Download Torque expression of self-ligating brackets

ORIGINAL ARTICLE
Torque expression of self-ligating brackets
Hisham M. Badawi,a Roger W. Toogood,b Jason P. R. Carey,c Giseon Heo,d and Paul W. Majore
Edmonton, Alberta, Canada
Introduction: The labiolingual inclination of maxillary and mandibular incisors is considered by many
orthodontists to be an important determinant of pleasing dental esthetics and ideal stable occlusion. In
contemporary fixed appliances, attaching a rectangular orthodontic archwire to a bracket with a rectangular
slot makes third-order control possible. The purpose of this study was to measure the difference in
third-order moments that can be delivered by engaging 0.019 ⫻ 0.025-in stainless steel archwires to 2 active
self-ligating brackets (In-Ovation, GAC, Bohemia, NY; Speed, Strite Industries, Cambridge, Ontario, Canada)
and 2 passive self-ligating brackets (Damon2, Ormco, Orange, Calif; Smart Clip, 3M Unitek, Monrovia, Calif).
Methods: A bracket/wire assembly torsion device was developed. This novel apparatus can apply torsion to
the wire while maintaining perfect vertical and horizontal alignment between the wire and the bracket. A
multi-axis force/torque transducer was used to measure the moment of the couple (torque), and a digital
inclinometer was used to measure the torsion angle. Fifty maxillary right central incisor brackets from each
of the 4 manufacturers were tested. Results: There was a significant difference in the engagement angle
between the 2 types of brackets; on average, torque started to be expressed at 7.5° of torsion for the active
self-ligating brackets and at 15° of torsion for the passive self-ligating brackets. The torque expression was
higher for the active self-ligating brackets up to 35° of torsion. Torsion of the wire past this point resulted in
a linear increase of the measured torque for the Damon2, the Smart Clip, and the In-Ovation brackets. The
torque was relatively constant past 35° of torsion for the Speed bracket. Conclusions: We conclude that
active self-ligating brackets are more effective in torque expression than passive self-ligating brackets. (Am
J Orthod Dentofacial Orthop 2008;133:721-8)
C
orrect buccolingual inclination of anterior teeth
is considered essential to providing good occlusal relationships in orthodontic treatment.
Inclination of the maxillary anterior teeth is particularly
critical in establishing an esthetic smile line, proper
anterior guidance, and a Class I canine and molar
relationship. Undertorqued maxillary anterior teeth affect the arch length and the space requirements. It has
been shown that for every 5° of anterior inclination,
about 1 mm of arch length is generated.1 Undertorqued
posterior teeth have a constricting effect on the maxillary arch, since they do not allow appropriate cusp-tofossa relationships between the maxillary and mandibular teeth.2
Torque expression can be achieved by filling the
bracket slot and gradually increasing the archwire
dimensions during treatment. However, the dimensions
From the University of Alberta, Edmonton, Alberta, Canada.
a
Resident, Orthodontic Graduate Program, Faculty of Medicine and Dentistry.
b
Associate professor, Department of Mechanical Engineering.
c
Assistant professor, Department of Mechanical Engineering.
d
Assistant professor, Orthodontic Graduate Program, Faculty of Medicine and
Dentistry.
e
Professor and director, Orthodontic Graduate Program, Faculty of Medicine
and Dentistry.
Reprint requests to: Paul W. Major, Orthodontic Graduate Program, Faculty
of Medicine and Dentistry, 4051B Dentistry/Pharmacy Centre, Edmonton,
Alberta, Canada, T6G 2N8; e-mail, [email protected].
Submitted, October 2005; revised and accepted, January 2006.
