nical properties
ontic wires
and clinical
Sunil Kapila, BDS, MS,* and Rohit Sachdeva, DDS, MS**
San Francisco,
Calif,, and Dallas,
Texas
This review article describes the mechanical properties and clinical applications
of stainless steel,
cobalt-chromium,
nickel-titanium,
beta-titanium,
and multistranded
wires. The consolidation
of this
literature will provide the clinician with the basic working knowledge on orthodontic wire
characteristics
and usage. Mechanical properties of these wires are generally assessed by tensile,
bending, and torsional tests. Although wire characteristics
determined
by these tests do not
necessarily reflect the behavior of the wires under clinical conditions, they provide a basis for
comparison
of these wires. The characteristics
desirable in an orthodontic wire are a large
springback,
low stiffness, good formability, high stored energy, biocompatibility
and environmental
stability, low surface friction, and the capability to be welded or soldered to auxiliaries. Stainless
steel wires have remained popular since their introduction to orthodontics
because of their
formability, biocompatibility
and environmental
stability, stiffness, resilience, and low cost. Gobaltchromium (Co-Cr) wires can be manipulated
in a softened state and then subjected to heat
treatment. Heat treatment of Co-Cr wires results in a wire with properties similar to those of stainless
steel. Nitinol wires have a good springback and low stiffness. This alloy, however, has poor
formability and joinability. Beta-titanium
wires provide a combination
of adequate springback,
average stiffness, good formability, and can be welded to auxiliaries. Multistranded
wires have a high
springback and low stiffness when compared with solid stainless steel wires. Optimal use of these
orthodontic
wires can be made by carefully selecting the appropriate
wire type and size to meet the
demands of a particular clinical situation. (AM J ORTHOD DENTOFAC
ORTHOP
1989;96:100-9.)
ecent advances in orthodontic wire alloys
have resulted in a varied array of wires that exhibit a
wide spectrum of properties. Up until the 193Os, the
only orthodontic wires available were made of gold.
Austenitic stainless steel, with its greater strength,
higher modulus of elasticity, good resistance to corrosion, and moderate costs, was introduced as an orthodontic wire in 1929, and shortly afterward gained popularity over gold.‘,’ Since then several other alloys with
desirable properties have been adopted in orthodontics.
These include cobalt-chromium, nickel-titanium, betatitanium, and multistranded stainless steel wires.
Presently the orthodontist may select, from all the
available wire types, one that best meets the demands
of a particular clinical situation. The selection of an
appropriate wire size and alloy type in turn would pro*Formerly Graduate Resident and Clinical Instructor in Orthodontics
at the
University of Oklahoma, College of Dentistry; presently member of the faculty,
Departmeni of Growth and Development, and doctoral student at the Universiry
of California San Francisco, School of Dentistry.
**Associate Professor in the Department of Orthodontics,
Baylor College of
Dentistry.
vide the benefit of optimum and predictable treatment
results. The clinician must therefore be conversant with
the mechanical properties and the relevant clinical applications of these properties for these wires. Although
several investigators have evaluated the mechanical
properties of various wire types, a cohesive clinical
interpretation of their findings is lacking. This article
reviews pertinent literature in order to describe the mechanical properties and optimal clinical applications of
stainless steel, cobalt-chromium, nickel-titanium, betatitanium, and multistranded wires. The objective of this
article is to provide the practicing clinician with the
basic working knowledge on orthodontic wire characteristics and usage.
LABORATORY TESTS
The properties of orthodontic wires are commonly
determined by means of various laboratory tests. Thus,
wires have previously been investigated under tension,3-9 in bending,5-7.9-‘6 and torsion6a’0,‘6*17(Table I).
Although these tests do not necessarily reflect the clinical situations to which wires are usually subjected: they
Mechanical
properties
and
clinical
of wires
applications
I
le P.Summary of wires tested and properties determined by previous investigators
Test
Tensile
Test type
Conventional
test*
tensile
Investigators
Goldberg
and
Burstone’
Drake et aL6
Miura et al7
Hurst’
Kusy and Stush’
Bending
Stiffness
Formability
Three-point
Torsion
tester?
test
Goldberg
and Burstone
Drake et a1.6
Andreasen
and
Morrowa’
Schwaninger
et al.”
Burstone et alI6
Goldberg
and Burstone’
Miura et al.’
