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MECHANICAL PROPERTIES & STRUCTURE OF STAINLESS
STEEL WIRE
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INDIAN DENTAL ACADEMY
Leader in continuing dental education www.indiandentalacademy.com
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ContentsIntroductionHistorical BackgroundKey termsStructural propertiesPhysical propertiesStainless Steel• Classification of stainless steel• Applications of Stainless Steel
Conclusionwww.indiandentalacademy.com
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Introduction
•Recent advances in orthodontics wire alloys have resulted in a varied array of wires that exhibit a wide spectrum of properties. Up until the 1930’s the only orthodontic wire available were made of gold.
•Austenitic stainless steel,was introduced as an orthodontic wire in 1929 and shortly afterwards gained popularity over gold.
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Historical Background of Stainless steel :
•1903 to 1921 was the year that stainless steel was
developed and perfected by Brearley of Sheffield and
Becket of the U.S.
•Stainless steel entered dentistry in 1919, being introduced
at Krupp’s Dental Polyclinic in Germany by the
Company’s dentists, F. Hauptmeyer. He first used it to
make a prosthesis and called it Wipla ( Wie platin; in German, like platinum),
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•Angle used it in his last year (1930) as ligature wire. By 1937 the value of Stainless steel as an orthodontic material had been confirmed.
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KEY TERMSLATTICES
•The three dimensional network of lines that can be visualized to connect the atoms in undisturbed crystal is called a lattice.
•In the 19th century BRAVAIS showed that only a limited number of crystal lattices exist.
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MECHANISM OF CRYSTALLIZATION
SOLID STATE OF DIFFUSION
ATOMIC DIFFUSION
GRAIN SIZE
NUMBER OF NUCLEI
RATE OF CRYSTALLIZATION
MOULD USED
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LATTICE IMPERFECTIONS
POINT DEFECT OR LINE DEFECT
DISLOCATIONS
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STRAIN HARDENING
INCREASE IN SURFACE HARDENESS STRENGTH PROPORTIONAL LIMIT
DECREASE IN DUCTILITY RESISTANCE TO CORROSION
RESIDUAL STRESS BUILTUP
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ANNEALING
RECOVERY482ºC 3Min
RECRYSTALLIZATION
GRAIN GROWTH
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Carbon steels:• Steels are iron-based alloys that usually contain less than 1.2%
carbon.
• The major classes of carbon steel are based on three possible
crystal structures that can occur for the iron carbon alloys:-
• Pure iron at room temperature has a body-centered cubic (BCC)
structure and is referred to as ferrite. This phase is stable in
temperatures as high as 912° C.
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•The spaces between atoms in the BCC structure (interstices)
are small and oblate; hence, carbon has a very low solubility in
ferrite (maximum of 0.02 wt%).
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• At temperatures between 912° C and 1394° C, the stable
form of iron is a face centered cubic (FCC) structure called
austenite. The interstices in the FCC lattice are larger than
those in the BCC structure. However, the size of the carbon
atom limits the maximum carbon solubility to 2.1 wt%.
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• When austenite is cooled slowly from high temperatures, the
excess carbon that is not soluble in ferrite forms iron carbide
(Fe3C) to yield a microstructural constituent referred as
pearlite which consists of alternating fine scale lamellae of
ferrite and iron carbide (Fe3C), called cementite / simply
carbide. The Fe3C phase has an orthorhombic crystal
structure and is much harder and more rigid than austenite /
ferrite.
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• If the austenite is cooled rapidly (quenched), it
undergoes spontaneous, diffusionless
transformation of a body-centered tetragonial (bct)
structure called martensite.
• The arrangement of the iron atoms in martensite is
highly distorted by the carbon atoms, resulting in a
very hard, strong, brittle alloy.
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•The formation of martensite is an important
strengthening mechanism for carbon steels. The
cutting edge of carbon steel instruments are
ordinarily martensitic because the high hardness of
this structure allows the grinding of a sharp edge
that is retained in use.
