ONLINE ONLY

Force characteristics of nopen-coil springs

Allyson Bourke,a John Daskalogiannakis,b Bryan Tompso

Hamilton and Toronto, Ontario, Canada

Introduction: The objective of this study was to quantify thtitanium open-coil springs. Methods: Eleven springs from

d togthcordrovirespsignmlycoilsforc

which42.e

Therefore, tightly wound coils (small pitch) generally

en-coiland 3Mm longniversald maxi-ed con-ing theitude athorten-plateaubatches.

Manhartsberger and Seidenbusch evaluated uni-

than the labeled forces. According to the manufacturer,

duced 300 g of force when compressed to 80% of theiroriginal length. They concluded that a new activation

Toronto, Ontario, Canada.

dProfessor, Faculty of Dentistry, University of Toronto, Toronto, Ontario,

0889-5406/$36.00

Copyright 2010 by the American Association of Orthodontists.doi:10.1016/j.ajodo.2010.01.026142.e1the coils should have produced the labeled forces whenthey were compressed up to 80% of their original lengthand maintained these forces during deactivation. Theauthors found that the Sentalloy open-coil springswith a suggested force delivery of 150 g actually pro-

Canada.

The authors report no commercial, proprietary, or financial interest in the

products or companies described in this article.

Reprint requests to: Allyson Bourke, 875 Upper Wentworth street, Hamilton,

Ontario, L7P 0M9, Canada; e-mail, allysonbourke@hotmail.com.

Submitted, March 2009; revised and accepted, January 2010.formly wound Sentalloy coil springs from GAC andfound that the generated forces were actually greater

cAssociate professor and head, Department of Orthodontics, Faculty of

Dentistry, University of Toronto; head, Division of Orthodontics, Toronto,

Ontario, Canada.fabricated from heat-activated superelastic NiTi (Sen-talloy, GAC International, Bohemia, NY) and stainlesssteel (3M Unitek, Monrovia, Calif). The Sentalloyopen-coil springs (0.010 3 0.035 in) produced forcesof 55 to 70 g with 9 mm of activation, whereas the stain-less steel springs (0.0103 0.030 in) produced forces of200 g when activated by just 1 mm.

The variables that determine the force produced byan open-coil spring are its lumen size, wire type, andwire size. All other things being equal, the larger the

produce lower forces.1

Schneevoight et al2 investigated 32 NiTi opsprings from 7 manufacturers, including GACUnitek. Segments of coil springs 20 6 2 mwere tested at 27C, 37C, and 47C with a utesting machine. The springs were compressemally. Only GAC Sentalloy springs demonstratstant forces on the unloading curve. Raistemperature caused an increase of the magnwhich the force plateaued by 0.4 to 0.9 N, and a sing of the plateau width by 4% to 15%. Themagnitudes varied by as much as 18% between

3

aPrivate practice, Hamilton, Ontario, Canada.bAssistant professor, Department of Orthodontics, University of Toronto,a 12-week period. A universal testing machine was usesprings were compressed to half of their original lenResults: The average forces generated at the initial reGAC International (Bohemia, NY) and 3M Unitek (Mon38.8% below the manufacturers labeled force levels,wound coils demonstrated statistically and clinically12-week experimental period (P\0.0001). GACs uniforload-deflection ratios (23.7 g/mm). Conclusions: Openone-third of their original length to produce the labeledlower load-deflection ratios and maximum forces,movement. (Am J Orthod Dentofacial Orthop 2010;138:1

Nickel-titanium (NiTi) coil springs can producecontinuous, light forces over a large range ofactivation. They have significantly limited the

use of stainless steel coil springs as force-generatingmodules in orthodontics, since the latter can onlyproduce initial forces of high magnitude that quicklydissipate even with small deactivations. Von Fraunhoferet al1 compared the forces generated by open-coil springsickel-titanium

n,c and Philip Watsond

e properties of commercially available nickel-3 manufacturers were tested 5 times over

measure the force generated when open-coiland then gradually allowed to decompress.ing session for uniformly wound springs froma, Calif) were 19.3% to 42.7% and 9.7% toectively. GACs 100-, 150-, and 200-g stop-ificant stepwise force degradation over thewound light (100 g) coils generated the lowest

might need to be compressed by more thanes. Uniformly wound coils generally produce

are generally more acceptable for tooth1-142.e7)

lumen size and the smaller the wire cross-section, thelighter the force produced, for the same activation.Another important physical parameter when consider-ing biomechanical properties of open-coil springs isthe winding pitch. Pitch is the distance between individ-ual coils in the spring. As the pitch decreases, theamount of wire incorporated into the wire is increased.

oral environment. Based on the desired power of the

base that was bolted in place. The base was designedto allow the springs to be compressed along a lengthof wire. During the testing, springs were compressedon a 0.020-in diameter wire, which was attached toa custom-designed crosshead (Fig 2).

