INTRODUCTION

To secure arch wires in orthodontic bracket slots, clinicians may use stainless-steel ligatures, self-ligating spring clips, or elastomeric ligatures. Some of the advantages of elastomeric ligatures include ease of application, patient comfort, and availability in a variety of colors. Disadvantages include microbial accumulation, lack of complete seating of the wire in the slot, friction, and rapid force decay which drives orthodontists to replace them at 4-week intervals.^1–8 Factors that accelerate force decay include a basic pH,⁹ heat,¹⁰ and moisture.¹¹

Two types of elastomers are used in orthodontics. The first is natural elastomers, which are used in interarch mechanics and are usually referred to as “elastics.” The second type is synthetic elastomers, which are used in elastomeric chains, elastomeric ligatures, and elastic threads and are usually referred to as “alastiks.”^1,3,6 Synthetic elastomers are made mainly from polyurethanes^8,12 and can either be thermoplastic (TP) or thermoset (TS).⁶ TP materials can be made plastic and are moldable at high temperatures, while TS materials are irreversibly cured during the process of manufacturing, are not remoldable, and will burn at high temperatures. TP materials have weak dipole or Van der Waal bonds between their polymers, while TS materials have stronger covalent chemical bonds.⁶ Both TP and TS elastomers can be manufactured by either injection molding or die cut stamping. The specimens used in this study were all die cut stamped.

Some companies make elastomeric chains in both TP and TS materials, and a recent study by Masoud et al.⁶ compared the two in vitro. They showed that TS elastomeric chains decayed less than did TP elastomeric chains. On the other hand, to the best of our knowledge elastomeric ligatures are currently only available from TP materials. The aim of this study was to compare force decay and dimensional changes of TP and TS elastomeric ligatures made to order by Rocky Mountain Orthodontics–RMO™ (Denver, Colo) in simulated oral environment over a period of 8 weeks.

MATERIALS AND METHODS

The study was an in vitro laboratory study in which elastomeric ligatures were stretched to predetermined distances in a simulated oral environment. The remaining forces and the dimensional changes were measured at different time points. The study was undertaken to compare the following between time points over a period of 8 weeks: (1) force decay between TP and TS elastomeric ligatures and (2) dimensional changes between TP and TS elastomeric ligatures.

Sample

Rocky Mountain Orthodontics–RMO™ (Denver, Colo) provided the TP and TS elastomeric ligatures. The TS ligatures were made by RMO specifically for this study. The sample included 72 clear TP and 72 clear TS elastomeric ligatures. Once received, the elastomeric ligatures were heated to confirm that the TP ligatures melted and that the TS ligatures burned (Figure 1). The ligatures were then stored in a drawer at room temperature away from light and extreme humidity. The sample of 72 TP and TS elastomeric ligatures was divided into six groups, with 12 specimens per group, to record the force loss and dimensional changes at six time points, as follows: 24 hours (24h), 1 week (1w), 2 weeks (2w), 4 weeks (4w), 6 weeks (6w), and 8 weeks (8w). At each time point, the specimen was discarded once measurements were completed.

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Setting

Elastomeric ligatures were stretched over the handles of stainless-steel hand instruments. The ends of the handles were tapered to facilitate easy application and removal of the ligatures (Figure 2). The handles were 3.937 ± 0.002 mm in diameter to simulate the stretch necessary to apply ligatures over maxillary central incisor twin brackets with 0.022-inch slots. Ligatures stretched over brackets do not make perfect circles. Using hand instruments instead of brackets eliminates the effect of bracket design and the potential for overstretching the ligatures upon application and removal.⁸

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Artificial saliva was formulated from 20 mM Hepes [4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid], 0.538 mM calcium chloride dihydrate (CaC₂.2H₂0), 0.451 mM potassium phosphate (KH₂PO₄), and 43.330 mM potassium chloride (KCl). A pH of 6.75 was selected and adjusted using 4 M NaOH. Two sealed plastic containers were used, one for the TP elastomeric ligature and the other for the TS elastomeric ligature to avoid possible contamination from chemicals released into the media. The artificial saliva was replaced weekly with fresh formulated saliva. The containers were placed in an incubator (Lab-Line Incubator model 403, Melrose Park, IL) with the temperature set at 37°C.

