Evaluation of the force generated by gradual deflection of orthodontic wires in conventional metallic, esthetic, and self-ligating brackets

ABSTRACT Objective: The purpose of this study was to evaluate the deflection forces of Nitinol orthodontic wires placed in different types of brackets: metallic, reinforced polycarbonate with metallic slots, sapphire, passive and active self-ligating, by assessing strength values variation according to gradual increase in wire diameter and deflection and comparing different combinations in the different deflections. Material and Methods: Specimens were set in a clinical simulation model and evaluated in a Universal Testing Machine (INSTRON 3342), using the ISO 15841 protocol. Data were subjected to One-way ANOVA, followed by Tukey tests (p<0.05). Results: Self-ligating brackets presented the most similar behavior to each other. For conventional brackets there was no consistent behavior for any of the deflections studied. Conclusions: Self-ligating brackets presented the most consistent and predictable results while conventional brackets, as esthetic brackets, showed very different patterns of forces. Self-ligating brackets showed higher strength in all deflections when compared with the others, in 0.020-inch wires.


INTRODUCTION
Nowadays, having a natural and pleasant smile even during orthodontic treatment is one of patients' main concerns. Devices combining acceptable esthetic and adequate technical performance, satisfying both the patient and the clinician expectations, have been developed 2 . Nevertheless, esthetic brackets show higher friction the desired movement 35 .
Self-ligating brackets, introduced as Russel's accessories in the mid-1930s, are systems that present a mechanical device designed to close the edgewise slot 33 . Their use has become common in recent years. Manufactures claim several advantages in using these accessories, and the low friction seems to be the most studied among them 11,15,18 . Some studies confirm that there in a shorter treatment time 15,19 . However, some studies showed that this reduced friction depends on the type and caliper of the wire and the degree of crowding 30 . Further studies and clinical evaluations 24 . Therefore, effectiveness of orthodontic movement results not only from different bracket systems, but also from a series of other factors, related to both the patient (teeth and supporting structures) and the type of mechanics applied. Teeth movement also depends on the action of orthodontic wires, which varies according to their structural and mechanical properties 3 .
Consequently, it is necessary to assess not only the friction related to different bracket systems, but also the behavior of different currently available orthodontic materials regarding the forces applied during orthodontic mechanics. Furthermore, the development of esthetic brackets with metal 2016;24(5):496-502 J Appl Oral Sci. 497 8 . To make the best choice among the various brackets and orthodontic wires available, it is essential to know the magnitude of forces released by these wires and their behavior regarding the gradual 7 . This in vitro of round Nitinol orthodontic wires, placed in conventional metallic, reinforced polycarbonate, sapphire (Ormco, Glenda, CA, USA), and selfligating brackets by using a clinical simulation model and following ISO 15841 as protocol.

Material -experimental groups
The sample used in this study consisted of 400 round-section Nitinol wires (Standard or Medium, GAC ® , Bohemia, NY, USA) with 0.014, different bracket types: conventional metallic, reinforced polycarbonate with metallic slots, sapphire, passive and active self-ligating brackets ( Figure 1). Wires that showed very different behavior from the others were excluded 21 : they statistically represent outliers that would negatively interfere with the results. In conventional brackets, the wires were tied with "O" shaped elastomeric ligatures (GAC ® ; Bohemia, NY, USA). In the self- Active self-ligating (In-Ovation R) * All groups were tested at a controlled temperature of 36±1°C  ligating brackets the wires were tied by closing the passive (Damon Q) or active (In-Ovation R) systems.

Methods -clinical simulation device
In order to internationally standardize the tests as adequately as possible, the methods used in this study followed the ISO 15841 Standard: Dentistry -Wires for use in orthodontics 16 .
in a clinical simulation device representing all 10 teeth of the maxillary arch 11 . Figure 2 shows the clinical simulation device that was used in this study. This device consists of a parabola shaped acrylic the maxillary teeth ( Figure 2). The parabola shape was determined by the wires, reducing the risk of in an unexpected way. Brackets were bonded with cyanoacrylate ester gel (Super Bonder, Loctite, São Paulo, SP, Brazil), positioned in the long axis of the acrylic devices parallel to the ground and at the same height. The wires had the same length 12 . The screw in the bottom of the acrylic resin plate.
The tests were performed on the structure corresponding to the right maxillary central incisor ( Figure 3). Unlike the others, this structure was not screwed, enabling its labio-lingual movement. It had a perforation, in which a metal cylinder was placed to activate it. The tip of the activation head, attached to the testing machine, had a rounded was performed without changing the inter bracket depends, among other things, on this distance. The Records of the force released by the wire clinically corresponds to the beginning of treatment, when the teeth are poorly positioned and the wire is forced into the accessories slots. Depending on the degree of crowding, teeth experience more or less force to align the teeth.
Universal Testing Machine (Instron 3342), with a load cell of 10 N 10 ( Figure 3). This load cell has an accuracy of 0.5% of the reading value with the temperature of 25°C. In this study, the load cell was maintained at this temperature. Also according to the ISO standard, the tests were always performed at the same testing temperature of 36±1°C for all test groups 26 . To obtain this, an acrylic container with water at a temperature of 36±1°C, maintained with the aid of submersible heater with integral thermostat (Electronic Atman Heater, China) and checked by a decimal precision thermometer, with a limit of error of ± 0.2°C (Incoterm, reference 5097, São Paulo, SP, Brazil), was adapted to the testing machine 24 (Figure 3).

