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Scientific Section |
Eastman Dental Hospital, London, UK
Claire Nightingale, 74 Antrim Mansions, Antrim Road, London NW3 4XL, UK. E-mail: claire{at}nightingales.org.uk
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Abstract |
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 Top Abstract IntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Design: Randomized clinical trial.
Setting: Eastman Dental Hospital, London and Queen Mary’s University Hospital, Roehampton, 1998–2000.
Subjects, materials and methods: Twenty-two orthodontic patients, wearing the pre-adjusted edgewise appliance undergoing space closure in opposing quadrants, using sliding mechanics on 0.019 x 0.025-inch posted stainless steel archwires. Medium-spaced elastomeric chain [Durachain, OrthoCare (UK) Ltd., Bradford, UK] and 9-mm nickel titanium coil springs [OrthoCare (UK) Ltd.] were placed in opposing quadrants for 15 patients. Elastomeric chain only was used in a further seven patients. The initial forces on placement and residual forces at the subsequent visit were measured with a dial push–pull gauge [Orthocare (UK) Ltd]. Study models of eight patients were taken before and after space closure, from which measurements were made to establish mean space closure.
Main outcome measures: The forces were measured in grammes and space closure in millimetres.
Results: Fifty-nine per cent (31/53) of the elastomeric sample maintained at least 50 per cent of the initial force over a time period of 1–15 weeks. No sample lost all its force, and the mean loss was 47 per cent (range: 0–76 per cent). Nickel titanium coil springs lost force rapidly over 6 weeks, following that force levels plateaued. Forty-six per cent (12/26) maintained at least 50 per cent of their initial force over a time period of 1–22 weeks, and mean force loss was 48 per cent (range: 12–68 per cent). The rate of mean weekly space closure for elastomeric chain was 0.21 mm and for nickel titanium coil springs 0.26 mm. There was no relationship between the initial force applied and rate of space closure. None of the sample failed during the study period giving a 100 per cent response rate.
Conclusions: In clinical use, the force retention of elastomeric chain was better than previously concluded. High initial forces resulted in high force decay. Nickel titanium coil springs and elastomeric chain closed spaces at a similar rate.
Key words: Elastomeric chain, force decay, in vivo study, nickel titanium coil springs, space closure
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Introduction |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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and has been in widespread use since. When a polymer is stretched and the stress within it increases proportionally to the applied strain, the polymer is described as behaving elastically. In such circumstances, the unloading curve of the resultant stress/strain graph is identical to the loading curve. However, when elastomeric chain is stretched, it does not behave as a perfectly elastic material, because it loses energy and its unloading curve demonstrates less stress for a given stretch compared to the loading curve. This is called a hysteresis curve and is important because it is the unloading curve that is of interest to orthodontists. Indeed, it is well known that elastomeric systems lose force during the duration of their use.1–3 This is thought to be due to a combination of water causing the weakening of intermolecular forces and chemical degradation,4 and tooth movement resulting in decreasing stretch placed upon the elastomeric chain.5,6 However, most investigations have been performed under laboratory conditions, which cannot simulate the oral environment.7 It is not known how much force remains in a length of elastomeric chain at the end of its clinical use or for how long it may remain active.
While stainless steel coil springs have been in use since the 1930s,8 nickel titanium (NiTi) coil springs were introduced more recently.9 Increasingly, nickel titanium coil springs are used for space closure as they are thought to retain more force over a given time period and also provide a constant force.10,11 This may be a more effective tooth moving force than that provided by elastomeric chain. Certainly previous studies have concluded that nickel titanium coil springs are more effective in space closure than either elastomeric modules12,13 or intra-oral elastics,14 although no statistically significant difference has been found between the rate of space closure with elastomeric chain or nickel titanium coil springs.13 Force delivery from nickel titanium coil springs has been found to vary in response to the amount of activation15,16 and temperature.16,17 The composition of nickel titanium wires has been found to vary within batches, which has produced variable forces from custom made springs18 and this may account for batch variation found also within coil springs15.
Despite their potential superiority, nickel titanium coil springs remain relatively expensive and elastomeric chain remains popular in clinical practice.
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Aims |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Hypotheses |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Method |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Randomization
A split mouth study design was used, in which nickel titanium coil springs and elastomeric chain were attached in opposing quadrants. The quadrant to receive a nickel titanium coil spring was determined initially by the toss of a coin and alternated sequentially between left and right sides for the remainder of the sample.
