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© 2020 Journal of Orthodontic Science | Published by Wolters Kluwer ‑ Medknow 1
Impact of recycling on the mechanical
properties of nickel‑titanium alloy
wires and the ecacy of their reuse
after cold sterilization
Ankit Yadav, Poonam K Jayaprakash, Rajeshwar Singh1, Meeta Dawer,
Palash Modi2, Bhumika Sehdev3 and Kiran K. Ganji4
Abstract:
OBJECTIVE: This study aimed to assess the feasibility of reusing nickel–titanium (NiTi) alloy
wires after 6 weeks of intraoral use by evaluating the changes in the load‑deection properties and
surface characterization of these alloy wires after cold sterilization by immersion in 2% of acidic
glutaraldehyde for 10 h.
MATERIAL AND METHODS: Twenty wires each in three groups of G1‑as‑received wires (ARW),
G2‑unsterilized used wires, and G3‑sterilized used wires (SUW) were tested by the three‑point
bending test and scanning electron microscopy (SEM). The data were subjected to statistics, one‑way
analysis of variance, and Bonferroni posthoc test for comparison.
RESULTS: Recycling of NiTi wires produced statistically insignicant changes in both the loading and
unloading properties of the wires. The forces needed to twist the used wires, that is, G2‑(UUW) and
G3‑(SUW) were lower than G1‑(ARW), suggesting lowering of the stiffness of the wires. Superelasticity
is well‑maintained by G2‑(UUW) and G3‑(SUW) although there is an insignicant lowering of the
forces exerted by them during loading and unloading. SEM demonstrated no increase in the pitting
of surfaces in both G2‑(UUW) and G3‑(SUW); multiple areas were seen to be more smoothened
over G2‑(UUW) and G3‑(SUW) NiTi wires surfaces.
CONCLUSION: The ndings of this study support the reuse of NiTi wires after 6 weeks of use in oral
conditions followed by cold sterilization by immersion in 2% acidic glutaraldehyde for 10 h.
Keywords:
Bending test, cold sterilization, nickel‑titanium alloy wires, recycling, superelasticity
Introduction
Nickel–titanium (NiTi) alloy wires gained
popularity because of their properties
like superelasticity and shape memory.
However, because of the higher cost value,
more than 50% of the orthodontists recycle
these wires for economic reasons.[1,2] To
eliminate the potential health hazards to
patients on whom these recycled archwires
are used, effective sterilization methods
must be used.[3‑5] Approximately 80% of
these orthodontists use chemical solutions,
that is, a cold method for disinfecting or
sterilizing these wires. The most popular
disinfectants and sterilants, authorized
by the American Dental Association,
include 2% glutaraldehyde and chlorine
dioxide for the 2% acidic glutaraldehyde
(Banicide) sterilization time is 10 h without
any dilution. Most of the disinfectants and
sterilants are reportedly corrosive and attack
the metallic substances that are immersed
in them.[1]
Address for
correspondence:
Dr. Kiran K. Ganji,
Faissalyia,
Sakaka 72721,
Al Jouf Province, KSA.
E‑mail: kiranperio@gmail.
com
Submitted: 15‑Jul‑2019
Revised: 23‑Nov‑2019
Accepted: 29‑Feb‑2020
Published: 18‑Aug‑2020
Department of
Orthodontics and
Dentofacial Orthopedics,
Teerthankar Mahaveer
Dental College,
Moradabad,
Uttar Pradesh,
2Consultant Orthodontist,
Phoenix Hospital,
Panchkula, Haryana,
India, Department
of 1Orthodontics and
Dentofacial Orthopedics
and 3Periodontology,
Mekelle University,
Ethiopia, 4Department
of Preventive Dentistry,
College of Dentistry, Jouf
University, KSA
Original Article
Access this article online
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Website:
www.jorthodsci.org
DOI:
10.4103/jos.JOS_45_19
How to cite this article: Yadav A, Jayaprakash PK,
Singh R, Dawer M, Modi P, Sehdev B, et al. Impact
of recycling on the mechanical properties of
nickel‑titanium alloy wires and the ecacy of their
reuse after cold sterilization. J Orthodont Sci
2020;9:10.
