American Journal of ORTHODONTICS Founded in 1915 Volume 81 Number 6 June, 1985

Copyright 0 1985 by The C. V. Mosby Company

ORIGINAL ARTICLES

Chinese NiTi wire-d new orthodontic alloy Charles J. Burstone, D.D.S., MS.,* Bai Qin, MS.,** and John Y. Morton, B.S.* Farmington, Corm., and Beijing, China

Dr. Burstone

Chinese NiTi wire was studied by means of a bending test to determine wire stiffness, springback, and maximum bending moments. Chinese NiTi wire has an unusual deactivation curve (unlike steel and nitinol wires) in which relatively constant forces are produced over a long range of action. The characteristic flexural stiffness of NiTi wire is determined by the amount of activation. At large activations NiTi wires has a stiffness of only 7% that of a comparable stainless steel wire, and at small activations 28% of steel wire. For the same activation at large deflections, the forces produced are 38% that of a comparable nitinol wire. Chinese NiTi wire demonstrates phenomenal springback. It can be deflected 1.8 times as far as nitinol wire or 4.4 times as far as stainless steel wire without appreciable permanent deformation. NiTi wire is highly useful in clinical situations that require a low-stiffness wire with an extremely large springback.

Key words: Nickel, titanium, stiffness, bending moment, springback

T he first nickel-titanium orthodontic alloys were introduced to the profession by Andreasen. They are known as Nitinol wirest and are based on the orig- inal research of Buehler,, 3 who developed specialized nickel-titanium alloys that have unique shape-memory characteristics. Although Andreasen has suggested that the shape-memory effect of nitinol wire could be useful and has carried out experimentation to demonstrate this possibility, nitinol wire has won wide clinical accep- tance because of its high springback and its low stiff- ness, rather than its thermal characteristics.4.

A new nickel-titanium alloy has been developed especially for orthodontic applications by Dr. Tien Hua Cheng and associates at the General Research Institute for Non-Ferrous Metals in Beijing, China. This alloy

This research was supported by NIHiNIDR Grant DE023953. *Department of Orthodontics. School of Dental Medicine, University of Con- necticut Health Center. **Beijing Medical College, Beijing, China. Wnitek, Monrovia, Calif.

has unique characteristics and offers significant poten- tial in the design of orthodontic appliances. Its history of little work hardening and a parent phase which is austenite yield mechanical properties that differ signif- icantly from nitinol wire. In addition, Chinese NiTi wire has a much lower transition temperature than nitinol wire.

It is the purpose of this article to describe the me- chanical properties of Chinese NiTi wire with particular reference to its orthodontic applications. Since stainless steel is the most commonly used arch wire material, NiTi wire will be compared to both stainless steel and nitinol wires in contrasting their mechanical properties.

METHOD

Nominal 0.016-inch stainless steel,* nitinol, and Chinese NiTi wires were submitted to a flexural test using a cantilever configuration. A torque gauge ap-

*Unitek, Monrovia, Calif.

445

446 Burstone, Qin, and Morton

Fig. 1. Apparatus for cantilever bending test. Force is always normal to the free end of the wire. The torque gauge measures the moment. The magnitude of the activation is measured by the protractor.

paratus was used to apply an angular deflection to the wires at the fixed ends. The angular deflection of the wires at this support was measured with a protractor. The couple necessary to create the angular displacement was measured by the torque gauge. The couple was resisted by a force at the free end through an anvil placed against the wires. The force remained normal to the wires throughout the range of activation. The ap- paratus is shown in Fig. 1. Two torque gauges were used, depending on the magnitude of the moments to be recorded. Ranges were 0 to 800 gm-mm and 0 to 6,000 gm-mm with an accuracy of 2% of full scale. The low stiffness of the wires required a .5-mm span length instead of the IO-mm span used in a previous study.6 Angular displacements to 80 were used. For each data point of the curve, at least three separate wires were measured. At critical points of the loading and unloading curve-particularly where a marked change in slope would occur-up to eleven separate wires were measured to further define the shape of the curve.