Yield diameter
Simulated arch wire
Three- and four-point
test
Automated
spring tester
Ingram et al.‘”
Schaus and NikolaiI
Kusy and Stush9
Kusy and Dilley”
Goldberg
et al. I4
Torsional
Drake et aL6
Andreasen
and
Morrow”
Burstone et al.“’
Larson et al.”
test$
Torque gauge
Torsional
test”
Wires
tested
Properties
evaluated
SS, B-ti
YS, E, YSIE,
spring
rate
SS, Niti, B-ti
SS, Niti, B-b, Jpn
Niti
Niti wires
Niti, B-b
YS, E, YSIE
Deflection/load
SS, B-ti
SS, Niti,
SS, Niti
YS, E, YSIE, spring rate
YS, E, YSIE, spring rate, MR
Bending moment,
spring rate, M,
graphs
Shape memory
YS, UTS, % elongation
B-ti
Niti
SS, Niti, Chinese Niti
SS, B-ti
SS, Niti, B-ti, Jpn
Niti
SS, Co-Cr, Niti
SS, Niti, B-ti, Msd
Niti, B-ti
Msd
SS, Co-Cr, Niti, B-ti,
Msd
Flexural YS
Bending moment, springback
Formability
Deflection/load
graphs
SS, Niti,
SS, Niti
B-ti
M,,
M,,
SS, Niti,
SS, Niti,
Chinese
B-ti
Niti
Springback
Flexural stiffness
E
E
Flexural E
spring
sprmg
rate
rate
Angular deflection
Shear modulus,
torsional
YS
Wires-SS
= stainless steel, Niti = nitinol,
B-ti = beta-titanium,
Co-Cr = cobalt-chromium,
Jpn Niti = Japanese nitinol, Msd =
multistranded;
Properties-YS
= yield strength, YSiE = relates to springback,
UTS = ultimate tensile strength, E = moduius of eiasticity,
M, = modulus of resilience
or stored energy.
*Instron Universal
Testing Machine,
Canton, Mass.
tTinius
Olsen Testing Machine Co., Philadelphia,
Pa.
ZMaillefer
Torsiometer,
Switzerland.
EY:
provide a basis for comparison of these wires.5-7 Tests
in bending provide some information on the behavior
of wires when subjected to first- and second-order
bends. Similarly, results of torsional tests reflect, to
a certain degree, wire characteristics in a third-order
direction I
Tension, bending, and torsion are uniquely different
stress states and place varied demands on wire performance. ‘*.19 The properties of wires under these three
stress states are therefore considered independently.
Graphic description of stress against strain can be used
to determine yield strength, modulus of elasticity,
stored energy, and springback when the wire is subjected to tensile loading. Similarly, graphic plots of
bending moment against angular deflection or torsional
moment against torque angle are used for the evaluation
of these wire characteristics under conditions of bending
and torsion, respectively (Fig. ‘1).
WIRE CHARACTERISTICS
CLINICAL RELEVANCE
OF
Several characteristics of orthodontic wires are considered desirable for optimum performance during treatment.5Jo,zoThese include a large springback, low stiffness, high formability, high stored energy, biocompatibility and environmental stability, low surface friction,
and the capability to be welded or soldered to auxiliaries
and attachments. A brief description of each of these
desirable wire characteristics is provided.
I. Spuingback. This is also referred to as maximum
elastic deflection, maximum flexibility, range of activation, range of deflection, or working range. Springback is related to the ratio of yield strength to the modulus of elasticity of the material (Y S / E) . Higher springback values provide the ability to apply large activations
with a resultant increase in working time of the appliance. This, in turn, implies that fewer arch wire changes
Am. J Orthod.
2 Kapiln and Sachdeva
0
15
A
30
Angular
45
DeflectIon
60
75
Denrofac. Orthop.
Augm 1989
90
(Degree)
B
Torque
Angle (Degree)
Fig. 1. Diagrammatic
representation
of graphs
obtained
in bending
(A) and torsional (B) tests. These
graphs
demonstrate
the differences
in properties
of stainless
steel, beta-titanium,
and nitinol wires and
the slightly different
responses
of these specimens
under conditions
of bending and torsion. The slope
of the straight
line on the graph represents
the stiffness
or load deflection
rate (E) of the wire; the
shaded
area under each plot is the stored
energy
at a fixed bending
or torsional
moment.
