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Laboratory and clinical analyses of nitinol wire
Laboratory and clinical nitinol wire
analyses of
George F. Andressen, D.D.S., M.S.D.,* and Ray E. Morrow, M.S.M.E.** Iowa City, Iowa, and Monrovia,
Calif.
M
an’s long history of technologic development has been marked by a continuing search for improved materials. This effort has resulted in vast arrays of new materials which have affected nearly every aspect of contemporary life, including orthodontics. A significant advance in orthodontic materials was made in the late 1930’s and 1940’s when stainless steel wire and appliances became widely available, Since that time there has been continuous evolutionary improvement in the strength and resilience of wires used for orthodontic treatment. The recent development of nitinol wire is another improvement which has emerged from the orthodontist’s search for lighter forces and greater working range. Nitinol was invented in the early 1960’s by William F. Buehler, a research metallurgist at the Naval Ordnance Laboratory in Silver Springs, Maryland (now called the Naval Surface Weapons Center). Mr. Buehler spent the next few years doing extensive research and publishing his findings on the properties and uses of his new alloy. The name nitinol is an acronym derived from the elements which comprise the alloy, ni for nickel and ti for titanium, and nof from Naval Ordnance Laboratory. “Shape memory” wire Most orthodontists are aware of nitinol because of a unique property of the alloy called “shape memory.” Nitinol has the characteristic of being able to return to a previously manufactured shape when it is heated through a transition temperature range (TTR). To use this property, the wire must first be set into the desired shape and held while undergoing a high-temperature heat treatment. After the wire has cooled to room temperature, it may be deformed within certain strain limits, When heated to its unique TTR, it will “remember” its shape and return to the original configuration. Though the orthodontic wires available today do not fully utilize this characteristic, research into orthodontic applications for the “memory” aspects of nitinol wire are continuing at the University of Iowa and at Unitek. *Rofessor, Department of Orthodontics, College of Dentistry, University of Iowa. **Member of Subcommittee on Orthodontic Wires Not Containing Precious Metals Council on Dental Materials and Devices; Senior Orthodontic Product Manager, Development, Unitek Corporation, Monrovia, Calif.
142
of the ADA’s Research and
Volume 13 Number 2
Laboratory and clinical analyses of nitinol wire 143
Table I Material Alloy Ultimate Modulus
propertty
Nitinol Nickel, titanium 230,OOOto25O,OOOp.s.i. 4.8 x lo6 p.s.i.
strength ofelasticity
“Elastic” orthodontic
Stainless
steel
Iron, chrome, nickel 280,OOOto300,COOp.s.i. 28.5 X lo6 p.s.i.
wire
While this characteristic of shape “memory” has attracted the most public interest, the wire also has another unique property which is, at present, of practical use to orthodontists. This property is nitinol’s outstanding elasticity when drawn into high-strength wire for orthodontic use. Compared with stainless steel, in normal handling, nitinol wire is more difficult to deform permanently. It can almost be bent back upon itself without taking a permanent set. It is this characteristic of exceptional elasticity currently being manufactured into preformed arch wires which offers the clinician a real advancement in orthodontic materials application. Clinical use of nitinol wire started in May, 1972. Since that time, the material has been evaluated by both clinicians and researchers, and the results have been quite similar. It was generally concluded during these long-term evaluations that, in orthodontic applications, nitinol (1) requires fewer arch wire changes, (2) requires less chair time, (3) shortens the treatment time required to accomplish rotations and leveling, and (4) produces less patient discomfort. Physical properties
The physical properties of nitinol are quite different from conventional stainless steel alloys, as illustrated in Table I. Its low modulus of elasticity, coupled with moderately high strength, accounts for nitinol ‘s extreme resilience, or working range. Bend testing
A series of bend and torsion tests have been performed in accordance with the new ADA Specijication No. 32 on Orthodontic Wires to determine the characteristics of nitinol and compare them to those of stainless steel wire (Figs. 1 and 2). In this test, a short length of wire was clamped in a Tinius Olsen stiffness tester with the free end of the wire resting on a fixed anvil, as shown in Fig. 3. As the clamp was rotated relative to the anvil, the applied bending moment at each corresponding angle was recorded, up to the maximum bend angle of 90 degrees. At this point, the bending moment was reduced to zero as the wire returned to a rest position. The angle of permanent bend was then measured. Fig. 1 illustrates that an 0.018 inch (0.457 mm.) round nitinol wire has a stiffness similar to a 0.014 inch (0.356 mm.) round stainless steel wire until the deflection angle reaches 35 to 40 degrees and the 0.014 inch (0.356 mm.) wire starts to take a permanent set. In a 90 degree bend, the applied moment is less than that produced by an 0.016 inch (0.406 mm.) stainless steel wire. The 0.018 inch (0.457 mm,) nitinol has a permanent set, or bend angle, of only 5 degrees while the stainless steel wires have a permanent set of from 39 to 40 degrees or greater. In a similar manner, Fig. 2 compares 0.017 by 0.025 inch (0.432 by 0.635 mm.) and
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Fig. 1. Comparison of round nitinol and stainless steel arch wires in a bending test, similar to the proposed ADA specifications on orthodontic wire. The solid line indicates the applied bending moment for the corresponding deflection up to 90 degrees. The dotted line shows the load-deflection curve as the applied moment is reduced. At zero bending moment, the dotted tine indicates the amount of permanent bend in the wire.