The basic study was carried out with instantaneous loading at room temperature. In addition, in a similar study the temperature was varied. Temperatures of 22 C, 37 C (mouth temperature), and 60 C were used. Time-dependent effects were also studied.

Fig. 2. Bending moment/deflection characteristics of stainless steel, nitinol, and NiTi wires. Both loading (activation) and un- loading (deactivation) curves are shown. NiTi wire produces lower moments and forces than nitinol wire.

To establish the moment of yield, wires were cycled through a loading and unloading sequence until 1 of permanent deformation was recorded.

RESULTS

It is useful to compare the properties of the Chinese NiTi wire with both stainless steel and nitinol wires. Three wire characteristics will be described: (1) the springback (the range of action of the wire), (2) stiffness (the force or moment produced for each unit activation), and (3) the maximum moment (the largest bending cou- ple that a wire is capable of delivering).

Springback

The moment deflection characteristics of the stain- less steel, nitinol, and Chinese NiTi wires are shown in Fig. 2. The amount of springback is defined here as the difference between the deflection (activation) of 80 and the residual deformation after unloading to 0 gm- mm. Based on the 80 activation, the springbacks for 0.016 inch wires are 16 for steel, 52 for nitinol, and 73 for Chinese NiTi. Chinese NiTi wire has 1.4 times the springback of nitinol wire and 4.6 times the spring- back of stainless steel wire for 80 of activation; at 40 of activation NiTi wire has 1.6 times the springback of nitinol wire.

Stiffness

A bending test was used to evaluate the moment- angular deflection characteristic of the wires. The stiff- ness was determined from the unloading curve, which is analogous to clinical use.

The clinician is interested in the amount of force or moment produced for any given deflection. This prop- erty of the material is its stiffness. In this study stiffness is measured as induced bending moment per degree of

Volume 87 Number 6

Chinese NiTi wire 447

- - - - - Daactiallc.

moo-

E ISOO-

d E E

I IOOO- @ 5

s

SW-

0 20 40 60 w

Fig. 3. Activation and deactivation curves for nitinol wire. The average unloading stiffness (straight line fit by linear regression) is the same for all activations.

deflection. Stainless steel and beta-titanium exhibit ap- proximately linear relations between moment and de- flection during unloading; hence, a single constant can describe the relationship.

The nickel-titanium alloys, particularly NiTi, ex- hibit nonlinear relationships between bending moment and angular deflection. Therefore, a single constant does not give an adequate measure of wire stiffness.

In Fig. 2 the loading (activation) curve is shown for stainless steel wire, with an initial linear and a nonlinear portion. As the wire returns to a passive po- sition, the unloading relation is a curve. The curvature is slight and, therefore, a straight line may be fit, to the data. A straight line established by connecting the data point at 80 with the unloaded point at 0 gm-mm or by linear regression gives approximately the same stiffness of 191 gm-mm per degree. As might be expected for steel wire, the same deactivation stiffness is obtained independent of the amount of activation produced.

Nitinol wire has a much lower deactivation stiffness than stainless steel wire (Fig. 2). The average stiffness from full load to the point of complete unloading is 39 gm-mm per degree. The average stiffness can be ob- tained by connecting the point of maximum loading with the point of complete unloading or by carrying out a linear regression of all points on the unloading curve. As with steel either method gives approximately the same result. It should be noticed that the unloading curve for nitinol wire is less linear than that for steel wire. The average stiffness may be somewhat mis- leading because the stiffness between the deflection at 80 to deactivation at 70 is 72 gm-mm per degree; the stiffness from 40 to complete deactivation is 20 gm- mm per degree; and the intermediate stiffness between 40 and 70 is 39 gm-mm per degree. It can also be

L E

P

0 m 40 w W

Fig. 4. Comparison of NiTi and Respond wires. Note unusual unloading curve for NiTi wire. The average stiffness in the mid- dle range of deactivation for NiTi wire is the same as Respond wire. The moment level is higher.