(Modified
from Drake SR, Wayne
DM, Powers
JM, Asgar K. AM J ORTHOD 1982;82:206-10.)
or adjustments will be required. Springback is also a
measure of how far a wire can be deflected without
causing permanent deformation” or exceeding the limits of the material.‘*
2. Stiffness or load dejection rate. This is the force
magnitude delivered by an appliance and is proportional
to the modulus of elasticity (E)14(Fig. 1). Low stiffness
or load deflection rates provide (1) the ability to apply
lower forces, (2) a more constant force over time as
the appliance experiences deactivation, and (3) greater
ease and accuracy in applying a given force.5s2o
3. Formability. High formability provides the ability to bend a wire into desired configurations such as
loops, coils, and stops without fracturing the wire.
4. Modulus of resilience or stored energy (MR).
This property represents the work available to move
teeth. It is reflected by the area under the line describing
elastic deformation of the wire (Fig. 1).
5. Biocompatibility and environmental stability.
Biocompatibility includes resistance to corrosion and
tissue tolerance to elements in the wire. Environmental
stability ensures the maintenance of desirable properties
of the wire for extended periods of time after manufacture. This, in turn, ensures a predictable behavior
of the wire when in use.
6. Joinability. The ability to attach auxiliaries to
orthodontic wires by welding or soldering provides an
additional advantage when incorporating modifications
to the appliance.
7. Friction. Space closure and canine retraction in
continuous arch wire techniques involve a relative motion of bracket over wire. Excessive amounts of
bracket/wire friction may result in loss of anchorage
or binding accompanied by little or no tooth movement.
The preferred wire material for moving a tooth relative
to the wire would be one that produces the least amount
of friction at the bracket/wire interface.
MECHANICAL
PROPERTIES
CLINICAL IMPLICATIONS
AND THEIR
Tables II and III summarize the composition and
important mechanical and clinical characteristics of
orthodontic wire alloys and will be used to describe the
findings of various investigators.
Stainless
steel wires
Carbon interstitial hardening and cold working cantribute to the high yield strength and modulus of elasticity of stainless steel (Table II). Residual stressespresent in a wire subsequent to bending can markedly affect
the elastic properties of the wire.” Heat treatment is
therefore used in stress-relieving stainless steel after
bending the wire into an arch, loops, or coils. This
helps to enhance the elastic properties of the wire.22-2s
The recommended temperature-time
schedule for
stress-relieving stainless steel is 750” F (399” C) for 1 I
minutes.26x27FunkZ3 recommends the use of a color index to determine when adequate heat treatment is
achieved. He suggests that a straw-colored wire indicates that optimum heat treatment has been attained.
Mechanical
properties
and clinical
applications
qf wires
1
gable ill. Composition, yield strength (YS), modulus of elasticity (E), and the ratio of yield strength to
modulus of elasticity (YS /E) of orthodontic wires in tension and bending as reported by various
investigators6,9”9.“‘.53
(YS/E is an indication of the springback of the wire; data from Asgharnia and
13rantley’9, Drake et a1.,6 Kusy et al. ,29 and Kusy and Stush’ are calculated means from various wire sizes)
Reference
Wire type and composition
Stainless steel-71%
18% Cr, 8% Ni,
CO.28
c
Fe,
Goldberg
Asghamia
Kusy
and Burstonex3
and Brantley”
et al.29
Test type
YS
X l@psi
(X IO9 Nfm’j
E
x Ioipsi
(x IO’ Nltr?)
YSIE x 10-j
Tensile
Tensile”
Bending*
Tensile?
Bendingt
E: Bending*
YS: Tensile*
275
188
287
226
330
227
(1.89)
(1.30)
(1.98)
(1.56)
(2.27)
(1.56)
25,000(172.4)
24,400(168.3)
30,800(212.4)
25,000(172.4)
32,800(226.2)
28,000(193.0)
11.0
7.1
9.3
9.0
10.1
8.1
319
130
210
180
290
(2.20)
(0.90)
(1.45)
(1.24)
(2.00)
29,000(200.0)
25,000(172.4)
31,000(213.8)
28,000(193.1)
36,000(248.3)
11.0
5.2
6.8
6.4
8.0
Cobalt-chromium-40%
Co,
20% Cr, 15% Ni; 15%
Fe, 7% MO, 2% Mn
Goldberg
and Burstone”
Asgharnia
and Brantley”
(blue Elgiloy)
Tensile
Tensile*
Bending*
Tensile7
Bending?
Nickel-titanium-52%
45% Ti, 3% Co
Asghamia
Tensile*
Bending*
Tensile*
Bending*
E: Bending*
YS: Tensile”
49
82
45
65
86
69
(0.34)
(0.56)
(0.31)
(0.45)
(0.59):
(0.48)s
5,000
7,000
4,300
4,300
6,440
4,850
(34.5)
(48.3)
(29.7)
(29.7)
(44.4)$
(33.4)s
9.8
il.7
10.4.
