Wire selection in orthodontics / Orthodontics wires

Dr Abbas Naseem
drabbasnaseem@gmail.com
abbas_naseem@yahoo.com
BASIC

Learning Outcomes ……..
1. Basic Terminology
2. Mechanical properties of orthodontic wire
3. Elastic properties of orthodontic wire
4. Properties of ideal orthodontic wire
5. What are the ideal orthodontic alloys
6. Characteristics/factors that influence selection of desirable
orthodontic wire
• Wire cross-section
• Wire length
• Amount of wire
• Selection of proper wire (Alloy and Cross Section)
7. History/Evolution of archwires
8. Types of archwires

Reference:
• Contemporary Orthodontics, 5th Edition, Proffit & Fields & Sarver.
Basic Terms
Stress: Force per unit area.
Stress is the internal distribution of the load.
Strain: Deflection per unit length.
Internal distortion produced by the load.

Reference:
1. Orthodontics, the art and science, 4th Edition, S.I.Bhalaji
2. Textbook of orthodontics, 2nd Edition, Gurkeerat singh
Basic Terms
Stiffness: The presence of low stiffness provides the ability to apply lower forces and a
more constant force over time. ¹
Stiffness / Load defection rate:
This is the force magnitude delivered by an appliance and is proportional to modulus of
elasticity (E). ²
Low stiffness leads to an ability to apply lower forces, a more constant force to be delivered
overtime and greater ease and accuracy in applying a given force.
Spring back: It is the measure of how far a wire can be deflected without causing permanent
deformation. It is also called elastic deflection. The archwire should ideally
possess high springback, which results in an increase in its range of action. ¹
Spring back / maximum elastic deflection / working range:
ratio of yield strength(YS) to modulus of elasticity(E) of the material. ²
Higher springback provides ability to apply large activations with a resultant increase in
working time of appliance, thus decreasing number of archwire changes.

Reference:
The range is the distance along the X-axis to the point at which permanent deformation occurs
(usually taken as the yield point, at which 0.1%permanent deformation has occurred).
Basic Terms
Stress and strain are internal characteristics that can be calculated from measurements of
force and deflection, so the general shapes of force–deflection and stress–strain curves are
similar.
The stiffness of the material is given by the
slope of the linear portion of the curve.
Clinically useful springback occurs if the
wire is deflected beyond the yield point (as
to the point indicated here as “arbitrary
clinical loading”), but it no longer returns to
its original shape.

Reference:
1. Contemporary Orthodontics, 5th Edition, Proffit & Fields & Sarver.
Basic Terms
Resilience: Area under the stress-strain curve out to the proportional limit.
Represents the energy storage capacity of wire, which is a combination of strength
and springiness. ¹
Resilience is the amount of force the wire can withstand before permanent deformation.
Archwires should exhibit high resilience so as to increase working range of appliance.²
Formability: Amount of permanent deformation that a wire can withstand before failing.
Represents the amount of permanent bending the wire will tolerate before it
breaks. ¹
The archwire material should exhibit high formability so as to bend the arch wire into desired
configuration such as coils, loops without fracturing the wire. ²

Reference:
• Applied Dental Materials, 9th Edition, John F. Mcable (Chapter:2. page:8-9)
Resilience:
The energy absorbed by a material in undergoing
elastic deformation up to the elastic limit.
Toughness:
The total amount of energy which a material can
absorb upto point of fracture.
Basic Terms

Reference:
• Applied Dental Materials, 9th Edition, John F. Mcable (Chapter:2. page:11)
Stress & Strain Relationship (Cause and
effect Relationship)• Use to characterise the mechanical properties of materials.
• The application of an external force, producing a stress within a material, results in a
change in dimension or strain within the body.
P: Propotional Limit: Linear relationship
between stress and strain upto point P.
• Further increases in stress cause
proportionally greater increases in strain
until the material fractures at point T.
E: Yield Stress
• Corresponds to the stress beyond which
strains are not fully recovered.
• Maximum stress which a material can
withstand without undergoing some
permanent deformation.
T: fracture stress (Tensile/Compressive strength)

Reference:
1. Applied Dental Materials, 9th Edition, John F. Mcable (Chapter:2. page:11)
Yield stress is difficult to characterise experimentally… ¹
It requires a series of experiments in which stress is gradually increased then
released and observations on elastic recovery made.
As a consequence of these experimental difficulties, the proportional limit is
often used to give an approximation to the value of the yield stress.
Hence, High value of proportional limit indicates a sample of material is more
likely to withstand applied stress without permanent deformation.
Proportional limit VS. Yield stress
Precisely determining proportional point can be difficult, so a more
practical indicator is the yield strength. ²
True elastic limit ?

