Wires in Orthodontics

Wires In OrthodonticsPresented by : Dr. Anuj Suri M.D.S. Part IIDept. Of Orthodontics ,B.V. Dental College & Hospital

Introduction The development of the various orthodontic treatment modalities available today would not have been possible without the introduction of metal and alloy in the form of wires in different dimensions, shapes and configurations.

A BRIEF REVIEWBefore Angle began his search for new materials , orthodontists made attachments from noble metals and their alloys . Gold (at least 75%, to avoid discoloration ), platinum, iridium, and silver alloys were esthetically pleasing and corrosion resistant , but they lacked flexibility and tensile strength .

In 1887 Angle tried replacing noble metals with German silver , a brass . His contemporary , J.N. Farrar , condemned the use of the new alloy, showing that it discoloured in the mouth .

In 1888, “by varying the proportion of Copper, Nickel, Zinc” around the average composition of Neusilber brass(German silver : 65% Cu, 14% Ni, 21% Zn), as well as by applying cold working operations at various degrees of plastic deformation , as a result , Angle made German silver rigid enough for expansion arches , or malleable enough for bands .

The material that was to truly displace noble metals was stainless steel . As with German silver ,it had its opponents . As late as 1934 Emil Herbst held that gold was stronger than stainless steel in wire form , and he complained that he could not gild stainless steel without exfoliation. If forced to choose , he even preferred German silver to stainless steel .

BASIC PHYSICAL PROPERTIES OF ARCHWIRE MATERIALS

Andreasen and Morrow (1978) Goldberg and Burstone (1979) Burstone and Goldberg (1980) and Kapila and Sachedeva (1989) have described a number of characteristics of archwires which are desirable for optimum performance during treatment.

STRESS (S, I) Stress is the internal distribution of the load measured as force per unit area i.e., Force/Original area. For simple compression or tension the stress is given by the expression,

Stress =F/A

Where, F= force applied A= cross-sectional area

Stress is measured in common units of psi or Mpa (Mega Pascal).

1 Pascal – stress resulting from a force of 1 Newton (N) acting upon 1 sq. meter of surface and is equal to 1.145 x 10–3 psi, (1000 psi = 6.894 Mpa).

One test method commonly used for dental materials is the three point bending test or transverse test. When an external force is applied to the mid point of the test beam, the stress can be resolved. The numerical value of stress is given by the expression,

Stress = 3FL where, 2bd L = distance between the supports b = width of the specimen d = depth of the specimen

When a wire is compressed across the diameter ,a tensile stress is set up in the specimen, the value of stress is being given by

Stress = 2F D² T At the axis of cylinder/wire. F = applied force D = diameter of wire T = length of the wire This type of test is referred to as a diametral

compressive tensile test and is usually used when conventional tensile testing is difficult to carry out.

TYPES OF STRESS Tension or Tensile stress : It tends to pull the material apart or tends to stretch or elongate a body. Compression or compressive stress : It is the direct opposite of tension stress. If a body is placed under a force, that tends to compress or shorten it, the internal resistance to such a force is called as compressive strain.

Shear stress : A stress that is applied by two forces acting in opposite directions but not in the same line. These stresses tend to slide one part of the material past another along planes parallel to the applied force.

STRAIN (γ) Strain is the internal distortion produced by load or a stress, i.e., change in length per unit length when stress is applied.

The numerical value of strain is given by

the expression Strain = L’ = change in length

L original length The common units of strain are inch per

inch or centimeter per centimeter.

Hooke’s Law Within the elastic range, the material deforms in direct proportion to the stress applied, i.e.,

f = E γ orstress = Modulus of elasticity x

strain

MODULUS OF ELASTICITY (Young’s modulus) (E)

It is defined as the ratio between a unit stress and a unit strain, usually expressed as pound/square inch (psi) or mega pascal (Mpa).

It is an index of stiffness or flexibility of a

material within the elastic range. And is given by :

Stress (I) E = --------------------- Strain (γ)

PROPORTIONAL LIMIT (IPL) or (P)

It is the point at which the first deformation occurs. It is the maximum stress at which the straight line relationship between stress and strain (Hooke’s Law) is valid

ELASTIC LIMIT (IEL) (E)

It corresponds to the stress beyond which strains are not fully recovered. It is the maximum stress that a material can withstand without permanent deformations.

YIELD STRENGTH (Ys) or (IYS)

It is the practical indicator at which the first deformation is measured. It is measured by pounds per square inch.the point at which a plastic deformation of 0.1 % is measured and is called yield strength (YS).

ULTIMATE TENSILE STRENGTH (UTS)

It is the maximum load carrying capacity of the wire before it fractures.

It represents the maximum stress required to fracture a material.

DUCTILITY

It is the ability of a material to be plastically strained in tension i.e., ability of a material to withstand permanent deformation under a tensile load without rupture.

ELONGATION It is the deformation as a result of tensile force application.It is usually expressed as percentage elongation and is equal to

L increase in length --------------- x 100 or --------------------- x 100 L0 original length

MALLEABILITY The ability of a material to withstand permanent deformation without rupture under compression as in hammering or rolling into sheets.

Gold -Most ductile and most malleable

Silver -next to most ductile and malleable

Platinum -Third most ductile

Copper -Third most malleable

RESILIENCE ( stored or spring energy)

Resilience represents the energy storage capacity of a wire, when it is stressed not to exceed its proportional limit. ie., It is the energy absorbed by a wire in undergoing elastic deformation upto the elastic limit.The energy stored is released when the wire springs back to its original shape after removal of an applied stress.

FORMABILITY Formability is amount of permanent deformation that a wire will withstand before failing i.e. before breaking or fracture.

FLEXIBILITY It is the measure of the strain that a wire can withstand without undergoing plastic deformation. A material is said to be flexible if it withstands the strain or the load up to its proportional limit without deforming permanently. It is a non-specific term denoting the ease of bending. It may indicate low stiffness, low strength, high working range or low brittleness, either singly or in any combination.

LOAD DEFLECTION RATE

For a given load (force) the deflection observed within the elastic limit is known as load deflection rate.

With regard to active members, a low load deflection rate is desirable for the reasons given below :-

A mechanism with a low load deflection rate will maintain a more desirable stress level in the periodontal ligament, since the force on a tooth will not radically change magnitude any time the tooth as been displaced.

Also a low load deflection rate member offers greater accuracy in control over force magnitude eg. If we use a high load deflection spring, it is possible that the load deflection rate might be 1000 gm/mm. This would mean that an error in adjustment of 1 mm could produce an error in force value of approximately 1000 grams.

If a low load deflection rate is desirable for an active member of the appliance, the opposite is true for a reactive member. The reactive member should be relatively rigid i.e., it should have a high load deflection rate.

SPRINGBACK (Range of action) AND SPRINGINESS Springback ability of a wire is a

measure of its ability to undergo large deflections without permanent deformation. In other words if a wire can be deflected over long distances without permanent deformation, it has a high spring back value.

It is expressed as YS/E i.e., the ratio of yield strength to modulus of elasticity which represents the approximate amount of elastic strain released by the wire on unloading.

GLOSSARY OF TERMS “Stabilised” nickel titanium –

The alloy has a fixed composition, which is incapable of demonstrating changes in its crystal structure. Its elastic properties are the result of its inherently stable structure.

“Active” nickel titanium – The alloy has a fixed composition, but is capable of undergoing changes in its crystal structure when stress or temperature is applied .

“Active” austenitic – On application of stress, the nickel titanium demonstrates a change in crystal structure from austenitic to martensitic.

