498

The use of nickel-titanium (NiTi) wires inorthodontics has created considerable interest becauseof the unique and useful mechanical properties of thewires. The original NiTi wires were martensitic atbody temperature and displayed high spring-backproperties.1-4 Superelastic NiTi wires introduced theconcept of applying a superelastic unloading curvethat potentially could deliver more constant force.5-7With the introduction of the newer higher transitiontemperature NiTi wires, shape memory became animportant parameter in the use and understanding offorce delivery.

Compression and tension coil springs made of NiTihave been recommended because of their 2 advan-tagesa minimum of permanent deformation and thepossibility of a more constant force during unload-ing.8-11 It was the purpose of this investigation to studyin depth 3 commercial closed coil (tension) springsthat have clinical targeted values of 100g, 150g, and200g.

The study was designed to investigate the forcesproduced not only at body temperature but also at tem-peratures that may vary considerably in the mouth dur-ing eating and the drinking of fluids. Of particularinterest was the determination of any differencebetween the targeted force values that were suggestedby the manufacturer and the delivered forces. Finally,

the slope of the unloading curve was studied to seewhether a relatively constant force was produced.

METHODSA special apparatus was constructed that allowed

accurate temperature control of the springs during theiractivation. Components included a Zwick Z010 mater-ial testing system, a model 091251 temperature testchamber climatic test system, and a Baldor IM-3363-DC Servomotor to deliver the force. Temperature wasvaried from 15C to 60C. The equipment used duringthe test is shown in Fig 1.

Three types of springs (GAC International, CentralIslip, NY) were tested and were denoted as heavy,medium, and light. The details of the spring design aregiven in Table I. Each spring was activated 5 times, andthe data were averaged for each temperature. The num-ber of springs at each temperature varied with thelargest concentration of springs used near body tem-perature (37C). The number of springs tested for eachtemperature is given in Tables II, III, and IV.

Each spring was activated from its passive positionto a total of 15 mm of activation and then deactivated tothe zero or passive position. The deactivation forces onlyare given in Tables II, III, and IV because they are more

ORIGINAL ARTICLE

Force characteristics of nickel-titanium tension coil springsHeinz Tripolt, MD,a Charles J. Burstone, DDS, MS,b Peter Bantleon, DDS, MD, MSc andWolfgang Manschiebel, MDaVienna, Austria, and Farmington, Conn

Nickel-titanium closed coil springs are commonly used for space closure. The springs possess a highresistance to permanent deformation and the potential for relatively constant force delivery. A study wasdesigned to determine whether relatively constant forces can be delivered and whether the forcemagnitudes approach the manufacturers targeted force values. Heavy, medium, and light springs wereactivated 15 mm at temperatures that ranged from 15C to 60C. The forces were measured duringdeactivation with a specially constructed force transducer temperature chamber. Relatively constant forcescan be achieved with an over-activation procedure that allows relaxation to the desired activation. The lightsprings delivered forces that were near the targeted force; no difference was found between the heavy andmedium springs in the constant force range. The force magnitudes varied markedly depending on mouthtemperature. (Am J Orthod Dentofacial Orthop 1999;15:498-507)

aDepartment of Orthodontics, University of Vienna.bProfessor, Department of Orthodontics, University of Connecticut.cProfessor, Department of Orthodontics, University of Vienna.Reprint requests to: Dr Charles J. Burstone, University of Connecticut HealthCenter, Department of OrthodonticsMC1725, 263 Farmington Ave, Farmington,CT 06032.Copyright 1999 by the American Association of Orthodontists.0889-5406/99/$8.00 + 0 8/1/92613

Table I. Spring designParameter Light Medium Heavy

Target force (g) 100 150 200Passive length (mm) 5 5 5Wire cross-section (mm) 0.21 0.24 0.5Helix diameter (mm) 0.93 0.92 0.91

American Journal of Orthodontics and Dentofacial Orthopedics Tripolt et al 499Volume 115, Number 5

relevant in representing the forces that act on the teethduring tooth movement. Forces given in newtons can beconverted to grams by multiplying a factor of 1.0197.