15.1
13.3
14.2
Tensile
Tensile*
Bending*
Tensile*
Bending*
E: Bending*
YS: Tensile*
170
95
165
125
141
105
(1.17)
(0.45)
(1.14)
(0.86)
(0.97)
(0.72)
9,400
9;400
12,500
9,960
8,370
10,500
(64.8)
(64.8)
(86.2)
(68.7)
(57.7)
(72.4)
LX.0
IO.1
13.2
12.5
16.8
10.0
Ni,
Beta-titanium-79%
Ti,
11% MO, 6% Zr, 4% Sn
and Brantley’g
Drake
et aL6
Kusy
and Stush’
Goldberg
Asgharnia
and Burstonex3
and Brantley’9
Drake
et ah6
Kusy
and Stush’
*As received.
tHeat treated to 900” F (482” C)
f Round wires.
§ Square and rectangular
wires.
Commercially available stainless steel wires demonstrate a range of values both for the modulus of elasticity and yield strength.29 The large modulus of elasticity of stainless steel (Table II) and its associated high
stiffness necessitate the use of smaller wires for alignment of moderately or severely displaced teeth. A reduction in wire size results in a poorer fit in the bracket
and may cause loss of control during tooth movement.
However, high stiffness is advantageous in resisting
deformation caused by extra- and intraoral tractional
forces.6
The yield strength to elastic modulus ratio (YS /E)
indicates a lower springback of stainless steel than those
of newer titanium-based alloys (Tables II and III). The
stored energy of activated stainless steel wires is substantially less than that of beta-titanium and nitinol
wires?” (Fig. 1). This implies that stainless steel wires
produce higher forces that dissipate over shorter periods
of time than either beta-titanium or nitinol wires, thus
requiring more frequent activations or arch wire
changes.
Joinability with stainless steel is possible by soldering but may be demanding.’ Stainless steel wires
also can be fused together by welding, but this generally
requires reinforcement with solder. Corrosion resistance of stainless steel wires is good,30 although solder
joints may corrode in the oral cavity. Park and Shearer?’
have demonstrated the release of nickel and chromium
from stainless steel appliances. The authors further note
that although the amounts of nickel and chromium Rleased are below the average dietary intake, the liberated elements may sensitize patients or produce reactions in already sensitized persons. Low levels of
bracket/wire friction have been reported with experi-
1
Kapila
Stainless
and
steel
Cobalt-chromium
Nickel-titanium
Beta-titanium
Multistranded
Am. .I. Or&d.
Sachdeva
Low
High
Good
Low
Good
Low
High
Good
Low
Good
High
Average
High
Low
Average
Low
Poor
Good
Poor
High
Average
High
t
Good
Good
Soldered
Welded
Soldered*
Welded?
Not joinable
Welded
Soldered
Welded
Dent&.
OrOioy.
Augusr 1989
LOW
Low-moderate
Low-moderate
High
Not kmwn
*Soldered
with some difficulty.
‘r Blue and yellow Elgiloy only.
$Some corrosion
and failures noted.
ments using stainless steel wires .32-34This signifies that
stainless steel wires offer lower resistance to tooth
movement than other orthodontic alloys.
obalt-chromium
wires
Cobalt-chromium (Co-Cr) alloys are available commercially as Elgiloy,* Azura,? and Multiphase.$ Elgiloy is manufactured in four tempers: soft (blue), ductile (yellow), semiresilient (green), and red (resilient)
in increasing order of resilience. Blue Elgiloy is the
softest of the four wire tempers and can be bent easily
with fingers or pliers. It is recommended for use when
considerable bending, soldering, or welding is required.
Heat treatment of blue Elgiloy increases its resistance
to deformation (Table II). Yellow Elgiloy is relatively
ductile and more resilient than blue Elgiloy. It can also
be bent with relative ease. Further increases in its resilience and spring performance can be achieved by heat
treatment. Green Elgiloy is more resilient than yellow
Elgiloy and can be shaped with pliers before heat treatment. The most resilient Elgiloy is marked red and
provides high spring qualities. Careful manipulation
with pliers is recommended when using this wire because it withstands only minimal working. Heat treatment makes red Elgiloy wire extremely resilient. Since
this wire fractures easily after heat treatment, all adjustments should be made before this precipitationhardening process.
