Influence of mandibular movements during speech on force loss of orthodontic elastics: An in vitro study

INTRODUCTION

Despite ongoing transformations in dentistry, which encompass a range of advancements in devices and diagnostics, the use of elastics remains a significant part of orthodontic biomechanics today. This practice is common regardless of the type of device used, whether fixed, removable, or in aligners.^[1,2] Recently, with the increasing demand for more comfortable orthodontic treatments with great esthetic appeal, a large number of clear aligner systems have emerged, and several treatment possibilities have been suggested; however, one issue remains intact: Patient collaboration is essential, either with the use of aligners, or with the use of removable accessory resources, including intraoral elastics. With the help of elastics, additional strength systems could be built, thus improving the biomechanical systems of clear aligners.^[3] The advantages of elastics in the clinical context include orthodontic dynamicity, low cost, high flexibility, ease of manipulation by the patient, and hygiene aspects.^[4-6] However, its main disadvantage is the decreased strength over time, requiring the patient’s collaboration for daily changes, which are often neglected.^[2,5-8]

The deterioration of elastics is influenced by several oral conditions, such as salivary composition and its immersion influence, and oral temperature, among others.^[2,5-7,9-11] Furthermore, elastomers suffer mechanical degradation, mainly due to mandibular movement during speech.^[12]

Although articles address the deterioration of intermaxillary elastics under conditions of dynamic stretching, no studies have adjusted their methodology to the human characteristics of speech. Therefore, this work seeks to evaluate whether the number and capacity of words spoken daily influence the decay of the force of the intermaxillary elastic.

MATERIAL AND METHODS

Based on a previous study^[13], a sample calculation was performed for P ≤ 0.05 and α ≥ 0.80, with an expected effect size of 0.82, resulting in a total of 320 elastics (n = 320) divided into groups of 20 elastics. Elastic bands from the company Morelli (Dental Morelli Ortodontia®, Sorocaba, Brazil) were selected and divided into 16 equal groups, of light (2.47 oz) and medium (4.59 oz) force, and sizes of 1/8”, 3/16”, 1/4” and 5/16” inches [Table 1]. Two different manufacturing batches were used in this study, corresponding to 50% of the sample in each group.

All elastics were obtained from sealed packaging, within the expiration date, and were stored at the same room temperature as recommended by the manufacturer.

Experimental devices

Two experimental devices were developed for this study. The first device, used to read the force of the elastic bands, consisted of a structure with a 500 g load cell and a reading accuracy of 0.1 g [Figure 1]. This device has two hooks, developed with stainless steel with a diameter of 0.8 mm (the average size of the hooks present in orthodontic brackets). One hook is fixed to the rigid structure of the device and the other is fixed to the load cell, spaced 3 times the internal diameter, according to the recommended standard extension index,^[13] with the size of the hook changing according to the diameter of the elastic. To measure the distances established in this research, an electronic digital caliper (Mitutoyo U.K. Ltd, United Kingdom) was used with an accuracy of ±0.001 mm. To calibrate this device, standard weights were used. The elastic force readings occurred immediately (T0), after 1 h (T1), and after 24 h (T2).

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Figure 1:

Elastic force measurement device (1. Latex elastic, 2. fixing hooks, and 3. hydraulic pump.)

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The second device is the mandibular movement simulator [Figure 2]. It was designed from a metallic structure, submerged in a tank with distilled water, at a constant temperature of 37°C controlled by a thermostat and a water circulator, simulating the oral cavity. The submerged part had two acrylic plates with pairs of steel pins with a diameter of 0.8 mm, where the elastic bands were positioned under a stretch of 3 times their internal diameter. The first acrylic plate was attached to the lower structure and was changed according to the diameter of the elastics using the side screw. The second acrylic plate was fixed to the pneumatic cylinder which, when activated, moved the plates 11 mm apart (in addition to the previous distance of 3 times the internal diameter), which is the average distance of mandibular displacement during speech and at a frequency of 5Hz [Figure 3 and Supplementary Video 1].

Supplementary Video 1.

