Introduction
Over recent decades, an increasing number of patients have accepted clear aligner treatment (CAT) for both cosmetic and comfort reasons. With the advancement of biomechanics and material science, significant progress has been made in CAT; this has led to an improvement in therapeutic efficacy. A recent study showed that the overall mean accuracy of the Invisalign aligner was 50% [
1,
2].
Upper-molar distalization is a typical Class II non-extraction treatment strategy that is used to acquire dental space, establish Class I molar relationships and obtain a normal overjet. During distalization, conventional orthodontic methods persistently generate undesirable outcomes such as molar extrusion and tipping [
3]. Over recent years, CAT has been used to improve vertical dimension control, rendering this a superior alternative for the treatment of patients with hyperdivergent or open bites [
4]. Upper molar distalization is associated with the highest predictability of 88% when a bodily movement of at least 1.5 mm was prescribed [
1]. However, due to the fact that molar distalization produces a reciprocal force on the anterior teeth, anchorage loss is inevitable [
5], which can represent a substantial cause of alveolar defects, such as dehiscence and fenestration in some patients with thin cortical plates in the anterior region [
6].
Class II elastics are commonly employed in conventional fixed multibracket therapy (FMB) for anchorage reinforcement and molar correction during maxillary molar distalization. Despite their popularity, recent investigations have revealed that the adverse effects of Class II elastics outweigh the intended objectives, such as clockwise rotation of the occlusal plane and the mandible, and worsening smile esthetics due to greater exposure of the gums [
7]. In clinical conditions, clear aligners, as an overlay device, may avoid the undesirable vertical movement of Class II elastics caused by coverage of the entire dentition with the biting force. Thus, the mutual reinforcement of clear aligners and Class II elastics can facilitate the anchorage reinforcement in conjunction with vertical control [
8]. A multicenter retrospective study conducted by Ravera et al. [
9] demonstrated that the combination of clear aligners with composite attachments and Class II elastics caused distalization of the maxillary first molars by 2.25 mm without remarkable tipping and vertical movements of the crown, and there was also good buccolingual control of the upper incisors with respect to the palatal plane. However, this evaluation was performed at the end of treatment rather than focusing on the immediate effect at the end of the distalization phase. Class II elastics exploited in the previous study were only utilized after the distalization of the molars. In such a complicated force system, the biomechanical analysis of the maxillary and mandibular dentition has not yet been fully examined. Additionally, there are no clinical guidelines for the selection of traction methods for Class II elastics, including the bonding of buttons directly onto the tooth surface and the precise cutting of clear aligners.
The finite element method (FEM) is a numerical engineering technique used to instantly calculate initial tooth movement after force loading. This method is widely used in biomechanical investigations to evaluate displacement and stress responses in a variety of applications. Over recent years, the FEM has proven to be an effective tool to simulate tooth displacement patterns in orthodontics. However, the previous FE-based models only involved a single dentition within their study purposes [
10,
11].
As far as the upper-molar distalization is performed with clear aligners in combination with Class II elastics, the maxillary dentition is inherently associated with the mandibular dentition by elastics. Therefore, it is necessary to establish rational FE-based models involving the whole dentition for comprehensive analysis, and the process of sequential molar distalization in clinical practice should be taken into consideration [
12]. This is the first attempt to biomechanically evaluate the effect of clear aligners in combination with Class II elastics on both the maxillary and mandibular dentitions during upper-molar sequential distalization by FEM and further provide reference guidelines for the selection of traction methods.
Discussion
CAT is known to exert distinct advantages for molar distalization. In a previous study, anterior flaring of teeth was reported in almost every patient to differing extents in patients who did not use auxiliaries other than composite attachments during the molar distalization process [
25]. Class II elastics are extensively employed in FMB for anchorage reinforcement in the treatment of Class II malocclusion. However, the treatment with clear aligners and intermaxillary elastics is unexplored in many aspects. The previous clinical literature analyzing this treatment method in molar distalization is prevalently made of several retrospective studies and case reports [
8,
26]. Several FE-based models about molar distalization with clear aligners only involved a single teeth or a single dentition, so that little attention was paid to the mandibular dentition. For instance, Rossini G, et al. [
11] assessed the force system resulting in the upper arch during second maxillary molar distalization with clear aligners and variable attachment settings. Their FE analysis only involved a maxillary dentition without auxiliaries for anchorage reinforcement, and the sequential molar distalization process was not considered. Ayidaga C, et al. [
13] analyzed the effect of different attachment configurations on the efficacy of bodily movement of the upper maxillary molar. Their simulation was limited to a single tooth, and the stress and movement analysis were only in the sagittal plane. Thus, further evidence is required for the specific effects of clear aligners in combination with Class II elastics on the entire dentition.