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IRON (bcc)
Heat 9120 C to 13940 C
AUSTENITE (fcc)
Gradually cooled
Ferrite + cementite
Rapidly cooled
Martensite
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HARDNESS OF STEEL
carbon content
percent of austenite which transforms to martensite during cooling
CRITICAL COOLING RATE
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Classification of Stainless Steel (depending on the lattice)
• Ferrtic
• Martensitic
• Austenitic
Other
• Duplex steels
• PH steel
• Cobalt containing steels
• Manganese containing steels
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STAINLESS STEEL:
When approximately 12% to 30% chromium is added to iron, the alloy is commonly called stainless steel. Elements other than iron, carbon and chromium may be present, resulting in a wide variation in composition and properties of the stainless steel.
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Composition ( %age ) of three types of stainless steel
Type of stainless steel (crystal structure formed by iron atoms)
Chromium
Nickel Carbon
Ferritic (bcc) 11.5 - 27.0
0 0.20 max
Austenitic (fcc)
16.0 - 26.0
7.0 - 22.0 0.25 max
Martensitic (bct)
11.5 - 17.0
0 to 2.5 0.15 - 1.20
Note : Silicon phosphorous, sulfur mangnese, tantalum may also be present in small amounts. Balance is iron.
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Function of each component
Carbon : Provides strength.
Chromium : Passivating property.
Nickel : Stabilizes at lower temperature-depresses the
martensitic formation temperature and slows atomic
diffusion.
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Silicon : Improves resistance to oxidation and carbonization.
Sulfur : Allows easy machining of wrought parts.
Phosphorous : Allows use of lower temperature for sintering.
Manganese : Is used as replacement for nickel to stablize the
austenite.
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FERRITIC STAINLESS STEEL:
• These alloys are designated as American Iron and Steel
Institute (AISI) series 400 stainless steel. This series number is
shared with the martensitic stainless steel.
• These stainless steel provide good corrosion resistance at low
cost.
• These stainless steels are not readily work-hardenable, these
stainless steels have numerous industrial uses; they have little
application in dentistry.
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MARTENSITIC STAINLESS STEEL:
• Because of their high strength and hardness, martensitic
stainless steels are used for surgical and cutting
instruments.
• Corrosion resistance is low-Dec with heat treatment
• Low ductility-2% elongation
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AUSTENITIC STAINLESS STEELS:
The austenitic stainless steels are the most corrosion resistant
alloys of the three major types and are the stainless steels used
for:-
• Orthodontic wires
• Endodontic instruments
• Crowns in pediatric dentistry
The austenitic stainless steel is achieved by the addition of
Nickel to the iron-chromium - carbon composition.
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Austenitic type
Chromium Nickel Carbon Molybdenum
Type 302 17% to 19% 8% to 10% 0.15% (max) -
Type 304 18% to 20% 8% to 12% 0.08% (max) -
Type 316L( low carbon)
16% to 18% 10% to 14% 0.03% (max) 2% to 3%
AUSTENITIC TYPES OF STAINLESS STEEL
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Austenitic stainless steel is preferable to ferritic stainless steel
• Greater ductility and ability to undergo more cold work without fracturing.
• Substantial strengthening during cold working ( some transform into a martensite phase )
• Greater ease of welding
• Ability to overcome sensitization
• Less critical grain growth
• Comparative ease in forming
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CORROSION RESISTANCE AND PROPERTIES OF AUSTENITIC STAINLESS STEEL:
• The resistance of stainless steel to tarnish and corrosion is
associated with the passivating effect of chromium.
• A very thin, transparent, adherent layer of Cr2O3 forms on
the surface of stainless steel when it is exposed to oxidizing
atmosphere such as room air.
• This protective layer provides a barrier to oxygen diffusion
and other corrosion environments and prevents further
corrosion of the underlying alloy.
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SENSITIZATION
•Austenitic stainless steel may lose its resistance to corrosion if
it is heated between approximately 4000C and 9000C, the exact
temperature depending on its carbon content. Such temperature
are within the range used by the orthodontist for soldering and
welding.
•The decrease in corrosion resistance is caused by the
precipitation of chromium-carbide at the grain boundaries at
these high temperatures.
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•The small carbon atoms rapidly diffuse to the grain boundary
region to combine with the chromium and iron atoms to form
(CrFe)4C, resulting in loss of the corrosion resistance provided
by chromium. This is called as sensitization.
