INTRODUCTION

Elastomers have been widely used in orthodontic treatment since the 1960s. In addition to their elastic property, elastomers are easy to use and economical.1 There are two types of elastomers used in orthodontics. The first type is natural elastomers, which are used in interarch mechanics and which are usually referred to as “elastics.” The second type is synthetic elastomers, which are used in elastomeric chains, ligatures, or elastic threads and which are usually referred to as “alastiks.”2,3

Alastiks are made mainly from polyurethanes and can either be thermoplastic or thermoset.1,4 Thermoplastic materials can be made from plastic and are moldable at high temperatures, while thermoset materials are irreversibly cured during the process of manufacturing, are not remoldable, and will burn at high temperatures. Thermoplastic materials have weak dipole or Van der Waal bonds between their polymers, while thermoset materials have stronger covalent chemical bonds.4 Some companies make elastomeric chains in both thermoplastic and thermoset materials. To the authors' knowledge, no study has compared the force decay of these chains in vivo or in vitro.

Orthodontic elastomeric chains are used in any movement that requires pulling, such as tooth retraction and protraction, space closure, and rotation correction. One of the biggest shortcomings of elastomers is the rapid decay of force over time. Clinically, orthodontic elastomeric chains are replaced at 3- to 4-week intervals as a result of force decay.2,3,5–9 The effect of initial force on force decay is debatable,5,10–14 and prestretching elastomeric chains before using them does not seem to produce a meaningful difference.8,15,16

Canine retraction can be done by direct chains or chain loops. A chain loop was described by Balhoff et al.9 as a chain stretched from the first molar hook, looped around the canine hook, and attached back to the molar hook. They reported that the direct chain designs had less force decay compared to the chain loop design. They noted, however, that the initial force values for the chain loop design were approximately two times that of the direct chain designs, and they stated that the chain loop design “should be repeated utilizing more units of elastomeric chain to determine the proper number of units which will generate appropriate physiological forces.”9

MATERIALS AND METHODS

The present study was a laboratory study in which elastomeric chains were stretched to predetermined distances and in which the remaining force was measured at different time points. This study was undertaken to compare the following over a period of 8 weeks: (1) force decay between thermoplastic and thermoset elastomeric chains; (2) force decay between light and heavy initial forces; and (3) force decay between two different canine retraction designs.

Sample

The sample included elastomeric chains from American Orthodontics™ (Sheboygan, Wisc) and Orthodontic Research and Manufacturing Company–Ormco™ (Glendora, Calif). Four elastomeric chain groups were selected for evaluation: (1) American Orthodontics™ Plastic Chains–thermoplastic (AOTP); (2) American Orthodontics™ Memory Chains–thermoset (AOTS); (3) Ormco™ Colored Power Chains–thermoplastic (OrTP); and (4) Ormco™ Generation II Power Chains–thermoset (OrTS). Two spools were ordered for each group, each with a different lot number. Forty elastomeric chain specimens for each group were used; 20 from spool a and 20 from spool b. The total number of chains used was 160. The spools were ordered no more than 2 months before the experiment. Once received, the elastomeric chains were stored in a drawer at room temperature away from light and extreme humidity. Gray closed elastomeric chains were selected for this experiment.

Each of the four elastomeric chain groups was subdivided into four subgroups to test two canine retraction designs and two force magnitudes, as follows: (1) direct chains light force (direct LF); (2) direct chains heavy force (direct HF); (3) chains loops light force (loop LF); and (4) chain loops heavy force (loop HF) (Figure 1). There were 10 specimens per subgroup, five from batch a and five from batch b. Two force magnitudes were selected to represent light forces (200 g) and heavy forces (350 g). A force of 350 g was used since several reports1,12,16 suggest that initial forces extending beyond 300 g can affect the force delivered by the elastomeric chain and the force decay. For the direct chain groups each specimen contained six modules. An extra module was left at both ends to assist in transferring the specimens. Chain loops were stretched from one pin around the second pin and back to the first pin. The specimens for the chain loop groups contained 14 modules plus an extra module at both ends (Figure 2).

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Testing Apparatus

A metal stretch jig was made of two halves joined by a jack screw with 10 pins on each half (Figure 3A). Each jig had a knob with markings on it where one-eighth of a turn separated the platforms by 0.1 mm. Sixteen jigs were made so that the testing of 160 chains could be done simultaneously. Artificial saliva was formulated from 20 mmol Hepes [4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid], 0.538 mmol calcium chloride dihydrate (CaC₂.2H₂0), 0.451 mmol potassium phosphate (KH₂PO₄), and 43.330 mmol potassium chloride (KCl). A pH of 6.75 was selected and adjusted using 4 M NaOH. Four plastic containers were used to contain the artificial saliva, which was replaced weekly. A label listing the elastomeric chain group and the stretching distances required for each subgroup was attached to the lid of each container. The containers were placed in an incubator (Lab-Line Incubator model 403, Melrose Park, Ill) with the temperature set at 37°C. Each plastic container had one elastomeric chain group to avoid possible contamination from chemicals released into the media.

