ORTHODONTIC WIRES INTRODUCTION Teeth move in response to forces imposed on them i.e. the elasticity of the surrounding tissues, direct action of the remote muscles or external forces like oral habits and dental appliances. The position of each teeth, from day to day and minute to minute is determined entirely by the combined effects of environmental forces. Orthodontic tooth movements are brought about by appliances consisting of orthodontic wires. In the 18th and early 19th century gold wires were used. The constant search for newer wires with better properties has lead to the development of wires made of different materials which are superior to the gold arch wires. TERMINOLOGY The commonly used terms are as follows: Mechanics: As an area of study with in the physical sciences, is concerned with the state of rest or motion of bodies, subjected to forces. Force: Force is defined as an act upon a body that changes or tends to change the state of rest or the motion of that body. Stress: Displacing force measured across a given area, it is known as stress (force per unit area, pound/sq. inch) Strain: The change in dimension is called a strain change in length / unit length. Strain could be 1) Plastic 2) Elastic Tensile stress: A tensile stress is caused by a load that tends to stretch or elongate a body. It is always accompanied by a tensile strain. Compressive stress: If a body is placed under a load that tends to compress or shorten it. the internal resistance to such a load in called compressive stress. It is accompanied by a compressive strain. Shear stress: A stress that tends to resist a twisting motion or a sliding of one portion of a body over another, is called as shear or shearing stress. The elastic limit of a material is the greatest stress to which a material can be subjected, such that is will return to its original dimensions when the forces are released. Proportional limit: If the wire is loaded in tension in small increments until the wire ruptures, with out removal of the load each time and if each stress is plotted on a vertical coordinate and the corresponding strain in plotted on the horizontal coordinate a curve is obtained. Hook’s law: The stress in directly proportional to the strain in elastic deformation. Yield strength: The yield strength is the stress required to produce the particular offset chosen (plastic strain). Modulus of elasticity: If any stress value is equal to or less than the proportional limit is divided by its corresponding strain value, a constant of proportionality will result, this constant of proportionality is known as the modulus of elasticity or Young’s modulus. The maximal flexibility is defined as the strain that occurs when the material is stressed to its proportional limit.
Resilience: Can be defined as the amount of Energy absorbed by a structure when it is stressed not to exceed its proportional limits.
Permanent deformation: The stress strain curve is no longer a straight line above P (proportional limit) but rather curves until the structure fractures. The stress is no longer proportional to strain. If the load is removed at any point prior to fracture. The wire remains bent, stretched or otherwise deformed. Strength is the maximal stress required to fracture a structure. It is called tensile strength, compressive strength or shear strength. Fatigue: Cyclic loading at stress values well below those determined in ultimate strength measurement can produce about failure of a structure. This type of failure is called fatigue. Toughness:Toughness is the property of being difficult to break. It can be defined as the energy required to fracture a material. Brittleness:A brittle material is apt to fracture at or near its proportional limit. Ductility:The ability of a material to withstand permanent deformation under a tensile load without rupture. Malleability: The ability of a material to withstand permanent deformation without rupture under compression, as in hammering or rolling into sheet, is termed malleability. Hardness:Surface hardness is the result of the interaction of numerous properties. Among the properties that influence the hardness of a material is its strength, proportional limit, ductility, malleability and resistance to abrasion and cutting because numerous factors influence hardness, hence the term is difficult to define. Friction: The materials touching one another share a contact area. The resistance to movement tangent to this area, of one material to the other is known as friction. CLASSIFICATION OF ARCHWIRE According to material used According to cross-section 1. Gold archwires 1. Round 2. Stainless steel archwires 2. Rectangular 3. Chrom-cobalt archwires 3. Rounded rectangular 4. Nickel-Titanium archwires 4. Square a. Martensitic 5. Braided b. Austenitic 6. Stranded c. Superelastic d. Japans NiTi e. Chinese NiTi f. Beta titanium – TMA g. Alpha NiTi h. Reverse curve NiTi 5. Copper NiTi 6. Ceramic coated/optiflex archwires
MECHANICAL PROPERTIES AND CLINICAL APPLICATION OF WIRES Optimum orthodontic tooth movement is produced by light, continuous force. It is particularly important that the light forces do not decrease rapidly decaying away either because the material itself loses its elasticity or because a small amount of tooth movement causes a larger change in the amount of force delivered. The basic properties of orthodontic materials • The elastic behavior of any material is defined in terms of its stress strain response to an external load. • Stress is the internal distribution of the load defined as force per unit area, whereas strain is the internal distortion produced by the load, defined as deflection per unit length. • Orthodontic archwires and springs can be considered as beam supported either only on one end or on both ends. • If a force is applied to such a beam its responses can be measured as the deflection produced by force. For orthodontic purpose three major properties of beam material are critical in defining their clinical usefulness i.e. strength, stiffness and range. Strength, Stiffness and Range • Stiffness and springiness are reciprocal properties springiness = 1/ stiffness • Each is proportional to the slope of the elastic portion of the force-deflection curve. The more vertical the slope the stiffer the wire. • Range is defined as the distance that the wire will bend elastically before permanent deformation occurs. If the wire is deflected beyond its yield strength, it will not return to its original shape but clinically useful spring back will occur unless the failure point is reached. These three major properties have an important relationship. Strength = Stiffness x Range Resilience • Represents the energy storage capacity of the wire, which is a combination of strength and springiness. • Formability is the amount of permanent deformation that a wire can withstand before failing. An ideal wire material for orthodontic purposes Should posses 1) High strength 2) Low stiffness 3) High range 4) High formability. The material should be weldable or solderable, so that hooks or stops can be attached to the wire. It should also be reasonable in cost.
