Review of Orthodontic Archwires

Chapter 2 Review of Orthodontic Archwires 2.1 Introduction The archwire, through mechanical interaction with the bracket slots, are designed to move teeth from malocclusion to a preferred dental occlusion. In order to move teeth, it is necessary to apply an orthodontic force, which produces a pressure above a dental root film capillary blood pressure of about 15 g/cm 2 and below 20 g/cm 2, onto the dental root film (US Patent 5759029). Thus, the magnitude of the optimal orthodontic force required is normally within the range of 0.5 to 3 N. This relatively low force should be applied continuously in order to achieve correction of teeth. Such forces may reduce the potential for patient discomfort, tissue hyalinization and undermining resorption (Kusy, 1997). Hence, the ideal wire should behave elastically and be able to produce light and continuous forces over the period of use. 2.2 Characteristics of Clinical Relevance The desirable characteristics in an archwire for optimum performance are springback, formability, modulus of resilience, biocompatibility and low friction (Evans et al, 1998). Moreover, the duration of the use and desired mechanical properties of the wire varies with the stage of treatment; namely, initial, intermediate and final stages of treatment. As such, it must be noted that to date, no single archwire is best for all stages. 2.2.1 Springback Springback, also referred to as the range of activation or working range, is the measure of how far a wire can be deflected without causing permanent deformation (Burstone and Goldberg, 13

1980). Higher springback values provide the ability to apply larger activation with a resultant increase in working time of the appliance. This would imply that fewer archwire changes or adjustments are required (Ingram et al, 1986). 2.2.2 Stiffness Stiffness (or load deflection rate) is the force magnitude delivered by an appliance and is proportional to the modulus of elasticity. Low stiffness provides (Kapila and Sachdeva, 1989): a. The ability to apply lower forces, b. A more constant force over time as the appliance experiences deactivation and c. Greater ease and accuracy in applying a given force 2.2.3 Biocompatibility Biocompatibility includes the resistance to corrosion and tissue tolerance to elements in the wire. Based on these criteria, the requirements for dental material biocompatibility include the following (Anusavice, 1996): a. It should not be harmful to the pulp and soft tissues. b. It should not contain toxic diffusible substances that can be released and absorbed into the circulatory system to cause a systemic toxic response. c. It should be free of potentially sensitizing agents that are likely to cause an allergic response. d. It should have no carcinogenic potential. Also the stability in the oral environment ensures the maintenance of desirable properties over extended periods of time. In addition to biocompatibility, the wire should also have poor biohostability. The ideal archwire should neither actively nurture nor passively act as a substrate for micro-organisms 14

that smell foul, cause color changes that detract from aesthetics, or remove and/or build up material that compromise mechanical properties (Kusy, 1997). 2.2.4 Friction Continuous archwire techniques involve a relative motion of bracket over archwire. Excessive bracket/wire friction may result in loss of anchorage or binding accompanied by little or no tooth movement (Kusy and Whitley, 1999). A preferred wire material would be one that produces the least amount of friction at the bracket/wire interface (Ireland et al, 1991). 2.3 Characterization The above mentioned mechanical characteristics (stiffness, springback, modulus of resilience) in the preceding section can be evaluated through a bending test. The mechanical properties of orthodontic wire are typically determined under bending conditions because this mode of deformation is considered more representative of clinical use than the conventional tensile test (Asgharnia and Brantley, 1986). More importantly, it is necessary to know the manner of bending of an archwire during the unloading process. This simulates the force that the wire exerts on a tooth as it is moved into the desired dental arch from a position of malocclusion. The performance and characteristics of conventional metallic archwires have been predominantly evaluated under bending conditions, such as a 3-point bending test (Asgharnia & Brantley, 1986; Wilcock, 1989; Oltjen et al., 1997). 2.4 Other influencing parameters Factors such as inter-bracket distance, wire curvature, direction of activation relative to the curved arch form, bracket width and dimensions of bracket slot relative to wire size substantially affect the flexural stiffness of the archwire (Rock and Wilson, 1988; Kusy and Whitley, 2000). The stage of treatment also dictates the choice of archwire used. 15