0889-5406/$34.00
Copyright © 2008 by the American Association of Orthodontists.
doi:10.1016/j.ajodo.2006.01.051
of the final working archwire never reach the full
dimensions of the bracket slot; therefore, a percentage
of the torque built into the bracket is lost because of the
play between the archwire and the bracket slot. This
amount of play has been theoretically calculated and
experimentally measured.3-17 Theoretical third-order
moments can be calculated from the nominal dimensions of archwires and brackets from the manufacturers. It has been shown that there is a considerable
discrepancy between the theoretical and the measured
bracket/archwire play.8 This play often extends to
100% of the prescribed torque; this is equivalent to
using round wires.2
Deviations from those calculations can be attributed
to intrinsic variations in archwire cross-sectional diameters,3,11,13 bracket slot dimensions,7,8,13 archwire edge
beveling,8,10 and bracket deformations.4,18 Other factors also have an impact on third-order moments,
including bracket placement errors19 and irregularities
in tooth morphology.20,21 Therefore, due to these variations in torque expression resulting from variations in
brackets and archwires, accurate calculation of thirdorder moments with the manufacturer’s archwire and
bracket dimensions is impossible.
Self-ligating orthodontic brackets are gaining popularity because of their advantages in reducing friction
and treatment time in orthodontic mechanotherapy.22
However, no studies have investigated the torque expression of self-ligating orthodontic brackets. In our
721
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American Journal of Orthodontics and Dentofacial Orthopedics
May 2008
Fig 2. Bracket alignment assembly.
Fig 1. Orthodontic torque measurement device. a, Inclinometer, b, wire support substructure; c, conical wire
support and alignment component; d, bracket wire
assembly area; e, dual turn table alignment system; f,
work gear-based torsion system; g, load cell; h, springs;
i, bushings; j, height adjustment system; k, guide rails; l,
base support.
study, we developed a comprehensive experimental
technique to measure the torque expressed from orthodontic brackets. This technique was used to test the
torque expression of 2 passive and 2 active commercially available self-ligating brackets.
MATERIAL AND METHODS
An experimental device was designed to measure
the torque expression of orthodontic brackets as accurately as possible. The orthodontic torque measurement
device (Fig 1) was developed with the collaboration of
members from the Departments of Dentistry and Mechanical Engineering at the University of Alberta,
Edmonton, Alberta, Canada. This novel apparatus has
the unmatched capability to apply known torsion to an
orthodontic wire while maintaining perfect vertical and
horizontal alignment between the wire and the bracket.
The design uses a multi-axis force/torque transducer
that can measure forces and moments in 3 dimensions.
The use of the multi-axis force transducer facilitated
isolating and recording the moment generated by thirdorder wire activations (torsion), and it allowed us to
Fig 3. Schematic of the torsion application system.
measure the forces and moments in all other planes to
ensure that they are as close to zero as possible. As a
result, we could generate and measure pure orthodontic torque.
The device consists of a digital inclinometer. The
inclinometer (model T2-7200-1N; US Digital, Vancouver, Wash) (Fig 1, a) was used to measure the torsional
rotation of the wire. The inclinometer has a 360°
rotation range with a 0.05° resolution (manufacturer’s
specification). The wire support substructure, which
includes the conical wire support and alignment dies,
holds the wire and is designed to ensure near-perfect
rotation of the wire around its long axis.
The torsion device has an alignment assembly (Fig 2)
that guarantees perfect alignment between the archwire
and the bracket slot. This assembly is made of a base
turntable and a secondary turntable (the stainless steel
cylinders). Rotation of the base turntable adjusts the
position of the bracket slot perpendicular to the wire.
Rotation of the secondary turntable adjusts the rotational alignment of the bracket slot to the wire to
achieve a perfect parallel relationship between the
archwire and the bracket slot.
Badawi et al 723
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 133, Number 5
A worm gear-based torsion system (Fig 1, f ) with
the inclinometer allowed us to place well-controlled
torsion of known degrees onto the brackets. The wire
substructure (Fig 3), a rigid assembly, is fully rotated
by the worm gear (Fig 1, f ), thus imposing the same
rotation at both ends of the structure. Therefore, both
worm gear and inclinometer undergo the same torsional
rotation. The gear ratio of the gear system is 1:120. In
other words, a full turn of the handle results in a 3° of
rotation of the torsional assembly.
The ATI Industrial Automation Nano 17 MultiAxis force/torque transducer (Apex, NC) was used to
measure the 3 force and 3 moment components of the
applied load. The load cell is a silicon strain gauge.
