Clinical outcomes of arch expansion with Invisalign: a systematic review

Objective This study aims to assess the scientific evidence regarding the clinical outcomes of Invisalign therapy in controlling orthodontic tooth movement. Materials and methods An electronic search was conducted on PubMed, Cochrane Library, Web of Science, Embase, and Scopus from November 2015 to November 2022 to identify relevant articles. Methodological shortcomings were highlighted, and an evaluation of the quality of the included studies was completed using the Risk of Bias in Non-randomized Studies of Interventions (ROBINS-I) tool. Results Fifteen non-randomized controlled trials were included in the analysis. Most non-randomized controlled trials (n=11; 73%) were rated with a moderate risk of bias according to the ROBINS-I tool. There were statistically significant differences between the pretreatment and posttreatment arches. The average expansion was significantly different from that predicted for each type of tooth in both the maxilla and mandible. Furthermore, the efficiency decreased from the anterior area to the posterior area in the upper arch. Conclusion Despite the fact that arch expansion with Invisalign® is not entirely predictable, clear aligner treatment is a viable option for addressing dentition crowding. The efficacy of expansion is greatest in the premolar area. More research focusing on treatment outcomes with different materials of aligners should be conducted in the future. Overcorrection should be considered when planning arch expansion with Invisalign. In the maxilla, the expansion rate decreases from the anterior to the posterior, and presetting sufficient buccal root torque of posterior teeth may result in improved efficiency of expansion. Supplementary Information The online version contains supplementary material available at 10.1186/s12903-023-03302-6.


Introduction
In recent decades, orthodontic technology has made continual progress.Since Clear Aligner Therapy (CAT) was introduced in 1997, it has become an important option in orthodontic treatment [1].Compared with traditional fixed orthodontic appliances, CAT has certain advantages, including fewer clinical emergencies, better aesthetic effect, more comfort, improved periodontal health and reduced irritation to soft tissues [2].Now, CAT is an increasingly common orthodontic treatment option [3,4].
Expanding the dental arch is one way to solve transverse problems and depending on the degree of maxillary compression, clinicians could choose different expansion methods such as dentoalveolar expansion or jaw expansion [5,6].Aligners, one common orthodontic therapy, can effectively expand the dental arch, reduce dental crowding and help achieve orthodontic treatment goals [7].Aligners have acceptable efficiency in arch expansion both in permanent and mixed dentitions and have been recommended for selected cases with mild to moderate malocclusion [8][9][10][11].However, aligners do not display the same accuracy as traditional fixed orthodontic appliances when it comes to transverse arch expansion [12,13].Moreover, although some studies have focused on improving the arch expansion effects of aligners [5,9], traditional orthodontic compliance still falls short on beauty, oral hygiene and mucosal health [14,15].In this regard and to further optimize arch expansion effects and overcome the shortcomings associated with traditional orthodontic compliance, Invisalign was designed for use in clinical practice.
Recently, in order to increase the indications and efficiency of Invisalign, diversification and evolution of its primary characteristics (including material, gingival margin design, attachments, divots and auxiliaries) have been done and combined with the application of digital technology [16][17][18].For instance, ClinCheck software, based on the crown of teeth, is used to analyze the efficiency and accuracy of tooth movement and can simulate dentition models before or after treatment and facilitate measurement [19].With the benefit of gingival margin design and the absence of toxicity, the health of periodontal tissue could be well protected while using aligners [2,15] , allowing the effects of treatment to last for a long time.
The most accurate type of tooth movement produced by aligners is the buccolingual tipping movement.This movement is achieved because the materials of the appliance are mainly bent along the buccolingual direction, which fully aligns with the logical mechanics of tooth movement [20].Therefore, the use of software such as ClinCheck allows a precise design of Invisalign and makes the arch expansion effective [20].
There has been a lack of systematic analysis regarding the arch expansion effects of Invisalign for in nearly five years.Despite the presence of a body of literature pertaining to clear treatment, its clinical performance has not been analyzed thoroughly and a synthesis of the results remains vague.Five systematic reviews of the clinical outcomes of clear aligners exist in the literature.These reviews are not only focused on the oblique movement of the teeth but also on other types of tooth movement during treatment [1,[21][22][23][24].
However, there are only a few studies on the efficiency of arch expansion with aligners.Therefore, the purpose of the present review is to systematically search the literature and summarize the currently available scientific evidence regarding the effectiveness of arch expansion using the Invisalign system.