To the best of our knowledge, this is the first biomechanical study to establish comprehensive FE-based models involving both maxillary and mandibular dentition to simulate the sequential distalization of the upper molars using clear aligners in combination with Class II elastics. Uncontrolled tipping movement was observed for the entire dentition. Biomechanically, bodily movement and torque are the most demanding movements to achieve since plain aligners cannot establish the force required without modifications [
27]. In such cases, the point of force application is passed above the resistance center of the teeth to produce a moment of rotation. Traditionally, the center of rotation was estimated by moment to force (M/F) ratios to predict the pattern of root movement, translation, or tipping around the apex [
28]. In the present study, the rotation centers of the incisors were demonstrated visually by generating color maps that were roughly situated at the intersection of the apical and middle thirds of the roots for maxillary anterior teeth and at the apical third of the roots for mandibular anterior teeth. Consequently, the stresses on the PDL and alveolar bone were mostly concentrated on the cervical region of the labial surface. It is suggested that if the PDL hydrostatic pressure exceeds the capillary pressure in the area, the vessels will collapse and blood flow to that area will be impaired, increasing the risk of root resorption. As reported by previous investigations [
23,
24], the threshold for capillary pressure in the PDL is estimated to be − 0.0047 MPa, which represents a substantial growth of the risk of external root resorption. Although the incidence is lower with CAT than with fixed appliances, root resorption cannot be avoided, particularly for maxillary and mandibular incisors [
29]. In our experimental simulations, the highest compressive stresses of maxillary anterior PDL in all models were all concentrated on the labial cervical region and apex for upper incisors as well as mesio-buccal cervical and apex area of canines with values above − 0.0047 MPa, indicating a high risk of root resorption. With the class II elastics, models B and C have a relatively lower root resorption risk with more evenly distributed and relatively lower PDL hydrostatic stress value compared to model A. However, the root resorption risk could not be eliminated with the anchorage reinforcement of Class II elastics. As for the mandibular anterior, the highest compressive stress of the PDL was below the threshold level in all models, indicating a low risk of root resorption whether the Class II elastics were used or not. Additionally, the corresponding higher stress on the labial alveolar crest of the anterior area can lead to higher risks of bone defects, such as bone fenestration and dehiscence, which are frequently encountered in clinical practice, particularly with regard to the incisors [
30]. By utilizing Class II elastics for anchorage reinforcement, proclination of anterior teeth has been controlled effectively with lower stress in models B and C, which may minimize the risks of root resorption and bone defects. Additionally, Class II elastics attached directly to the aligner by precision cutting result in superior anterior anchorage control when compared with buttons. Class II elastics applied to the tooth surface by buttons would cause more extrusion tendency with rotation of the specific canine compared with that applied to the aligner body. Under the conditions of Class II elastics, the mandibular anterior teeth also experienced undesirable labial movement, this was consistent with a recent retrospective study [
26]. Nevertheless, the highest stress of the PDL in the mandibular anterior area was within the safe range with little discrepancy between models B and C. Comparing two group sets in our study, anterior anchorage loss grew by the initial distalization of the 1
st molar. It could be assumed that with the reduction in the distance between the anterior anchorage and the distalized molar, the force required magnifies, thus resulting in increased loss of anchorage. Therefore, enhanced protection of the anterior anchorage may be expected during the process of sequential distalization. In our observation, the proclination angles was measured and magnified according to clinical situation, in which the whole process of molar distalization was about 50 steps. Therefore, an optimized torque design can be proposed to facilitate the management of anchorage control [
31].