2.4.1 Stages of treatment Orthodontic treatment can be separated into three stages: initial, intermediate and final stages of treatment. Each stage of treatment imposes different requirements of the archwire. 2.4.1.1 Initial stage At the beginning of treatment, tooth displacement will be at its greatest. In order to optimize the biological environment for tooth movement and minimize patient discomfort, the principle requirements are minimum stiffness and maximum range (Oltjen et al, 1997). This will enable the archwire to apply force of appropriate magnitude over relatively large distances. 2.4.1.2 Intermediate stage In this stage, wires of increasing stiffness, offering progressively greater control over tooth position, replace the highly flexible wires used in the initial stage. The wires have to be sufficiently stiff to enable the molars to resist unwanted movement. 2.4.1.3 Final stage When the principle tooth movements have been achieved in the intermediate stage, it is necessary to complete final detailing of tooth position and then to provide retention. Although round wires are used in the initial stages, rectangular archwires are required during the final stages of treatment, because the tight fit of a rectangular slot permits more accurate threedimensional control of teeth (Proffit and Fields, 1993). The archwire requirements at this stage are high stiffness and low range. It can be deduced from above that the stage of treatment does indeed influence both the physical and mechanical demands of an orthodontic archwire. 16

2.4.2 Bracket-wire interaction The amount of play between bracket and wire is not dictated by the desired wire stiffness but is under the full control of the clinician. This implies that the orthodontist determines the amount of bracket/wire play desired before selection of wire. Low moduli of elasticity of the newer 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 orientated in the bracket in such a way that forces work out in proper directions. They also maintain better control over root position by delivering both moments and forces (Kapila and Sachdeva, 1989). This highlights that the geometry of the wire is an influencing factor in its use. 2.5 Materials Almost all the commercially available archwires are metal alloys: stainless steel, cobaltchromium, nickel-titanium and beta-titanium. The metallic wires are manufactured by a series of proprietary steps, typically involving more than one company. Initially the wire alloy is cast in the form of an ingot, which must be subjected to successive deformation stages, until the cross-section becomes sufficiently small for wire drawing. Several deformation stages and intermediate heat treatments are required because considerable work hardening of alloy occurs during wire manufacturing (O Brien, 1997). In the following sections, the merits and demerits of each alloy group will be discussed. 2.5.1 Stainless Steel Up until the 1930s, the only orthodontic wires available were made of gold alloy. Austenitic stainless steel, with its greater strength, higher modulus of elasticity, resistance to corrosion and moderate costs was introduced as an orthodontic wire in 1929 (Kapila and Sachdeva, 1989). Stainless steel continues to be a popular wire because of its outstanding combination of 17

mechanical properties, corrosion resistance in the oral environment and cost. The stainless steel types 302 and 304 are most commonly used by the orthodontist in the form of bands and wires. These are commonly designated as 18-8 stainless steel because of the percentages of chromium (18%) and nickel (8%) in the alloys. These wires have a relatively higher modulus of elasticity, stiffness and a lower springback as compared to the other alloys used in orthodontics. Higher modulus of elasticity of stainless steel and 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 intra-oral tractional forces (Drake et al, 1982). Lower springback implies that the wires produce higher forces that dissipate over shorter periods of time, thus requiring more frequent activations or archwire changes. Thus, the high stiffness and strength of stainless steel wires make it an ideal choice for final stage treatment where more arch stability and small tooth movements are required. 2.5.2 Cobalt-Chromium (Co-Cr) Alloys These alloys were originally developed for use as watch springs (Elgiloy), but their properties are also excellent for orthodontic applications. More importantly, the formability of the alloy can be modified by heat treatment. The wires are available in four tempers: soft, ductile, semiresilient and resilient. Soft-temper wires are popular with clinicians because they are easily deformed and shaped into appliances; then heat treated to increase its yield strength and resilience. The effect of heat treatment on mechanical properties (Fillmore and Tomlinson, 1979) has been attributed to complex precipitation processes. 18