Reference:
• Applied Dental Materials, 9th Edition, John F. Mcable (Chapter:2. page:11)
• Rigid
• Strong
• Tough
• Ductile
• Rigid
• Strong
• Brittle
• Flexible
• Weak
• Brittle
• Flexible
• Tough
• Rigid
• Weak
• Brittle
• Flexible
• Resilient

“Stress and strain relationship used to characterise the mechanical
properties of materials.”
- Applied Dental Materials, 9th Edition, John F. Mcable (Chapter:2. page:8)
The Basic Properties of Elastic Materials – “The elastic behavior of
any material is defined in terms of its stress–strain response to an
external load.”
- Contemporary Orthodontics, 5th Edition, Proffit & Fields & Sarver. (Chapter:9. page:312)
MECJANICAL PROPERTIES
VS.
ELASTIC PROPERTIES

Reference:
• http://en.wikipedia.org/wiki/List_of_materials_properties

Reference:
• Orthodontics current principles and techniques; Grabber, 5th edition
The mechanical properties of an alloy to be used in an
orthodontic wire can be described on at least three levels.
1. Observational level
2. Stress-strain level
3. Atomic and molecular level
Mechanical Properties of
the Orthodontic wires

Reference:
• Basic Behavior of Alloys
• Elastic Limit
• Modulus of Elasticity

Reference:
Active and Reactive Members
The active member is the part involved in tooth movement.
The reactive member serves as anchorage and involves the teeth that will not be displaced.
Specifically of interest are three important characteristics involving
active and reactive members:
1. Moment-to-Force Ratio
2. Load-deflection Rate / Torque Twist Rate
3. Maximum elastic load / moment

Reference:
All three of the important characteristics of an
orthodontic appliance
are found within the elastic range of an orthodontic
wire and therefore may be called spring
characteristics.

Reference:
On the subclinical level the three primary objectives:
1. Control the center of rotation of the tooth.
2. Produce desirable stress levels in the PDL.
3. Maintain a relatively constant level of stress.
At the clinical level of observation:
the focus becomes the forces and moments produced by an
orthodontic appliance.

Reference:

Reference:
Force—a load applied to an object that will tend to move it to a different position in space.
Moment—a measure of the tendency to rotate an object around some point.
A moment is generated by a force acting at a distance.

Reference:
Couple—two forces equal in magnitude and opposite in direction. The result of applying two
forces in this way is a pure moment, since the translatory effect of the forces
cancels out.
A couple will produce pure rotation, spinning the object around its center of resistance.

Reference:
Center of resistance—The center of resistance (CR) for any tooth is at the approximate
midpoint of the embedded portion of the root.
Center of rotation—The point around which rotation actually occurs when an object is being
moved.

Reference:
• To produce different types of tooth movement, the ratio between the applied moment
and the force on the crown must be changed.
• As the M/F ratio is altered, the center of rotation changes.
• Crown tipping, translation, and root movement are examples of different types of
tooth movement that can be produced with the proper M/F ratio.
• The M/F ratio determines the control the orthodontic appliance
has over the active and reactive units.
• Specifically, it controls the center of rotation of the tooth or a
group of teeth.

Reference:
• Force produced per unit activation.
OR The force required per unit deflection.
• is a factor in the delivery of a relatively constant force.
• As the load deflection rate declines for a tooth that is moving under a continuous
force, the change in force value is reduced.
For active members a low load-deflection rate is desirable for two important reasons:
1. Maintains a more desirable stress level in the PDL because the force on a tooth does
not radically change magnitude every time the tooth has been displaced.
2. Offers greater accuracy in controlling force magnitude.

Reference:
The reactive member it should have a high load-deflection rate. (should be relatively
rigid)

Reference:
Greatest force or moment that can be applied to a member without causing

Reference:
• Basic Behavior of Alloys
the Orthodontic wiresPmax:
• A point where load and deflection are
no longer proportionate.
• Near Pmax, permanent deformation is
being produced in the spring, which will
not return to its original shape.
• Pmax represents the highest load that can
be placed on the spring without
permanent deformation; that is, the
maximal elastic load.
Hooke’s Law

Reference:
the Orthodontic wiresElastic Limit (EL) The EL is the greatest
stress that can be applied to the alloy
without permanent deformation.
Modulus of elasticity (E) this
mechanical property determines the
load-deflection rate of a spring.
Stress in a wire is force per unit area applied to a cross section.
Strain is deflection per unit length of the wire.
Modulus of elasticity (E) is the ratio of stress to strain.

Reference:
The EL is analogous to the maximal elastic load (Pmax) and therefore is the mechanical
property that determines the ability of a member to withstand permanent deformation.

Reference:
A number of other terms describe this general part of the curve (O-
EL), such as yield point, yield strength, and proportional limit;
these points are close to the EL, although they differ by definition.

Reference:
Fundamentally, elastic behavior involves interatomic bonding.
Because atoms are pulled apart, a fairly definite relationship exists
between stress and strain.
However, plastic behavior involves displacement along slip planes,
which are molecular, not atomic.
Plastic behavior therefore is not as linear as elastic behavior.