To measure the force, a digital force measurement gauge (50.00 ± 0.01 N) (DS2-11, IMADA, Tokyo, Japan) mounted on a test stand was used (Figure 3A). The elastomeric ligatures were stretched between two hooks: one was attached to the gauge and the other to the base of the test stand (Figure 3B). A digital distance meter on the test stand was used to determine the distance between the hooks. The distance between the hooks was set to 4.21 mm based on the following equation adopted from Taloumis et al.,⁸ where D is the diameter of the hand instrument (which simulates the diameter of a maxillary central incisor bracket), X is the distance between the hooks, Dh1 is the diameter of hook 1 (which was 1.10 mm), and Dh2 is the diameter of hook 2 (which was 1.40 mm):

A digital caliper (200 ± 0.03 mm) (Series EC16, ID: 111-102B, TRESNA, Guangxi Province, China) was used to measure the outer diameter (OD), the inner diameter (ID), the wall thickness (WT), and the width (W) of the unstretched and stretched specimens recorded in millimeters (to the nearest thousandth of a millimeter) (Figure 4). This was performed as previously described by Taloumis et al.⁸ in 1997. The digital caliper was also used to recheck distances measured by the digital distance meter on the force gauge stand. To prevent relaxation of the elastomeric ligatures while transferring them from the handles of the hand instruments to the force gauge and vice versa, a modified divider was used as a transfer jig (Figure 3C). The divider tips were replaced with longer segments of 0.036-inch (0.914-mm) stainless-steel wire that were modified using a rotary instrument (Figure 3D).

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RESULTS

Student’s t-tests revealed significant mean differences in percent force loss, percent change in OD, percent change in ID, and percent change in WT between TP and TS elastomeric ligatures across all time points (P < .001). The means and standard deviations at each time point are shown in Figures 5 through 7. However, the same could not be said about W. A significant difference was found between TP and TS in W at 24h, 1w, 2w, and 4w, with P values of .00, .012, .003, and .011, respectively. No statistically significant difference was noted at 6w and 8w, with P values of .118 and .252, respectively. The means and standard deviations for percent changes in W at each time point are shown in Figure 8. The mean difference in force loss between TP and TS across all time points was 22.91%.

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The result of the two-way ANOVA showed a statistically significant mean interaction between the material (TP or TS) and the time points (P < .001). The results of the one-way ANOVA, when comparing each time point to the time point that preceded it, are shown in Table 1. A sample of TP and TS specimens at initial, 24h, 1w, and 4w time points is shown in Figure 9.

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Table 1.  P Values and Percent Change in Force and Dimension for Time Points Showing Significant Changes when Compared to the Preceding Time Points

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DISCUSSION

The force decay of elastomeric ligatures has not been as extensively studied as has been the force decay of elastomeric chains and interarch elastics. To our knowledge, this was the first study to compare TP and TS elastomeric ligatures. Based on our results, we found statistically significant mean differences in force decay between TP and TS elastomeric ligatures at all time points. We also found statistically significant mean differences in OD, ID, and WT between TP and TS elastomeric ligatures at all time points.

One of the few studies on force decay in elastomeric ligatures was the study by Taloumis et al.⁸ in 1997. The sample in their study comprised TP elastomeric ligatures only, and the stretch distance was almost identical to the stretch distance used in the present study. Their average force losses across companies for specimens immersed in saliva bath (test group 3 in their study) were 62% and 74% for 24 hours and 28 days, respectively. Our results for the TP group showed force losses of 85.1% and 93.04%, respectively, for the same time points. An explanation as to why the results in the present study showed greater force loss was the use of a transfer jig and the fact that we did not allow the specimens to relax before measuring the force, as was the case in previous studies. Moreover, the study by Taloumis et al.⁸ demonstrated that groups that were allowed shorter periods of relaxation showed more force decay. In the present study, the specimens were not allowed to relax since elastomeric ligatures do not relax in the oral cavity. Not allowing the specimens to relax gives a true representation of the force applied clinically.

