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INTRODUCTION
Achieving appropriate anterior torque after incisor retraction is an essential treatment goal for clinicians. Conventional methods for maintaining anterior torque include direct application of moment/force to the lingual brackets and lever-arm extension.1 Loss of control of anterior segment torque could more easily occur with lingual techniques than with conventional labial techniques.2 Moreover, compared with the labial approach, the lingual approach enables the use of a longer hook on the lingual side because of the width and depth of the palate.3 However, using a longer hook may cause irritation to the patient's tongue.
Lingual brackets with two main slots, such as Fujita's Duet brackets,4 were developed to overcome this drawback. The new design allows for the engagement of an auxiliary wire into six anterior brackets in the course of en masse retraction and enables the use of shorter lever arms.1 Although these approaches seem useful for torque control, few studies have reported findings in support of this idea.
Orthodontic treatment requires a long time, usually more than 2 years.5 Furthermore, when conducting clinical research in orthodontics, limiting the differences between experimental and control groups to a single factor can be quite challenging.6 For education and research purposes, many methods of simulating orthodontic movements have been developed, but each method has its own shortcomings. The finite element method has been used in numerous previous orthodontic studies. However, most of these studies only evaluated the initial tooth movement.7 In the conventional typodont, which is widely used in dental education, heat is derived from an outside source; therefore, the wax surrounding the root apexes is the last to soften. This differs from the clinical situation, wherein bone resorption occurs around the tooth and the outer bone remains intact. To overcome these limitations, the Calorific machine system has been developed, but this system has a complicated design and requires wires to connect the tooth to the machine.8 The Heat Induction Typodont System (HITS), which has been used in some recent studies,910 has a distinct advantage over other tooth movement simulation methods. Our study used an upgraded version of the HITS machine with design modifications to improve energy efficiency.
Therefore, the purpose of this study was to compare inclination and vertical changes between single-wire and double-wire techniques during en masse retraction with different lengths of lever arms in lingual orthodontics using an upgraded version of the HITS.
MATERIALS AND METHODS
Heat Induction Typodont System simulation
A maxillary typodont with the first premolars extracted was prepared for the in vitro simulation study (Figure 1A). All anterior teeth were manufactured using aluminum for ensuring heat conductivity and light weight to prevent unwanted movement due to gravity. In contrast, resin was used for the posterior teeth. A typodont wax block (CALWAX, La Mirada, CA, USA) was used to form the alveolar bone.
The prepared typodont was placed in the HITS (Dentos, Daegu, Korea) (Figure 1B). The machine provided a rapid alternating magnetic-field environment, creating eddy currents inside the aluminum teeth. Eddy currents are circulating flows of electrons within the body of the conductor. The resistance of the teeth converted this electrical energy into heat, which finally softened the wax surrounding the teeth, resulting in tooth movement due to the designed force. Meanwhile, the posterior resin teeth were not supposed to be moved in order for them to be used as the superimposition reference. Our study utilized an upgraded version of the HITS machine with design modifications to improve the energy efficiency for increasing the speed of heating the metal teeth.
Duet lingual brackets (Dentos), which have two main slots: one occlusal and one lingual, were used in this study (Figure 1C). Bracket positioning was determined using the Lingual Plain Wire Mushroom Bracket Positioner (Dentos).11 Pattern Resin (GC Corporation, Tokyo, Japan) was used to manufacture an individual tray for each bracket on a setup model.12 A silicone impression (Vertex™ Castasil 21; Vertex Dental, Zeist, The Netherlands) was obtained to guarantee the reproducibility of the procedure, thereby enabling every initial typodont to be identical.
Forty samples were equally divided into four groups according to the length of the lever arm and the retraction technique. Groups 1 and 2 used a single wire with 3-mm and 6-mm hook lengths, respectively. Groups 3 and 4 used double wires with 3-mm and 6-mm hook lengths, respectively. For the single-wire technique, a 0.016 × 0.022-inch (in) stainless steel archwire (U2 size; Dentos) was engaged in all lingual slots. For the double-wire technique, an additional segmented 0.018 × 0.018-in stainless steel archwire was engaged into the occlusal slot of the six anterior teeth brackets (Figure 2). Four-millimeter en masse retraction, which was controlled by a resin stop, was performed through the force vector from the microimplant position between the first and second molars to two different vertical levels of the anterior retraction hooks by using an 8-mm-long NiTi coil spring (Dentos), thereby applying a force of 150 g per side.13
During space closure, the temperature of the six maxillary anterior aluminum teeth was maintained at approximately 51–53℃. Approximately 15 minutes was required to reach the target temperature and another 10 minutes to complete space closure. The entire procedure was repeated 10 times for each group.
Measurement
After applying an anti-reflection coating (DMAX, Daegu, Korea), the typodont was scanned using a 3Shape R1000 scanner (3Shape A/S, Copenhagen, Denmark) and stored as standard tessellation language files. The files were imported into Blender (Blender Foundation, Amsterdam, The Netherlands), an open-source 3-dimensional computer graphics software, for analysis.
The scanned images of each typodont before (T1) and after retraction (T2) were superimposed using posterior segments, which maintained their position during space closure. At T1, the plane that passed through the central incisors' edges and the right and left lingual cusp tips of the first molar was defined as the original occlusal plane. Images on the superimposition and data for inclination and vertical position changes were analyzed (Figure 3). The former was defined as the difference between T1 and T2 inclination angle formed by the long axis of the tooth and the original occlusal plane, and the latter was the perpendicular distance of the tips of the teeth between T1 and T2, referenced to the original occlusal plane.
