Effectiveness and Stability of Treatment with Orthodontics Clear Aligners: What Evidence?

Clear aligners, as a transparent and removable appliance, offer an alternative to conventional fixed appliance to patients with high demands for esthetics and comfort. Only a few investigations have focused on the efficacy of clear aligner therapy in controlling orthodontic tooth movement. Furthermore, the stability after treatment has not been thoroughly investigated. The purpose of this chapter was to update the knowledge of the available evidence about effectiveness and stability of clear aligners in non-growing subjects. Searches was made in different databases from January 2015 to January 2021. Relevant articles that met the inclusion criteria were selected. The level of evidence of the studies was moderate. The vertical movements of tooth were difficult to accomplish. Mesiodistal tipping showed the most predictability (82.5%) followed by vestibulolingual tipping. Molar distalization was also recorded as the highest accuracy. Derotation was difficult to accomplish with aligners especially of rounded teeth. The effectiveness of aligners in achieving the simulated transverse goals was 45%. The stability of clear aligner therapy was assessed by only two studies. Refinements are likely needed in almost all cases and to ensure treatment stability a retention period using a specific protocol is necessary.


Introduction
Orthodontic developments, especially during the last years, have been accompanied by a significant increase in the esthetic demands of the patients [1]. With the significant recent improvements in computer-aided design/computer-aid-ed manufacturing (CAD/CAM) and dental materials, there has been an increase in the demand for plastic systems [2]. Clear aligners provide an esthetic and comfortable treatment experience, facilitate oral hygiene, cause less pain as compared to fixed orthodontic appliances, and reduce the number and duration of appointments [3][4][5]. The aligner therapy also involves a lower incidence of demineralization, enamel abrasion, periodontal lesions, and mucosal irritations [6].
The concept of clear aligners was introduced by Kesling in 1946 with a tooth positioner fabricated by thermoplastic material molding technology and designed for minor tooth movements during the finishing stages of orthodontic treatment. In 1993, Sheridan and colleagues developed a technique of giving new clear retainers to the patient at each visit, incorporating interproximal reduction to provide the necessary space for tooth movement [3,7]. With further advancement in orthodontic technology, Align Technology introduce the clear aligner treatment (CAT) rendering Kesling's concept a feasible orthodontic treatment option [8]. A series of removable polyurethane aligners were introduced as an esthetic alternative to fixed labial appliances. Scanned images are converted to physical models by using different stereolithography (STL) techniques to fabricate a series of aligners that sequentially reposition the teeth. Each aligner is programmed to move a tooth or a small group of teeth 0.25-0.33 mm every 14 days [9,10]. Align Technology provides orthodontists with ClinCheck (Align Technology Inc., Santa Clara, Calif) models, which reflect the treatment outcomes. The aligners incrementally shift the teeth into place based on the outcome the orthodontist expects to achieve [11].
The primary focus of the clear aligner system was initially to solve cases of low and moderate crowding and to close small spaces [1]. However, it has continually evolved through the development of new aligner materials, attachments on teeth, as well as new auxiliaries, such as "Precision Cuts" and "Power Ridges" to address a wider range of malocclusions and to enable additional treatment biomechanics [2,5,12].
Despite the available body of literature pertaining to aligner technology, only a few investigations have focused on the efficacy of clear aligner therapy in controlling orthodontic tooth movement. Furthermore, the stability after treatment has not been thoroughly investigated.
The purpose of this chapter was to update the knowledge of the available evidence about effectiveness and stability of clear aligners and to answer the following clinical research question: "Are clear aligners effective in controlling the orthodontic movement in non-growing subjects and what about stability of this treatment modality?"

Search strategy
A systematic search in the medical literature produced between January 2015 and January 2021 was performed to identify all peer-reviewed articles potentially relevant to the review's question.
The following databases have been used: CENTRAL, MEDLINE, MEDLINE in Process, Embase and Cochrane Library databases.
The search strategy comprised use of the following terms: (invisalign OR clear aligners OR aligners OR transparent aligners) AND (effectiveness OR efficacy) AND (dental changes OR treatment outcome) AND (stability).
Additionally, a manual search was conducted in orthodontic journals of interest, such as The Angle Orthodontist, the American Journal of Orthodontics and the European Journal of Orthodontics. Title and abstract screening was performed to select articles for full text retrieval.