Between tests, the springs were kept compressed to70% of their original length (7 mm) in custom-fabricated compression racks (Fig 3). Each compressionrack could hold 40 springs. The rackswere submerged inphysiologic saline solution (0.9% sodium chloride) andstored in a sealed plastic container (Ziploc, S. C. John-son & Son, Racine, Wis). The sealed containers werestored at 37C in an incubator unit (Isotemp Incubator,senior model 205, Fisher Scientific, New York, NY).

The springs of various force groups were tested inthe same order at every testing session. Testing sessionswere performed at the beginning of the experiment (T0),at 24 hours (T1), 4 weeks (T2), 8 weeks (T3), and 12

142.e2 Bourke et al American Journal of Orthodontics and Dentofacial Orthopedicsstudy (approximately 80%, with an estimated magni-tude of difference of 10% at a level of significance ofP\0.05), a sample size of 20 coil springs for each forcelevel and each design was deemed adequate.

The uniformly wound coils from GAC came inlengths of 15 mm. According to GAC, the springs couldbe compressed by 12 mm (down to 3 mm) and stilldeliver the labeled forces. All other springs came in7-in segments, with no manufacturer-recommendedamount of compression.

All spring segments were cut to a length of 10 mm.This length was chosen for several reasons. First, 10mmwas considered an approximate length of coil used clin-ically to expand an interdental space to the full width ofa tooth. Second, the uniformly wound coils could havebeen cut to any length, whereas the stop-wound coilshad to be cut within the closed (tightly wound) regionsof the coil. A 10-mm length was one that could apply toboth stop-wound coil designs from the 2 manufacturers.

A universal testing machine (model 4301, Servorange should be determined for Sentalloy springs, andthey suggested activation ranges for each force levelof open and closed coils.

Studies of closed-coil NiTi springs have shownsimilar results to the study of Manhartsberger andSeidenbusch.3 Chow4 found that closed coil springsmanufactured by GAC and 3M Unitek produced lowergenerated forces than the labeled values (31% and36% lower, respectively). In the same study, springsmanufactured by TP Orthodontics (LaPorte, Ind)produced forces that were 24% higher than the manu-facturers labeled values. The 3M Unitek springsproduced the most constant forces throughout an activa-tion range of 13 mm and, therefore, were considered themost ideal for clinical tooth movement.

The objectives of this study were to quantify the bio-mechanical properties of commercially available NiTiopen-coil springs from 3 manufacturers and comparethem with their manufacturers specifications.

MATERIAL AND METHODS

Eleven open-coil springs from 3 manufacturers (3MUnitek, TP Orthodontics, and GAC International) wereselected. Springs of 3 force levels and 2 designs (uni-formly wound and stop-wound) were tested (Fig 1,Tables I and II). The following parameters weremeasured for each spring: maximum force, averageforce during deactivation, load-deflection ratio, andforce degradation over a 12-week period in a simulatedHydraulic, Instron, Canton, Mass) was used for all mea-surements. The static load cell was set at a constant 50 N.The speed with which the springs were compressed wasset at 15 mm per minute. The springs were compressedto half of their original relaxed lengths (from 10 to5 mm). This maximum amount of compression waschosen based on the methodology of Chaconas et al.5

The testing machine was programmed to cycle once,by compressing the springs from 10 to 5 mm and thento return to the original length at the same speed.

The data were automatically recorded on a personalcomputer by using Lab View graphical programmingfor instrumentation software (version 5.0,National Instru-ments, Austin, Tex). The outputs were compiled as Excelfiles (Microsoft, Redmond, Wash). Values were recordedto .001 mm and .001 N (1 N 5 approximately 100 g).

The testing machine was fitted with a customized

Fig 1. The 2 types of open-coil springs tested: stop-wound (top) and uniformly wound (bottom).

August 2010weeks (T4). One investigator (A.B.) completed alltesting sessions.

In

American Journal of Orthodontics and Dentofacial Orthopedics Bourke et al 142.e3Table I. Details of uniformly wound coils

Length

3M Unitek Nitinol 7 in

GAC International Sentalloy 0.59 in

Volume 138, Number 2The testing machine produced 2000 to 2500 datapoints per recording. The data sets were converted toExcel files. Only the data collected during the decom-pression (deactivation) of the springs were used toevaluate the force characteristics of the springs.

