ORTHODONTIC WIRES

Introduction Optimum orthodontic tooth movement is produced by light continuousforce. The challenge in designing and using an orthodontic appliance isto produce force system with these characteristics, creating forces thatare neither too great nor too variable over time.

Ideally archwires are designed to move teeth with light, continuousforces. Such forces may reduce the potential for patient discomfort,tissue hyalinization and undermining resorption.

So In order to provide the efficient appliance the knowledgeabout wires is essential because wires are the fundamentaldevices that produce the force.

BASIC PROPERTIES OF WIRES

STRESS Force per unit area within a structure subjected to an external force. (The internal resistance of the body to the external force) Force Stress = -------- Area

Stress can be simple or complex

3 Types of Simple Stress: 1. Compressive stress 2. Tensile stress 3. Shear stress

STRAIN Deformation or change in length per unit length of the body when a stress is applied.

Change in length Strain = -------------------- Original length

Strain may be elastic or plastic or a combination of booth.

Elastic strain is reversible and it disappears when the force is removed.

Plastic strain represents a permanent deformation of the material that never recovers even when the force is removed

ELASTIC LIMITThe greatest stress to which a material can be subjectedsuch that it returns to it original dimensions when forcesare released

PROPORTIONAL LIMITIt is the maximum stress at which stress is proportional to strain .If the stress and strain were to be plotted in a graph , the graphgives a straight line upto the point where stress and strainprogressed in a definite proportion. Beyond this point of stress, theline curves meaning that the strain is no more proportional to thestress.

MODULUS OF ELASTICITY (E )It represents relative stiffness or rigidity of the wire within elastic range.Also called YOUNG S MODULUS . So less the strain for the given stressgreater will be the stiffness. Stress E = --------- StrainAn ideal arch wire should have sufficient stiffness so as to withstandmasticatory forces. But it should also be low enough so as to provide 1. The ability to apply lower forces 2. A more constant force over time as the appliance experiences deactivation. 3. Greater ease & accuracy in applying a given force.

YIELD STRENGTHIt is the point of stress at which thematerial undergoes a SLIGHTbut permanent deformation.Yield strength is slightly more thanproportional limit and for practicalpurposes the same as proportionallimit.

RESILIENCEIt is defined as the amount of energyabsorbed by a material when it isstressed upto its proportional limit.

R

RANGE It is the distance the wire will bend elastically before permanentdeformation occurs. It is associated with SPRING BACK YS Spring back ------ EAn ideal arch wire should have large spring back , so it can bedeflected over larger distances without permanent deformation.

This provides the ability to apply large activations with resultantincrease in working time of the appliance .This enables to use fewerarch wires & it also minimizes intervals of adjustment , thusdecreasing no., of patient appointments. STRENGTH = STIFFNESS X RANGE

FORMABILITY It is the amount of permanent deformation that a wire canwithstand before it breaks.An arch wire should have high formability that provides the abilityto bend the wire into desired configurations such as loops,coils &stops.

SPRINGBACKThis is the ability of a wire to return to its original shape after aforce is applied. High values of springback means that it is possibleto tie the wire in a displaced tooth, without permanent distortion.

STIFFNESSThe amount of force required to deflect or to bend a wire. The greater the diameter of an archwire the greater the stiffness.

JOINABILITY This is whether the material can be soldered or welded.

BIOCOMPATABILITY It includes resistance to corrosion & tissue tolerance toelements in the wire .

ENVIRONMENTAL STABILITY It ensures the maintenance of desirable properties of the wire forextended periods of time after manufacture. This in turn ensures predictable behavior of the wire when in use.

CRITERIA FOR IDEAL ARCHWIRES kusy- Angle-97In contemporary practice, no one arch wire material meets all these requirements and the best results are obtained by using specific arch wire material for specific purposes.

Although the properties required in an orthodontic wire will vary,depending upon its application, generally three characteristics are important for a superior wire .

First, it should be possible for the wire to be deflected over long distanceswithout permanent deformation; hence, a large springback. This ensures thatthe clinician can activate his appliances without permanent deformation, whichassures better control over tooth movement and minimizes intervals foradjustment.

Second, the wire should have a stiffness that is lower than that of stainless steel,which would allow wires to fill the bracket for control and at the same timeproduce lighter forces.

Third, the wire should be highly formable, that is, capable of being easilyshaped, bent, and formed into complicated configurations, such as loops, withoutfracture.

FACTORS AFFECTING WIRE PROPERTIES1.Cross section of the wireChanging the diameter of the wire no matter how it is supportedgreatly affects its properties but vary in their magnitude.In doubling the cross section Strength - 8 times stronger( i.e increases by cubic function) Springiness Decreases by a factor of 16 (i.e- Decreases by 4th power function) Range Decreases by factor of 2 (i.e it decreases proportionately) Pascal Garrec et al- Angle 2004 Oct

2. Length of the wire:Changing the length of the wire dramatically affects its properties In doubling the length of the wire Strength Decreases in half Springiness Increases by a factor of 8 Range Increases by a factor of 4.3.Type of attachmentThe elastic properties of the wire is affected by whether the wire istied tightly or held loosely in a bracket.Supporting the wire in both ends Strength Increases twice Springiness - decreases by a factor of 4 Range - Decreases in half

CLASSIFICATION OF ORTHODONTIC WIRES1. By design or cross sectional form 2. By no of wires 3. By diameter 4. By composition a) Gold wires b) Stainless steel wires c) Co Cr wires d) Titanium based wires e) Composite wires

GOLD ALLOYS

The composition of the alloys used in gold orthodontic wires issimilar to the Type IV gold casting alloys.

CompositionGold - 55 % to 65 % is typical ( can also have as low as 15 %)Copper 11% to 18% Ag - 10 % to 25%Palladium 5% to 10%Platinum 5 % to 10 %Nickel 1 % to 2%.