WIRE CHARACTERISTICS OF CLINICAL RELEVANCE Several characteristics of orthodontic wires are considered desirable for optimum performance during treatment. These include a large springback, low stiffness high formability, high stored energy, biocompatibility and capability to be welded or soldered to auxiliaries and attachments. 1) Springback: This is also referred to as maximum elastic deflection, maximum flexibility, range, Springback is related to the ratio of yield strength to the modulus of elasticity of the material. Higher springback values provide the ability to apply large activations with a resultant increase in working time of the appliance. Springback is also a measure of how far a wire can be deflected without causing permanent deformation or exceeding the limits of material. 2) Stiffness or load deflection rate: This is the force magnitude delivered by an appliance and is proportional to the modulus of elasticity. Low stiffness or load deflection rate provides: • The ability to apply lower forces • A more constant force over time as the appliance experiences deactivation. • Greater ease and accuracy in applying a given force. 3) Formability: High formability provides the ability to bend a wire into desired configurations such as loops, coils and stops without fracturing the wire. 4) Modulus of resilience of stored energy: This property represents the work available to move teeth. It is reflected by the area under the line describing elastic deformation of the wire. 5) Biocompatibility and environmental stability: Biocompatibility includes resistance to corrosion and tissue tolerance to elements in the wire. Environmental stability ensures the maintenance of desirable properties of the wire for extended period of time after manufacture. 6) Jointability: The ability to attach auxiliaries to orthodontic wires by welding or soldering provides an additional advantage when incorporating modification to the appliance. 7) Friction: Space closure and canine retraction in continuous arch wire technique involves a relative motion of bracket over wire. Excessive amount of bracket/wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement. The preferred wire material for moving a tooth relative to the wire would be one that produces the least amount of friction at the bracket wire interface.
IDEAL ORTHODONTIC ALLOY The ideal orthodontic wire for an active member is one that gives a high maximal elastic load and low deflection rate. The mechanical properties that determines these characteristics are elastic limit and modulus of elasticity. The ratio between the elastic limit and modulus of elasticity determines the desirability of the alloy. The higher the ratio the better will be the spring properties of the wire. For an alloy to be markedly superior in spring properties. It must posses a significantly higher ratio. In the reactive member of an appliance a sufficiently high elastic limit is required and a high modulus of elasticity is also desirable. Four other properties of wire should be mentioned in evaluating an orthodontic wire. 1) The alloy must have a reasonable resistance to corrosion caused by fluids of the mouth. 2) It should have sufficient ductility so that it will not fracture under accidental loading in the mouth or during fabrication of an appliance. 3) It is desirable to have a wire that can be fabricated in a soft state and later heat treated to a hard temper. 4) A desirable alloy is one to which attachments can easily be soldered. WIRE CROSS SECTION The most critical factor in the design of an orthodontic appliance is the cross section of the wire to be used. Small changes in cross section can dramatically influence both the maximal elastic load and load deflection rate. The maximal elastic load varies directly as the third power of the diameter of round wire, and the load deflection rate varies directly as the fourth power of the diameter. Hence for an active member the wire should be of a smaller diameter but with a safety factor so that permanent deformation doesn’t occur. This holds vice versa for rigid reactive members of the appliance. Flexible member: 1) Multidirectional activation: The optimal cross section for a flexible member for multidirectional activation in which the structural axis is bent in more than one plane, is a circular cross section (round wires).. Disadvantage of round wires is if it is not properly oriented activation may not operate in the bracket and if certain loops are incorporated into the configuration, these can then roll into either the gingiva or the cheek. 2) Unidirectional activation Flat wire is the cross section of choice. Advantages of flat wires: Low deflection rate without permanent distortion because more energy can be absorbed into spring made of flat wire than of any other cross section. Flat wire can be anchored into a tube or a bracket so that it will not spin during deactivation of a given spring. Greater tooth movement is achieved Reactive member: square or rectangular wire is preferred because of ease of orientation and multidirectional rigidity. Wire length • The length of a member may influence the maximum elastic load and the load deflection in a number of ways depending on the configuration and loading of the spring.
