Force degradation of closed coil An in vitro evaluation
springs:
Padmaraj V. Angolkar, BDS, MDS, ~ Janet V. Arnold, DDS, b Ram S. Nanda, DDS, MS, PhD, c and Manville G. Duncanson Jr., DDS, PhD d Oklahoma Cit), Okla.
This in vitro study was designed to determine the force degradation of closed coil springs made of stainless steel (SS), cobalt-chromium-nickel (Co-Cr-Ni) and nickel-titanium (Niti) alloys, when they were extended to generate an initial force value in the range of 150 to 160 gm. The specimens were divided into two groups. Group I included SS, Co-Cr-Ni, and two nickel-titanium spring types (Niti 1 and Niti 2), 0.010 x 0.030 inch with an initial length of 12 mm. Group II was comprised of SS, Co-Cr-Ni, and Niti 3 0.010 x 0.036-inch springs, with an initial length of 6 ram. A universal testing machine was used to measure force. A pilot study determined the extension required for each spring type, so that the initial force was in the range of 150 to 160 gm. Initial force was recorded, and then the springs were extended to the respective distances at 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days resulting in a total of eight time periods. Between the time intervals, all springs were extended to the same initial extension on specially designed racks and stored in a salivary substitute at 37 ~ C. Means and standard deviations of force values, percent force loss, and mean extension were statistically analyzed. All springs showed a force loss over time. Of the total, the major force loss for most springs was found to occur in the first 24 hours. The SS and Co-Cr-Ni springs showed relatively higher force decay in group I (0.010 x 0.030 inch) compared with Niti 1 and Niti 2. The Niti 3 springs of group II (0.010 x 0.036 inch) showed higher force degradation than the SS and Co-Cr-Ni springs of this group. The least force decay was found in the Niti 1 springs. In general, the total force loss after 28 days was in the range of 8% to 20% for all springs tested. This was considered to be relatively less compared with force loss shown by latex elastics and synthetic elastic modules as reported in the literature. (AMJ ORTHOO DENTOFACORTHOP 1992;102:127-33.)
O v e r the years, a variety of materials have been used to close spaces between teeth as in the case of canine retraction after the extraction of premolars. These include, latex elastics, t-4 coil springs, 2"5'6 synthetic elastic modules, 4"7 headgear, 8 and recently, magnets. 9 Nagamoto" suggested that the "pulling action" delivered by closed coil springs is more delicate, and such a force is desirable during the course of orthodontic treatment. Orthodontic coil springs were, primarily, made of stainless steel (SS) and cobalt-chromiumnickel (Co-Cr-Ni) alloys. However, nickel-titanium (Niti) coil springs have recently been introduced and were indicated to have better springback and superelastic properties than the SS coil springs. ~~ Various parameters affect the force produced by the coil springs. The effect of alloy, wire size, lumen size, From the University of Oklahoma, College of Dentistry. "Visiting Assistant Professor, Department of Orthodontics. ~'Graduate Resident, Department of Orthodontics. 'Professor and Chairman, Department of Orthodontics. dProfessor and Chairman, Department of Dental Materials. 811130018
pitch angle (angle at which coils deviate from a perpendicular line to the long axis of the spring) of the coils and length of the springs on the load-deflection rate, and stiffness have been investigated. 2't't6 In general these studies concluded that: (1) for a constant lumen size, an increase in wire size increases the loaddeflection rate, (2) for a constant wire size, an increase in lumen size reduces the load-deflection rate, (3) an increase in pitch angle leads to a higher load-deflection rate, and (4) the Co-Cr-Ni springs are stiffer than the SS springs. During orthodontic tooth movement, light, continuous forces are desirable for optimum tissue response and rapid tooth movement. Force loss over time of the various materials currently used to close spaces has been documented. 2-4"'72~The latex elastics lose 20% to 30% of the initial force in the first two days. 2"~7"~9'2~ Similarly, the force loss of synthetic elastic modules has been reported to be as high as 74% in the first 24 hours.t7 Application of a relatively higher initial force has been suggested to keep the force level constant when using these materials. '7"2~ Although the current knowledge on the various 127
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Angolkar e t a / .
Am. J. Orthod. Dent@w. Orthop.
9
Fig. 1. Instron universal testing machine with hooks on load cell
(A) and crosshead (B). The eyelets on the spring (C) were attached to hooks, and the crosshead was moved downward to determine force during each test.
properties of the coil springs is documented and useful, the information on their force degradation over time, if any, is lacking. This study was designed to determine the force loss over time of the closed coil springs made of SS, Co-Cr-Ni, and Niti alloys.
MATERIALS AND METHODS Closed coil springs made of stainless steel (Unitek/3M Corp., Monrovia, Calif.), cobalt-chromium-nickel (Rocky Mountain, Denver, Colo.), and nickel-titanium alloys were tested. The nickel-titaniumsprings, from three manufacturers, were designated as Niti 1 (Ortho Organizers, San Marcos, Calif.), Niti 2 (Masel, Bristol, Pa.), and Niti 3 (GAC International, Inc., Central lslip, N.Y.). The Niti I and Niti 2 were preformed 0.010 x 0.030-inch springs with a length of 12 ram. Similarly, the Niti 3 springs were 0.010 x 0.036 inches in diameter and 6 mm long. The Niti springs ',,,'ere available only in these sizes, and to keep the sample uniform., the SS and the Co-Cr-Ni springs of similar length and i:lib.~aeter were selected, ttence, the specimens were divided in two groups. Group I had four types of springs (SS, Co-Cr-Ni,
August 1992
Niti 1, and Niti 2) with dimensions of 12 mm in length and 0.010 x 0.030 inches in diameter. Group II included three types (SS, Co-Cr-Ni, and Niti 3) which were 0.010 • 0.036 inches in diameter and 6 mm in length. The manufacturers attached eyelets to all the Niti springs so that the same length and lumen size ',,,'ere fabricated for proper comparison. The SS and the Co-Cr-Ni springs were cut 1 mm longer on either side and formed to receive eyelets so that the same length and lumen size were fabricated for proper comparison. An universal testing machine (Instron Corp., Canton, Mass.), equipped with a 2000 gm tensile load cell, carried hooks on the load cell, as well as the crosshead. The springs ',,,'ere attached to these hooks with the eyelets (Fig. 1), and the crosshead was moved downward at a rate of 0.20 in/min (5.1 mm/min). The force was recorded graphically on a X-Y recorder (Model 7005B, llewlett Packard, Anaheim, Calif.). A pilot study, using five springs of each type, was performed to determine the extension required for each spring type so that the initial force would be in the range of 150 to 160 gin. This force level was selected to sinmlate the customary clinical practice in retracting canines.2=''-~The extension was recorded by designing a metal arm that carried a digital extension indicating meter (IDU Digimatic Indicator, Series 575, MTI, Inc., Paramus, N.J.) the accuracy of which is rated at 1/100 of a millimeter and a 2000 gm load cell is rated to 1/10 of a gram (Fig. 2). Thirty springs of each type were extended to the respective distances determined from the pilot study, and the initial force was recorded. To simulate oral conditions, the springs were extended to the same extension on specially designed metal racks with stainless steel posts (Fig. 3) and stored in salivary substitute (Xero-Lube, Scherer Labs, Inc., Dallas, Texas) at 37~ C. Thereafter, the springs were returned to the testing machine and extended to the original extension at intervals of 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days, and the level of force generated was recorded. During these tests, care was taken to keep the extension similar to the initial extension for each spring. Means and standard deviations of force and extension were calculated for each spring type at each of the time intervals. Percent force loss at each time interval was also calculated. A one-way analysis of variance (ANOVA) was performed to determine whether the means were significantly different for each time interval for each spring type. Specific significant differences were noted by means of the Duncan's multiple range test.