They acquire additional strengthening through the cold-working incorporatedduring the wire-drawing process. These wires can potentially be strengthenedwith the proper heat treatment, although they are typically used in the as-drawncondition.

Properties1.Yield strength - 50,000 1,60,000 psi2.Modulus of elasticity 100 GPa

Advantages1.With the same cross section,Gold wires has less force delivery than stainless steel wires.2.Very formable3.Good joinability4.High corrosion resistance

Disadvantages1.High cost2.Decrease spring back

Stainless steel alloysHistorically, relatively few metallic alloys have been used in thefabrication of orthodontic appliances. Although at one time goldwas widely used for arch wires, in recent years austenitic stainlesssteel has been the mainstay of orthodontic wires.

It has maintained its popularity because of a good balanceof environmental stability,stiffness, resilience, and formability.Economic factors no doubt play a role in its wide acceptance incomparison to gold.

With the advent of stainless steels in World War I and therefinement of drawing processes to form wires in the late1930s, gold archwires gradually lost favor to the smallercross-sectional areas that stainless steel archwires couldprovide. By the 1950s the type 300 series of stainless steel alloyswere used for most orthodontic materials.

Composition: Chromium 18 % to 25 % Nickel 8 % to 12% Carbon - < 0.2 % Balance Mainly Iron

With the presence of chromium in the alloy, a coherent oxide layerformed that passivated the surface, thereby rendering the alloystainless.When at least 8% nickel was present, the single phase structure ofaustenite was stabilized, and the overall corrosion resistance wasenhanced. Carbon content was purposely maintained below 0.20% to reducethe formation of chromium carbides structures that can ultimatelyfoster the corrosion of austenitic steels.

As the 50s came to a close, Rocky MountainOrthodontics was offering two tempers of cold-workedstainless steels: 1.standard 2. extra hard grade. Today, American Orthodontics advertises three grades ofstainless steel wire: 1.Standard, 2.Gold Tone, 3.Super Gold Tone.

The most commonly used types are AISI 302 and 304 stainlesssteels, which contain approximately 18 percent chromium, 8 percent nickel, and less than 0.20 percent carbon.

The type 304 stainless steel has a slightly lower carbon and higherchromium specification. These alloys derive most of their strength from coldworking.

The microstructure demonstrates the typical ''fibrous" appearanceassociated with extensively elongated grains. This microstructurecan be altered by short exposures to high temperatures, which iswhy soldering procedures have to be undertaken carefully.

Properies YS 1100 to 1500 MPa E - 160 to 180 GPa

Advantages1. Good formability2. Good joinability3. Least friction 4. Good corrosion resistance5. Decreased cost

Disadvantages1 .High Modulus of Elasticity causes high forces. So smallerdiameter wires are used to decrease force levels .But this results inPoorer fit in brackets and may cause loss of control during toothmovement. 2. Needs soldering to reinforce joints. These soldered joints corrodein oral cavity.

Stainless steel, however, was not destined to enjoy thepredominance in the marketplace that gold had enjoyed for somany years before.

Cobalt-chromium alloysIn the 1950s, the Elgin Watch Company developed a complex alloywhose primary ingredients were cobalt, chromium, iron, and nickel .

This cobalt-chromium alloy was marketed as Elgiloy by RockyMountain Orthodontics.

PropertiesYield Strengh - 830 to 1000 MPaElastic modulus - 160 to 190 GPa

AdvantagesSpring back similar to SSHigh formabilityCan be soldered but technique demandingExcellent corrosion resistance

In order resilience is desired to capitalize on the inherent elasticityof the material, which could be achieved by heat treating the alloyat 482C for 7 to 12 minutes. This so called precipitation hardening heat treatment increasesthe ultimate strength and resilience of these archwires withoutchanging the stiffness.

So after heat treatment the softest Elgiloy becomes equivalent toregular stainless steel, while harder initial grades are equivalent tothe super grade. Along with Stainless steel it is considered the mostideal and economic finishing wire.

Cobalt chromium alloys (Co-Cr) are available commercially asElgiloy, Azura and Multiphase. Manufactured in four tempers in increasing order of resilience. Wires of different tempers are colour coded.

Soft - Blue Ductile - Yellow Semi resilient - Green Resilient - Red

Blue ElgiIoy - Type I - Heart resistant - Softest temperedSoftest of the four wire tempers and can be bent easily withfingers,or pliers.Recommended for use when considerable bending, soldering orwelding is required. Heatt treatment of Blue Elgiloy increases itsresistance to deformation.

Yellow EIgiloy - Type II - Ductile temperedIt is relatively ductile and more resilient than blue Elgiloy. Can alsobe bent with relative ease. Further increases in its resilience andspring performance can be achieved by heat treatment.

Green Elgiloy - Type III - Semi resistant temperedMore resilient than yellow elgiloy and can be shaped with pliersbefore heat treatment.

Red Elgiloy - Type IV - High Spring Temper

Most resilient and provide high spring qualities. Careful manipulation with pliersis recommended because it withstands only minimal working.

Heat treatment of 900 F ( 482 C ) for 7 to 12 mins in a dental furnacecauses precipitation hardening of the alloy , Increasing the resistance of thewire to deformation & results in properties similar to that of SS

Heat treatment at temp above 1200 f ( 749 c ) results in a rapid decline inresistance to deformation because of partial annealing. Optimum levels of heat treatment are confirmed by dark straw coloured wire orby use of temp indicating paste. Caution should be exercised when soldering attachments to these wires sincehigh temp causes annealing with resultant loss of yield & tensile strength.

The advantage of Co Cr wires over SS include a) greater resistance to fatigue & distortion b) longer function as resilient spring.