Effects of length and attachment: • If the length of the beam is doubled. Cantilever type: • Strength: bending strength reduces to by ½ • Springiness: increases 8 times • Range: increases 4 times Supported beam: If a beam is rigidly attached, it is twice as strong and I/4 springy and ½ the range. Increasing the length of the cantilever reduces the load deflection rate, yet maximum elastic load is not radically altered. Since load deflection rate varies linearly with the length. Adding length with in the practical confines of the oral cavity is an excellent way to improving spring properties. The length of the wire can be increased by the addition of loops or helices in the wire. Direction of loading: If a straight piece of wire is bent so that permanent deformation occurs and an attempt is made to increase the magnitude of the bend, bending in the same direction as had originally been done, the wire is more resistant to permanent deformation than if an attempt had been made to bend in the opposite direction. When a wire is bend so that it permanently deforms and an increase in the bend is desirable it should be done in the original direction of bending and twisting (has the greatest elastic load). This is term as the Bauschinger effect. Hence the operator should be sure of the last bend made in the archwire is in the same direction as the bending produced during its activation. Prevention of fatigue failure: Minute scratches or surface flaws leads to excess load in the metal around the defect leading to failure. During arch designing: wire must not be marked or notched with a file or other sharp instruments. Smooth beaked pliers to be used so that marking of the wires are avoided. Sharp bends to be avoided: causes increase in the work hardening leads to failure. Repeated bending at the same spot is to be avoided. Adjustments to be made away from high stress areas and bends at soldered joints to be avoided. Work hardening or strain hardening: In polycrystalline metal, dislocations tend to build up at the grain boundaries. The barrier action to slip at the grain boundaries causes the slip to occur on the intersecting slip planes leading to point defects and the grain is distorted. Greater stress is required to produce further slip and the metal becomes stronger and harder, This is known as work hardening or strain hardening. Work softening or reverse straining or bauschinger effect: Wires straightened by the process of reverse straining which is the flexing in a direction opposite to that of the original bend the yield point decreases. This is known as work softening or reverse straining. Pulse straightening of the wire is carried out which permits even the wires with the highest tensile strength to be straightened without loss of yield strength. Pulse straightened wires are costly. GOLD WIRES • Precious metal alloy was used before 1950’s due to it stability in intraoral conditions. • In modern dentistry wrought gold alloy is used. Gold in its pure state is very soft, malleable and ductile. Composition: • ADA specification no.7 for gold wires. Type I: High precious metal alloy containing atleast 75% of Au and Pt. Type II: low precious metal alloys containing atleast 65% of Au and Pt. • Basic composition: Au, Pt, Pd, Ag, Cu, Ni and Zn. • Gold alloys work harden much more slowly and to a lesser degree than steel. • Hence lesser intermediate anneals are required to the orthodontist. Nowadays usage is reduced due to alloy being soft for orthodontic appliances, high cost and low yield strength. Chrom cobalt nickel alloy - archwires Cobalt-chromium-nickel alloys drawn into wire were first marketed for use in orthodontic appliances during the 1950s and these were originally developed for use as watch springs (Elgiloy). They are available commercially as Elgiloy, Rocky mountain orthodontics, Azura and multiphase. Composition: Cobalt : 40% Manganese : 2% Chromium : 20% Carbon : 0.5 % Nickel : 15% Beryllium : 0.4% Molybdenum : 7% Iron : 1.5% They are heat treated before being supplied to the user and are available in several degrees of hardness: soft, ductile, semi resilient and resilient. Heat treatment: increases yield strength and decreases ductility. The wires are colour-coded for the clinicians convenience as follows: Soft – Blue Ductile – Yellow Semiresilient – Green Resilient – Red The most widely used is the soft temper (Elgiloy Blue) which is easily manipulated and is then heat treated to achieve increased resiliency. Clinicians can easily perform heat treatment using an electrical resistance welding apparatus and a special paste provided by the manufacturer to indicate the optimum period of time. Alternatively, furnace heat treatment at approximately 4800C for 7 to 12 min can be employed. Physical properties: 1) Tarnish and corrosion resistance is excellent. 2) Hardness, yield strength and tensile strength is comparable to 18-8 stainless steel. 