Statistical analysis

The mean forces and standard deviations werecalculated for each group of springs. The mean maxi-mum force, the mean average force, and the meanload-deflection ratio were also calculated. The average

Table II. Details of stop-wound coils

Length Inner d

TP Orthodontics

Reflex

7 in 0.35 in (0.

0.55 in (0.

GAC International

Sentalloy

7 in 0.035 in

Fig 2. Jig attached to the testing machine.iameter Labeled force level (catalog number)

012-in wire)

010-in wire)

(210-508)

(210-515)

Light, 100 g (10-000-21)

Medium, 150 g (10-000-22)

Heavy, 200 g (10-000-23)

ner diameter Labeled force level (catalog number)

0.030 in Light, 100 g (345-100)

Medium, 200 g (345-200)

Heavy, 275 g (345-275)

0.035 in Light, 100 g (10-000-09)

Heavy, 200 g (10-000-07)

Extra heavy, 300 g (10-00-17)force was defined as the force magnitude producedbetween 20% and 80% compression during unloading(deactivation) of the springs.

The Student t test was used to determine whether themeasured average force was significantly different fromthat labeled by the manufacturer. The significance levelwas set at P\0.05. Repeated-measures analysis of var-iance (ANOVA) was used for intragroup comparisonsbetween test sessions (T0, T1, T2, T3, and T4) andintergroup comparisons between manufacturers. Thiscomparison only involved springs from GAC and 3MUnitek, since TP Orthodontics springs were not labeledwith specific force values.

In each group of springs, comparisons were made ofmean maximum force, mean average force (20%-80%),and load-deflection ratio (slope). The Prasad-Rao-Jeske-Kackar-Harville post-hoc test was thenperformed to determine differences between testingsessions and manufacturers. Repeated-measuresANOVA was also used to determine whether therewere significant differences between GAC stop-woundand GAC uniformly wound coils at T0. The significancewas set at P\0.05.

Fig 3. Compression rack with GAC International stop-wound coils.

over the 3 months of 10% or more. The manufacturers

rs l

Differ

rs labels for stop-wound open-coils

Difference from labeled value (%) SD P

30.3 37.246 0.0018

37.1 35.991 0.0001

28.6 39.433 0.0001

NA 34.483 NA

NA 25.586 NA

GAC International stop-wound (100g)

100

150

200

250

)s

ma

rG

(

ec

ro

Maximum force (g)

Average force (g)

Load/deflection ratio(g/mm)

142.e4 Bourke et al American Journal of Orthodontics and Dentofacial Orthopedicslabels were described as clinically accurate if the mea-sured mean force was within 10% of the labeled force.

RESULTS

All springs had average force values that were sig-Force degradation was considered clinically signif-icant if there was a total decline in measured force

Table III. Comparison of average force with manufacture

Spring type Average force (g)

GAC International 100 g 71.203

GAC International 200 g 114.64

GAC International 300 g 241.99

3M Unitek 100 g 90.245

3M Unitek 200 g 122.48

3M Unitek 275 g 191.19

Table IV. Comparison of average force with manufacture

Spring type Average force (g)

GAC International 100 g 69.719

GAC International 150 g 94.346

GAC International 200 g 142.79

TP Orthodontics 0.010 3 0.035 127.85

TP Orthodontics 0.055 3 0.055 162.82

NA, not applicable.nificantly lower than the manufacturers values at T0(Tables III and IV). The 3M Unitek 100-g springsproduced average forces that were only 10 g lowerthan the labeled value. The GAC stop-wound 200-gsprings produced average forces that were less thanhalf of the labeled value.

The maximum force changed to a statisticallysignificant degree over time for the 3M Unitek 100-and 200-g uniformly wound springs, and the GAC100-, 200-, and 300-g springs, and for the TP Orthodon-tics stop-wound springs. The changes did not occur ina consistent stepwise pattern, with occasional increasesin force between time periods.

The light (100 g), medium (150 g), and heavy (200 g)stop-wound open-coil springs from GAC all had similarchanges in force characteristics over time. Statisticallysignificant changes in force magnitude occurred formaximum force (P \0.0001). The changes followeda stepwise pattern with degradation between mosttime periods (Fig 4). No significant changes were foundfor average forces and load-deflection ratios.

