AmericanJournal ofORTHODONTICS Volume 77, Number 2 February, 1980

ORIGINAL ARTICLES

Beta titanium: A new orthodontic alloy

Charles J. Burstone and A. Jon Goldberg Farmington, Corm.

Historically, few alloys have been used in the fabrication of orthodontic appliances. This article reviews the gold-based, stainless steel, chrome-cobalt-nickel, and nitinol alloys, as well as beta titanium, a new material for orthodontics. Mechanical properties and manipulative characteristics are summarized to develop a basis for the selection of the proper alloy for a given clinical situation. The beta titanium wire has a unique balance of low stiffness, high springback, formability, and weldability which indicates its use in a wide range of clinical applications. A number of such applications are described.

Key words: beta, titanium, resilient, formable alloy

H istorically, relatively few metallic alloys have been used in the fabrication of orthodontic appliances. Although at one time gold was widely used for arch wires, in recent years austenitic stainless steel has been the mainstay of orthodontic wires. It has maintained its popularity because of a good balance of environmental stability, stiffness, resilience, and formability. Economic factors no doubt play a role in its wide acceptance in comparison to gold. Chrome-cobalt-nickel wires have also been used in appliance therapy. Although the mechanical properties of Elgiloy and stainless steel are similar, the

From the Department of Orthodontics and the Department of Restorative Dentistry, University of Connecticut School of Dental Medicine.

0002-9416/80/020121+12$01.20/0 @ 1980 The C. V. Mosby Co. 121

122 Burstone rind Goldberg

orthodontist can give the former a strengthening heat treatment which allows manipulation of the wire in a softened state. This can be followed by a hardening heat treatment to obtain the desired resilience. More recently Nitinol, a stoichiometric nickel-titanium alloy, has been introduced. This alloy has excellent springback characteristics and a low stiffness; unfortunately, its low formability limits applications where considerable bend- ing of an appliance is required.

It has been our aim to develop an orthodontic alloy which offers an over-all balance in superiority of properties over those that are currently in use. Although the properties required in an orthodontic wire will vary, depending upon its application, generally three characteristics are important for a superior wire. First, it should be possible for the wire to be deflected over long distances without permanent deformation; hence, a large springback. This ensures that the clinician can activate his appliances without permanent deformation, which assures better control over tooth movement and minimizes intervals for adjustment. Second, the wire should have a stiffness that is lower than that of stainless steel, which would allow wires to fill the bracket for control and at the same time produce lighter forces. Third, the wire should be highly formable, that is, capable of being easily shaped, bent, and formed into complicated configurations, such as loops, without fracture.

It is the purpose of this article to present the clinical applications of a new orthodontic alloy, beta titanium, a material which has an excellent balance of properties, including high springback, low stiffness, and high formability. In addition, it is a material that allows joining of components by direct welding without appreciably reducing the resil- ience of the material.

Characterization of orthodontic wires

To develop the perspective on the clinical applications of beta titanium, the charac- teristics of this wire will be compared to those of the other orthodontic alloys. This analysis will develop the rationale for the selection of the most effective wire for a given clinical situation, based on material properties per se and not aspects of the size and geometry of the wires.

Orthodontic wires can be classified according to chemical composition, microstruc- ture, or mechanical properties. The first two factors determine the third. It is important to remember that composition alone does not predetermine properties, since the mi- crostructural arrangement of the various components has a significant secondary influence.

Numerous physical and mechanical properties can be used to describe orthodontic wires. The intent of any such list is to characterize clinically significant parameters. Therefore, yield strength (YS) and modulus of elasticity (E) are important, not only because they are basic material properties which can be measured with standardized laboratory procedures but also because they are closely associated with appliance proper- ties. Springback, or maximum elastic deflection, is related to the ratio of YS/E. Higher springback values allow increased activation, which is always desirable, unless other properties such as formability are being sacrificed excessively. The force magnitude delivered by an appliance is proportional to the modulus of elasticity itself. Formability and resistance to fracture are important since most appliances require at least minor modification by the practitioner and many situations demand extensive bending and wire

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forming. The cold-bend test2 described in American Dental Association Specification 32 is indicative of a wires resistance to fracture during bending, but ease of forming is still probably best described in terms of practical experience with representative orthodontic pliers. The ability and ease of joining is an important clinical parameter, and, finally, the corrosion resistance of such joints and the wires themselves should be satisfactory.