Outcome measures
The initial and residual force measurements were made in grammes (g) and the space closure measured in millimetres.
Appliance design and data collection
Upper and lower 0.019 x 0.025-inch posted stainless steel wires were left passive for one visit (minimum 1 month) before commencing space closure and checked to ensure free sliding. No special measures (e.g. figure of eight modules or archwire stops) were taken to prevent the archwire spinning around and this was not found to be a problem. The archwires were removed and alginate impressions taken for study models prior to starting space closure in eight patients who had both force delivery systems used. Study models were repeated at the next visit. The force delivery systems under investigation were either 9 mm nickel titanium closed coil springs or medium-spaced elastomeric chain.
Nickel titanium coil springs
The nickel titanium coil spring was placed directly between two hooks, i.e. the hook on the molar band and the archwire just distal to the lateral incisor bracket. The initial force delivered was measured using a dial push–pull gauge, calibrated in 5-g increments. The nickel titanium coil spring was not renewed at subsequent visits, unless showing signs of distortion. It was simply reattached between the hooks as is usual in clinical practice. If the extraction space was too great for a nickel titanium coil spring to be stretched directly between the two hooks, then elastomeric chain only was applied (seven additional patients).
Elastomeric chain
Elastomeric chain was placed in the opposing quadrant, stretched to provide as similar a force to that delivered by the nickel titanium coil spring as possible. The elastomeric chain was renewed at each visit and the force matched to the residual force generated by the nickel titanium coil spring.
Timing of next measurement
The patients were asked to return for routine adjustments every 4–6 weeks. However, some failed to attend at this interval, one patient returning as late as 15 weeks after application of the force delivery system. On this following visit, the residual force was measured. The procedure was repeated until space closure was complete.
Space closure measurement method
Mean space closure was assessed using the method described by Dixon et al. (2002).13 Using Vernier callipers, the distance between the canine tip and the buccal groove of the first molar was measured. This measurement was repeated three times and the mean value taken. The mean space closure per week for each force delivery system was calculated for each patient and then summated for the sample as a whole.
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Measurement error and statistics |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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calculation was made and found the mean difference between the measurements was -0.6 g (SD 2.4) and the range within which 95 per cent of the differences lay was -5.2 to +4.1 g. Therefore, the force gauge was calibrated.
Measurement error in assessing space closure
To reduce method error associated with the measurement of the study models, the examiner was blind as to the method of space closure used in each quadrant. The study models were measured randomly, so that no start and finish models of the same patient were measured consecutively. Additionally, measurements were taken three times to reduce random error.
Intra–examiner reliability
The study models were re-measured 1 week later and these measurements compared to the mean of the initial measurements. There was no statistically significant difference between the two (P = 0.41).
Space closure with each force delivery system
The data were compared using a paired t-test after checking the data for normality. There was no statistically significant difference between these two methods of space closure (P = 0.46).
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Results |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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). Fifty-nine per cent (31/53) of the sample maintained at least 50 per cent of their initial force over a time period of 1–15 weeks. No sample lost all of its force and in fact the mean force loss was only 47 per cent (range of force loss was 0–76 per cent). The chain left in place for 15 weeks retained 53 per cent of its initial force (Figure 2). The higher the initial force, the greater the force loss experienced (Figure 3).
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). Forty-six per cent (12/26) maintained at least 50 per cent of their initial force over a time period of 1–22 weeks. No spring lost all of its force and the mean force loss was 48 per cent (range = 12–68 per cent). Again, the higher the initial force, the greater the force decay (Figure 5).
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). There was no statistically significant difference between these two methods of space closure (P = 0.46). The relationship between the initial force applied and the rate of space closure was also investigated (Figure 6). The wide scatter demonstrates no relationship between the two parameters.
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Discussion |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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How long does the force last?
The patients were asked to return every 4–6 weeks for routine adjustments. However, some failed to return until much later, which provided useful information about the activity of elastomeric chain over longer periods of time, which would have been ethically unacceptable if this had been planned. Elastomeric chain provided acceptable levels of force for substantial periods of time, challenging the view that it rapidly loses all of its force. In fact, all the samples retained some residual force at each visit and the elastomeric chain in situ for 15 weeks had 53 per cent of initial force remaining. The in vivo mean force loss of 47 per cent compared well with a mean force loss of 45.6 per cent found in vitro by Killiany and Duplessis, (1986),2 despite the view that elastomeric chain in the oral environment would be less force retentive than that in a laboratory. The nickel titanium coil springs also performed well over a time period of up to 22 weeks. This is encouraging, as it suggests that the clinical practice of using the same nickel titanium coil springs over several visits is sound.