RESULTSFor simplicity in presentation, the loading curves

are not shown. Plots of NiTi coil springs, as with non-

formed wires, exhibit characteristic hysteresis duringthe loading-unloading cycle.

Force Characteristics at Mouth Temperature (37C)At 15 mm of activation, the heavy, medium, and

light springs delivered 4.72 N, 3.85 N, and 2.1 N at37C, respectively (Fig 2). A rapid force drop off

A

B CFig 1. Temperature-controlled spring tester. A, Overall view of equipment. Temperature chamber onright. B, Passive spring. C, Activated spring.

500 Tripolt et al American Journal of Orthodontics and Dentofacial OrthopedicsMay 1999

Table II. Forces from heavy nickel-titanium springs during deactivation (newtons)15C

N/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.8 1 1.3 1.5 2 3 4.5Spring 2 0.6 1.1 1.3 1.5 2 3.3 3.9Spring 3 0.6 1.1 1.3 1.5 2 2.2 3.7Spring 4 0.8 1 1.1 1.3 1.7 2.5 3.6Spring 5 0.6 1.1 1.3 1.5 1.8 2.5 3.6Mean 0.68 1.06 1.26 1.46 1.9 2.7 3.86

20CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.7 1.1 1.3 1.5 2 3 4.5Spring 2 0.7 1.3 1.5 1.7 2.2 2.9 4Spring 3 0.7 1.3 1.4 1.6 2 2.7 3.8Mean 0.70 1.23 1.40 1.60 2.07 2.87 4.10

25CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.8 1.2 1.4 1.65 2.2 3 4.8Spring 2 1 1.4 1.5 1.7 2 2.7 4Spring 3 1 1.4 1.5 1.7 2.2 2.9 4Spring 4 1 1.5 1.6 1.8 2.3 2.9 4Spring 5 1 1.3 1.5 1.6 2 2.7 4Mean 0.96 1.36 1.5 1.69 2.14 2.84 4.16

30CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1 1.7 1.8 2 2.5 3.2 4.4Spring 2 1 1.7 1.8 2.1 2.5 3.2 4.5Spring 3 1.2 1.5 1.7 2 2.5 3.3 4.7Spring 4 0.9 1.5 1.7 1.95 2.3 2.95 4Spring 5 1 1.5 1.7 1.9 2.3 3 4.2Spring 6 1.2 1.4 1.55 1.9 2.5 3.5 4.8Spring 7 1.1 1.5 1.6 2 2.5 3.5 4.8Mean 1.06 1.54 1.69 1.98 2.44 3.24 4.49

35CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.2 1.9 2 2.3 2.7 3.4 4.4Spring 2 1.3 1.9 2 2.2 2.5 3 4.1Spring 3 1 1.55 1.7 1.95 2.3 3 4Mean 1.17 1.78 1.90 2.15 2.50 3.13 4.17

37CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.1 1.5 1.7 2 2.5 3.4 4.7Spring 2 1.1 1.5 1.7 2 2.5 3.5 4.8Spring 3 1.2 1.5 1.7 2 2.5 3.5 4.7Spring 4 1.1 1.45 1.55 1.8 2.4 3.2 4.7Spring 5 1.1 1.5 1.7 2 2.5 3.45 4.7Mean 1.12 1.49 1.67 1.96 2.48 3.41 4.72

40CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.5 2 2.2 2.5 2.8 3.5 4.1Spring 2 1.5 1.9 2.2 2.3 2.7 3.3 4.4Spring 3 1.5 1.8 2 2.2 2.7 3.5 4.8Mean 1.50 1.90 2.13 2.33 2.73 3.43 4.43

50CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.7 2.2 2.4 2.5 3 3.8 5.1

60CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.9 2.45 2.7 3 3.5 4.2 5.6

American Journal of Orthodontics and Dentofacial Orthopedics Tripolt et al 501Volume 115, Number 5