Gold alloys. The composition of the alloys used in gold orthodontic wires is similar to the Type IV gold casting alloys, although there can be a wide variation between particular materials, 3 These alloys may contain as little as 15 percent gold, although 55 to 65 percent gold is more typical. The wires are alloyed with 11 to 18 percent copper, 10 to 25 percent silver, 5 to 10 percent palladium, 5 to 10 percent platinum, and 1 to 2 percent nickel and acquire additional strengthening through the cold-working incorporated during the wire- drawing process. These wires can potentially be strengthened with the proper heat treat- ment, although they are typically used in the as-drawn condition. The yield strength of wrought-gold wires can range from 50,000 to 160,000 p.s.i., depending on the alloy and condition, with corresponding elongations of 16 to 3 percent.3 The modulus of elasticity of gold-copper alloys is approximately 15,000,OOO p.s.i. This combination of properties makes gold very formable and capable of delivering lower forces than stainless steel. These features are very desirable; however, the commercial products typically have yield strengths in the lower end of the range, which limits springback. These wires are easily joined by soldering, and the joints are very corrosion resistant. The gold-wires have decreased usage in orthodontics because of their low yield strength and increasing cost.

Stainless steel. In the 1940s, austentic stainless steel began to displace gold as the primary alloy for orthodontic wires. The most commonly used types are AISI 302 and 304 stainless steels, which contain approximately 18 percent chromium, 8 percent nickel, and less than 0.20 percent carbon.4 The type 304 stainless steel has a slightly lower carbon and higher chromium specification. These alloys derive most of their strength from cold- working and carbon interstitial hardening. The microstructure demonstrates the typical fibrous appearance associated with extensively elongated grains. This microstructure can be altered by short exposures to high temperatures, which is why soldering procedures have to be undertaken carefully. The only heat treatments used with this wire are for stress relieving, which is typically done at 850 F. (454 C.) for less than 10 minutes.5 These wires have very high yield strengths of 50,000 to 280,000 p.s.i. Slightly higher yield strengths are possible; however, this can lead to brittleness. Reports of the modulus of elasticity of orthodontic stainless steel wires range from 23,000,OOO to 29,000,OOO p.s.i. with values closer to 23,000,OOO p.s.i. than to the higher values usually given.6 The high modulus necessitates the use of smaller-diameter wires for alignment procedures where lower forces are indicated. Unfortunately, decreased wire size results in poorer fit in the bracket and loss of control. The ratio of yield strength to modulus indicates that stainless steel wire can have slightly greater springback properties than gold. In general, stainless steel has excellent formability, although the wires with higher yield strength may be somewhat brittle. Stainless steel can be soldered, albeit the technique is moderately demanding. Finally, these wires have good corrosion resistance, although the solder joints do corrode in the oral cavity. Now this does not imply that stainless steel is unsatisfactory. On the contrary, for particular treatment modalities, it may be the wire of choice. An attempt is made, however, to recognize that all materials have limitations, and the criteria for selection should be the over-all balance of properties and the specific clinical applica-

124 Burstone and Goldhrrg

Table I. Springback* of beta titanium and stainless steel wire (degrees)

Mode t Stainless steel $ Beta titanium 8 Percent increase

A 16.0 32.8 105% B 16.5 31.3 90% C 17.5 37.3 113%

*0.017 by 0.025 inch wires, /4 inch span lengths, deflected to 60 degrees and released. t Modes described by Lopez, Goldberg, and Burstone. I0 $Chrome alloys, Ormco Corp., Glendora, Calif. TMA, Ormco Corp., Glendora, Calif.

tion. The combined adequate springback, good formability, and moderate cost account for the popularity of stainless steel as an orthodontic arch wire.