0.019 by 0.025 (0.483 by 0.635 mm.) rectangular nitinol wires with three square and rectangular stainless steel wires. from the graph, it is apparent that both 0.017 by 0.025 inch (0.432 by 0.635 mm.) and 0.019 by 0.025 inch (0,483 by 0.635 mm.) nitinol wires have a lower stiffness than 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire up to about 43 degrees. At that point, the 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire begins to yield. At the full 90 degree deflection, the 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol produces a lower bending moment than the 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel, and the 0.019 by 0.025 inch (0.483 by 0.635 mm.) nitinol produces a lower bending moment than the 0.016 by 0.022 inch (0.406 by 0.559 mm.) stainless steel. The permanent set angle is approximately 5 to 7 degrees for nitinol wire, compared to 48 degrees or greater for stainless steel wire. Torsion testing Fig. 4 shows a comparison between nitinol and stainless steel wires tested in torsion. In this instance, a wire was clamped at one end while a rotating jaw gripped the other end of the wire. A 1 inch gauge length between jaws was used. As the rotating jaw turned, the torsional moment in inch-ounces was measured as a function of the torque angle. Each of the wires was rotated through an angle of 720 degrees (2 complete revolutions). At this point, the rotating jaw was permitted to return to a neutral position and the permanent set angle of the wire was measured. The 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire took a permanent set of 450 degrees as compared to a permanent sef angle of approximately 220 degrees for the 0.017 by 0.025 inch (0.432 by 0.635 mm.) stainless steel and only 45 degrees for the nitinol wire. In both the bending and torsion tests, nitinol develops a lower bending moment or torsional load under the same conditions when compared with the 0.016 by 0.016 inch
Volume 73 Number 2
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and clinical
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SPAN=.50in.
400--
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loo--
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Fig. 2. Comparison of rectangular nitinol and stainless steel arch wires in a bending test, similar to the proposed ADA specifications on orthodontic wire. APPLIED
DEFLECTION ANGLE
Fig. 3. Schematic drawing of bend test fixture. The wire is clamped at left and is resting on the anvil at right. The clamp is rotated clockwise about the center through an angle of 90 degrees. The deflection angle and applied moment were measured in 10 degree increments.
(0.406 by 0.406 mm.) or 0.017 by 0.025 inch (0.432 by 0.635 mm.) stainless steel wires. Also, the amount of permanent set in the nitinol wire is remarkably lower than in either of the stainless steel wires. “Stored”
energy comparisons
Fig. 5 shows a curve similar to that in Fig. 2 which compares the stored energy in a 0.017 by 0.025 inch (0.435 by 0.635 mm.) stainless steel wire with a similar nitinol wire. In a bending application as depicted here, the amount of energy stored in the wire is shown by the area of the triangle representing the change in bending moment as the load is
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Fig. 4. Comparison of rectangular nitinol and stainless steel arch wires in a torsion test similar to the proposed ADA SpecificaQon No. 32 on orthodontic wire. Gauge length: 1 inch. Fig. 5. Comparison of the spring energy in nitinol and stainless steel. Energy for a wire and bending is one-half times load times deflection. In these examples, the stored or spring energy is equivalent to the triangular area under the loaddeflection curve as the wire returns to rest.
reduced from the maximum to zero and the change in deflection accompanying this reduction in bending moment. In this instance, the recoverable energy stored in a stainless steel wire is represented by the triangle 1, 2, 3, while the recoverable energy stored in a similar nitinol wire is represented by the triangular area 2, 4, 5. In this example, the stored energy of the nitinol wire is significantly greater than the stored energy in an equivalent stainless steel wire. However, this comparison was based upon the wires-being bent 90 degrees, which is an unlikely clinical occurrence. A more typical case is one in which the patient’s comfort puts a fixed limit on the amount of force which can be applied. In Fig. 6 the maximum force is chosen low enough so that both wires are capable of achieving full recovery without any permanent set. The stored energy of the stainless steel wire is represented by the area 0, 1, 2, while the stored energy of the equivalent size nitinol wire is represented by the much larger area 0, 3,4. This increased energy, which is work available to move teeth, is what accounts for nitinol’s increased clinical efficiency.
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Laboratory .017 x ,025 STAINLESS STEEL ,017 i ,025 NITINOL
and clinical
analyses of nitinol wire
147
A=O-l-2 1=0-3-d
STAINLESS STEEL
NITINOL
Fig. 6. Comparison of the spring energy in nitinol and stainless steel wire when the force is limited. Fig. 7. Comparison of the change in force between nitinol and stainless steel when undergoing a constant change in deflection.