With the exception of red temper Elgiloy, nonheattreated Co-Cr wires have a smaller springback than
stainless steel wires of comparable sizes, but this property can be improved by adequate heat treatment.‘2*24
The ideal temperature for heat treatment is 900” F
*Rocky Mountain Orthodontics,
Denver, Cola
tOnnco Corporation, Glendora, Calif.
Silmericat~ Orthodontics Corporation, Sheboygan,
Wis.
(482” C) for 7 to 12 minutes in a dental furnace.35 This
causes precipitation-hardening of the alloy, increasing
the resistance of the wire to deformation36 and results
in a wire that demonstrates properties similar to those
of stainless steel (Tables II and III). Heat treatment at
temperatures above 1200” F (749” C) results in a rapid
decline in resistance to deformation because of partial
annealing.3 Optimum levels of heat treatment are confirmed by a dark straw-colored wire or by use of temperature-indicating paste. 35
The advantages of Co-Cr wires over stainless steel
wires include greater resistance to fatigue and distortion, and longer function as a resilient spring.37 In most
other respects, the mechanical properties of Co-Cr wires
are very similar to those of stainless steel wires. Therefore, stainless steel wires may be used instead of Co-Cr
wires of the same size in clinical situations in which
heat-hardening capability and added torsional strength
of Co-Cr wires are not required.‘*
The high moduli of elasticity of Co-Cr and stainless
steel wires suggest that these wires deliver twice the
force of beta-titanium wires and four times the force of
nitinol wires for equal amounts of activation.6 The resultant undesired force vectors are therefore greater with
Co-Cr and stainless steel wires than with both types of
titanium alloys. Clinically, this may translate into
faster rates of mesial movement of posterior teeth,
thus placing greater demands on intra- and extraoral
anchorage.
Co-Cr and stainless steel wires have good formability and can be bent into many configurations relativeiy easily. Caution should be exercised when soldering attachments to these wires since high temperatures cause annealing with resultant loss in yield and
tensile strengths. 1.3Low-fusing solder is recommended
for this purpose.35
Although larger frictional forces have been noted
Mechanical
previously3’ between brackets and Co-Cr wires than
between brackets and stainless steel wires, a recent
report34 on zero torque/zero angulated brackets indicates similar values for friction between brackets and
these two types of alloys. This implies that resistance
to tooth movement along stainless steel and Co-Cr wires
may be comparable.
Nitinol, a stochiometric nickel-titanium alloy, was
first introduced for use in orthodontics in 19713* and is
available as NiTi,* Nitinol,? Orthonol,$ SentinolD and
Titanal. Commercially available nitinol wires from
various manufacturers demonstrate differences in some
properties. The most advantageous properties of nitinol
are the good springback and flexibility, which allow for
large elastic deflections’0.16 (Tables II and III). The high
springback of nitinol is useful in circumstances that
require large deflections but low forces. Although not
confirmed by values of YS/E in Table II, it has generally been noted that nitinol wires have greater springback and a larger recoverable energy than stainless steel
or beta-titanium wires when activated to the same
amount of bending or torquing6,17.3g(Fig. 1). This results in increased clinical efficiency of nitinol wires
since fewer arch wire changes or activations are required.” Similarly, for a given amount of activation,
wires made of titanium alloys produce more constant
forces on teeth than stainless steel wires. A distinct
advantage of nitinol is realized when a rectangular wire is inserted early in treatment. This accomplishes simultaneous leveling, torquing, and correction
of rotations.
Heat treatment of nitinol results in substantial alterations in mechanical properties of the alloy. Changes
in crystallographic arrangement caused by heating produce the “memory” effect in this alloy. Andreasen and
Morrow” described the “shape memory” phenomenon
as the capability of the wire to return to a previously
manufactured shape when it is heated through its transitional temperature range (TTR). This effect is realized
by holding the wire in the desired shape while undergoing high-temperature heat treatment. When subsequently cooled, the wire can be deformed within certain
strain limits, from which it recovers its original shape
if heated through its unique TTR. This change from
distorted to original form involves a transformation of
nitinol from the martensitic to the austenitic phase.
*Ormco Corporation, Glendora, Calif.
Wnitek Corporation, Monrovia, Calif.
*Rocky Mountain Orthodontics,
Denver, Cola
@AC International, Inc., Commack, N.Y.
I/Lancer Pacific. Inc., Carlsbad. Calif.
properties
and
clinical
applications
of wires
3
Hurst’ evaluated the percentage recovery of five
commercially available nitinol wires after subjecting
these wires to tensile deformation followed by heating
beyond their TTRs. He noted that the percentage recovery for NiTi, Nitinol, Orthonol, light and medium
Sentinol, and Titanal was about 90% and varied only
slightly among these wires. Some clinical uses of the
“shape memory” phenomenon have been suggested.