“Active” martensitic – On application of heat at the relevant transition temperature, the nickel titanium demonstrates a change in crystal structure from martensitic to austenitic

Transition Temperature – The temperature range over which the alloy structure changes from the martensitic to the austenitic phase.

Hysteresis – The temperature lag between the gelatin temperature and the liquefaction temperature of the gel is known as hysteresis.

Hysteresis curve – A non linear stress/strain curve, where the loading curve differs from the unloading curve.

Super elasticity – A term confined to those materials demonstrating unique hysteresis curves under conditions of varying temperature or stress.

Shape memory - This indicates that a material will return to its desired shape. In order for the term to have clinical meaning the means by which the material is able to demonstrate this effect needs to be specified, for example, thermodynamic shape memory, or superelastic shape memory.

Thermodynamic – This refers to

the ability of an archwire to return to its intended shape once heated through its transition temperature. To be of clinical value, thermodynamic archwires must have a transition range close to mouth temperature.

THE PHYSICAL PROPERTIES OF METALS AND ALLOYS AND THEIR APPLICATION IN WIRE FORM IN ORTHODONTICS

the flexural rigidity (EI) ;

the resistance to distortion ;

the susceptibility to fracture

THE FLEXURAL RIGIDITY (EI)

The flexural rigidity of a wire is the product of the Young’s modulus (E) and a factor (I) known as the second moment of inertia of the cross-section of the wire. The factor I depends on the shape and dimensions of the cross-section of a wire, and determines how stiff a wire (with a given Young’s Modulus) will be.

For a circular cross-section, for example,

I = π R4 / 4

Where, R is the radius of the cross-section.

The second moment of inertia of the cross-section (I) increases greatly as the radius of the wire is increased.

Doubling the radius (holding everything else constant) therefore increases the force applied by a spring by (2)4, i.e., by 16 times.

More practically, for a given deflection, the use of 0.6 mm diameter wire instead of 0.5 mm will double the force applied.

Again, replacing 0.5 mm wire by 0.7

mm will increase the stiffness of an appliance by nearly 4 times.

The Flexural Rigidity may be determined experimentally for a wire by mounting the latter as a simple cantilever and measuring the deflection for various loads applied to the free end.

The value of Flexural Rigidity (EI) can be deduced from the relationship between force applied (p) and the deflection (y) of the free end of the cantilever, i.e.

P/y = 3EI / l3

THE RESISTANCE OF WIRES TO DISTORTION

If a load is applied to the free end of a simple cantilever, the upper layers of the wire are extended and the lower layers are compressed. At any given cross-section of the wire the variation in the magnitude and direction of these internal stresses, from the outer to the inner surfaces of curvature, constitutes a series of couples whose resultant, the moment of resistance, is equal in magnitude and opposite in sense to the Bending Moment at the crosssection considered.

The maximum fibre stress occurring in the outermost layers of a bent wire at any point may be calculated from the expression

σmax = GR/I where, σmax is the maximum fibre stress. G is the Bending Moment at that

point, R is the radius of the wire, I is the second moment of inertia of

the cross-section.

Provided the maximum fibre stress, σmax, is less than the effective yield stress, σeff, the wire will behave elastically and return to its rest position when released. If, however, σmax is greater than σeff at any point along a wire, permanent plastic deformation will take place.

The necessary condition for distortion to occur may therefore be written

GR ≥ σeff I

SUSCEPTIBILITY TO FRACTURE

It is usually assumed that appliances occasionally fail because of metal fatigue induced by the repeated stressing of the wire.

A study, however, of the fatigue life of 2-cm finger springs by Bass and Stephens (1970) showed that these springs were capable of withstanding over 1,00,000 flexes when the distance of flexure was 7.5 mm.

Harcourt and Munns (1967) have investigated finger springs fractured in use and conclude that fatigue failure is less likely to occur than failure due to surface defects produced by the pliers during fabrication or by an abrasion wheel when the appliance is being trimmed, finished or fitted. They also emphasize that an appliance should be adjusted by bending an unstressed part of the wire, to avoid deforming any part of the appliance which has already been cold worked.

STRENGTH It is a force value that is a measure of the maximum possible load,i.e., the greatest force that a wire can sustain or deliver, if it is loaded to the limit of the material.

It is equivalent to the proportional limit (PL) or approximately the yield strength (YS) of the wire segment.

Considering the graphic representation of the stress – strain curve three points can be taken as representative of the strength of a material

- elastic limit- yield point- -ultimate tensile

strength

STIFFNESS

It is the rate of force delivery required for a unit activation . It is the measure of the force required to bend or otherwise deform the material to a definite distance.

Stiffness is proportional to the modulus of elasticity and crosssection of a given wire and is not appreciably influenced by any hardening treatment.

Stiffness and springiness are reciprocal properties.

Springiness = 1 / stiffness.

According to the force deflection curve, stiffness and springiness are proportional to the slope of the elastic portion of the curve.

The more horizontal the slope, the springier the wire, the more vertical the slope the stiffer the wire.

Stiffness of wire depends on two fundamental factors –

1. Composition and structure of wire alloy

2. The wire segment geometrically i.e., the cross section, shape, size and the segment length, i.e.,

Ed/L

IMPORTANCE OF STIFFNESS

Stiffness should be the first criteria in the selection of wire as it determines the relationship between force and deflection in ideal working range.

Stiffness tells how far the tooth will be moved by a specific force and conversely how much force will be applied at a certain deflection.

If only stiffness is considered, any of the commonly used material can be used interchangeably by merely selecting a suitable diameter of wire and for each material.

Wires of equal stiffness will behave identically below the point at which they take a permanent set, regardless of the material.

RANGERange is defined as the maximum amount of elastic activation before the onset of a permanent or plastic deformation.

Range is usually determined from the 0.1% offset point on the force – deflection diagram.

Strength,Stiffness and Range have an important relationship,i.e.,

Strength = Stiffness x Range

The stiffer a wire, the springiness comparatively decreases and vice versa.

Stiffness = Ed/L, higher the elastic modulus, stiffer the wire.

FACTORS AFFECTING STIFFNESS, STRENGTH AND RANGE The mechanical arrangement by which force is applied to the teeth , i.e.,

-bracket width -length of archwire -interbracket span -loop configuration

The second factor is the form of the wire itself – the size and shape of crossection.

The third factor is the material. Including the alloy formula, its hardness and the state of heat treatment.

Effects of size and shape on elastic properties A beam is any relatively slender structure subjected to lateral (bending) loads. An orthodontic archwire functions mechanically as a beam

With just a moderate understanding of the principles of beam behavior, an orthodontist can predict the function of an archwire in the mouth without resorting to mathematical calculations.

Beams function by bending, whether they are supporting a building or moving a tooth.

A beam in a building must bend very little, but in orthodontics, we are more often looking for the exact opposite i.e. , an archwire with the greatest possible flexibility.

Each of the major elastic properties – strength , stiffness and range- is substantially affected by a change in geometry of the beam.

Both the cross-section and length of beam are of great significance in determining its properties.

Changes related to size and shape are independent of the material. In other words, decreasing the diameter of a steel beam by 50% would reduce its strength to a specific percentage of what it had been previously.

Decreasing the diameter of the TMA beam by 50% would reduce its strength by exactly the same percentage.

Effects of diameter or cross section

Effects of length and attachment

Springiness increases as a cubic function of the increase in length of the beam, while strength decreases only in direct proportion.

Thus a relatively large wire selected for its strength, can be given the desired spring qualities by increasing its length.

Effects of varying materials on orthodontic force

The best balance of strength , springiness and range must be sought among the almost innumerable possible combinations of beam materials, diameter and length.