Spring rate Although the last analysis assumed that the only limitations were the maximum forces that could be applied to a patient, there are many instances in which the limitation is the amount of deflection required. This occurs where the clinician is seeking a specific depth of bracket engagement. Fig. 7 shows the same bending curves shown in Figs. 2, 5, and 6; only this time we have chosen an equal change in deflection d, and are showing the change in force A F which would accompany that change in deflection. The stainless steel wire undergoes a much larger change in force compared to the change in force of nitinol wire when both wires are deflected an equivalent amount. This property of the wire (that is, the change in load), divided by the change in deflection, is called the spring rate of the material. The graph shown in Fig. 7 demonstrates that the spring rate of stainless steel is approximately twice that of nitinol. Clinically, this means that, for any given malocclusion, nitinol wire will produce a lower, more constant, and continuous force on the teeth than would a stainless steel wire of equivalent size.
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F~Q. 8. This typodont shows a 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol wire ligated into a simulated malocclusion. Every other tooth is positioned in an ideal arch form, while each alternate tooth is displaced lingually and increasing 0.5 mm. The lower right second molar is in the ideal arch form, the lower right first molar is inset lingually 0.5 mm., and so on around the arch (with the exception of the two central incisors) to the lower left first molar, which is inset 3 mm. Note that the wire is severely deformed on the left side of the arch.
Fig. 9. nitinol
Same as Fig. 8 except wire into brackets have
that the ligature ties holding the 0.017 by 0.025 (0.432 by 0.635 been cut to permit the wire to return to its original arch form.
mm.)
Clinical applications Nitinol wire can be used in Class I, Class II, or Class III malocclusions, in both extraction and nonextraction cases. In selecting cases that benefit most from the use of nitinol wire, the primary criterion is the amount of malalignment of the teeth from the ideal arch form. The more the wire has to be deflected from the ideal arch form when ligated into the bracket, the greater benefit nitinol wire has over stainless steel wire. even in extreme cases, to permit Figs. 8 and 9 demonstrate how easily nitinol deflects, secure ligation without deforming. Fig. 8 illustrates an 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol wire ligated into a simulated malocclusion on a typodont. In this model, every other tooth was positioned in an ideal arch form. The alternate teeth were
Volume 13 Number 2
Fig.
10.
Fig.
11.
Laboratory and clinical analysesof nitinol wire 149
Nitinol wire used as a torquing auxiliary in uprighting impacted canines. Nitinol auxiliary torquing wire activated to the stabilizing arch wire by hooking and tying it in
place. Fig. 12. Sectional arch wire tied beneath bracket tie wings to reposition a displaced canine.
displaced lingually with increasing depth at 0.5 mm. increments. That is, the first tooth is inset 0.5 mm., the second 1 mm., the third 1.5 mm., and so on, around the arch to a maximum displacement of 3 mm. Fig. 9 shows the arch wire after the ligatures had been removed so that the wire could return to its original form. Note that there are only minor permanent bends in the wire. The most important benefits from nitinol wire are realized when a rectangular wire is inserted early in treatment. Simultaneous rotation, leveling, tipping, and torquing can be accomplished earlier with a resilient rectangular wire, such as nitinol. Clinicians have been successful in beginning treatment of certain carefully selected cases with full-size 0.017 by 0.025 inch (0.432 by 0.635 mm.) and 0.019 by 0.025 inch (0.483 by 0.635 mm.) rectangular arch wires that nearly fill the bracket slot. In a few instances, the entire case has been treated with just one arch wire. It is ideally suited for use with most pretorqued and preangulated appliances, because tipping and uprighting of the teeth can be initiated in the early stages of treatment. When the case is nearing completion with a nitinol arch wire, there is very little to be done in the way of placing compensating bends to upright roots, once the spaces have been closed. In the treatment of extraction cases with pretorqued and preangulated twin brackets and nitinol, conventional auxiliary methods of closing spaces may be used along with headgear when needed. The use of nitinol with pretorqued and preangulated brackets requires careful monitoring of tooth movement because of the wire’s high elasticity and more continuous force. Therefore, time intervals between appointments cannot be extended . Cross-bite correction. Nitinol wire has been successfully used in the correction of cross-bites. If the clinician pliers progressive first-order offsets in the buccal segment of
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nitinol wire, so that each tooth will be loaded in approximately equal amounts, and then expands and contracts the two arch wires opposite the direction of cross-bite, nitinol wire will produce a direction of force having a low load-deflection rate that is more desirable than cross-the-bite elastics. Uprighting impacted canines. Another clinical use of nitinol wire is the uprighting of impacted canines by the use of a couple or torque in the second order of space. The torque is accomplished by inserting an auxiliary nitinol wire into the bracket slot of the canine. This auxiliary extends % inch mesial and distal to the canine bracket (Fig. 10). After both ends of the nitinol wire are annealed, auxiliary hooks can be bent in the ends. The auxiliary is then deflected to a stablizing stiff arch wire and the auxiliaries are hooked to it by either ligating it to the stablizing wire or hooking the annealed bent hooks under and over the stablizing arch wire (Fig. 11). This reaction, in turn, produces a tipping action about the bracket and produces an uprighting force