Table III. Forces from medium nickel-titanium springs during deactivation (newtons)15C

N/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.4 0.6 0.75 1 1.45 2 3Spring 2 0.45 0.6 0.75 1.05 1.5 2.2 3.4Spring 3 0.4 0.55 0.7 1 1.4 2 3.2Spring 4 0.6 0.75 0.9 1.25 1.5 2.3 3.4Spring 5 0.6 0.8 1 1.3 1.7 2.5 3.6Mean 0.49 0.66 0.82 1.12 1.51 2.20 3.32

20CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.5 0.7 0.95 1.2 1.5 2.3 3.4Spring 2 0.65 0.75 1 1.3 1.5 2.3 3.3Spring 3 0.7 0.9 1 1.3 1.8 2.5 3.5Spring 4 0.7 0.9 1.1 1.4 1.7 2.5 3.5Spring 5 0.5 0.8 1 1.3 1.6 2.3 3.3Spring 6 0.7 1 1.15 1.5 1.9 2.6 3.7Mean 0.63 0.84 1.03 1.33 1.67 2.42 3.45

25CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.6 1 1.15 1.4 1.7 2.4 3.4Spring 2 0.7 1.1 1.3 1.5 2 2.7 3.8Spring 3 0.8 1.1 1.3 1.5 1.8 2.5 3.5Spring 4 0.6 1 1.3 1.5 1.9 2.5 3.5Spring 5 0.8 1.3 1.3 1.5 1.8 2.5 3.5Mean 0.70 1.10 1.27 1.48 1.84 2.52 3.54

30CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.7 1.1 1.3 1.45 1.7 2.3 3.5Spring 2 0.9 1.3 1.5 1.7 2.1 2.6 3.6Spring 3 0.9 1.3 1.5 1.8 2.3 3 4.1Spring 4 0.6 1.3 1.4 1.7 2 2.7 3.7Spring 5 0.7 1.3 1.4 1.6 2 2.5 3.6Mean 0.76 1.26 1.42 1.65 2.02 2.62 3.70

35CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.1 1.4 1.5 1.7 2.1 2.6 3.7Spring 2 1 1.3 1.5 1.7 2 2.6 3.7Spring 3 0.7 1.3 1.4 1.5 1.9 2.5 3.6Mean 0.93 1.33 1.47 1.63 2.00 2.57 3.67

37CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.1 1.4 1.6 1.7 2.1 2.6 3.7Spring 2 1.1 1.4 1.6 1.7 2.2 2.7 3.7Spring 3 1.1 1.5 1.6 1.95 2.4 3 4Spring 4 1.1 1.5 1.7 2 2.3 3 4Mean 1.10 1.45 1.63 1.84 2.25 2.83 3.85

40CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1 1.5 1.7 1.9 2.2 2.9 3.9Spring 2 1.1 1.6 1.7 2 2.4 3 4Spring 3 1.1 1.6 1.8 2.1 2.6 3.3 4.3Mean 1.07 1.57 1.73 2.00 2.40 3.07 4.07

50CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.3 1.8 2 2.3 2.7 3.4 4.3

60CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.5 2 2.3 2.6 3 3.6 4.6

502 Tripolt et al American Journal of Orthodontics and Dentofacial OrthopedicsMay 1999

Table IV. Forces from light nickel-titanium springs during deactivation (newtons)15C

N/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.1 0.25 0.35 0.45 0.55 0.8 1.3Spring 2 0.15 0.25 0.3 0.4 0.55 0.95 1.5Spring 3 0.25 0.4 0.5 0.55 0.75 1 1.7Spring 4 0.2 0.3 0.45 0.5 0.7 1.1 1.6Spring 5 0.2 0.35 0.4 0.5 0.65 1 1.5Mean 0.18 0.31 0.4 0.48 0.64 0.97 1.52

20CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.1 0.3 0.45 0.5 0.65 0.9 1.35Spring 2 0.3 0.4 0.5 0.55 0.65 1 1.6Spring 3 0.4 0.5 0.6 0.7 0.9 1.25 1.9Spring 4 0.45 0.5 0.6 0.7 0.9 1.3 1.9Spring 5 0.35 0.4 0.5 0.65 0.75 1.2 1.7Mean 0.32 0.42 0.53 0.62 0.77 1.13 1.9

25CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.2 0.45 0.55 0.7 0.9 1.2 1.6Spring 2 0.45 0.5 0.7 0.9 1.1 1.5 1.9Spring 3 0.4 0.7 0.8 1 1.2 1.5 2Spring 4 0.4 0.6 0.7 0.9 1 1.4 1.9Spring 5 0.5 0.55 0.6 0.75 1 1.3 1.8Mean 0.39 0.56 0.67 0.85 1.04 1.38 1.84

30CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.2 0.7 0.8 0.9 1.1 1.4 1.9Spring 2 0.4 0.9 0.95 1.01 1.3 1.5 2.2Spring 3 0.25 0.55 0.7 0.9 1.1 1.4 1.8Spring 4 0.4 0.5 0.55 0.65 0.9 1.2 1.7Spring 5 0.5 0.9 1 1.1 1.4 1.6 2.2Spring 6 0.4 0.7 0.8 0.9 1.1 1.4 1.9Mean 0.36 0.71 0.80 0.91 1.15 1.42 1.95

35CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.6 0.75 0.9 1 1.15 1.5 2Spring 2 0.7 0.9 1 1.1 1.25 1.5 1.9Spring 3 0.5 0.7 0.9 1 1.1 1.4 1.8Mean 0.60 0.78 0.93 1.03 1.17 1.47 1.90

37CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.7 0.9 1 1.2 1.4 1.7 2.2Spring 2 0.7 1.05 1.2 1.3 1.45 1.7 2.3Spring 3 0.5 0.95 1.1 1.2 1.4 1.6 2.2Spring 4 0.5 0.9 1 1.1 1.3 1.5 2Spring 5 0.7 1 1.2 1.3 1.4 1.5 1.9Mean 0.62 0.96 1.10 1.22 1.39 1.60 2.12

40CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.7 0.9 1 1.2 1.4 1.7 2.2Spring 2 0.7 1.1 1.3 1.4 1.5 1.8 2.3Spring 3 0.6 0.9 1 1.1 1.3 1.5 2.1Mean 0.67 0.97 1.10 1.23 1.40 1.67 2.20

50CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 0.9 1.4 1.5 1.6 1.8 2.1 2.6

60CN/mm 0.1 2.5 5 7.5 10 12.5 15Spring 1 1.1 1.5 1.7 1.9 2.05 2.4 2.8

American Journal of Orthodontics and Dentofacial Orthopedics Tripolt et al 503Volume 115, Number 5

occurs between 15 mm and 12.5 mm during deactiva-tion: heavy, 1.31 N; medium, 1.02 N; and light, 0.52 N.From 7.5 mm to 2.5 mm during deactivation, the dropoff is less: heavy, 0.47 N; medium, 0.39 N; and light,0.26 N (Tables II, III, and IV).

The data show that, with an initial activation of 15mm, there is a rapid drop off in force magnitude, par-ticularly with the heavy and medium springs. It is farmore advantageous to initially activate 15 mm beyondthe desired activation and to let the spring return to anactivation of perhaps 7.5 mm. If this procedure is car-ried out, the force magnitude would remain more con-stant within a deactivation range of 7.5 mm to 2.5 mm.

Fig 2 shows that the light spring delivers a more con-stant force during unloading than do the medium andheavy springs. There is little difference between theheavy and the medium springs in their useful range ofdeactivation after about 7.5 mm.