•Corrosion resistance is reduced in regions adjacent to the
grain boundaries in which the chromium level is depleted
below that necessary for protection (approximately 12%). The
stainless steel becomes susceptible to intergranular corrosion,
and partial disintegration of the weakened alloy may result.
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STABILIZATION
• Reduce the carbon content of the steel to an extent that
such carbide precipitation cannot occur, but this remedy is
not economically feasible.
• By stabilization if the stainless steel in which some
element is introduced that precipitates as a carbide in
preference to chromium. Titanium is commonly used. If
titanium is introduced six times the carbon content, the
precipitation of chromium carbide be inhibited at soldering
temperatures. These are called stabilized stainless steel.
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•If the stainless steel is severely cold worked, the carbides
precipitate along the slip planes. As a result, the areas deficient
in chromium are less localized and the carbides are more
uniformly distributed so that the resistance to corrosion is
greater than when they precipitate only along the grain
boundaries.
•Such a method is presumably used in the processing of
orthodontic stainless steel wires.
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•Any surface inhomogeneity is a potential source of tarnish or
corrosion- CORROSION CELL
•A common cause of the corrosion of stainless steel is the
incorporation of bits of carbon steel or similar metal in its surface.
•Such a situation results in an electric couple that may cause
considerable corrosion
•Chlorine-containing cleansers should not be used to clean
removable appliances fabricated from stainless steel.
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MECHANICAL PROPERTIES OF FOUR TYPES OF ORTHODONTIC WIRES
Alloy Modulus of elasticity (Gpa)
Yield strength(0.2% offset)Gpa
Ultimate tensile strength (Gpa)
Number of 900 cold bends without fracture
Stainless steel
179 1.6 2.1 5
Cobalt-chromium-Nickel
184 1.4 1.7 8
Nickel-titanium
41 0.43 1.5 2
Bita-titanium
72 0.93 1.3 4
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STRESSStress = Force/Area
STRAIN Strain = Deformation/Original length
MODULUS OF ELASTICITY:MOE = Stress/Strain
ELASTIC LIMIT
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MECHANICAL PROPERTIES OF FOUR TYPES OF ORTHODONTIC WIRES
Alloy Modulus of elasticity (Gpa)
Yield strength(0.2% offset)Gpa
Ultimate tensile strength (Gpa)
Number of 900 cold bends without fracture
Stainless steel
179 1.6 2.1 5
Cobalt-chromium-Nickel
184 1.4 1.7 8
Nickel-titanium
41 0.43 1.5 2
Bita-titanium
72 0.93 1.3 4
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YIELD STRENGTHDeformation of 0.1% is measured
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MECHANICAL PROPERTIES OF FOUR TYPES OF ORTHODONTIC WIRES
Alloy Modulus of elasticity (Gpa)
Yield strength(0.2% offset)Gpa
Ultimate tensile strength (Gpa)
Number of 900 cold bends without fracture
Stainless steel
179 1.6 2.1 5
Cobalt-chromium-Nickel
184 1.4 1.7 8
Nickel-titanium
41 0.43 1.5 2
Bita-titanium
72 0.93 1.3 4
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ULITIMATE TENSILE STRENGTH
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MECHANICAL PROPERTIES OF FOUR TYPES OF ORTHODONTIC WIRES
Alloy Modulus of elasticity (Gpa)
Yield strength(0.2% offset)Gpa
Ultimate tensile strength (Gpa)
Number of 900 cold bends without fracture
Stainless steel
179 1.6 2.1 5
Cobalt-chromium-Nickel
184 1.4 1.7 8
Nickel-titanium
41 0.43 1.5 2
Bita-titanium
72 0.93 1.3 4
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FORMABILITY FLEXIBILITY & RESILIENCE strength and springiness
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STIFFNESS OF THE WIRE
A knowledge of the stiffness of wire, tells us how far the tooth
will be moved by a specific initial force and how much force will
be applied at a certain deflection.
SPRINGINESSKapila, Sachdeva (1989), Drake et a1., (1992) referred to spring back as the maximum elastic deflection.
Springback = ys / E
Where, ys = Yield strength E = Modulus of elasticity
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STRENGTH
KUSY (1997) defines it as the force required to activate an arch wire to a specific distance. Profit - Defines strength as the product of stiffness and range
Strength = Stiffness x range. The size and shape of the cross section of a wire have profound effects on the stiffness, strength and working range of a wire.
