The effect of increasing the gaps between the front teeth on torque and intrusion control of the incisors for anterior retraction with clear aligners: a prospective study

Clear aligners have clear advantages in terms of visibility, comfort, aesthetics, and reducing of chair-side operation time. Nevertheless, it is important to be considered. For example, the relatively poor ability to precisely control complex teeth movement; this represents a major drawback of clear aligners from a clinical perspective [1]. In the 1980s, clear aligners were deemed inappropriate for patients who were undergoing teeth extraction [2]. Due to advancements in biomechanics and the progress in materials science, the use of clear aligners in cases requiring teeth extraction cases is no longer considered a contraindication. However, it does create a challenge for clinicians. The primary considerations in cases involving tooth extraction relate to the retraction of the anterior teeth and closing the extraction gap. Inadequate control of torque and three-dimensional movement of anterior teeth by clear aligner appliances heightens the likelihood of teeth inclination, torque loss, lingual inclination, extrusion, and overbite deepening during the teeth retraction process [3]. This, in turn, results in a complex treatment plan that prolongs the duration of treatment.

Various techniques are employed to improve dental control in clinical practice. Firstly, attachments that closely fit the appliance to the crown can enhance the retention force and increase the contact area. Therefore, the application of accessories can improve the efficiency of treatment [4]. However, the shape and position of the attachments are artificially established, and the operation mechanisms and reliability of this method are still uncertain [5]. Furthermore, employing various retraction strategies, such as the stepwise retraction of canines and incisors, may lead to improved control of teeth; however, it could also result in a substantial extension of the treatment duration. Thirdly, over-correction may be pre-set to improve our ability to achieve efficient tooth movement. Nevertheless, there is a lack of established methods for determining an appropriate level of over-correction.

Clear aligners generate orthodontic forces through elastic deformation of the appliance, differing from the traditional fixed appliances. Additionally, the transmission of force depends on the proper wrapping around the crown. Therefore, it is crucial to focus on the wrapping and fitting of the crown with the appliance. In a previous study, Tepediion et al. introduced interproximal enamel reduction to create gaps between anterior teeth, and subsequently assessed the changes in torque in the anterior teeth of subjects following the use of 12 pairs of appliances. The authors reported that there was no statistically significant difference between the actual torque and the preset torque [6]. In clinical practice, he stepwise strategy of canines distalization and incisors retraction is frequently employed to close the extraction gap, thereby increasing the wrapping area and achieving effective control of the canines. Hence, we formulated a hypothesis that the expansion of the wrapping area of the appliance in the mesial-distal direction of the incisors could be enhanced by sequentially distalizing the anterior gaps. This enhancement may improve our ability to control teeth and prevent the “pendulum effect” or “roller coaster effect” during retraction. Therefore, a prospective clinical trial was conducted on extraction cases to examine the impact of retraction with or without increasing anterior teeth gaps on the control of anterior teeth torque and three-dimensional teeth movement in patients utilizing clear aligners.

One-way analysis of variance (ANOVA) was utilized to evaluate the variations in torque changes for the maxillary central and lateral incisors among different groups, as outlined in Table 1. The torque changes for the middle and lateral incisors decreased significantly during the retraction process in the group with additional gaps (P < 0.05). Subsequent pairwise comparisons indicated significant differences between the group with a 0.5 mm gap, the group with a 1.0 mm gap, and the group without any gaps. However, there was no statistically significant difference observed between the group with a 0.5 mm gap, and the group with a 1.0 mm gap (P > 0.05) (Table 2).

One-way ANOVA was employed to assess the differences in the three-dimensional displacement of the central and lateral incisors between groups (Table 3). During the process of retraction, changes in the labial-lingual and vertical directions for the middle and lateral incisors decreased significantly in the groups where gaps were added (P < 0.05). There was no statistical significance in terms of displacements between the central and lateral incisors in the mesial and distal directions (P > 0.05). The statistically significant data were subsequently analyzed using the SNK-q test (Tables 4 and 5). The labial-lingual and vertical displacements of the central and lateral incisors differed significantly between the groups with additional gaps and the groups without gaps. There was no statistically significant difference observed between the group with a 0.5 mm gaps and the group with a 1.0 mm gaps (P > 0.05).