The variation among 5 light springs is shown in Fig3. Although there is variation, it is remarkably small.At a 15-mm force, the values vary from 1.9 N to 2.3 N,and, at 7.5 mm, from 1.4 N to 1.5 N. Similar results canbe seen for the medium spring (at 15 mm, 3.7 N to 4.0N and at 7.5 mm, 1.7 N to 2.0 N; Table III). The heavysprings varied from 4.7 N to 4.8 N at 15 mm and from1.8 N to 2.0 N at 7.5 mm (Table II). Because all the

Fig 2. Medium and heavy springs deliver similar forces.

Fig 3. Variation among springs of given batch is small.

504 Tripolt et al American Journal of Orthodontics and Dentofacial OrthopedicsMay 1999

Fig 4. Variation in force magnitude of heavy spring with temperature.

Fig 5. Variation in force magnitude of medium spring with temperature.

American Journal of Orthodontics and Dentofacial Orthopedics Tripolt et al 505Volume 115, Number 5

springs were from the same batch, it has not beenshown that a small variation might be present betweendifferent batches from the manufacturer.

Force Variation with TemperatureForce magnitude varied considerably in the range

of temperatures studied from 15C to 60C. The totalforce variation for the heavy spring was 1.74 N, 1.54 N,and 1.39 N at 15 mm, 7.5 mm, and 2.5 mm of deacti-vation points, respectively (Fig 4). The medium springhad a force differential at 15 mm, 7.5 mm, and 2.5 mmof 1.28 N, 1.48 N, and 1.34 N (Fig 5). The light springshowed a smaller force differential with 1.28 N at 15mm, 1.42 N at 7.5 mm, and 1.19 N at 2.5 mm (Fig 6).

The percentage of force loss or gain with tempera-ture decrease or increase is much less at the smallerdeactivation positions of the spring. Nevertheless, thechange in force magnitude is large with varying tem-peratures.

It could be argued that with most clinical situationsthe variation in temperature in the mouth and on thewire would not be as extreme as shown in this study. Itis useful, therefore, to compare the variation in forcewith temperatures close to body temperature at 37C.From 30C to 40C, all the springs exhibited a varia-tion between 0.25 N and 0.37 N. This variation is rela-

tively small considering the overall force magnitude,particularly of the medium and heavy springs. With thelight spring, which at 2.5 mm on the deactivation curvedelivers 0.96 N at 37C, a variation of 0.2 N can havesome clinical significance because it represents morethan 20% of the average force delivered.

DISCUSSIONThese studies support the conclusion that, with the

use of a superelastic NiTi spring or wire, a more con-stant force range can be reached by overactivatingbefore setting the spring to the desired amount of acti-vation. This is much easier to accomplish with a coilspring than with an arch wire. For instance, if a deacti-vation position of 7.5 mm gives the desired force level,then initial activation should be to 15 mm. This ensuresthat the phase change from martensitic to austenitic,which gives the more constant force magnitude, occurswithin the clinical range of use. In our study, it wasshown that, by allowing the spring to deactivate 5 mmfrom position 7.5 mm to 2.5 mm on the unloadingcurve, a relatively constant force is produced. The timefor reactivation would be after 5 mm of deactivation atthe 2.5-mm position. Differentiation is made betweenan initial activation (overactivation) and the desiredactivation of the spring for optimal use. Desired activa-

Fig 6. Variation in force magnitude of light spring with temperature.

506 Tripolt et al American Journal of Orthodontics and Dentofacial OrthopedicsMay 1999

tion in millimeters should be considerably less thatthan the overactivation in millimeters that is used tostretch the spring before placement.

When the NiTi springs are used at near body tem-perature, they show the typical superelastic curve,which allows for a range of relatively constant force. Itis not known biologically how force should vary inmagnitude with a continuous force system. Should theforce be constant as a tooth moves, or should the forceincrease in magnitude or decrease in magnitude? In thepast, springs with lowload deflection rates (a decreas-ing gradient force) were advocated because alowforce deflection rate made it easier to predict themagnitude of force used and to ensure that the toothwould not be subjected to radical changes in the mag-nitude of force during movement. On the other hand, itcould be argued that, once the periodontal ligamentthickens and biological changes occur (both cellularand extracellular), there could be a slight increase inthe magnitude of force. The concept of relatively con-stant force is intriguing, and it might intuitively bethought of as desirable because, with displacement, thestress in the periodontal ligament may not be markedlyaltered from 1 time period to another.