Variable Crosssection OrthodonticsVersus Variable Modulus Orthodontics

Variable cross-section orthodontics, the traditional orthodontics, involves the use of small wires for light force and larger for heavier forces.Variable modulus orthodontics involves taking advantage of the different materials in respect of stiffness and load deflection rate while maintaining the same cross-section.

The overall stiffness of the appliance (S) is determined by two factors:

one factor relates to wire itself, (Ws)

The other factor is the design of the appliance (As)

Therefore, S= Ws x As

One can obtain full range of forces by varying the material of the wire and keeping the cross-section constant.

THANK YOU

WIRESComposition

Manufacturing & Heat Treatment

Properties

GOLD ALLOY WIRES The first wire introduced for orthodontic purpose was made of gold

Gold arch wires were the ideal choice of arch wires with good bio-compatibility.

Composition of many gold alloy wires corresponds to the type IV gold casting alloys

They are also subjected to softening and hardening heat treatments.

Many wires appear to contain less than 60% gold with some containing less than 25 to 30% or even less.

The palladium content of the alloy is relatively high, which gives a composition closely resembling white gold casting alloys.

Palladium and platinum cause rise in the melting point, improves corrosion resistance and increases hardness and strength during heat treatment.

the copper content of most wires is well above 9%, which suggests that gold wires should be subjected to ordered gold copper formation when heated at appropriate low temperatures.

Gold alloys used, can be called to a large extent as binary alloys, as only gold and copper are major metals used.

These binary alloys to a large extent exhibit severe grain growth on heating and have poor ductility in the hardened state.

The addition of rhodium, iridium or cobalt reduces the rate of grain growth on heating.

After age hardening, these complex gold alloys, will have higher values of tensile strength and hardness.

HEAT TREATMENT OF GOLD WIRES

The changes that are produced in the strength and ductility of a wrought gold alloy by heat treatment are due to the alterations in the gold copper compound present in the alloy.

Softening heat treatment is undertaken initially by heating the wire to 1300° F, for approximately ten minutes and then quenching it. Softening of the alloys is produced as the gold copper alloy enters into solid solution at 1300° F. All of the hardening elements are completely dissolved in each other in solid solution, the space lattice is free to move on the slip planes without interference.

Increased number of slip planes, causes increased ductility of the wire.

After the wire is heated to 1300°F, it is immediately quenched to retain the super saturated condition of the gold copper compound that tends to come out of the solution. This causes the formation of segregated molecules which produce a locking effect on the space lattice and causes resistance to slip.

The space lattice itself is also distorted to some degree, thus decreasing the number of planes on which slip can occur. In this way the material becomes stronger and more resilient.

Since the composition of gold wires differ phenomenally, no one heat treatment will produce optimal results for all wires.

Hence it is best for the orthodontist to follow manufacturers recommendations when the alloys are used.

Besides (age) precipitation hardening, cold working of gold alloys increases strength of the wrought gold wires. The alloy hardens as the grain structure becomes broken up and the space lattice is distorted during cold working.

This type of hardening is easily relieved by heating the wire to recrystallization temperatures, recrystallization will take place and allow the atoms to return to normal position in the space lattice.

PROPERTIES OF GOLD WIRES

Yield strength of the gold wires range from 50,000 to 1,60,000 psi, depending on the alloy and condition, with corresponding elongations of 3-16%.Modulus of elasticity of gold copper alloys is approximately 15,000,000 psi. The combination of these properties makes gold very formable and capable of delivering lower forces than stainless steel

STAINLESS STEEL ALLOY

Stainless steel wires began to replace gold wires in the 1930’s .

Steels are iron – based alloys that usually contain less than 1.2% carbon.

When 12-30% chromium is added to steel the alloy is commonly called STAINLESS STEEL.

COMPOSITION Type(Space Lattice)

Chromium

Nickel

Carbon

Ferritic (BCC) 11.5-27 0 0.20 max

Austenitic(FCC) 16.0-26 7-22 0.25 max

Martensitic(BCT) 11.5-17 0-2.5 0.15-1.2 max

Silicon , phosphorous ,sulphur, manganese, tantalum, and niobium may also be present in small amounts. The balance is iron.

Ferritic Stainless Steel These alloys are often designated as American Iron and Steel Institute(AISI) Series 400 stainless steels. The ferritic alloys provide good corrosion resistance at a low cost , provided that high strength is not required. Because temperature change induces no phase change in the solid state , the alloy is not hardenable by heat treatment. Also , ferritic stainless steel is not readily workhardenable . This series of alloys finds little application in dentistry.

Martensitic Stainless Steel Martensitic stainless steel alloys share the AISI 400 designation with the ferritic alloys. They can be heat treated in the same manner as plain carbon steels , with similar results. Because of their higher strength and hardness, martensitic stainless steels are used for surgical and cutting instruments.

Austenitic Stainless Steel The austenitic stainless steel alloys are the most corrosion resistant of the stainless steels.

AISI 302 is the basic type , containing 18% chromium , 8% nickel , and 0.15% carbon . Type 304 has a similar composition , but the chief difference is its reduced carbon content (0.08%).

Both 302 and 304 stainless steel may be designated as 18-8 stainless steel ; they are the types most commonly used by the orthodontist in the form of band and wires .Type 316L (0.03% maximum carbon) is the type ordinarily employed for implants.

The alloying elements Chromium and Nickel maintains austenite at room temperature and prevents conversion of face centered cubic lattice structure of austenite to a martensitic cubic lattice structure.

By nature austenite is mallable and ductile whereas martensite is hard and brittle. By maintaining austenite at room temperature, several uses of austenitic stainless stell are made use of in orthodontics, such as wires, bands, instruments etc.

Tarnish and corrosion are resisted by stainless steel due to the passivating effect of chromium. A thin , transparent but tough impervious chromiumoxide layer forms on the surface of the alloy when it is subjected to an oxidizing temperature as mild as clean air or atmospheric room air. This protective oxide layer prevents tarnish and corrosion, but can be ruptured by mechanical or chemical means resulting in corrosion. However ,the passivating oxide layer eventually forms again in an oxidizing environment

Chromium loss is called sensitization.

A procedure to introduce some element that precipitates carbide in preference to chromium preferably titanium is called stabilization.

HEAT TREATMENT OF STAINLESS STEEL ALLOY

The physical properties of orthodontic stainless steel wires improve by heat treatment at low temperatures between 750° C to 820° C for ten minutes and at a lower temperature of 250° C for twenty minutes. By heat treatment residual stresses are removed, hence further application of stresses will not exceed the elastic strength of the metal.

CORROSION OF STAINLESS STEEL

Any surface inhomogenecity, surface roughness, incorporation of bits of carbon steel, soldered joints and treatment with chlorine causes corrosion of the stainless steel.

PROPERTIES OF STAINLESS STEEL

Alloy Modulus of Elasticity (10³ MPa)

0.2% Offset Yield Strength (MPa)

Ultimate Tensile Strength (MPa)

Number of 90-degree Cold Bends without fracture

Stainless Steel

179 1579 2117 5

AUSTRALIAN ORTHODONTIC ARCHWIRE

Claude, Arthur. J. Wilcock of Victoria, Australia, produced the orthodontic archwire to meet Dr. Begg’s needs for use in Begg technique.

The wire produced has certain unique characteristics different from usual stainless steel wires .

Grading and colour coding of Australian Orthodontic Arch wires

REGULAR GRADE : White label. Lowest grade and easiest to bend. Used for practice bending or forming auxillaries. It can be used for archwires when distortion and bite opening are not a problem.

