Stress distribution analysis in anterior teeth caused by several retraction mechanics

ABSTRACT Introduction: Orthodontic retraction of the anterior teeth is indicated when the patient has a malocclusion with protrusion of the incisors. Several mechanics are indicated to perform this retraction. Objective: The objective of this study was to compare the strains generated by four different types of retraction mechanics along the roots of the anterior teeth. Methods: A photoelastic model simulating an arch with first premolars extraction was made. Sixty retraction archwires were prepared, including fifteen for each type of mechanics evaluated: sliding, teardrop loop spring, T-loop spring and double key loop archwire. The strains were observed in two perspectives: occlusal and oblique. In the occlusal perspective, strains were compared among the six anterior teeth. From the oblique perspective, strains were compared among the thirds of the left canine root. Results: In the occlusal perspective, the teardrop loop spring mechanics presented greater strains, followed by T-loop spring, double key loop archwire and sliding mechanics. In all mechanics, strains were more concentrated in the canines than in the incisors. From the oblique perspective, the teardrop loop mechanics generated greater strains in the cervical regions of the canine, and in the apical regions, no differences were found in strains among the four types of mechanics. In the same mechanics, greater strains were present in the cervical zones. Conclusion: The teardrop loop spring retraction mechanic presented the greatest mean strain, and the sliding retraction mechanic presented the lowest mean strain on the root of anteroinferior teeth in the occlusal and oblique perspectives.


INTRODUCTION
Patients with excessive incisors buccal inclination frequently present abnormalities in masticatory function, such as lack of anterior guidance and unsatisfactory facial aesthetics due to lips protrusion and unaesthetic teeth exposure at rest. [1][2][3][4][5] Intending to correct these features, orthodontic treatment plans have been designed to reduce incisor protrusion. In this context, first premolar extractions are often proposed, followed by retraction of the anterior teeth. 1,3 The orthodontist is responsible for managing the particularities of the existing retraction techniques for space closure after extractions. The ideal approach is identified during treatment planning, aiming at achieving an effective dental movement. 5 Among the existing techniques to retract the anterior teeth, the following techniques can be cited: sliding mechanics, [6][7][8] teardrop loop spring, 9-12 T-loop spring 5,13-15 and double key loop archwire. [16][17][18] Photoelasticity is one of the available methods for stress analysis over teeth roots. It belongs to a group of convenient techniques for studying the effects caused by orthodontic retraction of anterior teeth, and it is based on birefringence, which is an optomechanical characteristic of transparent polymers. 19 Domingos RG, Lima Júnior A, Laganá DC, Rino Neto J, Abrão J, Paiva JB -Stress distribution analysis in anterior teeth caused by several retraction mechanics 5 Numerous studies in orthodontics have used this laboratory method to analyze the strain fields created on the structures of interest after a mechanical load. 17, [19][20][21][22][23][24][25][26] Despite the frequent evaluation of retraction mechanics by several studies, 1,2,[5][6][7][8][9][10][11][12][13][14][15][16][17][18] it is difficult to find articles that have compared the effects of the stress produced in periodontal tissues on anterior teeth with several space closing techniques. Therefore, this investigation aimed at analyzing and comparing the stress distributions in the following four anterior retraction techniques: sliding mechanics, teardrop loop spring, T-loop spring and double key loop archwire.