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To measure the force, a digital force measurement gauge (50.00 N ± 0.01 N) (DS2-11, IMADA, Tokyo, Japan) mounted on a test stand was used (Figure 3B). The chains were stretched between two hooks: one was attached to the gauge and the other to the base of the test stand (Figure 3C). A digital distance meter on the test stand was used to determine the distance between the hooks. A digital caliper (200 ± 0.03 mm) (Series EC16, ID: 111-102B, TRESNA, Guangxi Province, China) was used to measure the lengths of the unstretched specimens and to recheck distances measured by the digital distance meter on the force gauge stand. To prevent elastomeric chain relaxation while transferring them from the stretch jig to the force gauge and vice versa, a modified divider was used as a transfer jig (Figure 3D). The divider tips were replaced with longer segments of 0.036-inch (0.9144-mm) stainless-steel wire that were modified using a rotary instrument (Figure 3E).

Determination of Stretching Required to Reach Selected Magnitudes of Force

Measurements were obtained to estimate how long each elastomeric chain group needed to be stretched to reach the desired force for each of its four subgroups. Specimens from AOTP were cut from spools a and b. For subgroup 1 (direct LF), 20 specimens were stretched so that the force recorded was 200 g (after 15 seconds of being stretched). The average stretch distance between the hooks from these specimens was used as the distance to stretch AOTP subgroup 1. Once used, these specimens were discarded and no specimens were reused. The same process was repeated by using 20 specimens for each of the remaining three subgroups of AOTP. The whole process was repeated for the three remaining elastomeric chain groups (AOTS, OrTP, and OrTS). Tables 1 and 2 show the required stretching for each subgroup.

Table 1. Required Stretching for Direct Chains^a

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Table 2. Required Stretching for Chain Loops^a

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RESULTS

Percentages of elongation were determined by dividing the differences between the lengths of the stretched and unstretched specimens by the original lengths × 100. Tables 3 and 4 show the elongation percentages.

Table 3. Elongation Percentages of Direct Chains^a

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Table 4. Elongation Percentages of Chain Loops^a

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The results of the independent samples t-tests showed that the thermoplastic subgroups from American Orthodontics and Ormco had significantly (P < .001) more force decay when compared to the corresponding thermoset subgroups at all time points (Figures 4 and 5; Table 5). Combining all subgroups, AOTP had 19.75% more force decay compared to AOTS, while OrTP had 20.11% more force decay compared to OrTS.

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Table 5. Percentages of Initial Forces for All Subgroups (with Standard Deviations in Parentheses)^a

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Table 5. Extended

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A comparison between subgroups 1 (direct LF) and 2 (direct HF) tested the effect of initial force on force decay. A comparison between subgroups 1 (direct LF) and 3 (loop LF) and between subgroups 2 (direct HF) and 4 (loop HF) tested the effect of retraction design on force decay. No significant differences were found between the four subgroups of AOTP or AOTS. P values were .152 and .079, respectively. No one-way ANOVAs were done between the subgroups.

In OrTP, pairwise comparisons showed no significant difference between subgroups 1 and 2. All other comparisons showed significant differences (P < .001). One-way ANOVA was used to analyze the differences between subgroups 1 and 3 and between subgroups 2 and 4 at weeks 2, 3, 4, 5, and 6 (Table 6). While in OrTS, pairwise comparison showed that subgroup 4 had significantly more force decay when compared to the other three subgroups (P < .001). No significant differences were found between subgroups 1 and 2 or between subgroups 1 and 3. One-way ANOVA was used to analyze the differences between subgroups 2 and 4 at weeks 2, 3, 4, 5, and 6. Subgroup 4 had significantly more force decay when compared to subgroup 2 only at weeks 5 and 6. The P values were .008 and .000, respectively, and the mean differences were 0.0386 and 0.0638 at weeks 5 and 6, respectively. Finally, Table 7 shows the time points with significant force decay when compared to the time points preceding them.

