Elastomeric chains are used in orthodontics for many purposes such as space closure. ¹ They are extensively used due to low cost, being hygienic and easy application. ² , ³ Despite the advantages of elastomeric chains, their properties are often affected by heat and moisture. ⁴ The main drawback of elastomeric chains is their force decay due to the breakage of cross-links between the long-chain molecules during clinical application. ⁵

It has been reported that elastomeric chains lose a great portion of their force (around 50%–70%) within the first 24 h of use, and this force decay continues at a slower pace in the next 3–4 weeks and can be influenced by a number of factors. ⁶ , ⁷ , ⁸ Rapid force decay in elastomeric chains results in inadequate tooth movement, necessitating additional appointments for activation of the appliance. ⁹

The level of elastomeric chain force depends on several factors such as the materials used by the manufacturer for their fabrication, dyes added to the chains, form of the chain (open or closed), being subjected to tension before use, sterilization techniques, storage techniques, ¹⁰ environmental factors such as tooth movement, thermal alterations, acidity of the environment, use of mouthwashes, salivary enzymes, and masticatory forces. The abovementioned factors can all affect the trend of force decay and relaxation behavior of elastomeric chains. ⁵

Oral cavity has a complex environment, and orthodontic appliances in the oral cavity are subjected to thermal, chemical, and mechanical alterations over long periods of time, which may result in their degradation. ¹¹ The effect of environmental factors on the pattern of force decay of elastomeric chains has been previously evaluated. ¹² , ¹³ Salivary enzymes and masticatory forces change the properties of elastomeric chains. ¹⁴ , ¹⁵ The effect of different environmental factors such as pH and thermal alterations and the effects of commonly consumed drinks on the properties of elastomeric materials have also been studied. ¹⁶ , ¹⁷ , ¹⁸ , ¹⁹

Citric acid is a naturally occurring weak organic acid found in all citrus fruits and vegetables. ²⁰ It is used widely as an acidulant and pH regulator. Furthermore, it is added to foods and drinks as a flavor enhancer, preservative and antioxidant synergist. ²¹ It has been approved by the FAO/WHO as a safe additive and is, therefore, extensively used in the food industry. ²²

This in vitro study aimed to assess the effect of citric acid on elastomeric chain force decay. In case of finding a positive effect, orthodontic patients should be advised to decrease the consumption of products containing citric acid.

In this in vitro, experimental study, a total of 60 short elastomeric chains from two major manufacturers, namely American Orthodontics (Sheboygan, Wisconsin, USA) and Ortho Technology (Tampa, Florida, USA) were selected (n = 30 from each brand). They were then cut into five-piece segments. To prevent possible damage to the chains during the process of cutting, the sections were made at the center of adjacent links at the two ends of the five-piece segments such that at each end of the five-piece segment, a half-link was present Figure 1a. The segments from each brand were then randomly divided into two groups of control (artificial saliva) and citric acid. Thus, the following four groups were evaluated in this study: Figure 1

(a and b) Chains connected to the fixtures. (c) Electromechanical universal testing machine for measurement of chain force.

As shown in Figure 2, elastomeric chain force gradually degraded in all the groups during the 3 weeks such that the difference in force in all the groups between baseline and 24 h and 1 week and 2 weeks was significant (P < 0.001). The difference in force measured at 2 and 3 weeks was not significant in any group (P > 0.05). Figure 2

American Orthodontics and Ortho Technology elastomeric chain force in the artificial saliva and citric acid groups.

Elastomeric chains are extensively used in orthodontics for tooth movement and space closure. However, their force decay is inevitable. ¹ Sensitivity of elastomeric chains to alterations in the oral environment is the most important cause of their plastic deformation and force decay. ²⁵ Acids produced by the oral bacteria and acids present in foods and drinks are important environmental factors that affect the physical and mechanical properties of elastomeric chains. ²⁶ This study aimed to assess the effect of citric acid on elastomeric chain force decay.

The shape and size of elastomeric chains (short, moderate, or long) also affect their mechanical properties. ²⁷ Short chains were evaluated in this study since they retain a higher percentage of force overtime. ²⁸ Furthermore, the chains were cut into five-piece segments; evidence shows that this particular length of a short elastomeric chain provides the required tension for retraction of canine in case of extraction of premolars and is suitable for the 25-mm distance between the canine and first molar teeth. ²⁹

Force decay tests that are performed at room temperature under dry conditions are weak representatives of oral clinical conditions, and the displayed force values by these tests are different from the actual chain forces in water, artificial saliva, aqueous environments, and distilled water. ³⁰ Thus, in the present study, the samples were immersed in artificial saliva and incubated at 37°C to simulate the clinical setting.