With regards to the molars, the 2
nd molar exhibited a tendency for mesial and palatal inclination during the initial distalization of the 1
st molar. This relapse phenomenon could be attributed to the mesial reciprocal force produced by the distal displacement of the 1
st molar and the stress produced by the distal end of the aligner. This might explain why the maxillary 1
st molar exhibited relatively higher efficiency than the maxillary 2
nd molar during molar distalization [
25]. In the present study, we found that Class II elastics reduced the relapse tendency of the 2
nd molar. Furthermore, attachment to the aligner by precision cutting demonstrated superior anchorage control which loaded and transmitted the anchorage force directly by the aligner. The attachment of Class II elastics to the teeth by buttons, however, loaded force onto the canines, so that the anchorage force was communicated primarily by the squeezing force between the neighboring teeth which was weakened by the gap between the teeth during transmission. Regardless of the traction method selected, some degree of anchorage loss was unavoidable, necessitating the use of an optimized attachment design to facilitate anchorage control.
Based on past research works and our current observations, the combined use of Class II elastics during maxillary molar distalization with aligners might effectively reinforce anchorage. From our biomechanical analysis, precision cutting might be a superior alternative when better anchorage control for both anterior teeth and molars is required, as well as the extrusion of upper canines are unwanted. For instance, Class II malocclusion division 1 with a deep overjet, which is often associated with labial inclined incisors and thin cortical bone in the maxillary anterior region, proclination of upper incisors is undesired when molars are designed to be distalized for a considerable distance. For another, hyperdivergent patients are not appropriate for extrusion of upper canines, which could cause the occlusal plane to incline and the Spee curve to deepen. However, in some cases, such as Class II division 2 malocclusion with a deep bite and retroclined incisors, the proclination and extrusion of anterior teeth are considered as a desired movement. Class II elastics attached by buttons would be more suitable. No matter what traction method is selected, mandibular anterior teeth will encounter proclination and intrusion to some extent, which is an underlying risk for patients with thin cortical plats in the mandibular anterior region. In the vertical direction, the extrusion tendency of the molars when moving distally, and that of the lower molars may cause the occlusal plane to incline and the Spee curve to deepen. However, the model was built with free space regardless of biting force on the aligners as an overlay appliance, which may prevent extrusion of the posterior teeth. For both traditional fixed multibracket therapy and clear aligner, the displacement patterns of the teeth are functions of the relationship between the center of resistance and the line of force. However, along a continuous archwire, a single distalizing force at the archwire level induced lingual inclination of the anterior segment due to the rigid connection between the segments [
32]. In contrast, while clear aligners transmit force through appliance deformation, anterior teeth shifted in the opposite direction due to the reciprocal force created by molar distalization.
Nevertheless, as a biomechanical research tool, the FEM has intrinsic limits. In this study, the material properties of the subjects were simplifed based on the hypotheses derived from the average properties. The thickness of the periodontal ligament was assumed to be uniform, whereas in reality, it has an hourglass shape with the narrowest zone at the mid-root level [
33]. As reported by Hohmann et al. [
34], it is challenging to reconstruct PDL accurately from 3D image data obtained in vivo with its small dimensions. But they also proposed that for low loads as applied during typical orthodontic treatment, no noticeable discrepancies were found between the results generated with and without taking into account the geometric nonlinearities. Although the staging, loading methods, and traction methods were all designed in accordance with clinical practice and current studies, the efficacy of clear aligners in clinical practice is lower than predicted, owing to the materials of aligners and the manufacturing process. Clear aligners are made of thermoplastic resin polymers and they are susceptible to change in the oral environment due to heat, humidity, constant forces, and saliva [
35,
36]. The aligner material properties and deformation require more investigation in future studies to better simulate the clinical condition. Unfortunately, most of the aligner mechanical properties are patented by companies which cannot be disclosed to improve the FEM analysis [
11]. Moreover, we will explore the stress distribution at TMJ area in future researches.
Furthermore, this is a static analysis which only revealed an initial movement tendency independent of the clinical dynamic process, as well as an alternate force for tooth movement. Multiple biological processes are involved in root resorption, including the root’s long-term contact with cortical bone [
37]. Many factors, including patient cooperation, periodontal health, and root length can impact practical tooth movement. Therefore, the clinical translation of the conclusions should be taken with caution. In the future, more animal and clinical experiments should be carried out to acquire evidence at a higher level.
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