Comparison of deflection forces of esthetic archwires combined with ceramic brackets

Abstract Coated archwires and ceramic brackets have been developed to improve facial esthetics during orthodontic treatment. However, their mechanical behavior has been shown to be different from metallic archwires and brackets. Therefore, the aim of this study was to compare the deflection forces in coated nickel-titanium (NiTi) and esthetic archwires combined with ceramic brackets. Material and Methods Non-coated NiTi (NC), rhodium coated NiTi (RC), teflon coated NiTi (TC), epoxy coated NiTi (EC), fiber-reinforced polymer (FRP), and the three different conventional brackets metal-insert polycrystalline ceramic (MI-PC), polycrystalline ceramic (PC) and monocrystalline ceramic (MC) were used. The specimens were set up on a clinical simulation device and evaluated in a Universal Testing Machine (Instron). An acrylic device, representative of the right maxillary central incisor was buccolingually activated and the unloading forces generated were recorded at 3, 2, 1 and 0.5 mm. The speed of the testing machine was 2 mm/min. ANOVA and Tukey tests were used to compare the different archwires and brackets. Results The brackets presented the following decreasing force ranking: monocrystalline, polycrystalline and polycrystalline metal-insert. The decreasing force ranking of the archwires was: rhodium coated NiTi (RC), non-coated NiTi (NC), teflon coated NiTi (TC), epoxy coated NiTi (EC) and fiber-reinforced polymer (FRP). At 3 mm of unloading the FRP archwire had a plastic deformation and produced an extremely low force in 2; 1 and 0.5 mm of unloading. Conclusion Combinations of the evaluated archwires and brackets will produce a force ranking proportional to the combination of their individual force rankings.


Introduction
In modern society, the esthetic aspect of orthodontic appliances is important, particularly because more adult patients are seeking for orthodontic care 2  conditions. Since esthetic archwires have shown nearly the same level of biocompatibility as metallic wires, their clinical use may be considered safe 26 . Efforts have been made to investigate and develop fiberreinforced composite archwires suitable for use in clinical orthodontics, but commercial availability has been slowly progressing 4,5,15 .
During the coating application process on the archwire, a previous heat treatment is needed on its surface to produce an effective adhesion of the coating layer. As a result, the mechanical properties of metallic archwires could be affected during this process.
The mechanical properties of orthodontic archwires can be assessed by a 3-point bending test or a clinical simulation device, which evaluates the load-deflection properties, considered the most important parameters to determine the biologic nature of tooth movement.
Considering the difficulty to directly evaluate periodontal ligament stresses, the only way to estimate these parameters is by knowing the magnitude of forces applied to the teeth. Thereby, in vitro studies try to aid orthodontists to design and select an orthodontic mechanics that is not only efficient and biologically safe, but also esthetic pleasant to patients.
The aim of this study was to compare the load-

Methods
In order to internationally standardize the tests as adequately as possible, this study followed the ISO 15841 standard 16 . The tests were performed on the structure corresponding to the maxillary right central incisor.

Archwire deflection was performed by a clinical
Unlike the others, this structure was loose, enabling its bucco-lingual movement. It had a perforation that allowed a metal cylinder to be placed inside it for the activations. The tip of the activation device, attached to the testing machine had a rounded cut to fit the metal cylinder ( Figure 3).
The inter-bracket distance was kept constant at 6 mm 29 , since the relation force/deflection is dependent, among other things, on this distance. The speed of the testing machine for the deflection was 2 mm/min.

Statistical analyses
The sample size was calculated based on the ISO 15841 standard, which recommends six specimens for each group. However, to minimize the chances of any technical error and to increase the results reliability, a number of ten specimens were used for each group.   Results Inter-bracket comparisons (Table 1) The NC NiTi archwire, at 3 mm unloading, presented the statistically highest force in MC brackets.
However, at 2 mm unloading, the NC NiTi archwire presented the statistically lowest force in the same brackets ( Table 1).
The RC NiTi archwire, at 3, 1 and 0.5 mm unloading, presented the highest force in MC brackets ( Table 1).
The TC NiTi archwire, at 3 mm unloading, in MC brackets, and at 0.5 mm unloading, in MI-PC bracket, presented the statistically highest force. At 2 mm unloading, in MC brackets, it showed the lowest force.
At 1 mm unloading, in MI-PC brackets, it showed the highest force ( Table 1).
The EC NiTi archwire, at 3 mm unloading, presented the statistically highest force in MC brackets ( Table 1).
The FRP archwire, at 3 mm unloading, presented statistically higher forces in MC brackets. From this point, this archwire had a plastic deformation (crack), producing extremely low forces, near zero, meaning that the archwire stopped exerting force (Table 1).
Inter-archwire comparisons (Table 2) The MI-PC bracket presented the highest forces with RC NiTi and NC NiTi archwires for all deflections evaluated ( Table 2 Although the elastic deflection test in the threepoint machine is widely used by several authors 3,8,13 , this research employed a clinical simulation device, as reported by other authors 4,9,18 , including variables, such as brackets and elastomeric ligatures to best reproduce the clinical environment 1 .
We used super slick elastomeric modules able to generate significantly less static frictional force at the module/archwire interface than regular modules when tied normally.

