Background
The increasing demand of esthetic and comfortable appliances has prompted the development of clear aligners [
1]. Compared to fixed appliances, clear aligners have advantages of better periodontal maintenance and lesser root resorption [
2,
3]. Nowadays, clear aligners can effectively achieve various tooth movements, such as molar distalization and arch expansion [
4,
5]. However, due to limitations in material properties, the control deficiency of clear aligners in complex tooth movements limits its application in extraction cases. A successful treatment depends on the well-controlled anchorage. However, anchorage loss is frequently seen and manifests as mesial tilt and intrusion of the posterior teeth [
6,
7].
Overtreatment has been widely employed in clear aligners to improve its efficiency [
8]. In extraction cases of the posterior region, overtreatment manifests as preset distal tipping [
9]. During overtreatment, the attachment design is critical and is typically located on the buccal surfaces of the teeth. We previously found that the combined use of buccal and palatal attachments produced better results in molar intrusion than using single buccal attachment (BA) [
10]. However, Ahmed et al. reported the opposite results in torque control [
11]. Whether or not the bucco-palatal attachment (BPA) design is effective in anchorage control or further amplifies the effect of overtreatment remains unexplored.
Attachments are not curative in clinical practice. Anchorage loss occurs because of shortage of overtreatment. Dai et al. reported mesial tipping of the molars with overtreatment [
12]. Due to the lack of relevant studies, the adequate amount of overtreatment has not been determined.
As an effective and reliable method, the finite element analysis (FEA) is widely used in biomechanical studies [
13‐
15]. To simplify the analysis, a complicated assembly is divided into a finite number of units. The overall mechanical properties are obtained by analyzing and integrating the properties of each unit. Under a virtual clinical condition, the displacement tendencies of teeth and stress distributions of periodontal ligaments (PDLs) can be calculated and visualized [
16], which helps to better understand the mechanisms of clear aligners.
The aim of the present study was to compare the biomechanical effect of overtreatment between three groups: without attachment (WOA), BA and BPA groups. We established a model of the maxillary dentition with first premolars extracted. Through the parameter settings and calculations, the relevant results were presented in the software. And the conclusions could be obtained in the subsequent data collection and analysis.
Discussion
FEA, an effective and reliable tool in biomechanics, was employed in the present study for a visual analysis. This study explored how attachment positions affected anchorage in extraction cases. Moreover, we ascertained the adequate amount of overtreatment for maxillary teeth. If the amount of retraction was 0.25 mm, the appropriate degrees of overtreatment on maxillary second premolars were 2.8° with BA and 2.4° with BPA.
Interference fit was applied to simulate the force loading [
22,
24,
25]. During treatment planning, the amount of tooth movements was pre-designed in the aligners. In other words, the surfaces of the teeth and aligners were not in close contact with each other. Once the aligner was placed, the interference between them generated a force perpendicular to the contact surfaces. Further, the teeth moved toward the target positions due to the deformation of the aligners.
In the present study, we selected the second premolars as the subject. This design was based on certain considerations. First, the second premolars were adjacent to the extraction sites, presenting the highest anchorage loss [
22,
26]. Second, the anchorage of the distal molars could be protected with the overtreatment added on the second premolars. In addition, designing BPA on each posterior tooth did not conform with clinical practice. Although increasing attachments would improve the efficacy [
27], insertion and removal of the aligner might become difficult.
In extraction cases, the methods for anchorage enhancement include two-step retraction [
28], overtreatment [
9], intermaxillary traction [
29], and mini-screw [
26]. Compared to the potential joint injury with intermaxillary traction [
30] and invasive mini-screw implantation, the noninvasive overtreatment can be directly designed into the treatment protocol and achieved by replacing aligners step-wise. In clear aligners, the anchorage overtreatment manifests as an angle between the inner surface of the aligner and the outer surface of the teeth. However, this means that the aligner is not in close contact with the teeth, which reduces retention and effectiveness. As a frequently used tool to enhance retention force [
31], the application of attachments can improve this situation. The regular shape of the attachment helps exert orthodontic forces and facilitates the realization of complicated movements, such as bodily movement [
32]. Therefore, attachments play a crucial role in designing anchorage overtreatment.