Reference:
Elastic Limit (EL):
• The EL is the greatest stress that can be
applied to the alloy without permanent
deformation.
• The EL determines the maximal elastic load
(Pmax) of a configuration.
• With respect only to the mechanical
properties of the wire, the maximal elastic
load varies directly and linearly with the EL.
In a given alloy (e.g., 18-8 stainless steel), a number of factors determine the
elastic limit.
1. Cold working

Reference:
the Orthodontic wiresCold working
The amount of work hardening produced during cold drawing of the wire sharply influences
the EL.
• Wires that have been considerably cold worked have a hard temper and therefore a
high EL.
• Small, round wires may have particularly high ELs because the percentage of reduction
by cold working is high.
• Also, the cold worked outer core becomes proportionately greater in a wire of smaller
cross section.

Reference:
the Orthodontic wiresCold working
Too much work hardening, however, produces a structurally undesirable wire that becomes
highly brittle and may fracture during normal use in the mouth.
Far better is to have a slightly lower EL so that an orthodontic member can deform
permanently rather than break under accidental loading.

Reference:
the Orthodontic wiresWork hardening VS. Anodic reduction
Because the work hardening required to reduce the diameter of a wire increases the EL,
Anodic reduction is a poor method for reducing the size of the wire.
Anodic reduction does not cold work a metal; therefore, the wire produced by that method
has a lower EL than a work-hardened one, a circumstance that could lead to permanent
deformation.

Reference:
Raising EL
Many orthodontic alloys, such as Elgiloy and gold, can be heat treated to raise the EL,
but the most commonly used alloy, 18-8 stainless steel, cannot.
Stress relief process at 8508°F for 3 minutes or longer raises the apparent elastic limit of 18-8
stainless steel.
Stress relief removes undesirable residual stress introduced during manufacturing and during
fabrication by the orthodontist.

Reference:
Modulus of Elasticity(E):
• Modulus of elasticity (E) is the
ratio of stress to strain.
• The mechanical property that
determines the load-deflection
rate of an orthodontic member is
the modulus of elasticity (E).
Load-deflection varies directly and linearly with Modulus of elasticity (E).
(in torsion, linearly and directly as the modulus of rigidity).

Reference:
Modulus of Elasticity(E):
• Steel has an Modulus of elasticity
(E) approximately 1.8 times
greater than that of gold.
• Unlike the Elastic limit (EL), the
Modulus of elasticity (E) is
constant for a given alloy and is
not influenced by work hardening
or heat treatment.

Reference:
The Basic Properties of Elastic Materials
The elastic behavior of any material is defined in terms of its stress–strain response to an
external load.
For orthodontic purposes, three major properties of beam materials are critical in defining
their clinical usefulness: strength, stiffness (or its inverse, springiness), and range.
Elastic Properties of the
Orthodontic wires

Reference:
1.Strength
2.Stiffness/Springiness
3. Range
4.Resilience
5.Formability
First three major properties have an
important relationship:
Strength = Stiffness X Range
Elastic Properties of the
Orthodontic wires

Reference:
1. Strength:
Three different points on a stress–strain diagram can be taken as representative of the
strength of a material.
1. Proportional
limit
2. Yield strength
3. Ultimate tensile
strength

Reference:
2. Stiffness:
Stiffness and springiness are reciprocal properties:
Each is proportional to the slope of the elastic portion of the force–deflection curve.
The more vertical the slope, the stiffer the wire;
The more horizontal the slope, the springier the wire.

Reference:
3. Range:
Range is defined as the distance that the wire will bend elastically before
permanent deformation occurs. (distance is measured in millimeters)
• If the wire is deflected beyond this point, it will not return to its original shape, but
clinically useful springback will occur unless the failure point is reached.

Reference:
The relative strength, stiffness, and range for stainless steel, TMA, and M-NiTi
wires (which would be the same for any wire size).
• Note that both TMA and M-NiTi have half the strength of steel;
• M-NiTi has slightly less stiffness but much more range than TMA.

Reference:
4. Resilience: Area under the stress-strain curve out to the proportional limit.
Represents the energy storage capacity of wire, which is a combination of
strength and springiness. ¹
Resilience is the amount of force the wire can withstand before permanent deformation.
Archwires should exhibit high resilience so as to increase working range of appliance.²
5. Formability: Amount of permanent deformation that a wire can withstand before failing.
Represents the amount of permanent bending the wire will tolerate before it
breaks. ¹
The archwire material should exhibit high formability so as to bend the arch wire into desired
configuration such as coils, loops without fracturing the wire. ²

Reference:
Each of the major elastic properties—strength, stiffness, and
range—is substantially affected by the geometry of a beam.
Both the cross-section (whether the beam is circular, rectangular, or
square) and the length of a beam are of great significance in
determining its properties.
Geometry:
• Size and Shape (changes are independent of the material)
• Length and attachment
Effects on Elastic Properties of Beams

Reference:
Changing the diameter (d) of a beam, no matter how it is supported,
greatly affects its properties.
Doubling the diameter of a cantilever beam makes it 8 times as strong, but it is then only
1/16 as springy and has half the range.