Eligibility criteria
The following inclusion and exclusion criteria were used:

Inclusion criteria
Study design: meta-analysis, systematic reviews, randomized and non-randomized clinical trials, prospective and retrospective studies were included.
Participants: non growing patients. Intervention: articles that studied dental movement of cases treated with clear aligners.
Results: the efficacy of clear aligners in performing dental movements and the stability of treatment, superimposing virtual models or radiographs.

Exclusion criteria
We excluded for our study articles older than 6 years, samples with growing patients, articles written in a language other than English, in-vitro studies, author opinions, letters to the editor, isolated cases, series of cases, surgical cases, or reports of patients with syndromes.

Level of evidence
The grading system described by the Swedish Council on Technology Assessment in Health Care (SBU) [13] was used to assess the methodological quality and the level of evidence of the articles (Tables 1 and 2). Grade

Study selection
The selection of articles included in this review is shown in the PRISMA flow chart (Figure 1). Study selection procedure was comprised of title-reading, abstractreading, and full-text-reading stages. After exclusion of not eligible studies, the full report of publications considered eligible for inclusion by the authors was assessed. Eleven studies were included in the qualitative synthesis.
Data collected from each of the included articles are described in Tables 3 and 4. Nine of the covered studies assessed predictability of tooth movements comparing   post-treatment patient models to the predicted digital planned tooth movement models [2,6,7,11,[14][15][16][17][18]. Two studies assessed the stability of the clear aligner therapy [19,20].

Level of evidence of studies
According to the SBU tool (Tables 1 and 2), among the selected studies, the methodological quality was low for four studies [6,11,16,17], moderate for four others [7,14,15,19] and high for one study [18] (Table 5). Thus, conclusions with a moderate level of evidence could be drawn from the review process. • Treatment effects were stable throughout a short-term retention period using a specific retention protocol.
• Effectiveness and stability were equally achieved in mild, moderate, and rather severe cases.