One of the disadvantages of a constant force springthat is not dependent on a lowforce deflection rate isthat it is more difficult to vary the magnitude of theforce. With a typical material, like stainless steel, theforce-deflection rate could be reduced by reducing thecross-section of the wire, increasing the diameter of thehelix, and increasing the passive length of the spring.With this type of spring, a low deflection rate could the-oretically be developed, which means that, duringunloading, the force would be relatively constant. Therewould be the advantage that, if heavier forces weredesired, the amount of activation would be increased.This is not true of a NiTi superelastic spring whereincreasing the activation may not increase the magnitudeof force. In fact, the nature of the phase change curveusually suggests that a smaller activation may actuallyincrease force magnitude. This means that, to developdifferent magnitudes of force with a superelastic NiTispring, it is necessary to either change design or alter theheat treatment of the wirethus, the need for differentsprings for different force magnitudes.

The data that are presented in this study are onlyon the basis of a 15-mm activation of standardizedspring length and helix dimensions. The points on thedeactivation curve and their corresponding force mag-nitudes would vary significantly if other activationsare used. At small activations, little or no superelasticcurve would be observed and so a higher less-constantforce would be produced. For this reason, superelastic

NiTi springs are sensitive to the amount of the fullactivation and to the amount of relaxation allowed toreach the desired activation. This is not the case withsteel, beta-titanium, or other traditional materials inwhich the force-deflection rate is independent of themagnitude of the activation, provided that no perma-nent deformation occurs. There was a significant dif-ference between the light spring and the heavy ormedium springs in force magnitude. At 7.5 mm (deac-tivation curve point), the heavy spring delivered 200gand the light spring delivered 122g. This is close tothe targeted goal that was listed as the desired forcemagnitude for the springs. In this targeted range ofuse (activation at 7.5 mm to 2.5 mm), there was nodifference between the force magnitudes of the lightand medium springs.

As pointed out, it would be possible to design astainless steel spring to deliver a relatively constantforce with the principle of a lowforce deflection rate.Although this might be achieved, the stainless springwould have many disadvantages when compared withthe NiTi spring. The stainless steel spring would belarger in both helix diameter and spring length and thusnot as adaptable to the conditions in the mouth. Thecross-section of the steel wire could be reduced tolower the force, but then there is the risk of permanentdeformation. NiTi springs are superb in their resistanceto permanent deformation and so offer superiority tostainless steel in this respect.

NiTi springs, which are fabricated of superelasticmaterials, undergo transformations during activationfrom austenitic to martensitic and during deactivationfrom martensitic to austenitic. This phase transforma-tion is related to the superelastic curve, which is usedfor the relatively constant force magnitude. In addition,these materials exhibit a shape memory effect. As thetemperature increases, a change from martensitic toaustenitic occurs. This can be seen in the temperaturevariation data where, at points along the deactivationcurve, different forces are produced depending on howmuch deactivation has occurred. As the wire approach-es full deactivation, the austenitic phase is reached.Thus, increases in temperature will have a lesser effecton force magnitude.

Airoldi et al12 have shown that, during drinking ofhot and cold fluids, transient variations occur in themouth temperature adjacent to areas where an archwire or spring is placed. This change in temperature,which could last many seconds, varied from 8C tomore than 50C. Although they did not study solidfoods placed in the mouth, it is possible that hot or coldfoods could produce even greater temperature changesin a longer period of time. What is the clinical signifi-

American Journal of Orthodontics and Dentofacial Orthopedics Tripolt et al 507Volume 115, Number 5

cance of this temperature change? It can be argued thatit is both good and bad. Perhaps a continually changingenvironment of forces allows the periodontal ligamentto undergo some desirable physiologic changes. As theforce is lowered, the blood vessels could open up andallow for tissue repair. We do not know whether a con-stant force measured in minutes or hours is desirable.Perhaps some type of oscillating force would allow abetter physiologic response. On the other hand, thenegative aspect of the force variation is that it makesthe force system less predictable if we are trying todeliver from our appliance a known force magnitude.