Co-Cr orthodontic wires are very similar in appearance, mechanical properties and joining characteristics to stainless steel. The advantages of Co-Cr wires to stainless steel wires are greater resistance to fatigue and distortion, and longer function as a resilient spring. It is recommended for use when considerable bending, soldering or welding is required. Unfortunately, the true potential of these wires have not been tapped into. Most practitioners have used these alloys as a direct substitute for stainless steel wires only. Kusy et al (2001) carried out a study on commercially available Co-Cr wires of the four tempers and found that though formability varied as expected, resilience and flexibility characteristics were variable and independent of temper. The as-received wires do not meet their potential as a variably formable and variably resilient alternative to stainless steel. This could be due to lack of control of the processing variables. Perhaps this is why Co-Cr wires have never made the impact that was expected of them when they were first introduced. 2.5.3 Nickel-Titanium Alloys Nickel-titanium has remained a strong focus of material research in orthodontics since it was first introduced in 1971. This alloy was originally developed in the Naval Ordanance Laboratory (USA) and was known as Nitinol. It is based upon the intermetallic compound NiTi, which has weight percentages of 55% Ni and 45% Ti. Nickel-titanium alloy possesses two features of considerable importance for clinical orthodontics: good springback and flexibility, which allow for large deflections but low forces. Compared to stainless steel, nickel-titanium has a greater recoverable energy when activated to the same amount of bending or torque. This results in increased clinical efficiency since fewer archwire changes are required. Further research into this family of alloy led to the discovery of its shape memory effect and the study of its response to heat treatment (Andersen and Morrow, 1978). Heat treatment results in substantial alterations in mechanical properties of the alloy. Changes in 19

crystallographic arrangement, a reversible transformation between the austenitic and martensitic NiTi phases caused by heating, produce this shape-memory effect. This occurs over a certain transformation temperature range or when the stress is decreased below the appropriate level. Andersen and Morrow (1978) 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. Many researchers have tried to harness this property of the alloys. A highly convenient electric resistance method has been developed for the heat treatment of wires (Sentalloy, GAC) and a commercial apparatus (GAC International) is available that enables clinicians to heat treat superelastic nickel-titanium wires as desired for the treatment of individual patients. Nickeltitanium alloys with shape-memory behavior activated at body temperature have been recently introduced by manufacturers. Recently, the use of hollow super-elastic nickel-titanium wires was proposed by Shima et al (2002a, 2002b) as orthodontic wires. Though much of the research interest for the alloys is focused on its shape-memory effect, the first wire (Nitinol, Unitek/3M) was not associated with this phenomenon. Currently there is a wide range of nickel-titanium alloy wires available in the market (Ni-Ti by Ormco/Sybron; Sentalloy by GAC, Nitinol by Unitek/3M). However these alloys are not without drawbacks. Low stiffness of nickel-titanium alloys 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. High cost, relative to other metallic wires is also its drawback (Baldwin et al; Nakano et al, 1999). Nickel hypersentitivity 20