Discussion
In this review, we aimed to provide data on the effectiveness and stability of treatment with clear aligners. The level of evidence was moderate as we identified one study with level «A» and four studies with level «B».
The effectiveness of clear aligners was judged by the predictability of tooth movement which varies with the type of tooth and the type of movement. Lopez et al. [2] found that the expression of the programmed movement was not fully accomplished with Invisalign®.
Concerning vertical movements, the study by Lopez et al. [2] revealed that vertical movements are difficult to accomplish with aligners. Extrusion of a single tooth is moderately difficult using clear aligners when compared to fixed-appliance systems, however, some auxiliaries such as buttons, elastics and optimized extrusion attachments can be used to facilitate this movement [5,21].
Many studies showed that intrusion was the most unpredictable movement especially for the maxillary central and lateral incisors [16,21]. Invisalign has a bite-block effect, because 2 aligners of 0.38-mm width are interposed between posterior teeth throughout treatment. Unexpected intrusion of the molars would cause the incisors to appear extruded on the posttreatment models after superimposition [16]. In fact, according to Grunheid et al. [22], mandibular incisors tend to be positioned more occlusally than predicted. The bite-block effect may make open bites easier to treat with Invisalign [16].
Concerning horizontal movements, mesiodistal tipping showed the most predictability especially of upper molars and lower premolars (82.5%) followed by vestibulolingual tipping [14]. Lingual crown tip (53%) was significantly more accurate than labial crown tip (38%), particularly for maxillary incisors [23]. According to Rossini et al. [8], aligners can easily tip crowns but cannot tip roots because these appliances cause tooth movement by tilting motion rather than bodily movement. In the anterior region, the elasticity of the aligner at the gingival margin results in difficulty in controlling the applied forces [24]. With the use of Power Ridges (Align Technology, Amsterdam, The Netherlands), the aligner can accurately control root torque according to the crown position in the virtual setup [17]. Tepedino et al. [15] also concluded that with Nuvola® aligners, in patients with moderate crowding up to 6 mm, the torque movements for central and lateral incisors and canines of both arches predicted in the digital setup were, in general, clinically achieved. However, molar torque may not be fully achieved, with maxillary second molars often having a clinically relevant magnitude of more facial crown torque than predicted [22].
Molar distalization was recorded as the highest accuracy with no need for attachments. Simon et al. [25] also reported a high accuracy (88%) of the bodily movement of upper molars when a distalization movement of at least 1.5 mm was prescribed.
Several studies agreed that derotation of rounded teeth especially canines was difficult to achieve with aligners [16,22,26]. An amount of rotation greater than 15° has been identified as a risk factor for decreased accuracy for rotational prediction [25]. Interproximal contacts of rotated canines might also be considered a significant predictor for the diminished efficacy of tooth movement, especially in the absence of interproximal reduction of the enamel (IPR) [26]. The direction of derotation has been also documented to influence the accuracy of the maxillary canine, with distal movement demonstrating less accuracy than mesial [21]. This is possibly due to the actual contact area between canine and premolar and the potential challenges of providing enamel reduction in this area. DOI: http://dx.doi.org /10.5772/intechopen.99998 It has been recommended to plan overcorrections, especially if rotations exceed 15°, to use attachments, and to reduce staging to less than 1.5° per aligner [8,16,25]. However, although various types or shapes of attachment grips or practices of interproximal enamel reduction have been reported as potential prognostic factors for better efficacy of rotational tooth movement, this does not necessarily translate into an identified substantial effect in practice [26].
Concerning transverse movements, the effectiveness of achieving the simulated transverse goals was 45% [6]. Aligners could increase the arch width, but expansion was achieved by tipping movement of posterior teeth rather than bodily expansion. In fact, Invisalign becomes less accurate going from the anterior to the posterior region being more effective in premolar area [27,28]. Thus, according to the initial torque of the posterior teeth, an appropriate amount of negative torque in the crown could be preset in ClinCheck to improve bodily expansion efficiency. For patients who need a large amount of expansion, clinicians should consider reducing the amount of expansion for each aligner to ensure periodontal health [7].
According to Lopez and al. [2], Invisalign® was also able to alter intercanine, interpremolar, and intermolar width in the presence of crowding. Kravitz et al. [23] recommended to treat cases with severe lower crowding mostly by interproximal reduction (IPR) instead of dentoalveolar expansion. This recommendation comes from the finding that retraction is more accurate than dentoalveolar expansion of the lower anterior teeth. The expansion of the mandibular intercanine width also poses the greatest risk of relapse following treatment [29].
Concerning the effectiveness of the occlusal contacts with clear aligners, the study by Izhar et al. [10] found that the software models do not accurately reflect the patient's final occlusion immediately at the end of active treatment. Kassas et al. [30] also stated that clear aligners were not sufficient for providing ideal occlusal contacts. The deterioration in occlusal contacts was caused by the thickness of aligners, which interferes with the settling of the occlusal plane.
As far as the malocclusion type is concerned, the study by Graf et al. [19] showed that Invisalign® treatments are able to significantly reduce malocclusions in adult patients. The study found that all types of sagittal malocclusion (class I, class II, and class III) were 'greatly improved' with a rate of 77.44%. Graf and al. [19] also concluded that conventional attachments and the combination with optimized attachments equally led to treatment effectiveness regarding the total PAR score reduction with equally achieved effectiveness in mild, moderate, and rather severe cases. However, for Class II malocclusion, Patterson et al. [31] reported that there was no significant Class II correction or overjet reduction with elastics for an average of 7-month duration in the adult population. Additional refinements may be necessary to address problems created during treatment mainly posterior open bite.
