Materials Transactions, Vol. 46, No. 7 (2005) pp. 1551 to 1554
Special Issue on Medical, Healthcare, Sport and Leisure Materials
#2005 The Japan Institute of Metals
Bending Properties of Co–Ni–Cr–Mo Alloy Wire for Orthodontic Application
Takayuki Yoneyama1;2; * , Osamu Takahashi3 , Equo Kobayashi1 , Takao Hanawa1 and Hisashi Doi1
1
Department of Metallurgy, Division of Biomaterials, Institute of Biomaterials and Bioengineering,
Tokyo Medical and Dental University, Tokyo 101-0062, Japan
2
Department of Materials Engineering, School of Engineering, The University of Tokyo, Tokyo 113-8656, Japan
3
Development Department, SII Micro Parts Ltd., Sendai 989-3124, Japan
In order to evaluate the clinical performance of the Co–Ni–Cr–Mo alloy wire in orthodontic application, bending properties of Co–Ni–Cr–
Mo alloy wire was investigated in comparison with the conventional stainless steel wire and Co–Cr alloy wire. Three-point bending test was
carried out for the wires with varied drawing rate up to 3.0 mm in deflection and then unloaded with 0.2 mm/s of loading/unloading speed.
Bending yield load of the Co–Ni–Cr–Mo alloy wire increased and the residual deflection decreased with increasing wiredrawing rate. The best
processing condition was decided as the wiredrawing rate of 78% with the age-hardening treatment. The bending elastic modulus of the Co–Ni–
Cr–Mo alloy wire was higher than those of stainless steel wire and Co–Cr alloy wire. The bending strength of the Co–Ni–Cr–Mo alloy wire was
lower than that of stainless steel wire and was comparable to that of Co–Cr alloy wire.
(Received January 20, 2005; Accepted March 1, 2005; Published July 15, 2005)
Keywords: cobalt nickel chromium molybdenum alloy, orthodontic wire, dental application, three-point bending test, drawing rate, heat
treatment, bending yield, bending strength, bending elastic modulus
1.
Introduction
Many types of metallic materials are used for many dental
applications in restorative dentistry, prosthodontics, orthodontics and oral surgery, such as gold alloys and Co–Cr
alloys for dental castings, titanium and titanium alloys for
dental implants and bone plates, etc. Among the varieties of
clinical purposes of metallic materials in dentistry, orthodontic wires are used to produce the force to move malaligned teeth to their proper positions. Although the main
orthodontic force derives from elastic or super-elastic
recovery in bending, torsional moment also works for
correcting tooth rotation by means of rectangular wires.
These orthodontic wires are connected to the slots of
orthodontic brackets adhered to each tooth.
The main alloys of orthodontic wires are stainless steel,
Co–Cr alloy, Ti–Ni alloy and Ti–Mo alloy. The wire
selection by orthodontists is based on the clinical purpose
and treatment method. Recently, super-elastic Ti–Ni alloy
wires are widely used for the early dynamic stage, where
relatively large tooth movement is required. Ti–Ni alloy
wires are capable of exhibiting a super-elasticity which
provides light, continuous force for physiological and
efficient tooth movement.1,2)
On the other hand, high elastic modulus is required in the
finishing stage to control the final slight movement of teeth.
Hard stainless steel wires and Co–Cr alloy wires are normally
used for this purpose. However, the former are easily broken
and have rough surface, while the latter need to be heattreated after bending in clinics to be hardened and consequently discolor.
It was reported that a Co–Ni–Cr–Mo alloy, developed as a
new spring material, possessed superior mechanical properties and high corrosion resistance.3) The purpose of this study
is to investigate the bending properties of this Co–Ni–Cr–Mo
alloy wire in comparison with conventional orthodontic wires
*Corresponding
author, E-mail: yoneyama.met@tmd.ac.jp
with high elastic modulus in order to evaluate the clinical
performance of the Co–Ni–Cr–Mo alloy wire in orthodontic
application.
2.
Experimental Procedures
Co–Ni–Cr–Mo alloy wires (SPRON 510, SII Micro Parts
Ltd., Japan), 0.402 mm in diameter, were used in this study.
The chemical composition of the alloy was; Ni: 30, Cr: 21,
Mo: 10, Fe: 2, Nb: 1.5, Ti: 0.8, Mn: <0:5, Bal: Co (mass%).
Three wiredrawing rates were applied to process the wires:
70, 78 or 86%, then straightened. Age-hardening treatment at
873 K for 7.2 ks in vacuum was performed. For the references, a hard stainless steel wire (Australian Wire, Special
plus, TP Orthodontics Inc., USA) and a Co–Cr alloy wire
(Red Elgiloy, Resilient, RMO, USA) were used, of which the
specimen diameters were 0.412 and 0.400 mm, respectively.