RESULTS In all, 210 springs were tested at eight time intervals for a total of 1680 readings. A pilot study was used to determine the extension required for each spring type to keep the force levels in the range of 150 to 160 gm, yet when the experiment was conducted, the forces ranged from 140 to 178 gm, possibly the result of individual variations among the springs. Tables I and II depict the means and standard deviations of the force value in grams along with mean
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extension in millimeters and percent force loss for all springs in group I and group II, respectively. The specific comparison according to the Duncan's multiple range is also shown. In group I, the SS springs lost 17.3% of the initial force in the first 24 hours. Thereafter, the force decay was minimal for a total force loss of 20% to 21% by the end of 28 days. The Co-Cr-Ni springs of this group showed 7.8% less force after 4 hours and 10% by the end of 24 hours. The force loss then remained minimal for the next four time intervals. ttowever, between 21 and 28 days, force decay increased to 19.4%. The Niti 1 springs showed a loss of 3.3% force after 4 hours, after which the force level remained relatively constant for the next two intervals. At the end of 7 days, this loss was increased to 8.6%, and the force level did not change significantly thereafter. A similar pattern was noted in the Niti 2 springs for the first 3 days after which the force decay was 7.9%. However, the force loss continued gradually to 9.9% by the end of 21 days. Between 21 and 28 days, the force decay increased steeply to 14.6%. Fig. 4 graphically illustrates the performance of all springs included in group I. In contrast to group I, the force degradation was relatively less in the SS and Co-Cr-Ni springs than in the nickel-titanium springs in group II. In this latter group, the SS springs showed 8% force decay in the first 24 hours, which gradually increased to about 11.3% by the end of 21 days. Then there was a sharp increase in force loss to 15.5% by the end of 28 days. The Co-Cr-Ni springs of group II lost minimal force in the first 7 days (2.8%). The decay increased gradually to about 6.6% by the end of 21 days, after which there was a steep increase in force loss, which reached ! 1.3% by the end of 28 days. The Niti 3 springs showed a sharp decline in force after the first 4 hours (10.6%). The force loss increased gradually from this point to reach 17% by the end of 28 days. Fig. 5 shows the performance of all group II springs. The variation in force values within each spring type was found to be approximately the same as evident from the standard deviations (Table 1 and II). The meanextensions of SS and Co-Cr-Ni springs in group I were 1.02 and 1.50 mm, respectively. Niti 1 showed a mean extension of 5.50 mm, and the Niti 2 showed 5.00 mm. In group II, extensions of 2.00 and 2.50 mm, respectively, were noted for the SS and Co-Cr-Ni springs. The mean extension for Niti 3 was I mm. DISCUSSION
Many investigators have indicated the importance of using the proper magnitude of force during orthodontic treatment to attain optimal tissue response and
Force degradation of closed coil springs
129
Fig. 2. Digital extension indicator as attached to a specially designed metal arm (,4). This arm was attached to the crosshead above. The lower part of the indicator (B) rested on a platform as shown. When the crosshead was moved downward, the part (B) compressed on the platform, which was equivalent to the extension of the spring above. This was indicated on the instrument digitally.
Fig. 3. Metal racks with stainless steel posts showing group of springs attached. This assembly was stored in a salivary substitute at 37 ~C between various time intervals of the experiment.
rapid tooth movement. 2225 Some of the materials used to apply orthodontic forces have been shown to lose force over time. 24'~72z Knowledge of this force loss becomes critical if constant forces are desired. Closed
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Angolkar et al.
Am..1. Orthod. Dentofac. Orthop. August 1992
Table I. Means and standard deviations of force in grams along with mean extension in millimeters and percent force loss of all springs of group I (0.010 • 0.030 inch) at various time intervals Co-Cr-Ni
SS
Time Initial 4 hours 1 day 3 days 7 days 14days 21 clays 28days
N
( • SD)
(ram)
30 152.63 ( • 8.75) 30 144.46 (• 30 126.16 ( • 16.84) 30 121.63 (• 30 119.36 (-+ 16.44) 30 121.36 ( • 10.39) 30 118.96 ( • 10.98) 30 119.96 ( - 8.94)
loss
1.02 1.02
5.35
1.02
17.3
1.03
20.3
1.02
21.79
1.02
20.48
1.02
22.05
1.02
21.40
Force ( • SD) 178.26 (--20.72) 164.21 (--- 14.46) 159.28 ( - 12.40) 155.89 ( • 13.10) 154.48 ( • 11.99) 156.33 ( • 10.99) 151.55 (+-- 15.90) 143.59 ( • 12.69)
Niti I
Niti 2
Fxtension Percent Force Extension Percent (,nnl) loss ( • SO) (mnl) loss 1.50 1.50
7.88
1.51
10.64
1.50
12.54
1.51
13.34
1.50
12.30
!.50
14.98
1.50
19.44
148.00 (-'-5.53) 143.10 (-'-5.73) 142.90 (---8.61) 141.43 (• 135.24 (• 137.03 (• 138.13 (• 135.62 (-+5.47)
5.50 5.49
3.31
5.50
3.44
5.50
4.43
Force I Extension I Percent (• [ (ram) loss 140.40 (• 134.85 (• 134.87 ( • 6.59) 129.25
5.00 5.01
3.95
5.02
3.39
5.02
7.94
5.03
9.77
5.01
11.41
5.01
9.98
5.01
14.65
( • 5.70)
5.51
8.62
5.50
7.41
5.52
6.66
5.51
8.36
126.68 (+-6.60) 124.38 ( • 10.19) 126.38 ( -+-7.72) 119.83 (•
The values connected by ( ) were statistically similar at p < 0.05 level as determined by Duncan's multiple range test.
