Force level of small diameter nickel-titanium orthodontic wires ligated with different methods
Higa et al. Progress in Orthodontics
Force level of small diameter nickel- titanium orthodontic wires ligated with different methods
Rodrigo Hitoshi Higa 0 2
José Fernando Castanha Henriques 0 2
Guilherme Janson 0 2
Murilo Matias 0 2
Karina Maria Salvatore de Freitas 1
Fernanda Pinelli Henriques 0 2
Manoela Fávaro Francisconi 0 2
0 Department of Orthodontics, Bauru Dental School, University of São Paulo , Alameda Octávio Pinheiro Brisolla 9-75, Bauru, SP 17012-901 , Brazil
1 Department of Orthodontics, Ingá Dental School , Maringá , Brazil
2 Department of Orthodontics, Bauru Dental School, University of São Paulo , Alameda Octávio Pinheiro Brisolla 9-75, Bauru, SP 17012-901 , Brazil
Background: The aim of this study was to compare the deflection force in conventional and thermally activated nickel-titanium (NiTi) wires in passive (Damon Q) and active (Bioquick) self-ligating brackets (SLB) and in conventional brackets (CB) tied by two different methods: elastomeric ligature (EL) and metal ligature (ML). Methods: Two wire diameters (0.014 and 0.016 in.) and 10 specimens per group were used. The specimens were assembled in a clinical simulation device and tested in an Instron Universal Testing Machine, with a load cell of 10 N. For the testing procedures, the acrylic block representative of the right maxillary central incisor was palatally moved, with readings of the force at 0.5, 1, 2, and 3 mm, at a constant speed of 2 mm/min and temperature of 36.5 °C. Results: The conventional NiTi released higher forces than the thermally activated NiTi archwires in large deflections. In general, the SLB showed lower forces, while the ML had higher forces, with both showing a similar force release behavior, constantly decreasing as the deflection decreased. The EL showed an irregular behavior. The active SLB showed smaller forces than passive, in large deflections. Conclusions: The SLB and the ML exhibit standard force patterns during unloading, while the elastomeric ligatures exhibit a randomly distributed force release behavior.
Orthodontic wires; Orthodontic brackets; Comparative study; Mechanical phenomena
The orthodontic wires used in the alignment and
leveling phase have undergone a great evolution in recent
years. Nickel-titanium (NiTi) wires presented great
emphasis because of their properties of superelasticity
and shape memory, which make their use proper for the
initial stages of orthodontic treatment [
]. With the
development of metallurgy, NiTi wires with improved
properties have been developed.
For a controlled tooth movement, light and continuous
forces have been indicated [
]. In order to achieve the force
levels suitable for alignment and leveling phase, it is
necessary to know the force-deflection characteristics of the
wires. Currently, with access to technology, it is possible to
measure the forces released by the different wire types.
Several factors related to bracket/wire combination
can influence the force released to the teeth, such as
arch dimension, amount of deflection, ligation method,
and frictional forces [
]. There are several ways to
connect the wire to the bracket, and depending on the
form chosen, the frictional force will be different. The
frictional force acts as a counterforce to the forces
exerted by orthodontic wires. Thus, the higher the
friction, the lower the force dissipated to the teeth [
The wire can be ligated to the bracket by means of
metal ligature (ML) of different diameters, elastomeric
ligature (EL), or by the specific closure system in the
case of self-ligating brackets (SLB) [
]. Among the EL,
the most common way to tie is the “ring” shape.
Another tying option with elastomeric ligatures is the
“figure 8” shape, which promotes greater pressure of the
wire in the slot, increasing friction [
properties include light continuous force, consistent
longlasting seating archwire, resistance to water sorption,
and shape memory . Furthermore, they can be
applied quickly, are comfortable for the patient, and
have a variety of colors. However, the EL allows greater
microbial accumulation on the surface of the teeth
adjacent to the bracket, compared to the other ligation types,
besides the fact that the archwires may not completely
seat during torquing or rotational corrections, and
binding may occur with sliding mechanics [
studies have evaluated the influence of the ligation type
in the force exerted by the wire on the tooth [
6, 11, 12
The use of SLB has become common in recent years.
