17100681 Mechanical Properties of Wires and Its Orthodontic Application

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Source: AJO-DO on CD-ROM (Copyright © 1998 AJO-DO), Volume 1989 Aug (100 109): Mechanical properties and clinical applications of orthodontics wires - Kapila and Sachdeva -------------------------------Mechanical properties and clinical applications of orthodontic wires Sunil Kapila, BDS, MS, and Rohit Sachdeva, DDS, MS San Francisco, Calif., and Dallas, Texas This review article describes the mechanical properties and clinical applications of stainless steel, cobalt-chromium, nickel-titanium, beta-titanium, and multistranded wires. The consolidation of this literature will provide the clinician with the basic working knowledge on orthodontic wire characteristics and usage. Mechanical properties of these wires are generally assessed by tensile, bending, and torsional tests. Although wire characteristics determined by these tests do not necessarily reflect the behavior of the wires under clinical conditions, they provide a basis for comparison of these wires. The characteristics desirable in an orthodontic wire are a large springback, low stiffness, good formability, high stored energy, biocompatibility and environmental stability, low surface friction, and the capability to be welded or soldered to auxiliaries. Stainless steel wires have remained popular since their introduction to orthodontics because of their formability, biocompatibility and environmental stability, stiffness, resilience, and low cost. Cobaltchromium (CoCr) wires can be manipulated in a softened state and then subjected to heat treatment. Heat treatment of Co-Cr wires results in a wire with properties similar to those of stainless steel. Nitinol wires have a good springback and low stiffness. This alloy, however, has poor formability and joinability. Beta-titanium wires provide a combination of adequate springback, average stiffness, good formability, and can be welded to auxiliaries. Multistranded wires have a high springback and low stiffness when compared with solid stainless steel wires. Optimal use of these orthodontic wires can be made by carefully selecting the appropriate wire type and size to meet the demands of a particular clinical situation. (AM J ORTHOD DENTOFAC ORTHOP 1989;96:100-9.) Recent advances in orthodontic wire alloys have resulted in a varied array of wires that exhibit a wide spectrum of properties. Up until the 1930s, the only orthodontic wires available were made of gold. Austenitic stainless steel, with its greater strength, higher modulus of elasticity, good resistance to corrosion, and moderate costs, was introduced as an orthodontic wire in 1929, and shortly afterward gained popularity over gold.1,2 since then several other alloys with desirable properties have been adopted in orthodontics. These include cobalt-chromium, nickel-titanium, beta-titanium, and multistranded stainless steel wires. Presently the orthodontist may select, from all the available wire types, one that best meets the demands of a particular clinical situation. The selection of an

appropriate wire size and alloy type in turn would provide the benefit of optimum and predictable treatment results. The clinician must therefore be conversant with the mechanical properties and the relevant clinical applications of these properties for these wires. Although several investigators have evaluated the mechanical properties of various wire types, a cohesive clinical interpretation of their findings is lacking. This article reviews pertinent literature in order to describe the mechanical properties and optimal clinical applications of stainless steel, cobalt-chromium, nickel-titanium, beta-titanium, and multistranded wires. The objective of this article is to provide the practicing clinician with the basic working knowledge on orthodontic wire characteristics and usage. LABORATORY TESTS The properties of orthodontic wires are commonly determined by means of various laboratory tests. Thus, wires have previously been investigated under tension,3-9 in bending,5-7,9-12 and torsion6,10,16,17 (Table I). Although these tests do not necessarily reflect the clinical situations to which wires are usually subjected, they provide a basis for comparison of these wires.5-7 Tests in bending provide some information on the behavior of wires when subjected to first- and second-order bends. Similarly, results of torsional tests reflect, to a certain degree, wire characteristics in a third-order direction. Tension, bending, and torsion are uniquely different stress states and place varied demands on wire performance.18,19 The properties of wires under these three stress states are therefore considered independently. Graphic description of stress against strain can be used to determine yield strength, modulus of elasticity, stored energy, and springback when the wire is subjected to tensile loading. Similarly, graphic plots of bending moment against angular deflection or torsional moment against torque angle are used for the evaluation of these wire characteristics under conditions of bending and torsion, respectively (Fig. 1). WIRE CHARACTERISTICS OF CLINICAL RELEVANCE Several characteristics of orthodontic wires are considered desirable for optimum performance during treatment.5,10,20 These include a large springback, low stiffness, high formability, high stored energy, biocompatibility and environmental stability, low surface friction, and the capability to be welded or soldered to auxiliaries and attachments. A brief description of each of these desirable wire characteristics is provided. 1. Springback. This is also referred to as maximum elastic deflection, maximum flexibility, range of activation, range of deflection, or working range. Springback is related to the ratio of yield strength to the modulus of elasticity of the material (YS/E). Higher springback values provide the ability to apply large activations with a resultant increase in working time of the appliance. This, in turn, implies that fewer arch wire changes or adjustments will be required. Springback is also a measure of