Reference:
Changing either the length of a beam (whatever its size or the
material from which it is made ) OR the way in which it is attached
dramatically affects its properties.
Doubling the length of a cantilever beam cuts its strength in half (?) but makes
it 8 times as springy and gives it 4 times the range.

Reference:
Changing either the length of a beam OR the way in which it is
attached dramatically affects its properties.
If a beam is rigidly attached on both ends, it is twice as strong but only one-fourth as springy
as a beam of the same material and length that can slide over the abutments.
For this reason, the elastic properties of an orthodontic archwire are affected by whether it is
tied tightly or held loosely in a bracket.

Reference:
The best balance of strength, springiness,
and range must be sought among the almost
innumerable possible combinations of beam
materials, diameters, and lengths.

Reference:
Properties of Ideal Orthod
It should possess:
1) High Strength
2) Low Stiffness (in most applications)
3) High Range
4) High Formabililty
5) Material should be weldable or solderable
6) Reasonable in cost

Reference:
• Ravichandra Sekhar Kotha et. Al, An Overview of Orthodontic Wires, Trends Biomater. Artif. Organs, 28(1), 32-36 (2014)
Properties of Ideal Orthod

Reference:
Ideal orthodontic wire for an active member:
gives a high maximal elastic load and a low load-
deflection rate.
What are Ideal
Orthodontic Alloys?
The orthodontist should look for alloys that have a
high EL and a low E.
In the reactive member of an appliance:
however, not only is a sufficiently high EL required,
but also a high E is desirable.
The ratio between the EL and E determines the desirability of the alloy:
the higher the ratio, the better the spring properties (M/F ratio, Load deflection
rate/Torque twist rate, Maximum elastic load/ moment) of the wire.

Reference:
Four other properties of wire should be considered when evaluating an orthodontic wire:
1. resistant to corrosion
2. sufficiently ductile
3. fabricated in a soft state and later heat treated to hard temper.
4. easy soldering of attachments
What are Ideal
Orthodontic Alloys?

Reference:
Factors / Characteristics
that influence Selection of
desirable orthodontic wire
?
1. Wire Cross Section
2. Wire Length
3. Amount of Wire
4. Selection of the Proper Wire (Alloy
and Cross Section)

Reference:
Small changes in cross section can influence the maximal elastic load
and the load-deflection rate greatly.
The maximal elastic load varies directly as the third power of the
diameter of round wire, and the load deflection rate varies directly
as the fourth power of the diameter.

Reference:
The fact that the load-deflection rate varies as the fourth power of the diameter in round
wires suggests the crucial importance of selecting a proper cross section.
In selecting a proper cross section for the rigid reactive members of
an appliance, the load-deflection rate, rather than the maximal
elastic load, is the prime consideration.
Drawbacks to round wire:
1. Round wire must be properly oriented or activations may not operate in the intended
plane.
2. Round wire may rotate in the bracket and if certain loops are incorporated into the
configuration, these can roll into the gingiva or the cheek.

Reference:
What is the optimal cross section for a flexible member?
Generally, for multidirectional activations in which the
structural axis is bent in more than one plane, a circular
cross section is the structure of choice.

Reference:
• Many orthodontic wire configurations undergo unidirectional
bending.
For example, an edgewise vertical loop used for anterior retraction has a structural axis
that bends in only one plane.
• For unidirectional bending, flat wire is the cross
section of choice.
• More energy can be absorbed into a spring made of a flat wire
than with any other cross section.

Reference:
Advantages of Flat wires:
• Problem of orientation is much easier to solve than with a round
cross section.
• Can be anchored definitely into a tube or bracket so that it will not
spin during deactivation of a given spring.
• Can also be used in certain situations when considerable tooth
movement is required in one plane but limited tooth movement is
needed in the other.

Reference:
For the reactive member, square or rectangular wire is
superior to round wire because of the ease of orientation
and greater multidirectional rigidity of the former, which
leads to more definite control of the anchorage units.
In the edgewise mechanism the assumption may be made that greater rigidity is needed
buccolingually or labiolingually than occlusogingivally because an edgewise wire is used.
This may or may not be true, depending on the intended use of the edgewise mechanism.