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Figure 2:

Mandibular movement simulator device (1. Thermometer, 2. controlled heater, and 3. frequency controller).

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Figure 3:

Mandibular movement simulator device in operation.

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RESULTS

The measurement in grams of the initial force of the elastics, the force indicated by the manufacturer, the distance in millimeters from the internal diameter and the distance after stretching by 300%, and the decay of force after 1 h and 24 h of testing are presented in Table 2. We observed that the greatest decay of the elastic occurs in the 1^st h of the test and maintains a constant average loss after 24 h.

With the exception of the 1/8” elastics, which ruptured during the cyclic test, all light-force elastic sizes showed no significant differences between the two measurement periods (1 h and after 24 h). For the 3/16”, 1/4”, and 5/16” medium-force elastic sizes, the strength loss was significantly greater in the tests after 24 h. The 1/8” elastic showed significantly greater strength loss after 24 h in the static test.

Regardless of the test mode (cyclic or static), the greatest decay of the elastics occurs in the 1^st h for all elastics. The average decay of elastic bands after 24 h is 18.64%, with a maximum of 23.70% and a minimum of 13.57%. The 3/16” (light and medium) and 1/4” light elastics showed greater force decay in the cyclic test. The 1/4” medium and 5/16” elastics (light and medium) showed greater decay in the static test.

Individualizing the analysis between the same size, and evaluation after 24 h, there is a significant difference between static and cyclic testing for all elastics, except for the 1/4” elastic with medium force intensity, as shown in Table 3.

DISCUSSION

The ability to speak is vital in human life, including communication, socialization, learning, cognitive development, and expression of cultural identity. If the orthodontist does not have a complete understanding of the elastic degradation process, significant clinical problems may arise.^[2] Although some studies^[7,9,10,12] have been based on stretching the elastic to evaluate its decay, to date, no methodology was exclusively related to mandibular movements caused by speech.

Current work shows that elastics in dynamic movement present a greater loss of resistance compared to static elastics.^[2,7] This can be explained by the slippage of the molecular chain caused by repetitive movement, or the breakage of some chains due to the elastic limit.^[2,17]

When analyzing the data shown in Table 2, it is possible to observe a significant discrepancy between the results obtained in the static and dynamic speech tests. It was found that elastics with lower rubber content, such as the light and medium 3/16” elastics, and the light 1/4” elastic, showed a notable reduction in strength after 24 h of exposure to the dynamic test to simulate the speech, compared to the static test. On the other hand, elastics with a larger diameter and, consequently, a greater amount of rubber, such as the light and medium 5/16” elastics, showed a more pronounced loss of strength in the static test compared to the dynamic one. The same observation was found in other studies^[6,9] which described that the degradation of the strength of the elastic with a larger diameter was lower than the other types, collaborating with the finding in this research.

One of the hypotheses of what happened with the elastics with a larger amount of rubber (5/16” light and medium) is that the presence of saliva causes the elastomers to swell, leading to the absorption of saliva and the separation of the polymer chains, a known phenomenon such as plasticization.^[17] The greater the amount of rubber, the greater the lubrication of the molecules present, which reduces the breakage of the molecules during stretching. On the other hand, elastomers with lower rubber content have less lubricity and are more prone to fracture of molecular bonds, resulting in an increased loss of strength. Therefore, in relation to orthodontic clinical behavior, it is recommended to prioritize the use of elastomers with a greater amount of rubber when using intermaxillary elastics.

The ideal force for tooth movement stimulates cellular activities without completely obstructing the blood vessels in the periodontal ligament. Therefore, elastics with light and medium force intensities are the most commonly used in clinical practice^[10,12] and were also chosen as the elastics of choice in this research. Considering that the results indicated that elastics with a greater volumetric quantity of polymers present better results, future research could be carried out with heavy elastics to evaluate their behavior.