Table II. Means and standard deviations of force in grams along with mean extension in millimeters and percent force loss of all springs of group II (0.010 x 0.036 inch) at various time intervals
Time
N
Initial
30
Hours
30
1 day
30
3 days
30
7 days
30
14 days
30
21 days
30
28 days
30
Force ( -+SD) 165.70 ( + 10.48) 159.50 (• 152.43 (---8.41) 153.56 ( ---7.95) 149.80 (• 148.76 ( • 12.44) 146.89 (• 139.93 (+__8.99)
Niti 3
Co-Cr-Ni
SS E x t e n s i o n Percent (mm) loss 2.01 2.01
3.74
2.00
8.00
2.01
7.32
2.01
9.59
2.01
10.22
2.00
11.35
2.01
15.55
Force (• 176.66 ( '"- 1! .88) 174.27 ( • 10.85) 173.39 (-'-8.10) 171.77 ( • 10.85) 171.70 (+__9.84) 169.42 ( • ! 1.22) 164.88 (.+. 13.42) 165.32 (__- 15.42)
I
Extension ] Percent Ohm) I (mm) 2.50 2.50
1.30
2.50
1.85
2.49
2.84
2.50
2.80
2.49
4.09
2.50
6.66
2.50
11.34
Farce ( • SD)
I
Extension ] Percent (ram) ] loss
163.93 ( • 15.70) 146.48 ( • 15.80) 139.80 ( • 13.83) 139.58 ( • 1 ! .98) 140.62 ( • 17.82) 136.58 ( • 13.74) 133.03 ( • 15.33) 136.00 (__. 14.83)
! .00 i.00
10.64
1.00
14.71
1.01
14.85
1.01
14.21
1.01
16.68
1.01
18.84
!.01
17.03
The values connected by ( ) were statistically similar at p < 0.05 level as determined by Duncan's multiple range test.
coil springs have been used by many clinicians to retract canines.and close extraction spaces. 2"s'6 Although various properties of these springs have been investigated, 2'n'~6 the values of force degradation over time were not available. This investigation attempted to study, in vitro, the force delay of various coil springs currently used to close extraction spaces. The optimal force levels to close extraction spaces
by retracting canines has been indicated to be in the range of 150 to 200 gm. 22'2~Hence to simulate clinical conditions, all springs from both groups were extended to a distance so that the forces fell within this range. In addition, the springs were stored in a salivary substitute at 37 ~ C extended on specially designed racks. The results of the study indicated that all springs
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Force degradation of closed coil springs
Number 2
Focoe (Gnu)
Group I
180--
160.
140120. 100. 80. 60-
40-
20. 0
1
Initial
I
4 HN
I
24 His
( ~ Co-Cr.Ni
I
3 days
I
7 days
[]
[ ] SS
I
14 days
I
21 days 28 days
Nifi 1
~
Nili 2
Fig. 4. Graph showing force in grams versus time for all springs in group I,
Force ( G ~ ) Group II
1800
C
0
160-
140-
120.
1008060-
40200
I
Initial
4 Hrz
I
24 Hrs
O Co.-Cr-Ni
I
.3 days
[]
I
7 days
SS
I
14 days
9
I
I
21 days 28 days
Ni. s
Fig. 5. Graph showing force in grams versus lime for all springs in group II.
131
132
A n g o l k a r et al.
lost force over time to varying degrees. Most of the springs showed a major force reduction in the first 24 hours to 3 days. After that there was a gradual but small force decay until 21 days. Between 21 and 28 days, a sharp increase in force loss was noted in most of the springs (Figs. 4 and 5). These observations may be compared with the performance of synthetic elastic modules and latex elastics, which showed maximum force decay in the first 24 h o u r s . 2't7"2~ However, the reported total percent force decay of these materials was considerably higher at the end of 28 days. 2~.t7-2~ The maximum force decay noted in this study was 21.4% for the SS springs of group I. In contrast, the synthetic elastic modules lose as high as 74% ~7 and latex elastics 20-30% of the initial force by the end of 28 days. 3"17"2~ It appears that the use of closed coil springs may be advantageous for canine retraction and to close extraction spaces when a constant force level is desirable. However, a force decay of 15% to 20%, in general, should be taken into consideration. The performance of the springs in the two groups was significantly different. The total force loss of SS and Co-Cr-Ni springs of group I was higher (19% to 21%) as compared with the group II springs (11% to 15%). This difference may be related to the differences in the initial length and lumen size of the springs of the two groups. By keeping the wire size constant, an increase in lumen size has been shown to reduce the load deflection rate and to increase the spring back properties of SS and Co-Cr-Ni springs, t6 In contrast to the SS and Co-Cr-Ni springs, the Niti springs of group I (Niti 1 and Niti 2) showed lesser force decay than the Niti 3 springs of group II. The reasons for the difference in the performance of Niti springs could not be identified. The Niti 1 and Niti 2 springs were manufactured from American nickel-titanium alloys. The Niti 3 springs were made of a Japanese nickel-titanium alloy. The Niti 3 springs appeared to be relatively stiff compared with other two nickel-titanium alloys as only 1 mm activation was required to achieve the force level of 150 to 160 gm. Within each group, the performance of SS and CoCr-Ni springs was found to be similar. This observation was in contrast to other properties of these springs, such as load deflection rate and stiffness, which have been reported to be higher in the Co-Cr-Ni springs than the SS springs. ~~ The Niti springs of group I showed lesser force decay compared with the springs of the other two alloys. This may be related to the superelastic property of nickel-titanium alloy. ~~ However, the Niti 3 springs of group II showed higher force decay, than the SS and Co-Cr-Ni springs, the reasons for which could not be clearly identified.
