Download Influence of Temperature Changes on Loading and Unloading

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10.5005/jp-journals-10021-1197
ORIGINAL ARTICLE
Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study
Influence of Temperature Changes on
Loading and Unloading Characteristics of
NiTi Wires: An in vitro Study
1
Ch Sudheer Kumar, 2Shyamala Chandrasekar, 3Sridhar Kodumuru
ABSTRACT
Aims and objectives: The study was undertaken to evaluate the influence of temperature changes on loading and unloading characteristics
of NiTi wires.
Materials and methods: NiTi wires included in the study were divided into six groups respectively (group-1 to group-6). Each group consisted
of 10 NiTi wires. Three point bending test was done to study the effect of loading and unloading in each group of NiTi wires (Conventional NiTi,
HANT and Cu NiTi) using universal testing machine at 37°C and 0°C.
Results: Showed that HANT wires exhibited less amount of force during loading and unloading compared to conventional and Cu NiTi at 37°C,
whereas, Cu NiTi wires require less force for loading and exhibited gradual recovery during unloading compared to conventional and HANT wires
at 0°C.
Keywords: NiTi wires, Loading and unloading forces, Universal testing machine.
How to cite this article: Kumar CHS, Chandrasekar S, Kodumuru S. Influence of Temperature Changes on Loading and Unloading Characteristics
of NiTi Wires: An in vitro Study. J Ind Orthod Soc 2013;47(4):411-416.
INTRODUCTION
Orthodontic wire history starts from the use of piano wires,
gold wires, stainless steel wires, to the recently introduced
transformation thermo NiTi. According to the availability of
the arch wire the strategy of the mechanotherapy also keeps
on changing.
In mechanotherapy the periodic change of wire from the
initial phase of treatment to finishing phase is mandatory. In
the period prior to seventies when gold and stainless steel were
the only available materials, this was affected by altering the
cross section of the wire, i.e. from small to large and its
geometry, i.e. round to rectangular. Complicated loop designs
were required to alter the stiffness characteristics of the wire
for a chosen tooth movement. The strategy of wire selection
and usage is called variable cross section orthodontics.1
In the mid seventies a host of new arch wire became
available, i.e. nitinol and beta-titanium. With the availability
1,3
Reader, 2Professor
Department of Orthodontics, Kothiwal Dental College, Moradabad, Uttar
Pradesh, India
2
Department of Orthodontics, Saveetha Dental College, Chennai, Tamil
Nadu, India
3
Department of Orthodontics, People’s College of Dental Sciences
Bhopal, Madhya Pradesh, India
1
Corresponding Author: Ch Sudheer Kumar, Reader, Department of
Orthodontics, Kothiwal Dental College, Moradabad, Uttar Pradesh
India, Phone: 07500656169, e-mail: drsudheercholleti@ gmail.com
Received on: 22/7/13
Accepted after Revision: 1/10/13
of wires with widely varying moduli, it becomes possible for
the clinician to select wires with lower moduli for the early
stages of treatment and increase the moduli to higher levels
toward the finish of the treatment. This approach has been
termed by Burstone as variable modulus orthodontics.1
In the nineties, nickel-titanium archwire, that have super
elastic and thermodynamic properties were introduced, i.e.
Cu NiTi, NeoSentalloy, etc. By taking advantage of the body
temperature and setting the alloys transformation temperature
(Af) for the martensitic transformation, precise control of
the memory phenomenon can be affected. This is called as
varying transformation temperature orthodontics.
In stress induced phase transformation the austenitic phase
of nitinol wires is transformed to martensitic phase during
activation. Upon deactivation the reverse occurs. Thermo
elastic nitinol is a martensitic active alloy which is influenced
by temperature that ultimately exhibit a thermally induced
shape memory effect. Transient phase transformation from
martensitic to austenite occurs in ambient oral temperature.
Experimental prediction of the force exerted by the various
wires is necessary to avoid any untoward pressure to the
periodontal tissue of the individual patient. Hence, this study
was undertaken to evaluate the influence of temperature changes
on loading and unloading characteristics of NiTi wires.