MATERIAL AND METHODS
The tests were performed with a photoelastic model simulating a dental arch in the initial orthodontic anterior retraction phase with first premolars extraction. Roth 0.022-in fixed orthodontic appliance (Abzil, São José do Rio Preto, Brazil) was installed on artificial teeth (Orto-Art; Piracicaba, Brazil) and a dental simulator (typodont) was created with the teeth correctly positioned along a 0.021 x 0.025-in steel archwire (Morelli, Sorocaba, Brazil) without the first premolars.
Next, an impression of the typodont was made with silicone (Redelease, São Paulo, Brazil). After silicone polymerization, the wax was removed and a flexible epoxy resin was poured In Group 1 (sliding mechanics), 0.019 x 0.025-in steel archwires were used. Long hooks were installed bilaterally on the wire in the canine region. [6][7][8] An elastic power chain was loaded bilaterally from the first molar tube to the hook, to make the retraction force (Fig 1). In Group 2, 0.019 x 0.025-in steel retraction archwires were used with bilateral teardrop loop springs positioned 2 mm distal from the canine brace. A Gable bend of 15 degrees was made 9-12 (Fig 2), and the retraction force was made by loop activation. In Group 3, a 0.021 x 0.025-in steel arch was sectioned in three parts, which were installed in the two posterior segments and in the anterior segment.
A cross tube was welded to the wire between the canine and the lateral incisor. Bilaterally, the activation was performed Domingos RG, Lima Júnior A, Laganá DC, Rino Neto J, Abrão J, Paiva JB -Stress distribution analysis in anterior teeth caused by several retraction mechanics 7 with a T-loop spring made of 0.017 x 0.025-in titanium molybdenum alloy (TMA) wire that was attached to the anterior section cross tube and loaded onto the auxiliary slot of the first molar tube (Fig 3). This loop was displaced to the anterior portion of the interbrackets space from the canine to the second premolar with a 45-degree Gable bend close to the molar tube. 5,[13][14][15] In Group 4, prefabricated 0.019 x 0.025-in steel retraction archwires with double key loops (DKL) positioned 1 mm mesial to and 1 mm distal from the canine brackets were used. Bilaterally, these loops were tied by metallic ligatures until the segment between the mesial loops reached 0.5 mm of deflection. The retraction force was applied by an elastic power chain loaded from the distal canine loop to the hook of the first molar tube [16][17][18] (Fig 4). A total of sixty retraction archwires were prepared (fifteen for each group). Figure 5 shows a diagram that illustrates the moment of force expected for the applied mechanics.      cervical-mesial (CM), apical-mesial (AM), apical (A), distal-apical (DA) and cervical-distal (CD). In the qualitative analysis, the photoelastic fringes were described according to their density and morphology -high strained zones are indicated by small space and fine fringes. 19 In the quantitative analysis, the photoelastic fringes expressed in each evaluated zone were classified with an ordinal number, as described by the American The different coloration among the lines shows the transition from one to another fringe. The first one, black colored, is the one of order zero; the one of violet color, order one; the violet/blue transition, order two; transition from red to green, order three. From the fringe of order three, the subsequent fringes are always counted in the transition from red to green. 19

STATISTICAL ANALYSIS
The photoelastic fringe data were analyzed twice by the intraclass correlation coefficient (ICC) with 95% confidence intervals, to verify the agreement/reproducibility between the first and second measurements, and the measurement repeatability was calculated. 28 The values for the first evaluation were subjected to the Kolmogorov-Smirnov test and classified as nonparametric, so the Friedman test 29 was chosen to compare the outliers of the four types of mechanics. If statistical significance was shown by this test, multiple nonparametric paired comparisons were performed to verify the types of mechanical differences in the fringes. All tests were performed with a significance level of 5%.