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Yadav, et al.: Nickel‑titanium reuse after cold sterilization
2 Journal of Orthodontic Science | 2020
To reuse NiTi wires following cold sterilization
treatment in patients, these must be assessed for
selected properties like stiffness, strength, and surface
characteristics. Few studies exist on the effects of cold
sterilization with chemical solutions on NiTi wires. This
study was conducted to assess the feasibility of reusing
NiTi alloy wires by evaluating the changes in mechanical
properties and their surface characteristics after cold
sterilization by immersion in 2% acidic glutaraldehyde
for 10 h.
Methods
Sixty 0.016 NiTi alloy archwires (Nitinol Superelastic
Wire, 3M Unitek, CA) were divided into three groups
of 20 samples each. Twenty as‑received wires (ARW)
served as control (G1). The remaining 40 wires were
placed intraorally for a period of 6 weeks in patients
undergoing orthodontic treatment. After 6 weeks, these
wires were taken out and cleaned with 70% isopropyl
alcohol for the removal of any debris. Out of these 40
wires, 20 unsterilized used wires (UUW) formed the
second group (G2). A third group (G3) comprised the
remaining 20 sterilized used wires (SUW) that were
sterilized using 2% acidic glutaraldehyde for a duration
of 10 h. A three‑point bending test was conducted to
ascertain the load‑deection properties of the nickel–
titanium archwires.[6] A typodont set with brackets
bonded on it using adhesive was used as a jig [Figure
1]. The test wires were secured to the brackets with the
elastomeric modules. The rst premolar was removed
from the typodont set. The distance kept between the
midaxes of brackets from the canine and the second
premolar was 14 mm apart. A bracket bonded to the
metallic rod, which gets attached to the load cell [Figure
2], was used to apply force to deect the wire section
between the canine and premolar brackets. The jig is
attached to the crosshead of an Instron machine (Instron
Corp., Canton Mass) with 50 kg force on load cell [Figure
3]. The speed of the crosshead of the testing machine was
set at 1 mm/min for a total of 2 mm deection for the
loading of the wire. The crosshead was then reversed and
the wire was unloaded. The forces required to deect the
wires for 0.2 mm intervals during loading and unloading
were recorded and plotted for displacement on the X–Y
recorder. After every test run, the next wire to be tested
was relegated and the entire procedure was repeated.
A scanning electron microscope (SEM, ZEISS EVO 50)
was used to arbitrarily choose and examine six different
segments of the wire specimens, two specimens from
each group. Representative SEM images of the wire
specimens were studied at a magnication of 1000× to
expose any changes in the surface texture of the NiTi
wires after using 2% acidic glutaraldehyde. Ethical
approval was obtained from the institutional ethical
committee (TMDC/18/34‑456). The Statistical Package
for the Social Sciences software (version 21) and Epi Info
version 3.0 were used for the statistical analysis.
Figure 1: Typodont set used as a jig
Figure 2: Metallic rod with bracket
Figure 3: Universal testing machine crosshead with jig attached for three‑point
bending test
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Yadav, et al.: Nickel‑titanium reuse after cold sterilization
Journal of Orthodontic Science | 2020 3
Results
The values obtained for G1, G2, and G3 wires during
loading and unloading were tabulated, and the average
mean at each interval was calculated [Figures 4 and 5].
Table 1 shows the comparison of the mean peak load at
2 mm deection between the three groups done using
a one‑way analysis of variance. Test results revealed a
highly signicant difference (P < 0.01) between the three
groups. G1‑(ARW) showed the highest mean peak load
value of 304.60 ± 38.15 g. G2‑(UUW) showed the mean
peak load value of 254.20 ± 42.51 g, which was the least
of all the three tested groups. The mean peak load value
of G3 was 268.90 ± 44.11 g.
Table 2 for intergroup comparison revealed a highly
signicant difference (P < 0.01) in the mean peak load
values between the G1‑(ARW) and G2‑(UUW). The
difference in the mean peak load between the G1‑(ARW)
and G3‑(SUW) is also signicant (P < 0.05), but the mean
peak load difference value between the G2‑(UUW) and
G3‑(SUW) is nonsignicant (P > 0.05).