Based on the manufacturer’s specifications, it is rated
for maximum loads of 25 N of transverse forces (Fx,
Fy), 35 N axial force (Fz), and 0.25 Nm moments in all
3 axes. The resolutions of the measurements are 1/1280
and 1/256 for forces and moments, respectively. The
error of measurement of the force/torque transducer
was 1.5%. The load cell was used in conjunction with
a data acquisition card (DAQ 16-Bit E series NI
PCI-6033E; National Instruments, Austin, Tex) recommended by the manufacturer. The transducer signal was
imported to a personal computer via the data acquisition card with 16 bits of input resolution. LabView data
acquisition software (National Instruments, Austin,
Tex) was used to acquire the signal from the transducer
and log it to file.
Fifty maxillary right central incisor brackets of each
of 4 types of brackets (total, 200 brackets) were
included in the study (In-Ovation, GAC, Bohemia, NY;
Speed, Strite Industries, Cambridge, Ontario, Canada;
Damon2, Ormco, Orange, Calif; Smart Clip, 3M
Unitek, Monrovia, Calif) and mounted on stainless steel
cylinders (secondary turntable, Fig 2). Epoxy adhesive
(Loctite, E-20HP, Hysol; Henkel, Rocky Hill, Conn)
was used to bond the brackets to the cylinders after
treating the surfaces with methyl ethyl ketone. A
bracket mounting jig was used for bonding all brackets.
The epoxy was allowed to fully cure before testing the
brackets. The brackets’ torque prescription did not
affect our method, since the true zero torque position
was used as a baseline reference for all brackets.
RESULTS
A pilot study was conducted with 7 Speed brackets
and 10 Damon2 brackets. For detecting a difference
between the brackets ⱖ5 Nmm with ␣ ⫽ 0.05, ␤ ⫽ 0.1,
the sample sizes were calculated to be 81, 40, 19, and
54 for angles 12°, 24°, 36°, and 48°, respectively.
Those calculations were averaged, and a sample size of
50 was determined to be adequate for this study.
Table I.
Bracket 1
Bracket 2
Bracket 3
Bracket 4
Bracket 5
Coefficient of variation (CV) of error analysis
Angle
Mean
SD
CV
36
48
36
48
36
48
36
48
36
48
12.703
28.809
8.703
23.863
3.0745
23.184
9.644
24.443
6.638
28.398
0.207
0.162
0.317
0.107
0.2417
0.239
0.438
0.311
0.347
0.203
1.63
0.56
3.64
0.45
7.86
1.03
4.54
1.27
5.23
0.72
Error analysis was conducted to assess the variability in our measurements after controlling for the variation resulting from the brackets and the wires. Five
wire/bracket combinations were tested (0.019 ⫻
0.025-in stainless steel with Damon2 brackets). For
each bracket/wire combination, the test was repeated 10
times, recording torque measurements for 4 angles
(12°, 24°, 36°, and 48°). Greater variability in data is
usually expected when the mean increases; therefore, the
coefficient of variation was used to evaluate the standard
deviation of the data as a percentage of the mean (Table I).
The coefficient of variation for angle 48° was 0.45% to
1.27%. The average coefficient of variation was 2.7%.
During the error analysis, care was taken to avoid
plastic deformation of the wires or the brackets. Figure
4 shows that there was no decrease in the moments as
repeated measurements were made with the same
bracket/wire combination. The measurement error of
the force/torque transducer was 1.5% (manufacturer’s
specification).
The Kolmogorov-Smirnov test was used to evaluate
the data statistically to confirm normal distribution. The
data were distributed normally among all the groups
except for the Speed bracket at angle 12°. The mean
moment of couple (torque) for the 4 self-ligating
brackets was compared at 4 angles (12°, 24°, 36°, and
48°). Repeated measures analysis of variance
(ANOVA) and multivariate ANOVA were conducted
with a statistical package (version 13; SPSS, Chicago,
Ill) to identify any significant differences between the
brackets (Table II).