Efgiloy. * Elgiloy, a cobalt-chromium-nickel alloy, offers the potential for significant changes in mechanical properties with the appropriate heat treatment. This alloy, with a nominal composition of 40 percent cobalt, 20 percent chromium, 15 percent nickel, 7 percent molybdenum, and 16 percent iron, has excellent formability in the soft condition and can be heated to 480 C. to obtain strength properties comparable to stainless steel.7 The modulus of elasticity of these wires is 28500,000 to 30,000,OOO p.s.i. Thus, the eventual spring characteristics can be similar to those of a stainless steel appliance. In addition to solid solution hardening and cold working, these systems are amenable to precipitation hardening, which is the mechanism responsible for effective heat treatment. Elgiloy can be soldered, but, as with stainless steel, the technique is demanding. Corro- sion resistance of the wire is excellent.

Nitinol. t Several years ago a stoichiometric nickel-titanium alloy was introduced for orthodontic application.* Nitinol is approximately 52 percent nickel, 45 percent titanium, and 3 percent cobalt. Solid-state solution hardening and cold working are the basic strengthening mechanisms employed with this alloy. With proper heat treatment, the alloy demonstrates significant changes in mechanical properties and crystallographic arrange- ment. The latter phenomenon is responsible for the noted memory effect of this material; however, such transformations are currently not employed in clinical practice. Because of a low modulus of elasticity of 4,800,OOO p.s.i., combined with a tensile strength of 240,000 p.s.i., this wire can sustain large elastic deflections, as reflected in its very high springback characteristics.g This feature makes the wire desirable for applica- tions where large deflections and low forces are required. Nitinol has limited formability, which contraindicates its use for situations where bends with a small radius are required. Furthermore, springback properties are decreased after bending. lo Nitinol is not amenable to joining operations. Sarkar and associates reported that nitinol is somewhat less CO~TO- sion resistant than the other orthodontic wires, although the clinical significance of this difference has not yet been established.

Beta titanium. Beta titanium is the newest alloy to be introduced to the orthodontic profession. Titanium has been used as structural metal since 1952, and its possible use in orthodontics has been suggested periodically. The lack of success of such an application until now can be explained by the springback characteristics and chronologic development

*Rocky Mountain Orthodontics, Denver, Cola. tUnitek Corporation, Monrovia, Calif.

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Fig. 1. Beta titanium arch wire used for detailed alignment of upper arch. Ductility allows placing of bends and tie-back loop. Cross section 0.018 by 0.025 inch. Force produced is less than half that of stainless steel, allowing light and oriented forces for finishing.

of titanium metallurgy. To compete with stainless steel, a wire must possess at least comparable formability and springback, which is proportional to the ratio of yield strength to modulus of elasticity (YS/E). This ratio for typical stainless steel orthodontic wire is approximately 1.1 x 10m2, as it is for some of the gold-based and cobalt-chromium- nickel alloys. The early industrial applications of titanium employed commercially pure material (99.2 percent titanium). At temperatures below 1,625 F. this metal has a hexa- gonal close-packed (HCP) crystal form, with room-temperature modulus and yield strength values of 15.5 x lo6 p.s.i. and 55 x IO3 p.s.i., respectively.* The ratio of these values is 0.35 x IO-, which would imply that an appliance constructed from pure titanium would have only one third the maximum elastic deflection of a comparable stainless steel appliance. The second phase of titaniums chronology saw the development of titanium alloys, but still based on the HCP structure. Ti-6Al-4V, a representative commercial alloy, has a YS/E ratio of 0.87 x lo-*, still below stainless steel. In the 1960s an entirely different high-temperature form of titanium alloy became available. At temperatures above 1,625 F. pure titanium rearranges into a body-centered cubic (BCC) lattice, referred to as the beta phase. With the addition of such elements as molybdenum or columbium, a titanium-based alloy can maintain its beta structure even when cooled to room temperature. Such alloys are referred to as beta-stabilized titaniums. The alloying and body-centered cubic structure impart a unique set of properties.

