Bending characteristics of nitinol wire
`
`I. Lopez,* J. Goldberg,** and C. J. Burstone* Farmington. Conn.
`
`Until recently only a limited group of alloys have been used for orthodontic
`wires, namely, gold-coper, austenitic stainless steel, and chromium-cobalt-nickle (Cr(cid:173)
`Co-Ni) based materials. In general, the last two groups have similar mechanical properties
`although processing, stress·relieving, heat-treating, and minor alloying adjustments allow
`for small product differences. On the other hand, the gold-based wires have a modulus of
`elasticity approximately one half that of the other two groups. Unfortunately, the yield
`strength of gold alloys is less by approximately the same factor, which gives them a ratio
`of yield strength to modulus of elasticity that is nearly the same as that of stainless steel
`and Cr-Co-Ni alloys. Since springback or resilience is proportional to this ratio, all three
`categories of alloys are approximately the same in regard to this clinical characteristic.
`When used in orthodontic appliances, wires with low moduli of elasticity in combina(cid:173)
`tion with high resilience aid in delivering clinically desirable low continuous forces and
`increased working time. 1 It is these benefits, coupled with technologic advances in metal(cid:173)
`lurgy and wire processing, which have promulgated the introduction of several low(cid:173)
`stiffness, high-springback orthodontic wires. Braided wires use traditional stainless steel
`but achieve their improved properties through unique design of the wire's cross section. A
`second approach to obtaining "high-deflection" wires is through the use of novel alloys,
`the most notable being nitinol, a nickel-titanium alloy. 2
`Nitinol was developed by William F. Buehler in the early 1960's. The original alloy
`contained 55 percent nickel and 45 percent titanium, which resulted in a one-to-one
`stoichrometric ratio of these elements. The most unique feature of this NiTi intermetallic
`compound is the "memory" phenomenon, which is a result of temperature-induced
`crystallographic transformations.~ Andreasen-! suggested that these shape changes might
`be used by the orthodontist to apply forces. This memory principle is not used clinically,
`although it would appear plausible.
`The commercially available nitinol orthodontic wire contains 1. 6 percent cobalt to
`modify the transition temperature and mechanical properties. Even without the "mem(cid:173)
`ory" effect, the unusually low modulus of elasticity of 4.8 x 106 p.s.i. and high resil(cid:173)
`ience offer desirable features to the orthodontist. Using a clinical model, Andreasen and
`Barrett5 demonstrated that nitinol had a lower stiffness than stainless steel and could be
`deflected further without permanent deformation when tied into malaligned brackets.
`Andreasen and Morrow6 have evaluated the bending characteristics and spring rate of
`
`From the University of Connecticut Health Center, School of Dental Medicine.
`This study was supported in part by Research Grant DE03953 from the National Institute of Dental
`Research, National Institutes of Health. Bethesda. Md.
`*Department of Orthodontics.
`** Department of Restorative Dentistry.
`
`0002-9416/79/050569+07$00.70/0 © 1979 The C. V. Mosby Co.
`
`569
`
`GOLD STANDARD EXHIBIT 2042
`US ENDODONTICS v. GOLD STANDARD
`CASE PGR2015-00019
`
`

`
`570 Lope;, Goldberg. and Burstone
`
`"lrn
`
`j Orthod.
`'''"'. Jn~
`
`straight sections of nitinol wire and have demonstrated ih w,c in Class I. Clas~ II. and
`Class III malocclusions.
`Although nitinol cannot be formed over a sharp edge, tir-.,t- and second-order bends
`can be placed in the wire. Because of its unusual metallurgic structure, however. it cannot
`be assumed, a priori. that bent-wire appliances have the same properties as unbent wires.
`Indeed, even stainless steel demonstrates differences in properties after a bend is placed.
`depending on the direction of subsequent loading. Placement of permanent bends in wires
`and activation in the opposite direction is clinically important in all orthodontic tech(cid:173)
`niques, since straight wires must be modified for exact finishing detail and to deliver the
`necessary forces during treatment.
`The purpose of this study was to characterize further the bending characteristics of
`nitinol wire. In addition to straight-wire sections, standardized bends were also evaluated.
`Finally, the time dependence of these measurements was considered.
`
`Materials and methods
`
`Nitinol* and a standard austenitic stainless steel wire, t each 0.18 inch in diameter,
`were evaluated in three different modes. Permanent deformation versus reflection charac(cid:173)
`teristics of straight-wire sections were measured with a Tinius Olsen stiffness tester:j: by
`the procedure outlined in the new ADA specification No. 32 for orthodontic wires.; Three
`samples of each wire were tested. A 0.25 inch span length was used to minimize the
`length of wire being fed into the testing span during bending. This mode of testing is
`illustrated schematically in Fig. I, A.
`A second series of bending tests was made on straight sections. but instead of the
`"instantaneous" loading conditions used in the r~ocedure referred to above, samples were
`loaded until 60 degrees of deflection had occurred and were maintained for 0, I. 5, 20, 40,
`or 60 minutes and then released. A Tinius Olsen tester was again used, and permanent
`deformation was measured for three different samples at each of the above-listed times,
`for a total of eighteen samples per material. This experiment was repeated, but the same
`three samples were reused at each of the indicated times, for a total of three samples per
`material.
`The third set of tests considered the permanent deformation versus deflection charac(cid:173)
`teristics of nitinol and stainless steel wire after permanent bending. Samples were first
`given a permanent bend of 35 degrees. Care was taken to bend in only one direction, and a
`template was used to ensure reproducibility. The samples were then gripped in the Tinius
`Olsen stiffness tester with the initial permanent bend 0.5 mm. from the edge of the vise.
`Standard test procedures were then followed, with the force being applied in a direction
`opposite that of the initial bend, as shown in Fig. I, B. In another series of tests,
`schematically represented in Fig. I. C, the wires were first permanently deformed to 90
`degrees (P 1) and then returned to the 35 degree permanent bend (P2 ). Samples were then
`tested in the same direction as the last bend. P2 • In Fig. I. F indicates the direction of
`loading used during the test.
`
`*Unitek Corporation, Monrovia, Calif.
`tStandard Permachrome, Unitek Corporation, Monrovia, Calif.
`tTinius Olsen Testing Machine Corporation, Philadelphia, Pa.
`
`