Finally, a one-way analysis of variance (ANOVA) was employed to compare the alterations in displacement in the three-dimensional direction of the anchorage molars among the three groups (Table 6). There were no statistically significant differences observed in the displacement changes of the second premolars, first molars, and second molars in the bucco-lingual or vertical directions when comparing across the three groups (P > 0.05). Significant differences were observed in the mesial and distal displacements of the second premolar, the first molar, and the second molar when comparing the three groups (P < 0.05). There were significant differences in the mesial and distal displacements of the second premolar among the three groups. The mesial and distal displacements of the first molar and the second molar exhibited significantly greater values in the groups with additional gaps. Nevertheless, no significant disparity was observed between the group with a 0.5 mm gap and the group with a 1.0 mm gap (Table 7).

Prior to obtaining the model, the patients were instructed to thoroughly clean their oral cavity. Then, we verified that the morphology of the palatal rugae in each participant corresponded to that of the plaster model. Any discrepancies or imperfections were identified and rectified to guarantee the authenticity and precision of the research model, with particular attention to the palatal rugae and tooth surfaces [8]. If necessary, we obtained the research model again. Recent evidence indicates that alterations in the anatomical positioning of the rugae palatine remain relatively stable throughout craniofacial growth, development, and orthodontic treatment, and they are commonly considered as reference points for the alignment of three-dimensional images [9, 10]. In cases involving extraction, it is important to note that the position of the maxillary first rugae palatine is unstable and therefore should not be relied upon as a reliable landmark. Therefore, the area encompassing the second and third palatine folds was chosen as the overlapping region in this study. The accuracy of the 3D invisible appliance may be affected by the precision of the prototyping technology, leading to variations in the preset gap among different 3D printing technologies/systems [11]. Antonino Lo Giudice et al. [12] stated that consumer-grade LCD-based 3D printers at the entry level exhibit lower accuracy compared to professional-grade 3D printers, yet their precision remains comparable to the clinically accepted threshold values in orthodontics.

The main concern and obstacle in the use of clear aligners for treatment are mainly related to the control of tooth movement, particularly in cases involving tooth extraction. The overall stiffness of clear aligners is comparable to that of the nickel-titanium round wire used in fixed appliances, enabling effective tooth alignment. However, achieving a tooth movement effect similar to that of a stainless-steel wire in fixed appliances is challenging when closing the tooth extraction space, primarily due to the long-distance deformation of the appliance and the limited control of the teeth by the edge of the appliance’s edge. The lack of precise control over the three-dimensional movement of the teeth leads to greater movement of the crown compared to the root of the anterior teeth. This results in oblique tooth movement, loss of torque, and increased susceptibility to lingual inclination, extrusion, and deepening of the overbite of the anterior teeth [13].

At present, it is a prevailing belief that clear aligners have limited capacity to control the root movement during orthodontic treatment [14]. Prior research has identified a notable disparity between the predetermined torque and the actual torque expression value in clear aligner treatment [15]. In a prior investigation, Hahn et al. found that the use of clear aligners for correction result in labial-palatine-oriented force as well as side forces along the long axis of the tooth. These side forces can exert a depressing force on the root of the tooth, thereby hindering the establishment of effective coupling achieve a predetermined root movement control [16]. In order to enhance the efficiency of torque expression, a “torque ridge” was incorporated on the cervical incisor to facilitate effective coupling. Furthermore, the torque of the anterior teeth was regulated to facilitate comprehensive tooth movement. Simon et al. found that in cases of tooth extraction with a preset torque exceeding 10°, there was an approximate 50% reduction in torque during the retraction phase. This phenomenon occurred regardless of whether the attachment or the torque ridge was added to the orthodontic device [17]. Prior research has also demonstrated that the application of integral retraction to a maxillary incisor by 0.15 mm resulted in a tendency for distolingual inclination of the tooth. The addition of 5° of lingual torque resulted in an increase in the equivalent stress on the periodontal membrane. Additionally, the trend for the inclination movement of the maxillary incisors approached whole tooth movement, although it was not entirely aligned with it [18]. Therefore, t additional clinical studies are required to validate the efficacy of this approach in enhancing torque on the anterior tooth. In a separate case report on clear aligners and reduction therapy, Meng et al. [19] reported that when retracting the anterior teeth, they designed a 0.5–1.0 mm loose gap between the anterior teeth. This design increased the coverage area of the aligners on the anterior teeth, improved control of the anterior tooth axis, and resulted in a favorable corrective outcome. However, no systematic experimental study has further investigated this effect. In a separate investigation, Tepediion et al. [6] recruited 39 patients with dental crowding < 6 mm and implemented interproximal enamel reduction (IPR) to create space. After wearing 12 sets of clear aligners, the researchers assessed the alterations in the torque of the patients’ anterior teeth. The authors found that there was no statistically significant difference between the torque variation specified in the correction plan and the actual value.