To determine whether there were differences be-tween springs from the different manufacturers, theabels for uniformly wound open-coils

ence from labeled value (%) SD P

28.8 32.141 0.0008

42.7 35.559 0.0001

19.3 86.84 0.0076

9.7 11.725 0.0015

38.8 23.015 0.0001

30.5 18.205 0.0001

August 2010uniformly wound springs from 3M Unitek and GACwere compared. Statistically significant differenceswere found between these springs for both the 100-and 200-g force levels. The differences in force charac-teristics between the springs from these manufacturersare summarized in Table V. GAC uniformly woundand stop-wound coils were then compared to determinewhether spring design affects the force characteristicsof open-coil springs. A significant difference betweenthe GAC 100-g uniformly wound and stop-wound coilswas found with regard to maximum force (P\0.0001)at T0. Significant differences were found between the

0

50

T0 T1 T2 T3 T4Time period

F

Fig 4. Change in force magnitude over time for GACInternational stop-wound light springs. The GAC Inter-national medium and heavy stop-wound coil springshad similar changes in force magnitude over time.

with a near-horizontal deactivation curve (ie, a low

American Journal of Orthodontics and Dentofacial Orthopedics Bourke et al 142.e5load-deflection ratio) would be preferable.1,6

In some clinical situations, springs are compressedmore than the amount considered in this study to beaverage force. Therefore, the maximum force generatedGAC 200-g uniformly wound and stop-wound coilsin average forces (P \0.004), maximum forces(P\0.0001), and load-deflection ratios (P\0.004) atT0 (Fig 5, Table VI).

DISCUSSION

NiTi has a unique crystalline structure that can shiftbetween 2 phases: austenitic and martensitic. For someof these alloys, conversion from the austenitic to themartensitic phase can occur by lowering the tempera-ture, in others by submitting the wire to stress.

When using a NiTi coil spring to produce toothmovement (open space at a location in the arch), thepart of the load-deflection diagram that is useful is thedeactivation or unloading part of the diagram (ie, asthe coil spring decompresses, part of the dynamicenergy stored in it is used to produce the force for toothmovement). Therefore, the reverse transformation(from martensite to austenite), evident in the unloadingcurve of austenitic NiTi, is of interest when consideringthe force the teeth will feel. The more horizontal thedeactivation curve, the more uniform and continuousthe tooth-moving force will be. If we accept the wide-spread, yet somewhat arbitrary, notion that a force oflow magnitude and continuous action is most advanta-geous for tooth movement, then a NiTi coil spring

Table V. Differences in force characteristics betweenuniformly wound open coils from 3M Unitek andGAC International

3M Unitek vsGAC International

Averageforce

Maximumforce

Load-deflectionratio

100 g Higher Higher No difference

200 g No difference Lower No difference

Volume 138, Number 2by springs was also an important characteristic exam-ined in this study. Maximum force was defined as theforce produced when the springs were compressed to50% of their resting length. This definition was basedon the investigators ability to manipulate the heaviestsprings in the study to the bottom-out point. Schnee-voight et al2 produced force-compression diagrams for32 commercially available open-coil springs, includingsprings from GAC and 3M Unitek. The springs werecompressed to what they termed the maximumextent. This maximum compression was reported tobe 76.37% of the original length of 20 mm. Therefore,it is possible that the 50% compression we used under-estimates the maximum force for some springs thatwere evaluated. Despite this, the maximum force levelsgenerated at the initial recording session by the coilsfrom GAC and 3M Unitek springs were from 19.9%to 120.0% higher than the manufacturers labeledforces. No manufacturers included any informationwith their products warning the clinician of the possiblemaximum force magnitudes produced by the springs.Based on the ability of the heavier springs to produceforces as high as 527 g, the clinician must use caution,especially when activating heavier springs.

No published studies have evaluated the force deg-radation of NiTi open-coil springs over a long periodwith the same testing machine. At the outset of thestudy, one objective was to measure force degradation.However, after evaluating the results, the term forcedegradation was deemed to be a misnomer. The forceover time did not show a consistent stepwise decline forall uniformly wound coils and the TPOrthodontics stop-wound coils. On the contrary, in some instances, forcemagnitude increased over time (this was particularlytrue for maximum force). So, perhaps a more appropri-ate term for this part of the investigation should bechange in force magnitude over time. The maximumforce values were evaluated by taking the highest forcevalue at 50% compression from resting length. The in-troduction of error was possible because of the potentialoutlier effect. It was speculated that taking several forcevalues at maximum compression and averaging thevalues could moderate the outlier effect. Reevaluatingthe data and using multiple values to calculate the max-imum force tested this hypothesis. When the recalcu-lated maximum forces were compared with theoriginal maximum force, no significant differenceswere evident. Also, standard deviations calculated formaximum forcewere comparablewith those for averageforce. The standard deviation for the maximum forcewould most likely be high if there was significant vari-ation because of outliers in the maximum force values.