Both Co-Cr & SS wires has same high modulus of elasticity E =160 to 190 GPa which suggest that these wires deliver twice theforce of - Titanium & four times the force of NiTi wires forequal amount of activation. The resultant undesired force vectorsare greater with Co- Cr & SS wires.Clinically this may translate intofaster rates of mesial movements of posterior teeth, thus placing greater demand on intra & extraoral anchorage.

Multi Stranded Wires ( BRAIDED AND TWISTED WIRES) Kusy & Stevens -Angle 1987 Jan Initial orthodontic leveling arch wires require great working rangeto accommodate the usual malalignment of bracket slot in theuntreated malocclusion. Low stiffness is advantageous so that theforce can be kept as gentle as possible. High strength is desirable sothat normal masticatory forces will not deform or fracture the wire.

Before the advent of Titanium alloy wires , various methods wereemployed to maximize these desirable properties in SS wires. Onemtd is to bend loops into the wire to increase the inter bracket wirelength there by increasing the range & decreasing the stiffness. But this posed problems of hygeine & tissue impingement,as well asincreased chair time

Another method involved the use of multiple strand wires.

This takes advantage of small cross section which provides theflexibility & also has sufficient strength provided through the use ofmany strands of wires.

Very small diameter ( 0.005 inch to 0.010 inch) SS can be braided ortwisted together by the manufacture for clinical uses. The finalinterwined wires may be either round or rectangular in shape &between 0.016 inch to 0.025 inch in overall cross section.

TYPES:1. Triple stranded wires2. Coaxial wires 5 or 6 stranded wires wrapped around one central strand. The cross section might be round or flat.

ADVANTAGES : Multistranded wires are able to sustain large elastic deflection inbending. Because of their low apparent modulus of elasticity inbending , theses wires apply low forces for a given deflection whencompared with single solid SS wires.

Rose ,Frucht & Jonas AJO 2003 April did a study on clinicalcomparison of Multistranded SS wire & a direct bonded polyethylene ribbon reinforced resin composite wire used forlingual retention.

They concluded that 50% of MS wires stayed over 24 monthswhereas mean survival time for RRC wire is only 11.5 months. So multistranded wires have excellent retention as it allowsminimal physiological movement of teeth which is desirable.

AUSTRALIAN WIRESMr. Arthur j wilcock of whittlesea, Victoria., originally developedthis special orthodontic wire at the request of Dr. P.R. Begg.

Available in a variety of diameter sizes, grades of resiliency,coiled or in St. lengths . St. Length wire is usually not considered tobe as resilient as coiled wire due to the straightening process. Eachgrade is easil identified by a coloured label.

Regular grade- White labelLowest grade easier to bend, used for practice bending or formingauxiliaries.Can be used for archwires when distortion or bite opening is not aproblem.

Regular plus grade -Green labelRelatively easy to form yet more resilient than regular grade, usedfor auxiliaries and arch wires when more pressure and resistance todeformation are desired.

Special grade - Black labelHighly resilient, yet Can be formed into intricate shapes with littledanger of breakage. Used (.016" diameter) for starting arches inboth the light wire and other techniques.

Special plus grade Orange labelSpecial plus wire is routinely used by experienced operators.Hardness and resiliency of .016" size are excellent for supportinganchorage and reducing deep overbites. Must be bent with care.

Extra special plus grade - Blue labelThis grade is unequaled in resiliency and hardness. It is moredifficult to bend and more subjected to fracture. However, manyOrthodontists feel E.S.P.'s ability to move teeth, open bites andresist deformation far outweighs the inconvenience caused by anoccasional breakage while bending.

The new grades and sizes of wire available now are;Premium - .020"Premium plus - .010", .011", .012", .014", .016 & .018"Supreme - .008", .009", .010", .011They are available in spools or straight lengths.

TITANIUM BASED WIRESNi Ti wires B- Titanium wires or TMA wiresAlpha Titanium wiresJapanese NiTi wiresChinese NiTi wiresCopper NiTi wiresTimolium wires

CONVENTIONAL NITINOL In the late 60s, the Office of the Navy was activelystudying new types of alloys that exhibited a shapememory effect (SME).

One of these, a nickel-titanium alloy, showedgreat promise and was dubbed nitinol, an acronymfor nickel-titanium Naval Ordnance Laboratory.

This alloy was capable of being deformed, clamped, heated, andcooled into a specified shape, so that when it was later deformedinto a new shape and subsequently heated, the material wouldremember its previous post-heat treatment shape.

Around 1970, Dr. George Andreasen recognized the potential ofthis alloy which were based on the original research of Buehler.Largely through his efforts, the first nitinol alloy was marketed toorthodontists as Nitinol.

Composition: Stoichiometric equiatomic nickel titanium wires contain

Ni 52% Ti 45% Co 3% Solid-state solution hardening and cold working are thebasic strengthening mechanisms employed with this alloy.

PHASE TRANSFORMATION :There are 2 major NiTi phases in nickel titanium wires

1. Austenitic NiTi 2. Martensitic NiTi

Austenitic NiTi has a ordered BCC structure that occurs at Hightemperature & Low stresses. It is relatively rigid & unyielding.

Martensitic NiTi has a distorted monoclinic , triclinic or hexagonalstructure that forms at High stresses & Low temperatures.In this phase the wire is said to be ductile & readily capable ofplastic deformation.

Shape Memory and Super elastic properties of NiTi based wires areattributed to a phase transformation of the crystal structure fromaustenitic form to martensitic form. This transformation occurs as aresult of temperature change or the application and removal ofstress.

When the wire is loaded, an anatomic shift occurs from theaustenitic BCC structure through an intermediate rhomboidalphase(R-phase) to the martensitic hexagonal close-packed latticestructure.

Upon unloading, the martensitic wire will return to the austeniticlattice , passing again through the R-phase.

TEMPERATURE TRANSITION RANGE ( TTR ):The phase transformations do not occur at a particular temperaturebut rather within a range known as temperature transition range. Itrefers to the temperature range for the start & completion for thatparticular structure.