3) Ductility : Greater in soft compared to 18- 8. Lesser in hardened condition. Mechanical properties: 1) Greater resistance to fatigue and distortion 2) Non heat treated co-cr wire have smaller spring back than stainless steel wires of comparable size. 3) The mechanical properties of co-cr wires are very similar to those of stainless steel wires. 4) The high modulus of elasticity of co-cr: deliver twice the force of Beta Titanium wires and four times the force of Nitinol wires for equal amount of activation. 5) Therefore, this would lead to faster rate of mesial movement of posterior teeth: therefore increase on intra and extraoral anchorage. 6) Co-cr wires have good formability and can be bent into many configurations relatively easily. 7) Care to taken during soldering attachments since high temperature cause annealing with loss in yield and tensile strength (Low fusing solder is recommended). 8) Resistance to tooth movement in relation to frictional forces between brackets and co-cr wires is comparable to stainless steel wires.
RECENT ADVANCES: 1) G & H Wire company: Nickel cobalt wires can be heat treated in bent areas and easily soldered without annealing. They have good ductility and strength, highly flexible and resistant to fatigue and corrosion. The wire also offers reduced bracket friction and greater spring efficiency than typical stainless steel wires. 2) Masal orthodontics, inc. Heat-treatable blue Masiloy chrome cobalt Arches can accept sharp, intricate bends with out breaking. Heat treatment increases the resiliency by 20 percent. STAINLESS STEEL WIRES Steels are iron based alloys that contain less than 1.2 percent carbon. When approximate 12 percent to 30 percent chromium is added to iron the alloy is commonly called stainless steel. Pure iron at room temperature has a body centered cubic (BCC) structure called as ferrite. This phase is stable upto 9120C (16740F). At temperatures between 9120C and 13940C the stable form of iron is a face centered cubic, structure called austenite. If the austenitic alloy is cooled very rapidly it will undergo a spontaneous, diffusionless transformation to a body centered tetra gonal (BCT) structure called ‘Martensite’. This lattice is highly distorted and strained, resulting in a very hard, strong, brittle alloy. Austentic alloy: On slow cooling from high temperature, the excess carbon forms iron carbide. This hard brittle phase adds strength to the relatively soft and ductile ferrite and austenitic forms of iron. Therefore, there are three types of stainless steels, on the basis of lattice arrangement of iron: 1) Ferritic stainless steel: a) Provides good corrosion resistance b) Low cost c) Has less strength d) Not hardenable by heat treatment e) Not readily work hardenable Hence, finds little application in dentistry. 2) Martensitic stainless steel: a) High strength b) Can be heat treated c) Decrease corrosion resistance d) Decrease ductility Used for surgical and cutting instruments. 3) Austenitic stainless steel: a) Most corrosion resistant of the stainless steel. b) Composition: Chromium – 18% Nickel – 8% Carbon – 0.15% AISI 302 is the basic type. Type 304 has similar composition the chief difference is in the carbon content which is limited to 0.08%. This is the most commonly used type. Austenitic stainless steel is preferable to the ferritic alloys because of: 1) Greater ductility and ability to undergo more cold work without breaking. 2) Substantial strengthening during cold working. 3) Greater ease of welding 4) The ability to fairly readily overcome sensitization 5) Comparative ease in forming. Disadvantages: Annealing temperatures: In the soldering and welding temperature ranges hence, low fusing silver solders are recommended for soldering purpose. Clinical implication: • Cold working and carbon interstitial hardening leads to high yield strength, high modulus of elasticity and hardness. • Residual stresses present in the wire due to bending need to be eliminated hence heat treatment essential for stress relief. After bending the wire into arch, loops or coils heat treatment is carried out. • Funk has suggested that straw coloured wire indicates that optimum heat treatment has been achieved. • High stiffness of stainless steel wires leads to usage of smaller diameter of wire. • The yield strength to elastic modulus ratio indicates decreased spring back of stainless steel compared to newer titanium alloys. • Therefore, stainless steel wires produce higher forces that dissipate over shorter periods than nitinol wires, thus requiring more frequent activation or arch wire changes. • Lower levels of friction between bracket- wire: offers lower resistance to tooth movement than other orthodontic alloys. Intergranular corrosion of stainless steel Carbon is an undesirable impurity in austenitic stainless steel, but it is difficult to remove it completely. Carbon does not enter into the physical structure of these steels except in the