on the canine roots. Nitinol is also successful when used as a sectional arch for bringing high or lingually displaced canines into position. Such arches can be ligated beneath the tie wings (Fig. 12) or, better, used with any band-side slotted bracket. Opening the bite. Nitinol wire can be used effectively in opening the bite by either intruding the maxillary and mandibular anterior segments and extruding the posterior segments. When opening a deep bite, in order to get interbracket clearance in the mandibular arch for complete anterior maxillary retraction, one can place first-order steps in the anterior segment to produce intruding forces on the anterior teeth and extruding forces on the premolars and molars. Another method of opening the bite is to plier into the wire about one half inch or more reverse curve of spee and use either a crimpable loose tie-back or a loose cinch-back mesial or distal to the buccal tube. This allows nitinol’s highly elastic forces to exert depressing forces on the anterior teeth and molars and extruding forces on the premolars. For the maxillary arch, the same two principles apply, that is to say either a second-order step in the desired segment of anterior teeth that need to be intruded and posterior teeth that need extrusion or the use of a more pronounced curve of Spee in the anterior segment only. The forces exerted in this example are intrusive forces on the anterior segment and extrusive forces on the posterior segment. Limitations The most significantly different clinical characteristic of nitinol wire, when compared to stainless steel wire, is its resistance to taking a bend. It can be deflected far out of plane without losing its ability to spring back. This can cause some problems in placement of desired bends, in-and-out steps, loops, and torques. Successfully using nitinol in a conventional edgewise practice requires some changes in the way orthodontists use their wires. Nitinol cannot be bent with “sharp-cornered ” instruments. Although nitinol feels quite flexible and gives the impression that it is very ductile, it will readily break when bent over a sharp edge. Clinicians generally have no difficulty in putting in first- and second-order bends or the desired torque, although the wire has to be severely overformed in order to get it to take the desired permanent bend. The bending of loops or omega bends in nitinol is not recommended. They are time consuming to bend, and are a potential source of failure when one is ttying to close the loops. Closing loops in particular are not practical for nitinol wire.
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Laboratory
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Because nitinol cannot be soldered or successfully welded to itself without annealing the wire, and because the bending of tie-back hooks entails a high risk of failure, practitioners have found crimpable hooks and stops a successful alternative. “Cinch-backs” distal to the buccal tubes are easily accomplished by either resistance-annealing or flameannealing the end of the nitinol wire. Once the wire is dead soft, it is very easy to bend into any desired configuration, but care must be taken not to overheat the wire. A dark blue color indicates that the temperature is sufficient to anneal the wire. If nitinol wire is heated to a cherry red, as with stainless steel wire, there is a risk of making it brittle. In addition, by its very nature, nitinol is not a stiff wire, which means that it can easily be deflected. Occasionally the use of auxiliary retraction forces will have the effect of opening the patient’s posterior bite. There are two ways to remedy this: (1) simply remove the auxiliary force mechanism at the completion of treatment and the nitinol arch wire will relevel the bite or (2) take the nitinol wire out and go to conventional posterior bite-closing mechanics with a steel arch wire. This same limitation is also evident when one wishes to provide maximum stability in the arch at the completion of treatment. Such stability is often best maintained by using a stiffer stainless steel arch wire tailored to the desired finished occlusion. Conclusion
Nitinol orthodontic wire has physical properties quite different from stainless steel which make it easy to deflect and ligate while providing it with an exceptional working range. This means that large-dimension nitinol wires, which more completely fill bracket slots, may be used earlier in treatment when indicated. In turn, the number of graduated arch wire changes normally required can be reduced or, in certain cases, completely eliminated. At the same time, these larger nitinol arch wires readily accomplish and maintain desired rotations and leveling without increasing the patient’s discomfort. In conclusion, our findings are that, when applied with skill and professional judgment, nitinol arch wire represents a significant improvement over conventional orthodontic arch wire and is a valuable addition to the orthodontist’s armamentarium. REFERENCES 1. 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. 2. Andreasen, G. F., and Brady, P. R.: A use hypothesis for 55 nitinol wire for orthodontics, Angle Orthod. 42: 172-177, 1972. 3. Andreasen, G. F., and Barrett, R. D.: An evaluation of cobalt substituted nitinol wire in orthodontics, AM. J. ORTHOD. 63: 462-470, 1973. 4. Heisterkamp, C. A., Buehler, W. J., and Wang, F. E.: 55Nitinol: A new biomaterial, paper presented at the 8th International Conference on Medical and Biomedical Engineering (Chicago), 1969. 5. Castleman, L. S., Motzkin, S. M., Alicardri, F. P., and Bonawit, V. L.: Biocompatibility of nitinol alloy as an implant material, J. Biomed. Mater. Res. 10: 695-731, 1976. 6. Civjan, S., Huget, E. F., and De Simon, L. B.: Potential applications of certain nickel-titanium (nitinol) alloys, J. Dent. Res. 54: 89-96, 1975. 7. Council on Dental Materials and Devices: New American Dental Association specification No 32 for orthodontic wires not containing precious metals, J. Am. Dent. Assoc. 95: 1169-l 171, 1977. Dr. Andreasen: College Mr. Morrow: 862 Hugo
of Denrisny (52242) Reid St., Arcadia, Calif.