On Cooling : Ms ( martensite start ) The start of martensite formation Mf ( martensite finish ) - The end of martensite formation

On Heating: As ( austenite start ) Here the martensite begins to decline & austenite begins to form. Af ( Austenite finish )- Till here the whole structure is austenite Md ( martensite deformation )- SIM

Shape memoryIt is a phenomenon occurring in the alloy that is soft and readilyamenable to change in shape at a low temperature, but it can easilybe reformed to its original configuration when it is heated to asuitable transition temperature.

Hurst & Nanda in AJO 1990. According to them the specific TTR depends on the chemicalcomposition of the alloy & its processing history. The TTR can be changed by altering the proportion of Ni to Ti or by substituting Cofor Ni.

The memory configuration of the alloy must be first set in thematerial by holding it in desired shape while annealing it at 450 Fto 500 F for 10mins. Through deflection & repeated temp cyclesthe wire in the austenite phase is able to memorize the preformedshape including specific orthodontic archforms.

Once a certain shape is set , the alloy can then be plasticallydeformed at temperatures less than TTR.

To obtain maximum shape recovery the amount of plasticdeformation should be limited to 7% to 8% of the original linearlength.

Super elasticityThe "super-elastic property" is a phenomenon that can bedescribed briefly. The stress value remains fairly constant up to acertain point of wire deformation. At the same time, when the wiredeformation rebounds, the stress value again remains fairly constant.

This can be produced by stress, not by temperature difference, andis called stress-induced martensitic transformation.

Martensitic transformation begins when an external force is appliedin such a manner that the stress exceeds a given amount. Evenwhen strain is added, the rate of stress-increase levelsoff due to theprogressive deformation produced by the stress-induced martensitictransformation, indicating a movement similar to the slipdeformation. This phenomenon is the so-called super-elasticity.

On the other hand, if the stress is diminished, the NiTi alloy returnsto the previous shape without retaining the permanent deformationbecause of the characteristics of returning to the austenite phasewithin a given temperature range.

Hence it is necessary to manufacture wire in the austenitic phase forit to possess Superelastic behavior. These wires are called A NiTiwires.

The non super elastic NiTi wires contain substantial quantities ofheavily cold worked & stable martensite. As temp of these alloys are much higher than room temp & thetemp of oral environment .They are now commercially available &are referred to as M NiTi wires.

CLASSIFICATION OF NI -TI WIRES Kusy has classified nickel titanium wires as

1.Martensite stabilized alloys ( M NiTi ) These do not possess shape memory or superelasticity,because the processing creates a stable martensitestructure.( ex- Nitinol )

2.Martensite active alloy.These alloys employ thermoelastic shape memory effect.

The oral environment raises the temp of the deformed archwirein martensite phase to transform into stable austenite form.

This can be observed by the clinician if a deformed archwiresegment is warmed in the hands.( ex - Neo-Sentalloy & Copper NiTi)

3. Austenitic- active wires.( A NiTi ) These alloys under go a Stress Induced Martensite ( SIM )transformation when activated. These alloys have both super elastic& shape memory effect. They do not exhibit thermoelastic behaviorwhen deformed wire segment is warmed in the hands.( ex ChineseNiTi & Japanese NiTi ).

In A-NiTi wire, over a considerable range of deflection the forceproduced hardly varies. This means the wire exerts about the sameforce whether it is deflected a relatively small or a largedistance ,which is a unique & extremely desirable property.(ex Chinese & Japanese NiTi.) Burstone CJ et al AJO 1985 June

The unique force deflection curve for A NiTi wire is thatits unloading curve differs from loading curve i.e reversibility hasan energy loss associated with it termed HYSTERESIS .This means the force delivered is not the same as the force appliedto activate.

The different loading & unloading curves produce the remarkableeffect that the force delivered can be changed by merely releasingthe wire & retying it.

PROPERTIES :Good spring backExcellent flexibilityLow stiffnessHigh range

ADVANTAGES:Large elastic deflections is capable in these wires because of their flexibility & spring back effect.Because of their low stiffness these wires produce low forces. Also for a given amount activation they produce more constant force than that produced by SS wires.Andreasen & morrow In AJO 1980 Nov indicate that these wires are associated with fewer arch wire changes , less chair side time , reduction in time required to accomplish rotations & leveling & less patient discomfort.

DISADVANTAGES:

1. Poor formability allows the wire best suitable only for pre adjustedsystems & so cannot be used in Begg or similar techniques which requireformation of loops & coils etc.2. Any first , second or third order bends have to be over prescribed toobtain the desired permanent bend.3.The low stiffness provides inadequate stability at the completion oftreatment4.Some patient are sensitive to Nickel. 5. NiTi also fractures readiliy when bent over a sharp edge. In additionbending also adversely affects the spring back property, & so loops &stops or not recommended in these wire.

Since hooks cannot be bent or attached to NiTi ,CRIMPABLE HOOKS& stops are recommended.

CINCH BACKS distal to molar buccal tubes can be obtained by resistanceor by flame annealing the end of the wire. This makes the wire dead soft& it can be bent into any preferred configuration. A dark blue colorindicates the attainment of desired annaeling temp.

Care should be taken not to over heat the wire because this makes itbrittle.

Chinese NiTi wire (AJO- 1985 Jun Burstone)

Developed by Dr. Tien Hua Cheng and associates at the GeneralResearch Institute for Non-Ferrous Metals in Beijing, China.

This alloy has unique characteristics and offers significant potential in thedesign of orthodontic appliances. Its history of little work hardening anda parent phase which is austenite yield mechanical properties that differsignificantly from nitinol wire.

In addition, Chinese NiTi wire has a much lower transition temperaturethan nitinol wire.

Physical Properties(Burstone et al AJO 1985 June )

1.SpringbackChinese NiTi wire has 1.4 times the springback of nitinol wire and4.6 times the springback of stainless steel wire.