In the present study, the exposure time of chains to acid was 90 s, which corresponds to the time it takes for the pH to drop following the consumption of foods containing citric acid. ³¹

In this study, the initial force (10 s) values ranged from 366 ± 13 g to 282 ± 12 g. However, after 24 h, the force levels of all elastic chains ranged from 159 ± 42 g to 98 ± 10 g. This shows a 56%–65% force decay that is in agreement with prior studies that reported 50%–70% force decay after 24 h. ⁶ , ⁷ , ⁸ Such differences are clinically important because 100–300 g forces are clinically acceptable for movement of a group of teeth or a single tooth. ³²

However, the subsequent force decay of the tested chains was more important than the initial force levels. The current results showed that the chain force in all the groups gradually decreased within 3 weeks such that in all the groups, significant differences were noted between the values at baseline, 24 h, 1 week, and 2 weeks. These results were in agreement with the results of previous studies that showed that passage of time was a factor that caused a reduction in chain force. ²⁵ , ³³ , ³⁴ , ³⁵

Water is known as a factor causing a reduction in elastomeric chain force as well. The reason is the affinity between the hydrogen atoms in the water molecule and oxygen atoms in the chain. This affinity results in water sorption by the polymer chain. The absorbed water creates a space in the polymer chain and expands the gaps. The polymer can absorb water to a certain amount until reaching a peak. ³⁶ In this study, the difference in the magnitude of force at 2 and 3 weeks was not significant in any group, which can be probably due to the fact that water sorption by the chains reached its peak.

At baseline, there was no significant difference in chain force between the control and citric acid groups in any brand. After initiation of the experiment, the magnitude of force in the citric acid group experienced a greater reduction compared with that in the control group, and the difference between the two groups was significant in both the brands at all time points until the end of the 3-week study period. This finding indicates that citric acid can effectively decrease the elastomeric chain force. This finding was in agreement with the result of Nattrass et al., who suggested that acidity can cause force decay. ³⁷ However, our result in this respect was in contrast to those of Suprayugo et al., Santos et al., and Teixeira et al., who believed that acidity cannot effectively decrease elastomeric chain force. ¹⁷ , ²⁴ , ³⁸ Furthermore, this result was in contrast to the findings of Ferriter et al. and Pithon et al., who reported a smaller reduction in force of elastomeric chains stored in environments with higher acidity. ³⁴ , ³⁹ The difference between our results and those of the abovementioned studies can be due to the different environments used in each study. Suprayugo et al. and Pithon et al. immersed the elastomeric chains in soft drinks, which may contain other additives and compounds. Thus, the results cannot be directly attributed to the acidity of the solutions. Santos et al. and Ferriter et al. used environments with higher pH than our study. The same results could be obtained in lower pH values.

Elastomeric chains of different brands show variable degrees of reduction in force, which may be attributed to their different manufacturing processes such as printing system, raw materials, additive chemical agents, and shape and size of the loops. ³⁸ In the present study, the magnitude of force of Ortho Technology elastomeric chains in both the control and citric acid groups was significantly higher than the corresponding value in American Orthodontics elastomeric chains at all time points.

It is important to know the magnitude of primary force that elastomeric chains of each manufacturer can retain since knowledge in this respect enables the dental clinicians to initiate treatment with a more suitable magnitude of force that lasts and maintains a minimum desired value until the next appointment session. ¹⁰ In this study, the American Orthodontics elastomeric chains immersed in citric acid showed a considerable decrease in the amount of force generated after 24 h. Considering the force value required for orthodontic movement, the American Orthodontics elastomeric chains immersed in citric acid after 24 h, the American Orthodontics elastomeric chains immersed in artificial saliva after 1 week, and the Ortho Technology elastomeric chains immersed in both citric acid and artificial saliva after 2 weeks with remaining force <100 g, lost their effectiveness to close the spaces. Therefore, the Ortho Technology elastomeric chains presented better results in this study.

One of the main limitations in analyzing the results of this study was lack of similar in vivo studies. One of the limitations of in vitro studies including this one is that the fluctuations of intraoral temperature and acidity due to the consumption of different foods and beverages cannot be simulated in vitro. Although in vitro studies are valuable, direct generalization of their results to the clinical situations should be performed with caution. Despite these limitations, this study provided the orthodontists with additional information regarding some considerations that need to be taken into account by orthodontic patients such as limiting the consumption of fruits and vegetables containing citric acid during the appliance activation period by the use of elastomeric chains.

Elastomeric chains experienced force decay following immersion in artificial saliva and citric acid overtime. After the study onset, the magnitude of force in the citric acid group experienced greater decay, which indicates that citric acid can effectively cause elastomeric force decay. The magnitude of force of Ortho Technology elastomeric chains in both the citric acid and artificial saliva groups was significantly higher than that of American Orthodontics elastomeric chains at all time points.

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Conflicts of interest

The authors of this manuscript declare that they have no conflicts of interest, real or perceived, financial or non-financial in this article.