It is questioned whether “in vivo” research presents ideal characteristics and results closer to clinical reality.^[6] However, some research models, when conducted in vivo, may incur biases, such as participant collaboration, measurement difficulties, changes in ideal conditions, and changes in behavior, among other factors, resulting in discrepant values that are not representative of the real behavior of the product.^[18] Due to this consideration, the standardization of methodologies carried out in an in vitro environment can generate more accurate data, providing greater credibility and confidence in the use of the product. Notaroberto et al.^[19] evaluated and compared the behavior of latex and non-latex elastics regarding the loss of force over time in 15 volunteer patients at time intervals of 0, 1, 3, 12, and 24 h using a distension testing machine. They concluded that latex elastics had more stable behavior during the study period, compared to non-latex elastics in all periods within 24 h. Although the objective was to compare the different types of materials, the degradation of force over the periods was very similar to that observed in our study. Furthermore, Ptáčková et al.^[20] compared the degradation of the strength of intermaxillary latex elastics under in vitro and in vivo conditions, at distensions of 3 times their diameter, using the collaboration of 10 volunteers who received orthodontic buttons glued at individualized distances, and a temperature-controlled testing machine. They concluded that the degradation of the strength of intermaxillary elastics occurs both in vivo and in vitro, being relatively similar between the two environments. Specifically, the greatest force decay occurs in the 1^st h of use (in the first 2 h), with a progressive reduction over time up to 48 h.

Our research carefully simulated the ideal conditions found in the oral cavity for the study of intermaxillary elastics, such as humidity temperature and orthodontic bracket hook size.^[6] We chose to use distilled water instead of artificial saliva due to its ability to maintain a constant temperature. The size of the hook can influence friction, especially during dynamic tests, and potentially interfere with the structural integrity of the elastic.^[7]

Another important point in changing the strength of the elastic is the manufacturing period and storage.^[6] All elastic bands tested were in equal packaging, maintaining their characteristics, as recommended by the manufacturer.

The degradation of elastics of the same diameter, subjected to the same test, but with different levels of force intensity, indicates a propensity for greater wear in elastics with a higher concentration of rubber, with the exception of elastics 5/16”. This theory of more pronounced degradation in elastics with a greater amount of rubber is controversial in the literature. For example, a study conducted by Qodcieh et al.^[12] reported a greater loss of strength in heavy rubber bands compared to medium rubber bands. However, previous studies suggest that thicker elastics can maintain higher forces for a prolonged period.^[9]

The 24-h period for evaluating elastic degradation is in line with several authors.^[9-12] In these studies, it is reported that the initial strength is lost in the first few hours, but that in the following hours, the loss of strength decreases, which does not require immediate replacement of the elastic bands.^[12] One hypothesis for this phenomenon is that the changes caused by stretching are not cumulative; once the rubber band is stretched, these changes stabilize sequentially.^[12,17]

Clinically, changing elastics more than once a day may be related to esthetic color changes that occur with latex elastics, as observed by the authors of this research and reported in the literature.^[12,21]

In this research, unlike other studies^[6,9] the 1/8” elastic bands ruptured during the tests. According to the methodology adopted in this study, the 1/8” elastics, when stretched to 11 mm, had an increase of approximately 6.5 times the size of the internal diameter, which may explain their inability to support the test.

From a clinical point of view, based on the results of this study, it is suggested that it is not recommended the use of 1/8” elastics during the day, due to the frequent rupture that can occur resulting from jaw movements during speech. This often makes it difficult for the patient to continually replace the elastics, which can interfere with the consistent biomechanics of forces standardized by the orthodontist. These intermaxillary elastics are recommended for post-orthognathic surgery patients, in whom there is limited intermaxillary opening or for use only during the night during sleep.

It is important that orthodontists have knowledge of biomechanics and the behavior of the materials that they use for better treatment performance. This study, along with Qodcieh et al.,^[12] recommends that manufacturers better document the degradation properties of their products.

CONCLUSION

• Regardless of the test performed, intermaxillary elastics tend to lose strength after 24 h

• 1/8” (light and medium) broke during cyclic testing, suggesting they are not suitable for use during speech

• 1/4” (medium) did not show a significant difference between static and cyclic testing suggesting that it is the most suitable size for both nighttime and daytime use, especially in patients with high verbal communication needs.