reactions to nickel-titanium wires have been observed in orthodontic patients who are nickelsensitive, although such cases are rare (Justin et al, 1993). It is also difficult to place permanent bends and the wire cannot be bent over sharp edges or into a complete loop. Furthermore, it cannot be soldered and must be joined by mechanical crimping process. The wires also have a high bracket/wire friction. However, newer nickel-titanium wires with ion-implanted surfaces have been developed to obtain reduced bracket friction (GAC International). 2.5.4 Beta-Titanium (beta-ti) The last major alloy to have an impact on orthodontics is beta-titanium alloy, introduced in 1980. This alloy is commercially available as titanium-molybdenum alloy (TMA, Ormco). The nominal composition of TMA is 77.8% titanium, 11.3% molybdenum, 6.6% zirconium and 4.3% tin. The presence of molybdenum causes the elevated temperature body-centered cubic beta polymorphic phase of titanium to be metasable at room temperature, rather than the hexagonal close-packed alpha phase. This results in its excellent formability or capability for permanent deformation. It is also the only orthodontic wire alloy that possesses true weldability (Nelson et al., 1987). Beta-Ti has a modulus of elasticity less than that of stainless steel and about twice that of nickel-titanium. 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 nickeltitanium alloy is inadequate to produce the desired force magnitudes (Burstone and Goldberg, 1980). A beta-titanium wire can be deflected almost twice as much as stainless steel without permanent deformations. It has a corrosion resistance comparable to stainless steel and Co-Cr alloys (Goldberg and Burstone, 1988). 21

Beta-titanium was almost a perfect wire except for a fundamental drawback. Beta-titanium wires demonstrate higher levels of bracket/wire friction than either stainless steel or Co-Cr wires. Its coefficients of friction were the worst of any of the orthodontic alloys (Kusy and Whitley, 1989), and consequently its ability to accommodate the sliding of teeth was limited (Kusy and Whitley, 1990). It can be inferred from the above discussions that even the conventional metallic wires differ in mechanical properties and hence their effective use varies with the stage of treatment. Table 2.1 summarizes the mechanical properties of these four alloy groups. Table 2.1: Mechanical Properties of four main groups of alloys used as archwires Wire alloy Modulus of elasticity (GPa) Yield Strength (MPa) Stainless steel 160-180* 1100-1500* Cobalt-chromium alloy (sotf-temper) 160-190 830-1000 Beta-Titanium (TMA) 62-69 690-970 Nickel-titanium (nitinol) 34 210-410 *Data from Asgharnia and Brantley, 1989 and Drake et al, 1982. 2.6 Aesthetic Wires Recently, orthodontic treatment has become more common in adult patients, and the demand for improvement in the aesthetics quality of braces has been increasing. Many individuals regard metallic braces as unsightly when placed in the mouth. Although aesthetic brackets have brought a dramatic improvement in the appearance of appliances, metal archwires are still visible. This limitation in the improvement in appearance has led many manufacturers and researchers to attempt to produce durable aesthetic archwires. These wires have to be visually unobtrusive and at the same time perform the essential function of aligning teeth. 22

2.6.1 Coated metallic wires The first attempt to make aesthetic wires was to camouflage the archwire by covering it with a plastic layer. Polymer-coated metallic wires (Rocky Mountain Teflon-coated stainless steel wires) were introduced in the 1970s. Although the appearance of the wires was greatly improved, experience with the Teflon-coated archwires showed that the coating tend to stain and split with usage, revealing the underlying metal. Another alternative is using a spray-coat, which has the advantage of adding only a thin layer to the archwire, but the coat tends to have a rather grey tinge and often chips off with use (Postlethwaite, 1992). 2.6.2 Optiflex The first, completely non-metallic archwire was introduced into the orthodontic market, called a totally aesthetic labial archwire. The commercial name is Optiflex (Ormco/Sybron). The wire comprises of three layers: a silica core, which is surrounded by a moisture protection silicone resin middle layer and a stain-resistant nylon outer layer. The outer layer has the dual purpose of preventing damage to the archwire and further increase the strength of the archwire. This wire was aesthetically very pleasing. However, its orthodontic force is too light for clinical use (Lim et al, 1994). Further improvement in the stiffness and resilience of Optiflex would be needed in order for the archwire to be clinically efficient. 2.6.3 Composite archwires As composites are displacing metallic alloys as structural components in the aerospace industry, the expectation is that the attractive properties and characteristics of these aesthetic composites will capture a significant share of the orthodontic market as well. Currently there are no composite wires available in the market. However, it has been recognized that an optimal and aesthetic archwire can be developed using composite technology from continuous 23