how far a wire can be deflected without causing permanent deformation20 or exceeding the limits of the material.12 2. Stiffness or load deflection rate. This is the force magnitude delivered by an appliance and is proportional to the modulus of elasticity (E)14 (Fig. 1). Low stiffness or load deflection rates provide (1) the ability to apply lower forces, (2) a more constant force over time as the appliance experiences deactivation, and (3) greater ease and accuracy in applying a given force. 5,20 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 or stored energy (MR This property represents the work available to move teeth. It is reflected by the area under the line describing elastic deformation of the wire (Fig. 1). 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 periods of time after manufacture. This, in turn, ensures a predictable behavior of the wire when in use. 6. Joinability. The ability to attach auxiliaries to orthodontic wires by welding or soldering provides an additional advantage when incorporating modifications to the appliance. 7. Friction. Space closure and canine retraction in continuous arch wire techniques involve a relative motion of bracket over wire. Excessive amounts 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. MECHANICAL PROPERTIES AND THEIR CLINICAL IMPLICATIONS Tables II and III summarize the composition and important mechanical and clinical characteristics of orthodontic wire alloys and will be used to describe the findings of various investigators. Stainless steel wires Carbon interstitial hardening and cold working contribute to the high yield strength and modulus of elasticity of stainless steel (Table II). Residual stresses present in a wire subsequent to bending can markedly affect the elastic properties of the wire.21 Heat treatment is therefore used in stress-relieving stainless steel after bending the wire into an arch, loops, or coils. This helps to enhance the elastic properties of the wire.22-28 The recommended temperature-time schedule for stress-relieving stainless steel is 750° F (399° C) for 11 minutes.26,27 Funk23 recommends the use of a color index to determine when adequate heat treatment is achieved. He

suggests that a straw-colored wire indicates that optimum heat treatment has been attained. Commercially available stainless steel wires demonstrate a range of values both for the modulus of elasticity and yield strength29 The large modulus of elasticity of stainless steel (Table II) and its associated high stiffness necessitate the use of smaller wires for alignment of moderately or severely displaced teeth. A reduction in wire size results in a poorer fit in the bracket and may cause loss of control during tooth movement. However, high stiffness is advantageous in resisting deformation caused by extra- and intraoral tractional forces.6 The yield strength to elastic modulus ratio (YS/E) indicates a lower springback of stainless steel than those of newer titanium-based alloys (Tables II and III). The stored energy of activated stainless steel wires is substantially less than that of beta-titanium and nitinol wires6,10 (Fig. 1). This implies that stainless steel wires produce higher forces that dissipate over shorter periods of time than either betatitanium or nitinol wires, thus requiring more frequent activations or arch wire changes. Joinability with stainless steel is possible by soldering but may be demanding.1 Stainless steel wires also can be fused together by welding, but this generally requires reinforcement with solder. Corrosion resistance of stainless steel wires is good,30 although solder joints may corrode in the oral cavity. Park and Shearer31 have demonstrated the release of nickel and chromium from stainless steel appliances. The authors further note that although the amounts of nickel and chromium released are below the average dietary intake, the liberated elements may sensitize patients or produce reactions in already sensitized persons. Low levels of bracket/wire friction have been reported with experiments using stainless steel wires.32-34 This signifies that stainless steel wires offer lower resistance to tooth movement than other orthodontic alloys. Cobalt-chromium wires Cobalt-chromium (Co-Cr) alloys are available commercially as Elgiloy, Azura, and Multiphase. Elgiloy is manufactured in four tempers: soft (blue), ductile (yellow), semiresilient (green), and red (resilient) in increasing order of resilience. Blue Elgiloy is the softest of the four wire tempers and can be bent easily with fingers or pliers. It is recommended for use when considerable bending, soldering, or welding is required. Heat treatment of blue Elgiloy increases its resistance to deformation (Table II). Yellow Elgiloy is relatively ductile and more resilient than blue Elgiloy. It can also be bent with relative ease. Further increases in its resilience and spring performance can be achieved by heat treatment. Green Elgiloy is more resilient than yellow Elgiloy and can be shaped with pliers before heat treatment. The most resilient Elgiloy is marked red and provides high spring qualities. Careful manipulation with pliers is recommended when using this wire because it withstands only minimal working. Heat treatment makes red Elgiloy wire extremely