formation of a small amount of martensite in cold-working, but at temperatures between 8000 and 12000 F it starts a chain of corrosion which can result in actual physical failure of the metal. At these temperatures carbon reacts with chromium to form chromium carbide. This reaction is called sensitization. A temperature above the 12000 F range chromium carbide breaks up into the component elements. Below sensitizing temperatures, the reaction between carbon and chromium cannot take place, thus at either higher or lower temperature, the reaction between carbon and chromium cannot take place, the metal is safe from sensitization. Chromium carbide is harmless in itself but chromium tied up as the carbide cannot contribute to the corrosion resistance in the metal. Prevention of intergranular corrosion This is the most important problem in manipulating stainless steel at high temperature. This can be prevented by: 1) Keeping out of the sensitizing temperature range (8000 to 12000 F) 2) Controlling the carbon content.
Controlling temperature to prevent intergranular corrosion: Speed in handling the metals in the sensitizing temperature range, as during soldering can be very effective means of minimizing sensitization. Stainless steel should always be quenched immediately after soldering to bring it down to a safe temperature as rapidly as possible. If the metal is cooled rapidly from annealing to room temperature, there is no opportunity for chromium carbide to form. With the usage of a low temperature silver solder the objective is to heat to soldering temperature, solder, and then quench as quickly as possible. High temperature solder also can be used, but only if the entire piece of steel can be heated to this high temperature. The metal is then above the sensitizing range while it is being soldered, and thus it is perfectly safe. It is a must that it be quenched immediately after soldering. Stabilization to prevent intergranular corrosion: The second possibility for control of intergranular corrosion is not making carbon available for the sensitizing reaction. This is done at the time that the alloy is manufactured, either by keeping the carbon content exceptionally low or by adding other metals which tie up in other compounds. These compounds include niobium or titanium plus tantalum which form carbide precipitates in preference to chromium. Stainless steel that have been treated in this manner are said to be stabilized. Columbium and titanium are commonly used for this purpose.
Triple stranded stainless steel arch wire: • Beside single standard round, square and rectangular wires there are now multistandard wires of varying size, shape and number are available. • The separate strands may be as small as 0.178 mm. • Significance: o These wires sustain large elastic deflection in bending. o These wires apply low forces for a given deflection compared to solid stainless steel wires. Theory for the general multistranded arch wires: Classic mechanical theory shows that as the diameter of a wire strand is reduced, the stiffness decreased as a function of the fourth power, and the range increased proportionally. To improve the strength and at the same time to maintain the desirable stiffness and range properties many small wires are twisted together and even swagged or spot welded. The result is an inherently high elastic modulus material behaving as a low – stiffness member because of its co-axial spring like nature. These wires deliver higher forces per unit of activation over a greater distance. AUSTRALIAN HEAT TREATED ARCH WIRES One of the outstanding properties or characteristics of the Australian wire is its resilience or ability to spring back after having been deflected. Variations in the types of wires is made by fluctuations in the rate at which the wire passes the heat source. Australian wires are available in the following forms Color code 1) Regular grade White 2) Regular plus grade Green 3) Special grade Black 4) Special plus Orange 5) Extra special plus Blue 6) Supreme Blue Regular grade: Lowest grade and easiest to bend . It is used for practice or forming auxiliaries. It can be used for arch wires when distortion and bite opening is not a problem. Regular plus grade: • Relatively easy to form, yet more resilient than regular grade. • Used for auxiliaries and arch wires when more pressure and resistance to deformation is desired. • Available in sizes 0.014”, 0.016”, 0.018”, 0.020”. Special grade: • Highly resilient yet can be formed into intricate shapes with little danger of breakage. • The 0.016” is often used for starting arches in many techniques. • Available in sizes 0.014”, 0.016” 0.018” and 0.020”. Special plus grade: • Special plus wire is routinely used by experienced operators. • Hardness and resiliency of 0.016” size are excellent for supporting anchorage and reducing deep overbites. • Wire must be bent with care. • Available in sizes 0.014”, 0.016”, 