These include the possible consolidation of extraction
spaces40-42and alignment of crowded teeth.43*M However, further tests and improvements are required for
the “shape memory” phenomenon to become widely
accepted for clinical purposes.
Garner, Allai, and Moore33 and Kapila and
associates34 have noted that bracket/wire frictional
forces with nitinol wires are higher than those with
stainless steel wires and lower than those with betatitanium wires in zero torque/zero angulated 0 .O1g-inch
brackets. In 0.022~inch brackets, nitinol and betatitanium wires demonstrated similar levels of friction
that were greater than those with stainless steel or
Co-Cr wires. 34 Furthermore, some investigators32’45
have reported higher bracket/wire friction with nitinol
than with stainless steel wires at bracket angulations
of up to 3”. However, at higher bracket angulations, these authors demonstrated significantly lower
bracket/wire friction with nitinol than with stainless
steel wires.
Andreasen and Morrow” indicate that nitinol wires
are associated with advantages such as fewer arch wire
changes, less chairside time, reduction in time requ.ired
to accomplish rotations and leveling, and less patient
discomfort. However, several other properties of nitinol
impose limitations on its use. The poor formability of
these wires implies that they are best suited for preadjusted systems. Any first-, second-, and third-order
bends have to be overprescribed to obtain the desired
permanent bend. Nitinol fractures readily when bent
over a sharp edge.” In addition, bending also adversely
affects the springback property of this wire.46 The bending of loops and stops in nitinol is therefore not recommended .
Since hooks cannot be bent or attached to nitinol,
crimpable hooks and stops are recommended for use.
Cinch-backs distal to molar buccal tubes can be obtained by resistance or flame-annealing the end of the
wire. This makes the wire dead soft and it can be bent
into the preferred configuration. A dark blue color indicates the desired annealing temperature. Care should
be taken not to overheat the wire because this makes
it brittle.
The low stiffness of nitinol provides inadequate stability at the completion of treatment. This stability can
1
Kapila and Sachdeva
be attained by means of stainless steel wire tailored to
the desired final occlusion.
Findings on resistance to corrosion of nitinol wires
have been inconsistent. Although some investigat0rs47,48 report that nitinol is as resistant to corrosion as
stainless steel, various authors30,49have found nitinol to
be more susceptible to corrosion than other orthodontic
alloys. Further, whereas Schwaninger, Sarkar, and
Foster” have noted that corrosion does not affect flexural properties of nitinol wires, some reports46-50indicate an increase in permanent deformation and a decrease in elasticity caused by corrosion or the cumulative effects of cold-working.
Recycling of nitinol wires is often practiced because
of their favorable physical properties and the high cost
of the ~ire.~l Recycling involves (1) repeated exposure
of the wire for several weeks or months to mechanical
stresses and elements of the oral environment and (2)
sterilization between uses. Although Mayhew and
Kusy5’ and Buckthal and Kusy” have demonstrated no
appreciable loss in properties of nitinol wires after as
many as three cycles of various forms of heat sterilization or chemical disinfection, the effects of the oral
environment on the wire properties are still inconclusive.“.30,45-50The combined effects of repeated clinical
use and sterilization on the properties of nitinol wires
require further investigation before recycling of these
wires is recommended.
Beta titanium has been popularized as an orthodontic alloy only in the current decade.5%20It is commercially available as TMA* (titanium-molybdenum
alloy). Beta titanium has a modulus of elasticity that is
less than that of stainless steel and about twice that of
nitinol17 (Fig. 1 and Table II). This makes its use ideal
in situations in which forces less than those of stainless
steel are necessary and in instances in which a lower
modulus material such as nitinol is inadequate to produce the desired force magnitudes.‘O Furthermore, the
relatively lower forces generated by beta-titanium wires
imply that the counterproductive force vectors generated by beta-titanium wires can be counteracted by
smaller forces than those required for comparable stainless steel wires. Extraoral anchorage demands with
beta-titanium wires will therefore be less than those for
stainless steel wires.