Our results showed that on average and across all time points there was 22.91% more force loss in the TP group. Masoud et al.⁶ compared TP and TS elastomeric chains. Despite the fact that their specimens did not include elastomers from Rocky Mountain Orthodontics and that elastomeric chains were used instead of elastomeric ligatures, they showed a 20% greater force loss in the TP compared to the TS group, which closely resembles our findings. In their study the average force loss at the 28th day (4 weeks) was 60.9% for the TP elastomeric chains and 40.63% for the TS elastomeric chains. In our study, specimens exhibited more force loss, with 93.04% and 77.41% for TP and TS elastomeric ligatures, respectively, at the 28th day.

This study measured force decay for up to 8 weeks. There was only one other study⁶ that extended beyond 6 weeks, which was done using elastomeric chains. It showed that the elastomeric chain groups, both TP and TS, shared the same general pattern of force decay, with significant force decay up to week 2 and no significant force decay between weeks 2 and 6, followed thereafter by significant force decay at weeks 7 and 8. In our study, TP and TS specimens showed significant force decay at 24 hours, 1 week, and 4 weeks, with no significant force decay thereafter. One explanation for why the elastomeric ligatures did not show significant force decay after week 4 was that, as noted above, elastomeric ligatures exhibited more force decay during the first 4 weeks compared to elastomeric chains. This means there was not much force left in the elastomeric ligatures to exhibit further significant force decay.

In the present study we used percent force loss rather than the remaining percent of initial forces (remaining force) commonly used when studying elastomeric chains. Initial forces by elastomeric chains can be much more predictable than initial forces by elastomeric ligatures. When retracting a canine using an elastomeric chain, changing the number of modules changes the initial force. Initial forces exerted by elastomeric ligatures are primarily not under the control of the clinician and are affected by multiple factors. Some of these factors include the width of the bracket, the interbracket distance, the cross-sectional size and shape of the arch wire, the material properties of the arch wire, and the position of the bracket slot which is determined by the three-dimensional configuration of the malocclusion. The position of the bracket affects the load, which can be in the form of twist, deflection, or both.¹³ All of these factors also affect elastomeric chains, but to a lesser extent. To put that into prospective, if we assume a simple unipoint bracket contact and a flat curve of Spee, 10 equations must be solved simultaneously to find the force exerted by the wire.¹⁴ However, in a simulated clinical situation,¹⁵ it was shown that at a 1.5-mm deflection the forces ranged from 150 g to 850 g. The initial forces for the specimens in the present study ranged from 208 g to 277 g for the TP specimens and from 250 g to 347 g for the TS specimens. Optimum forces for canine retraction have been proposed by multiple authors to be in the range of 100 g to 300 g.¹⁶ However, optimum forces required to produce all movements seen in leveling and aligning have been proposed to be between 10 g and 100 g.¹⁷ If one considers root ratings, proposed by Ricketts,¹⁸ for optimum forces required for tooth movements then all movements seen in leveling and alignment require forces between 20 g and 120 g. After 4 weeks the average remaining force in the TP specimens was 17 g and 68 g for the TS specimens. Remaining forces might diminish even more clinically since more force decay has been shown in vivo.^19,20 Furthermore, as the bracket moves closer to the wire intraorally, the forces exerted by the elastomeric ligature might decrease even further. The remaining forces in the TP ligatures might be suitable for some but not all orthodontic tooth movements.

Significant differences were found at all time points between TP and TS elastomeric ligatures in the percent change in OD, ID, and WT, but not in W. An explanation for the limited difference in W (Figure 8) was that the other dimensions (OD, ID, and WT) were all in the same plane of the ligature stretch. The W dimension was vertical to the other three dimensions and vertical to the plane of stretching, which might have been the reason for smaller dimensional changes in W (Figure 4). In the same context, the greatest dimensional change was in ID (Figure 6), since it was the combination of the increased OD and decreased WT (Figure 4).

CONCLUSIONS

• A novel TS elastomeric ligature showed significantly less force decay and dimensional changes over time when compared to a commercially available elastomeric ligature. This novel elastomeric ligature might be superior during initial leveling and aligning and during finishing stages. The dimension with the least change was the width of the elastomeric ligatures.

• Using a transfer jig to prevent relaxation of the specimens before force measurement showed that force decay of commercially available elastomeric ligatures was greater than that which had previously been published.

• The elastomeric ligatures, both TP and TS, did not show significant force decay after week 4 and did not exhibit the same force decay pattern of elastomeric chains.