resilient. since this wire fractures easily after heat treatment, all adjustments should be made before this precipitation-hardening process. With the exception of red temper Elgiloy, nonheat-treated Co-Cr wires have a smaller springback than stainless steel wires of comparable sizes, but this property can be improved by adequate heat treatment.12-24 The ideal temperature for heat treatment is 900° F (482° C) for 7 to 12 minutes in a dental furnace.35 This causes precipitation-hardening of the alloy, increasing the resistance of the wire to deformation36 and results in a wire that demonstrates properties similar to those of stainless steel (Tables II and III). Heat treatment at temperatures above 1200° F (749° C) results in a rapid decline in resistance to deformation because of partial annealing.3 Optimum levels of heat treatment are confirmed by a dark strawcolored wire or by use of temperature-indicating paste.35 The advantages of Co-Cr wires over stainless steel wires include greater resistance to fatigue and distortion, and longer function as a resilient spring.37 In most other respects, the mechanical properties of Co-Cr wires are very similar to those of stainless steel wires. Therefore, stainless steel wires may be used instead of Co-Cr wires of the same size in clinical situations in which heat-hardening capability and added torsional strength of Co-Cr wires are not required.18 The high moduli of elasticity of Co-Cr and stainless steel wires suggest that these wires deliver twice the force of beta-titanium wires and four times the force of nitinol wires for equal amounts of activation.6 The resultant undesired force vectors are therefore greater with Co-Cr and stainless steel wires than with both types of titanium alloys. Clinically, this may translate into faster rates of mesial movement of posterior teeth, thus placing greater demands on intra- and extraoral anchorage. Co-Cr and stainless steel wires have good formability and can be bent into many configurations relatively easily. Caution should be exercised when soldering attachments to these wires since high temperatures cause annealing with resultant loss in yield and tensile strengths.1,3 Low-fusing solder is recommended for this purpose.35 Although larger frictional forces have been noted previously32 between brackets and Co-Cr wires than between brackets and stainless steel wires, a recent report34 on zero torque/zero angulated brackets indicates similar values for friction between brackets and these two types of alloys. This implies that resistance to tooth movement along stainless steel and Co-Cr wires may be comparable. Nickel-titanium wires Nitinol, a stochiometric nickel-titanium alloy, was first introduced for use in orthodontics in 197138 and is available as NiTi, Nitinol, Orthonol, Sentinol and Titanal. Commercially available nitinol wires from various manufacturers demonstrate differences in some properties. The most advantageous properties of nitinol are the good springback and flexibility, which allow for large elastic

deflections10,16 (Tables II and III). The high springback of nitinol is useful in circumstances that require large deflections but low forces. Although not confirmed by values of YS/E in Table II, it has generally been noted that nitinol wires have greater springback and a larger recoverable energy than stainless steel or betatitanium wires when activated to the same amount of bending or torquing6,17,39 (Fig. 1). This results in increased clinical efficiency of nitinol wires since fewer arch wire changes or activations are required.39 Similarly, for a given amount of activation, wires made of titanium alloys produce more constant forces on teeth than stainless steel wires. A distinct advantage of nitinol is realized when a rectangular wire is inserted early in treatment. This accomplishes simultaneous leveling, torquing, and correction of rotations. Heat treatment of nitinol results in substantial alterations in mechanical properties of the alloy. Changes in crystallographic arrangement caused by heating produce the "memory" effect in this alloy. Andreasen and Morrow10 described the "shape memory" phenomenon as the capability of the wire to return to a previously manufactured shape when it is heated through its transitional temperature range (TTR). This effect is realized by holding the wire in the desired shape while undergoing high-temperature heat treatment. When subsequently cooled, the wire can be deformed within certain strain limits, from which it recovers its original shape if heated through its unique TTR. This change from distorted to original form involves a transformation of nitinol from the martensitic to the austenitic phase. Hurst8 evaluated the percentage recovery of five commercially available nitinol wires after subjecting these wires to tensile deformation followed by heating beyond their TTRs. He noted that the percentage recovery for NiTi, Nitinol, Orthonol, light and medium Sentinol, and Titanal was about 90% and varied only slightly among these wires. Some clinical uses of the "shape memory" phenomenon have been suggested. These include the possible consolidation of extraction spaces40-42 and alignment of crowded teeth.43,44 However, further tests and improvements are required for the "shape memory" phenomenon to become widely accepted for clinical purposes. Garner, Allai, and Moore33 and Kapila and associates34 have noted that bracket/wire frictional forces with nitinol wires are higher than those with stainless steel wires and lower than those with beta-titanium wires in zero torque/zero angulated 0.018-inch brackets. In 0.022-inch brackets, nitinol and beta-titanium wires demonstrated similar levels of friction that were greater than those with stainless steel or Co-Cr wires.34 Furthermore, some investigators32,45 have reported higher bracket/wire friction with nitinol than with stainless steel wires at bracket angulations of up to 3°. However, at higher bracket angulations, these authors demonstrated significantly lower bracket/wire friction with nitinol than with stainless steel wires. Andreasen and Morrow10 indicate that nitinol wires are associated with advantages such as fewer arch wire changes, less chairside time, reduction in time required to