Table 3 represents the mean force lost during unloading
of the wires in the groups G1‑(ARW), G2‑(UUW), and
G3‑(SUW), when the wire deection was decreased by 0.6
mm, that is, from 1.6 mm to 1.0 mm. Mean values of the
force lost during unloading for G1‑(ARW), G2‑(UUW),
and G3‑(SUW) were 21.50 ± 17.30 g, 14.75 ± 18.39 g, and
15.70 ± 15.22 g, respectively. A comparison of mean force
lost during unloading between G1‑(ARW), G2‑(UUW),
and G3‑(SUW) shows a nonsignicant difference among
the three groups of wires.
Figure 6 graph for G1‑(ARW) shows a nonlinear
load‑deection curve. In the loading curve of G1‑(ARW),
the average mean force at 0.4 mm of deection is 151.6
g at 1.0 mm of deection force value is 234.6 g, thus,
force value increased by 83 g (54.96%). During the
unloading of the wire, the average mean force at 1.0
Figure 4: Average mean values of force (in grams) at an interval of 0.2 mm
deection during loading of wires
Figure 5: Average mean values of force (in grams) at an interval of 0.2 mm
deection during unloading of wires
Figure 6: Graphical representation of force values (in grams) and their average
means obtained at intervals of 0.2 mm deection during loading and unloading of
G1‑as‑received wires (ARW)
Table 1: Mean peak load (in grams) at 2 mm deection
Mean SD Standard error F P
ARW (G1) 304.6 38.15 8.53 7.740 0.001**
UUW (G2) 254.2 42.51 9.50
SUW (G3) 268.9 44.11 9.86
**P<0.01 (highly signicant). ARW: as‑received wire; UUW: unsterilized used
wire; SUW: sterilized used wire
Table 2: Intergroup comparison of mean peak load
(in grams) at 2 mm deection
(I) GPs (J) Gps Mean peak load P
G1‑(ARW) G2‑(UUW) 50.40 0.001**
G2‑(UUW) G3‑(SUW) 14.70 0.808
G3‑(SUW) G1‑(ARW) 35.70 0.027*
*P<0.05 (signicant), **P<0.01 (highly signicant)
Table 3: Force (in grams) lost on unloading curve
between the interval from 1.6 mm to 1.0 mm
Mean SD Standard error F P
ARW (G1) 21.50 17.3 3.87 0922 0.404
UUW (G2) 14.75 18.39 4.11
SUW (G3) 15.70 15.22 3.40
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Yadav, et al.: Nickel‑titanium reuse after cold sterilization
4 Journal of Orthodontic Science | 2020
mm of deection of the wire was 190.1 g, and then, the
force value decreased by 91.7 g (48.23%) from 98.4 g at
0.4 mm of deection.
Figure 7 for the G2‑(UUW) shows a nonlinear
load‑deection curve. The mean peak load value of
G2‑(UUW) is 254.2 ± 42.51 g at 2 mm deection. In
the loading curve of G2‑(UUW), the average mean
force at 1 mm of deection was 228.80 g, and the force
value became 266.5 g at 1.6 mm of deection, that is,
it increased by 8.10 g (3.49%) while loading. During
the unloading of the wire, the average mean force at
1.6 mm of deection of the wire was 185.45 g, and it
was 170.7 g at 1.0 mm interval of deection curve, that
is, it decreased by 14.75 g (7.97%). This shows that the
wire shows superelastic property in this region. In the
unloading curve of G2‑(UUW), the average mean force
at 0.4 mm of deection was 144.6 g, and the force value
became 228.8 g at 1.0 mm of deection while loading.
Thus, the force value increased by 82.2 g (56.07%) for
further 0.6 mm increase in deection. During unloading
of the wire, the average mean force at 1.0 mm of
deection of the wire was 170.7 g, and then, the force
value decreased by 64.5 g (37.78%) from 106.2 g at 0.4
mm of deection.
Figure 8 representing G3‑(SUW) shows a superelastic
plateau during loading and unloading of the wire. In
the loading curve, the average mean force at 1 mm of
deection was 229.20 g, and then, it increased by 15.20
g (6.63%) for further 0.6 mm increase in deection (i.e.,
244.40 g at 1.6 mm). During the unloading of the wire, the
average mean force at 1.6 mm of deection of the wire
was 197.70 g, and then it, decreased by 15.70 g (7.94%)
to 182.00 g at 1.0 mm interval of the deection curve. In
the loading curve of G2‑(SUW), the average mean force
at 0.4 mm of deection was 146.50 g, and then, the force
value increased by 82.70 g (56.45%) for a further 0.6 mm
increase in deection, that is, the force value became
229.20 g at 1.0 mm of deection while loading. During
the unloading 40 of the wire, the average mean force at
1.0 mm of deection of the wire was 182.00 g, and then,
the force value decreased by 78.90 g (43.35%) from 103.10
g at 0.4 mm of deection.