0 m 40 60 W

Fig. 5. Activation and deactivation curves for NiTi wire. Unlike the stainless steel and nitinol wires, unloading curves change at different activations.

seen from Fig. 3 that the average unloading stiffnesses for different activations from 25 to 80 are approxi- mately the same if represented by a straight-line fit. These average stiffnesses represented by the straight line can be useful in describing the stiffness of nitinol wire. The stiffness of the wire will vary from this line primarily in the range of initial deactivation and final deactivation with fairly good predictive values for the middle range of deactivation.

The stiffness pattern for Chinese NiTi wire differs significantly from stainless steel and nitinol wires. In Fig. 4 the loading curve begins as a straight line and the wire exhibits linear elastic behavior. At 10 the slope of the line changes and continues as a straight line to 80. The unloading curve (which has more clinical sig- nificance) is particularly unusual. Initially, the moment drops very rapidly during unloading. This is followed by a long range of deactivation whereby a relatively constant moment is produced. Finally, just before total

448 But-stone, Qin, and Morton Am. .I. Onhod. Junr 1985

Table 1. Moments at yield (based on 1 permanent deformation)

Fig. 6. Comparison of average NiTi wire stiffness at activations from 80 to 5 from unloading curves shown in Fig. 5. Stiffness increases 3.8 times from the largest to the smallest activations.

Fig. 7. Activation (original 80) and reactivation (to 40) curves for NiTi wire. The moment decreases to 383 gm-mm after 40 of deactivation. If the wire is untied and retied into a bracket (reactivation), the moment increases to 700 gm-mm.

deactivation, the stiffness increases as the moment val- ues drop rapidly. For 80 of activation the average stiff- ness (based on a linear regression) is 14 gm-mm per degree-only 36% that of nitinol wire. For the first 5 of unloading the stiffness is 61 gm-mm per degree, and for the final 8 of unloading it is 27 gm-mm per degree. The unloading stiffness in the middle range from 15 to 75 is 11 gm-mm per degree. Thus, through most of the range of deactivation, the stiffness of the Chinese Niti wire is about 11 gm-mm per degree. The loading characteristic of a stainless steel braided wire (Re- spond,* 0.0155 inch) is also shown in Fig. 4. Although the braided wire also has an average slope of 11 gm- mm per degree, the clinical force system delivered would be entirely different. During unloading the mo- ments produced by the braided wire are much smaller.

*ORMCO, Glendora, Calif.

Wire M )Cld

I I Degrees oj

(gm-mm) SD activation

Stainless steel, 0.016 inch

Nitinol, 0.016 inch

Chinese NiTi, 0.016 inch

1,400 27 9

97.5 69 25

805 24 40

Although the moments would decrease at a rate equiv- alent to that of NiTi wire, they would be delivered at very low force levels. Note that the moment produced at 80 of activation with the Respond wire is approxi- mately one half that of the NiTi wire.

With the steel and nitinol wires, the average un- loading stiffness is the same regardless of the amount of activation. This is not true for Chinese NiTi wire. Fig. 5 shows the loading and unloading curves for ac- tivations between 5 and 80. Fig. 6 plots the average stiffness (using linear regression) for activations of 5 to 80. The average stiffness varies from 53 gm-mm per degree at 5 to 14 gm-mm per degree at So. For activations of 10 or less, the unloading curve and the loading curve are identical. Because linear behavior is occurring, use of a modulus of elasticity (E) in this range is valid for predicting forces or moments.

The change in stiffness among different activations is related to another clinically interesting finding; namely, that the magnitude of force increases if a wire is retied into a bracket. If one were to use a stainless steel or nitinol wire, a certain amount of force would be produced if one engaged an arch wire into a given bracket. If the tooth moved toward the arch wire, and the clinician then untied the wire and retied it, the force would be the same after retying. This would not be true with the Chinese NiTi wire. Following an 80 activa- tion, if a tooth moved to the 40 position, 380 gm-mm would remain (Fig. 7). If the wire is then untied and retied, a higher moment (700 gm-mm) is produced- almost twice the moment as is produced when the wire is left in place. As the wire continues to deactivate, the moment produced by the twice-activated wire ap- proaches the moment from a single activation.