Table 6. One-Way Analysis of Variance (ANOVA) for Subgroups of ORMCO Thermoplastic (OrTP) at Weeks 2, 3, 4, 5, and 6

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Table 7. P Values and Mean Force Decay for Time Points Showing Significant Force Decay^a

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DISCUSSION

Since the 1970s, several studies have been published on the decay of elastomers.1,2,5–9 These results have shown wide ranges of force decay (24–85%) after 28 days. Some of the reasons behind these variable results may include a difference in the media in which the sample was tested, the quality of the elastomeric chains used, and the result of combining the results of thermoplastic and thermoset elastomeric chains. For example, in several studies5,6 elastomeric chains from RMO showed less force decay compared to those from other manufacturers. After some investigation it was noticed that RMO elastomeric chains are made from thermoset materials. To our knowledge, this was the first study to address differences between thermoplastic and thermoset elastomeric chains and the first to extend beyond the 6-week time point.

The thermoset specimens needed to be stretched more than the thermoplastic ones to reach the desired forces. This was most evident in the direct chain specimens stretched to 350 g, in which the thermoset specimens needed around 190–200% of elongation, while the thermoplastic specimens needed 70–107%. Another finding was that chain loops needed very little stretching to reach the desired forces. All four elastomeric chain groups required stretching of less than 20% to exert 200 g of force and stretching of less than 40% to exert 350 g of force. In the study by Balhoff et al.,9 initial forces for the chain loop groups ranged from 553.60 g to 753.20 g.9 In the present study care was taken to generate more appropriate initial forces in the chain loop subgroups.

The thermoplastic subgroups exhibited around 20% more force decay when compared to the corresponding thermoset subgroups of the same company. Studies usually publish the results of force decay at the 28th day. In the present study the average force decay at the 28th day was 60.9% for the thermoplastic chains and 40.63% for the thermoset chains.

No significant differences in force decay were found between direct LF and direct HF in any of the chain products. However, when comparing direct chains and chain loops, a significant difference was found between the subgroups in OrTP and OrTS. In OrTP, there was significantly higher force decay in direct LF compared to loop LF and in direct HF compared to loop HF. These results suggested that in OrTP, chain loops had less force decay compared to direct chains. Looking at OrTS, the results showed that subgroup 4 (loop HF) had significantly more force decay compared to the other three subgroups. No conclusion could be drawn from this since loop HF showed more force decay compared to direct HF, which was stretched more, and more force decay compared to loop LF, which was stretched less.

Huget et al.13 showed that chains stretched 50% of their length lost less force than did chains stretched to 100% or 200% of their length. This was not the case in our study, even in the thermoplastic subgroups. In the present study, a closer look at OrTP subgroups 1 and 2 shows that subgroup 1 was stretched 32.28% (below 50%) and subgroup 2 was stretched 106% (above 100%). There was no statistically significant difference in force decay between these two subgroups. However, loop OrTP (stretched below 30%) decayed less than direct OrTP, which led us to believe that there might be a stretch limit at around 30% only for OrTP below which the decay is less, probably as a result of less deformation. If one wishes to stretch a chain less than 30% and still apply 200 g of initial force, a chain loop should be considered.

Our results showed that significant force decay occurred until the first week in the Or chains and until the second week in the AO chains. The force then remained fairly constant until week 6. Most studies1–3,6–9,17–19 investigated force decay in elastomeric chains up to 3 or 4 weeks, while two studies5,10 extended to 6 weeks. Similarly, they showed that after the first week reasonably constant force remained throughout their study period. Our results showed that all four chain products exhibited significant force decay at weeks 7 and 8. These amounts of force decay might be greater clinically since the distances between brackets decrease as teeth move closer.5,9–11 Moreover, researchers17,18 have shown that more force decay occurs in vivo.

Forces in the range of 100–300 g were suggested as optimal forces for canine retraction.20 In the present study, all thermoset subgroups still had mean forces in the range of 115–220 g remaining after 4 weeks. In the thermoplastic subgroups when the initial force was around 350 g, the means of the remaining forces after 4 weeks were between 122 and 149 g. However, in the thermoplastic subgroups when the initial force was around 200 g, the means after 1 week were below 100 g, ranging from 73 to 95.5 g.

CONCLUSIONS

• Contrary to the interchangeable use of thermoplastic and thermoset elastomeric chains in the literature and in clinical practice, they perform differently under stress, and a clear distinction should be made between the two.

• Thermoset elastomers exhibited less force decay over time compared to thermoplastic elastomers and required more stretching to reach the desired forces.

• There was no difference in force decay when using light or heavy initial forces.

• Direct chains and chain loops performed similarly in the thermoset elastomeric chain groups. However, chain loops did perform favorably in OrTP, suggesting that chain loops might be superior when using thermoplastic elastomers. Additionally, all elastomeric chain groups shared the same general pattern of force decay, with no significant force decay between weeks 2 and 6, followed thereafter by significant force decay at weeks 7 and 8.