Statistical analyses
The sample size, according to the ISO 15841 standards, is of six specimens in each group 16 . To minimize the chances of any technical error and increase reliability of the results, 20 specimens were chosen for each group. Outliers were excluded through a statistical program that provides the values to be deleted 22 . Normal distribution was evaluated with Kolmogorov-Smirnov tests. Because all variables showed a normal distribution, one-way ANOVA and Tukey tests were used.
All statistical analyses were performed with Statistica software (Statistica for Windows -Release 7.0 -Copyright Statsoft Inc., Tulsa, OK, USA). Results were considered significant at p<0.05.

RESULTS
The deactivation forces were generally self-ligating brackets, with 0.014 and 0.016-inch nickel-titanium wires, except with the activation of 2 mm, in which the opposite occurred (Tables  1 and 2). Overall, Inspire ICE showed the highest deactivation forces while In-Ovation R showed the lowest.
The deactivation forces were generally ligating brackets, with 0.018-inch nickel-titanium wires (Table 3). Overall, Inspire ICE showed the highest deactivation forces while In-Ovation R showed the lowest.
There was an inversion of the previous  tendency with 0.020-inch nickel-titanium wires.
higher with self-ligating than with conventional brackets (Table 4). Overall, Damon Q showed the highest deactivation forces while Spirit showed the lowest. The forces with the Damon Q with 3 mm of activation exceeded 1000g.

Sample and methodology
A clinical simulation device was used to approximate the laboratory results to clinical situations, providing more practical applications 32 .
was used for orthodontic wires laboratory tests 16 .
it is clinically closest to the orthodontists' interests, since that is what they do when adapting a wire to the patient's teeth. Although engineers work with parameters such as elastic modulus and yield value, the orthodontist is more concerned about knowing the force released regarding the amount

Results
In general, it was observed that the deactivation force increased with the increase in amount of deflection (Tables 1 to 4). These findings are consistent with other studies 29 .
The results found in different combinations of the brackets with 0.014-inch nickel-titanium wires are in agreement with other studies in the literature that large variation in the difference of force values, and devices 36 (Table 1). mm, presented the highest forces when compared with conventional brackets (Table 1). This result can be explained by assuming that part of the force is used to overcome the greater resistance to sliding, generated in tests with conventional bracket systems during unloading 6,34 NiTi wires, along with self-ligating bracket systems, with conventional brackets 1,9 . However, in deflection of 3 mm, the selfligating brackets presented the smallest forces, contradicting previous investigation 27 (Table 1). This occurs because smaller diameter wires release smaller forces, since they are not completely pressed against the slots. In conventional brackets, even small diameter wires are pressed into the slots by the elastic tie. 31 , there were no differences in the discharge forces between Damon and In-Ovation self-ligating brackets with 0.014may be a consequence of the more uniform wire mechanical locking system than the wire tying process of conventional brackets, with elastomeric ligatures 36 . There may be more variation in the process of tying the elastomeric ligatures than when closing the self-ligating brackets.
The results were not standardized between different bracket combinations with 0.016-inch Nitinol wires, showing that the design of the experimental testing qualitatively and quantitatively affects the discharge forces during leveling and alignment with superelastic NiTi wires 28,36 (Table  2). Similarly, when these forces were evaluated in an experimental model with embedded bracket systems, the wires tended to lose their superelastic properties, showing variations of force 27   studies 4,21 also noticed higher forces when passive (Smart Clip) were compared with active self-ligating brackets (Time3). Finally, different bracket combinations with 0.020-inch Nitinol wires showed again that Damon Q generated greater forces than In-Ovation R brackets showed greater forces than the others. This result is consistent with another study in which friction was responsible for reducing the amount of released force 36 .This partially explains the fact that the highest average was generated by the simulation device with self-ligating brackets 14,17 .
Clinically, this explanation makes sense because friction increases the released force during loading, but decreases it during unloading 23 . Therefore, the device with higher friction generated less force, because friction hinders the return of the wire to its initial position during the discharge 36 . The presence of the brackets, the distance between them, the bands, and the crowding itself are factors that increase friction in the clinical setting. Thus, this large friction would be able to decrease the released force by the wire. Other studies also found higher frictional resistance of conventional brackets when compared with self-ligating ones 5,13 .
Conventional and esthetic brackets showed very different patterns of forces due to bracket composition. Thus, this result should be considered when choosing the type of brackets to be used according to the type of mechanics necessary in orthodontic treatment of each case.
An important aspect to be considered is the difficulty in extrapolating laboratorial findings regarding frictional forces to the clinical environment. Many different factors, such as occlusal forces and tightening or loosening the archwire in the brackets during treatment, may produce different force 20 .

CONCLUSIONS
Conventional and esthetic brackets showed very different patterns of forces due to bracket composition; Self-ligating brackets presented similar behavior to each other and showed higher strength in all 0.020-inch wires.