Nickel titanium coil springs do not provide a low, constant force
The nickel titanium coil springs gave variable forces, even when stretched the same distance, and so do not appear to provide a predictable force. This is an unexpected finding as nickel titanium has traditionally been considered to be more reliable in its behaviour than elastomeric chain. Certainly, the high initial forces suggest the superelastic plateau (Figure 4
) was exceeded for some of the sample, which would explain why it took 6 weeks before the plateau of constant force delivery was witnessed.
How much force is clinically necessary?
It is not known exactly how much force is required to move teeth. Indeed, clinicians apply a wide range of forces for space closure20 and there is no evidence of an optimal force level in the orthodontic literature.21 Quinn and Yoshikawa (1985)22 suggested that 100–200 g is optimal for canine retraction and the residual force provided by nickel titanium coil springs during the plateau period of force delivery after 6 weeks fell into this range. Elastomeric chain provided residual forces lower than this, mostly between 50 and 150 g, yet space closure continued successfully. The initial forces applied were generally outside this range, especially with nickel titanium coil springs. It is surprising that no apparent relationship was demonstrated between the initial force placed and the space closure achieved. This may reflect the small sample size but is more likely to indicate the wide range of individual response to orthodontic forces. One patient achieved no space closure at all over a period of 10 weeks, despite a range of forces (135–270 g) being applied.
Nickel titanium coil springs and elastomeric chain close space at a similar rate
The rate of space closure of the two systems was very similar, which is comparable to findings by Dixon et al. (2002),13 who, in a larger sample of patients, found mean monthly (4 weeks) rates of space closure to be 0.58 mm for elastomeric chain and 0.81 mm for nickel titanium coil springs. This compares with rates of 0.84 and 1.04 mm, respectively, in this current study. Whilst this difference was not statistically significant this may be considered clinically significant when large spaces are to be closed. Theoretically, a 10-mm space (remaining after the loss of a first molar) would take 47.6 weeks for closure with elastomeric chain, but 38.8 weeks with nickel titanium coil springs.
Method issues
Force loss is due to both the inherent properties of the force delivery system and the decreasing stretch placed upon it as the extraction space closes. For clinical effectiveness it is necessary for a system to deliver sufficient force to move teeth throughout its period of use and the composite final force is most relevant. Due to the many variables in this study (differing inter-hook distances, lengths of time between appointments, oral environment) it is not feasible to draw absolute comparisons between the two force delivery systems with respect to force retention. Instead, this study aims to describe how the two perform in a normal clinical environment.
Sample size
The modest sample size in this study reflected the availability of patients from the single operator (CN) and was further influenced by the exclusion of patients using inter-maxillary elastics from the study. The conclusion of Dixon et al. (2002)13 that inter-maxillary traction had no effect on the rate of space closure, suggests that these inclusion criteria may not have needed to be so rigid. A power calculation on the sample size gave a result of 0.45, i.e. there was only a 45 per cent chance of having a statistically significant result. To increase the power of the study to 0.85, 100 samples of both elastomeric chain and nickel titanium coil springs would have needed to be tested.
Fixed hooks
Two fixed hooks were required for clear identification of the extension of the force delivery system. This was essential to measure the force generated. Therefore it was not possible to use nickel titanium coil springs attached to a ligature or to control the force delivery from the springs in the manner described by Manhartsberger and Seldenbrusch (1996).15 Hence, this reduced the sample size further.
Split mouth design
Whilst it might be anticipated the archwires may swivel asymmetrically under the influence of differing forces from the two force delivery systems, this was not identified as a problem in this study.
Space closure
The complete space closure in four quadrants was considered a potential problem as it was not known when space closure occurred, although it had happened within one visit (4–6 weeks). However, recalculation of the mean space closure excluding these quadrants did not alter the results and these quadrants have remained in the sample. The investigation of rate of space closure was incorporated latterly into the study, hence only patients recruited later on had study models taken, which may skew the results. Therefore, the data should be interpreted cautiously, but it is still comparable to the results of previous studies.
The purpose of this study was to describe the performance of elastomeric chain and nickel titanium coil springs in typical clinical use, hence, the study design was not rigid with respect to initial forces applied or the time period over which the space closure system was used. Nevertheless, useful information has been summated which may be helpful for future understanding of the role of forces within orthodontics and the selection of force delivery systems for use in sliding mechanics.