AIMS AND OBJECTIVES
The aims and objectives of this study were:
1. To evaluate the loading and unloading properties of the
following different types of round and rectangular nickeltitanium wires, at 37°C.
a. 0.016" round conventional NiTi
The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416
411
Ch Sudheer Kumar et al
b. 0.016" round copper NiTi
c. 0.016" round heat activated NiTi
d. 16" × 22" rectangular conventional NiTi
e. 16" × 22" rectangular copper NiTi
f. 16" × 22" rectangular heat activated NiTi.
2. To evaluate the influence of temperature changes on
loading (at 0°C) and unloading (at 37°C) properties of the
above mentioned group of wires.
MATERIALS AND METHODS
Wires and their Configurations2
The wires tested included three different commercially
available superelastic nickel-titanium orthodontic archwires
from different manufacturers most commonly used in the
Department of Orthodontics, Saveetha Dental College.
The sample wires included in the study were divided in to
the following six groups.
Groups
No. of
samples
Type of
wire
Group I
10
Conventional NiTi
0.016"
Group II
Group III
10
10
Copper NiTi 35°C
Heat activated NiTi
0.016"
0.016"
Group IV
10
Conventional NiTi
16" × 22"
Group V
Group VI
10
10
Copper NiTi 35°C 16" × 22"
Heat activated NiTi 16" × 22"
Fig. 1: Acrylic block
Dimensions Manufacturer
American
orthodontics
Ormco
Lancer
orthodontics
American
orthodontics
Ormco
Lancer
orthodontics
Fig. 2: Hounsfield universal testing machine
METHOD
Three point bending test was done to study the effect of loading
and unloading of 10 specimens from each group of wires at
37°C and 0°C. A total of 120 samples were tested for the
same, for the deflection of 1 and 2 mm respectively.
Following armamentarium were used to do the three point
bending test at 37°C and 0°C.
• Specially designed acrylic block (Fig. 1)
• Hounsfield universal testing machine (Fig. 2)
• A water bath (Fig. 3)
• NiTi archwires (Fig. 4)
• Twin edgewise brackets (Fig. 5)
• Ligature director and other necessary instruments required
for bonding (Fig. 6)
• Loading of NiTi wire at 37°C (Fig. 7)
• Loading of NiTi wire in water bath (Fig. 8)
• Operating unit (Fig. 9)
• Digital display meter (Fig. 10)
• Light cure composite.
Fig. 3: Water bath
Acrylic Block
An acrylic block measuring 60 × 40 × 20 mm was fabricated
using self-cure acrylic. A cut was made in the block to a depth
412
Fig. 4: NiTi archwires
JIOS
Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study
Fig. 5: Twin edgewise brackets
Fig. 8: Loading of NiTi wire in water bath at 0°C
Fig. 6: Armamentarium
Fig. 9: Operating unit
Fig. 10: Digital display meter
Fig. 7: Loading of NiTi wire at 37°C
of 10 × 10 mm in the center of the block to allow the deflection
of wire samples for the beam test.3
Four 0.018 inch standard medium twin edgewise brackets
3.5 mm wide with 0° torque and angulation were fixed using
light cure composite at a distance of 9 and 8 mm.3
From each batch, a 30 mm long piece of wire was cut from
the nearly straight, posterior section of the individual archwire
blanks. The specimens were tested in orthodontic bending with
a hounsfield universal testing machine, fitted with a 500 kg
compression load cell calibrated on the 2 kg range with a 2 kg
standard weight traceable to the National Bureau of standards.
The deflecting rod of the machine was fitted with the common
ligature director instrument, to simulate a clinical condition.
The testing machine was operated at crosshead speed of
2.0 mm per minute and the deflection was fixed for 2 mm.
The jig was placed in the water bath which maintains the
water at 37°C and ice cold water at 0°C and testings were
conducted for 120 samples to find out loading and unloading
at 37°C and 0°C. An industrial thermometer with calibrations
from –5°C to 100°C was used to monitor the temperature
variations every minute.