2.Stiffness With steel & Nitinol the avg unloading stiffness is same regardlessof amount of activation. But not for Chinese NiTi.At 80activation the average stiffness is 73% that of SS& 36% that of Nitinol.

3.Temperature-dependent effectsChinese NiTi wire, exhibits some small differences at varyingtemperatures because material components have lower transition temperatures. The stiffness is approximately the same betweenroom temperature at 22C and mouth temperature at 37C.

At a temperature of 60C, the loading curve is slightly higher andthe unloading curve loses its S shape and exhibits greater permanentdeformation and less springback.

Since the wire is normally used between room temperature andmouth temperature, these temperature-dependent effects areclinically insignificant.

4.Time-dependent effects

Stainless steel wires are resistant to additional permanent deformationthat occurs with time. Some stress relaxation may occur, but the effectsare not significant.

When 0.016-inch stainless steel, nitinol, and Chinese NiTi wires wereengaged in brackets placed interproximally 3 mm apart with a 6.5 mmocclusogingival discrepancy between the center bracket and the adjacentones the wires remained tied in for periods of 1 minute, 1 hour, and 72hours.

It showed that, over 1 minute, the Chinese NiTi wire deformed a limitedamount,compared to the nitinol and stainless steel wires which deformedconsiderably.

Furthermore, the nitinol wire continued to show a timedependent deformation past the initial 5 minutes.

Although NiTi wires show some time-dependent effects,these are insignificant at room temperature.

Clinical significance and discussion

Because of its high range of action or springback, ChineseNiTi wire is applicable in situations where large deflections arerequired.

Applications include straight-wire procedures when teeth arebadly malaligned and in appliances designed to deliverconstant forces during major stages of tooth movement.

Japanese NiTi alloy wire In 1986, Miura F et al reported on Japenese NiTi.

In 1978, Furukawa Electric Co., Ltd. of Japan produced JapaneseNiTi alloy, possessing all three properties (excellent springback,shape memory, and super-elasticity).

Japanese NiTi exhibited an unusual property termed "superelasticity," which no other orthodontic wire has shown.

The wire delivered a constant force over an extended portion of thedeactivation range. Among all the wires compared, Japanese NiTialloy wire was the least likely to undergo permanent deformationduring activation.

The new alloy exhibited a specific stress-strain curve unlike thoseof the other tested materials. Stress remained nearly constantdespite the strain change within a specific range. This unique featureis the manifestation of so-called super-elasticity.

Commercial NiTi alloy wire, does not possess super-elasticity, eventhough it belongs to the same category as NiTi alloy wire. In Nitinolthe stress was increased in proportion to the strain increase, whichis similar in pattern to the stainless steel and Co-Cr-Ni wires.

Super-elasticity is especially desirable because it delivers a relativelyconstant force for a long period of time, which is considered aphysiologically desirable force for tooth movement.

COPPER NI TI WIRES

In 1994 Copper NiTi wires were introduced by Ormcocorporation.The addition copper to nickel titanium enhances thethermal reactive properties of the wires thereby enabling theclinician to provide optimal forces for consistent tooth movement.

It is available in three temperatures variants: 27C , 35 C &40 C corresponding to the austenite finish temperature (Af ) forthe completion of martensite to austenite transformation. Shape memory behavior is reported by the manufacturer to occurfor each variant to occur to each variant at temperatures exceedingthe specified temperature.

COMPOSITION:All three Copper NiTi wires on elamental analysis indicate that theyhave very similar compositions. 1. Nickel 44% 2. Titanium 51% 3. Copper - < 5% 4. Chromium 0.2% to 0.3%

Kusy has reported that Copper NiTi contains nominally 5to6 Wt % cu & 0.2 to 0.5 wt% Cr.

The 27C variant contains 0.5% Cr to compensate for the effectof copper in raising the Af temp above that of the oralenvironment.

The 35C & 40C variant contains 0.2% Cr.

Addition of copper:

It not only modifies the shape memory , but also increases the stability of transformation . It also helps to control hysteresis width & improves corrosion resistance.The superelastic wire contains Cu of 5 to 6 wt % to increase strength & to reduce energy loss.

Unfortunately these benefits are associated with an increase in phase transformation temp above that of the ambient value in mouth.Thus necessitates addition of 0.55 Cr.

27C Copper NiTi :

1. Differential Scanning Calorimetry ( DSE ) demonstrates thatthis wire contains a single peak both on heating & cooling.This indicates a direct transformation from martensite to austeniteon heating & reverse transformation on cooling without anintermediate R phase.

2. It generates forces in the high range of physiological force limits& produces constant unloading forces that can result in rapid toothmovement.

Engagement force is lower than that of other superelastic wiresbecause of the lower loading forces built into the Cu alloy.At the same time unloading force levels are comparable.

3. This varient would be useful in mouth breathers.

35 C Copper NiTi :

1. DSC on this alloy shows two overlapping peaks on heating ,corresponding to transformation from martensite to R phase followed by transformation from R phase to austenite.

2. They generate mid- range constant force levels when the wire reaches oral temp,. Early engagement is easier with full size arch wire due to lower loading forces.Unloading forces are higher & more sustained than other shape memory wires when it reaches body temp,.

3. This variant is activated at normal body temp.

40C Copper NiTi :

1. DSC is similar to 35 C Copper NiTi with two peaks. 2. It provides intermittent forces that are activated when oral temp exceeds 40C. It is useful as an initial arch wire & can be used to engage severely malaligned teeth like highly placed canine without creating damage or painful levels of force or unwanted side effects. It is also the wire of choice for patients scheduled for long intervals between visits when control of tooth movement is a concern.

3.This variant would be activated only after consuming hot food or beverages.