RESULTS
Qualitative analysis of the four groups in this study allowed the observation of the following characteristics: » Group 1 (sliding mechanics): In the occlusal perspective, few photoelastic fringes appeared on the canine distal zones, extending until the second premolar mesial area. In the incisor lingual zones, less intense fringes were observed when compared to canine distal zones. In the oblique perspective, mild color alterations were observed in the three apical zones.
Nevertheless, a more evident fringe sequence of colors was observed in the cervical zones, principally in the cervical-distal zone (Fig 6).
» Group 2 (teardrop loop spring): In the occlusal perspective, more intense colors and a greater sequence of fringes were observed bilaterally in the canine distal zones. In the incisor lingual zones, less intense fringes appeared when compared to the canine distal zones. In the oblique perspective, a more perceptible color alteration was observed in the apical zones. In the cervical zones, an evident increase in the fringe sequence was observed (Fig 7).  In the oblique perspective, mild alterations appeared in the apical zones. On the cervical zones, a more evident increase in fringes was observed in the cervical-distal zone (Fig 9).
Tables 1 to 4 describe the results obtained from the quantitative analysis. Table 1 describes the mechanics type comparisons according to the previously chosen zones for classification of the fringes in the occlusal perspective. Table 2 describes the comparisons in the oblique perspective. Table 3 shows the paired nonparametric multiple comparisons of the fringes among the mechanics studied in the occlusal perspective. Table 4 shows the comparisons in the oblique perspective.
A chart showing the comparison among the fringe sequence means of each mechanical group can be seen in Figure 10 (occlusal perspective) and Figure 11 (oblique perspective).         A body movement of the teeth during anterior retraction allows a more diffuse distribution of the tension in the periodontal ligament and other alveolar tissues, minimizing the risks for cell death and hyalinization of the extracellular matrix, preventing adverse effects, such as root resorption. 30 When intending to create a final force vector for tooth body movement, the retraction force applied to the anteroinferior teeth was complemented by an additional vertical control, to generate an opposite intrusion moment of force, which attempted to annul the extrusion moment of force caused by the resistance to the retraction in relation to the tooth resistance center, suggesting a final resultant force without intrusion or extrusion of the anterior segment (Fig 5). 4,5,[10][11][12][13][14][15] For this reason, in the sliding mechanics long hooks were added to the archwire to activate the system close to the tooth resistance center [6][7][8] . For the mechanics applying loops (teardrop Domingos RG, Lima Júnior A, Laganá DC, Rino Neto J, Abrão J, Paiva JB -Stress distribution analysis in anterior teeth caused by several retraction mechanics 22 loop spring and T-loop spring), a Gable bend was incorporated for each respective loop. [5][6][7][8][9][10][11][12][13][14][15] For the mechanics using the DKL archwire, the mesial loops were tied aiming a deflection of the anterior segment. [16][17][18] For effective retraction, a correct load of force applied to the anterior teeth is essential. In the literature, varied intensities are found for achieving body tooth movement: 200 g on each side; 5 70 g per anterior tooth, thus 210 g of activation for each side; 4 240 g on each side for anteroinferior retraction; 10,11 and 150 g to 300 g for a space closure of 0.5 to 1 mm per month. 7,8 This great variation is due to the variety of techniques proposed for such movement. In this study, 240 g of retraction force in each side was applied.
Considering the occlusal perspective, the canine distal zones presented higher strains when compared to the incisors in all evaluated mechanics. This evidence is expected because these teeth are the closest to the point of retraction force, which would lead to a higher incidence of stress on them. When performing the quantitative analysis by examining the means of the fringe sequences, the teardrop loop spring mechanics generated statistically significant higher strains, followed by the T-loop spring, DKL archwire and sliding mechanics. In this perspective, all tensions appearing along the dental root were observed at the same point, because the root was examined Again, the greatest means were observed in the cervical zones when the teardrop loop spring was used. In the apical areas, the differences were small among the groups, and there was no statistically significant difference in the apical-distal (AD) zone.
Reviewing the results of this investigation, the safest retraction technique seems to be the sliding mechanics, when compared to the other three evaluated groups.
The retraction mechanics performed with loops presents a total force obtained from the combination of the retraction force with the intrusion force generated by the Gable bend effect. 4,5,12 In the mechanics performed with sliding movement and long hooks, the retraction force is decomposed into two vectors: one related to the retraction force itself and one related to the anterior segment intrusion. 7,8 Therefore, if the same amount of retraction force is applied to the systems, in the mechanic that uses a Gable bend a greater load acts on the teeth, because an additional intrusive force is made.
The teardrop loop spring mechanics showing statistically higher strains does not confirm that it is more harmful, but Photoelasticity is an illustrative approach for assessing the strains produced after the test specimen activation. It was the chosen method for this investigation because the methodology is reproducible and easy to execute, 19,25 which allowed the