Descriptive statistics for the 4 bracket types are
given in Table III. There was considerable variation
among the torque measurements in each type of
bracket. For the Speed brackets, this variation was 1.97
to 11 Nmm; for the In-Ovation brackets, it was 3.7 to
16.7 Nmm; for the Damon2 brackets, it was 1.4 to 11.2
Nmm; and for the Smart Clip brackets, it was 2.8 to 14.2
Nmm. This variation was small for small torsion angles
and increased as the torsion angle increased (Fig 5).
724 Badawi et al
American Journal of Orthodontics and Dentofacial Orthopedics
May 2008
Fig 4. Moments measured for error analysis showing no plastic deformation of wire or bracket.
Table II.
Repeated measures ANOVA (pairwise comparison)
95% CI for mean
Angle
12°
24°
36°
48°
Bracket comparisons
Damon2
In-Ovation
Smart
Speed
In-Ovation
Smart
Speed
Smart Clip
Speed
Damon2
In-Ovation
Smart
Speed
In-Ovation
Smart
Speed
Smart Clip
Speed
Damon2
In-Ovation
Smart
Speed
In-Ovation
Smart
Speed
Smart Clip
Speed
Damon2
In-Ovation
Smart
Speed
In-Ovation
Smart
Speed
Smart Clip
Speed
Mean difference
Significance
Lower bound
Upper bound
⫺2.36036
.07858
⫺1.54796
.001
1.000
.000
⫺3.89228
⫺1.12382
⫺2.48965
⫺.82844
1.28098
⫺.60627
2.43894
.81240
.002
.685
.67954
⫺.79206
4.19834
2.41686
⫺1.62654
.007
⫺2.92264
⫺.33044
⫺6.90882
⫺.99844
⫺6.34030
.000
.874
.000
⫺9.94213
⫺3.53284
⫺8.82384
⫺3.87551
1.53596
⫺3.85676
5.91038
.56852
.000
.998
2.57153
⫺2.73360
9.24923
3.87064
⫺5.34186
.000
⫺8.19997
⫺2.48375
⫺4.53588
2.23154
.88280
.240
.816
.996
⫺10.53013
⫺2.90407
⫺3.58141
1.45837
7.36715
5.34701
6.76742
5.41868
.018
.049
.80477
.00833
12.73007
10.82903
⫺1.34874
.959
⫺5.76834
3.07086
1.09862
8.09258
27.47506
.999
.012
.000
⫺6.55311
1.22711
21.51632
8.75035
14.95805
33.43380
6.99396
26.37644
.147
.000
⫺1.32748
18.75947
15.31540
33.99341
19.38248
.000
12.55642
26.20854
Badawi et al 725
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 133, Number 5
Table III.
Angle
12°
24°
36°
48°
Descriptive statistics
Bracket
Mean
SD
n
Damon2
In-Ovation
Smart
Speed
Damon2
In-Ovation
Smart
Speed
Damon2
In-Ovation
Smart
Speed
Damon2
In-Ovation
Smart
Speed
.45336
2.81372
.37478
2.00132
5.52642
12.43524
6.52486
11.86672
23.23314
27.76902
21.00160
22.35034
55.83124
54.73262
47.73866
28.35618
1.488591
3.702820
2.772042
1.978374
3.894859
6.922984
5.402589
5.239754
9.632597
12.478563
9.492177
6.686943
11.181229
16.690271
14.180880
11.008746
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
50
Clinically effective torque has been suggested to be
5 to 20 Nmm.9,23,24 The angles of torsion at which the
lower limit of that range (5 Nmm) is achieved were 15°
for the active self-ligating brackets and 22.5° for the
passive self-ligating brackets. For the active self-ligating brackets, the angle of torsion at which the upper
limit of that range (20 Nmm) was achieved was 31°, but
it was 34.5° for the passive self-ligating brackets (Fig 6).