0.018” 0.020” and 0.022”. Extra special pulse grade • This grade is unequaled in resilience. • It is more difficult to bend and more subject to fracture. However many orthodontists feel that the ability of this wire to move teeth, open deep overbites and resist deformation far outweighs the inconvenience caused by an occasional breakage while bending. • This wire can be easily broken it not bend properly, there is no margin for bending errors. • Available in size of 0.016” only. Supreme grade: • This wire is also referred to as premium plus in Australia. • Primarily used only in treatment of rotations, alignment and leveling. • Though supreme grade exceeds the yield strength of extra special plus it is intended for use in either short sections or full arches where sharp bends are not required. • Available in 0.010”, 0.012” and 0.016”. Newer grades of Australian wires • During the last two decades, 3 more grades have been introduced namely Premium, Premium Plus and Supreme (P, P+ and S) in an order of increasing yield strength. • Increase in the yield strength: by work hardening and material with the appropriate composition and properties. Properties: • Have a greater springback property due to increase yield strength. • Resiliency: Shows greater resiliency thereby greater resistance to permanent deformation. • These wires have the ability to deliver over long periods a constant force when subjected to an external load. • Formability: Greater resiliency leads to lesser formability and therefore they are more brittle than lower grade wires. Clinical usage of newer grades: 1) High load deflections rate required a) Relatively larger forces in stage I: 0.016” OR 0.018” PREMIUM OR PREMIUM PLUS WIRES INDICATED. b) Relatively large resistance to deformation required i.e. for maintenance of the arch form 0.018” P and P+ OR 0.020” P indicated. 2) Low load deflection rate is required a) Generating smaller forces (alignment). 0.014”P or Special plus wire is used or sectionals in 0.012” P+ wires can be used. b) High springback and resiliency (root torquing and uprighting) c) 0.012” Premium Plus or smaller diameter Supreme wires can be used. Wire size: they are available as follows. Premium : .008”, .009”, .010”, .011”, .012”, .014”, .016”,.018” ,.020” Premium plus: .008”, .009”, .010”, .011”, .012”, .014”, .016”,.018” Supreme : .008”, .009”, .010”, .011” . Due to extreme hardness of A. J. Wilcock Australian wire special attention must be given to bend it successfully. a. Pre-warm the wire by sliding between the thumb and forefinger. Do not attempt to straighten the wire by stripping betweens the plier breaks. b. Hold pliers very lightly when bending the wire. Do not squeeze or pull the wire. Pliers must have smooth breaks, carbide tips are not recommended. c. Bend the wire very slowly pressing with the thumb or fore finger, do not rotate the pliers while bending loops and circles should be formed against the square beak and beaks should be apart slightly. d. Never pinch the wire with the pliers before or during bending. e. Do not scratch the wire to locate bends. Australian wire becomes hard from bending hence there is no need for heat treating and no margin to permit back bending to correct mistakes. They are difficult to bend and they are stiffer than stainless steel wires even after months in the oral cavity. NICKEL TITANIUM ALLOY Nitinol was invented in the early 1960’s by William F. Buehler, a research metallurgist at the Naval ordinance. Nitinol: Ni for Nitinol and Ti for titanium and nol for Naval ordanance laboratory. Composition: Nickel – 54% Titanium – 44% Cobalt – 02% • The wire has low stiffness in combination with moderately high strength which leads to large elastic deflection or working range. • The alloy has limited formability. • Alloy can exist in various crystallographic forms. At high temperature body centered cubic lattice (BCC) referred to as the austenitic phase. • Appropriate cooling induces transformation to a close packed hexagonal martensitic lattice. • This transition can also be induced by stress. • Austenitic NiTi is the high-termperature, low stress form, and martensitic NiTi is the low-temperature, high stress form. • Transformation occurs by a twinning process, which is reversible below the elastic limit. • This transition leads to two potential properties shape memory, and super elasticity or pseudoelasticity. Shape memory:
• Shape memory refers to the ability of the material to ‘remember’ its original shape after being plastically deformed while in the martensitic form. • Hence wire is set into the desired shape and held while undergoing a high temperature heat treatment near 4820C. • Then cooled and formed into a second shape. • Subsequent heating through a lower transition temperature i.e. near mouth temperature leads to returning of the wire to its original shape. • Inducing the austenitic to martensitic transition by stress can produce superelasticity a phenomenon – NiTi wires. On a stress sufficient to induce the phase transformation there is a significant increase in strength referred to as superelasticity which occurs due to a volumetric change in crystal structure. • At the completion of the phase transformation, behavior reverts to conventional elastic and plastic strain with increasing stress. • Unloading results in reverse transition and recovery. • Therefore, NiTi alloy can be produced with either austenitic or martenstic structure with varying degrees of cold work and variations in transition temperature. • NiTi has low modulus value and larger working range. Less formability and can neither soldered nor welded. • Crimpable hooks and stops like clinchback distal to molar buccal tube is recommended. Clinchback is performed by flame annealing which leads to making the wire dead soft and it can be bent into the preferred configuration. • Dark blue colour indicates the desired annealing temperature. Clinical application: • High springback, flexibility, low constant forces ,shape memory and elasticity are the important and advantageous properties for clinical applications of NiTi. • Frictional forces are higher than stainless steel and lower than those with beta-titanium. Uses: 1) Crossbite correction 2) Uprightening impacted canines 3) Opening the bites The primary criteria is the amount of malalignment of the teeth from the ideal arch from. Greater malpositions results in Niti wires being advantages over stainless steel Problems encountered in Nickel titanium arch wires 1) The difficulty of placing bends, steps and stops in the majority of the wires. 2) Brittleness and breakage of the wire especially when subjected to biting force. 3) Tendency of archwire to slide from side to side sometimes causing them to stick out beyond a terminal molar. Annealing the ends to allow bending appeared to be an unsatisfactory solution to the last problem because the ends often frayed or broke. 4) Not self limiting – frequent visits necessary. 5) High cost.
BETA – TITANIUM ALLOY In 1960 a high temperature form of titanium alloy which above 16250F rearranges into a body centered cubic lattice, referred to as the beta phase, with the addition of elements as molybdenum or columbium was developed. These titanium based alloys are referred to as beta-stabilized titanium. Composition: Titanium – 79% Molybdenum – 11% Zirconium – 06% Tin – 04% Mechanical properties: • Has low modulus of elasticity therefore lighter forces with large deflections. • Modulus of elasticity of beta-titanium is approximately twice that of nitinol and less then one half that of stainless steel. • Greater spring back property and good formability due to their ability to be highly cold worked and because of the BCC structure of the beta phase. • Heat treatment is not recommended for the current orthodontic beta titanium wires. • Has good corrosion resistance it can be joined by welding.
Clinical application: • Deflection approximately twice of stainless steel therefore greater range of action for either initial tooth alignment or finishing arches. • Beta-titanium is ductile hence loops and complicated bends can be given • Gentle delivery of forces with edgewise wire. • Larger activation is possible due to low forces produced. • Beta-titanium has higher co-efficient a friction due to the increase surface roughness as compared to stainless steel and Elgiloy wires. • Nitrogen ion – implantation techniques employed by the manufacturer has lead to decrease in the bracket section for beta titanium orthodontic wire. CHINESE NiTi ARCHWIRE A new nickel-titanium alloy was developed specially for orthodontic applications by Dr. Tien Hua Cheng and associates. Its parent phase is austenite and work hardening yield mechanical properties that differ significantly from Nitinol wire. Mechanical properties: 1) Chinese NiTi wire has 1.4 times the springback of nitinol wire and 4.6 times the springback to stainless steel wire for 800 of activation, at 400 of activation NiTi wire has 1.6 times the springback of Nitinol wire. 2) Stiffness – At 800 of activation of the average stiffness, Chinese NiTi wire is 73% that of stainless steel wire and 36% that of nitinol wire. Temperature: The Chinese NiTi wire have much load transition temperature than Nitinol wire. The stiffness is approximately the same between room temperature at 220C and mouth temperature at 370C. At a temperature at 600C, the loading curve is slightly higher and the unloading curve loses its ‘S’ shape and exhibits greater permanent deformation and less springback. Time dependent effects: Although NiTi wires show some time – dependent effects, these are insignificant at room temperature.
Clinical significance: • Indicated were large deflections are required. • Straight wire procedures with extensive malalignment of teeth. • Appliances designed to deliver constant forces during major stages of tooth movement.