Accurate prediction of orthodontic forces from NiTi wire is difficult because considerable nonlinearity oc- curs during deactivation and stiffness depends on the degree of activation. The average stiffness values for the NiTi wire given in this article are based on the linear regression method. A straight line connecting points

Volume 87 Number 6

Chinese NiTi wire 449

Table II. Moments and springback at 80 deflection

Wire

Stainless steel, 0.016 inch

Nitinol, 0.016 inch

Chinese NiTi, 0.016 inch

Moment (gm-mm)

3,067

2,112

1,233

SD

29

38

29

Permanent deformation Springback (degrees) (degrees)

64 16

28 52

7 13

% Recovery

20

65

91

from the beginning to the end of the unloading curve gives a slightly higher stiffness. The overall conclusions remain the same.

The maximum moment

Varying types of tooth movement require the deliv- ery of different magnitudes of force. Unless an ortho- dontic wire is capable of delivering an adequate moment before permanently deforming, it may not be satisfac- tory for a given application. It has been suggested pre- viously that two maximum moments should be consid- ered-the point of yield measured at 1 of permanent deformation (M,) and the highest moment produced after considerable yielding (M,,,).6 In this study the mo- ment for stainless steel at 1 of permanent deformation (M,) was found at 9 of activation and its magnitude was 1,400 gm-mm (Table I). The nitinol wire exhibited M, at 25 with a moment value of 975 gm-mm and the Chinese NiTi wire. at 40 with 805 gm-mm. The ulti- mate moment (M,,J, which occurs after considerable permanent deformation, is somewhat easier to establish with stainless steel wires. M,it occurs where the change of the slope of the loading curve becomes minimum or when an increase in deflection produces little or no increase in the measured moment. This ultimate mo- ment is much more difficult to determine with nitinol and NiTi wires because the geometry of loading changes with the large deflections required. Therefore, for convenience, we have used the moment produced at 80 of activation instead of the maximum ultimate moment that can be produced by the wire. At 80 the ultimate moments produced were: 3,067 gm-mm for stainless steel wire, 2112 gm-mm for Nitinol wire, and 1,233 gm-mm for Chinese NiTi wire. As shown in Table II, these values should be taken in the context of the amount of permanent deformation produced in the wire. Although the stainless steel wire delivers 3,067 gm-mm, the percent of the recovery of the wire is only 20%.* The nitinol wire at 2,112 gm-mm has a 65%

Springback *Percent recovery = ~ x loo.

Activation

recovery. The Chinese NiTi wire has a recovery of 91% for 1,233 gm-mm. Thus, the NiTi wire, in comparison to other wires, has a wide range of useful springback beyond the point where initial permanent deformation is observed.

Temperature-dependent effects

The mechanical properties of stainless steel do not vary at the temperatures commonly used for clinical purposes. Nitinol wires show negligible differences in stiffness or springback between room temperature and mouth temperature (Fig. 8). Chinese NiTi wire, on the other hand, exhibits some small differences at varying temperatures because material components have lower transition temperatures. In Fig. 9 the stiffness is ap- proximately the same between room temperature at 22 C and mouth temperature at 37 C. At a temperature of 60 C, the loading curve is slightly higher and the unloading curve loses its S shape and exhibits greater permanent deformation and less springback. Since the wire is normally used between room temperature and mouth temperature, these temperature-dependent ef- fects are clinically insignificant.

Time-dependent effects

Stainless steel wires are resistant to additional per- manent deformation that occurs with time. Some stress relaxation may occur, but the effects are not significant. The 0.016-inch stainless steel, nitinol, and Chinese NiTi wires were engaged in brackets placed interprox- imally 3 mm apart with a 6.5 mm occlusogingival dis- crepancy between the center bracket and the adjacent ones (Fig. 10). The wires remained tied in for periods of 1 minute, 1 hour, and 72 hours. It should be noted that, over 1 minute, the Chinese NiTi wire deformed a limited amount, compared to the nitinol and stainless steel wires which deformed considerably. Furthermore, the nitinol wire continued to show a time-dependent deformation past the initial 5 minutes. This has been reported previously. Although NiTi wires show some time-dependent effects, these are insignificant at room temperature.