Am. J. Orthod. Dentofiw. Orthop. August 1992
The mean extension values of springs tested showed interesting observations. Webb et al. ~' and Miura et al. t~ suggested that closed coil springs should be extended a half to a third of the original length to achieve a force level of 300 gm. The force level of 150 to 160 gm, suggested for canine retraction, 22'2~was maintained in this experiment. The extension for SS springs was only in the range of 1 mm for an initial length of 12 mm in group I (Table I). Similarly, an extension of 2 mm was required for SS springs with an initial length of 6 mm in group II (Table I1). These extensions were considerably below the level suggested by the Webb et al. '~ and Miura et al.~~Similar observations were noted for the Co-Cr-Ni springs. Clinically, these findings may have two implications when the SS and Co-Cr-Ni springs are used. First, minimal extension may be required to achieve the force levels of 150 to 160 gm. Hence, the use of some force gauge may be warranted to keep the force levels in optimal range. This will also prevent overextension that may lead to either excessive forces or permanent deformation of springs. Second, since only 1 to 2 mm of activation may be needed, this high load-deflection rate may cause slower tooth movement. When the teeth involved move 1 to 2 mm, the force level may drop considerably. Hence, frequent activation may be required to keep the force level constant. In comparison, the Niti 1 and the Niti 2 showed an extension in the range of 5 to 6 mm to achieve the force level of 150 to 160 gm. This may be related to the lower load-deflection rate of these springs as a result of the superelastic properties of the nickel-titanium alloys. Also, the load-deflection curves of these alloys have shown that even at higher extensions, the stress or force levels relatively remains the same. ~~ This means that these springs may not require frequent activations. In this regard, the Niti springs appear to have a clinical advantage over SS and Co-Cr-Ni springs. SUMMARY AND CONCLUSIONS
This in vitro study was designed to determine the force degradation of closed coil springs made of stainless steel, cobalt-chromium-nickel and nickel-titanium alloys, when they were extended to a distance such that the initial force was in the range of 150 to 160 gm. The coil springs were divided into two groups. Group I included SS, Co-Cr-Ni, and two nickel-titanium springs (Niti 1 and Niti 2), all of 0.010 • 0.030 inches and 12 mm long. Similarly, SS, Co-Cr-Ni, and Niti 3 springs, 0.010 • 0.036 inches and 6 mm long comprised group II. A universal testing machine was used to measure force. A pilot study determined the extension required for each spring type so that the initial
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force was in the range o f 150 to 160 gin. A t the beginning o f e x p e r i m e n t , the springs were e x t e n d e d to their respective distances, and the initial force recorded. T h e n force was m e a s u r e d at 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days, resulting in a total o f eight time periods. B e t w e e n the time intervals, all springs were extended to the s a m e initial extension on specially designed racks and stored in a salivary substitute at 37 ~ C. M e a n s and standard deviations o f force values, percent force loss, and m e a n extension w e r e statistically analyzed. The f o l l o w i n g c o n c l u s i o n s w e r e drawn f r o m this study 1. All springs s h o w e d force loss o v e r time. 2. T h e m a j o r force loss was found to o c c u r in the first 24 hours for most springs. 3. T h e SS and C o - C r - N i springs s h o w e d relatively higher force d e c a y in group I (0.010 • 0 . 0 3 0 inch) as c o m p a r e d with the Niti 1 and Niti 2 springs. 4. T h e Niti 3 springs o f g r o u p II (0.010 • 0.035 inch) s h o w e d higher force degradation than the SS and C o - C r - N i springs o f this group. 5. T h e least force d e c a y was found in the Niti 1 springs. 6. In general, coil springs s h o w e d a 8% to 2 0 % force loss at the end o f 28 days, w h i c h is relatively l o w e r than the force loss s h o w n by latex elastics and synthetic elastic modules. REFERENCES I. Paulich F. Measuring of orthodontic forces. AM J ORTHODORAL SURG 1939;25:817-49. 2. Bell WR. A study of applied force as related to use of elastics and coil springs. Angle Orthod 1951;21:151-4. 3. Newman GV. Biophysical properties of orthodontic rubber elastics. J New Jersey Dent Soc 1963;35:95-103. 4. Wong AK. Orthodontic elastic materials. Angle Orthod 1976;46:196-205. 5. Arnold EB, Cunningham JS. Coil springs as an application of force. AM J OR'nIOD 1934;20:577-9. 6. Nagamoto G. Contraction coil springs: its uses and how to make it. AM J ORT,OD ORALSURG 1947;33:392-5. 7. Hershey HG, Reynolds WG. The plastic module as an orthodontic tooth moving mechanism. AM J OR'roODDeN'rOFACOR"mOP 1975;67:554-62.
Force degradation of closed coil springs
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8. Jacobson A. A key to understanding of extraoral forces. AM J ORmoo 1979;75:361-86. 9. Gianelly AA, Vaitas AS, Thomas WM, Berger DG. Distalization of molars with repelling magnets. J Clin Orthod 1988;22:40-4. 10. Miura F, Masakuni M, Ohura Y, Karibe M. The super-elastic Japanese NiTi alloy wire for use in orthodontics. Part I11. Studies on Japanese NiTi alloy coil springs. A.~ J ORIIIOD DF2q'I'OFAC O~ntoP 1988;94:89-96. 1!. Born HS. Some facts concerning the open coil spring. A.~! J ORmoo 1955;917-25. 12. Pletcher EC. Simplified management of space closure. A.~I J O~t'ntOD 1959;45:278-86. 13. Kobayashi K. Studies on mechanical properties of orthodontic coil springs. J Dent Mater 1971;12:172-84. 14. Webb RI, Caputo AA, Chaconas SJ. Orthodontic force production by closed coil springs. AM J ORTHOD1978;74:405-9. 15. Chaconas SJ, Caputa AA, Harvey K. Orthodontic force characteristics of open coil springs. AM J ORTHOD1984;85:494-7. 16. Boshart BF, Currier FG, Nanda RS, Duncanson MG Jr. Load deflection rate measurements of activated open and closed coil springs. Angle Orthod 1990;60:27-34. 17. Andreason GF, Bishara S. Comparison of Alastik chains involved with intra-arch molar to molar forces. Angle Orthod 1970;406:151-8. 18. Andreason GF, Bishara S. Comparison of time related forces between plastic alastiks and latex elastics. Angle Orthod 1970;40:319-28. 19. Chaconas SJ, Caputo AA, Belting CW. Force degradation of orthodontic elastics. J Dent Res lAbstr]. 1977;56:B-84. 20. Angolkar PV. An in vitro evaluation of orthodontic elastics IThesis]. Bombay, India: University of Bombay, 1980. 21. DeGenova DC, Mclnnes-Ledoux P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains--a product comparison study. AM J ORrHOD 1985;87:377-84. 22. Storey E, Smith R. Force in orthodontics and its relation to tooth movement. Aust Dent J 1952;56:11-8. 23. Reitan K. Some factors determining the evaluation of forces in orthodontics. AM J ORTIIOD 1957;43:32-45. 24. Schwartz AM. Tissue changes incidental to orthodontic tooth movement, l:,rr J OR'n{OD 1932;18:331-52. 25. Quinn TB, Yoshikawa DK. A reassessment of force magnitude in orthodontics. A.~l J OR'ntOD 1985;88:252-60. Reprint requests to: Dr. Ram Nanda University of Oklahoma College of Dentistry Department of Orthodontics P.O. Box 26901 1001 Stanton L. Young Blvd. Oklahoma City, OK 73190
springs:
Padmaraj V. Angolkar, BDS, MDS, ~ Janet V. Arnold, DDS, b Ram S. Nanda, DDS, MS, PhD, c and Manville G. Duncanson Jr., DDS, PhD d Oklahoma Cit), Okla.