accomplish rotations and leveling, and less patient discomfort. However, several other properties of nitinol impose limitations on its use. The poor formability of these wires implies that they are best suited for preadjusted systems. Any first-, second-, and third-order bends have to be overprescribed to obtain the desired permanent bend. Nitinol fractures readily when bent over a sharp edge.10 In addition, bending also adversely affects the springback property of this wire.46 The bending of loops and stops in nitinol is therefore not recommended. Since hooks cannot be bent or attached to nitinol, crimpable hooks and stops are recommended for use. Cinch-backs distal to molar buccal tubes can be obtained by resistance or flame-annealing the end of the wire. This makes the wire dead soft and it can be bent into the preferred configuration. A dark blue color indicates the desired annealing temperature. Care should be taken not to overheat the wire because this makes it brittle. The low stiffness of nitinol provides inadequate stability at the completion of treatment. This stability can be attained by means of stainless steel wire tailored to the desired final occlusion. Findings on resistance to corrosion of nitinol wires have been inconsistent. Although some investigators47,48 report that nitinol is as resistant to corrosion as stainless steel, various authors30,49 have found nitinol to be more susceptible to corrosion than other orthodontic alloys. Further, whereas Schwaninger, Sarkar, and Foster11 have noted that corrosion does not affect flexural properties of nitinol wires, some reports46,50 indicate an increase in permanent deformation and a decrease in elasticity caused by corrosion or the cumulative effects of cold-working. Recycling of nitinol wires is often practiced because of their favorable physical properties and the high cost of the wire,51 Recycling involves (1) repeated exposure of the wire for several weeks or months to mechanical stresses and elements of the oral environment and (2) sterilization between uses. Although Mayhew and Kusy51 and Buckthal and Kusy52 have demonstrated no appreciable loss in properties of nitinol wires after as many as three cycles of various forms of heat sterilization or chemical disinfection, the effects of the oral environment on the wire properties are still inconclusive.11,30,46-50 The combined effects of repeated clinical use and sterilization on the properties of nitinol wires require further investigation before recycling of these wires is recommended. Beta-titanium wires Beta titanium has been popularized as an orthodontic alloy only in the current decade.5,20 It is commercially available as TMA (titanium-molybdenum alloy). Beta titanium has a modulus of elasticity that is less than that of stainless steel and about twice that of nitinol17 (Fig. 1 and Table II). This makes its use ideal in situations in which forces less than those of stainless steel are necessary and in instances in which a lower modulus material such as nitinol is inadequate to