91006
analyses of
George F. Andressen, D.D.S., M.S.D.,* and Ray E. Morrow, M.S.M.E.** Iowa City, Iowa, and Monrovia,
Calif.
M
an’s long history of technologic development has been marked by a continuing search for improved materials. This effort has resulted in vast arrays of new materials which have affected nearly every aspect of contemporary life, including orthodontics. A significant advance in orthodontic materials was made in the late 1930’s and 1940’s when stainless steel wire and appliances became widely available, Since that time there has been continuous evolutionary improvement in the strength and resilience of wires used for orthodontic treatment. The recent development of nitinol wire is another improvement which has emerged from the orthodontist’s search for lighter forces and greater working range. Nitinol was invented in the early 1960’s by William F. Buehler, a research metallurgist at the Naval Ordnance Laboratory in Silver Springs, Maryland (now called the Naval Surface Weapons Center). Mr. Buehler spent the next few years doing extensive research and publishing his findings on the properties and uses of his new alloy. The name nitinol is an acronym derived from the elements which comprise the alloy, ni for nickel and ti for titanium, and nof from Naval Ordnance Laboratory. “Shape memory” wire Most orthodontists are aware of nitinol because of a unique property of the alloy called “shape memory.” Nitinol has the characteristic of being able to return to a previously manufactured shape when it is heated through a transition temperature range (TTR). To use this property, the wire must first be set into the desired shape and held while undergoing a high-temperature heat treatment. After the wire has cooled to room temperature, it may be deformed within certain strain limits, When heated to its unique TTR, it will “remember” its shape and return to the original configuration. Though the orthodontic wires available today do not fully utilize this characteristic, research into orthodontic applications for the “memory” aspects of nitinol wire are continuing at the University of Iowa and at Unitek. *Rofessor, Department of Orthodontics, College of Dentistry, University of Iowa. **Member of Subcommittee on Orthodontic Wires Not Containing Precious Metals Council on Dental Materials and Devices; Senior Orthodontic Product Manager, Development, Unitek Corporation, Monrovia, Calif.
142
of the ADA’s Research and
Volume 13 Number 2
Laboratory and clinical analyses of nitinol wire 143
Table I Material Alloy Ultimate Modulus
propertty
Nitinol Nickel, titanium 230,OOOto25O,OOOp.s.i. 4.8 x lo6 p.s.i.
strength ofelasticity
“Elastic” orthodontic
Stainless
steel
Iron, chrome, nickel 280,OOOto300,COOp.s.i. 28.5 X lo6 p.s.i.
wire
While this characteristic of shape “memory” has attracted the most public interest, the wire also has another unique property which is, at present, of practical use to orthodontists. This property is nitinol’s outstanding elasticity when drawn into high-strength wire for orthodontic use. Compared with stainless steel, in normal handling, nitinol wire is more difficult to deform permanently. It can almost be bent back upon itself without taking a permanent set. It is this characteristic of exceptional elasticity currently being manufactured into preformed arch wires which offers the clinician a real advancement in orthodontic materials application. Clinical use of nitinol wire started in May, 1972. Since that time, the material has been evaluated by both clinicians and researchers, and the results have been quite similar. It was generally concluded during these long-term evaluations that, in orthodontic applications, nitinol (1) requires fewer arch wire changes, (2) requires less chair time, (3) shortens the treatment time required to accomplish rotations and leveling, and (4) produces less patient discomfort. Physical properties
The physical properties of nitinol are quite different from conventional stainless steel alloys, as illustrated in Table I. Its low modulus of elasticity, coupled with moderately high strength, accounts for nitinol ‘s extreme resilience, or working range. Bend testing
A series of bend and torsion tests have been performed in accordance with the new ADA Specijication No. 32 on Orthodontic Wires to determine the characteristics of nitinol and compare them to those of stainless steel wire (Figs. 1 and 2). In this test, a short length of wire was clamped in a Tinius Olsen stiffness tester with the free end of the wire resting on a fixed anvil, as shown in Fig. 3. As the clamp was rotated relative to the anvil, the applied bending moment at each corresponding angle was recorded, up to the maximum bend angle of 90 degrees. At this point, the bending moment was reduced to zero as the wire returned to a rest position. The angle of permanent bend was then measured. Fig. 1 illustrates that an 0.018 inch (0.457 mm.) round nitinol wire has a stiffness similar to a 0.014 inch (0.356 mm.) round stainless steel wire until the deflection angle reaches 35 to 40 degrees and the 0.014 inch (0.356 mm.) wire starts to take a permanent set. In a 90 degree bend, the applied moment is less than that produced by an 0.016 inch (0.406 mm.) stainless steel wire. The 0.018 inch (0.457 mm,) nitinol has a permanent set, or bend angle, of only 5 degrees while the stainless steel wires have a permanent set of from 39 to 40 degrees or greater. In a similar manner, Fig. 2 compares 0.017 by 0.025 inch (0.432 by 0.635 mm.) and
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30 DEFLECTION
40 ANGLE
50
60
70
80
90
DEGREES
Fig. 1. Comparison of round nitinol and stainless steel arch wires in a bending test, similar to the proposed ADA specifications on orthodontic wire. The solid line indicates the applied bending moment for the corresponding deflection up to 90 degrees. The dotted line shows the load-deflection curve as the applied moment is reduced. At zero bending moment, the dotted tine indicates the amount of permanent bend in the wire.