450 Burstone, Qin, and Morton Am. J. Orthod. June 1985

Fig. 9. Effect of temperature on the mechanical properties of nitinoi wire. Negligible increases in stiffness occur with rising temperatures.

Clinical significance and discussion

Because of its high range of action or springback, Chinese NiTi wire is applicable in situations where large deflections are required. Applications include straight-wire procedures when teeth are badly mal- aligned and in appliances designed to deliver constant forces during major stages of tooth movement. The amount of deformation without notable permanent set is remarkable-4.4 times that of the stainless steel wire and 1.6 times that of the nitinol wire (based on 1 of permanent deformation). I-6

Achievement of relatively constant forces has been obtained traditionally by lowering the load-deflection rate of the orthodontic appliance. This has been accom- plished by configurational design; for instance, placing helices or additional wire in the appliance. The newer wire materials such as those used in nitinol and TMA* reduce the load-deflection rate without a large reduction in the maximum moment. This is caused by their ex- cellent ratios between yield strength and modulus of elasticity. Another approach is possible with Chinese NiTi wire because of its unusual loading-unloading curve. In the middle range of unloading, the load-de- flection rate is low. The higher stiffness found in the NiTi wire during the final stage of unloading helps assure that not only are the forces delivered at a more constant rate, but a higher magnitude of force level is maintained. One might compare the NiTi and the Re- spond wires as they are charted in Fig. 4. If given the full 80 activation, then each wire was allowed to relax to 70. The NiTi wire at 70 would deliver 800 gm- mm and the Respond wire would deliver 439 gm-mm-

*ORMCO, Glendora, Calif.

Flg. 9. Effect of temperatures on the mechanical properties of NiTi wire. Very small reduction in springback at mouth temper- ature. Higher temperature (So0 C) reduces springback and in- creases stiffness. Higher temperatures are beyond usual clinical range.

approximately one half of the moment. Nevertheless, from this point on, with continued unloading, the stiff- ness of the wires would be the same. Thus, the NiTi wire in its middle range of deactivation could deliver a higher level of force than a given braided wire, al- though both deliver equivalently constant forces for the same activation.

The moment at yield of the NiTi wire, although lower than that of the stainless steel wire, is comparable to the nitinol wire and considerably higher than what would be available in braided steel wires of comparable stiffness. The ultimate maximum moment of the nitinol wire is much higher than that of the NiTi wire. This may or may not be advantageous since, at these levels, nitinol wire exhibits much permanent deformation.

The prediction of force magnitudes delivered by Chinese NiTi wire is more difficult than with other alloys when the modulus of elasticity can be used with appropriate formulas.8 Nitinol wires unloading char- acteristic is somewhat problematic because of its non- linearity. Nevertheless, a linear regression line for the unloading curve could approximate the force conditions during clinical use, recognizing the inherent inaccuracy. Chinese NiTi wire is even more problematic because the unloading curve is complex and the stiffness de- pends upon the amount of activation. If one used the linear regression lines from the bending data to deter- mine the stiffness, the stiffness of a 0.016-inch NiTi wire at large activations (80) would be 7% that of stainless steel wire; however, the wire stiffness for a small activation (10) would be 28% that of stainless steel wire (Fig. 11). In other words, for a small acti- vation NiTi wire would feel more like a 0.015inch nitinol wire, and for a large activation it would have

Volume 87 Number 6

Chinese NiTi wire

c Fig. 10. Time dependent effects. A, The 0.016~inch wires placed into three brackets. B, The shape of the wires after removal. Top row-stainless steel wire, middle row-nitinol wire, bottom row-NiTi wire:A, 1 minute. S, 1 hour. C, 3 days. Note small amount of permanent deformation of NiTi wire and its increasing deformation over time with nitinol wire.

the stiffness of a 0.008-inch stainless steel wire. In the future, if NiTi wire is used for calibrated appliances with known dimensions and activations, the actual force system can be determined experimentally and, there- fore, the problems of force prediction can easily be resolved.