From the patients’ perspective, these brackets are more
comfortable and easier to clean due to the absence of
elastic or metal ligatures. Many studies have been
published evaluating the frictional force produced by SLB
], since the manufacturers have claimed that in
these accessories there is a lower resistance force to
sliding, decreasing treatment time. Although friction is
not the only factor that determines treatment efficiency,
it has been associated with the forces dissipated by the
archwires. The different SLB designs, active or passive,
can show a different behavior in relation to the friction
properties. Passive brackets have shown lower friction
than active brackets [
Due to the influence of the ligation methods in the force
exerted on the teeth and to the extensive variation of them
in the market, further studies become essential to evaluate
the behavior of each wire/bracket combination. This way,
the aim of the study was to evaluate the forces exerted by
conventional and thermally activated NiTi wires in
different ligation types, in SLB and conventional brackets (CB).
Three sets of brackets were selected for this study: Damon
Q passive self-ligating (Ormco, Orange, California),
Bioquick active self-ligating (Forestadent, Pforzheim,
Germany), and Morelli conventional (Dental Morelli, São
Paulo, Brazil). All brackets had a nominal 0.022-in. slot
size. Two different NiTi wires were tested: conventional
and thermally activated (Dental Morelli, São Paulo, Brazil),
with 0.014- and 0.016-in. diameters (Table 1).
The wires were ligated to the CB by means of “ring”
shaped elastomeric ligature (RSEL) and metal ligature
(ML). The wires, brackets and ligatures used belonged
to the same batch, so that there were no influences in
the results. The standard ISO 15841, which recommends
six specimens of each sample, was used. However, to
minimize the chance of any technical error and increase
reliability of the results, a number of 10 specimens were
chosen for each group.
For the elastomeric ligatures, tying a needle holder
was used, and after insertion of the elastic, a 3-min
waiting period before the tests was determined, to enable
initial relaxation of the material, as recommended in
other studies [
]. For ML, the ligature was initially
tightened with a needle holder around the wings of the
bracket, and then loosened by one turn to allow free
movement of the archwire.
The evaluation tests of the force released through
deflection of the orthodontic wire were performed in a clinical
simulation device representing the maxillary teeth,
extending from the right second premolar to the left
second premolar [
10, 30, 31
Figure 1 shows the clinical simulation device that was
used in this study. This device was composed of an
acrylic resin plate with parabolic shape where blocks
which represent the maxillary teeth were affixed. The
parabola shape was determined by the wire to be tested,
reducing the risk of generating diverse forces beyond the
deflection applied in this study.
The blocks that represent the teeth were affixed to the
acrylic plate respecting a standard distance of 6 mm
between brackets [
], corresponding to the average
distance between slots considering the bracket size and
the average size of dental crowns mesiodistally, since the
force/deflection relation is dependent, among other
things, on this distance [
]. Brackets were bonded with
cyanoacrylate adhesive (Super Bonder, Loctite) on acrylic
blocks. These blocks were fixed by means of threaded
screws to the bottom of the acrylic resin plate.
The tests were performed on the block corresponding
to the right maxillary central incisor. This block was not
screwed, enabling its bucco-palatal movement. It
received a perforation, in which a metal cylinder was
placed, allowing its activation. The tip of the activation
attached to the testing machine had rounded cut to fit
the metal cylinder (Fig. 2). The speed of the testing
machine was 2 mm/min.
To evaluate the wire deflection, an Instron 3342
universal testing machine (Norwood, MA, USA) with load cell
of 10 N (1 kgf ) was used. Very high load cells have no
accuracy befitting with the forces dissipated by orthodontic
treatment. To maintain a constant temperature of 36.5 °C
in order to get closer to the reality of the oral
environment, the tests were done in an acrylic container filled
with water, where the temperature was controlled by a
submersible electric resistance connected to a digital
thermostat (TIC 17RGTi/9 model, Full Gauge Controls,
Canoas/RS, Brazil) previously scheduled to stay in the
desired temperature range (Fig. 3).