0.019 by 0.025 (0.483 by 0.635 mm.) rectangular nitinol wires with three square and rectangular stainless steel wires. from the graph, it is apparent that both 0.017 by 0.025 inch (0.432 by 0.635 mm.) and 0.019 by 0.025 inch (0,483 by 0.635 mm.) nitinol wires have a lower stiffness than 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire up to about 43 degrees. At that point, the 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire begins to yield. At the full 90 degree deflection, the 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol produces a lower bending moment than the 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel, and the 0.019 by 0.025 inch (0.483 by 0.635 mm.) nitinol produces a lower bending moment than the 0.016 by 0.022 inch (0.406 by 0.559 mm.) stainless steel. The permanent set angle is approximately 5 to 7 degrees for nitinol wire, compared to 48 degrees or greater for stainless steel wire. Torsion testing Fig. 4 shows a comparison between nitinol and stainless steel wires tested in torsion. In this instance, a wire was clamped at one end while a rotating jaw gripped the other end of the wire. A 1 inch gauge length between jaws was used. As the rotating jaw turned, the torsional moment in inch-ounces was measured as a function of the torque angle. Each of the wires was rotated through an angle of 720 degrees (2 complete revolutions). At this point, the rotating jaw was permitted to return to a neutral position and the permanent set angle of the wire was measured. The 0.016 by 0.016 inch (0.406 by 0.406 mm.) stainless steel wire took a permanent set of 450 degrees as compared to a permanent sef angle of approximately 220 degrees for the 0.017 by 0.025 inch (0.432 by 0.635 mm.) stainless steel and only 45 degrees for the nitinol wire. In both the bending and torsion tests, nitinol develops a lower bending moment or torsional load under the same conditions when compared with the 0.016 by 0.016 inch
Volume 73 Number 2
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SPAN=.50in.
400--
I 300-
er 6 H
200-.
loo--
0
10
20
30 DEFLECTION
40 ANGLE
50
60
70
60
90
DEGREES
Fig. 2. Comparison of rectangular nitinol and stainless steel arch wires in a bending test, similar to the proposed ADA specifications on orthodontic wire. APPLIED
DEFLECTION ANGLE
Fig. 3. Schematic drawing of bend test fixture. The wire is clamped at left and is resting on the anvil at right. The clamp is rotated clockwise about the center through an angle of 90 degrees. The deflection angle and applied moment were measured in 10 degree increments.
(0.406 by 0.406 mm.) or 0.017 by 0.025 inch (0.432 by 0.635 mm.) stainless steel wires. Also, the amount of permanent set in the nitinol wire is remarkably lower than in either of the stainless steel wires. “Stored”
energy comparisons
Fig. 5 shows a curve similar to that in Fig. 2 which compares the stored energy in a 0.017 by 0.025 inch (0.435 by 0.635 mm.) stainless steel wire with a similar nitinol wire. In a bending application as depicted here, the amount of energy stored in the wire is shown by the area of the triangle representing the change in bending moment as the load is
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ANGLE
Fig. 4. Comparison of rectangular nitinol and stainless steel arch wires in a torsion test similar to the proposed ADA SpecificaQon No. 32 on orthodontic wire. Gauge length: 1 inch. Fig. 5. Comparison of the spring energy in nitinol and stainless steel. Energy for a wire and bending is one-half times load times deflection. In these examples, the stored or spring energy is equivalent to the triangular area under the loaddeflection curve as the wire returns to rest.
reduced from the maximum to zero and the change in deflection accompanying this reduction in bending moment. In this instance, the recoverable energy stored in a stainless steel wire is represented by the triangle 1, 2, 3, while the recoverable energy stored in a similar nitinol wire is represented by the triangular area 2, 4, 5. In this example, the stored energy of the nitinol wire is significantly greater than the stored energy in an equivalent stainless steel wire. However, this comparison was based upon the wires-being bent 90 degrees, which is an unlikely clinical occurrence. A more typical case is one in which the patient’s comfort puts a fixed limit on the amount of force which can be applied. In Fig. 6 the maximum force is chosen low enough so that both wires are capable of achieving full recovery without any permanent set. The stored energy of the stainless steel wire is represented by the area 0, 1, 2, while the stored energy of the equivalent size nitinol wire is represented by the much larger area 0, 3,4. This increased energy, which is work available to move teeth, is what accounts for nitinol’s increased clinical efficiency.
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A=O-l-2 1=0-3-d
STAINLESS STEEL
NITINOL
Fig. 6. Comparison of the spring energy in nitinol and stainless steel wire when the force is limited. Fig. 7. Comparison of the change in force between nitinol and stainless steel when undergoing a constant change in deflection.