The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416
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Ch Sudheer Kumar et al
The loading and unloading force values measured in
Newtons for all the 120 samples were recorded in the
computer and noted down. The readings were tabulated, and
subjected to statistical analysis.
STATISTICAL ANALYSIS
The data values tabulated were subjected to the following
statistical analysis:
• An analysis of variance.
• One-way ANOVA was used to compare the mean values
between different study groups.
• A multiple range test by Tukey-Honesty significantly
different procedure to identify the significant groups at
5% level.
Keeping the different wire types constant and varying the
temperature, the p-value was calculated. p < 0.05 denoting
statistical significance.
Mean, standard deviation and test of significance were
calculated between different temperatures within each study
group.
RESULTS
Inference
Round and rectangular heat activated NiTi wires requires less
amount of force (2.36N, 5.68N) to deflect the wire compared
to conventional NiTi (2.64, 7.84N) and copper NiTi (2.84,
6.12N) which is statistically significant (Table 1).
While unloading, round conventional NiTi (2.00N) and
heat activated NiTi (2.00N) has recovered gradually by
releasing slow and constant forces compared to copper NiTi
(2.08N). During unloading procedure round copper NiTi wire
at 37°C return to 0-level suddenly, on the other hand both
rectangular conventional NiTi (4.72N) and heat activated NiTi
(3.56N) wires reached 0-level instantly, but rectangular copper
NiTi (3.28N) wires recovered gradually (Table 2).
Round heat activated NiTi wire requires less amount of
force (2.52N) to deflect the wire compared to conventional
NiTi (3.13N) and copper NiTi (3.36N). Rectangular copper
NiTi wire requires less amount of force (7.27N) to deflect
the wire compared to conventional NiTi (9.65N) and heat
Table 1: Mean, standard deviation and test of significance of mean values between different study groups
Temperature
Loading at 37°C
Unloading at 37°C
Groups
Mean ± SD
p-value
Significant groups
5% level
I
II
III
IV
V
VI
2.64 ± 0.21
2.84 ± 0.34
2.36 ± 0.21
7.84 ± 0.77
6.12 ± 0.10
5.68 ± 0.17
0.001 (S)
II vs III
<0.0001 (S)
IV vs V, VI
I
II
III
IV
V
VI
2.00 ± 0.00
2.08 ± 0.10
2.00 ± 0.00
4.72 ± 0.51
3.28 ± 0.62
3.56 ± 0.34
0.007 (S)
II vs I, III
<0.0001 (S)
IV vs V, VI
*One-way ANOVA was used to calculate the p-value; Note: Study 1; deflection = 1 mm
#Multiple range test by Tukey–HSD procedure was employed to identify the significant groups at 5% level
Table 2: Mean, standard deviation and test of significance of mean values between different study groups
Temperature
Groups
Mean ± SD
p-value
Significant groups 5% level
Loading at 37°C
I
II
III
IV
V
VI
3.13 ± 0.57
3.36 ± 0.60
2.52 ± 0.23
9.65 ± 1.96
7.27 ± 1.20
7.45 ± 1.89
<0.0001 (S)
II vs III
I vs III
<0.0001
IV vs V, VI
I
II
III
IV
V
VI
2.19 ± 0.20
2.38 ± 0.33
2.05 ± 0.05
6.32 ± 1.70
4.20 ± 1.14
5.07 ± 1.60
0.0001 (S)
II vs I, III
0.0002
IV vs V, VI
Unloading at 37°C
Note: Study 1; deflection = 2 mm
414
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Influence of Temperature Changes on Loading and Unloading Characteristics of NiTi Wires: An in vitro Study
Table 3: The mean, standard deviation and test of significance of mean values of loading (at 0°) and
unloading (at 37°C) of different types of round and rectangular NiTi archwires upto 1 mm deflection
Temperature
Groups
Mean ± SD
p-value
Loading at 0°C
I
II
III
IV
V
VI
I
II
III
IV
V
VI
2.92 ± 0.29
2.12 ± 0.17
1.20 ± 0.19
5.72 ± 0.94
2.28 ± 0.10
2.96 ± 0.57
2.32 ± 0.29
2.00 ± 0.00
1.00 ± 0.00
4.76 ± 0.08
3.60 ± 0.38
4.84 ± 0.41
<0.0001 (S)
I vs II, III
II vs III
<0.0001 (S)
IV vs V, VI
<0.0001 (S)
I vs II, III
II vs III
<0.0001 (S)
IV vs V
VI vs V
Unloading at 37°C
Significant groups 5% level
Note: Study II; deflection = 1 mm
activated NiTi (7.45N) wires. While unloading, round heat
activated NiTi has recovered gradually by releasing slow and
constant forces (2.05N) compared to conventional NiTi
(2.19N) and copper NiTi (2.38N). Rectangular copper NiTi
recovered gradually (4.20N) compared to conventional NiTi
(6.32N) and heat activated NiTi wires (5.07N) (Table 3).