REGULAR PLUS GRADE : Green Label Relatively easy to form, yet more

resilient than regular grade. Used for auxillaires and archwires when more pressure and resistance to deformation is required.

SPECIAL GRADE : Black Label. Highly resilient, yet can be formed into

intricate shapes with little danger of breakage.

SPECIAL PLUS GRADE : Orange Label Hardness and resiliency of the wire are excellent for supporting anchorage and reducing deep overbites.

EXTRA SPECIAL PLUS GRADE :Blue Label.

Highly resilient and hard, difficult to bend and subjects to fracture.

Supreme Grade : Blue Label. Primarily used in early treatment for

correction of rotations, alignment and leveling. Although supreme wire exceeds the yield strength of E.S.P. It is intended for use in either short sections or full arches where sharp bends are not required.

The flexibility of supreme wire is comparable to that of Nickel- Titanium wires and has the added advantage of good formability.

Each grade of wire is available in diameters of 0.010″, 0.012″, 0.014″, 0.016″, 0.018″, 0.020″, 0.022″. They are supplied in the form of spools or cut lengths of the wire.

With the demand from the orthodontic faculty for more harder wires , even higher grades , premium and premium plus wires were developed .

The fundamental difference for the superior properties of these new wires is the use of a new manufacturing process called pulse straightening as against the spinner straightening procedure used earlier .

The new grades and sizes of wire makes available are:

Sizes Available Premium : .020″

Premium Plus :.010″,.O12″,.014″,.016″, .018″

Supreme :.008″, .009″, .010″, .011″.

HEAT TREATMENT OF AUSTRALIAN WIRE

Since 1970’s preformed archwires , torquing auxillaries and uprighting springs have been available commercially . Attempting to straighten the high tensile wire , for subsequent forming into appliance leads to frequent breakage . The low and medium grade wires exhibit better formability as they are subjected to less work hardening and hence are more ductile .

Till then the wires were straightened by what is called spinner straightening, usually in the cold hard drawn condition . The wire is pulled through high speed rotating bronze rollers which torsionally twist the wire into a straightened condition This can result in permanent deformation.

Presently the premium and supreme wires are straightened by a process called pulse straightening .Though the exact procedure , presumably remains a trade secret , it enables to straighten these high yield strength wires , without structural deformation and altering the physical properties.

The properties of the wire are affected by the way the wire is straightened before bending it to form any component of the appliance . If the wires are straightened by the process of reverse straining, meaning flexing in a direction opposite to that of the original bend, the yield point of the wire reduces(this is what is done manually in a clinical setting). The phenomenon is known as work softening due to reverse straining or the ‘Bauschinger Effect’ , named after the person who described it for the first time .

PROPERTIES OF AUSTRALIAN WIRES These are ultra high tensile austenitic stainless steel arch wires.

The wires are resilient, certain bends when incorporated into the arch form and pinned to the teeth become activated by which stresses are produced within the wires which generates forces. The magnitude and continued application of the resolved sum of these forces are vital for efficient functioning of the appliance.

The wires must be sufficiently resilient to resist permanent deformation and maintain their activation, for maximum control of anchorage.

All these properties make these

wires very hard and brittle.

CHROME COBALT ALLOY A cobalt-chromium-nickel

orthodontic wire alloy was developed during the 1950’s by the Elgiloy Corporation(Elgin, IL,USA). Initially it was manufactured for watch springs by Elgin watch company, hence the name Elgiloy. Marketed as Elgiloy, Azura, Multiphase etc.

COMPOSITIONChrome cobalt alloy is a cobalt base

alloy containing 40% cobalt, 20% chromium, 15% nickel, 7% Molybdenum, 2% manganese, 0.16% carbon, 0.04% beryllium and 15.8% iron.

TYPES OF CHROME COBALT ALLOY WIRES Blue(soft) elgiloy : Can be bent easily with finger pressure and pliers. Heat treatment of blue elgiloy increases its resistance to deformation.

Yellow elgiloy : Relatively ductile and more resilient than blue elgiloy. Further increase in its resilience and spring performance can be achieved by heat treatment.

Green elgiloy : More resilient than yellow elgiloy and can be shaped with pliers before heat treatment.

Red elgiloy : Most resilient of elgiloy wires, with high spring qualities, withstands only minimal work hardening. Heat treatment makes it extremely resilient.

HEAT TREATMENT OF COBALT- CHROME ALLOY The ideal temperature for heat treatment is 900° F or 482°C for 7 to 12 minutes, in a dental furnace.

This causes precipitation hardening of the alloy increasing the resistance of the wire to deformation.This heat treatment would increase the yield strength and decrease the ductility.

Electrical heat treatment using a heat treatment unit can also be used with a temperature indication paste, wet cotton has to be placed at the ends of the wire.

Heat treatment(of Blue Elgiloy) increases flexural yield strength(20-30%), modulus of elasticity(10%), reduces failure to corrosion in localized areas where stresses can get concentrated.

PROPERTIES OF CHROME COBALT ALLOY

Alloy Modulus of Elasticity(10³ MPa)

0.2% Offset Yield Strength(MPa)



Chrome Cobalt

184 1413 1682 8

These wires have the following advantages:-

1.Tarnish and corrosion resistance are excellent

2.Hardness ,yield strength ,and tensile strength are approximately same as those of 18"-8" stainless steel.

3. Ductility in the softened condition is greater than that of 18-8 stainless steel alloys and less than the alloys in the hardened condition.

NICKEL TITANIUM ALLOY

William F. Buehler in 1960’s invented Nitinol

Ni – Nickel ti-titanium Nol-Naval Ordinance

Laboratory,U.S.A.,

Andreasen G.F. and co-workers introduced the use of nickel-titanium alloys for orthodontic use in the 1970’s.

COMPOSITION55% nickel, 45% titanium

resulting in one to one steiochromatic ratio of these elements.

1.6% cobalt also is added to obtain desirable properties.

PROPERTIESTransition Temperature Range : TTR Shape Memory Super elasticity

Transition Temperature Range : TTR

Transition temperature range is a specific temperature range when the alloy nickel titanium on cooling undergoes martensitic transformation from cubic crystallographic lattice.

It is found to be in martensitic crystallographic lattices consisting of lesser symmetric lattices like monoclinic, orthor hombic, tetragonal crystallographic structures at lower temperatures

In martensitic phase the alloy cannot be plastically deformed.

At higher temperatures the alloy is found to be in cubic crystallographic lattice consisting of body centered cubic crystallographic structures.

It is also known as Austenitic phase of the alloy. Plastic deformation can be induced, in austenitic phase of the alloy.

The same plastic deformation induced at the higher temperature returns back when the alloy is heated through a temperature range known as reverse transformation (transition) temperature range, RTTR.

Any plastic deformation below or in the TTR is recoverable when the wire is heated through RTTR.

TTR of nickel titanium alloy is between 482 - 510° C when the alloy is cooled from higher temperature which is very high for clinical usage.

Substitution of 1.6% cobalt results in formation of TiNi and TiCo which have transition temperature ranges of +164.6° C (+330° F) for TiNi and

–237.2° C (-395° F) for TiCo giving a very wide transition temperature range.

Shape Memory It is the phenomenon, where in

if a plastic deformation incurred within or below the TTR, it is recoverable within certain strain limits of 8%, which is the outer fibre strain limit of the wire.

The nitinol wire should be plastically deformed at a lower temperature and casted. The casted wire should be placed in the oven and heated between 482° C to 510° C. Plastic deformation occurs and the wire is then placed in the refrigerator.