Figure 9 shows that GI‑(ARW), G2‑(UUW), and G3‑(SUW)
NiTi wires demonstrate Pseudoplasticity, as the wires are
displaced from 1 mm to 1.6 mm during loading of the
wires, and pseudoelasticity, as the wires reverted from
1.6 mm to 1 mm deection during unloading.
Surface Topography
The SEM image of G1‑(ARW) at 1000× magnication
is shown in Figure 10a which reveals that the ARW
shows many round or oval pitting and relatively wider
depressions that must have been created during the
manufacturing process of these wires. The SEM image
of G2‑(UUW) at 1000× magnication is shown in Figure
10b, which reveals a smoother surface of the wire than
the ARWs. The presence of deep indentations was seen
on the wire surface, which was because of the drawing
Figure 7: Graphical representation of force values (in grams) and their average
means obtained at intervals of 0.2 mm deection during loading and unloading of
G2‑unsterilized used wires (UUW) wires
Figure 8: Graphical representation of force values (in grams) and their average
means obtained at intervals of 0.2 mm deection during loading and unloading of
G3‑sterilized used wires (SUW)
Figure 9: Graphical representation of the comparison of the load‑deection curves
of G1‑(ARW), G2‑(UUW), and G3‑(SUW)
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Yadav, et al.: Nickel‑titanium reuse after cold sterilization
Journal of Orthodontic Science | 2020 5
process of the wire while the manufacturing process.
SEM image of G3‑(SUW) at 1000× magnification is
shown in Figure 10c, revealing a smoother surface of
wire as compared with the ARW group, which could be
explained by the wear occurring between the wire and
the bracket surfaces. Notches were present at certain
places. There was a presence of certain prominences and
depression which could be because of the defect in the
manufacturing process.
Discussion
Many studies have reported an intraoral deterioration
of NiTi wires because of corrosion in the uoride‑rich
environment.[4,7‑9] The feasibility to use these wires after 4
to 6 weeks of intraoral use is a matter of concern. Dry heat,
cold sterilization, and autoclaving of the used NiTi wires
are the reported methods for sterilization.[10,11] Various
studies have reported that the nitinol and titanium
alloy wires can be heat sterilized without deteriorating
their mechanical properties. Nickel–titanium wires
undergo phase changes because of heat treatment
which alters their properties.[4] Temperatures greater
than 60°C increased the vulnerability of these wires to
deform plastically and reduced their springiness.[8,10,11]
Intraoral use exposes the wire to physical stresses and
oral conditions like thermodynamic changes and forces
from mastication and occlusion. G2‑(UUW) has been
included in our study to understand these changes in
the mechanical and surface characteristics of the used
NiTi wires. In G3‑(SUW), the wires were exposed to the
effects of physical stresses in oral conditions as well as
cold sterilization, which subjected it to corrosion attack
from 2% glutaraldehyde and cold working, which can
both alter its properties.
To demonstrate the differences between the first
nitinol wire and the superelastic NiTi wires in 1986,
a three‑point bending test was introduced by Miura.
[12] However, many other authors have also advocated
that the archwire should be tested under restraint
so that the wire is not free at both ends to simulate
the clinical situation. In addition, higher force
values during loading and unloading are obtained,
as compared with previous methods.[13] Beyond 2
mm of deflections, permanent deformation starts
to set in; thus, most of the studies use a range of 0.2
mm of deflections.[1,2,5,12] The loading portion of the
graph simulates the activation of the wires; whereas,
the unloading section of the graph denotes the
deactivation of force, which causes tooth movement
during clinical performance.[1‑3,12]
The G1‑(ARWs) were found to have signicantly higher
loading and unloading forces than G2 and G3 wires.