This in vitro study was designed to determine the force degradation of closed coil springs made of stainless steel (SS), cobalt-chromium-nickel (Co-Cr-Ni) and nickel-titanium (Niti) alloys, when they were extended to generate an initial force value in the range of 150 to 160 gm. The specimens were divided into two groups. Group I included SS, Co-Cr-Ni, and two nickel-titanium spring types (Niti 1 and Niti 2), 0.010 x 0.030 inch with an initial length of 12 mm. Group II was comprised of SS, Co-Cr-Ni, and Niti 3 0.010 x 0.036-inch springs, with an initial length of 6 ram. A universal testing machine was used to measure force. A pilot study determined the extension required for each spring type, so that the initial force was in the range of 150 to 160 gm. Initial force was recorded, and then the springs were extended to the respective distances at 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days resulting in a total of eight time periods. Between the time intervals, all springs were extended to the same initial extension on specially designed racks and stored in a salivary substitute at 37 ~ C. Means and standard deviations of force values, percent force loss, and mean extension were statistically analyzed. All springs showed a force loss over time. Of the total, the major force loss for most springs was found to occur in the first 24 hours. The SS and Co-Cr-Ni springs showed relatively higher force decay in group I (0.010 x 0.030 inch) compared with Niti 1 and Niti 2. The Niti 3 springs of group II (0.010 x 0.036 inch) showed higher force degradation than the SS and Co-Cr-Ni springs of this group. The least force decay was found in the Niti 1 springs. In general, the total force loss after 28 days was in the range of 8% to 20% for all springs tested. This was considered to be relatively less compared with force loss shown by latex elastics and synthetic elastic modules as reported in the literature. (AMJ ORTHOO DENTOFACORTHOP 1992;102:127-33.)
O v e r the years, a variety of materials have been used to close spaces between teeth as in the case of canine retraction after the extraction of premolars. These include, latex elastics, t-4 coil springs, 2"5'6 synthetic elastic modules, 4"7 headgear, 8 and recently, magnets. 9 Nagamoto" suggested that the "pulling action" delivered by closed coil springs is more delicate, and such a force is desirable during the course of orthodontic treatment. Orthodontic coil springs were, primarily, made of stainless steel (SS) and cobalt-chromiumnickel (Co-Cr-Ni) alloys. However, nickel-titanium (Niti) coil springs have recently been introduced and were indicated to have better springback and superelastic properties than the SS coil springs. ~~ Various parameters affect the force produced by the coil springs. The effect of alloy, wire size, lumen size, From the University of Oklahoma, College of Dentistry. "Visiting Assistant Professor, Department of Orthodontics. ~'Graduate Resident, Department of Orthodontics. 'Professor and Chairman, Department of Orthodontics. dProfessor and Chairman, Department of Dental Materials. 811130018
pitch angle (angle at which coils deviate from a perpendicular line to the long axis of the spring) of the coils and length of the springs on the load-deflection rate, and stiffness have been investigated. 2't't6 In general these studies concluded that: (1) for a constant lumen size, an increase in wire size increases the loaddeflection rate, (2) for a constant wire size, an increase in lumen size reduces the load-deflection rate, (3) an increase in pitch angle leads to a higher load-deflection rate, and (4) the Co-Cr-Ni springs are stiffer than the SS springs. During orthodontic tooth movement, light, continuous forces are desirable for optimum tissue response and rapid tooth movement. Force loss over time of the various materials currently used to close spaces has been documented. 2-4"'72~The latex elastics lose 20% to 30% of the initial force in the first two days. 2"~7"~9'2~ Similarly, the force loss of synthetic elastic modules has been reported to be as high as 74% in the first 24 hours.t7 Application of a relatively higher initial force has been suggested to keep the force level constant when using these materials. '7"2~ Although the current knowledge on the various 127
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Angolkar e t a / .
Am. J. Orthod. Dent@w. Orthop.
9
Fig. 1. Instron universal testing machine with hooks on load cell
(A) and crosshead (B). The eyelets on the spring (C) were attached to hooks, and the crosshead was moved downward to determine force during each test.
properties of the coil springs is documented and useful, the information on their force degradation over time, if any, is lacking. This study was designed to determine the force loss over time of the closed coil springs made of SS, Co-Cr-Ni, and Niti alloys.
MATERIALS AND METHODS Closed coil springs made of stainless steel (Unitek/3M Corp., Monrovia, Calif.), cobalt-chromium-nickel (Rocky Mountain, Denver, Colo.), and nickel-titanium alloys were tested. The nickel-titaniumsprings, from three manufacturers, were designated as Niti 1 (Ortho Organizers, San Marcos, Calif.), Niti 2 (Masel, Bristol, Pa.), and Niti 3 (GAC International, Inc., Central lslip, N.Y.). The Niti I and Niti 2 were preformed 0.010 x 0.030-inch springs with a length of 12 ram. Similarly, the Niti 3 springs were 0.010 x 0.036 inches in diameter and 6 mm long. The Niti springs ',,,'ere available only in these sizes, and to keep the sample uniform., the SS and the Co-Cr-Ni springs of similar length and i:lib.~aeter were selected, ttence, the specimens were divided in two groups. Group I had four types of springs (SS, Co-Cr-Ni,
August 1992
Niti 1, and Niti 2) with dimensions of 12 mm in length and 0.010 x 0.030 inches in diameter. Group II included three types (SS, Co-Cr-Ni, and Niti 3) which were 0.010 • 0.036 inches in diameter and 6 mm in length. The manufacturers attached eyelets to all the Niti springs so that the same length and lumen size ',,,'ere fabricated for proper comparison. The SS and the Co-Cr-Ni springs were cut 1 mm longer on either side and formed to receive eyelets so that the same length and lumen size were fabricated for proper comparison. An universal testing machine (Instron Corp., Canton, Mass.), equipped with a 2000 gm tensile load cell, carried hooks on the load cell, as well as the crosshead. The springs ',,,'ere attached to these hooks with the eyelets (Fig. 1), and the crosshead was moved downward at a rate of 0.20 in/min (5.1 mm/min). The force was recorded graphically on a X-Y recorder (Model 7005B, llewlett Packard, Anaheim, Calif.). A pilot study, using five springs of each type, was performed to determine the extension required for each spring type so that the initial force would be in the range of 150 to 160 gin. This force level was selected to sinmlate the customary clinical practice in retracting canines.2=''-~The extension was recorded by designing a metal arm that carried a digital extension indicating meter (IDU Digimatic Indicator, Series 575, MTI, Inc., Paramus, N.J.) the accuracy of which is rated at 1/100 of a millimeter and a 2000 gm load cell is rated to 1/10 of a gram (Fig. 2). Thirty springs of each type were extended to the respective distances determined from the pilot study, and the initial force was recorded. To simulate oral conditions, the springs were extended to the same extension on specially designed metal racks with stainless steel posts (Fig. 3) and stored in salivary substitute (Xero-Lube, Scherer Labs, Inc., Dallas, Texas) at 37~ C. Thereafter, the springs were returned to the testing machine and extended to the original extension at intervals of 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days, and the level of force generated was recorded. During these tests, care was taken to keep the extension similar to the initial extension for each spring. Means and standard deviations of force and extension were calculated for each spring type at each of the time intervals. Percent force loss at each time interval was also calculated. A one-way analysis of variance (ANOVA) was performed to determine whether the means were significantly different for each time interval for each spring type. Specific significant differences were noted by means of the Duncan's multiple range test.