Materials and methods
We conducted this systematic review following the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines.The PRISMA checklist can be found in Additional File 1.Moreover, the protocol for this systematic review was registered in PROSPERO 2023 (Registration number: CRD42023420285).

Eligibility criteria
The inclusion and exclusion criteria for this review are as follows:

Types of studies
Both prospective and retrospective studies were considered eligible for inclusion in this review.These studies were concerned with the outcomes of arch expansion with Invisalign.Only studies published in English were included in the review.

Participants
Only orthodontic adult patients with permanent dentition and who have expanded the dental arch after Invisalign therapy were included in this review.

Interventions
Studies using Invisalign therapy to expand the dental arch were included in this review.All other aligner systems were excluded.

Comparison group
The control method used in most relevant studies was self-control, which compares the patients' conditions before and after treatment .

Outcome
The review encompassed the evaluation of any effect on clinical efficiency, predictability of ClinCheck, treatment outcomes and movement accuracy after arch expansion.Studies that evaluated arch width on actual and virtual models were included in the review.

Exclusion criteria
The following criteria were used to exclude studies: studies older than 15 years, patients with mixed dentition, studies written in a language other than English, animal studies, case reports and studies that did not provide data and reviews of literature.

Information sources, search strategy, study selection and data collection
An electronic search was conducted on PubMed, Cochrane Library, Web of Science, Embase and Scopus.The search was performed until November 30, 2022.An additional manual search of references in the included studies was also conducted.We used the following search term combinations: ((((aligner[Title/Abstract]) OR (invisalign system[Title/Abstract])) OR (invisalign[Title/ Abstract])) OR (orthodontic appliances, removable[Title/ Abstract])) AND ((expansion[Title/Abstract]) OR (arch development[Title/Abstract])).

Selection of studies
An initial screening of titles and abstracts was conducted independently by two researchers, who then crosschecked and reviewed the text in full to decide whether the studies were eligible.Disagreements were resolved through discussion and, when necessary, by seeking the opinion of a third researcher.

Data collection process
Data collection was conducted independently by two researchers followed by a discussion in order to determine the eligibility of the studies to be addressed.

Risk of bias (RoB) assessment and effect measurement
RoB assessment was done by two independent researchers using the ROBINS-I tool.The checklist included the following three main domains of bias: preintervention, intervention and postintervention.The RoB was judged for each domain and an overall evaluation was made, categorizing RoB as low, moderate, serious, critical or having no information.The main methods used to measure effects was the mean difference.

Study selection
An independent search was performed by two of the authors (Ma and Wang).The study selection procedure comprised of title-reading, abstract-reading and fulltext-reading stages.After excluding studies that were not eligible, a full report of publications considered eligible for inclusion by either author was obtained and assessed independently.Finally, 15 articles were included in the analysis (Fig. 1).

Characteristics of included studies
Data collection forms were used to record the desired information.The following data were collected: title, year of publication, names of authors, study design, number of participants, type of intervention, comparative groups, clinical outcomes and conclusions (Table 1).

Quality assessment
To determine the methodological quality and level of evidence, the classification system described by the Swedish Council on Technology Assessment in Health Care was used [38].Table 2 provides the criteria used to judge each study.
Therefore, conclusions obtained from this review were based on a limited level of evidence.The most recurrent sources of bias were related to the study type and the lack of blinded outcome assessment.However, only two retrospective studies [30,34] and one prospective study [36] were rated as having evidence of low value.Furthermore, the reason why they were rated as low was the lack of reproducibility tests or well-defined patient materials.
Finally, we also used the ROBINS-I tool for RoB assessment to evaluate the bias of included studies in Table 5.Among the included studies, 2 [30,34] had a serious RoB and 2 [20,29] of the remaining 13 had a critical RoB (Table 5).The most recurrent sources of bias were related to the selection of patients and the lack of blinded outcome assessment.Besides, because the comparison was between the patient's post-treatment models and "their own" predicted models, the RoB of confounding was low.However, Zhao's study [29] and Haouili's [20] study showed a critical RoB due to the lack of well-defined patient material.In addition, the patient's age range was relatively large in Riede's study [34].In contrast, other studies selected patients with a smaller age range, which could affect the predictability of arch expansion.Most authors chose the cuspid of a tooth as the landmark for measurements, but Grünheid [26] and Zhao [29] did not.Furthermore, Zhao did not perform reliability testing [29].Overall, the bias in measuring outcomes was serious.