Spring rate Although the last analysis assumed that the only limitations were the maximum forces that could be applied to a patient, there are many instances in which the limitation is the amount of deflection required. This occurs where the clinician is seeking a specific depth of bracket engagement. Fig. 7 shows the same bending curves shown in Figs. 2, 5, and 6; only this time we have chosen an equal change in deflection d, and are showing the change in force A F which would accompany that change in deflection. The stainless steel wire undergoes a much larger change in force compared to the change in force of nitinol wire when both wires are deflected an equivalent amount. This property of the wire (that is, the change in load), divided by the change in deflection, is called the spring rate of the material. The graph shown in Fig. 7 demonstrates that the spring rate of stainless steel is approximately twice that of nitinol. Clinically, this means that, for any given malocclusion, nitinol wire will produce a lower, more constant, and continuous force on the teeth than would a stainless steel wire of equivalent size.
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F~Q. 8. This typodont shows a 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol wire ligated into a simulated malocclusion. Every other tooth is positioned in an ideal arch form, while each alternate tooth is displaced lingually and increasing 0.5 mm. The lower right second molar is in the ideal arch form, the lower right first molar is inset lingually 0.5 mm., and so on around the arch (with the exception of the two central incisors) to the lower left first molar, which is inset 3 mm. Note that the wire is severely deformed on the left side of the arch.
Fig. 9. nitinol
Same as Fig. 8 except wire into brackets have
that the ligature ties holding the 0.017 by 0.025 (0.432 by 0.635 been cut to permit the wire to return to its original arch form.
mm.)
Clinical applications Nitinol wire can be used in Class I, Class II, or Class III malocclusions, in both extraction and nonextraction cases. In selecting cases that benefit most from the use of nitinol wire, the primary criterion is the amount of malalignment of the teeth from the ideal arch form. The more the wire has to be deflected from the ideal arch form when ligated into the bracket, the greater benefit nitinol wire has over stainless steel wire. even in extreme cases, to permit Figs. 8 and 9 demonstrate how easily nitinol deflects, secure ligation without deforming. Fig. 8 illustrates an 0.017 by 0.025 inch (0.432 by 0.635 mm.) nitinol wire ligated into a simulated malocclusion on a typodont. In this model, every other tooth was positioned in an ideal arch form. The alternate teeth were
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10.
Fig.
11.
Laboratory and clinical analysesof nitinol wire 149
Nitinol wire used as a torquing auxiliary in uprighting impacted canines. Nitinol auxiliary torquing wire activated to the stabilizing arch wire by hooking and tying it in
place. Fig. 12. Sectional arch wire tied beneath bracket tie wings to reposition a displaced canine.
displaced lingually with increasing depth at 0.5 mm. increments. That is, the first tooth is inset 0.5 mm., the second 1 mm., the third 1.5 mm., and so on, around the arch to a maximum displacement of 3 mm. Fig. 9 shows the arch wire after the ligatures had been removed so that the wire could return to its original form. Note that there are only minor permanent bends in the wire. The most important benefits from nitinol wire are realized when a rectangular wire is inserted early in treatment. Simultaneous rotation, leveling, tipping, and torquing can be accomplished earlier with a resilient rectangular wire, such as nitinol. Clinicians have been successful in beginning treatment of certain carefully selected cases with full-size 0.017 by 0.025 inch (0.432 by 0.635 mm.) and 0.019 by 0.025 inch (0.483 by 0.635 mm.) rectangular arch wires that nearly fill the bracket slot. In a few instances, the entire case has been treated with just one arch wire. It is ideally suited for use with most pretorqued and preangulated appliances, because tipping and uprighting of the teeth can be initiated in the early stages of treatment. When the case is nearing completion with a nitinol arch wire, there is very little to be done in the way of placing compensating bends to upright roots, once the spaces have been closed. In the treatment of extraction cases with pretorqued and preangulated twin brackets and nitinol, conventional auxiliary methods of closing spaces may be used along with headgear when needed. The use of nitinol with pretorqued and preangulated brackets requires careful monitoring of tooth movement because of the wire’s high elasticity and more continuous force. Therefore, time intervals between appointments cannot be extended . Cross-bite correction. Nitinol wire has been successfully used in the correction of cross-bites. If the clinician pliers progressive first-order offsets in the buccal segment of
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nitinol wire, so that each tooth will be loaded in approximately equal amounts, and then expands and contracts the two arch wires opposite the direction of cross-bite, nitinol wire will produce a direction of force having a low load-deflection rate that is more desirable than cross-the-bite elastics. Uprighting impacted canines. Another clinical use of nitinol wire is the uprighting of impacted canines by the use of a couple or torque in the second order of space. The torque is accomplished by inserting an auxiliary nitinol wire into the bracket slot of the canine. This auxiliary extends % inch mesial and distal to the canine bracket (Fig. 10). After both ends of the nitinol wire are annealed, auxiliary hooks can be bent in the ends. The auxiliary is then deflected to a stablizing stiff arch wire and the auxiliaries are hooked to it by either ligating it to the stablizing wire or hooking the annealed bent hooks under and over the stablizing arch wire (Fig. 11). This reaction, in turn, produces a tipping action about the bracket and produces