On cooling the wire comes back to the original shape. It is then heated again and plastic deformation is induced after which the wire is again placed in the oven, followed with freezing.

This heating followed by freezing is continued until the wire retains the shape exhibited at higher temperature even at room temperature.

Hence the preformed nitinol arch will have a second Transition Temperature Range (TTR) which is lower and clinically applicable.

Andreasen introduced 0.019 inch thermal nitinol wire with a transition temperature range between 31° C and 45° C.

Body or mouth temperature acted as the RTTR to activate the wire after placement in the brackets.

Super elasticity It is the property of the wire

explained as even when the strain is added, the rate of stress increase levels off due to the progressive deformation produced by the stress induced martinsitic transformation.

This property can be produced by stress and not temperature difference.Therefore it is called as stress induced martensitic transformation.

Another wire called the Japanese Niti wire introduced by Fujio Miura is manufactured by a different process and demonstrates super elasticity.

Another nickel titanium alloy introduced by Burstone developed by Dr Tien Hua Cheng called as Chinese Niti alloy exhibits superior spring back property when compared to Nitinol due to little work hardening and presence of the parent phase which is austenite yielding better mechanical properties.

HEAT TREATMENT OF THE JAPANESE NI-TI WIRE

A new type of heat treatment was reported by Fujio Miura and associates which is known as Direct Electric Resistance Heat Treatment (DERHT). An electric current is directly passed through the wire, thus generating enough heat to make it possible to bend it as well as impart change in the super elastic property of the wire.

With DERHT method, excellent spring back properties of the wire are not diminished and it is possible to apply optimal force to each tooth with a single archwire.

Heat treating equipment consists of an electric power supply, a pair of electric pliers, an electric arch holder. Electric power supply consists of a transformer, a timer, an electric current meter and a foot switch. The amount of heat can be controlled by amperage and the heating time.

For smaller diameter wires lesser current is required. For eg : 0.022” wire requires 8.0A for 2.0 seconds, 0.014” wire requires 3.5A for 2.0 seconds. The DERHT method utilizes the electric resistance of the wire to generate heat.

In spite of resulting molecular re-arrangement the mechanical properties of the wire are unchanged.

The equipment in the DERHT method is light in weight, in addition a distinct advantage is with the pair of electric pliers and the electric arch holder, it is possible to heat treat only the desired section of the wire.

Load –Deflection curves for the three segments of the archwire,showing the effect of heat treatment

By utilizing DERHT method, it is possible to alter the super elastic characteristics of the wire in any desired section. An electric current of 3.5A transmitted through an electric arch holder was applied to the anterior segment of arch wire (A-A′) for a period of 45 min.

The same procedure was repeated by transmitting the same current to the premolar segment including the anterior segment of the arch wire B-B′ for 15 min. Hence current was transmitted through A-A′ for 60 min. The molar segments of the arch wire were not heat treated.

On testing it was found that the heat treated segments demonstrated better super elastic properties in relation to time. Hence it is possible to heat treat any desired section of the archwire by DERHT method and utilize optimally the super elastic property of the wire.

Nickel titanium are most commonly manufactured into Nickel Titanium alloy by the process of vacuum induction melting or vacuum arc melting process. Several re-melts are often needed to improve homogeneity of nickel titanium alloy. Powders are then made of the alloy.

The process of hot isostatically pressing is used by the manufacturer to form the powders into wires. Voids occur in the areas where the powders are not completely pressed together.

The wires obtain their final shape by the prcess of drawing or rolling.

The processes of drawing or rolling may leave scratch marks on the surface.

Because compositions can now be controlled in an ingot to within parts per million, martensitic (M), and austenitic (A) transformations, may start (s) , and finish (f) so that :

Ms = 14 ½ ºC, and Mf =7½ºC on cooling with

As= 34 ½º C and Af= 43 ½ ºC on heating .

COPPER Ni Ti WIRES In 1994 Ormco Corporation introduced a new orthodontic wire alloy, Copper NiTi Copper Ni Ti is a new quaternary ( nickel, Titanium copper and chromium ) alloy with distinct advantages over formerly available nickel titanium alloys.

Stress induced martensite is responsible for the super elastic characteristics of nickel titanium alloys. martensite transformation is also temperature dependent.

One of the most important makers is the materials austenitic finish temperature (Af) It is the differential between the Af temperature and mouth temperature that determines the force generated by nickel titanium alloys.

This temperature (Af) can be controlled over a wide range by affecting the composition , thermo mechanical treatment and manufacturing processes based on nickel titanium –copper Ni-Ti, this alloy has the advantage of generating more constant forces than any other superelastic nickel titanium alloy. It is more resistant to deformation as a result of thermo –mechanical insults in the mouth. Also it demonstrates a smaller mechanical hysteresis, that is, it does not lose its recovery load as do other nickel titanium alloys.

Orthodontic archwires fabricated from this alloy have been developed for specific clinical situations and are classified as follows:

Type I Af 15 0C Type II Af 27 0C

Type III Af 35 0C

Type IV Af 40 0C

These four alloys form the basis for “variable transformation temperature orthodontics” developed by Dr. Rohit C.L. Sachdeva, Texas.

These variants would be useful for different types of orthodontic patients. For example, the 27oC variant would be useful for mouth breathers; the 35oC variant is activated at normal body temperature; and the 40 o C variant would provide activation only after consuming hot food and beverages.

PROPERTIES OF COPPER NiTi WIRES 1. Copper Ni Ti generates a more

constant force over long activation spans than other nickel titanium alloys and does so on a consistent basis, from archwire to archwire.

2. For every small activations, Copper NiTi generates near constant force, unlike other nickel titanium alloys.

3.Copper NiTi is more resistant to permanent deformation compared with other nickel titanium alloys; it exhibits better spring back characteristics.

Copper Ni-Ti Exhibits smaller drop in unloading (tooth –driving) force than is true with other nickel titanium alloys.

.

The addition of copper combined with more sophisticated manufacturing and thermal treatment processes make possible the fabrication of four different Copper NiTi archwires with precise and consistent transformations temperatures: 15 0C, 27 0C, 350C and 400C. this enables the clinician to select archwires on a case specific basis

ALPHA TITANIUM The alpha titanium alloy is attained by adding 6% aluminum and 4% vanadium to titanium Because of its hexagonal lattice. It posses fewer slip planes making it less ductile from β- titanium. the hexagonal close pack structures of Alpha Titanium has only one active slip plane along its base rendering it less ductile.

β – TITANIUM – TITANIUM MOLYBDENUM ALLOY OR T.M.A.

In the 1960’s an entirely different “high temperature” form of titanium alloy became available. At temperature above 1625°F pure titanium rearranges into a body centered cubic lattice (B.C.C.), referred to as ‘Beta’ phase.

With the addition of such elements as molybdenum or columbium, a titanium based alloy can maintain its beta structure even when cooled to room temperature. Such alloys are referred as beta stabilized titaniums. The alloying and body – centered cubic impart a unique set of properties.

Goldberg and Burstone demonstrated that with proper processing of an 11% molybdenum, 6% Zirconium and 4% tin beta titanium alloy, it is possible to develop an orthodontic wire with a modulus of elasticity of 9.4 x 10 6 psi and yield strength of 17 x 10 4 psi. The resulting YS/E ratio of 1.8 x 10 2 is superior to 1.1 x 10 –2 for stainless steel.

PROPERTIES OF β-TITANIUM

Alloy Modulus of Elasticity(10³ MPa)

0.2% Offset Yield Strength(MPa)



β- Titanium

71.7 931 1276 4

1. The low elastic modulus yields large deflections for low forces. The high ratio of yield strength to elastic modulus produces orthodontic appliances that can sustain large elastic activations when compared with stainless steel devices of the same geometry.