Efficacy of expansion
In all the included studies, the efficacy of arch expansion with Invisalign could be evaluated by comparing the pretreatment and posttreatment models.It is clear that there were statistically significant differences between the pretreatment and posttreatment arches, indicating that Invisalign effectively expanded the dental arch [6, 19, 20, 28-34, 36, 37].Deregibus concluded that Invisalign class II treatment resulted in a significant increase in arch width at the molar and premolar levels in both arches [30].However, in Morales-Burruezo's study, expansion was more effective in the premolar area and less effective in the canine and second molar areas [31].Furthermore, the efficacy of expansion was different between the upper and lower arches [6].

Predictability of expansion
The predictability of expansion, which refers to the ability to predict final outcomes at the beginning of Invisalign treatment, could be examined by comparing the difference between the virtual posttreatment digital model simulated on the ClinCheck software and the actual digital model obtained by scanning the posttreatment model.The predictability of expansion is also called the efficiency or the accuracy of arch expansion.Among the included studies, 13 of them focused on the predictability of arch expansion with Invisalign [6, 20, 26-34, 36, 37].The average expansion was significantly different from that predicted for each type of tooth in both the maxilla and mandible, and both underexpansion and overexpansion were observed [26].However, no statistically significant changes between the ClinCheck model and the posttreatment model were discovered in Lione's study [36].Notably, in Zhou's study, the efficacy of crown expansion movement in the upper arch for the canine, first premolar, second premolar and first molar were 79.75%, 76.10%, 73.27% and 68.31%, respectively [32].Clearly, the efficiency decreased from the anterior area to the posterior area in the upper arch, which was similar to Lione's finding [33].

Types of materials
Invisalign appliances made of different materials were used to investigate whether there was a difference in the predictability of dental arch development.According to Riede, the effectiveness of achieving transverse values as planned was generally not increased with the use of SmartTrack compared to the previously used Ex30 material [34].However, the Invisalign system aligners (SmartTrack material) offered a high degree of predictability both in the upper and lower arches in Bernardez's study [6].Furthermore, during orthodontic treatment with Ex30 aligners, the predictability of expansion depending on the magnitude of the planned expansion was not predictable, while canine depth, arch depth, molar inclination and molar rotation were shown to be predictable [28].A progressive reduction in the expansion rate from the canine to the posterior regions, with the greatest net increase at the premolars.
The Invisalign system can increase arch width by increasing the buccal tipping of maxillary teeth.

Other findings
When treating crowded dentition, buccal arch expansion and interproximal reduction are important clinical tools [25].Besides, careful planning, including overcorrection and the use of other auxiliary methods of expansion, should be taken into consideration, which will result in a reduction in the rate of midcourse corrections and refinements [27].The Invisalign system can increase arch width by the tipping movement of posterior teeth, and no significant change was observed in maxillary basal bone width [32].Furthermore, the amount of preset expansion amount and initial maxillary first molar torque were significantly negatively correlated with the efficiency of bodily expansion movement [32].