The active self-ligating brackets produced higher
levels of torque (moment) up to 35° of torsion. Any
torsion beyond this point resulted in a linear increase in
the moments generated by the Damon2, Smart Clip,
and In-Ovation brackets. As for the Speed bracket, a
further increase in torsion beyond this point did not
result in increases in the moments generated at the same
rate as the other 3 brackets. The moment generated by
the Speed bracket reached a plateau at which the
increase in torsion did not result in increases in the
moments generated (Fig 6).
At 12° of torque, there were no significant differences between Damon2 and Smart Clip brackets, or
between Speed and In-Ovation brackets. There were,
however, statistically significant differences between
Damon2 and both Speed and In-Ovation, and between
Smart Clip and both Speed and In-Ovation (Table II).
At 24° of torque, again there were no significant
differences between Damon2 and Smart Clip brackets,
or between Speed and In-Ovation brackets. However,
there were statistically significant differences between
Damon2 and both Speed and In-Ovation, and between
Smart Clip and both Speed and In-Ovation. Speed and
In-Ovation brackets delivered statistically significant
higher torque than did Damon2 and Smart Clip brackets
at angles 12° and 24° (Table II).
At 36° of torque, the only statistically significant
difference was between In-Ovation and both Speed and
Smart Clip brackets. At 48° of torque, the most significant
difference was between Speed and the other brackets.
Speed delivered significantly less torque than the other
brackets. There was also a significant difference between
Damon2 and Smart Clip brackets (Table II).
DISCUSSION
Fig 5. Variation in the torque measurement increases
as angle of torsion increases.
On average, the angle of engagement (the angle at
which torque was first expressed) for both Speed and
In-Ovation brackets was at 7.5° of torsion. The same
angle of engagement for Damon2 and Smart Clip
brackets was at 15° of torsion (Fig 6).
The applied load in this study delivered a torsional
moment of a couple directly to the bracket to simulate
orthodontic torque application. This load was measured
for 4 commercially available self-ligating bracket systems. We found no statistically significant difference in
torque expression between Damon2 and Smart Clip
brackets at angles 12°, 24°, and 36°. There was,
however, a statistically significant difference between
the 2 brackets at angle 48°. There was no statistically
significant difference between In-Ovation and Speed
726 Badawi et al
American Journal of Orthodontics and Dentofacial Orthopedics
May 2008
Fig 6. Torque expression of the 4 brackets.
brackets at angles 12° and 24°, but there was a
statistically significant difference between the 2 brackets at angles 36° and 48° (Table I).
The torque expression of Damon2 and Smart Clip
brackets followed a similar pattern, characteristic of
passive self-ligating brackets. Torque started to be
expressed at an angle of 15° of torsion compared with
an angle of 7.5° for Speed and In-Ovation brackets,
characteristic of active self-ligating brackets. For
Damon2 and Smart Clip brackets, the value of the
deviation angle (the amount of this axial rotation that
the wire is permitted to undergo before contact with the
slot walls) was found to be greater than that of the
theoretically calculated one based on the nominal
archwire and bracket slot sizes. This is consistent with
previous research on torque expression of conventional
brackets.2,17 For the Speed and the In-Ovation brackets,
the actual deviation angles were less than the theoretically calculated ones. This can only be explained by the
active ligating mechanism that seems to reduce the
amount of archwire play in the bracket slot.
Clinically effective torque has been suggested to be
5 to 20 Nmm.9,23,24 This range of torque was expressed
at 15° to 31° of torsion for the active self-ligating
brackets, and at 22.5° to 34.5° of torsion for the passive
self-ligating brackets. To standardize the experiment,
we used rigid clamps to hold the wire on either side of
the tested brackets. In a clinical situation, the wire is
held by brackets on either side, and there would be
some play. Therefore, 2 clinically useful conclusions
can be drawn based on this finding. First, the range of
clinically useful torsion by using active self-ligating
brackets is likely to be larger than that for the passive
self-ligating brackets (16° for the active self-ligating
brackets and 12° for the passive self-ligating brackets).
Second, in vivo, meaningful torque control can only be
achieved if torsion in the wire is higher than 15° and
22.5° for active self-ligating brackets and passive
self-ligating brackets, respectively. This amount of
torque is much higher than most torque prescriptions
available commercially. Future research might need to
investigate ways to use brackets instead of rigid clamps
on either side of the tested brackets, without compromising the experimental technique.