Reference:
The length of a member may influence the maximal elastic load and the load-
deflection in a number of ways, depending on the configuration and loading of
the spring.
1. Loading the cantilever with a vertical force applied at the free
end.
2. Loading the cantilever with a couple applied at the free end.
2. Wire Length

Reference:
Figure 11-26 shows a cantilever attached at B with a vertical force applied at A.
The distance L represents the length of the cantilever measured parallel to its structural axis.
Load deflection rate ∝ 1/L³
Maximum elastic load ∝ 1/L
1.) Loading the cantilever with a vertical force applied at the free end:
2. Wire Length

Reference:
Increasing the length of the cantilever is a better way to reduce the
load-deflection rate than is reducing the cross section.
Increasing the length of the cantilever greatly reduces the load-deflection rate, yet the
maximal elastic load is not changed radically because it varies linearly with the length.
2. Wire Length

Reference:
2.) Loading the cantilever by means of a couple or moment applied to the free end:
Load deflection rate ∝ 1/L²
Maximum elastic load – no effect
The length may be doubled or tripled, but the maximal elastic moment remains the same.
This is a most desirable type of loading because additional length can reduce the moment-
deflection rate, but the maximal elastic moment is not reduced. However, the principle can
be applied only if moments alone are required for a given tooth movement.
2. Wire Length

Reference:
• Additional length of wire may be incorporated in the form of loops
or helices or some other configuration. This tends to lower the load-
deflection rate and increase the range of action of the flexible
member. The maximal elastic load may or may not be affected.
• When a member is to incorporate additional wire, the parts of the
configuration where additional wire should be placed must be
located properly, and the form the additional wire should take must
be determined.
• If location and formation are done properly, lowering the load-
deflection rate without changing the maximal elastic load should be
possible merely by adding the least amount of wire that will achieve
these ends.
3. Amount of wire

Reference:
Consider the problem of the cantilever in relation to the placement of the additional wire.
The bending moment represents an internal moment resisting the
100-g force applied to the free end of the cantilever.
3. Amount of wire

Reference:
Significance of the bendingmoment:
Amount of bending at each cross section of the wire is directly
proportionate to the magnitude of the bending moment.
in other words,
The greater the bending moment at any particular cross section,
the more the wire is going to bend at that point.
3. Amount of wire

Reference:
The optimal place for additional wire is at cross sections where the
bending moment is greatest.
In the case of the cantilever, the position for additional wire is at the point of
support because the bending moment is greatest there: 1000 g-mm.
• Helical coils can be used to reduce the load-deflection rate.
Figure 11-30 illustrates the proper positioning of a helical coil for this purpose.
3. Amount of wire

Reference:
• The load-deflection rate is maximally lowered for the amount of
wire used if the helix is placed at the point of support.
• Placement of additional coils at the point of support in a
cantilever does not change the maximal elastic load.
3. Amount of wire

Reference:
• A straight wire of a given length and a wire with numerous coils at
the point of support have identical maximal elastic loads,
provided they have the same length measured from the force to
the point of support.
• This should not be surprising because the maximal elastic load is
a function of this length of the configuration rather than of the
amount of wire incorporated into it.
This is true of many other configurations as well: the load-deflection rate can be lowered
without changing the maximal elastic load if additional wire is incorporated properly.
3. Amount of wire

Reference:
How to Determine where the bending moment is greatest?
A practical way of deciding where these parts of a wire might be is to activate a
configuration and see where most of the bending or torsion occurs.
The sections where the bending or torsional moments are greatest are the cross sections
with the greatest stress.
The configuration of the additional wire should be such that maximal advantage can be
taken of the bending and torsional properties of the wire.
3. Amount of wire

Reference:
• In short, the amount of wire used is not what is important in
achieving a desirably flexible member, but rather the placement
of the additional wire and its form.
• Additional wire should not be used in reactive or rigid members.
Loops and other types of configurations diminish the rigidity of
the wire and thus may be responsible for some loss of control
over the anchor units.
3. Amount of wire

Reference:
Selection of the Proper Wire
(Alloy and Cross Section)
Selection of the proper size wire should be based:
1. Primarily on the load-deflection rate required.
2. Secondarily on the magnitude of the forces and
moments needed.

Reference:
Many orthodontists select a cross section of wire based on two other factors, which,
although valid, are not as significant:
1) Eliminate the play between wire and bracket:
Some clinicians believe that increasingly heavier wires are needed in a replacement
technique to eliminate the play between wire and bracket.
2) Smaller the wire, the greater the maximum elastic deflection:
The smaller the wire, the more it can be deflected without permanent deformation. This
is true, but maximal elastic deflection varies inversely with the diameter of the wire.

Reference:
Small differences in cross section produce big changes in load-deflection rates
because in round wires the load deflection rate varies as the fourth power of the
diameter. (Table 11-1).

Reference:
In bending, the stiffness, or load-deflection rate, is determined by the
moment of inertia of the cross section of the wire with respect to the
neutral axis.

Reference:
Clinicians are interested in the relative stiffness of the wire they use,
but they have neither the time nor the inclination to use engineering
formulas to determine these degrees of stiffness.
Therefore, a simple numbering system has been developed, based on engineering theory,
that gives the relative stiffness of wires of different cross sections if the material
composition of the wire is the same.
The cross-sectional stiffness number (Cs) uses 0.1-mm (0.004-inch) round wire as a base of 1.
A 0.006-inch wire has a Cs of 5.0, which means that for the same activation, 5 times as much
force is delivered.