`
`Volume 75
`Number 5
`
`Bending characteristics of nitinol wire 571
`
`'' --r(cid:173)
`' ' ~,) P=35"
`' ' ' ' ') • F
`
`Fig. 1. The three modes of testing. P indicates the direction of bending of the configuration before
`loading, which is represented by dashed lines. In C, P, and P2 show change of loading direction to
`produce preload configuration. F shows the direction of loading used during the test.
`
`Results
`
`Test results for straight-wire sections are shown in Fig. 2, where permanent deforma(cid:173)
`tion is plotted as a function of deflection for the stainless steel and nitinol wires. The
`ability of the nickel-titanium alloy to undergo greater elastic deflections is clearly demon(cid:173)
`strated. At any given activation, the nitinol experiences less permanent set. For wires
`0.018 inch in diameter, the nitinol sustained approximately 23 degrees of activation and
`stainless steel 13 degrees before any permanent deformation was evident. These results
`are a further documentation of the flexibility and elasticity of nitinol described in earlier
`5
`6
`works. 2
`•
`•
`The behavior of these wires after the introduction of permanent bends is characterized
`in Fig. 3. The data from Fig. 2 have been replotted for comparison. The most notable
`feature of this figure is the large change in nitinol when tested in different modes. With
`respect to bending of straight sections (Fig. 1, A), nitinol demonstrates superior elastic
`properties as compared to stainless steel. However, when NiTi alloy is tested in a direction
`opposite to a permanent bend (Fig. 1, B), there is a considerable loss of elastic behavior.
`In fact, for activations of less than 40 degrees, nitinol exhibits greater permanent defor-
`
`

`
`572 Lope;, Go/dber;;. and Burst ow
`
`1 t'l J Orthod
`AfuY 1979
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`Fig. 2. Permanent deformation versus deflection for straight-wire sections, mode A in Fig. 1.
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`Fig. 3. Permanent deformation versus deflection for the three modes of testing illustrated in Fig. 1.
`
`marion than stainless steel, their relative performance switching for activations larger than
`50 degrees. When the nitinol wire was bent to 90 degrees and returned to 35 degrees prior
`to testing (Fig. I, C). it again demonstrated the desirable low permanent deformation,
`although its performance was not comparable to the straight-wire test.
`The magnitude of change associated with testing of stainless steel in different modes
`was not nearly as large as that of the NiTi alloy. The changes which did occur, however.
`document the desirability of overbending stainless steel. Even with small activations of 10
`to 35 degrees, stainless steel in straight and overbent sections showed less permanent
`deformation than the samples tested in a direction opposite the last permanent bend.
`The time dependence of nitinol and stainless steel is illustrated in Fig. 4, where
`
`

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`Volume 75
`Number 5
`
`Bending characteristics of nitino/ wire 573
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`43
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`42
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`41
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`40
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`20
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`19
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`17
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`
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`
`40
`
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`
`Fig. 4. Permanent deformation versus length of time samples held at a 6Q-degree bend. x-x-x repre(cid:173)
`sents data obtained with three independent samples at each time interval. · - · - · represents data
`obtained with the same three samples reused at each time interval. Range of values obtained at each
`data point is indicated.
`
`permanent deformation is plotted as a function of the length of time the samples were
`maintained at a 60-degree bend. Two curves are shown for each material. The lines
`labeled "independent samples" resulted from three replications at times of 0, I, 5, 20,
`40, and 60 minutes, new specimens being used for each test. Each line labeled "reused
`samples" was obtained with a total of three samples, each one being retested at the time
`intervals indicated.
`The unexpected result was the time dependency of nitinol, even when independent
`samples were used. The nitinol experienced approximately 5 degrees of additional perma(cid:173)
`nent set over 60 minutes, most of which occurred within the first 5 minutes. A one-way
`
`