JAPANESE NiTi ARCHWIRES In 1978, Furukawa Electric Co. Ltd. produced a new type of the Japanese NiTi alloys, possessing all three properties. • Excellent springback • Shape memory • Super elasticity Mechanical properties: Shape memory is the phenomenon occurring in the alloy that is soft and readily amenable to change in shape at a low temperature, but it can easily be reformed to its original configuration when it is heated to a suitable transition temperature. Superelastic property: When the strain was increased from 2% to 8% there was an increase in the stress values. When the strain is reduced 8% to 2% the stress did not reduce proportionally as compared to stainless steel and nitinol wires. There exists no permanent strain when the stress reaches zero. This is called as superelastic property Heat treatment: 200 degrees C–300 degrees C no significant change was observed in the mechanical properties of wire but at 500 degrees C treated for 120 minutes wire had a superelastic portion of approx 50 g load along with the gradual removal of the load.
Clinical applications: Superelasticity properties are to be used as an advantage to enhance the efficiency of the multibracketed technique. ALPHA TITANIUM ARCH WIRE It is the recent alloy in the family o titanium alloy. Composition: Titanium – 90% Aluminum – 6% Vanadium – 4% • Molecular structure is the alpha phase with a closely packed hexagonal lattice. • It possesses fewer slip planes thereby this wire is difficult to deform. Hence it is less ductile than beta-titanium. Clinical significance: 1) Rectangular wires in the sizes of 0.022” x 0.018” (ribbon mode) or 0.020” x 0.020” (square) are recommended by Mollenhauer for the finishing stage. 2) Alpha titanium combination wire with an anterior ribbon (0.022” x 0.018”) and posterior round (0.018”) sections in second stage of Begg treatment. COPPER – NiTi ARCHWIRE It was introduced by Rohit Sachdeva and Suhio Mriyasaki in 1994. Composition: wt % Atomic wt % Titanium 42.99 48.08 Nickel 49.87 45.39 Chromium 0.50 0.96 Copper 5.64 5.57 Properties: 1) Copper NiTi generates a more constant force over long activation spans and very small activations as compared to nickel titanium alloys. 2) Copper NiTi more resistant to permanent deformation compared to nickel titanium alloys. 3) Exhibits better spring back characteristics. 4) Exhibits a smaller drop in unloading force. 5) The addition of copper combined with more sophisticated manufacturing and thermal processes make possible the fabrication of four different copper NiTi archwires with precise and consistent transformation temperatures 150C, 270C, 380C and 400C. This enables the clinician to select archwires on a case-specific basis. Load deflection characteristics: • In comparison to the superelastic NiTi wires the cu-NiTi wires shows a significant lower hysteresis which can be clearly seen in the load deflection curves. • Unloading forces more closely approximate loading forces which means that the wire delivers more constant forces especially for small activations compared to superelastic wires. • It makes possible the insertion of larger size wires, and better bracket slot engagement early in treatment without causing pain and patient discomfort. Classification: Depending on austenitic finish temperatures they are classified into. Type I Af = 150C Type II Af = 270C Type III Af = 350C Type IV Af = 400C Variable transformation temperature thermo-mechanics: • One of the most important markers is the material’s austenitic finish temperature. • To exploit superelasticity to its fullest potential, the working temperature of the orthodontic appliance should be greater than the Af temperature. • It is the differential between the Af temperature and mouth temperature that determines the force generated by nickel titanium alloys. Type I is not used for clinical applications due to the high force level. Type II produces the highest force and is indicated in normal patients. Type III is indicated in patients with a low to normal threshold of pain, and also in periodontally compromised patients. Type IV produces the lowest level of force and are good in patients highly sensitive to pain and are periodontally compromised. A simple clinical trick is to apply ice to a section of the arch wire and it can be nudged into the bracket easily. CERAMIC ARCH WIRES Optiflex archwire: Optiflex is a recent new orthodontic archwire designed by Tallas. It combines unique mechanical properties with a highly esthetic appearance. It is made of clear optical fiber, it comprises of three layers. 1) A silicon dioxide core that provides the force for moving teeth. 2) A silicone resin middle layer that protects the core from moisture and adds strength. 3) A strain resistant nylon outer layer that prevents damage to the wire and further increases its strength. Properties: • Shape: Round or Rectangular • Has wide range of action • Ability to apply light continuous forces Clinical application: • Sharp bends to be avoided • It is a highly resilient archwire that is especially effective in the alignment of crowded teeth. • Lee White Wire is tooth coloured, epoxy-coated archwire that has superior wear resistance and stability of six to eight weeks. • A unique heat treatment bakes on the epoxy coating and makes it possible to offer a wide variety of sizes. • The preformed wires are designed in a natural archform.
_________________ kumar niwlikar
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