Goldberg and Burstone13 demonstrated that, with the proper processing of an 11 percent molybdenum, 6 percent zirconium, and 4 percent tin beta titanium alloy, it is possible to develop an orthodontic wire with a modulus of elasticity of 9,400,OOO p.s.i. and yield strength of 170,000 p.s.i. The resulting YS/E ratio of 1.8 x IO-* is superior to 1.1 X lo-* for stainless steel.

TO demonstrate springback characteristics, a usage test was performed using a Tinius Olsen stiffness tester.* Wires of % inch span were deflected 60 degrees and the amount of springback was measured. Three modes were tested: (1) straight wires, (2) wires with a 35

*Tinius Olsen Testing Machine Company, Philadelphia, Pa.

126 Burstonc and Goldberg

Fig. 2. Alignment loops in 0.016 inch beta titanium continuous arch. A, L vertical and helical loops. B, 7 loops and vertical loop tie-back.

degrees bend, and (3) wires which were overbent to 90 degrees and then bent back to 35 degrees. The three modes of testing were reported in detail previously.iO Data comparing 0.017 by 0.025 inch steel and beta titanium* are found in Table I. Note that beta titanium in straight-wire applications can be deflected 105 percent more than stainless steel without permanent deformation. In applications where bends are placed, if overbending is done the increase is 113 percent.

The modulus of elasticity of beta titanium is approximately twice that of nitinol and less than one half that of stainless steel. Its stiffness makes it ideal in applications where less force than steel is required but where lower modulus materials would be inadequate to develop required force magnitudes.

It has been shown that the formability of the beta titanium orthodontic wire, as measured by the ADA cold-bend test, is similar to that of stainless steel.13 However, the titanium alloy cannot be bent over as sharp a radius as stainless steel, so that some care in the selection of pliers and bending procedures is required. The beta titanium wire can be joined by welding alone and has good corrosion resistance.

In summary, the beta titanium wire possesses a unique balance of high springback and formability with low stiffness, making it particularly suitable for a number of treatment modalities as explained below.

*TMA, Ormco Corporation, Glendora, Calif.

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IFig. 3. Vertical loop segment formed in 0.0175 by 0.025 inch wire. Large deflection and low forces are Iused to rotate and change premolar axial inclination.

Clinical application

Because of its unique and balanced properties, beta titanium wire can be used in a number of clinical applications. Ideal edgewise arches fabricated of titanium have sig- nificant superiority over stainless steel. They can be deflected approximately twice as far without permanent deformation, which allows a greater range of action for either initial tooth alignment or finishing arches. The forces that are produced are approximately 0.4 that of steel, producing a more gentle delivery of forces with an edgewise wire; for example, an 0.018 by 0.025 inch wire in beta titanium delivers about the same force as an 0.014 by 0.020 inch steel wire when activated in a second-order direction. Furthermore, it would have the advantage of full bracket engagement and third order or torque control if used in an 0.018 inch slot bracket. Beta titanium is ductile, which allows for placement of tie-back loops or complicated bends. Springback properties are not lost during the bending operation, and complicated configurations can be placed if needed.

In Fig. 1, an 0.018 by 0.025 inch beta titanium arch has been placed in brackets with 0.022 inch slots for the purpose of finishing. The use of an undersized wire in an 0.022 inch slot allows some degree of freedom, particularly in a third-order direction, which can be highly desirable because of the ease of fabrication of the arch and its efficiency of action. The high ductility and formability of the titanium allowed the placement of a vertical loop tie-back mesial to the first molar as well as finishing bends with the arch.