Before each test, load cell calibration was achieved by
Bluehill Lite software (v.2.25, 2005). Assessments of wire
deflection in unloading were performed beginning in
3.1 mm, and from this point, generated values could be
measured in 3, 2, 1, and 0.5 mm. The deflection of the
wire attached to the bracket corresponds clinically to the
beginning of treatment, when the teeth are poorly
positioned and the wire is forced into the slots of the
accessories. Depending on the degree of crowding, teeth will
experience more or less force so proper alignment occurs.
The elastic deflection test was chosen because it is
clinically closest to the orthodontists’ interests, because
that is what they do when adapting a wire to the
patient’s teeth. Although engineers work with parameters
like elastic modulus and yield value, the orthodontist is
more concerned with knowing the force released in
relation to the amount of deflection.
The Kolmogorov Smirnov test was used to evaluate the
normal distribution of the variables, indicating that the
parametric statistical tests could be applied.
Descriptive statistics were calculated for each
Three- and one-way ANOVA and Tukey tests were used
to compare different wires, diameters, and brackets.
All statistical analyses were performed with Statistica
software (Statistica for Windows—Release 7.0 -
Copyright Statsoft, Inc. Tulsa, Okla), at the p < 0.05 level of
Fig. 3 Acrylic container filled with water containing temperature
control system, where the tests were conducted
Table 2 (end of the manuscript) represents the results of
the three-way ANOVA, considering the different
archwire type, ligation system, and diameter of the
archwire, in the evaluated deflections. It was found
that there was influence of the different combinations
in the greater deflections, but not in the smaller
Regarding the archwire types, the mean values of the
conventional NiTi archwires were greater than thermally
activated NiTi ones, but statistically significant
differences were found only in 1, 2, and 3 mm of deflection.
The same situation was found for the archwire diameter,
indicating that the wire diameter of 0.016 in. releases
greater forces than the 0.014 in.
Tables 3, 4, 5, and 6 show the means, standard
deviations, and comparison of the forces in the different ligation
systems by one-way ANOVA, at different amounts of
In general, for the smallest amount of deflection
(0.5 mm), there was a trend, in any diameter and type of
wire tested, that the force exerted by EL were much
smaller than those of other ligation types.
In 1 mm of deflection, the SLB, along with the EL,
showed smaller forces, while the ML showed a trend to
present greater forces.
For 2 mm of deflection, the active SLB tended to
have smaller forces compared to other systems. The
ML showed higher forces in most tests in this
deflection, while the Damon and the EL showed
intermediate forces in relation to the others. Only for the 0.014
thermally activated NiTi different results were
observed, with the ML releasing higher forces and the
other ligation methods showing significantly similar
forces among them.
In 3 mm of deflection, there was a trend for the SLB
to show smaller forces, especially the active system
showing smaller forces in most tests. The ML showed
intermediate forces and the EL showed the highest
forces for this deflection.
Comparing the force of the conventional NiTi wire with
the thermally activated Niti, a statistically significant
difference only in the two largest deflections (2 and 3 mm)
was observed. The highest forces of the conventional
NiTi wire are in agreement with other studies that also
found similar results when comparing the two types of
]. The values found in this study, however,
suggest that in small deflections there is no difference in
the force exerted by these two wires in the diameters
Based on these considerations, they should have
different use according to the biomechanical need.
In low friction mechanics, the thermally activated
NiTi wires are more suitable in the alignment stage,
*Statistically significant at p < 0.05
that the SLB release smaller forces than CB, when
smaller diameters are tested [
The ML produced greater forces in most tests.
Even so, its force release behavior was similar to the
SLB, where the forces constantly decreased as the
deflection decreased. It is possible that the ML
behaves as an active self-ligating bracket, with the
difference that it allows less freedom of the wire
within the slot, compared to other SLBs. On the
other hand, the force release behavior of the EL is
very different from the other ligation types, releasing
very high forces in large deflections and very low
forces in small deflections.
In turn, the self-ligating systems show low force
release rate at higher deflections, but they also release
forces in small deflections, in agreement with the
concept of light and continuous forces.