ADVANTAGES OVER TRADITIONAL NI-TIALLOYS

1. Copper NiTi is more resistant to permanentdeformation & exhibits better springback.

2. It demonstrates a smaller loading force for the same degree of deformation, making it possible to engage the wire to severely malposed tooth with less patient discomfort & potential for root resorption.

3. The decreased hysteresis & flatter unloading curve result in more consistent forces that are active longer within the optimal range for tooth movement.

4.It exhibits a more constant force/ deformation relationship, providing superior consistency from archwire to archwire.

5. As copper is an efficient conductor of heat, these wire demonstrates consistent transformation temperatures that ensure consistency of force. This equates to effectiveness in moving teeth

Beta titanium Developed by Dr.Burstone in 1980.

One of Dr. Burstones primary objectives was to producean alloy whose deactivation characteristics were aboutone-third that of stainless steel or twice that of aconventional martensitic stabilized nitinol. This led toOrmco Corporations introduction of the low-stiffnessbeta-phase titanium-molybdenum alloy known as TMA.

COMPOSITION : Titanium 77.8 % Molybdenum 11.3% Zirconium 6.6% Tin - 4.3

At temperatures above 1,625 F pure titanium rearranges into abody- centered cubic ( BCC) lattice, referred as beta phase.

With addition of elements like molybdenum or columbium, the Ti based alloy can maintain its beta structure even when cooled toroom temperature. So it is referred as Beta- stabilized titaniums.

The alloying and body-centered cubic structure impart a unique setof properties.

The BCC structure provides excellent formability to titanium wires.The many slip systems ( 12) available forthe dislocation movement in BCC crystal structureaccounts for the high ductility.

The addition of zirconium & tin contributes to increasedstrength & hardness & their presence avoids the formationof an embrittling phase during wire processing at hightemperatures.

ADVANTAGES:

For orthodontic use, Advantages of this alloy are several

1. when compared with stainless steels, TMA produced gentler linear forces per unit of deactivation and had substantially more range and higher springback.

2. Excellent formability The high formability of titanium allows the fabrication of closing loops with or without helices. The low stiffness of the material and its high springback improve a loop of any given design or allow for the maintenance of a given force system with simpler designs, as in the elimination of helices or loops.

It is the only wire with true weldability

It allows direct welding of auxiliaries to an arch wirewithout reinforcement by soldering. Using a lightcapacitance weld, a smaller cross section of titanium canbe welded directly to the main arch-on-arch segment .Finger springs and other auxiliaries of an active nature canalso be welded directly to an arch wire. The welding hasnot appreciably altered the mechanical properties of thespring, and it can be activated a full 90 degrees withoutany permanent deformation..

Welding should be performed with care. Unlike steel,where too much heat will produce softness in the wire,overheating of titanium could lead to brittleness of anenergy-imparting finger spring.

The flat-to-flat electrode configuration was found to bepreferable for welding the beta titanium alloy, anddistortion can be minimized by selection of optimumsettings- Donover ,Lin, Brantley & Conover AJO 1984- March

It appears that the welding of round wire to rectangularwires yields joints with best relative ductility.

4. Absence of nickel makes the wire more biocompatible & can be used in nickel sensitive patients.

5. Has excellent corrosion resistance & biocompatibilitywhich is due to the presence of a thin , adherent passivatingsurface layer of titanium oxide.

Indeed, TMA was almost the perfect wire, since itscharacteristics were so balanced.

DISADVANTAGES

1. The coefficients of friction were the worst of any of the orthodontic alloys, and consequently its ability to accommodate the sliding of teeth was limited .This causes highest friction.

2. High cost.

ION IMPLANTATION:

Recently , the ion implantation process has been applied toorthodontic wires. This process alters the surface composition ofthe wire. It has been proposed that this process decreases thefrictional forces produced during tooth movement.

Implantation of nitrogen ions into the surface of this wire causessurface hardening & can decrease frictional force by as much as70% This process tends to increase stress fatique , hardness, & wearregardless of the composition of the material.

Laboratory data suggest that ion implantation of nitrogen into TMAwire will reduce both static & kinetic coefficients of frictionsignificantly. however these reductions are significant only whenboth the wire & opposing bracket surface are implanted.

Ion implantation takes place in vacuum & involves the implantation ofoxygen & nitrogen ions.These ions penetrate the wire surface byreacting with tin in TMA to change the surface & immediate sub-surface of the material.This layer is very hard & creates considerablecompressive forces.These forces improve the fatique resistance &ductility while reducing the co efficient of friction roughly to that of SS.

Katherine Kula & Proffit AJO 1998 studied the effect of ionimplantation of TMA wires on rate of space closure. They concludedthat there was no significant difference between ion implanted &Unimplanted TMA wires in sliding mechanics clinically.

Although the mechanical properties of Elgiloy and stainless steel aresimilar, the orthodontist can give the former a strengthening heattreatment which allows manipulation of the wire in a softened state.This can be followed by a hardening heat treatment to obtain thedesired resilience.

ALPHA TITANIUM WIRES

Developed by Mr. A.J.Wilcock in 1988.They are pure Titanium in -phase.

Composition Titanium 90% Aluminium 6% ( formers ) Vanadium _ 4 %

The alloy is different in that its molecular structure resembles aclosely packed Hexagonal Lattice as against the BCC lattice of TMA.Because of this hexagonal lattice it possesses fewer slip planes. Themore slip planes, the easier it is to deform the material( Kusy et alAJO 2004 Nov )

Also - Titanium has Modulus of elasticity of 144 GPa wheras - Titanium has about 72.4 GPa .So it deliversmore force & has less working range. But Titaniumdelivers only 60% force / unit of deactivation & has 1.6times more working range when compared with Titanium .