RESULTS In all, 210 springs were tested at eight time intervals for a total of 1680 readings. A pilot study was used to determine the extension required for each spring type to keep the force levels in the range of 150 to 160 gm, yet when the experiment was conducted, the forces ranged from 140 to 178 gm, possibly the result of individual variations among the springs. Tables I and II depict the means and standard deviations of the force value in grams along with mean
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extension in millimeters and percent force loss for all springs in group I and group II, respectively. The specific comparison according to the Duncan's multiple range is also shown. In group I, the SS springs lost 17.3% of the initial force in the first 24 hours. Thereafter, the force decay was minimal for a total force loss of 20% to 21% by the end of 28 days. The Co-Cr-Ni springs of this group showed 7.8% less force after 4 hours and 10% by the end of 24 hours. The force loss then remained minimal for the next four time intervals. ttowever, between 21 and 28 days, force decay increased to 19.4%. The Niti 1 springs showed a loss of 3.3% force after 4 hours, after which the force level remained relatively constant for the next two intervals. At the end of 7 days, this loss was increased to 8.6%, and the force level did not change significantly thereafter. A similar pattern was noted in the Niti 2 springs for the first 3 days after which the force decay was 7.9%. However, the force loss continued gradually to 9.9% by the end of 21 days. Between 21 and 28 days, the force decay increased steeply to 14.6%. Fig. 4 graphically illustrates the performance of all springs included in group I. In contrast to group I, the force degradation was relatively less in the SS and Co-Cr-Ni springs than in the nickel-titanium springs in group II. In this latter group, the SS springs showed 8% force decay in the first 24 hours, which gradually increased to about 11.3% by the end of 21 days. Then there was a sharp increase in force loss to 15.5% by the end of 28 days. The Co-Cr-Ni springs of group II lost minimal force in the first 7 days (2.8%). The decay increased gradually to about 6.6% by the end of 21 days, after which there was a steep increase in force loss, which reached ! 1.3% by the end of 28 days. The Niti 3 springs showed a sharp decline in force after the first 4 hours (10.6%). The force loss increased gradually from this point to reach 17% by the end of 28 days. Fig. 5 shows the performance of all group II springs. The variation in force values within each spring type was found to be approximately the same as evident from the standard deviations (Table 1 and II). The meanextensions of SS and Co-Cr-Ni springs in group I were 1.02 and 1.50 mm, respectively. Niti 1 showed a mean extension of 5.50 mm, and the Niti 2 showed 5.00 mm. In group II, extensions of 2.00 and 2.50 mm, respectively, were noted for the SS and Co-Cr-Ni springs. The mean extension for Niti 3 was I mm. DISCUSSION
Many investigators have indicated the importance of using the proper magnitude of force during orthodontic treatment to attain optimal tissue response and
Force degradation of closed coil springs
129
Fig. 2. Digital extension indicator as attached to a specially designed metal arm (,4). This arm was attached to the crosshead above. The lower part of the indicator (B) rested on a platform as shown. When the crosshead was moved downward, the part (B) compressed on the platform, which was equivalent to the extension of the spring above. This was indicated on the instrument digitally.
Fig. 3. Metal racks with stainless steel posts showing group of springs attached. This assembly was stored in a salivary substitute at 37 ~C between various time intervals of the experiment.
rapid tooth movement. 2225 Some of the materials used to apply orthodontic forces have been shown to lose force over time. 24'~72z Knowledge of this force loss becomes critical if constant forces are desired. Closed
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Am..1. Orthod. Dentofac. Orthop. August 1992
Table I. Means and standard deviations of force in grams along with mean extension in millimeters and percent force loss of all springs of group I (0.010 • 0.030 inch) at various time intervals Co-Cr-Ni
SS
Time Initial 4 hours 1 day 3 days 7 days 14days 21 clays 28days
N
( • SD)
(ram)
30 152.63 ( • 8.75) 30 144.46 (• 30 126.16 ( • 16.84) 30 121.63 (• 30 119.36 (-+ 16.44) 30 121.36 ( • 10.39) 30 118.96 ( • 10.98) 30 119.96 ( - 8.94)
loss
1.02 1.02
5.35
1.02
17.3
1.03
20.3
1.02
21.79
1.02
20.48
1.02
22.05
1.02
21.40
Force ( • SD) 178.26 (--20.72) 164.21 (--- 14.46) 159.28 ( - 12.40) 155.89 ( • 13.10) 154.48 ( • 11.99) 156.33 ( • 10.99) 151.55 (+-- 15.90) 143.59 ( • 12.69)
Niti I
Niti 2
Fxtension Percent Force Extension Percent (,nnl) loss ( • SO) (mnl) loss 1.50 1.50
7.88
1.51
10.64
1.50
12.54
1.51
13.34
1.50
12.30
!.50
14.98
1.50
19.44
148.00 (-'-5.53) 143.10 (-'-5.73) 142.90 (---8.61) 141.43 (• 135.24 (• 137.03 (• 138.13 (• 135.62 (-+5.47)
5.50 5.49
3.31
5.50
3.44
5.50
4.43
Force I Extension I Percent (• [ (ram) loss 140.40 (• 134.85 (• 134.87 ( • 6.59) 129.25
5.00 5.01
3.95
5.02
3.39
5.02
7.94
5.03
9.77
5.01
11.41
5.01
9.98
5.01
14.65
( • 5.70)
5.51
8.62
5.50
7.41
5.52
6.66
5.51
8.36
126.68 (+-6.60) 124.38 ( • 10.19) 126.38 ( -+-7.72) 119.83 (•
The values connected by ( ) were statistically similar at p < 0.05 level as determined by Duncan's multiple range test.