β- titanium can be highly cold worked . The wrought wire can be bent into various orthodontic configurations and has formability comparable to that of austenitic stainless steel . The mechanical properties of many titanium alloys can be altered by heat treatments that use the transformation from α to β lattice structure. However , heat treatment of the current orthodontic β- titanium wire is not recommended.

Clinically satisfactory joints can be made by electrical resistance welding of β- titanium . Such joints need not be reinforced with solder. A weld made with insufficient heat fails at the interface between the wires , whereas overheating may cause a failure adjacent to the weld joint .

TOOTH COLOURED ORTHODONTIC WIRES

The new orthodontic materials of recent years have been adopted from those used in aerospace technology. The high performance aircraft of the 1970’s and 1980’s were titanium based, but the current generation are built of composite plastics, and there is every reason to believe that orthodontic wires of this type will move into clinical use in future.

It is interesting that one nonmetallic wire already has been offered for clinical use. Optiflex is a new orthodontic arch wire designed by Dr. Talass and manufactured by ORMCO. It has got unique mechanical properties with a highly esthetic appearance. Made of clear optical fibre, it comprises of three layers.

1. A silicon dioxide core that provides the force for moving teeth.

2. A silicon resin middle layer that protects that core from moisture and adds strength.

3. A strain resistant nylon outer layer

that prevents damage to the wire and further increases its strength.

CV NiTi WIRES Masel has announced its' new CV(tm) NiTi wires. CV NiTi is meant as an alternative to the copper NiTi wires used in many orthodontic procedures

CLINICAL IMPORTANCE OF VARIOUS WIRES

GOLD WIRES

Gold alloy wires have decreased usage in orthodontics due to high cost of gold alloy and also advent of stainless steel alloy which is cost effective and provides properties required for orthodontic wires

STAINLESS STEEL WIRES Orthodontic stainless steel is the most widely used alloy in orthodontics. It finds its application as arch wires, auxillaries , retainers, removable appliances,bands,etc. The wires are available both in round as well as rectangular cross-sections.

The Australian stainless steel wires described previously are used in the Begg’s technique as well as in the preadjusted edgewise technique

MULTI STRANDED 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” wires used as wrap wires. This produces an overall diameter approximately 0.165”.Multi stranded wires are available in round, rectangular nd square cross-sections.

The strand of stainless steel wire is more flexible due to the contact slip between adjacent wrap wires and the core wire of the strand. When the strand is deflected the wrap wires which are both under tension and torsion will slip with respect to the core wire and each other. If there is no elastic deformation each wire returns to its normal position, giving the elasticity to the strand of the wire.

According to studies conducted by Kusy and Barrows, multi-stranded wires have elastic properties similar to nickel-titanium arch wires. Hence they can be used as a substitute to the newer alloy wires considering the cost of the nickel titanium wires .Kusy and Dilley noted that the stiffness of a triple stranded .0175” (3x .008”) stainless steel arch wire was similar to that of 0.010” stainless steel arch wire. The multistranded arch wire was also 25% stronger than the 0.010” stainless steel wire. Then the 0.0175” triple stranded wire and 0.016” Nitinol demonstrated a similar stiffness.

Some of the multi stranded wires available are :

1. Dentaflex (DENTAURUM)- Dentaflex is available in triple strand , co-axial six strand and braided eight strand.

2. Twist flex- UNITEK3. Force 9 - ORMCO4. D-rect – ORMCO5. Respond – ORMCO

D-rect is an 8 stranded , interwoven braided rectangular wire . Its high flexibility , together with 3-dimensional control and slot filling capabilities make it ideally suitable for multiple applications like:-

1. Initial torque control2. Picking up second molars

later in treatment3. 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.

Force 9 is a 9 stranded , interwoven , braided rectangular wire. It delivers 50% more force than the 8 stranded D-rect wire. Its selection can be based upon similar applications where slightly more force seems to be indicated.

Respond is a 6 stranded ,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 arch wire in more severe dental malalignments.

COMBINATION WIRESCurrently, stainless steel combination wire is also available . It consists of an anterior rectangular wire and posterior round wire . The anterior rectangular wire gives better torque control and acts as brakes to burnout the anchorage . These wires are also known as Dual flex or Wonder wires.

CHROME COBALT WIRES The advantages of elgiloy over stainless steel wires include greater resistance to fatigue and distortion and longer function as a resilient spring. The high modulus of elasticity of elgiloy wire suggests that these wires deliver twice the forces of β- titanium wires and four times the force of nitinol wires for equal amount of activations.

The eligiloy blue alloy is very popular with many orthodontists because the as-received wire can be easily manipulated into desired shapes and then heat treated to achieve considerable increases in strength and resilience.This heat treatment can be performed easily with the aid of an electrical resistance welding apparatus , and the manufacturer provides a special paste that indicates when the appropriate conditions of temperature and time have been achieved.

The other three tempers of Elgiloy have mechanical properties that are similar to tempers that are available with the less expensive stainless steel wire alloys.

AZURLOY is a heat treatable alloy with excellent formability in its non heat treated form . Applications that take advantage of this formability followed by heat treating to increase the spring rate might include :Multiloop systemsUtility archesOverlay intrusion or base arches

NICKEL TITANIUM WIRES Nickel titanium wire can produce an uniform constant force which is delivered for a long period of time during the de-activation of the wire. The tooth alignment proceeds clinically during deactivation as a result of material superelasticity.

By increasing archwire dimensions in sequence, super elastic nickel titanium is theoretically capable of providing physiological force delivery over a wide range, by virture of its non linear force/ deflection characteristic through out the process of leveling and aligning in addition, reactivation of austenitic wire during alignment and leveling can alter the force delivered to the dentition simply by releasing and retying it.

The days are not far when we would be using only one or at the most two wires to finish a case with little or no wire bending at all.

The desirable mechanical properties of Nickel Titanium alloy wires and their relatively high cost has prompted many clinicians to recycle these wires. Many of the clinicians who recycled these wires indicated that a deterioration in the mechanical properties of the wire was their major concern.

Recycling involves repeated exposure of the wire for several weeks or months to mechanical stresses and elements of the oral environment, as well as sterilization between uses. The combined effects of repeated clinical use and sterilization may subject, the wire to corrosion and cold working with resultant alteration in the properties. Clinical recycling increases the loading and unloading forces associated with these wires and also reduces the superelasticity of these wires.

Uses of NiTi alloy wires 1. Because of its superior spring back,

superelasticity, shape memory, and its ability to produce light force for longer duration , NiTi is ideal wire for initial leveling and aligning. Rectangular NiTi allows full engagement of the bracket slot and give better torque control in the initial phase of treatment.

2. Reverse curve NiTi, also known as Rocking chair NiTi helps in bite opening and when placed down helps in bite closure along with leveling and aligning.

1. Titanium alloys offers a significant improvement over currently available materials for tooth separation, especially for adult patients or adolescents with tight contact and amalgam filling with broad contacts. Distortion is not a problem as it can be with tempered stainless steel or elastiomerics. The problems of breakage during insertion common with elastomeric modules is resolved. Re-use after autoclaving is also possible with NiTi springs.

Recently a NiTi palatal expander(ortho organizer) has been developed which is used for transverse expansion of the maxilla. It is a temperature activated palatal expander with the ability to produce light continuous pressure on the mid palatal suture while simultaneously uprighting, rotating and distalizing the maxillary first molar.