HEAT ACTIVATED WIRES

A Martensitic heat activated titanium wires exhibitexcellent shape memory & superelastic properties. It transforms to its Austenite form at 35C , delivering avery gentle continuous force.Because it is soft & pliable atroom temperature , it can be easily engaged to even themost severely malaligned teeth. Larger round or evenrectangular arch wires can be utilized much earlier intreatment with little patient discomfort.

Nitinol heat activated wire is a thermally activated super elastic archwire.It is the easiest of nitinol wires to engage& it delivers light continuous forces that effectively movetooth with minimal discomfort to the patient.

ADVANTAGES: It can be cooled or chilled resulting in a softer , more pliable wire for easy engagement.Provides light continuous forceForce activation at 27CAvailable in square sizes making it excellent for early torque control

COMPARISION OF IMPORTANT PROPERTIES

CLINICAL APPLICATIONS

ALIGNMENTPrinciples in the choice of alignment arch wires:Initial arch wires should provide light continuous force of approximately 50 grams, to produce the most efficient tipping tooth movement.The arch wire should be able to move freely within the brackets. So cross section of the wire should be small & should be loosely tied to the bracket to minimize friction.

3.Rectangular arch wires particularly those with tight fitwithin the bracket should be avoided.

4.Springier the arch wire crowding of symmetric naturecan be corrected without the danger of loosing the archform. If only one tooth is crowded out of line, a rigid archwire is needed to maintain the arch & auxiliary wireshould be used to reach the malalighned tooth.

So initial archwire should have Excellent strength. Good springiness Long range of action Small cross section

Super elastic A NiTi with the cross section of 14 mil or16 mil is ideal for this category.

If steel is used in this stage either multistranded wires orloops should be used to increase springiness.

Beta titanium is rarely used.

Alignment with Begg technique

The narrow brackets used in Begg tech, provide themaximum possible interbracket distance . Also the initialarch wires bypass the premolars. This long posterior spanof wire makes it difficult to use the highly flexible Tibased wires. So SS wires with loops are used. Latest combinationtechnique uses combined wires with flexible anteriorsegment & stiff posterior segment like Dual- Flex.

For levelling & space closure :

It depends mainly on biomechanics involving eitherintrusion of anteriors or extrusion of posteriors. For spaceclosure the wire should have least friction. Usually roundSS wires with a progressing increase in cross-section isused. This method takes advantage of increasing wire size to increase the stiffness & to get constant force delivery .This method is termed as REPLACEMENT APPROACH or VARIABLE CROSS SECTION ORTHODONTICS.

There is another method proposed by Charles .J.Burstone(AJO 1981 July) as early as 1980s called VARIABLE MODULUS ORTHODONTICS.

The author states that advances in orthodontic wire alloyshave made it possible to control wire stiffness by varyingmaterial properties namely the Modulus of elasticity,hence the name. Burstone formulates his concepts bystating that the over-all stiffness of the appliance (S) isdetermined by two factors; one factor relates to the wireitself, (Ws) and the other is the design of the appliance(As)

In general terms,

Appliance stiffness = Wire stiffness * Design stiffness

Design stiffness is dependent on factors like interbracket distancebrought by incorporating loops & coils.

Ws - Wire stiffness is determined by a cross-sectional property orby the material stiffness dependent on materials property such asthe modulus of elasticity.

Therefore an increase in appliance stiffness can brought about notonly by change in appliance design or increase in cross sectionthickness of the wire but also by selecting material with highermodulus of elasticity while maintaining the same cross section.

ADVANTAGES:1.The amount of play bt bracket & wire is not dictated by desiredwire stiffness but is under the total control of the clinician. once thecross sectional size & shape have been established, the desiredstiffness can be implemented by selecting the alloy with appropriatematerial stiffness.2.The low Moduli of elasticity of the newer alloys the use of light ,rectangular wire even during the early stages of treatment.Rectangular wires are preferable over round wires because they canbe better oriented in the bracket in such a way that forces work outin proper directions.They further increase patient comfort by avoiding loops.3.The selection of an appropriate alloy type & wire size may reducethe no of archwires needed for alignment by reducing bracket /wire play early in treatment.

Accuracy of 3rd order bends of NiTi & the effects of high and low pressure during memorizing heat treatment Thomas stamm-ajo-2004This study evaluated the Accuracy of 3rd order bends of NiTiwires and determined the effects of high and low pressure formaintaining the wire shape during memorizing heat treatment.

A computer aided bending machine was used to incorporate200 randomly determined torsional angles between 0 &60into 30linear 0.016* 0.022 in Neosentalloy wires.

Results showed that 3rd order bends

With bends > 30 but < 40, the method with higher pressureoffers greater precision than that with the lower pressure.

With torque bends > 40, the bending error with both methods isclinically unacceptable.

In general force applied to keep the wire in its bent shape duringthe memorizing heat treatment has an effect on the accuracy of 3rdorder bends. Heat treatment methods that cannot keep the wire inits 3D shape should not be used.

Characterization and cytotoxicity of ions released from SS & NiTi alloys- Theodore Eliades- Ajo 2004The purpose of this study was to qualitatively & quantitativelycharacterize the substances released from orthodontic brackets &NiTi wires and to comparatively assess the cytotoxicity of the ionsreleased from these orthodontic alloys.

Two full sets of SS brackets of 20 brackets each and 2 groups of 0.018*0.025 NiTi archwires of 10 wires each were immersed in 0.9% salinesolution for a month.

Human periodontal ligament fibroblasts & gingival fibroblasts wereexposed to various concentrations of the two immersion media, Nickelchloride was used as a positive control for comparasion purpose.

The results indicated that no ionic release for the NiTi agingsolution, where as measurable nickel & traces of chromium werefound in the SS bracket aging medium.

Concentration of the nickel chloride solution greater than 2mmwere found to reduce by more than 50% the viability & DNAsynthesis of fibroblasts. However neither orthodontic materialsderived media had any effect on the survival & DNA synthesis ofeither cell.