The buccal and palatal surfaces of teeth were concurrently covered by the aligners. However, the bodily movements were not achieved. This might be related to the tooth morphology. The contact area of the second premolar was close to the buccal surface, leading to the buccal aligner having a less coverage area and retention. Further, the flat buccal surface was conducive to force transmission. Therefore, if no attachment was designed, the anchorage of the palatal side was stronger than that of the buccal side. However, when BA was introduced, the control force of aligner to the buccal surface increased significantly. But this, in turn, created a new imbalance. The distinct difference between the buccal and palatal displacements showed the necessity of the palatal attachment. With the combined use of buccal and palatal attachments, the mesial displacement of the palatal cusp also decreased significantly, and the differences between the two cusps were the least among the three groups. Thus, BPA showed the best performance in bodily control of the second premolars.
With no overtreatment, the second premolars displacement in the BA group was similar to that in the BPA group. However, the BPA group showed the least displacement once the overtreatment was added. Significant differences existed between them. The BPA design enhanced the control force of the aligner to the teeth. And compared to the smooth surfaces, the sharp edges of attachments also increased the efficiency [
27]. Therefore, BPA performed the best in anchorage enhancement.
The second premolars were in contact with the first molars. When the second premolars began to tip distally, the first molars received the distal force and moved distally as a result. However, as the force gradually attenuated due to friction [
33], the magnitude of distal displacement of the first molars was less than that of the second premolars. Moreover, when the overtreatment in the disto-occlusal direction was added, a reciprocal force in the mesio-apical direction was also inevitably generated on teeth and transmitted to the aligner. In the canine region, the force could counter the distal tipping and intrusion tendencies of the canines. However, if no attachment was designed, the amount of overtreatment was not sufficient. As the second premolars remained tipping mesially, the tipping tendencies of the canines and first molars could not be reduced. This result suggested that the overtreatment designed on the second premolar with attachments might also be useful in preventing the roller coaster effect.
Although several studies have revealed that excessive force increases the risk of root resorption and alveolar defect [
22,
34‐
36], the present study is a FEA study and the inference should be prudent. Therefore, we just discussed the stress distribution. The differences in stress distribution might be attributed to the different movement patterns of the teeth. Without overtreatment, the mesial tipping of the second premolars caused the stress concentration on the cervical and apical areas [
37]. With overtreatment, the crown and root moved mesially together in the WOA group. For BA, the buccal crown moved distally while the root moved mesially. However, the PDL and alveolar bone showed reduced and even-distributed stress with BPA design, which could be attributed to the bodily movements of buccal and palatal crowns. The results also suggested that the BPA design have potential to achieve better stress distribution.
This study successfully utilized the interference fit to conduct the analysis. The obtained results indicated that the interference fit was a feasible and effective loading method. The optimal overtreatment degrees and BPA design could also provide a reference for treatment planning. However, the limitations of this study could not be ignored. In clinical practice, the four premolars extraction cases were more common, but we failed to construct the mandible. Liu et al. reported that less anchorage loss occurred in mandibular teeth [
38]. We assumed that less overtreatment is needed for the mandibular anchorage. If the Class II elastics is applied, the distal force on the upper aligner could reduce the maxillary anchorage overtreatment required, while the mesial force on the mandibular first molar might increase the mandibular anchorage overtreatment. However, the lingual elastics design might have potential to reduce the adverse effect of Class II elastics [
18]. In addition, the actual clinical settings cannot be replaced by virtual simulation. More work is needed to substantiate the conclusion. The practicality and effectiveness of this study should be examined in further clinical cases.
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