The springback for beta titanium is superior to that
of stainless steel (Tables II and III). A beta-titanium
wire can therefore be deflected almost twice as much
as stainless steel wire without permanent defor*Ormco
Corporation,
Glendora,
Calif
mation.5~‘7.20.53Beta-titanium wires also deliver about
half the amount of force as do comparable stainless
steel wires5,20.53;for example, an 0.018 X 0.025-inch
beta-titanium wire delivers approximately the same
force as a 0.014 X 0.020-inch stainless steel wire in
a second-order activation. The former configuration has
the added advantage of full bracket engage~l~nt and a
resultant greater torque control than the smaller stainless
steel wire.
The good formability of beta-titanium wire allows
stops and loops to be bent into the wire. IIowever,
Burstone and Goldberg” recommend that these wires
should not be bent over a sharp radius. Helices that are
commonly used with stainless steel to lower the load
deflection rate of the appliance may not be necessary
with beta-titanium wires because of their low modulus
of elasticity and high springback. This helps to simplify
appliance design by eliminating the need to place loops
and helices in the wire.
It is possible to attach stops, hooks, and active auxiliaries by welding to beta-titanium wires, thereby increasing the versatility of the wire.20,53,54However, adequate strength of the weld without loss in wire properties is achieved within a narrow optimal voltage
setting on a resistance spot welder. Nelson, Burstone,
and Goldberg54 have provided values for these optimal
voltage settings. A flat-to-flat electrode configuration is
recommended for welding because it produces a strong
joint with low levels of distortion.j5 Overheating of the
wire causes it to become brittle.20,54
Beta-titanium has a corrosion resistance comparable
to stainless steel and cobalt-chromium alloys.j3 Betatitanium wires demonstrate higher levels of bracket/wire
friction than either stainless steel or
Co-Cr wires.33,34This may imply slower rates of tooth
movement during canine retraction and space consolidation with beta-titanium wires than with stainless steel
or Co-Cr wires.
Multistranded
wires
Multistranded wires are made of stainless steel and
composed of specified numbers of thin wire sections
coiled around each other to provide a round or rectangular cross-section. Kusy and Dilley56 investigated the
strength, stiffness, and springback properties of multistranded wires in a bending mode of stress. They noted
that the stiffness of a triple-stranded 0.0175-inch
(3 X 0.008-inch) stainless steel arch wire was similar
to that of O.OlO-inch single-stranded stainless steel
wire. The multistranded wire was also 25% stronger
than the O.OlO-inch stainless steel wire. The 0.0175
inch triple-stranded wire and 0.016inch nitinol wire
demonstrated similar stiffnesses. Kowever, nitinol tol-
Volume 96
Number
Mechanical
2
crated more than 50% greater activation than the multistranded wire. The triple-stranded wire was also half
as stiff as a 0.016-inch beta-titanium wire. In a more
recent investigation, Kusy and Stevenss7 state that although the elastic properties of multistranded wires vary
widely, several of these wires compare favorably with
some of the beta-titanium and nitinol wires. Table III
summarizes the important characteristics of multistranded wires relative to those of other alloys.
Ingram, Gipe, and Smith” noted that titanium alloy
wires and multistranded stainless steel wires have low
stiffness when compared with solid stainless steel wires.
The investigators also found that most multistranded
wires had a springback similar to that of nitinol, but a
larger springback when compared with solid stainless
steel or beta-titanium wires. Unlike stainless steel wires,
in which springback decreases with increasing thickness, the titanium and multistranded wires have springback properties that are relatively independent of wire
size. These findings agree with those made by Kusy
and Dilley56 and KLIS~.~’In contrast, Schaus and Nikolai,13 using a simulated arch form, noted that multistranded wires were less flexible than suggested by
theory or previous tests. They indicated that factors such
as interbracket distances, wire curvature, direction of
activation relative to the curved arch form, bracket
width, dimensions of bracket slot relative to wire size,
and friction between bracket and wire substantially affect the flexural stiffness of the arch wire.
PTiMAL
RTMO
CLINICAL
APPLICATIONS
OF
The practical applications of orthodontic wires can
be optimized by carefully selecting the appropriate alloy
type and wire size to meet the demands of a specific
clinical situation. Kusy5’ and Kusy and Greenberg’8,59
have recommended a sequential use of arch wires selected for optimal use of the mechanical properties of
their constituent alloys. The authors suggest that for
initial leveling requiring wide-ranging tooth movements, a 0.016inch nitinol wire outperforms a 0.0175
inch triple-stranded stainless steel wire, an 0.018-inch
round nitinol wire is superior to a 0.014-inch round
stainless steel wire, and an 0.018-inch square nitinol
wire outperforms a 0.014-inch round stainless steel
wire. However, in a recent report, Kusy and Stevens57
noted that 0.015~inch triple-stranded wires demonstrate
a greater working range than either nitinol or betatitanium wires of similar or greater dimensions. The
authors also indicate that multistranded wires compare
more favorably with titanium wires than suggested by
previous research56 and may provide a viable alternative
to the more expensive titanium wires for initial leveling.