Heat treatment was performed to harden the Co–Cr wire in a
clinical condition, since this treatment is usually performed
to harden this wire after forming.
To investigate the bending property of the wires a threepoint bending test was carried out, which simulates the
clinical application of the wire on teeth.2,4,5) Figure 1 shows a
Wire
Steel pole
5 mm
Load
cell
Micrometer
7 mm
Displacement
transducer
Fig. 1 Schematic drawing of the three-point bending test device, viewed
from above.
T. Yoneyama, O. Takahashi, E. Kobayashi, T. Hanawa and H. Doi
schematic drawing of the device. The center pole was
combined with a load cell to measure the load on the wire.
The two side poles were mounted on a movable stage
connected with a displacement transducer to measure the
deflection of the wire. Neither bracket nor ligature wire was
used to avoid the influence of ligature condition. The
maximum deflection was set at 3.0 mm, then the load on
the wire was removed. The loading and unloading speed was
approximately 0.2 mm/s.
To compare the bending property of the wires, one-way
factorial analysis of variance was used for the detection of the
differences among conditions. Tukey–Kramer test was
performed as the post hoc test for the detection of the
differences between conditions. Statistical significance was
set at p < 0:01.
3.
10
(b)
8
(c)
Load, F /N
1552
(a)
6
4
2
0
Results and Discussion
0
To evaluate the clinical performance of the new Co–Ni–
Cr–Mo alloy wire, a bending test simulated the clinical
condition was selected, because the main orthodontic force
for tooth movement was derived from the elastic deformation
in bending of orthodontic wires. Typical load-deflection
diagrams of the Co–Ni–Cr–Mo alloy wires with different
wiredrawing rate before and after age-hardening treatment at
873 K for 7.2 ks are shown in Fig. 2. Within each specimen
group with or without age-hardening treatment, yield load
increased and residual deflection decreased with increasing
wiredrawing rate. With respect to the influence of agehardening, elastic modulus and yield load increased markedly and residual deflection decreased in addition to the
above-mentioned changes. Since the age-hardened wire with
wiredrawing rate of 86% showed rather brittle property in a
trial bending test to form clinical loops, the age-hardened
wire with wiredrawing rate of 78% was selected to be
compared with the referential wires.
1
2
3
Deflection, d /mm
4
Fig. 3 Load-deflection diagrams of (a) age-hardened Co–Ni–Cr–Mo alloy
wire with wiredrawing rate of 78%, (b) hard stainless steel wire, (c) heattreated Co–Cr alloy wire.
Figure 3 shows typical load-deflection diagrams of the
age-hardened Co–Ni–Cr–Mo alloy wire with wiredrawing
rate of 78%, the hard stainless steel wire, and the heat-treated
Co–Cr alloy wire. To eliminate the influence of the difference
in specimen diameter, three parameters were taken from
these diagrams, bending elastic modulus, bending strength,
and residual deflection to compare the bending properties of
the alloy wires.
The bending elastic moduli of the age-hardened Co–Ni–
Cr–Mo alloy wire with wiredrawing rate of 78%, the hard
stainless steel wire, and the heat-treated Co–Cr alloy wire are
shown in Fig. 4. The bending elastic modulus is a parameter
10
6
Bending elastic modulus, E b/GPa
8
Load, F /N
280
(f)
(e)
(d)
(c)
(b)
(a)
4
2
0
0
1
2
3
Deflection, d /mm
4
Fig. 2 Load-deflection diagrams of Co–Ni–Cr–Mo alloy wires. (a) wiredrawing rate of 70%. (b) 78%. (c) 86%. (d) 70% with age-hardening. (e)
78% with age-hardening. (f) 86% with age-hardening.
260
240
220
200
78A
HSS
HCC
Fig. 4 Bending elastic moduli of age-hardened Co–Ni–Cr–Mo alloy wire
with wiredrawing rate of 78% (78A), hard stainless steel wire (HSS), and
heat-treated Co–Cr alloy wire (HCC).
Bending Properties of Co–Ni–Cr–Mo Alloy Wire for Orthodontic Application
1.5
Residual deflectoin, d R/mm
5000
Bending strength, σ b/PMa
1553
4800
4600
4400
4200
4000
78A
HSS
exhibiting the functional force level in orthodontic treatment,
and high value in this modulus is effective at the final
treatment stage where the slight correction of teeth alignment
is required. The value of the age-hardened Co–Ni–Cr–Mo
alloy wire was significantly higher than those of the hard
stainless steel wire and the heat-treated Co–Cr alloy wire.
Therefore, the functional property of the new Co–Ni–Cr–Mo
alloy wire was evaluated to be superior to the wires currently
used.