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Comparison of some desirable clinical characteristics of orthodontic wires
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•In orthodontic wires strength and hardness may increase with a
decrease in the diameter because of the amount of cold working
induced in forming the wire.
•The property of being readily strain hardened is a
characteristic of austenitic stainless steel.
•Phase change from a face-centered to a body-centered lattice.
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•It is unfortunate that after strain hardening, a stainless steel wire
can become fully annealed in a few seconds at a temperature of
7000 C t6 8000 C. After such an annealing procedure, it has lost
much of the range of elasticity or working range that is necessary
to produce a satisfactory orthodontic appliance.
•Low-fusing solders and by confining the time for soldering and
welding procedures
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DUPLEX STEELS
• Duplex steels consists of an assembly of both austenite and
ferrite grains Besides iron these steels contain molybdenum,
chromium, and they have lower nickel content. As a result of the
ferrite content, these steels ( as opposed to austenitic ones ) are
attracted by magnets.
• Their duplex structure results in improved toughness and
ductility compared to ferritic steels. While their yield strength is
more than twice that of similar austenitic stainless steels.
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•They also are highly stress-corrosion resistant.
•Combining a lower nickel content with superior mechanical
properties, duplex steel has been used for the manufacture of
one-piece brackets.
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PRECIPITATION - HARDENABLE (PH) STEEL
Unlike most stainless steel, the PH steels can be hardened
by heat treatment. The process actually is an aging treatment,
which promotes the precipitation of some elements purposely
added. Because of its high tensile strength, PH stainless steel is
widely used for “mini” brackets.
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COBALT CONTAINING ALLOYS
• Several cobalt containing alloys are commonly used in orthodontics, both for wires and brackets.
• Some, such as Eligiloy and Flexiloy, still contain a large proportion a nickel.
• Others, however are almost nickel free, having been developed to replace their allergenic counterparts.
• Although both types are used to make archwires, nickel free steels are used primarily to manufacture attachments.
• These alloys generally are corrosion resistant.
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MANGANESE - CONTAINING STEELS
Known as an “austenitizing” element, manganese acts by
interstitially solubilizing the really” austenitizing” element
nitrogen, thus replacing nickel. Unfortunately, when used in a high
proportion, manganese increases the alloys susceptibility to
corrosion.
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Wires used in orthodontics may be classified in three ways.
BY DESIGN/CROSS SECTION
• Round• Rectangular• Square • Double cross sectioned ( wonder arches )
BY DIAMETER
•0.016”•0.018”•0.020” etc
BY ALLOY
•Stainless steel •NiTi•Elgiloy•TMA•B – Ti•Polymeric wires etc
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MANUFACTURING OF ORTHODONTIC WIRES
THUROW has described the various steps in manufacturing
process as follows:
a.Melting
b.Ingot Formation
c.Rolling
d.Drawing
BRANTLEY – Thermomechanical processing.
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MELTING
The selection and melting of the components of alloys influence
the physical properties of metals.
INGOT FORMATION
An ingot is produced by the powering of mother alloy into a mold.
It differs from any other casting, by being a non uniform
chunk of metal. Different parts of the ingot possess varying
degrees of porosities.
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ROLLING
• It is the first mechanical step in the manufacturer of a wire from the
ingot.
• The ingot is rolled into a long bar by a series of rollers that gradually
reduce it to a relatively small diameter.
• Orthodontic wires with rectangular/ square cross-sectional are
fabricated by rolling round wires, using a TURK’S head apparatus
that consist of pair of rollers positioned at right angles.
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•The squeezing and massaging action of rolling the ingot, alters
the shape and arrangement of the crystals. This action causes
them to elongate into finger like shapes, closely meshed with
each other.
•Each pass through the rollers increases the work hardening until
finally, the structure becomes so locked - up that it can no longer
adjust enough to adapt to the squeezing of the rollers.
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DRAWING
•It is a more precise process by which the ingot is reduced to its
final size.
•The wire is pulled through a small hole in a dye. The size of the
hole is slightly smaller than the starting diameter of the wire, in
order to facilitate uniform squeezing of the wire from all sides by
the walls of the die as it passes through, reducing the wire to the
diameter of the division.