properties
and
clinical
applications
of wires
1
The intermediate stages of treatment require closing
loops, gable bends, and attachments. Beta-titanium
wires meet these demands while providing greater range
of activation than stainless steel or Co-Cr wires. In
torsion, the formability and stiffness of stainless steel
and Co-Cr wires far exceed those of the titanium wires,
thereby making these alloys the finishing wires of
choice. The lower friction between stainless steel or
Co-Cr wires and brackets32-24suggest that these wires
may be more suitable than other alloys for movement
of teeth along a wire.
Until the recent introduction of new types of orthodontic alloys, increments in wire stiffness during treatment were instituted by progressively increasing the
cross-section of stainless steel wires. Burstone6’ refers
to this as “variable cross-section orthodontics.” The
author further states that advances in orthodontic wire
alloys have made it possible to control wire stiffness
by varying material properties-namely,
the modulus
of elasticity. This is known as “variable modulus orthodontics.”
Burstone6’ formulates his concepts by stating that
the overall stiffness of the orthodontic appliance (S) is
determined by the wire stiffness (Ws) and design stiffness (As) as represented by:
s = ws x As
Design stiffness (As) is dependent on factors such as
interbracket distance and the incorporation of loops and
coils into the wire. Changes in wire stiffness (Ws), on
the other hand, can be brought about by altering the
cross-sectional stiffness (Cs) and/or the material stiffness (MS) as designated by the formula:
Ws = MS x Cs
where the cross-sectional stiffness is determined by a
cross-sectional property such as moment of inertia of
the wire and the material stiffness is dependent on the
modulus of elasticity of the alloy. Therefore, an increase
in appliance stiffness (S) can be brought about not only
by change in appliance design or increase in crosssectional thickness of the wire, but also by selecting a
material with a higher modulus of elasticity. The relationships of material stiffnesses for stainless steel,
cobalt-chromium, nickel-titanium and beta-titanium
wires are in the ratio of 1:1.2:0.26:0.42.
Material
stiffness for multistranded wires range from M5 to l/5
of that for single-stranded stainless steel wires.6o
Several advantages of “variable modulus orthodontics” have been suggested@ as follows:
1. The amount of play between bracket and wire
is not dictated by the desired wire stiffness, but is under
full control of the clinician. This implies that the ortho-
4
Am. J. Orthod.
Kapila and Sachdeva
dontist determines the amount of bracket/wire play desired before selection of the wire. Once the crosssectional size and shape have been established, the desired stiffness can be implemented by selecting an alloy
with an appropriate material stiffness.
2. The low moduli of elasticity of the newer orthodontic alloys permit the use of light, rectangular wires
even during the early stages of treatment. Rectangular
wires are preferable over round wires because they can
be better oriented in the bracket in such a way that
forces work out in the proper directions. They further
aid in patient comfort by preventing loops from turning
into the cheeks and gingiva. Rectangular wires also
maintain better control over root position by delivering
both moments and forces.
3. The use of newer orthodontic alloys with their
lower moduli of elasticity offers substantial advantages
with a Q.Q22-inch bracket slot.
4. The selection of an appropriate alloy type and
wire size may reduce the number of arch wires needed
for alignment by reducing bracket/wire play early in
treatment. In addition, since the titanium wires also
work more efficiently and over longer periods of time
because of their greater springback, the number and
frequency of arch wire changes are reduced.
In the last few decades, a variety of new wire alloys
has been introduced into orthodontics. These wires
demonstrate a wide spectrum of mechanical properties
and have added to the versatility of orthodontic treatment. Appropriate use of all the available wire types
may enhance patient comfort and reduce chairside time
and the duration of treatment. The restricted use of only
stainless steel wires to treat an entire case from start to
finish therefore may be indicated only in relatively few
patients. It may be beneficial instead to exploit the
desirable qualities of a particular wire type that is specifically selected to satisfy the demands of the presenting clinical situation. This, in turn, would provide
the most optimal and efficient treatment results.
Orthop.
August 1989
erties of orthodontic
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Variable-modulus
Reprint requests to:
Dr. Sunil Kapila
HSW 604
University
of California
San Francisco,
CA 94143-0512
orthodontics.
AM
J ORTHOD