Bending strength of the wires was calculated with the
maximum bending load within the deflection range up to
3.0 mm. The bending strength values of the age-hardened
Co–Ni–Cr–Mo alloy wire with wiredrawing rate of 78%, the
hard stainless steel wire, and the heat-treated Co–Cr alloy
wire are shown in Fig. 5. Since the orthodontic force is
delivered in the elastic deformation range of the wires, large
stress exceeding the elastic limit is applied only in the
forming process of the wires to fit the individual dental arch.
Therefore, this value is a parameter to show the required
force in shaping as well as the maximum orthodontic force of
the wires. Although the bending strength value of the Co–Ni–
Cr–Mo alloy wire was the lowest among the three alloy
wires, it was significantly lower only than that of the hard
stainless steel wire. This means a relatively low elastic stress
range and easy shaping by orthodontists of the Co–Ni–Cr–
Mo alloy.
Figure 6 shows the residual deflection, after being unloaded from 3.0% deflection, of the age-hardened Co–Ni–
Cr–Mo alloy wire with wiredrawing rate of 78%, the hard
stainless steel wire, and the heat-treated Co–Cr alloy. This
value shows the amount of permanent deformation after fixed
condition of deformation and means the workability of the
wires. The residual deflection value of the age-hardened Co–
Ni–Cr–Mo alloy wire was significantly higher than those of
the other two, the hard stainless steel wire and the heattreated Co–Cr alloy wire, while the value of the hard stainless
steel wire was also significantly higher than that of the heat-
0.5
0
HCC
Fig. 5 Bending strength values of age-hardened Co–Ni–Cr–Mo alloy wire
with wiredrawing rate of 78% (78A), hard stainless steel wire (HSS), and
heat-treated Co–Cr alloy wire (HCC).
1
78A
HSS
HCC
Fig. 6 Residual deflection values of age-hardened Co–Ni–Cr–Mo alloy
wire with wiredrawing rate of 78% (78A), hard stainless steel wire (HSS),
and heat-treated Co–Cr alloy wire (HCC).
treated Co–Cr alloy wire. Consequently, it was evaluated that
the workability of the new Co–Ni–Cr–Mo alloy wire was
superior to the other wires.
Since orthodontic wires are used in the oral cavity of
patients for months, corrosion resistance is a very important
factor for the material. It was reported that this Co–Ni–Cr–
Mo alloy showed much better corrosion resistance than the
stainless steels of SUS 304 and 316L in chemically
accelerated conditions.3) Since SUS 304 and 316L are major
materials for orthodontic wires and surgical implants,
respectively, the corrosion resistance of the Co–Ni–Cr–Mo
alloy is thought to be sufficient for the dental application.
Co–Cr alloys have been used as major metallic materials in
dentistry and surgery for a long time.6) The composition of
the alloy used in this study is rather similar to the one of an
alloy for surgical implant, Co–Ni–Cr–Mo alloy.7,8) The
composition of the Co–Ni–Cr–Mo alloy used in this study is
different from the one in the standards in low nickel content
and niobium addition. The Co–Cr alloy wire used as a
referential material in this study also included additional
elements. The approximate basic compositions is reported to
be Co–20Cr–16Fe–15Ni–7Mo.9)
Among the dental and surgical alloys with similar
constituent elements, the superior bending properties of the
Co–Ni–Cr–Mo alloy used in this study is caused by the alloy
composition as well as the selection of the conditions for
work hardening and aging treatment. In addition, the Co–Ni–
Cr–Mo alloy wire showed clinically better surface condition
than the references, smooth surface to follow the tooth
movement for better clinical performance without being
discolored by additional heat treatment.
4.
Conclusions
The bending properties of the new Co–Ni–Cr–Mo alloy
wires were investigated in comparison with conventional
1554
T. Yoneyama, O. Takahashi, E. Kobayashi, T. Hanawa and H. Doi
hard stainless steel wires and the heat-treated Co–Cr alloy
wires. It was evaluated that the Co–Ni–Cr–Mo alloy wire
showed superior bending properties as an orthodontic
material as follows:
(1) Yield load of the Co–Ni–Cr–Mo alloy wire increased
and the residual deflection decreased with increasing
wiredrawing rate and age-hardening treatment at 873 K
for 7.2 ks. The best processing condition was decided as
the wiredrawing rate of 78% with the age-hardening
treatment.
(2) The bending elastic modulus of the Co–Ni–Cr–Mo
alloy wire was higher than those of the hard stainless
steel wire and the heat-treated Co–Cr alloy wire.
(3) The bending strength of the Co–Ni–Cr–Mo alloy wire
was lower than that of the hard stainless steel wire and
was comparable to that of the heat-treated Co–Cr alloy
wire.
(4) The residual deflection of the Co–Ni–Cr–Mo alloy wire
was higher than those of the hard stainless steel wire
and the heat-treated Co–Cr alloy wire.
It is concluded that the Co–Ni–Cr–Mo alloy wire is
evaluated to exhibit excellent clinical performance and
handling for the final stage of orthodontic treatment.
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