The targeted force magnitudes for a given springmay not be the force magnitudes that are actually deliv-ered under clinical conditions because of the great vari-ation in temperature in the mouth. Although the varia-tion in force magnitude is significant, it should benoted that this study showed that, in the temperaturerange from 30C to for 40C, force magnitude is rela-tively small. Biologic studies are necessitated to see ifthis variation in force magnitude with small or largetemperature changes has clinical and physiologic sig-nificance.

CONCLUSIONA valid approach to the delivery of relatively con-

stant force can be achieved by taking advantage of theunique nature of the unloading curve with superelasticNiTi coil springs. However, it is more difficult to deliv-er a desired magnitude because many variables influ-ence the force level, including the amount of activation,the manufacturing variation in transition temperature,and mouth temperature.

1. A standardized NiTi closed coil (tension) springcan produce a relatively constant force duringdeactivation, provided that the correct techniqueis used.

2. With the standardized springs, a 15-mm activa-tion, followed by a 7.5-mm deactivation to the

desired activation of 7.5 mm, delivers a relative-ly constant force if the spring is used for 5 mm oftooth movement.

3. Standard springs from the same batch producerelatively reproducible force magnitudes duringdeactivation.

4. Superelastic coil springs are extremely tempera-ture sensitive and thus produce a large force vari-ation at different mouth temperatures. However,in a narrow temperature range, this variation issmall. The clinical implications of the longertransient temperature ranges in the mouth are notknown.

5. Little difference was found between the heavyand medium springs. The light spring deliveredforces that were close to the targeted force mag-nitudes described by the manufacturer.

REFERENCES

1. Andreasen GF, Hilleman TB. An evaluation of 55 cobalt substituted nitinol wire foruse in orthodontics. J Am Dent Assoc 1971;82:1373-5.

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

3. Lopez I, Goldberg AJ, Burstone CJ. Bending characteristics of nitinol wire. Am JOrthod 1979;75:569-74.

4. Goldberg AJ, Morton J, Burstone CJ. The flexure modulus of elasticity of orthodon-tic wires. J Dent Res 1983;62:856-8.

5. Burstone, CJ, Morton JY. Chinese NiTi wirea new orthodontic alloy. Am J Orthod1985;87:445-52.

6. Miura F, Mogi M, Ohura Y, Hamanaka H. The super-elastic property of the JapaneseNiTi alloy wire for use in orthodontics. Am J Orthod Dentofac Orthop 1986;90:1-10.

7. Miura F, Ohura Y, Karibe M. The super-elastic Japanese NiTi alloy wire for use inorthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J OrthodDentofac Orthop 1988;94:89-96.

8. Bondemark L, Kurol J, Bernhold M. Repelling magnets versus superelastic nickel-titanium coils in simultaneous distal movement of maxillary first and second molarssimultaneous distal movement of maxillary first and second molars. Angle Orthod1994;64:189-98.

9. Erverdi N, Koyuturk O, Kucukkeles N. Nickel-titanium coil springs and repellingmagnets: a comparison of two different intra-oral molar distalization techniques. Br JOrthod 1997;24:47-53.

10. Sangkyu H, Quick, D. Nickel-titanium spring properties in a simulated oral environ-ment. Angle Orthod 1993;63:67-72.

11. Von Fraunhofer JA, Bonds PW, Johnson BE. Force generation by orthodontic coilsprings. Angle Orthod 1993;63:145-8.

12. Airoldi G, Riva G, Vanelli M, Filippi V, Garattini G. Oral environment temperaturechanges induced by cold/hot liquid intake. Am J Orthod Dentofac Orthop1997;112:58-79.