That round heat activated NiTi requires less amount of
force (1.20N) to deflect the wire compared to conventional
NiTi (2.92N) and copper NiTi (2.12N) wires. Rectangular
copper NiTi requires less amount of force (2.28N) to deflect
the wire compared to conventional NiTi (5.72N) and heat
activated NiTi (2.96N) wires. While unloading, round heat
activated NiTi (1.00N) wire recovered gradually by releasing
slow and constant forces compared to conventional NiTi
(2.32N) and copper NiTi (2.00N) wires. Rectangular copper
NiTi (3.60N) wire recovered gradually compared to
conventional NiTi (4.76N) and heat activated NiTi (4.84N)
wires (Table 4).
Round heat activated NiTi requires less amount of force
(1.40N) to deflect the wire compared to conventional NiTi
(3.41N) and copper NiTi (2.30N) wires. Rectangular copper
NiTi requires less amount of force (3.06N) to deflect the wire
compared to conventional NiTi (7.61N) and heat activated NiTi
(3.94N) wires. While unloading, round heat activated NiTi
(1.21N) recovered gradually compared to conventional NiTi
(2.73N) and copper NiTi (2.17N) wires. Rectangular copper
NiTi (4.71N) recovered gradually compared to conventional
NiTi (6.48N) and heat activated NiTi (6.29N) wires.
DISCUSSION
The nickel-titanium wires have the lightest force delivery and
widest elastic range, with outstanding spring back, particularly
for the shape memory alloys.
The physical behavior of the nickel-titanium wires can be
interpreted and explained from the metallurgic analysis.5 At
the high temperature range the crystal structure of nickeltitanium alloy is an austenitic phase which is body center cubic
lattice.4 The martensitic phase which is a closely packed
hexagonal lattice is at a low temperature range.6A change in
temperature of the wire produces a change in the crystal
structure called the martensitic transformation.4-6
Bending stiffness of the nickel-titanium alloy wire is
reduced when the wire is subjected to cold temperature which
makes it more ductile clinically.8 In this study the bending
stiffness was measured after the samples of wires subjected
to cold testings at 0°C. It was found that the bending stiffness
Table 4: The mean, standard deviation and test of significance of mean values of loading (at 0°C) and
unloading (at 37°C) of different types of round and rectangular NiTi archwires upto 2 mm deflection
Temperature
Groups
Mean ± SD
p-value
Loading at 0°C
I
II
III
IV
V
VI
3.41 ± 0.56
2.30 ± 0.24
1.40 ± 0.25
7.61 ± 2.15
3.06 ± 0.80
3.94 ± 1.14
<0.0001 (S)
I vs II, III
II vs III
<0.0001 (S)
IV vs V, VI
I
II
III
IV
V
VI
2.73 ± 0.50
2.17 ± 0.24
1.21 ± 0.22
6.48 ± 1.78
4.71 ± 1.19
6.29 ± 1.52
<0.0001 (S)
I vs II, III
II vs III
0.0001 (S)
IV vs V
VI vs V
Unloading at 37°C
Significant groups 5% level
Note: Study II; deflection = 2 mm
The Journal of Indian Orthodontic Society, October-December 2013;47(4):411-416
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Ch Sudheer Kumar et al
was considerably reduced when compared to the testings at
room temperature.7
Elastic recovery of nickel-titanium archwires were superior
to that of any other wires. Any permanent distortion of the formed
archwire will alter its characteristics and result in lack of tooth
movement. Rate of improvement in elastic recovery with
permanent deformation becomes very important. The better the
elastic recovery of the wire the more difficult it is to deform and
it is less ductile. The deterioration in elastic recovery can be
circumvented by stress relief annealing, although this eliminates
that part of the improvement in elastic properties of formed
components which are loaded favorbly.9,10