Therefore, increasing the gap between the adjacent teeth using IPR in clear aligners could enhance the torque expression and minimize torque loss. Through the application of three-dimensional limited analysis, Hu et al. [20] found that the inclination movement of the anterior teeth gradually decreased as the anterior tooth space increased during the retraction of anterior teeth. This phenomenon could be explained by the expansion of the anterior interdental space through sequential distant displacement before initiating retraction, as well as the enlargement of the mesial and distal coverage of the incisor tooth by the orthodontic device. These adjustments contribute to improved control over three-dimensional movement of the tooth. Therefore, the alterations in torque for the central and lateral incisors were marginally reduced in the cohort with expanded anterior space. This suggests that augmenting the anterior space facilitated the regulation of anterior torque, leading to a greater inclination towards overall movement in the anterior teeth. The results were in line with the three-dimensional finite element analysis that had been previously documented by Hu et al [20] Therefore, in clinical practice, enhancing our control of torque in relation to the anterior teeth can be achieved by expanding the anterior tooth space while simultaneously closing the extraction space. This approach has the potential to minimize the loss of torque during the retraction process.

Accurately controlling the three-dimensional movement of the anterior teeth is essential for successfully correcting extraction cases using clear aligners. The clear aligner appliance applies a therapeutic force by encasing the tooth crown and the modifying the appliance. The natural chewing force induces the “jaw cushion effect”, causing posterior tooth intrusion [21]. Furthermore, the reaction force from the posterior tooth intrusion leads to extrusion of the anterior teeth, resulting in increasing overbite and premature contact of the anterior teeth during the treatment process. However, modifying the increasing overlap during the later stage can be a time-consuming and laborious process. The solution to addressing a deepening overbite involves the intrusion of anterior teeth. The decrease in tooth extraction results in the formation of a cavitation structure within the space where the tooth was extracted; which disrupts the transmission of orthodontic force and diminishes the transmission of force for the anterior tooth intrusion [22]. Patients with bimaxillary protrusion may experience a more pronounced “bow effect” when using clear aligner devices, as noted in a study by [23]. This effect can lead to a “roller coaster effect,” which in turn complicates the management of vertical tooth positioning, as discussed in a study by [24]. Gu et al. found that there was a specific correlation between the retraction design and the efficiency of the anterior teeth intrusion. When the anterior teeth were not intended for retraction, the efficiency of the anterior tooth intrusion movement was 46.9%. however, when the retraction was incorporated into the design, the anterior tooth intrusion movement was not achieved; instead, tooth extrusion was observed. In another study, Song et al. [25] observed the absence of intrusion but the presence of extrusion when a 0.1 mm vertical intrusion displacement was designed for each step during anterior teeth retraction in cases involving tooth extraction (Table 2). Consequently, in the context of employing a clear aligner for extraction cases, it is crucial to augment the intrusion of the anterior teeth while undergoing the retraction process to mitigate the exacerbation of the overlap. However, the comprehensive treatment outcome frequently proves to be unsatisfactory. Ting et al. [26] conducted a three-dimensional finite element analysis and observed that a 0.2 mm retraction of an anterior tooth with a 0.15 mm intrusion per step resulted in a tendency for lingual movement of the tooth root. This movement contributed to the prevention of lingual inclination of the anterior tooth. However, the actual clinical impact of this phenomenon has not been investigated.

There was no planned anterior tooth intrusion for the three groups during retraction. Subsequent to conducting overlapping analysis, it was observed that the vertical alterations in the three groups of anterior teeth were elongated, which align with the results documented by Song et al. [25] in prior studies. However, it was observed that the vertical extrusion of the middle and lateral incisors in the groups with a 0.5 mm/1.0 mm gap compared to the group without gap. The observed variation in extrusion was determined to be statistically significant (Table 3). These findings showed that utilizing a clear aligner to widen the space between the anterior teeth had more favorable impact on vertical alignment. Tooth intrusions must occupy the space with the dental arch. In clinical practice, it is common to retract the anterior teeth without creating additional space between them. However, this approach does not accommodate tooth intrusions, resulting in a suboptimal aesthetic outcome for tooth intrusions. By expanding the anterior tooth space, the orthodontic device’s tooth wrapping area was increased, leading to enhanced control of tooth torque. This expansion also facilitated downward tooth movement and minimized tooth extrusion during the retraction stage. By implementing this strategy, dentists may enhance the efficiency tooth intrusion, minimize torque loss in the anterior teeth during the adduction process, and mitigate the excessive overlap resulting from inefficient expression of intrusion movement.