produce the desired force magnitudes.20 Furthermore, the relatively lower forces generated by beta-titanium wires imply that the counterproductive force vectors generated by beta-titanium wires can be counteracted by smaller forces than those required for comparable stainless steel wires. Extraoral anchorage demands with beta-titanium wires will therefore be less than those for stainless steel wires. The springback for beta titanium is superior to that of stainless steel (Tables II and III). A beta-titanium wire can therefore be deflected almost twice as much as stainless steel wire without permanent deformation.5,17,20,53 Beta-titanium wires also deliver about half the amount of force as do comparable stainless steel wires5,20,53; for example, an 0.018 ´ 0.025-inch beta-titanium wire delivers approximately the same force as a 0.014 ´ 0.020-inch stainless steel wire in a second-order activation. The former configuration has the added advantage of full bracket engagment and a resultant greater torque control than the smaller stainless steel wire. The good formability of beta-titanium-wire allows stops and loops to be bent into the wire. However, Burstone and Goldberg20 recommend that these wires should not be bent over a sharp radius. Helices that are commonly used with stainless steel to lower the load deflection rate of the appliance may not be necessary with betatitanium wires because of their low modulus of elasticity and high springback. This helps to simplify appliance design by eliminating the need to place loops and helices in the wire. It is possible to attach stops, hooks, and active auxiliaries by welding to betatitanium wires, thereby increasing the versatility of the wire.20,53,54 However, adequate strength of the weld without loss in wire properties is achieved within a narrow optimal voltage setting on a resistance spot welder. Nelson, Burstone, and Goldberg54 have provided values for these optimal voltage settings. A flat-to-flat electrode configuration is recommended for welding because it produces a strong joint with low levels of distortion.55 Overheating of the wire causes it to become brittle.20,54 Beta-titanium has a corrosion resistance comparable to stainless steel and cobaltchromium alloys.53 Beta-titanium wires demonstrate higher levels of bracket/ wire friction than either stainless steel or Co-Cr wires.33,34 This may imply slower rates of tooth movement during canine retraction and space consolidation with betatitanium wires than with stainless steel or Co-Cr wires. Multistranded wires Multistranded wires are made of stainless steel and composed of specified numbers of thin wire sections coiled around each other to provide a round or rectangular cross-section. Kusy and Dilley56 investigated the strength, stiffness, and springback properties of multistranded wires in a bending mode of stress. They noted that the stiffness of a triple-stranded 0.0175-inch (3 x 0.008-inch) stainless steel arch wire

was similar to that of 0.010-inch single-stranded stainless steel wire. The multistranded wire was also 25% stronger than the 0.010-inch stainless steel wire. The 0.0175-inch triple-stranded wire and 0.016-inch nitinol wire demonstrated similar stiffnesses. However, nitinol tolerated more than 50% greater activation than the multistranded-wire. The triple-stranded wire was also half as stiff as a 0.016-inch beta-titanium wire. In a more recent Investigation, Kusy and Stevens57 state that although the elastic properties of multistranded wires vary widely, several of these wires compare favorably with some of the beta-titanium and nitinol wires. Table III summarizes the important characteristics of multistranded wires relative to those of other alloys. Ingram, Gipe, and Smithl12 noted that titanium alloy wires and multistranded stainless steel wires have low stiffness when compared with solid stainless steel wires. The investigators also found that most multistranded wires had a springback similar to that of nitinol, but a larger springback when compared with solid stainless steel or beta-titanium wires. Unlike stainless steel wires, in which springback decreases with increasing thickness, the titanium and multistranded wires have springback properties that are relatively independent of wire size. These findings agree with those made by Kusy and Dilley56 and Kusy.58 In contrast, Schaus and Nikolai,13 using a simulated arch form, noted that multistranded wires were less flexible than suggested by theory or previous tests. They indicated that factors such as interbracket distances, wire curvature, direction of activation relative to the curved arch form, bracket width, dimensions of bracket slot relative to wire size, and friction between bracket and wire substantially affect the flexural stiffness of the arch wire. OPTIMAL CLINICAL APPLICATIONS OF ORTHODONTIC WIRES The practical applications of orthodontic wires can be optimized by carefully selecting the appropriate alloy type and wire size to meet the demands of a specific clinical situation. Kusy58 and Kusy and Greenberg18,59 have recommended a sequential use of arch wires selected for optimal use of the mechanical properties of their constituent alloys. The authors suggest that for initial leveling requiring wideranging tooth movements, a 0.016-inch nitinol wire outperforms a 0.0175-inch triple-stranded stainless steel wire, an 0.018-inch round nitinol wire is superior to a 0.014-inch round stainless steel wire, and an 0.018-inch square nitinol wire outperforms a 0.014-inch round stainless steel wire. However, in a recent report, Kusy and Stevens57 noted that 0.015-inch triple-stranded wires demonstrate a greater working range than either nitinol or beta-titanium wires of similar or greater dimensions. The authors also indicate that multistranded wires compare more favorably with titanium wires than suggested by previous research56 and may provide a viable alternative to the more expensive titanium wires for initial leveling. The intermediate stages of treatment require closing loops, gable bends, and attachments. Beta-titanium wires meet these demands while providing greater range of activation than stainless steel or Co-Cr wires. In torsion, the formability and