This concept of light and continuous forces is
important because the force released for orthodontic
movement is more biologically favorable, without
damaging the surrounding tissues. In addition, the force is
released since wire placement and remains until the
new appointment, promoting constant orthodontic
movement. In this sense, leveling and alignment will be
In addition, the ligatures may change the force
released due to loss of elasticity of the material, with
time. A previous study found that the force released
by “relaxed” elastomeric ligatures was higher than
due to their lower forces and superelastic properties
compared to the conventional NiTi. However, in
conventional mechanics, when the friction promoted
by the ligation system is greater, these wires may be
unable to overcome this resistance. Several studies
have mentioned friction as one of the factors that
dissipate the forces in orthodontic treatment [
]. These studies show that low friction results
in higher loads.
Behavior of the forces released was significantly
variable depending on the different ligation types. In
0.5 mm of deflection, it was observed that the EL
promoted very low forces in all tests (Tables 3 to 4). This
result probably occurred because the force exerted by
the NiTi wires was hardly enough to overcome the
friction generated by the ligatures. The EL pressures the
wire inside the bracket slot, increasing the friction. In
applying this concept in clinical practice, force values
released by this type of ligation probably would not
promote tooth movement.
However, in 3 mm of deflection, which was the
highest tested, the CB showed higher forces than the
SLB. This occurred because the SLB does not press
the smaller diameter wires inside the slot walls.
However, in CB, even these wires are pressed by the
elastomeric or metal ties, promoting greater deflection
of the wire, which in turn results in higher levels of
force. Previous studies that also compared the forces
in different ligation systems corroborate with the fact
*Statistically significant at p < 0.05
the new [
]. This probably occurs due to loss of
friction with relaxation of the elastomer. However,
further studies are necessary to evaluate the force
after a certain period of performance of elastomeric
Thus, it is hard to predict the amount of force
released by the wire when it is connected to the
bracket by means of EL. A study that examined the
effect of ligation on the load-deflection characteristics
of NiTi wires concluded that the EL act as a restraint
on superelastic wires [
]. Therefore, the results of
this study suggest that with ML and self-ligating
system, predictability of the released force is greater than
with the EL. The SLB has the advantage of releasing
When comparing the two self-ligating bracket
systems, the passive (Damon Q) presented higher forces
than the active system (Bioquick) in larger
deflections, except for the 0.014-in. thermally activated
NiTi wire. This may indicate that in situations where
there is great force release, the difference between
the systems appears. These situations can be related
to archwires of large diameter, large deflections
(larger crowding) and alloys with small
superelasticity and resilience.
This force difference between self-ligating brackets can
be justified by the smaller frictional force promoted by
this system, demonstrated by several studies [
This is in agreement with the concept that the smaller
the friction, the higher the forces [
]. The results of
this study suggest that in situations where there is
greater force release, friction tends to exert greater
In applying this concept to clinical practice, friction
might influence the force in the initial stage of
leveling and alignment, when crowding is severe, or in the
final stage, when using a larger diameter wire.
Another study compared the friction among different
brackets and smaller friction was found for the
Damon passive system only when larger diameter
wires were used [
➢ Conventional NiTi wire showed higher forces than
thermally activated NiTi, in large deflections.
➢ The sets of low friction (self-ligating and
conventional brackets tied with ML) showed more
standardized forces than conventional brackets with
elastomeric ligature. Metal ligature promotes greater
magnitude of forces than SLB.
➢ The active self-ligating showed smaller forces than
the passive system in large deflections.
CB: Conventional brackets; EL: Elastomeric ligature; ML: Metal ligature;
NiTi: Nickel-titanium; SLB: Self-ligating brackets
RHH performed the tests and prepared the manuscript. JFCH coordinated
the research project and participated in the review of the manuscript. GJ
reviewed the quality of the manuscript in detail, mainly in relation to the
intellectual content and language. MM participated in the elaboration of the
tests and design of the study. KMSF participated in the coordination of the
study and the statistical analysis review. FPH reviewed the manuscript in
detail and participated in the interpretation of the results. MFF collected the
necessary materials and assisted in performing the tests. All authors read and
approved the final manuscript.
The authors declare that they have no competing interests.
Springer Nature remains neutral with regard to jurisdictional claims in
published maps and institutional affiliations.
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