The analysis revealed that the anterior retraction expression effect was significantly lower in the groups with added gaps compared to the group without gap (Table 3). This phenomenon may be attributed to the expansion of the anterior interdental space, which results in a larger area covered by the orthodontic device. This expansion enhances the practitioner’s capacity to control the movement of the teeth, thereby aligning the mode of anterior teeth movement more closely with the overall movement pattern. Therefore, the displacement of the anterior teeth in the sagittal direction was smaller in the group without gap compared to the amount achieved in the other group. These findings indicate that adjusting the retraction amount for each step may be beneficial in reducing the treatment duration. Yuan et al. [27] performed a three-dimensional finite element analysis to investigate the effect of increasing anterior dental space in clear aligner. They observed that an increase in the anterior dental space led to a more pronounced tendency for the anterior teeth to move as a body rather than inclined to move, resulting in decreased periodontal membrane stress. This finding aligns with the results of our own experimental study.

Due to a lack of tooth control, we often insufficient torque control of the anterior teeth during the retraction stage with clear aligners. Furthermore, the anterior tooth intrusion efficiency is often low; this results in a “roller coaster” effect and a poor fit between the appliance and the teeth. This leads to an off-track effect which requires redesign or re-manufacture of the appliance and elongates the period of clinical treatment. In this study, despite the sequential remote displacement leading to increase the anterior dental space would increase the number of correction steps, this method had better control over anterior tooth torque and vertical direction. This made the anterior teeth less prone to torque loss and contributed to an increased deepening of the overlap during the retraction process; this also prolonged the period of treatment.

In the present study, it was observed that mesial, buccal inclination, and extrusion movement were evident in all groups, which aligns with the findings reported by Gu et al. [28]. Inter-group comparisons were conducted to analyze the changes in the three-dimensional direction of the three groups of anchorage molars. The results indicated that the inclined buccal movement and vertical extrusion of the anchorage molars in the groups with added gap were greater than those in group without gap. However, these differences were not found to be statistically significant (Tables 6, 7). The mesial movement of the anchorage molars in the groups with added gap was significantly higher compared to the group without gap (Tables 6, 7). However, there was no statistically significant difference in the mesial movement when comparing the groups with a 0.5 mm and 1.0 mm gap added.

Based on our analysis, it was found that augmenting the anterior space resulted in a more effective control of torque on the anterior teeth. Furthermore, the anterior teeth exhibited a mode of movement that was more similar to the overall movement. The principle of orthodontic biomechanics indicates that as the anterior retraction movement aims to model the entire process, there is a greater need for anchorage. Therefore, the loss of molar anchorage was higher in the groups where a gap was added compared to those in the group without gap. Notably, no methods were employed to enhance anchorage during the retraction stage. Consequently, it is necessary to incorporate additional anchorage-enhancing designs in the initial phases of treatment. Thus, increasing the anterior tooth space would not result in additional consumption of throughout the treatment process, and it would also prevent any extension of the treatment period.

As a prospective study, our investigation was grounded in the phased digital overlap model, and we established three experimental groups using clear aligners for extraction cases. By increasing the anterior tooth space during the retraction stage and closing the extraction space by 2 mm, alterations in torque, the three-dimensional displacement of the anterior teeth, and the three-dimensional displacement of the anchorage molar teeth were observed. Based on our analysis, the following conclusions can be drawn: (1) increasing the adduction mode of the anterior tooth space facilitated the regulation of torque in the anterior teeth and resulted in a movement mode more inclined to overall movement; (2) increasing the adduction mode of the anterior tooth space may decrease the extrusion of the anterior teeth and help to control the vertical direction of the anterior teeth; and (3) increasing the adduction mode of the anterior space will lead to an increased loss of molar anchorage. Hence, it is essential to focus on anchorage control during the clinical design period. However, with regards to the entire correction process, this method does not result in increased loss of anchorage.