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•Drawing is a more precise process them rolling, as it subjects
the entire surface of the wire to the same pressure instead of
squeezing it from only two opposite sides as in rolling.
•Before the wire is reduced to its orthodontic size, it is drawn
through a series of dies and annealed several times along the
way to retrieve work hardening.
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STAINLESS STEEL WIRES
• In the 1940’s Austenitic stainless steel began to displace gold as
the primary alloy for orthodontic wires.
• The most commonly used types are 302 and 304 stainless steel,
which contain approximately 18% chromium, 8% Nickel and
carbon of 0.08%. to 0.15 (max).
• 17-7 precipitation hardenable stainless steel alloy-high strength
& resiliency.
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•These alloys derive most of their strength from cold working
and carbon interstitial hardening.
•The microstructure demonstrates the typical ‘fibrous’
appearance associated with extensively strains.
•A two-phase structure was found for some SS wires. where
the austenitic phase was accompanied by a body-centered
cubic (bcc) martensitic phase.
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Ormco's H.T. Gold is a heat treated stainless steel wire which
provides higher force level and greater springback than traditional
stainless steel. It should be considered in applications where
resistance to deformation is a primary factor. The higher force
level and rigid nature of this wire makes it an excellent choice for
transverse arch form control.
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MULTISTRANDED STAINLESS STEEL WIRE
• Flexibility of stainless steel wire can be increased by building
up a strand of stainless steel wire around a core of 0.0065” wire
along with 0.0055” wire used as wrap wires. This produces an
overall diameter of approximately 0.0165”.
• The strand of stainless steel wire is more flexible due to the
contract slip between adjacent wrap wires and the core wire of
the strand.
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Twisted
Braided
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• When the strand if deflected the wires which are both under
tension and torsion will slip with respect to the core wire and
each other. If there is no plastic deformation wire returns to
its normal position giving the elasticity to the strand of the
wire.
• Multistranded wires are available in round, rectangular,
square cross sections.
• Multistranded wire can be used as a substitute to the newer
alloy wire considering the cost of nickel-titanium wire.
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•Kusy and Dilley noted that the stiffness of a triple stranded
0.0175" (3 X 008") stainless steel arch wire was similar to that
of 0:010" single stranded stainless steel arch wire.
•The multistranded archwire was also 25% stronger than
the .010" stainless steel wire.
•Then .0175" triple stranded wire and .016" Nitinol
demonstrated a similar stiffness.However nitinol tolerated 50%
greater activation that the multistranded wire.
•The triple stranded wire was also half as stiff as .016" B-
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Some of the multistranded wire available are:
• Dentaflex- Dentaurum is available in triple strand, co-
auxial, six strand, and braided eight strand.
• Twist flex-unitex
• Force-9-Ormco
• D-rect Ormco
• Respond- Ormco
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D-rect is an 8 stranded, interwoven braided rectangular
wire. Its high flexibility, together with 3-dimensional control and
slot filling capabilities, makes it ideally suitable for multiple
applications like:
1.Initial torque control
2.Picking up second molars later in treatment
3. A finishing arch wire, where torque control is desired yet
resilient to permit interarch occlusal settling.
4.Torque control with vertical or anterior Box elastics.
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FORCE 9 is a 9-strand, interwoven, braided rectangular wire. It
delivers 50% more force than the 8-stranded D-rect. Its selection
can be based upon similar applications where slightly more force
seems to be indicated.
RESPOND is a 6-strand, spiral wrap with a central core wire.
Respond can deliver light, initial forces while filling the arch
wire slot for greater control. Its resistance to permanent
deformation makes respond an excellent choice as an initial
archwire in more severe dental malalignments.
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Currently a stainless steel combination wire is also available. It
consists of an anterior rectangular wire and a posterior round
wire. The anterior rectangular wire gives better torque control
and acts as brakes to burn out the anchorage. These wires are
also known as dual flex wires or wonder wires.
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CONCLUSION
• 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 verstaility of orthodontic
treatment.
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•Appropriate use of all the available wire types may enhance patient
comfort and reduce chairside time and duration of treatment.
•The future lies in finding newer materials which gives more
physiologic tooth moving forces. This is just an milestone, But the
quest for newer orthodontic wires continues.
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