The high ductility of beta titanium allows it to be formed into arches or segments with complicated loop configurations. A continuous arch with T, vertical, helical, and L loops, formed in a small round wire, is shown in Fig. 2. In many applications, loop placement can better deliver the desired force system without side effects than straight continuous wires. One of the advantages of beta titanium, as used in loop configuration, lies with loop incorporation in larger cross sections of edgewise wire which allow the loop tlo be positively oriented within the brackets. In Fig. 3, an 0.018 by 0.025 inch wire with a vertical loop formed into a buccal segment is being used to rotate the premolar and change its axial inclination; note the large activation possible as low forces are produced without loss of orientation of the wire within the brackets. A more complicated configuration is

Fig. ,4. Rectangular loop with helices to rotate distal aspect of canine buccally. A, Pas B,l ried in place. Loop shown following completion of canine rotation.

si ve configuration.

Fig. 5. Intrusive arch. A, Passive. B, Activated to produce 60 grams. The propertie inate the need for placing helices mesial to the molar tube.

!S of beta titanium

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Fig. 6. Canine root spring with helices. A, Passive. 6, Active. Spring can be activated 180 degrees without permanent deformation.

shown in Fig. 4, where helices have been placed at the anterior end of a rectangular loop for rotation and alignment of a canine. Ninety degrees of rotation has been built into the loop relative to the canine bracket in its rotated position.

Specialized springs or auxiliaries fabricated from beta titanium allow for simplification in design in achieving identical force delivery. For example, with a base arch used for intrusion, helices can be placed immediately mesial to an auxiliary tube at the first molar to lower the load-deflection rate. A base arch of beta titanium is shown in Fig. 5. The low load-deflection rate produced by the low modulus of elasticity and the high springback allows a 12 mm. activation to produce 60 Gm. of force in the midline without the placement of helices posteriorly, thereby simplifying the design.

A root spring incorporating helices identical in design to steel but fabricated of titanium has almost 180 degrees of activation, in comparison to 90 degrees, which ensures a more constant delivery of the moment moving the root distally on the canine (Fig. 6). If a straight wire were used for root movement, although not as efficient as the foregoing mechanism, 120 degrees of activation could be incorporated in an 0.018 by 0.025 inch wire before permanent deformation would occur. Two wire segments, one of steel and the other of beta titanium, were fitted within a 13 mm. distance between an auxiliary tube on the first molar and the canine bracket. As seen in Fig. 7, after removal, no permanent deformation is seen in the beta titanium wire, whereas the steel wire has been deformed 62 degrees.

Fig. 7. Canine root spring without helices. An activation of 120 degrees was placed and then tied between the first molar and the canine. Upper steel wire demonstrated 62 degrees of permanent deformation. Lower beta titanium exhibited negligible deformation.

Fig. 6. High formability of beta titanium allows fabrication of closing loops of intricate design. Forces delivered are less than one half those of steel.

The high formability of titanium allows the fabrication of closing loops with or without helices. The low stiffness of the material and its high springback improve a loop of any given design or allow for the maintenance of a given force system with simpler designs, as in the elimination of helices or loops. Some typical closing loops are shown in Fig. 8.

Beta titanium is unique in comparison to commonly used orthodontic wire in that it allows direct welding of auxiliaries to an arch wire without reinforcement by soldering. Using a light-capacitance weld, a smaller cross section of titanium can be welded directly to the main arch-on-arch segment. An 0.014 inch tie-back and intermaxillary hook are shown in Fig. 9. Finger springs and other auxiliaries of an active nature can also be welded directly to an arch wire. Fig. 10 shows a helical finger spring which can be used to erupt an impacted canine. The distal portion of the spring has been folded upon itself to increase the welding area, and a light-capacitance weld was used. The welding has not appreciably altered the mechanical properties of the spring, and it can be activated a full 90 degrees without any permanent deformation. Welding should be performed with care.

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Beta titanium 131

Fig. 9. Hooks and tie-backs can be directly welded without solder reinforcement.

Fig. 10. Helical finger spring, 0.014 inch round, welded directly to 0.018 by 0.025 inch continuous arch. No solder is needed. Weld does not soften the spring wire as with stainless steel. A, Passive. 6, Active.

Unlike steel, where too much heat will produce softness in the wire, overheating of titanium could lead to brittleness of an energy-imparting finger spring.