fibers and polymer matrix to suit the varying degree of stiffness required for each stage of orthodontic treatment (Goldberg and Burstone, 1992). Currently, two research groups have developed prototype composite archwires, using two different fabrication methods. Kusy et al. developed a vertically disposed patented (US Patent 5869178) modified pultrusion system to produce composite profiles ranging from 0.012 inch (0.3 mm) to 0.025 inch (0.6 mm) in cross-section. In this set-up, the fibers are being pulled through a rigid die of a fixed cross-section, giving rise to longitudinally straight profiles (Figure 2.1). With this patented process, prototype glass fiber-reinforced bisphenol A diglycidylether methacrylate (Bis-GMA) composite wires were developed. The composite wires fabricated were subjected to a 3-point bending test and a bend stress relaxation to determine their flexural properties and viscoelastic behavior respectively (Zufall and Kusy, 2000a). It was observed that archwire recovery was not correlated with reinforcement level but the stress relaxation behaviour was strongly correlated. Also, the relaxed elastic moduli in bending of the composite wires were similar to the elastic moduli in bending of several conventional orthodontic archwire materials. A tribological (friction and wear) study was also designed to determine the effect of coating on the composite wires (Zufall and Kusy, 2000b). It was observed that although the coating did increase the frictional and binding coefficients of the wires, it was still within the limits outlined by conventional wire-bracket couples. In addition, it reduced the risk of glass fiber release into the oral cavity. Prototype glass fiber-reinforced poly-methyl methacrylate (PMMA) composite archwires have also been developed using hot-drawing (Imai et al, 1998) fabrication method, drawing through a glass die. Mould polymerization method (Watari et al, 1998) was also used to develop glass fiber-reinforced epoxy wires. These prototype wires were also subjected to a 3-point bending test to evaluate their mechanical properties. The fiber-reinforced polymer wires showed sufficient strength and good elastic recovery. The aesthetic in external appearance was evident 24

and its range of strength (by varying the volume fraction of fibers) corresponds to conventional wires. 2.7 Motivation It can be inferred from the above discussions that even the conventional metallic wires cannot produce a single wire that can be used throughout all stages of treatment. Moreover, the current trend in orthodontics is towards developing aesthetic appliances. Metallic wires have poor aesthetics. Although attempts had been made to coat these metallic wires, the coatings were not durable in the oral cavity and they increased the friction between the bracket and wire. Polymers and ceramics are not ideal candidates due to low stiffness (polymer) and brittleness (ceramics). However, using composite technology with careful selection of the constituents, it is possible to combine both aesthetics as well as favorable mechanical properties. As such, in this study the objective was to develop a composite orthodontic wire. Though attempts have been made by researchers to develop aesthetic composite wires, as mentioned in Section 2.6.3, there is still room for much improvement. In all the above mentioned methods, pulling the fibers through a small die induces stresses on the fibers, causing them to break. This becomes more prevalent if the cross-section is very small, like that required for an orthodontic wire (0.025 0.014 ). It also becomes more difficult to insert fibers into the die. It can be observed by looking at the cross-sections in Figure 2.2, obtained by the method described in US Patent 5869178, a high fiber volume fraction would be required for even fiber distribution. 25

Figure 2.1: Photo-pultrusion apparatus Figure 2.2: Cross-section of profiles obtained using method described in US Patent 5 869 178 Source: US Patent 5 869 178 Furthermore, to shape the profile longitudinally, such as to the dental arch, beta-staging needs to be carried out. Also, the methods do not produce very smooth surfaces. This roughness will give rise to friction between the archwire and bracket, not desirable in orthodontic treatment. Hence it is desirable to develop a novel method that will eliminate pre-stresses on the fibers as well as fiber breakage, with desired surface finish and mechanical performance. Most of the prior researches on composite archwires have only focused on varying the mechanical property of the composite through different volume fraction of reinforcement. No research has focused on controlling the interface of the composite wires developed. Hence, in this study, surface modification of fibers and its influence on the mechanical properties of the composite wire were explored. 26