It has been shown that there is a force difference if the appliance is left in place throughout the deactivation or if it is removed and retied. The clinician should be aware of this characteristic and should design his treat- ment accordingly. If no change is desired in the mag- nitude of a force, it is better to leave a wire in place. On the other hand, if it is thought that the force levels have dropped too low for a given type of tooth move- ment, then the simple act of untying and retying can increase the magnitude of the force.

The potenial uses of NiTi are many inasmuch as it offers a low-stiffness and high-springback wire for tooth alignment. In addition, if larger cross sections are used, they are capable of delivering the larger moments re- quired for major tooth movement, such as root move- ment and translation.

SUMMARY AND CONCLUSIONS

The new nickel-titanium alloy (Chinese Niti wire) described here has the following unique mechanical properties:

1. The wire has a springback that is 4.4 times that of comparable stainless steel wire and 1.6 times that of nitinol wire, if springback is measured at yield based on a S-mm span cantilever test.

1.0

0.8

0.6

a4

a2

~

S.S.

Wire Stiffness for Identical Cross Sections Reference Stiffness (s.s.) = 1.0

Nitinol NiTi N iTi (small A) (large A)

Respond

Fig. 11. Comparison of NiTi wire stiffness to that of wires com- posed of other materials. Stainless steel has a stiffness number of 1 .O. At small deflections NiTi wire delivers 0.28 x the force of steel. At large deflections only 0.07 x the force of steel is delivered for the same activation.

2. At 80 of activation the average stiffness of Chinese NiTi wire is 73% that of stainless steel wire and 36% that of nitinol wire.

3. The unusual nonlinear loading curve builds into the NiTi wire a constant force mechanism in the middle range of deactivation. This is potentially a significant design feature for constant-force appliances.

4. Unlike wires of other orthodontic alloys, the characteristic stiffness is determined by the amount of

452 Burstone, Qin, and Morton

activation. The load-deformation rate at small activa- tions is considerably higher than that at large acti- vations.

5. NiTi wire deformation is not particularly time- dependent and, unlike nitinol wire, will not continue to deform a significant amount in the mouth between adjustments.

6. Chinese NiTi wire is highly suitable if low stiff- ness is required and large deflections are needed. Its higher stiffness at small activations make it more ef- fective than wires of traditional alloys whose force lev- els may be too low (as teeth approach the passive shape of the wire).

REFERENCES 1. Andreasen GF, Hilleman TB: An evaluation of 5.5 cobalt substi-

tuted nitinol wire for use in orthodontics. J Am Dent Assoc 82: 1373-1375, 1971.

2. Buehler WJ: Proceedings of 7th Navy Science (ONR-16 Office of Technical Services, US Department of Commerce, Washing- ton, DC). Vol. 1, unclassified, 1963.

3.

4.

5.

6.

7.

8.

Buehler WJ, Gilfrick JV, Wiley RC: Effects of low temperature phase changes on the mechanical properties of alloys-near com- position TiNi. J Appl Physics 34: 1475-1484, 1963. Andreasen GF, Bigelow H, Andrews JG: 55 Nitinol wire: force developed as a function of elastic memory Aust Dent J 24: 146. 149, 1979.

Andreasen GF, Montagano L, Krell D: An investigation of linear dimensional changes as a function of temperature in an 0.010 inch %obalt-substituted annealed nitinol alloy wire. AM .I ORTHOD 82: 469-472, 1982. Burstone CJ, Goldberg AJ: Maximum forces and deflections from orthodontic appliances. AM J ORTHOD 4: 95-103, 1983. Lopez I, Goldberg AJ, Burstone CJ: Bending characteristics of nitinol wire. AM J ORTHOD 75: 569-574, 1979. Burstone CJ: Variable modulus orthodontics. AM J ORTHOD 80: l-16, 1981.

Reprint requests to: Dr. Charles J. Burstone Department of Orthodontics University of Connecticut Health Center School of Dental Medicine Farmington, CT 06032

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