One study of our review by Dai et al. [17] assessed the effectiveness of Invisalign in first premolar extraction treatment. According to this study, first molar anchorage control and central incisor retraction were not fully achieved as predicted. Only medium anchorage control was achieved as the first molars actually moved mesially. The G6-optimized attachment showed similar control in first molar angulation and mesiodistal translation as did 3-and 5-mm horizontal rectangular attachments. On the other hand, setting a distal tipping of 6.6 mm on the first molars might help clinically maintain the tooth angulation, leading to bodily tooth movement. According to the same study [17], the incisors inclined lingually under the retraction force. Accordingly, the use of power ridges or attachments as well as overcorrection by setting greater buccal crown inclination during the virtual setup should be considered to achieve optimal incisor torque control.
Current evidence does not support the clinical use of aligners as a treatment modality that is equally effective to the gold standard of braces [32]. However, clear aligners have advantage in segmented movement of teeth and shortened treatment duration, but are not as effective as braces in producing adequate occlusal contacts, controlling teeth torque, and retention [5,33].
Many variables influence the accuracy of dental movements, but very few studies have analyzed these parameters in treatments with clear aligners. According to Tepedino et al. [15], several factors determine successful tooth movement such as the attachment's shape and position, the aligner's material and thickness, the amount of activation present in each aligner, and the techniques used for the production of the aligners. Treatment outcomes depend also on the patient's characteristics, bone density and morphology, crown and root morphology of the teeth, as well as on factors related to the clinician. Orthodontists have to incorporate their expert knowledge in determining proper sequencing of tooth movements, tooth attachment design and placement, and prescribing overcorrection when needed for difficult tooth movements to increase efficiency and achieve better treatment outcomes [22,34]. Patient compliance is also mandatory to achieve good results by wearing the aligners 22 hours a day or more [28].
One study from this review with a high level of evidence [18] evaluated the impact of wear protocol on the accuracy of clear aligners. It has concluded that fourteen-day changes were statistically significantly more accurate in some posterior movements mainly maxillary intrusion, distal-crown tip and buccal-crown torque, and in mandibular intrusion and extrusion.
As in all types of orthodontic treatment, stability is one of the most important issues to discuss regarding clear aligners. According to the systematic review by Zheng et al. [20], only one study compared the post-retention dental changes between patients treated with Invisalign and those treated with conventional fixed appliances. They found that the change in the total alignment score in the Invisalign group was significantly larger than that for the Braces group. There were significantly larger changes in maxillary anterior alignment in the Invisalign group than in the conventional bracket group. Tamer et al. [5] also reported that maxillary anterior leveling relapsed in the Invisalign group. On average, the posttreatment models lost twice as many points for alignment than the respective ClinCheck models. In other words, a full finishing phase of treatment may be needed to achieve the results indicated in the ClinCheck model [11].
The type and degree of tooth movement, the duration of active treatment and the retention protocol are among major influencing factors of posttreatment stability and relapse. The study by Graf et al. [19] is the first one to assess the stability of clear aligners outcome throughout a retention period of 10 months. The retention protocol involved a mandibular multistrand fixed retainer (0.0155 inch; stainless steel, 24 K gold plated) bonded on each lingual surface from canine to canine and a removable modified Hawley retainer for the upper arch (with mandatory Adams clasps on first molars). The study showed that the treatment outcome can be stable throughout this retention protocol. It has also concluded that treating patients with respect to their physiological boundaries and maintaining their original arch form would be key to treatment stability. Overexpansion of the dental arch, especially in the lower arch and in adult patients, is a potential risk for stable results.

Conclusion
There is current evidence with a moderate level of certainty regarding the effectiveness of clear aligner therapy for certain tooth movements. Clear aligners can safely straighten dental arches in terms of leveling and derotating the teeth, except for canines and premolars. The crown tipping can be easily performed. However, important limitations include arch expansion through bodily tooth movements, extraction space closure, corrections of occlusal contacts, and larger antero-posterior and vertical discrepancies. The use of additional attachments might be more effective for various types of movement, such as bodily expansion of the maxillary posterior teeth, canine and premolar rotational movements, incisors torque control and extrusion of maxillary incisors. Overcorrections might also improve the effectiveness of orthodontic movement. However, overcorrections are not as simple for all movements and need to be made on a case-by-case basis depending on the goal of treatment.
Studies on effectiveness of clear aligners had methodological heterogeneity as they assessed predictability of different types of tooth movements for different teeth by using different materials like Invisalign, F22 aligner and Nuvola system. Retention and stability studies regarding aligners also remain limited in the literature. Therefore, further well-designed and reported researches are required on this subject.