The difference between the load deflection curves
of the Speed and In-Ovation brackets seems to be due
to the material from which the clip mechanism is made.
The Speed clip is made of nickel-titanium alloy, and the
load deflection curve of the Speed bracket was consistent with what would be expected of a nickel-titanium
alloy (Fig 6). The In-Ovation bracket clip is made of
stainless steel, and the load deflection curve of the
In-Ovation bracket was consistent with what would be
expected of a stainless steel alloy. The In-Ovation
bracket followed a similar load deflection curve as the
Damon2 and the Smart Clip brackets; however, the
torque expression for the In-Ovation bracket started
much earlier than the other 2.
In the data for these 4 brackets, the amounts of
variation were considerable. For the Speed brackets,
this variation was between 1.97 and 11 Nmm; for the
In-Ovation brackets, it was 3.7 to 16.7 Nmm; for the
Damon2 brackets, it was 1.4 to 11.2 Nmm, and for
the Smart Clip brackets, it was 2.8 to 14.2 Nmm (Table
III). This variation was small for small torsion angles
and increased as the torsion angle increased.
This variation can be attributed to the bracket, the
wire, or the experimental technique. Testing of the
wires is underway as a part of another study focusing
on orthodontic wire characteristics. The preliminary
results show that the 0.019 ⫻ 0.025-in stainless steel
wires are consistent. The error analysis showed that this
Badawi et al 727
American Journal of Orthodontics and Dentofacial Orthopedics
Volume 133, Number 5
experimental technique was reliable and consistent; the
measurement error of the transducer was 1.5%, and the
average coefficient of variation (standard deviation as a
percentage of the mean) was 2.7% (Table I). Repeated
measurements of the same bracket and wire showed a
small amount of variation. Figure 4 confirms that,
during data collection for the error analysis, the wires
and the brackets were not permanently distorted, and
the graph shows no pattern of decrease in the loads as
the measurements were made.
We concluded that the significant amount of variation in measurements resulted from structural variation
in the brackets, specifically the archwire slot size. As
with any other product, the manufacturing process of
brackets results in some variation in sizes and characteristics, including dimensional accuracy and torque
prescription consistency.2 Various bracket manufacturing processes such as injection-molding, casting, and
milling can affect the accuracy of the prescribed torque
values, and this has been reported to be about 5% to 10%.2
Cash et al17 investigated slot size of orthodontic brackets
and found that all brackets tested were oversized—
between 5% and 17%. Investigators who evaluated thirdorder moments generated by specific bracket/archwire
combinations concluded that there is considerable variation in the tolerances of orthodontic appliances, and
that these variances are clinically unacceptable because
they reduce the amount of third-order control that
otherwise should be present with those wire/bracket
combinations.10,11,25
CONCLUSIONS
1. The active self-ligating brackets seem to have better
torque control, a direct result of their active clip
forcing the wire into the bracket slot.
2. The amount of archwire bracket slop was considerably less for active self-ligating brackets than
passive self-ligating brackets.
3. The active self-ligating brackets expressed higher
torque values than the passive self-ligating brackets
at clinically usable torsion angles (0°-35°).
4. The passive self-ligating brackets produced lower
moments at low torsion angles and started producing higher moments at high torsion that cannot be
used clinically.
5. The clinically applicable range of torque activation
was greater for the active self-ligating brackets than
for the passive self-ligating brackets.
6. All the brackets showed significant variations in the
torque expressed; this seemed to be attributed to the
variation in bracket slot dimensions. Damon2 and
Speed brackets were relatively more consistent than
Smart Clip and In-Ovation brackets.
7. Based on our findings, future research should investigate torque expression on all maxillary anterior
teeth simultaneously.
8. This research warrants a closer look at the usefulness of multiple bracket prescriptions because of
the wide variations in torque expression and the
high degrees of bracket archwire slop in passive
self-ligating bracket systems.
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