Summary

Beta titanium is a new orthodontic alloy with unique properties and an excellent balance of properties suitable for many orthodontic applications. For a given cross sec- tion, it can be deflected approximately twice as far as stainless steel wire without perma- nent deformation. It delivers force values less than half that of stainless steel, which allows the orthodontist to fabricate arch wires which fill the bracket and yet deliver forces associated with smaller cross sections of stainless steel. Beta titanium is highly ductile, which allows it to be formed into complicated configurations when required. Wires can be directly welded together without appreciable lossess in mechanical properties, which simplifies the placement of stops, intermaxillary hooks, and active auxiliaries such as

finger springs. For many years stainless steel or chrome-cobalt (Elgiloy) were the two major arch wire materials used in orthodontics. Both of these materials have approxi- mately the same modulus of elasticity; hence, identical configurations with the same activation would deliver approximately identical forces. As new alloys, such as nitinol and now beta titanium, are being introduced into orthodontics, the clinician should be aware of the differences in the modulus of elasticity between the alloys. Wire materials are no longer interchangeable. If one selects a high-springback alloy, such as beta titanium, a larger cross section is needed to deliver identical forces using the same activation and configuration. For common applications, edgewise steel wires invariably deliver more force than is required; hence, replacement of steel edgewise wires with beta titanium should improve the force system. On the other hand, if small round wires are used in alignment procedures, it would be advisable to use larger cross sections of titanium so that optimal force magnitudes are produced and the play between arch wire and bracket is reduced.

Beta titanium not only offers an improvement in the properties of presently designed orthodontic appliances with its increased springback, reduced force magnitudes, good ductility, and weldability, but its excellent balance of properties should permit the design of future appliances which deliver superior force systems with simplified configuration.

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orthodontic concepts and techniques. I, ed. 2, Philadelphia, 1975, W.B. Saunders Company, pp. 23-258. 2. Council on Dental Materials and Devices, American Dental Association: Specification No. 32 for orthodon-

tic wires not containing precious metals, I. Am. Dent, Assoc. 95: 1169-l 171, 1977. 3. Craig, R.G., and Peyton, F.A. (editors): Restorative dental materials, ed.5, St. Louis. 1975. The C.V.

Mosby Company, pp. 298-340. 4. Phillips, R.W.: Science of dental materials, ed. 7, Philadelphia, 1973, W.B. Saunders Company, p. 643. 5. Marcotte, M.R.: Optimum time and temperatures for stress relief heat treatments of stainless steel wire, J.

Dent. Bes. 52: 1171-1175, 1973. 6. Goldberg, A.J., Vanderby, R., Jr., and Burstone, C.J.: Reduction in the modulus of elasticity in orthodon-

tic wires, J. Dent. Res. 56: 1227-1231, 1977. 7. Elgiloy, Rocky Mountain Orthodontics Brochure, Denver, Colo., 1977. 8. Anreasen, G.F., and Hilleman, T.B.: An evaluation of 55 cobalt substituted nitinol wire for use in

orthodontics, J. Am. Dent. Assoc. 82: 1373-1375, 1971. 9. Andreasen, G.F., and Morrow, R.E.: Laboratory and clinical analyses of nitinol wire, AM. J. ORTHOD. 73:

142-151, 1978. 10. Lopez, I., Goldberg, J., and Burstone, C. J.: Bending characteristics of nitinol wire, AM. J. ORTHOD. 75:

569-575, 1979. 11. Sarkar, N.K., Redmond, W., Schwaninger, B.M., and Goldberg, A.J.: The chloridecorrosion behavior of

four orthodontic wires, J. Dent. Res. 58: A98, 1979. 12. Titanium and titanium alloys. In Metals Handbook, ed. 8, Metals Park, Ohio, 1975, American Society for

Metals, vol. 1, pp. 1147-1156. 13. Goldberg, A.J., and Burstone, C.J.: An evaluation of beta titanium alloys for use in orthodontic appliances,

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