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Conclusions |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Contributors |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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Acknowledgments |
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References |
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TopAbstractIntroductionAimsHypothesesMethodMeasurement error and statisticsResultsDiscussionConclusionsContributorsReferences |
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2 Killiany DM, Duplessis J. Relaxation of elastomeric chains. J Clin Orthod 1985; 19: 592–593.[Medline]
3 Nattrass C, Ireland AJ, Sherriff M. The effect of environmental factors on elastomeric chain and nickel titanium coil springs. Eur J Orthod 1998; 20: 169–176.
4 Huget EF, Patrick KS, Nunez L. Observations on the elastic behaviour of a synthetic orthodontic elastomer. J Dent Res 1990; 69: 496–501.
5 De Genova DC, McInnes-Ledoux P, Weinberg R, Shaye R. Force degradation of orthodontic elastomeric chains—a product comparison study. Am J Orthod 1985; 87: 377–384.[CrossRef][Medline]
6 Lu TC, Wang WN, Tarng TH, Chen JW. Force decay of elastomeric chain—a serial study. Part II. Am J Orthod Dentofac Orthop 1993; 104: 373–377.[Medline]
7 Ash JL, Nikolai RJ. Relaxation of orthodontic elastomeric chains and modules in vitro and in vivo. J Dent Res 1978; 57: 685–690.
8 Arnold EB, Cunningham JS. Coil springs as application of force. Int J Orthod Oral Surg Rad 1934; 20: 577–579.
9 Miura F, Mogi M, Ohura Y, Karibe M. The super-elastic Japanese NiTi alloy wire for use in orthodontics. Part III. Studies on the Japanese NiTi alloy coil springs. Am J Orthod Dentofac Orthop 1988; 94: 89–96.[CrossRef][Medline]
10 Angolkar PV, Arnold JV, Nanda RS, Duncanson MG. Force degradation of closed coil springs. An in vitro evaluation. Am J Orthod Dentofac Orthop 1992; 102: 127–133.[Medline]
11 Miura F, Mogi M, Ohura Y, Hamanaka H. The super-elastic property of the Japanese NiTi alloy wire for use in orthodontics. Am J Orthod Dentofac Orthop 1986; 90: 1–10.[CrossRef][Medline]
12 Samuels RHA, Rudge SJ, Mair LH. A comparison of the rate of space closure using a nickel-titanium spring and an elastic module: a clinical study. Am J Orthod Dentofac Orthop 1993; 103: 464–467.[Medline]
13 Dixon V, Read MJ, O’Brien KD, Worthington HV, Mandell MA. A randomized clinical trial to compare three methods of orthodontic space closure. J Orthod 2002; 29: 31–36.
14 Sonis AL. Comparison of NiTi coil springs vs elastics in canine retraction. J Clin Orthod 1994; 28: 293–295.[Medline]
15 Manhertsberger C, Seldenbrusch W. Force delivery of Ni-Ti coil springs. Am J Orthod Dentofac Orthop 1996; 109: 8–21.[CrossRef][Medline]
16 Tripolt H, Burstone CJ, Bantleon P, Manschiebel W. Force characteristics of nickel-titanium coil springs. Am J Orthod Dentofacial Orthop 1999; 15: 498–507.[CrossRef]
17 Barwart O. The effect of temperature changes on the load value of Japanese NiTi coil springs in the superelastic range. Am J Orthod Dentofac Orthop 1996; 110: 553–558.[CrossRef][Medline]
18 Bourauel C, Drescher D, Ebling J, Broome D, Kananachos A. Superelastic nickel titanium alloy retraction springs—an experimental investigation of force systems. Eur J Orthod 1997; 19: 491–500.
19 Bland JM, Altman DG. Statistical methods for assessing agreement between two methods of clinical measurement. Lancet 1986; 1: 58–61.
20 Nattrass C, Ireland AJ, Sherriff M. An investigation into the placement of force delivery systems and the initial forces applied by clinicians during space closure. Br J Orthod 1997; 24: 127–131.[Abstract]
21 Ren Y, Maltha JC, Kuijpers-Jagtman AM. Optimum force magnitude for orthodontic tooth movement: a systematic literature review. Angle Orthod 2003; 73: 86–92.[Medline]
22 Quinn RS, Yoshikawa DK. A reassessment of force magnitude in orthodontics. Am J Orthod 1985; 83: 252–260.
Received September 20, 2002; accepted March 19, 2003
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