`
`574 Lopez. Goldberg. and Burstone
`
`Am }. Orthod
`Mal' 1479
`
`Fig. 5. Photograph showing nitinol on top and stainless steel wire directly underneath. A 35-degree
`permanent bend was placed in each wire (testing mode B). Each wire was then deflected 20 degrees in
`a direction opposite the original permanent bend and released. The nitinol wire experienced 5 degrees
`more permanent deformation.
`
`analysis of variance of the nitinol independent samples indicated a statistically significant
`difference (p < 0.05) between the various test times. A regression analysis of the same
`data resulted in the equation
`
`D =
`
`10 + 2.02t 2
`
`where D is permanent deformation in degrees and t is time in minutes. The average
`permanent deformation of three nitinol samples tested for a period of 48 hours was 21
`degrees, which compared favorably with the calculated value of 19.9 degrees.
`As one might anticipate, the recycled samples experienced even larger increases in
`permanent deformation, as the cold-working was apparently cumulative in its effect.
`
`Discussion
`
`The superior elastic behavior of nitinol wire as compared to conventional orthodontic
`stainless steel wire has been clearly verified with straight-wire sections. Cantilever bend(cid:173)
`ing of such specimens of nitinol up to 30 degrees caused insignificant permanent deforma(cid:173)
`tion, while a comparable deflection of the stainless steel resulted in a permanent set of
`approximately 10 degrees.
`Comparable superiority cannot be expected, however, when the wires experience
`more elaborate manipulation. When permanently deformed and activated in an opposite
`direction, nitinol actually undergoes more permanent deformation than stainless steel for
`activations of less than 40 degrees. An example is shown in Fig. 5. Clinically, if it is
`necessary to deform nitinol wire permanently, it should be overbent and permanently
`deformed in the direction that the appliance will ultimately be activated. This technique is
`similar to that recommended for stainless steel. 1 Even with this technique, however, the
`clinician must recognize that nitinol, although superior to stainless steel, does not have the
`degree of superiority demonstrated by comparisons of straight unbent wire sections.
`Nitinol appears to experience an unexpected time-dependent relaxation phenomenon.
`Increases in permanent deformation were small when independent samples were evaluated
`for periods of up to 60 minutes. However, the tests conducted for 48 hours indicated that
`the relaxation phenomenon continues for at least this time period. Furthermore, samples
`with a history of prior permanent bends appear to experience this relaxation effect to an
`
`

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`Volume 75
`Number 5
`
`Bending characteristics of nitinol wire 575
`
`even greater extent. For the test conditions used in the present study, the permanent
`deformation of the reused samples doubled in just 60 minutes, from 10 degrees to 20
`degrees (Fig. 4). This would suggest that repeated bending of the wire should be avoided.
`If springback (maximum elastic deflection) rather than permanent deformation is used for
`comparison, nitinol exhibits 2.5 times the springback of stainless steel wire when loading
`is instantaneous. However, if the load is maintained over 48 hours, which is more
`applicable to the clinical situation, the superiority in springback is reduced to a factor of
`2.0. Further research is needed to fully characterize the extent and cause of the time(cid:173)
`dependent behavior.
`The unorthodox mechanical behavior of nitinol is associated with the unusual metal(cid:173)
`lurgic structure of this nickel-titanium intermetallic compound. This same structure is also
`responsible for the memory effect and the favorable modulus of elasticity. As with any
`new dental material, the clinician must recognize its attributes and deficiencies in order to
`use it to full advantage in the clinical setting. Bending, time dependency, and the cumula(cid:173)
`tive effects of cold working can all have deleterious effects on the elastic characteristics of
`nitinol. With proper appreciation and understanding of these manipulative variables,
`however, the orthodontist can fully exploit the advantageous features of this new alloy.
`
`REFERENCES
`I. Burstone, C. J .. Baldwin, J. J., and Lawless, D. T.: The application of continuous forces to orthodontics,
`Angle Orthod. 31: l-14, 1961.
`2. Andreasen, 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.
`3. Buehler, W. J., and Wang, F. E.: A summary of recent research on the nitinol alloys and their potential
`application in ocean engineering, Ocean Eng. 1: 105-120, 1968.
`4. Andreasen, G. F., and Brady, P. R.: A use hypothesis for 55 nitinol wire for orthodontics, Angle Orthod. 42:
`172-177, 1972.
`5. Andreasen, G. F., and Barrett, R. D.: An evaluation of cobalt-substituted nitinol wire in orthodontics, AM. J.
`0RTHOD. 63: 462-469, 1973.
`6. Andreasen, G. F., and Morrow, R. E.: Laboratory and clinical analyses of nitinol wire, AM. J. 0RTHOD. 73:
`142-151, 1978.
`7. Council on Dental Materials and Devices: American Dental Association specification No. 32 for orthodontic
`wires not containing precious metals, J. Am. Dent. Assoc. 95: 1169-1171, 1977.