Table II. Means and standard deviations of force in grams along with mean extension in millimeters and percent force loss of all springs of group II (0.010 x 0.036 inch) at various time intervals
Time
N
Initial
30
Hours
30
1 day
30
3 days
30
7 days
30
14 days
30
21 days
30
28 days
30
Force ( -+SD) 165.70 ( + 10.48) 159.50 (• 152.43 (---8.41) 153.56 ( ---7.95) 149.80 (• 148.76 ( • 12.44) 146.89 (• 139.93 (+__8.99)
Niti 3
Co-Cr-Ni
SS E x t e n s i o n Percent (mm) loss 2.01 2.01
3.74
2.00
8.00
2.01
7.32
2.01
9.59
2.01
10.22
2.00
11.35
2.01
15.55
Force (• 176.66 ( '"- 1! .88) 174.27 ( • 10.85) 173.39 (-'-8.10) 171.77 ( • 10.85) 171.70 (+__9.84) 169.42 ( • ! 1.22) 164.88 (.+. 13.42) 165.32 (__- 15.42)
I
Extension ] Percent Ohm) I (mm) 2.50 2.50
1.30
2.50
1.85
2.49
2.84
2.50
2.80
2.49
4.09
2.50
6.66
2.50
11.34
Farce ( • SD)
I
Extension ] Percent (ram) ] loss
163.93 ( • 15.70) 146.48 ( • 15.80) 139.80 ( • 13.83) 139.58 ( • 1 ! .98) 140.62 ( • 17.82) 136.58 ( • 13.74) 133.03 ( • 15.33) 136.00 (__. 14.83)
! .00 i.00
10.64
1.00
14.71
1.01
14.85
1.01
14.21
1.01
16.68
1.01
18.84
!.01
17.03
The values connected by ( ) were statistically similar at p < 0.05 level as determined by Duncan's multiple range test.
coil springs have been used by many clinicians to retract canines.and close extraction spaces. 2"s'6 Although various properties of these springs have been investigated, 2'n'~6 the values of force degradation over time were not available. This investigation attempted to study, in vitro, the force delay of various coil springs currently used to close extraction spaces. The optimal force levels to close extraction spaces
by retracting canines has been indicated to be in the range of 150 to 200 gm. 22'2~Hence to simulate clinical conditions, all springs from both groups were extended to a distance so that the forces fell within this range. In addition, the springs were stored in a salivary substitute at 37 ~ C extended on specially designed racks. The results of the study indicated that all springs
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Force degradation of closed coil springs
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Focoe (Gnu)
Group I
180--
160.
140120. 100. 80. 60-
40-
20. 0
1
Initial
I
4 HN
I
24 His
( ~ Co-Cr.Ni
I
3 days
I
7 days
[]
[ ] SS
I
14 days
I
21 days 28 days
Nifi 1
~
Nili 2
Fig. 4. Graph showing force in grams versus time for all springs in group I,
Force ( G ~ ) Group II
1800
C
0
160-
140-
120.
1008060-
40200
I
Initial
4 Hrz
I
24 Hrs
O Co.-Cr-Ni
I
.3 days
[]
I
7 days
SS
I
14 days
9
I
I
21 days 28 days
Ni. s
Fig. 5. Graph showing force in grams versus lime for all springs in group II.
131
132
A n g o l k a r et al.
lost force over time to varying degrees. Most of the springs showed a major force reduction in the first 24 hours to 3 days. After that there was a gradual but small force decay until 21 days. Between 21 and 28 days, a sharp increase in force loss was noted in most of the springs (Figs. 4 and 5). These observations may be compared with the performance of synthetic elastic modules and latex elastics, which showed maximum force decay in the first 24 h o u r s . 2't7"2~ However, the reported total percent force decay of these materials was considerably higher at the end of 28 days. 2~.t7-2~ The maximum force decay noted in this study was 21.4% for the SS springs of group I. In contrast, the synthetic elastic modules lose as high as 74% ~7 and latex elastics 20-30% of the initial force by the end of 28 days. 3"17"2~ It appears that the use of closed coil springs may be advantageous for canine retraction and to close extraction spaces when a constant force level is desirable. However, a force decay of 15% to 20%, in general, should be taken into consideration. The performance of the springs in the two groups was significantly different. The total force loss of SS and Co-Cr-Ni springs of group I was higher (19% to 21%) as compared with the group II springs (11% to 15%). This difference may be related to the differences in the initial length and lumen size of the springs of the two groups. By keeping the wire size constant, an increase in lumen size has been shown to reduce the load deflection rate and to increase the spring back properties of SS and Co-Cr-Ni springs, t6 In contrast to the SS and Co-Cr-Ni springs, the Niti springs of group I (Niti 1 and Niti 2) showed lesser force decay than the Niti 3 springs of group II. The reasons for the difference in the performance of Niti springs could not be identified. The Niti 1 and Niti 2 springs were manufactured from American nickel-titanium alloys. The Niti 3 springs were made of a Japanese nickel-titanium alloy. The Niti 3 springs appeared to be relatively stiff compared with other two nickel-titanium alloys as only 1 mm activation was required to achieve the force level of 150 to 160 gm. Within each group, the performance of SS and CoCr-Ni springs was found to be similar. This observation was in contrast to other properties of these springs, such as load deflection rate and stiffness, which have been reported to be higher in the Co-Cr-Ni springs than the SS springs. ~~ The Niti springs of group I showed lesser force decay compared with the springs of the other two alloys. This may be related to the superelastic property of nickel-titanium alloy. ~~ However, the Niti 3 springs of group II showed higher force decay, than the SS and Co-Cr-Ni springs, the reasons for which could not be clearly identified.