Reference:
Tables 11-2 and 11-3 list, under the Cs column, stiffness numbers based on nominal cross
sections. Manufacturing variation or mislabeling of wires obviously can change the actual
Cs significantly. Two Cs numbers are given for rectangular wires—one for the first-order
direction and one for the second-order direction.

Reference:

Reference:
Any two sections of wire can be compared for
stiffness simply by dividing the Cs number of one
into the other.

Reference:
The overall stiffness of an appliance (S) is determined by two factors:
1. Wire itself (Ws)
2. Design of the appliance (As)
S = Ws × As
where S is the appliance load-deflection rate (Stiffness);
Ws, the wire stiffness; and
As, the design stiffness factor.
In general terms
Appliance stiffness = wire stiffness × design stiffness
Wire stiffness =material stiffness × cross-sectional stiffness
Wire stiffness is determined by a cross-sectional property (e.g., moment of inertia) and a
materials property (the E).

Reference:
A full range of forces can be obtained by varying the material of the wire while keeping the
cross section the same.

Reference:
1. Variable cross-section Principle:
The amount of play between the attachments and the wire can be varied, depending on
the stiffness required.
With small, low-stiffness wires, excessive play may lead to lack of control over tooth
movement.

Reference:
2. Variable-Modulus Principle:
Clinician determines the amount of play required before selecting the wire.
In some instances more play is needed to allow the brackets freedom of movement
along the arch wire.
In other situations minimal play is allowed to ensure good orientation and effective
third-order movement.
After the desired amount of play has been established, the correct wire stiffness
can be produced by using a material with a proper Ms.
In this way the play between the wire and the attachment is not dictated by the stiffness
required but rather is under the full control of the operator.

Reference:
2. Variable-Modulus Principle:
The variable-modulus principle allows the orthodontist to use oriented rectangular
wires or square wires in light force and heavy force applications and stabilization.
A rectangular wire orients in the bracket and thus offers greater control in delivering the
desired force system; it is easier to bend because the orientation of the wire can be
checked carefully.
More important, when placed in the brackets, the wire does not turn or twist,
allowing the forces to be dissipated in improper directions.

Reference:
• Orthodontics, the art and science, 4th Edition, S.I.Bhalaji
• 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.
History

Reference:
Stainless Steel (SS) Wires
Types of orthodontic wires
• Most commonly used: austenitic 18-8 stainless steel (contains chromium and nickel content of approximately 18% and 8%, respectively)
Characteristics:
• High resistance to Corrosion (by the formation of a passivated oxide layer, which blocks the further oxygen
diffusion to the underlying mass)
• Produce higher forces applied during shorter time periods (since they have
lower spring back ability).
• Store less energy compared to those of beta-titanium or nickel-
titanium.
• Can be soldered with different biomechanical attachments.
• The corrosion resistance of stainless steel is good in general (but releases nickel
and chromium in fewer amounts and may induce hypersensitivity reactions)
Kolokitha et al., concluded that orthodontic treatment is not related to an increased likelihood of
hypersensitivity reactions to nickel unless there is a history of skin piercing.

Reference:
• Stainless steel wires have a lower bracket-wire friction than other
types of wires (this friction can be further reduced by using nanotechnology applications)
• Australian wires are a kind of stainless steel wires available in
different grades with gradually increasing stored energy values
(resiliency).

Reference:
• Properties of Stainless steel wires can be controlled by varying the
amount of cold working and annealing during manufacture.
• Steel is softened by annealing and hardened by cold working.
• Fully annealed stainless steel wires are soft and highly formable.
• Steel ligatures are made from such “dead soft” wire.

Reference:
• “Super Grades” Steel wires:
possess impressive yield strength, are brittle and break if bend
sharply.
• “Regular Grades” Steel wires:
can be bent to almost any desired shape without breaking.

Reference:
Cobalt-chromiumWires
• Cobalt-chromium-nickel alloy known as elgiloy.
• These alloys were originally developed for use as watch spring by ELGIU national company.
• Available in different tempers depending on amount of cold work and are usually color-
coded.
Color codes
High spring tempers Red
Semispring temper Green
Soft or ductile tempers Yellow
High formability combined with increased elasticity and yield strength following heat
treatment by 10% and 20- 30%, respectively, have made Blue Elgiloy, a cobalt chromium
wire type, popular in clinical practice.

Reference:
Characteristics:
• Easy to bend.
• Can be heat hardened at 482° C for about 7 minutes after
manipulation to increase hardness (strength) approximately equal
to that of stainless steel.
• Non-heat treated cobalt-chromium wires have a smaller spring-
back than stainless steel wires
• Excellent resistance to tarnish and corrosion.
• Inexpensive and can be soldered (fluoride fluxes are used) and
welded.