Interbracket comparisons
Overall, the brackets presented the following decreasing force ranking: MC, PC and MI-PC, with small variations according to the amount of unloading.

Significant interbracket differences tended to occur
with large deflections. This happened because the frictional force of the PC brackets is greater, because of its rough surface 21 . Furthermore, the chemical characteristic of alumina on the ceramic surface may cause adherence on the archwire surface. This may generate a high friction and reduce the orthodontic force from 12% to 60% 17 . Because of these problems, a metal-insert has been developed in order to reduce the friction force generated by the ceramic brackets, which is the case of the MI-PC. Development of polycrystalline had already reduced the high friction forces of the ceramic brackets, but the forces were still higher than metal brackets. Therefore, the incorporation of a metal-insert reduced even further the forces generated by these esthetic brackets, decreasing this disadvantageous characteristic 6 .

Metal-insert Polycrystalline ceramic (MI-PC)
3  Previous studies have shown that MI-PC bracket produces less friction forces than PC brackets, resulting in higher forces during unloading 6  Only from the moment that it exceeds the static friction, the archwire will actually express its stored energy. This result reinforces other findings that found lower force generated by EC archwires 1,9,18 . The differences observed between teflon and epoxy coated archwires occur probably because teflon coating is

Clinical considerations
The results of this study do not allow a thorough comparison with previous studies since aspects, such as archwire size, deflection values, brackets and elastomeric rings and temperature are variables that need to be considered 9,13-15,18 .
The optimal deflection occurs at clinically useful displacements between 1 and 2.5 mm. These are the movements that predominate during leveling and aligning with low dimension archwires. This unloading region is the force value most likely to be applied in the clinical situation as soon as some movement of the teeth has occurred within the periodontal ligament 10 .
Reduction in the internal dimensions of NiTi archwire, to compensate for the coating thickness, seems to be responsible for the major changes in mechanical properties of the esthetic archwires, particularly in elastic deflection forces, as observed 18 .
The esthetic archwires employed in this study presented mostly deflection forces comparable to those obtained by NC NiTi archwires. Special attention should be given to the FRP archwire that, despite being highly esthetic, presented a permanent crack with 3 mm of deflection. This means that when the glass fiber archwire is employed in moderate to severe crowding, it may undergo permanent bending, interrupting tooth movement.
Since there are few published studies on the mechanical properties of esthetic archwires, additional studies need to be conducted, so that these mechanical properties are consistent with the desired force levels to induce tooth movement.
Furthermore, additional investigation is necessary to clarify whether the differences observed above reflect the actual influence of the coating material or if they are influenced by the coating manufacturing process.

Conclusions
• Overall, MC brackets presented the best results, because they produced the highest forces during unloading (lower friction), followed by PC brackets and, finally, by MI-PC brackets, with small variations according to the amount of unloading; • Overall, RC NiTi archwire presented the best results, because it produced the highest forces during unloading (lower friction), followed by NC NiTi, TC NiTi, Comparison of deflection forces of esthetic archwires combined with ceramic brackets J Appl Oral Sci. 2018;26:e20170220 9/9 EC NiTi and FRP archwires. Combinations of these archwires with the brackets will result in a proportional ranking; • The FRP archwire presented plastic deformation at 3 mm of deflection and produced extremely low forces at 2; 1 and 0.5 mm of deflection, not comparable with the mean forces generated by the other tested archwires; • Esthetic brackets and archwires, when used together, can exhibit very different patterns of forces because of the bracket composition and type of archwire coating.