Julio de A Gurgel11 conducted a study evaluating force
deflection behavior of 8 superelastic nickel-titanium
orthodontic wires (35°C Cu NiTi, 27°C Cu NiTi, nitinol heat
activated, neosent alloy F200, nickel-titanium, rematitan lite,
elastinol 35, elastinol 27) under controlled moment and
temperature using testing machine. He applied deflections of
0.2 to 2.0 mm at 35°C in the lateral incisor area for copper
NiTi 35°C, copper NiTi 27°C, elastinol 35, nickel-titanium,
neosentalloy and these wires that exhibited the lowest
activation, deactivation forces might be more adaptable
clinically in case of mild to moderate crowding.
The present study was conducted to evaluate the influence
of temperature changes on loading and unloading
characteristics of NiTi wires. In the present experiment it was
noted that less amount of force was exerted by round and
rectangular heat activated NiTi when deflected to 1 mm, but
in unloading both conventional and heat activated rectangular
NiTi wires recovered faster which could be injurious to
periodontal ligaments and painful for the patients.
Even though less amount of force was needed for
rectangular heat activated NiTi for loading, the behaviour of
recovery is not conducive biologically. Though rectangular
copper NiTi needed significantly excess force to load but it
exhibited a smooth and gradual recovery. A similar observation
was reported by Torstein R Meling.7
In the 2 mm activation, in loading, round heat activated
NiTi required less force. In unloading, round heat activated
NiTi recovered gradually as in 1 mm deflection. Similarly,
rectangular copper NiTi recovered gradually. So in the case
of minor crowding round and rectangular heat activated NiTi
where deflection is minimal and in case of severe crowding
where deflection is more round heat activated NiTi and
rectangular copper NiTi could be considered.
Loading at 0°C for 1 mm of activation round heat activated
NiTi and rectangular copper NiTi exerted less amount of force.
But in unloading both conventional and heat activated
rectangular NiTi wires recovered faster which could induce
more force and discomfort for the patients. But rectangular
copper NiTi wires while unloading exhibited a smooth and
gradual recovery.
CONCLUSION
The introduction of new alloys into orthodontics offers a new
approach in controlling the magnitude of forces used for tooth
416
movement. In this study the basic aim was to explore the effect
of temperature variation on bending stiffness of nickeltitanium arch wires without a change in the property of the
wires. The result can be summarized as:
• Both round and rectangular heat activated NiTi wires
required less amount of force for loading. However, though
the unloading force was also less, the recovery was faster
at 37°C for 1 mm and 2 mm deflections compared to
conventional and copper NiTi wires.
• At 0°C round and rectangular copper NiTi wires required
less amount of force for loading and exhibited gradual
recovery during unloading for 1 and 2 mm deflections
compared to conventional and heat activated NiTi wires.
• Apart from loading and unloading properties, two other
factors may be taken into consideration while making
choice of wire for initial alignment, namely- cost effectiveness and clinical viability. Copper NiTi is expensive and it
needs cooling conditions to 0°C. Taking the above factors
into consideration, it is concluded that - Heat activated NiTi
wires are preferable in mild to moderate crowding cases,
but in periodontally compromised cases copper NiTi wires
are preferable taking into account its light and continuous
deactivation forces.
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