Evaluation of degradation of force of esthetic elastomeric chains

Introduction: Elastomers are considered important allies to orthodontic treatment and due to the aesthetic demand, aesthetic elastomers are increasingly used in adult patients. Objective: The aim of this experimental in vitro study was to evaluate the degradation of force of esthetic elastomeric chains, of three different sizes: short, medium and long. Material and method: For evaluating the degradation of force, 90 elastomeric chain segments were used, divided into 9 Groups (n=10), as follows: Morelli short elastomer, Morelli medium, Morelli long, Orthometric short, Orthometric medium, Orthometric long, American Orthodontic short, American Orthodontic medium and American Orthodontic long. Acrylic plates with pins were used to provide stretching of the elastomeric chains. The initial stretching force of the elastomeric chain was 150 grams, measured by a tensiometer. After this, they were prestretched to 50% of their original length, measured in an Instron 4411 Universal test machine, and placed on the seating pin on the plate. The plate was immersed in artificial saliva at 37 °C in a plastic receptacle, and removed for measurement after a time interval of 21 days. For statistical analysis, the methodology of mixed models for repeated measures in time and Tukey-Kramer test were used. Degradation of the forces was analyzed by 2-way Evaluation of degradation of force... Rev Odontol UNESP. 2019;48:e20190111. DOI: https://doi.org/10.1590/1807-2577.11119 2/7 analysis of variance (ANOVA) and the Tukey test. Result: All the elastomeric chains showed statistically significant reduction in force (p<0.05). The American Orthodontic elastomeric chains showed higher reduction in force values, irrespective of the elastic size (p<0.05). Conclusion: The three commercial brands studied underwent significant reduction in the quantity of force released; the American Orthodontic brand of elastomer showed the highest value of degradation in force. Descriptors: Orthodontics; elastomers; elastic modulus; dental esthetics.


INTRODUCTION
Elastomers are considered important adjuvant materials in orthodontic treatment and have many clinical applications 1,2 . This refers especially to esthetic elastomers used in adult patients who are highly demanding about the esthetic appearance of orthodontic appliances 3,4 . Elastomer chains can be classified as short, medium or long according to the distance between the links 5,6 . They have the advantages of being practical, hygienic, having elastic memory, being easy to place and comfortable for the patient to use, biocompatible and inexpensive [7][8][9] . However, they also have disadvantages because they are incapable of releasing constant levels of force for a long period of time and undergo changes in their physical properties when exposed to the oral environment, where they absorb saliva and become deformed due to breakdown of internal bonds. In addition, they undergo changes due to chemical cleaning products and the chemical action of chewing and toothbrushing; and stains are directly related to daily diet and eating habits [10][11][12] .
During orthodontic treatment, it is desirable for the elastics to maintain adequate force, for the purpose of achieving the dental movement desired by the orthodontist. However, the elastomeric chains dissipate a decreasing force 6,13 . To achieve greater efficiency during orthodontic treatment, it is necessary to know about some important factors such as the force applied on the teeth to be moved, because continuous forces obtain better results than intermittent forces 6,13 .
Some authors have suggested prestretching of chain elastics before their clinical application. This is a mechanical resource used for minimizing the abrupt drop in force released by the elastic after its fixation in the dental arch. These elastics may be stretched at different speeds (slow or fast), with variation of 50%, 75% and 100% in distention in relation to their initial length 1,14 . The recommendation is that they should be changed every three weeks, and that elastomeric chains should not be used for a period longer than four weeks. This is because, on an average, the elastics lose 30% of the magnitude of initial force, in addition to the occurrence of bacterial plaque accumulation and chemical interaction with foods and oral fluids 1,6 .
When evaluating the degradation for force undergone by elastics of the chain type, because of stretching time to which they were submitted 1,6,13,15 , the orthodontist frequently has difficulty in determining the adequate force to be transmitted to the teeth by the elastomeric chains and their time interval of dissipating useful force 13 . In the literature there is a scarcity of studies about the degradation of force of esthetic elastomeric chains. This fact justified the hypothesis that elastomeric chains long in size would show a higher level of degradation of force than the short and medium sizes. The authors also expected to find difference in the degradation of force between the different commercial brands (Morelli, Orthometric and American Orthodontic). Therefore, the aim of this study was to evaluate the degradation of forces of esthetic elastomeric chains in three different sizes: short, medium and long.