Discussion
Most of the collected literature, including that for which full text cannot be obtained, was published in the past three years, indicating a trend in which dental arch expansion has been a focus of research on clear treatment.Six systematic reviews on Invisalign are currently available [1,[21][22][23][24]39] and three of them evaluate the efficiency of arch expansion [1,23,24].However, these reviews have not paid great attention to the changes in the transverse dimension, and they relied on studies published prior to ours.Therefore, we decided to submit a comparatively precise and innovative systematic review to assist in clinical practice.This review included 12 retrospective studies and 3 prospective studies.After an assessment of the quality of the included studies, limited conclusions were drawn because there were more than two studies having a grading of B. Besides the limited sample size, these 15 studies were drawn from different countries and regions and have different inclusion and exclusion criteria for subjects, which increases the bias of this systematic review.Moreover, the age differences of the patients included in each study would also influence the results.There is also a lack of studies published in authoritative journals among the included studies.In this systematic review, further mathematical analysis is needed to perform a comprehensive exploration of the clinical outcomes of arch expansion with Invisalign.The perspective and methods of interpreting data that we used were also not innovative enough.Finally, there is a lack of a multicenter study in the included studiesone will be needed in the future to clarify the clinical outcomes of arch expansion with Invisalign.
Two retrospective studies [29,32] reported that the expansion effect of the dental arch is mainly caused by the tipping movement of teeth, which is manifested as the change of transverse width.Furthermore, Duncan et al. [25] mentioned that the arch expansion achieved by the buccal tipping movement of teeth, which is a    kind of transversal movement, is one of the significant pathways to resolve dentition crowding.In Duncan's research [25], itis noteworthy that the greatest expansion efficacy occurred in the premolar area, which is supported by Zhou et al. [32] and Morales-Burruezo et al. [31].Furthermore, the results of one retrospective study [30] and one prospective study [36] showed a significant increase of arch width and functional and stable outcomes for patients who have undergone class II clear treatment.Lione et al. [36] and Grünheid et al. [26] stated that although maxillary arch expansion may not be fully attained, in nonextraction cases, Invisalign is able to achieve predicted tooth positions with high accuracy.They also reported that aligners made of different materials do not have a significant difference in efficacy [34].However, Solano-Mendoza [28] and Vidal-Bernardez [6] present a contrasting viewpoint.Among patients treated with Ex30 aligners the predictability of upper arch expansion is not predictable [28], but-on the contrary-among those treated with SmartTrack material, the predictability is shown to be high in both the upper and lower arches [6].Therefore, more research focusing on aligner materials should be conducted.Morales-Burruezo et al. [31], Lione et al. [33] and Tien et al. [37] stated that there were statistically significant differences between the predicted and actual treatment outcomes.Therefore, overcorrection should be considered on ClinCheck in order to obtain expected outcomes.Furthermore, discretion is required when overcorrecting to compensate for expansion inaccuracy.Notably, a progressive reduction in the expansion rate from the anterior area to the posterior region in the upper arch was observed in three retrospective studies [29,31,32] and one prospective study [33].The reasons for this reduction may be differences in root anatomy and cortical bone thickness, a higher occlusal load, great soft tissue resistance in the posterior region and a decline of mechanical efficiency from the anterior to the posterior [32].Furthermore, the amount of preset expansion and initial maxillary first molar torque are significantly negatively correlated with the efficiency of expansion movement.Thus, presetting sufficient buccal root torque of posterior teeth is an important strategy for improving the efficiency of expansion [32].
Above all, in this systematic review, we found that the use of Invisalign holds promise for orthodontic patients who undergo arch expansion treatment.In some studies, researchers have provided evidence indicating that the effects of Invisalign are superior to those of other common orthodontic treatments.Therefore, the use of Invisalign might be a suitable therapy for orthodontic patients who will undergo arch expansion.The available clinical studies have mainly concentrated on adolescents, and there is also a lack of reporting on adverse reactions associated with Invisalign.As the average age of patients receiving orthodontic treatment is gradually increasing, in order to provide patients with superior and effective orthodontic treatment, further studies are needed to address these problems .

Conclusions
• Despite the fact that arch expansion with Invisalign ® is not completely predictable, clear treatment is a viable option for resolving dentition crowding.• The efficacy of expansion is highest in the premolar area.• Research focusing on treatment outcomes with different materials of aligners should be conducted in the future.• Overcorrection should be considered when planning an arch expansion with Invisalign.• In the maxilla, the expansion rate decreases from the anterior to the posterior.
• In the maxilla, presetting sufficient buccal root torque of posterior teeth may enhance the efficiency of expansion.
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Table 1
Characteristics of included studies

Table 2
Swedish council on technology assessment in health care criteria for grading assessed studies

Table 3
Definitions of evidence level

Table 4
Grading of Included Studies

Table 5
Risk of bias of included studies by robins-i quality assessment scale