Reference:
COMPARISON WITH STAINLESS STEEL:
• Elgiloy, the cobalt–chromium alloy, has the advantage that it can be supplied in a
softer and therefore more formable state, and the wires can be hardened by
heat treatment after being shaped.
The heat treatment increases strength significantly.
• After heat treatment, the softest Elgiloy becomes equivalent to regular stainless
steel, while harder initial grades are equivalent to the “super” steels.
• This material, however, had almost disappeared by the end of the twentieth
century because of its additional cost relative to stainless steel and the extra step
of heat treatment to obtain optimal properties.

Reference:
Shape Memory Alloys:
1. Nickel titanium (nitinol) - William F. Buehler
2. Superelastic NiTi wires
3. TMA
4. Titanium niobium

Reference:
Beta TitaniumWires
• Introduced in 1979
• Also known as titanium-molybdenium alloy (TMA) or Titanium Niobium.
Characteristics:
• Modulus of elasticity of these wires is lower than half of stainless
steel wires and almost twice that of Nitinol.
• Demonstrate good formability, (but should not be strongly bent for there is a risk of breaking).
• Electrical welding of biomechanical attachments is possible, (but overheating
should not be done as it makes the wire more brittle).
• According to a recent study, beta-titanium wires are better in terms
of joinability than stainless steel wires (since they demonstrate higher resilience and better surface and
structural characteristics, which indicates only a minor change in wire properties after welding).

Reference:
Beta TitaniumWires
Characteristics:
• Resistance to corrosion is similar to that of cobalt chromium and
stainless steel wires.
• Good biocompatible material (due to the absence of nickel).
• Resistance to corrosion is due to the formation of a surface
passivation oxide layer (but exposure to fluoride agents leads to the degradation, subsequent corrosion, and qualitative alteration
of the wire’s surface).
• Highly expensive
• More bracket-wire friction than any other alloy
Alpha-beta titanium alloy is also called as TiMolium, it has stiffness and other characteristics (such as
elasticity and yield strength) are between the values set for stainless steel and beta-titanium wires

Reference:
Beta TitaniumWires
Characteristics:
• TMA has a modulus of elasticity between that of steel and nitinol
(approximately 0.4 times that of stainless steel).
• TMA can be deflected up to 2 times as much as steel without
• Unlike Nitinol, TMA is not significantly altered by the placement of
bends and twists and has good ductility, equivalent to or slightly
better than that of stainless steel, and can be welded without
significant reduction in yield strength.

Reference:
Nickel Titanium Wires
• Exceptional ability to apply light force over a large range of activations.
This phase transition allows certain NiTi alloys to exhibit two remarkable
properties found in no other dental materials—shape memory and
superelasticity.
Martensitic form
Lower temperatures
Higher stress
Austenitic form
Higher temperatures
Lower stress
The uniqueness of NiTi is that the transition between the two structures is fully reversible
and occurs at a remarkably low temperature.

Reference:
 The original Nitinol wires marketed under that name in the late 1970s by Unitek were M-
NiTi wires, with no application of phase transition effects.
 M-NiTi remains useful, primarily in the later stages of treatment when flexible but larger
and somewhat stiffer wires are needed.
M-NiTi: NiTi wires stabilized in the martensitic form are referred to as M-NiTi.
A-NiTi: NiTi wires displaying martensite–austenite transitions are referred to as A-NiTi.
 In the late 1980s, new nickel–titanium wires with an austenitic grain structure (A-NiTi)
appeared.
 These wires exhibit superelasticity and/or shape memory in various degrees.
 The properties of A-NiTi have quickly made it the preferred material for orthodontic
applications in which a long range of activation with relatively constant force is needed
(i.e., for initial archwires and coil springs).

Reference:
Shape memory: refers to the ability of the material to “remember” its original
shape after being plastically deformed while in the martensitic form.
Superelasticity: refers to the very large reversible strains that certain NiTi wires can
withstand due to the martensite-austenite phase transition.
• Materials displaying superelasticity are austenitic alloys that undergo a transition to martensite in
response to stress—a mechanical analogue to the thermally induced shape memory effect.
• Superelastic materials must exhibit a reversible phase change at a close transition temperature,
which must be lower than room temperature for the austenite phase to exist clinically.
Shape memory is a thermal reaction and superelasticity is a mechanical one, they are inherently linked.

Reference:
Thermoelasticity:
• In a typical application, a certain shape is set while the alloy is maintained at an elevated
temperature, above the martensite–austenite transition temperature.
• When the alloy is cooled below the transition temperature, it can be plastically
deformed, but the original shape is restored when it is heated enough to regain an
austenitic structure.
• This temperature-induced change in crystal structure (called thermoelasticity) was
important to the original nitinol use in the space program but proved difficult to exploit
in orthodontic applications.