MATERIAL AND METHOD
For conducting this experimental in vitro study, short, medium and long elastomeric chains of three commercial brands (Sorocaba, SP, Brazil), Orthometric (Marília, SP, Brazil) and American Orthodontic (Sheboygan, WI, USA) were used. In total, 90 elastomeric chain segments were used, divided into 9 Groups (n=10) (Figure 1). All the elastics were acquired from the same manufacturer and were stored in their original packaging, with adequate "use by" date, kept in a dry place and protected from light, thus eliminating eventual changes caused by variations in temperature and lighting.
To evaluate the degradation of force, transparent acrylic resin plates were used. Each plate measured 11 cm long × 4 cm wide × 0.5 cm thick, where perforations 0.5 cm deep were made with a carbide tip 151 XL (Fava, São Paulo, SP, Brazil) adapted to a low speed motor. The pins were fabricated of orthodontic archwire measuring 0.9 mm in diameter by 10 mm high, which were fixated with transparent selfpolymerizing acrylic resin (Jet, Campo Limpo Paulista, SP, Brazil) in the perforations that had been made at a distance of 15 to 16 mm between them. On each plate, 10 pairs of pins were fixated to keep the chain type elastics stretched, with the distance between the pins corresponding to a stretching force of 150 gf of the elastics, measured by a tensiometer. After this they were prestretched to 50% of their initial length, measured with an Instron 4411 Universal test machine 6,14 . The elastics were cut with an extra half chain on each side to prevent distortion at the time of cutting them.
The elastic was removed from the (Instron) test machine and taken to its seating pin on the plate, which was immersed in artificial saliva at 37 °C in a plastic receptacle 6 . After a time interval of 21 days, the elastics were removed from the receptacle for new force measurement. This time interval is used by orthodontists for replacing the elastics, thus simulating the clinical use of these orthodontic accessories 14 . The plates were identified by means of a label containing the brand name of the elastic manufacturer and were numbered.
Selection of adequate force to be applied for orthodontic movement is of fundamental importance for performing good orthodontic treatment. The clinician must be concerned about maintaining integrity of the capillary blood supply in the periodontal ligament to promote bone resorptions and avoid areas of hyalinization around the alveolar bone on the pressure side. Very heavy forces may retard bone formation in the zone of traction 6,16 .

Statistical Analysis
After exploratory analysis of the force data they underwent logarithmic transformation so that the data would meet with the presuppositions of a parametric analysis. Thus, the methodology of mixed models for repeated measures in time and Tukey-Kramer test were applied. The percentages of degradation of the forces were analyzed by 2-way analysis of variance (ANOVA) and the Tukey test. All the analyses were performed with the R program (R Foundation for Statistical Computing, Vienna, Austria) software program, considering a level of significance of 5%.

RESULT
There was significant reduction in force value in the final time interval in comparison with the initial time (p<0.05%) for all the brands and sizes of elastics (Figure 1). For the medium sized elastics, in the two (initial and final) time intervals, the highest mean force value was observed in the Morelli brand (P<0.05%) (Figure 2). There was no significant difference between the three sizes with regard to degradation of force in percentage (p>0.05). The American Orthodontic brand showed the highest percentage of degradation of force in the three sizes of elastics (p<0.05). (Figure 3).

DISCUSSION
During orthodontic treatment, it is desirable for the elastics to maintain adequate force, with the purpose of obtaining correct tooth movement 1,6,13 . The main finding of this study was that the three commercial brands studied (Morelli, Orthometric and American Orthodontic) and the three sizes of elastics (short, medium and long) showed significant degradation of force values, corroborating the findings in the study of Kochenborger et al. 14 .
In terms of percentages, at the end of the time interval of 21 days, the mean degradation of force values among the groups were as follows: Morelli 28%, Orthometric 35% and American Orthodontic 62%, Figure 2. In a previous study evaluated the American Orthodontic brand of elastomer being stretched at different speeds; the result was that there was degradation of force in all the tests 1 .
For the short size elastics, in the initial time the highest force was observed for American Orthodontic, followed by Orthometric and Morelli brands. The ideal orthodontic appliance must have the capacity to release constant forces during its activation. Our findings suggested that the elastomeric chains of the American Orthodontic brand (short, medium and long) did not fully satisfy this premise, because their force diminished. These results corroborated the findings of previous studies in which the elastomer of the American Orthodontic brand had the lowest degradation of force value when compared with the other brand (Unitek), however, this study had been conducted with elastomers of the chain type in gray color 2 .
Chain elastics may vary in thickness, elastic properties, manufacturing process, addition of pigments and in the distance between links 2 . These factors influence the magnitude of force released and the degree of deformation shown by these materials.
In the present study, a difference was observed between the sizes of the links of the medium size (MM 1.5mm, OM 2mm, AOM 1.5mm) and long (ML 2mm, OL 3mm, AOL 3mm) esthetic elastomers when compared with the studied commercial brands. This could explain the difference between the results obtained. There are studies in the literature that have been conducted with different methodologies, providing results of evaluating the degradation of force compared with colored and gray elastomers of the chain type 15 . Furthermore, there is a scarcity of studies that evaluated the degree of degradation of force undergone by esthetic elastomers of the chain type.
Because this was an in vitro study, it has limitations, because during clinical use, orthodontic elastics are submitted to other deformations resulting from the action of salivary enzymes, mastication and brushing, salivary pH associated with the ingestion of hot and cold foods, among others 6,17 . In addition, previous studies have found that the use of alcohol or chlorhexidine mouthwashes in their composition considerably decreased the force produced by elastomeric chains 18,19 .

CONCLUSION
It was concluded that all the elastomeric chains: short, medium and long underwent significant reduction in the quantity of force released; the American Orthodontic brand of elastomer showed the highest value of degradation in force. When analyzing the results of this research, it was concluded that the medium size esthetic elastomeric chains of the Morelli brand were the type that obtained the best results, in addition to showing a satisfactory cost-benefit to the orthodontist.