The action of the appliance is consequence of NiTi’s shape memory and transition temperature effects. The NiTi expander has a transition temperature of 94ºF. when it is chilled before insertion, it becomes flexible and can be easily bent to facilitate placement.

As the mouth begins to warm the appliance , the metal stiffness, shape memory is restored and the expander begins to exert a light, continuous force on the teeth and the midpalatal suture.

NiTi is also available in the form of coil springs. These NiTi coil springs manufactured by Ormco greatly enhance efficiency in both space closure and space opening. NiTi coil springs are also used for distalisation of molars.

Trade names of NiTi alloy wires manufactured by some companies:1. Elastinol – Masel orthodontics2.Bioforce Sentalloy – Gac International3. Nitanium – ortho organizers4. Neosentalloy – Gac International 5. BMA arch wire – Masel Orthodontics6. Titanal XR – Lancer orthodontics7. Rematitan – Dentaurum8. NiTinol SE – Unitek

1. NiTinol XL – Unitek2. Turbo – Ormco3. Orthonol – Rocky Mountain4. Marsenol – Glenroe Technologies5. Reflex – T.P. orthodontics6. Sentinol – Gac International7. Align – A company8. Force merican orthodonticsTurbo wire is the first braided NiTi wire manufactured by Ormco.

COPPER NiTi WIRES Type I wire – Af 15 0C

Sachdeva does not

recommend the frequent use of this alloy because it generates very heavy forces and clinical indications are few

Type II wire Af 270CThis wire generates the highest force

of the three ( type I , III, IV) and is the best used :

1. In patients who have an average or higher pain threshold.

2. In patients who have normal periodontal health.

3. In patients where rapid tooth movement is required and the force system generated by this orthodontic archwire is constant.

Type III wire – Af 350CThis wire generates force in the

midrange and is best used :1. In patients who have a low to

normal pain threshold.2. In patients whose periodotium is

normal to slightly compromised.3. When relatively low forces are

desired.

Type IV wire – Af 40 0C These wires generate “tooth driving”

forces when the mouth temperature exceeds 400C. these forces are intermittent in nature. The indications for use of this alloy includes:

1. Patients who are sensitive to pain.2. Patients who have compromised

periodontal conditions(smaller cross sections are indicated

for severe cases)

3. Where tooth movement is deliberately slowed down , i.e., when the patient may not be able to visit the orthodontist regularly or his/her cooperation is very poor and the orthodontist does not want “things to get out of hand”

This wire is very beneficial as an initial rectangular wire.

ALPHA TITANIUM WIRES Alpha titanium gets hardened by absorbing intraoral free hydrogen ions which turns it into titanium hydride at the oral temperature of 37 0c and 100% humidity. Mollenhauer reported that after six weeks in mouth the wire become brittle to bend. Therefore any modifications if required should be done within six weeks.

Presently the wire is available as a combination, the anterior section is .018 “ x .025” rectangular for torque control and braking while the posterior section which is oval, tapering from 0.018” to 0.017”. hence it can be used as closing wire.

BETA- TITANIUM WIRESBeta-titaniums balance of physical properties also makes it an ideal choice for utility arches.Its excellent formability makes the fabrication of utility arches fairly simple.

T.M.A appears to be well suited as a utility arch for three primary reasons:

1. It is highly formable and utility arches are easily formed.2. With its enhanced resiliency a single activation is all that is required to achieve vertical corrections3. With its reduced load/deflection rate the incisor torque control can be obtained while staying within accepted force ranges.

Preformed tear drop looped T.M.A. arch wire provides twice the working range of stainless steel and requires fewer activations for retraction. T.M.A.’s moderate forces create less trauma for the patient and increases patient comfort. Retraction can be accomplished more efficiently with reduced chair time. A stainless steel tear drop loop produces a force of 728 gm for 1 mm activation and a T.M.A. tear drop loop produces a force 367 gms for 1 mm activation.

Ormco has introduced a low friction T.M.A. with reverse curve of spee which is ideal for bite opening, arch leveling, space closure and early three dimensional manipulation and torque control.Through an exclusive spi-spectrum ion beam implantation process. Ormco has introduced T.M.A. colours.

T.M.A. colour gives patients some exciting new looks while at the same time providing the clinician with many clinical benefits of T.M.A. wire. The implantation of a mixture of oxygen and nitrogen ions into the wire surface is similar to the process used in the manufacturing of low friction T.M.A. This ensures a colour fastness not available in coated wire products. Patients will love the chance to choose a favourite colour, adding excitement to what was once a routine wire change.

TOOTH COLOURED ORTHODONTIC WIRES

It is interesting that one nonmetallic wire already has been offered for clinical use. Optiflex is a new orthodontic arch wire designed by Dr. Talass and manufactured by ORMCO.

Optiflex posses five advantages which make it a unique archwire in terms of esthetics and mechanics alike:1. Optiflex is the most esthetic orthodontic arch wire to date.2. Optiflex is completely stain resistant. The arch wire will not stain or loose its clear look even after several weeks in the mouth. The yellowish stain commonly seen in elastometric ligatures and chains will never the observed in optiflex.

3. Beyond esthetics, optiflex is very effective in moving teeth using light continuous forces. The force applied with optiflex is approximately equal to half the force applied with a respond archwire of similar size.4. Optiflex is very flexible. It has an extremely wide range of action. When indicated it can be tied with elastometric ligatures to severely malaligned teeth without the fear of fracturing the archwire.5. Due to its superior mechanical properties, optiflex can be used with any bracket systems.

Optiflex has got the following clinical applications:It is used in adult patients who wish that their braces not be really visible for reasons related to personal concerns or professional considerations.2. It can be used as an initial wire in cases with moderate amounts of crowding in one or both arches.

3. It should be used in cases to be treated without bicuspid extraction. Optiflex is not the ideal arch wire for major cuspid retraction. Retracting cuspids in the extraction cases with optiflex has been disappointing due to its limited ability to control the distal tipping and the labio lingual rotation of the retracted cuspids.4. Optiflex can be used in presurgical stage in cases which require orthognathic intervention as part of the treatment.

Optiflex archwires combine with translucent brackets to create the ultimate in labial appliance esthetics. Optiflex is available in ten 6 inch straight lengths of 017” and 021”.

MARSENOL is a tooth coloured Nickel titanium wire manufactured by GLENROE TECHNOLOGIES. It is an E.T.E. coated Nickel Titanium. E.T.E. is an abbreviation for ELASTOMERIC POLY TETRA FLORETHYLENE EMULSION. Marensol exhibits all the same working characteristics of an uncoated superelastic Nickel titanium wire. The coating adheres to the wire and remain flexible. The wire delivers constant force over long periods of activation and is fracture resistant.

Lee White Wire manufactured by LEE PHARMACEUTICAL is a resilient stainless steel or Nickel titanium arch wire bonded to a tooth coloured EPOXY coating, suitable for use with CERAMIC and PLASTIC brackets. The epoxy is completely opaque and does not chip, peel, stain or discolour.

GRADED ARCHWIRESContemproary opinion states that the

response of a tooth to force application and the rate of tooth movement is dependent on the surface area of the periodontium. This means that the ideal archwire would not only delivery a constant and low force to malaligned teeth, but it should also be capable of varying its level of force delivery, consistent with the area of periodontium involved.

To date, only one known ,manufacturer (GAC), has put this theory into practice by developing the “bio-force” archwire. It is already possible to produce variation in arch wire force delivery between archwires of identical dimensions by specifying trasition temperatures within group ranges. The manufactures have taken this process one step further, by introducing variable transition temperatures within the same archwire.