Am. J. Orthod. Dentofiw. Orthop. August 1992
The mean extension values of springs tested showed interesting observations. Webb et al. ~' and Miura et al. t~ suggested that closed coil springs should be extended a half to a third of the original length to achieve a force level of 300 gm. The force level of 150 to 160 gm, suggested for canine retraction, 22'2~was maintained in this experiment. The extension for SS springs was only in the range of 1 mm for an initial length of 12 mm in group I (Table I). Similarly, an extension of 2 mm was required for SS springs with an initial length of 6 mm in group II (Table I1). These extensions were considerably below the level suggested by the Webb et al. '~ and Miura et al.~~Similar observations were noted for the Co-Cr-Ni springs. Clinically, these findings may have two implications when the SS and Co-Cr-Ni springs are used. First, minimal extension may be required to achieve the force levels of 150 to 160 gm. Hence, the use of some force gauge may be warranted to keep the force levels in optimal range. This will also prevent overextension that may lead to either excessive forces or permanent deformation of springs. Second, since only 1 to 2 mm of activation may be needed, this high load-deflection rate may cause slower tooth movement. When the teeth involved move 1 to 2 mm, the force level may drop considerably. Hence, frequent activation may be required to keep the force level constant. In comparison, the Niti 1 and the Niti 2 showed an extension in the range of 5 to 6 mm to achieve the force level of 150 to 160 gm. This may be related to the lower load-deflection rate of these springs as a result of the superelastic properties of the nickel-titanium alloys. Also, the load-deflection curves of these alloys have shown that even at higher extensions, the stress or force levels relatively remains the same. ~~ This means that these springs may not require frequent activations. In this regard, the Niti springs appear to have a clinical advantage over SS and Co-Cr-Ni springs. SUMMARY AND CONCLUSIONS
This in vitro study was designed to determine the force degradation of closed coil springs made of stainless steel, cobalt-chromium-nickel and nickel-titanium alloys, when they were extended to a distance such that the initial force was in the range of 150 to 160 gm. The coil springs were divided into two groups. Group I included SS, Co-Cr-Ni, and two nickel-titanium springs (Niti 1 and Niti 2), all of 0.010 • 0.030 inches and 12 mm long. Similarly, SS, Co-Cr-Ni, and Niti 3 springs, 0.010 • 0.036 inches and 6 mm long comprised group II. A universal testing machine was used to measure force. A pilot study determined the extension required for each spring type so that the initial
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force was in the range o f 150 to 160 gin. A t the beginning o f e x p e r i m e n t , the springs were e x t e n d e d to their respective distances, and the initial force recorded. T h e n force was m e a s u r e d at 4 hours, 24 hours, 3 days, 7 days, 14 days, 21 days, and 28 days, resulting in a total o f eight time periods. B e t w e e n the time intervals, all springs were extended to the s a m e initial extension on specially designed racks and stored in a salivary substitute at 37 ~ C. M e a n s and standard deviations o f force values, percent force loss, and m e a n extension w e r e statistically analyzed. The f o l l o w i n g c o n c l u s i o n s w e r e drawn f r o m this study 1. All springs s h o w e d force loss o v e r time. 2. T h e m a j o r force loss was found to o c c u r in the first 24 hours for most springs. 3. T h e SS and C o - C r - N i springs s h o w e d relatively higher force d e c a y in group I (0.010 • 0 . 0 3 0 inch) as c o m p a r e d with the Niti 1 and Niti 2 springs. 4. T h e Niti 3 springs o f g r o u p II (0.010 • 0.035 inch) s h o w e d higher force degradation than the SS and C o - C r - N i springs o f this group. 5. T h e least force d e c a y was found in the Niti 1 springs. 6. In general, coil springs s h o w e d a 8% to 2 0 % force loss at the end o f 28 days, w h i c h is relatively l o w e r than the force loss s h o w n by latex elastics and synthetic elastic modules. REFERENCES I. Paulich F. Measuring of orthodontic forces. AM J ORTHODORAL SURG 1939;25:817-49. 2. Bell WR. A study of applied force as related to use of elastics and coil springs. Angle Orthod 1951;21:151-4. 3. Newman GV. Biophysical properties of orthodontic rubber elastics. J New Jersey Dent Soc 1963;35:95-103. 4. Wong AK. Orthodontic elastic materials. Angle Orthod 1976;46:196-205. 5. Arnold EB, Cunningham JS. Coil springs as an application of force. AM J OR'nIOD 1934;20:577-9. 6. Nagamoto G. Contraction coil springs: its uses and how to make it. AM J ORT,OD ORALSURG 1947;33:392-5. 7. Hershey HG, Reynolds WG. The plastic module as an orthodontic tooth moving mechanism. AM J OR'roODDeN'rOFACOR"mOP 1975;67:554-62.
Force degradation of closed coil springs
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8. Jacobson A. A key to understanding of extraoral forces. AM J ORmoo 1979;75:361-86. 9. Gianelly AA, Vaitas AS, Thomas WM, Berger DG. Distalization of molars with repelling magnets. J Clin Orthod 1988;22:40-4. 10. Miura F, Masakuni M, Ohura Y, Karibe M. The super-elastic Japanese NiTi alloy wire for use in orthodontics. Part I11. Studies on Japanese NiTi alloy coil springs. A.~ J ORIIIOD DF2q'I'OFAC O~ntoP 1988;94:89-96. 1!. Born HS. Some facts concerning the open coil spring. A.~! J ORmoo 1955;917-25. 12. Pletcher EC. Simplified management of space closure. A.~I J O~t'ntOD 1959;45:278-86. 13. Kobayashi K. Studies on mechanical properties of orthodontic coil springs. J Dent Mater 1971;12:172-84. 14. Webb RI, Caputo AA, Chaconas SJ. Orthodontic force production by closed coil springs. AM J ORTHOD1978;74:405-9. 15. Chaconas SJ, Caputa AA, Harvey K. Orthodontic force characteristics of open coil springs. AM J ORTHOD1984;85:494-7. 16. Boshart BF, Currier FG, Nanda RS, Duncanson MG Jr. Load deflection rate measurements of activated open and closed coil springs. Angle Orthod 1990;60:27-34. 17. Andreason GF, Bishara S. Comparison of Alastik chains involved with intra-arch molar to molar forces. Angle Orthod 1970;406:151-8. 18. Andreason GF, Bishara S. Comparison of time related forces between plastic alastiks and latex elastics. Angle Orthod 1970;40:319-28. 19. Chaconas SJ, Caputo AA, Belting CW. Force degradation of orthodontic elastics. J Dent Res lAbstr]. 1977;56:B-84. 20. Angolkar PV. An in vitro evaluation of orthodontic elastics IThesis]. Bombay, India: University of Bombay, 1980. 21. DeGenova DC, Mclnnes-Ledoux P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains--a product comparison study. AM J ORrHOD 1985;87:377-84. 22. Storey E, Smith R. Force in orthodontics and its relation to tooth movement. Aust Dent J 1952;56:11-8. 23. Reitan K. Some factors determining the evaluation of forces in orthodontics. AM J ORTIIOD 1957;43:32-45. 24. Schwartz AM. Tissue changes incidental to orthodontic tooth movement, l:,rr J OR'n{OD 1932;18:331-52. 25. Quinn TB, Yoshikawa DK. A reassessment of force magnitude in orthodontics. A.~l J OR'ntOD 1985;88:252-60. Reprint requests to: Dr. Ram Nanda University of Oklahoma College of Dentistry Department of Orthodontics P.O. Box 26901 1001 Stanton L. Young Blvd. Oklahoma City, OK 73190