stiffness of stainless steel and Co-Cr wires far exceed those of the titanium wires, thereby making these alloys the finishing wires of choice. The lower friction between stainless steel or Co-Cr wires and brackets22-24 suggest that these wires may be more suitable than other alloys for movement of teeth along a wire. Until the recent introduction of new types of orthodontic alloys, increments in wire stiffness during treatment were instituted by progressively increasing the crosssection of stainless steel wires. Burstone60 refers to this as "variable cross-section orthodontics." The author further states that advances in orthodontic wire alloys have made it possible to control wire stiffness by varying material properties— namely, the modulus of elasticity. This is known as "variable modulus orthodontics." Burstone60 formulates his concepts by stating that the overall stiffness of the orthodontic appliance (S) is determined by the wire stiffness (Ws) and design stiffness (As) as represented by: S = Ws ´ As Design stiffness (As) is dependent on factors such as interbracket distance and the incorporation of loops and coils into the wire. Changes in wire stiffness (Ws), on the other hand, can be brought about by altering the cross-sectional stiffness (Cs) and/or the material stiffness (MS) as designated by the formula:

Ws = Ms ´ Cs where the cross-sectional stiffness is determined by a cross-sectional property such as moment of inertia of the wire and the material stiffness is dependent on the modulus of elasticity of the alloy. Therefore, an increase in appliance stiffness (S) can be brought about not only by change in appliance design or increase in crosssectional thickness of the wire, but also by selecting a material with a higher modulus of elasticity. The relationships of material stiffnesses for stainless steel, cobalt-chromium, nickel-titanium and beta-titanium wires are in the ratio of 1:1.2:0.26:0.42. Material stiffness for multistranded wires range from 1/25 to 1/5 of that for single-stranded stainless steel wires.60

Several advantages of "variable modulus orthodontics" have been suggested60 as follows: 1. The amount of play between bracket and wire is not dictated by the desired wire stiffness, but is under full control of the clinician. This implies that the orthodontist determines the amount of bracket/wire play desired before selection of the wire. Once the crosssectional size and shape have been established, the desired stiffness can be implemented by selecting an alloy with an appropriate material stiffness.

2. The low moduli of elasticity of the newer orthodontic alloys permit the use of light, rectangular wires even during the early stages of treatment. Rectangular wires are preferable over round wires because they can be better oriented in the bracket in such a way that forces work out in the proper directions. They further aid in patient comfort by preventing loops from turning into the cheeks and gingiva. Rectangular wires also maintain better control over root position by delivering both moments and forces. 3. The use of newer orthodontic alloys with their lower moduli of elasticity offers substantial advantages with a 0.022-inch bracket slot. 4. The selection of an appropriate alloy type and wire size may reduce the number of arch wires needed for alignment by reducing bracket/wire play early in treatment. In addition, since the titanium wires also work more efficiently and over longer periods of time because of their greater springback, the number and frequency of arch wire changes are reduced. CONCLUSIONS In the last few decades, a variety of new wire alloys has been introduced into orthodontics. These wires demonstrate a wide spectrum of mechanical properties and have added to the versatility of orthodontic treatment. Appropriate use of all the available wire types may enhance patient comfort and reduce chairside time and the duration of treatment. The restricted use of only stainless steel wires to treat an entire case from start to finish therefore may be indicated only in relatively few patients. It may be beneficial instead to exploit the desirable qualities of a particular wire type that is specifically selected to satisfy the demands of the presenting clinical situation. This, in turn, would provide the most optimal and efficient treatment results.

Fig. 1. Diagrammatic representation of graphs obtained in bending (A) and torsional (B) tests. These graphs demonstrate the differences in properties of stainless steel, beta-titanium, and nitinol wires and the slightly different responses of these specimens under conditions of bending and torsion. The slope of the straight line on the graph represents the stiffnes or load deflection rate (E) of the wire; the shaded area under each plot is the stored energy at a fixed bending or torsional moment. (Modified from Drake SR, Wayne DM, Powers JM, Asgar K. AM J ORTHOD 1982;82:206-10.)

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