Reference:
• A wrought Ni-Ti alloy known as Nitinol (Nickel-Titanium naval ordinance laboratory) was
introduced in 1972.
Characteristics:
• High resiliency.
• Higher energy storage capacity
• Shape memory or thermal memory.
• Pseudoelastic or superelasticity.
• Limited formability.
• Cannot be welded or fused.
The friction develops at bracket-wire interface is more with Nitinol wires followed by beta-titanium,
stainless steel and chromium-cobalt wires

Reference:
Two major phases
One intermediate phase
Austenitic Phase
Martensitic phase
• At higher temperature
• Body centered cubic
structure (BCC)
• At lower temperature
• hexagonal close packed
structure (HCP)
Delays the transition from austenite to martensite
upon cooling until lower temperatures are achieved.

Reference:
Shape memory achieved by first establishing a shape
at temperatures near 482° C.
If the appliance wire is then cooled and formed into a
second shape and heated through a lower transition
temperature range (TTR), the wire will return to its original
shape.
The cobalt content is used to control the transition
temperature range, which can be near mouth temperature.

Reference:
Phase transformation (from BCC austenitic to HCP
martensitic) can be seen by decreasing the temperature
from an elevated temperature
and can also be induced by the application of stress and a
volumetric change is associated during this transition.
This transformation results in two unique features such as
shape memory and pseudoelasticity or superelasticity.

Reference:
Pseudoelasticity or Superelasticity Inducing the austenitic
to martensitic transformation by stress can produce
superelasticity, a phenomena that is employed with Ni –
Ti wires.

Reference:
The shape memory principle is not used clinically. Instead, nitinol is
used for its low force and high springback.
The most dramatic characteristic of nitinol, however, is its resistance
to permanent deformation.
NiTi wires can be activated over twice the distance of stainless steel,
with minimal permanent deformation.
Nitinol is more brittle than stainless steel and cannot be joined by
soldering or welding.

Reference:
Superelastic Ni-Ti Wires
 Unlike nitinol, these alloys have a much lower transition
temperature—either slightly below or slightly higher than mouth
temperature.
 Generally speaking, the austenitic form of these alloys has a slightly
higher springback than nitinol and may be less brittle.
 The superelastic NiTi wires are available in different degrees of
stiffness.

Reference:
Superelastic Ni-Ti Wires
 Although the final transition temperature of some superelastic NiTi
wires is below mouth temperature, others are not activated fully
until they reach 37°C or higher.
 These wires have both superelastic and shape memory properties.
 Generally, the heat treatment process performed during
manufacture to raise the transition temperature allows for wires
that deliver lower forces at mouth temperature;
 hence the orthodontist may be able to achieve full bracket
engagement with larger wires earlier in treatment.

Reference:
Titanium Niobium Wires
Characteristics:
• Low springback (equivalent to stainless steel)
• Much less stiff than TMA
• Useful when a highly formable wire with low forces in small
activations is required.

Reference:
Chinese Ni-Ti Wires
The Chinese NiTi wires were recommended by Burstone et
al in 1985.
They exhibited 4.4 times the spring-back of stainless steel
wires and 1.6 times the spring-back of the original Nitinol
wires if a constant force in the middle of its deactivation
range.

Reference:
Japanese Ni-Ti Wires
The Japanese NiTi wires was introduced by Miura et al. in
1986
Their properties were almost similar to Chinese NiTi wires.

Reference:
Multistranded Wires
• Made of a varying number of stainless steel wire strands coaxially placed or coiled around
each other in different configurations.
Characteristics:
• Deliver low forces
• Low stiffness
• Low resilience
• Inexpensive than titanium alloys
• Higher friction at bracket-wire interface compared to NiTi wires and
single-stranded stainless steel wires

Reference:
Esthetic Wires
Fiber-reinforced composite arch wires
Characteristics:
• Highly esthetic
• Biocompatible
• Hydrolytic stability
• Less water sorption
• Stiffness is same as metallic wires
• Post processing formability
• Sliding mechanics are good
• Wearing of these arch wires at the interface chances of leaching of
glass fibers within the oral cavity.

Reference:
Esthetic Wires
Teflon coated stainless steel arch wires
• Teflon is coated on stainless steel wire by an atomic process that
forms a layer of about 20-25ìm thickness on the wire that imparts
to the wire a hue which is similar to that of natural teeth.
• Teflon coating protects the underlying wire from the corrosion
process.
• However, corrosion of the underlying wire is likely to take place if it
is used for longer period in the oral cavity since this coating is
subject to flaws that may occur during clinical use.

Reference:
Other Esthetic wires:
 Optiflex
 Bioforce wires
 Marsenol
 Lee White wire

REFERENCES:
Articles:
• Ravichandra Sekhar Kotha et. Al, An Overview of Orthodontic Wires, Trends Biomater. Artif.
Organs, 28(1), 32-36 (2014)
Books:

Dr Abbas Naseem
abbas_naseem@yahoo.com
Dated: November 26th , 2014

Wire selection in orthodontics / Orthodontics wires

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