NEWER ARCH WIRES TIMOLIUM WIRES

A new entry into the area of titanium based alloys .It is also termed alloy.

Composition: Titanium 89.9% Aluminium 6.1 % Vanadium 3.2 % Molybdenum 0.3 % Zirconium 0.3 % Tin 0.4%

It is shown that this alloy has a smooth surface texture, less friction atarchwire - bracket interface & better strength & superior welding qualitiesthan existing Ti based alloys.

Kusy et al AJO 2004 Nov studied the surface roughness of 6 Tibased wires & found that Timolium wires exhibited poor surfacequalities with formation of sheets with visible steps or fissures onpulling the wire against resistance in a linear direction.

Vinod Krishnan et al Angle 2004 April evaluated the weldcharacteristics of 3 archwires namely SS, titanium &Timolium wires .The rank order of wires in descending order oftheir mean values for strength on tensile evaluation of the weldjoint was titanium, SS, Timolium . Thus timolium wiresexhibited very low tensile shear test values of the three wirestested.

On surface evaluation timolium has better surface properties . Theweld surface of timolium exhibited a smooth & symmetrical flow ofthe alloy , less surface distortions & an intact weld surface.

SUPERCABLE Hanson combined the mechanical advantage of multistrandedcables with properties of superelastic wires to create a superelasticNiTi Coaxial wire called Supercable.

It contains 7 individual strand woven together to maximizeflexibility and minimize force delivery.

ADVANTAGES: Eliminates archwire bending. Effective to control rotations, tipping & in levelling. Can use in severe crowding Minimal patient discomfort & fewer visits.

TITANIUM NIOBIUM WIRES This alloy is an innovative archwire designed for precision- tooth finishing . The unique metallargicalproperties of this wire make it the most precise intraoraldetailing wire.

This alloy has low spring back ( = SS ) & is much lessstiffer than TMA. At 80% of the stiffness of TMA , it isperfect for holding bends. Though it is soft & pliable itpossess a resiliency after bending equal to SS.

COMBINED WIRES Jose' L. Zuriarrain, et al -AJO 1996 Dec used Combining mechanics The key to success in a multi attachment straight wire system is to have the ability to use light tipping movements in combination with rigid translation and to be able to vary the location of either, at any time the need arises during treatment.

They used three specific combined wires for the technique; Dual Flex-l, Dual Flex-2, and Dual Flex-3 (Lancer Orthodontics).

The Dual Flex-1 consists of a front section made of 0.016-inchround Titanal and a posterior section made of 0.016-inch roundsteel. The flexible front part easily aligns the anterior teeth and therigid posterior part maintains the anchorage and molar control bymeans of the "V" bend, mesial to the molars. It is used at thebeginning of treatment.

The Dual Flex-2 consists of a flexible front segment composed ofan 0.016 0.022-inch rectangular Titanal and a rigid posteriorsegment of round 0.018-inch steel.

The Dual Flex-3, however, consists of a flexible front part of an0.017 0.025-inch Titanal rectangular wire and a posterior part of0.018 square steel wire. The Dual Flex-2 and 3 wires establishanterior anchorage and control molar rotation during the closure ofposterior spaces. They also initiate the anterior torque. All wires have elastic hooks.

COMPOSITE WIRES

The arch wire is one of the main component of multibracketed appliance, is usually made of metal materials like SS , NiTi etc which are esthetically inferior .The no of adult orthodontic patients are steadily increasing & there is a demand for more esthetic appliance. This laid the foundation for inventing esthetic composite wires .

There are two kind of arch wires have been produced to improveesthetics.

1. It is a metal wire with white colored teflon ( poly tetra fluoro ethylene ) or epoxy resin

2. Fiber Reinforced Plastic orthodontic wire: ( FRP ) Its made of translucent composite material like poly methyl metha acrylate (PMMA) as matrix

It is commercially available as OPTIFLEX.

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

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

3. A stain-resistant nylon outer layer that prevents damage to the wire and further increases its strength

The wire can be either round or rectangular and is manufacturedin various sizes. Its mechanical properties include a wide range ofaction and the ability to apply light, continuous force.Sharp bends must be avoided, since they could fracture the core.Otherwise, Optiflex has practically no deformation. It is a highlyresilient archwire that is especially effective in the alignment ofcrowded teethAdvantages:1.Improved esthetics2.Capability to vary the stiffness of the wire without changing the crosssection of the wire allows the clinician to use Variable modulusmethod.3.Allergic reactions to nickel is also avoided with composite wires.

COATED WIRES

An investigation of the frictional properties of composite wiresagainst several orthodontic brackets showed that reinforcementfibers were abrasively worn from the wire surfaces when tests wereconducted at high normal forces or angulations. This potentialrelease of glass fibers within the oral cavity was consideredunacceptable, and a polymeric surface coating was suggested as apotential remedy.

The prerequisites for this coating material was that it should beeasily applicable in thin layers, be wear-resistant, and have lowfrictional characteristics. In addition, the coating material needed tobe biocompatible and transparent. One material that exhibited all ofthese properties was poly chloro-p-xylylene, which has been wellestablished for use in biomedical coating applications, such ascatheters and cardiac pacemakers.

Kusy et al ANGLE 2000 studied Sliding Mechanics of CoatedComposite Wires to determine the effects of poly(chloro-p-xylylene)surface coatings on archwire sliding mechanics, the tri biological(friction and wear) characteristics of coated composite wires wereevaluated.

This finding implies that the risk of glass fiber release duringclinical use would be eliminated by the coating.

conclusionRecent advances in material science and technology has resulted in anarray of newer arch wire materials, opening new vistas in orthodontictreatment. Materials with widely diverging properties are on the markettoday and their usage has profound implications on appliance mechanics.So it is upto the orthodontist to clearly outline the phases of treatmentand select the arch wire most suited for attaining specific goal oftreatment.