The crystallographic behavior of the NiTi wires in our
study resembles that of austenitic NiTi alloys when
interpreted by the stress–strain graph. The initial linear
loading curve represents a purely elastic deformation
of the austenitic phase. The curve attens to a nonlinear
pattern at the same load (pseudoplasticity), where the
martensitic transformation begins. Level of the plateau
signifies the load exerted during the completion of
martensitic transformation, which is lower for recycled
wires. When the reverse transformation to the austenitic
phase begins during the unloading, the graph again
shows plateau (pseudoelasticity), at a particular load
from where stress‑induced martensitic structure exists.
In the nal part of the deactivation curve, the phase
transformation from the martensitic to austenitic phase
is completed [Figures 6–9]. The unusual nature of the
superelastic material is that the loading curve differs
from the unloading curve (hysteresis) is depicted by the
load‑deection curves of the three groups.
The recycled NiTi wires (G2 and G3) exerted reduced
forces while loading and unloading compared with G1
wires. There were statistically signicant changes in
loading and unloading forces in the interval between 0.6
mm and 1.0 mm during loading and 1.0–0.6 mm during
unloading. This showed a reduction in the pseudoelastic
Figure 10: (a‑c) SEM images of G1‑(ARW), G2‑(UUW), G3‑(SUW) at
1000× magnication
c
b
a
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Yadav, et al.: Nickel‑titanium reuse after cold sterilization
6 Journal of Orthodontic Science | 2020
characteristics during unloading in the lower ranges of
deection in the recycled NiTi wires, which exhibits as
work‑hardening caused by the summative effects of
masticatory forces and abrupt intraoral temperatures
changes. Similar ndings have been reported by other
authors.[12,14] It suggests that recycled NiTi wires must be
activated more frequently or earlier archwire changes
must be exercised. The comparison of mean peak
load at 2 mm deection reveals a highly signicant
difference (P < 0.01) among the tested groups.
G1‑(ARW) showed the highest mean peak load value
of 304.60 ± 38.15 g. Both G2‑(UUW) and G3‑(SUW)
groups showed the decreased value of mean peak load
and the difference was statistically signicant. These
ndings are in accordance with many other studies.
[1,14] They show that there is a reduction in loading and
unloading forces, after the clinical use of 6 weeks (G2)
and (G3). Kapila et al. have reported that the clinical
use contributes more than the sterilization in causing
the changes in the load‑deection characteristics.[1,5,15]
According to Segner et al., a plateau value of 0.5 mm
is considered a good value.[15] The recycled NiTi wires
(G2 and G3) have a clinical plateau of 0.6 mm length;
thus, the recycled wire showed very well‑maintained
superelastic characteristics that are needed for clinical
use. The nding of our study supports the recycling
of NiTi wires, as these wires retain their desirable
mechanical properties after cold sterilization with 2%
acidic glutaraldehyde. However, the testing procedure
used is a static environment, that is, thermal and
dynamic changes, such as forces e of mastication and
occlusion were nonexistent.[12,13]
Many investigators have reported increased sensitivity
of the recycled wires to corrosion; thus, surface
characteristics were assessed with SEM to study the
surface topography of the three groups.[9,12] SEM
specimens were examined at 1000× magnications. The
images obtained for representative segments of recycled
wires showed no signs of increased pitting. Instead,
G2 and G3 wires demonstrated areas of smoothness
and some surfaces of the wires were scored. This
smoothening and scoring results from abrasion because
of the sliding and rubbing of these wires within the
bracket slot, as explained by previous investigators.[15‑17]
Some reports have concluded that in‑vitro corrosion
does not affect the physical properties of recycled NiTi
wires.[18] However, nickel dissolution that occurs from
corroded surfaces of NiTi wires could have adverse
reactions in patients previously sensitized to nickel.
[19] The ndings of this study suggest the possibility of
reuse of recycled NiTi wires after 6 weeks following
cold sterilization using 2% acidic glutaraldehyde for
10 h; however, further research is needed to validate
its clinical application.
Conclusion
There is a reduction in stiffness exhibited by the recycled
NiTi wires after 6 weeks of clinical use. The surface
topography of the clinically exposed wires also shows
no increase in pitting of the surface, indicating no sign
of corrosion attack because of oral environment or
sterilization procedure with 2% glutaraldehyde. The
ndings of this study support the reuse of NiTi wires
after 6 weeks of use in oral conditions, followed by cold
sterilization by immersion in 2% acidic glutaraldehyde
for 10 h.
Financial support and sponsorship
Self‑funding.
Conicts of interest
There are no conicts of interest.
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