Background
In recent years, clear aligner therapy (CAT) has been used widely in orthodontic treatment due to its aesthetic appeal and comfort. However, clear clinical recommendations and standards for CAT based on scientific evidence have not yet been established [
1]. For example, designing the target position in CAT does not always result in the expected tooth movement [
2], with unpredictability being common [
3].
Expanding the upper arch with CAT is a viable option [
4] for many patients with a limited transverse maxillary deficiency or mild to moderate dental crowding as it provides space to solve crowding, and also improves the bite by matching the upper and lower dentition. Although CAT is widely popular, it still has limitations in the field of arch expansion. Some studies have pointed out that fixed appliances improve malocclusion more effectively than that achieved by Invisalign [
5,
6]. In addition, statistical analyses have shown that the efficiency of maxillary arch expansion correlates negatively with the preset amount of expansion movement [
7,
8]. Most importantly, maxillary arch expansion is achieved mainly through the tipping movement of posterior teeth, with the tilt angle increasing as the arch expands [
8,
9].
However, posterior buccal tilt can cause posterior palatal tip drooping, leading to occlusal disorders and vertical problems, and potentially periodontal problems, such as alveolar bone resorption. Although the efficiency of arch expansion and teeth movement patterns by CAT are not satisfactory, only a small number of studies have investigated the technique in detail. It is therefore crucial to gain a better understanding of the basic biomechanical mechanism of maxillary arch expansion caused by CAT in order to effectively control the movement of teeth during clinical treatment.
The 3D finite element method (FEM) is a computer technique that simulates the stress distribution of the PDL and alveolar bone after application of loads that simulate teeth displacement [
10]. This technique has been used extensively in the field of orthodontics to help gain a better understanding of the biomechanics involved in orthodontic movement and also to provide guidance for clinical operations.
The current study used FEM to analyze the biomechanics of maxillary arch expansion and to investigate the appropriate stride length and torque compensation angle. This information was then used to guide the clinical use of clear aligners in upper arch expansion.
Discussion
CAT is used widely used in the treatment of malocclusion. Therefore, the biomechanical analysis of teeth movement is crucial in CAT. In recent years, with the rapid development of computer technology, FEM has been used as an effective tool in orthodontic biomechanics [
10,
12,
18]. This study created a 3D finite element model of a patient with maxillary arch stenosis to investigate the appropriate stride length and torque compensation for maxillary arch expansion.
Clinical studies have demonstrated that the buccal tilt and efficiency of the posterior teeth decreases from the first premolar to the first molar when the upper arch expansion was created using CAT [
7‐
9]. These findings are consistent with those of our study, which showed CA produced buccal and distal tipping of the posterior teeth, lingual tipping of the anterior teeth, and extrusion of the incisors with maxillary arch expansion without torque compensation. However, these trends in teeth movement may easily cause an occlusal disorder and mandibular clockwise rotation, which are unfavorable to the profile of patients with a mandibular retraction. If the tilt degree reaches a certain degree, it will also cause alveolar bone absorption. In order to reduce the above adverse effects, many clinicians increase the torque compensation during maxillary arch expansion. However, there is no scientific evidence on the specific degree of increase, with this chosen based on the experience of the physician.
Nowadays, many research focus on the transverse arch dimension, which is enough to prove the importance of transverse dimension of arch in orthodontics. Some scholars report that Mixed Palatal Expansion (MPE) can improve the transverse dimension of upper and lower arch [
19]. Although there is evidence that CA is inferior to fixed orthodontics in orthodontic treatment for controlling teeth torque and increasing transverse width [
5], the advantage of CAT is that it sets specific stride lengths and torque compensation. Therefore, we set-up different upper arch expansion stride lengths and torque compensation to help orthodontists understand the biomechanical mechanism of maxillary arch expansion.
The tilt angle was used in the current study to directly reflect the buccal tilt of the posterior teeth after increasing torque compensation. As displayed in Table
4, when no torque compensation was added, the maximum tilt angle of group A did not exceed 1.5°. With an increase of the stride length, the degree of buccal tilt of the posterior teeth increased successively, with a maximum of more than 3° when the stride length reached 0.3 mm. Therefore, we consider that it is reasonable to reduce the stride length to 0.1 mm in upper arch expansion when the posterior dental axis of the patient is normal or the periodontal condition is not ideal. Although the degree of buccal tilt of the posterior teeth decreased with an increase in torque compensation, when the stride length was 0.1 mm and the torque compensation was 1.5°, the posterior teeth crown moved to the palatal side. This indicated that an increase in torque angle will also reduce the efficiency of the posterior teeth in maxillary arch expansion.