Only the GAC stop-wound coils demonstrated a sta-tistically significant force degradation in a stepwisefashion. The degradation was also deemed clinicallysignificant. The definition of clinically significantforce degradation, based on the previous study byChow,4 was arbitrarily set at a 10% reduction in forcemagnitude. All 3 GAC stop-wound coils showed statis-tically and clinically significant force degradationvalues over time for maximum force but not for averageforce or load-deflection ratio. There are 2 possibilities toexplain this phenomenon. First, it might be due to mea-

surement error. However, based on the recalculatedmaximum forces in which the maximum values were

woun

Table VI. Differences in force characteristics betweenGAC International uniformly wound and stop-woundopen coils (100 and 200 g)

GAC Internationalstop-woundvs uniformly wound

Averageforce

Maximumforce

Load-deflectionratio

100 g No difference Higher No difference

200 g Higher Higher Higher

142.e6 Bourke et al American Journal of Orthodontics and Dentofacial Orthopedicsaveraged, this is unlikely. Second, force degradationmight depend on the amount of activation.

In theory, if the springs exhibit the superelastic prop-erties of NiTi, the force should be constant regardless ofthe compression within a certain activation range. How-ever, some springs in this study did not demonstratea high level of superelasticity. Therefore, differences inforce levels existed at varying degrees of compression.We considered force degradation over time at a constant

Fig 5. Load-deflection diagram for GAC stop-compression. This is not representative of tooth move-ment. Although several springs did not show force degra-dation over time, a decrease in force magnitude wouldoccur clinically as the toothmoved, because of relaxationof the spring and lack of superelasticity. For the force toremain near constant despite tooth movement, the load-deflection ratio must be relatively low. The springswith the lowest load-deflection ratios were theGAC light(100 g) and the uniformly wound and stop-wound coils(23.7 and 23.2 g/mm, respectively).

GAC International suggests that Sentalloy productscannot be bench tested because small variations in thetemperature of the product will affect their force charac-teristics. The transition temperature for the Sentalloyproducts ranges from 26.8C to 31.6C; for TP Ortho-dontics Reflex products, it is approximately 27C; andfor 3M Unitek Nitinol products, it is 48C to 82C.7,8

It is possible that the Sentalloy and Reflex springscooled to these temperatures during testing. However,Michailesco et al9 suggested that mouth temperaturescan range from 18.9C to 48.8C. The springs in ourstudy were maintained within this temperature rangeat all times. Still, the reported force characteristicsd and uniformly wound coils (100 and 200 g).

August 2010might not be completely representative of those in theoral environment. However, due to the reported greatvariations in mouth temperatures, springs should beable to maintain their superelastic properties over a rel-atively large temperature range in the oral environment.

Statistically significant differences were foundbetween the 2 spring designs; yet it was difficult todetermine whether these differences were clinicallysignificant. The GAC 200-g stop-wound and uniformlywound springs demonstrated statistically significantdifferences for average force, maximum force, andload-deflection ratio. The stop-wound coils producedhigher average forces, maximum forces, and load-deflection ratios. The lower load-deflection ratio of theuniformly wound coil could be considered more clini-cally appropriate. In explaining these differences, oneshould consider that the stop-wound coil has a mechani-cal disadvantage compared with a uniformly wound coilof the same length. The winding configuration of thestop-wound coil consists of areas of open and closedcoil. Therefore, there is a smaller potential range of

compression with increased stress created in the activeportions of the coil during compression compared withthe uniformly wound coil. The characteristics of thestop-wound coil would be comparable with those ofa uniformly wound coil with a length equal to the sumof the uniformly wound (active) portions of the originalstop-wound coil. This explains the higher average andmaximum forces created during decompression.

CONCLUSIONS

1. The average force magnitudes produced by all NiTicoil springs in this study were significantly lowerthan their labeled values. In contrast, the maximumforces were significantly higher. Thus, the manu-facturers methods for determining the labeledvalues require clarification.

2. Statistically and clinically significant changes inmaximum force magnitude occurred for GACInternational 100-, 150-, and 200-g stop-woundopen-coil springs over the 12-week period.

widely advertised notion of a near-constant forceoffered by these springs over a long activation.

4. There are significant differences between the open-coil springs from different manufacturers (3MUnitek and GAC International), as well as betweenuniformly wound and stop-wound springs.

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Laboratory analysis of superelasic NiTi compression springs.

Force characteristics of nickel-titanium open-coil springsMaterial and methodsStatistical analysis

ResultsDiscussionConclusionsReferences