an uprighting force on the canine roots. Nitinol is also successful when used as a sectional arch for bringing high or lingually displaced canines into position. Such arches can be ligated beneath the tie wings (Fig. 12) or, better, used with any band-side slotted bracket. Opening the bite. Nitinol wire can be used effectively in opening the bite by either intruding the maxillary and mandibular anterior segments and extruding the posterior segments. When opening a deep bite, in order to get interbracket clearance in the mandibular arch for complete anterior maxillary retraction, one can place first-order steps in the anterior segment to produce intruding forces on the anterior teeth and extruding forces on the premolars and molars. Another method of opening the bite is to plier into the wire about one half inch or more reverse curve of spee and use either a crimpable loose tie-back or a loose cinch-back mesial or distal to the buccal tube. This allows nitinol’s highly elastic forces to exert depressing forces on the anterior teeth and molars and extruding forces on the premolars. For the maxillary arch, the same two principles apply, that is to say either a second-order step in the desired segment of anterior teeth that need to be intruded and posterior teeth that need extrusion or the use of a more pronounced curve of Spee in the anterior segment only. The forces exerted in this example are intrusive forces on the anterior segment and extrusive forces on the posterior segment. Limitations The most significantly different clinical characteristic of nitinol wire, when compared to stainless steel wire, is its resistance to taking a bend. It can be deflected far out of plane without losing its ability to spring back. This can cause some problems in placement of desired bends, in-and-out steps, loops, and torques. Successfully using nitinol in a conventional edgewise practice requires some changes in the way orthodontists use their wires. Nitinol cannot be bent with “sharp-cornered ” instruments. Although nitinol feels quite flexible and gives the impression that it is very ductile, it will readily break when bent over a sharp edge. Clinicians generally have no difficulty in putting in first- and second-order bends or the desired torque, although the wire has to be severely overformed in order to get it to take the desired permanent bend. The bending of loops or omega bends in nitinol is not recommended. They are time consuming to bend, and are a potential source of failure when one is ttying to close the loops. Closing loops in particular are not practical for nitinol wire.
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Because nitinol cannot be soldered or successfully welded to itself without annealing the wire, and because the bending of tie-back hooks entails a high risk of failure, practitioners have found crimpable hooks and stops a successful alternative. “Cinch-backs” distal to the buccal tubes are easily accomplished by either resistance-annealing or flameannealing the end of the nitinol wire. Once the wire is dead soft, it is very easy to bend into any desired configuration, but care must be taken not to overheat the wire. A dark blue color indicates that the temperature is sufficient to anneal the wire. If nitinol wire is heated to a cherry red, as with stainless steel wire, there is a risk of making it brittle. In addition, by its very nature, nitinol is not a stiff wire, which means that it can easily be deflected. Occasionally the use of auxiliary retraction forces will have the effect of opening the patient’s posterior bite. There are two ways to remedy this: (1) simply remove the auxiliary force mechanism at the completion of treatment and the nitinol arch wire will relevel the bite or (2) take the nitinol wire out and go to conventional posterior bite-closing mechanics with a steel arch wire. This same limitation is also evident when one wishes to provide maximum stability in the arch at the completion of treatment. Such stability is often best maintained by using a stiffer stainless steel arch wire tailored to the desired finished occlusion. Conclusion
Nitinol orthodontic wire has physical properties quite different from stainless steel which make it easy to deflect and ligate while providing it with an exceptional working range. This means that large-dimension nitinol wires, which more completely fill bracket slots, may be used earlier in treatment when indicated. In turn, the number of graduated arch wire changes normally required can be reduced or, in certain cases, completely eliminated. At the same time, these larger nitinol arch wires readily accomplish and maintain desired rotations and leveling without increasing the patient’s discomfort. In conclusion, our findings are that, when applied with skill and professional judgment, nitinol arch wire represents a significant improvement over conventional orthodontic arch wire and is a valuable addition to the orthodontist’s armamentarium. REFERENCES 1. 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. 2. Andreasen, G. F., and Brady, P. R.: A use hypothesis for 55 nitinol wire for orthodontics, Angle Orthod. 42: 172-177, 1972. 3. Andreasen, G. F., and Barrett, R. D.: An evaluation of cobalt substituted nitinol wire in orthodontics, AM. J. ORTHOD. 63: 462-470, 1973. 4. Heisterkamp, C. A., Buehler, W. J., and Wang, F. E.: 55Nitinol: A new biomaterial, paper presented at the 8th International Conference on Medical and Biomedical Engineering (Chicago), 1969. 5. Castleman, L. S., Motzkin, S. M., Alicardri, F. P., and Bonawit, V. L.: Biocompatibility of nitinol alloy as an implant material, J. Biomed. Mater. Res. 10: 695-731, 1976. 6. Civjan, S., Huget, E. F., and De Simon, L. B.: Potential applications of certain nickel-titanium (nitinol) alloys, J. Dent. Res. 54: 89-96, 1975. 7. Council on Dental Materials and Devices: New American Dental Association specification No 32 for orthodontic wires not containing precious metals, J. Am. Dent. Assoc. 95: 1169-l 171, 1977. Dr. Andreasen: College Mr. Morrow: 862 Hugo
of Denrisny (52242) Reid St., Arcadia, Calif.
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