Therefore, in addition to considering the degree of tilt, the effect of the expansion of the posterior teeth was also used as a reference factor. However, Fig.
6 shows that when the stride length was 0.1 mm and the torque compensation was 1.2°, the posterior teeth tended to demonstrate bodily movement to achieve the upper arch expansion effect. In addition, as shown in Fig.
5, the efficiency of upper arch expansion in this group was too low, with the highest being less than 18%, with the second premolar and molar teeth being lower. According to our speculation when the stride length was 0.2 mm, if we blindly pursued bodily movement, the efficiency of the maxillary arch expansion was greatly reduced. When the stride length was 0.3 mm, the degree tilt of the posterior teeth was severe and an increase in torque compensation did not improve it significantly. In addition, study have reported a risk of root resorption when hydrostatic pressure of PDL exceeds typical human capillary blood pressure. Root resorption may occur if the hydrostatic pressure rises above 0.0047 MPa, but other factors, such as the position of the pressure on the root, may also affect the severity of root resorption [
20]. However, the authors emphasize that the tooth movement rate was low with a hydrostatic stress of 0.0047 MPa [
21]. Although the highest Von-Mise in this study is different from the hydrostatic pressure mentioned above, based on the study on Von-Mise of tooth root resorption [
22] and the stride length used in this study, we consider that the highest equivalent von-Mises stress values of the root, PDL, and alveolar bone were still within the normal physiological range. According to Fig.
10, the highest Von-Mise of PDL, root and alveolar bone increased significantly when the stride length reached 0.3 mm. Thus, we decided not to expand the upper arch using a 0.3 mm stride length. In conclusion, according to our comprehensive analysis of upper arch expansion efficiency and degree of tilt, we suggest that the step length should be 0.1 mm and the torque compensation 0.5°, or when maxillary arch expansion is required, the stride length should be 0.2 mm and the torque compensation 1.5°.
However, in clinical applications, different torque compensation should be set according to the specific situation of the patient. For patients who require a posterior buccal tilt, the effect of maxillary arch expansion is to correct the axial of the patient’s posterior teeth. For patients with severe buccal tipping, micro-implant assisted maxillary arch expansion [
23] can be considered to avoid periodontal problems such as alveolar bone resorption. Some scholars have reported the difference between bone-borne Haas-inspired miniscrew-assisted maxillary expander (BB HIMAME) and bone-tooth-borne miniscrew-assisted rapid palatal expander (BTB MARPE) of mini-screw assisted maxillary expanders. The high stress around the fronto-maxillary suture of BB HIMAME, but the BTB MARPE will produce tipping movement of the anchor tooth [
24]. This also reminds us that it is necessary to consider comprehensively when choosing micro-implant assisted maxillary arch expansion.
In addition to controlling the maxillary arch expansion stride length and torque compensation, we may also improve movement of teeth of the upper arch expansion in other ways. For example, some scholars have proposed that alternating movement can achieve higher movement efficiency than whole movement [
25]. The thickness of the aligner also has a certain effect on teeth movement. Increasing the thickness of the appliance has been reported to result in more ideal movement of the target teeth [
26]. In addition, academic and clinical observations have shown that although the use of attachments in upper arch expansion results in no significant difference in torque control of the posterior teeth, the use of attachments can increase aligner retention. Moreover, buccal attachment is routinely added in maxillary arch expansion by CAT. Studies have shown that simultaneous use of buccal and palatal attachments can control teeth movement patterns in molar intrusion [
27]. This is also a concept that can be adopted in the future for maxillary arch expansion.
The current study also demonstrated that the anterior teeth will lingual tilt and the incisor will extrude during upper arch expansion. Interestingly, we showed that these movement trends were not affected by torque compensation, and only increased significantly when the expansion stride length was 0.2 mm (Fig.
3B). This suggested that corresponding treatment measures should be considered for the anterior teeth in CAT maxillary arch expansion, such as reducing the stride length and setting the torque compensation of the anterior teeth in order to avoid unpredictable tooth movement. Attention should also be paid to this issue in the future when using CAT to achieve maxillary arch expansion.
In summary, with a better understanding of the biomechanics of maxillary arch expansion by CAT we expect to achieve ideal movement of teeth in the future as CA materials advance. This improvement will make CA a convenient tool for orthodontists.
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