets (STEP
brackets; Leone Orthodontic Products), and by CEL
(silver mini modules; Leone Orthodontic Products) on
87
88 Franchi et al
American Journal of Orthodontics and Dentofacial Orthopedics
January 2008
Instron Calibration Laboratory in terms of crosshead
displacement/speed and load cell. The test wire was
inserted into the testing unit, and its bottom end was
clamped by a vise and mounted on the machine’s
crosshead. The elastomeric ligatures were placed immediately before each test run to avoid ligature force
decay. Frictional forces produced were tested 30 times
with new wires on each occasion.
A total of 180 tests (30 tests for each of the 6 types
of ligation systems) were carried out. Static and kinetic
friction forces were recorded while 15 mm of wire were
drawn through the brackets at a speed of 15 mm per
minute. Static friction was defined as the force needed
to start the wire moving through the bracket assembly.
This force was measured as the maximal initial rise on
the machine’s chart trace. Recommendations for usage
with the testing machine strongly suggested that, in
tests measuring kinetic friction, this must be evaluated
as an average of the frictional forces appraised at
subsequent time periods during displacement.12 For the
purpose of this study, measurements of kinetic friction
were performed at 2, 5, and 10 mm of displacement and
then averaged.
Statistical analysis
Fig. Experimental model and friction testing apparatus:
A, in-vitro model of right buccal segment of maxillary
arch; B, the test wire was ligated into the experimental
model clamped to the machine’s crosshead.
the same type of conventional stainless steel brackets.
The buccal segment model consisted of 5 brackets for
the second premolar, the first premolar, the canine, the
lateral incisor, and the central incisor. A section of
0.0215 ⫻ 0.028-in stainless steel wire was used to align
the brackets before fixing them with cyanoacrylate glue
onto an acrylic block (Fig, A). The interbracket distance
was set at 8.5 mm.
An 18-cm-long 0.019 ⫻ 0.025-in stainless steel
wire was tested. The wire was secured into the brackets
by using the self-ligating systems, the NCEL, or the
CEL. The frictional forces generated by the 0.019 ⫻
0.025-in stainless steel wire with the 2 types of ligation
systems were recorded by sliding the wire into the
aligned brackets. The friction generated by the testing
unit consisting of wire, brackets, and ligation systems
was measured under dry conditions and at room temperature (20°C ⫾ 2°C) with a testing machine (model
4301; Instron, Canton, Mass) with a load cell of 10 N
(Fig, B). The testing machine had been calibrated by the
Descriptive statistics, including means, medians,
standard deviations, and minimum and maximum values were calculated for the static and kinetic frictional
forces produced by the various ligation systems with
the 0.019 ⫻ 0.025-in stainless steel wire. Because
normal distribution of the data was not found (ShapiroWilks test), the comparisons between the ligation systems were carried out with analysis of variance
(ANOVA) on ranks with the Tukey post-hoc test
(P ⬍.05) (SigmaStat 3.1; Systat Software, Point Richmond, Calif).
RESULTS
The descriptive statistics and the comparisons of
static and kinetic frictional forces for the ligation
systems are shown in Tables I and II. The statistical
tests showed significantly smaller static and kinetic
forces generated by the SLBs and the NCEL when
compared with the CEL. No significant differences
were found among the different SLBs, or between these
and the NCEL.
The average amount of both static and kinetic
friction was minimal (⬍2 g) in the SLB and NCEL
groups with aligned brackets with 0.019 ⫻ 0.025-in
stainless steel wire, whereas it was greater than 500 g
with CEL.
Franchi et al 89
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 133, Number 1
Table I.
Descriptive statistics and statistical comparisons of static frictional forces (g)
Descriptive statistics
Damon 3 MX (1)
SmartClip (2)
Opal-M (3)
Carriere (4)
STEP w/slide (5)
STEP w/conventional elastastic ligature (6)
Mean
Median
SD
Minimum
Maximum
1.2
1.8
1.3
1.3
1.3
590.7
1.1
1.7
1.4
1.3
1.0
587.3
0.4
0.8
0.3
0.7
0.7
36.7
0.5
0.6
0.7
0.4
0.6
529.1
3.0
3.3
1.9
2.8
3.3
656.2
Significant statistical
comparisons*
1
2
3
4
5
vs
vs
vs
vs
vs
6
6
6
6
6
*P ⬍.05.
Table II.
Descriptive statistics and statistical comparisons of kinetic frictional forces (g)
Descriptive statistics
Damon 3 MX (1)
SmartClip (2)
Opal-M (3)
Carriere (4)
STEP w/slide (5)
STEP w/conventional elastastic ligature (6)
Mean
Median
SD
Minimum
Maximum
0.6
1.0
0.9
0.6
0.7
541.6
0.6
0.9
0.9
0.5
0.7
538.6
0.3
0.6
0.3
0.4
0.3
40.2
0.1
0.2
0.3
0.1
0.1
491.4
1.2
2.3
1.5
1.6
1.5
631.6
Significant statistical
comparisons*
1
2
3
4
5
vs
vs
vs
vs
vs
6
6
6
6
6
*P ⬍.05.
DISCUSSION
Clinical evidence of the beneficial effects of lowfriction archwire ligatures on the biomechanical characteristics of orthodontic treatment can be derived
from either passive SLBs4,5 or NCEL on conventional
brackets,10 in which the archwire is not compressed
into the slot by a ligation structure. Our aim in this
study was to compare the friction generated by these
innovative ligation systems (SLBs and NCEL) with the
friction produced by CEL. The research was tailored to
test the friction during the fundamental therapeutic
phase of sliding mechanics on aligned brackets with a
rectangular archwire. We used a device specifically
designed and manufactured to simulate the clinical
conditions of a dental arch section to study static and
kinetic attritions.10
Both the SLBs and the NCEL showed levels of
friction that were significantly lower than those produced by CEL during sliding mechanics with a 0.019 ⫻
0.025-in rectangular wire. The amounts of static and
kinetic frictional forces exerted by the SLBs and the
NCEL were minimal (⬍2 g) when compared with the
CEL that exhibited more than 500 g of force on average
for both frictional force types. The significant differences between SLBs and NCEL vs CEL are similar to
those reported by Pizzoni et al4 and Thomas et al.5
However, both studies used a single-bracket experimental model, whereas we attempted to reproduce the
clinical condition of 5 aligned brackets in a dental arch
segment.
The data concerning the elastomeric ligatures, however, should be considered with caution. Each test with the
machine was performed with new elastomeric ligatures.
We made no attempt to evaluate the effects of time and
oral environment on the amount of force released with
different types of elastomeric ligatures.13 Frictional resistance is reduced after presoaking elastomeric modules in
saliva for 1 week.14
This investigation demonstrated clearly that minimal amounts of friction are generated with 4 types of
passive SLBs that are commercially available. The
literature reports values of frictional forces for active
SLBs that are 5 times greater than passive SLBs.10
Based on the results of this study, NCEL also produce
significantly lower levels of frictional forces than CEL,
so that NCEL might be a valid alternative to passive
SLBs during sliding mechanics.
CONCLUSIONS
In this study, we found that both passive SLBs and
NCEL produce significantly smaller frictional forces
(⬍2 g) than CEL (⬎500 g).
We thank 3M Unitek, Ortho Organizers, Leone
Orthodontic Products, and Ultradent Products for supplying the test materials.
90 Franchi et al
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