Statistical analysis
The Kruskal–Wallis test and Dunn's post-hoc test were used for comparisons among the four groups. The Mann–Whitney U test was used to evaluate the effect of the retraction wire and the lengths of the lever arms. All statistical analyses were conducted using IBM SPSS Statistics for Windows, ver. 20.0 (IBM Corp., Armonk, NY, USA), and the level of significance for all the tests was set at p < 0.05. For reliability tests, 10 samples were extracted randomly, and the same investigator remeasured the values after a 2-week interval. No statistically significant differences were observed in intraexaminer error, as determined by the paired t-tests (p > 0.05). Dahlberg's formula was used to test for random errors; the average error level was 0.02 mm (error range, 0.01–0.03 mm) in linear measurements, and the average error level was 0.04° (error range, 0.02–0.06°) in angular measurements.
RESULTS
No significant difference was observed between the four groups in crown inclination and vertical position at T1 (p > 0.05) as shown in Table 1.
After 4-mm retraction, in the 40 samples, the mean value of inclination changes was 18.16 ± 1.46° for the central incisors, 14.92 ± 1.59° for the lateral incisors, and 9.82 ± 0.78° for the canines. The corresponding mean vertical position changes were 3.40 ± 0.45 mm, 2.39 ± 0.39 mm, and 1.35 ± 0.14 mm, respectively.
The analyses of the effect of the retraction wire and the lengths of the lever arms are shown in Table 2. The effect of hook length on inclination was significant in all tooth types (p < 0.01). However, between the single-wire and double-wire groups, significant differences were observed in the inclination of the central and lateral incisors (p < 0.01), but no significant difference was observed in the inclination of the canines. Hook length had a significant effect on vertical position changes of the central and lateral incisors (p < 0.001). However, between the single-wire and double-wire groups, vertical position change was significantly different in only the lateral incisors (p < 0.01), and no significant difference was observed in the central incisors and canines.
Multiple comparisons among groups (Table 3) revealed that Group 1 had the highest value, while Group 4 had the least amount of decrease in crown inclination and vertical position changes. Between Groups 2 and 3, no significant difference was observed in inclination changes of the central and lateral incisors. However, Group 2 showed significantly less inclination changes in the canines.
DISCUSSION
This study compared the inclination and vertical changes in anterior teeth after 4-mm en masse retraction between the single-wire and double-wire techniques with different hook lengths.
Compared to the single-wire technique, the double-wire technique provided better anterior torque control in the central and lateral incisors and less extrusion in the lateral incisors. This result suggested that during en masse retraction, using the double-wire technique is helpful in preventing side effects in the anterior teeth. In edgewise appliances, torque is controlled by a couple force moment, which could be generated when a rectangular wire engages into a rectangular bracket.14 Interbracket spans were shorter on the lingual side than on the labial side, and this resulted in increased difficulty in developing adequate force moment for rotation and torque control.715 In the two-slot bracket system, two couple force moments may be generated in both the lingual and occlusal slots to provide better control of tooth movement. However, our result suggested that the double-wire technique could not guarantee full control of tooth movement. A more cautious approach should be adopted in clinical situations, wherein the inclination of the canines or extrusion of the central incisors are important factors.
Two different hook lengths were used in this study: 3 mm and 6 mm. According to a previous study,16 both the 3-mm and 6-mm hook lengths provide a force vector under the central resistance of the six anterior teeth. With that kind of directional force, torque loss and extrusion should be higher in the short-hook group than in the long-hook group. Among the four groups, Groups 2 and 3 had different hook lengths but showed similar effects of torque control on the incisors. Our result suggested that instead of extending the hook length, using double wires with short hook lengths may yield a similar pattern of tooth displacement. Kim et al.,17 in a finite element analysis study, found that in order to prevent torque loss during en masse retraction using the single-wire technique, the length of the hook should be at least 15–20 mm. With lingual appliances, using a long hook is possible because of the width and depth of the palate, and a case report showed good lingual treatment outcome after en masse retraction using a single wire with that hook length.3 However, lever arms longer than 15 mm are subject to elastic deformation; thus, they may induce anterior transverse bowing and may diminish the translational effects on the incisors.17 With the double-wire technique, Lim and Hong1 reported one successful case, wherein the inclination of the incisors was well maintained after space closure using a shorter hook length.
Some previous studies have used a prototype of this HITS for research purposes.910 Our study used an upgraded version of the HITS machine with design modifications to improve energy efficiency. The metal teeth could be heated faster, and hence, less time was required to reach the desired temperature. In previous studies,810 sticky wax was used to manufacture the typodont, and the required temperature for the experiment was considerably high, ranging from approximately 60℃ to 65℃. Since most orthodontic appliances are manufactured from metal, they would also be heated during the experiment and their physical characteristics may change. In our study, the melting point of the wax was approximately 51–53℃. In the HITS, the anterior metal teeth should be heated up to 51–53℃ to melt the surrounding wax for enabling tooth movement. However, the rest of the experimental apparatus, such as the wax body, brackets, wires, and posterior resin teeth, were not heated up to that temperature. During the experiment, the temperature of the wire was measured as being below 35℃. Our results suggested that conventional typodont wax is an acceptable material to simulate tooth movement in the HITS.
Finite element analysis has been recommended as a useful tool that has been used in many previous orthodontic studies. However, most of these studies were limited to the evaluation of initial tooth movement, and designing a dynamic simulation with a long distance of movement was quite challenging in these studies.7 Nevertheless, the HITS machine used in this study could simulate a 4-mm retraction.
However, owing to the tapered structure of the model teeth, the amount of metal required to cover the cervical region was higher than that required for the apical area, which increased the difficulty of facilitating lingual root torque rather than crown tipping because of the uneven distribution of heat conducted by the HITS machine to the model teeth. The current version of the HITS cannot mimic the periodontal tissue and attachment apparatus. In the future, new wax materials and further improvements in experimental design may provide better orthodontic movement simulation.
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