This takes the form of graded force delievery with in the same alignment archwire providing light forces of approximately 80g anteriorly, and a heavier force of 100 g posteriorly. The level of force applied is therefore graded throughout the arch length according to tooth size.

THE CHOICE OF ARCH WIRE IN THE CLINICAL SITUATION

The demands placed on the arch wire depend upon the particular purpose for which it is intended , and the purpose will change at different stages of treatment.

For each arch wire , stiffness must be such that an appropriate force magnitude is delivered , strength must be sufficient to prevent distortion by masticatory forces , and range must make it possible to apply the force over a sufficient distance , so that frequent reactivation is not required .

The arch wire must be resistant to fracture , and of a material which is safe to use in the mouth.

ARCH WIRES FOR INITIAL ALIGNMENT

Early in treatment , tooth displacement will be at its greatest.

The principal arch wire requirement is for minimum stiffness and maximum range , in order that the arch wire can apply force of appropriate magnitude over relatively large distances.

The arch wires used in the initial stages of treatment are designed to enable full engagement of the arch wire in the bracket channel at the earliest opportunity.

Arch wire materials appropriate for initial alignment stage are round cross-section wires as follows:1.nickel- titanium (preferably

in its superelastic form)2.multistranded stainless steel3.Australian premium and

supreme grade wires (.009,.010,.011)

Where tooth displacements are marked , and wide brackets are used, the first arch wire should be particularly low in stiffness and high in range . ‘Superelastic’ nickel titanium wire of 0.012” to .016” diameter or six- strand multistranded stainless steel wire of 0.015 or 0.0175” diameter may be chosen.

When there are marked individual tooth displacements , it is necessary loosely to ligate the bracket to the archwire (rather than attempting to achieve full bracket engagement) , in order to avoid excessive arch wire deformation and to limit the force applied.

If 0.018” edgewise brackets are in use , 0.0175’multistrand archwire is not ideal , because the small clearance between the archwire channel and the archwire will restrain movement of the archwire through the channel. Even larger diameter nickel-titanium or multistrand round wires may be used to complete the period of initial alignment.

The very nature of initial alignment archwires means that they offer poor control over unwanted tooth movements . Their low stiffness means that it is inadvisable to use them in combination with elastic traction , because they will allow too much tipping of (otherwise unsupported) anchorage units.

Sliding mechanics , using elastic traction or coilsprings for instance , should be avoided because of bracket binding and because the archwire is likely to bend ,with loss of angular control over both the teeth being moved and the anchorage units.

In most cases , initial alignment is complete within three months of commencing treatment.

Indeed , considering the poor control offered and the dangers of producing unwanted tooth movement , initial archwires should be exchanged for the archwires of mid-treatment as soon as possible

MID TREATMENT ARCHWIRES

the highly flexible arch wires used for initial alignment are replaced by a series of ‘working’ archwires of increasing stiffness ,offering progressively greater control over tooth position

In the early stages of mid-treatment single strand , round, stainless steel archwires of small diameter are appropriate .

Archwires of 0.016”and then 0.018” diameter are used next .If movement of teeth along the archwire is required (sliding mechanics) , then the archwire must be capable of adequate control of tooth position , and frictional losses must be kept as low as possible .

When round wire is used then it should be of at least 0.016” in diameter , 0.018” brackets are preferable.

Mid-treatment archwires are sufficiently stiff to enable the molars to resist unwanted movement, and they therefore play an important part both in molar control and in anchorage management. Inter and intra-maxillary elastic forces can be used safely with stainless steel single strand round wires of 0.016” diameter and above.

If edgewise brackets are used and it is intended to progress to rectangular wire, the use of round as described above is to be recommended in the first instance. As an intermediate stage , it may be advantageous to step down to rectangular wires of low stiffness(multistrand,nickel-titanium,B-titanium) before using single- strand stainless steel rectangular wire.

the correct approach is to use a series of rectangular wires of increasing stiffness With 0.022”brackets nickel-titanium 0.018”x 0.025”, and then 0.019”x 0.025” stainless steel is an appropriate progression.

The sequence of the archwire changes during mid-treatment will depend of course upon the mechanical technique being followed and the treatment aim , but the overall concept of progressive increase in archwire stiffness still applies.

ARCHWIRES FOR DETAILING AND RETENTION

When the principal tooth movements have been achieved using the mid-treatment archwires it is necessary to complete final detailing of tooth position and then to provide retention.

The archwire requirements at this stage are for high stiffness and

low range

When rectangular wire has been used at the end of mid-treatment stage the detailing archwire should also be rectangular , possibly of increased stiffness, and retaining any torqueing adjustments which have been incorporated (non preadjusted edgewise brackets). β-titanium archwire of 0.021”x 0.025” size has a stiffness which is appropriate for detailing of tooth position.

Begg brackets present problems at the detailing and retention stage . The loose fit of the archwire in the channel , and the flexibility of the accessory springs makes precise positioning of teeth very difficult to achieveand retain.

If preadjusted edgewise brackets have been used then theoretically the detailing stage will be unnecessary because of the activation programmed into the brackets. However, minor errors in bracket positioning will become obvious in these final stages of treatment , and archwire modification may still be required.

In search In search of the ideal of the ideal archwirearchwire

From this description of contemporary archwire alloys we see that no ideal archwire exists. This conclusion is not surprising because the demands of the treatment plan require different characteristic stiffnesses and ranges.

Specific wires will do some things well and others poorly; but no wire will do it all.

The future

One promising approach toward achieving an esthetic archwire with excellent overall properties involves the use of composites, which can be composed of ceramic fibers that are embedded in a linear or crosslinked polymeric matrix.

Existing experimental prototypes are tooth colored, can be as strong as the strongest piano wire, and can vary in stiffness from that of the most flaccid multi-stranded archwire, to nearly that of a beta-titanium archwire

These characteristics can be varied during manufacture without any change in wire-slot engagement by pultrusion, in which the relative proportions of the fiber and matrix materials are adjusted appropriately and cured by electromagnetic radiation.

Mechanical tests show that such archwires are elastic until failure occurs. When compared with NiTi, resilience and springback are comparable. Moreover, when failure finally does occur, the wire loses its stiffness, but it remains intact.

Although the specifics of other characteristics, such as formability, weldability, and frictional coefficients, are unknown at this time, preliminary experiments suggest that preformed archwires and rectangular cross-sections should be possible by a process known as beta-staging and that low coefficients of friction and enhanced biocompatibility should be possible by modifying the surface chemistry of the polymer.

Indeed, as composites are displacing metallic alloys as structural components in the aerospace industry, the expectation is that the attractive properties and characteristics of these esthetic composites will capture a significant share of the marketplace within the next decade.

Summary of relative levels of important properties for selection of orthodontic wire alloys

References1. Backofen W.A. & Gales G.F. : the low temperature heat treatment of stainless steel for orthodontics. A.O. 1951, vol 21, 117 –124

2. Funk A.C. : heat Treatment of S. Steel . A.O. 1951, vol 21, 129-136.

3.Richman, G. Y. : Practical metallurgy for the orthodontist AJO 1956, ; vol. 42, 573-587

4.Burstone C.J. et al. Beta Titanium, A new orthodontic alloy, AJO 1980, Vol. 77, 212 –132

5.Orthodontic materials –William Brantley

6. Refined Begg for modern times- V.P. Jayade

7. Orthodontic treatment with removable appliances- Houston, Issacson

8. Ralph W. Philips Skinner’s Science of dental materials Ninth edition. 261-270, 537-551.

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Wires in Orthodontics