|
 
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| REVIEW ARTICLE |
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| Year : 2022 | Volume
: 14
| Issue : 3 | Page : 222-229 |
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Clear Aligners: Where are we today? A narrative review
Bushra Kanwal1, Keerthan Shashidhar2, MN Kuttappa2, US Krishna Nayak2, Akshai Shetty3, Karishma A Mathew4
1 Rawa Almas Splendid Diamond Clinic, Al Baha, Saudi Arabia
2 Department of Orthodontics and Dentofacial Orthopedics, A B Shetty Memorial Institute of Dental Sciences, NITTE (Deemed to be University), Mangalore, India
3 Department of Orthodontics and Dentofacial Orthopaedics, R V Dental College, Bangalore, India
4 Department of Orthodontics and Dentofacial Orthopedics, Meenakshi Institute of Craniofacial Surgery, NITTE (Deemed to be University), Mangalore, Karnataka, India
| Date of Submission | 28-Nov-2021 |
| Date of Decision | 12-Jan-2022 |
| Date of Acceptance | 20-Apr-2022 |
| Date of Web Publication | 28-Jun-2022 |
Correspondence Address:
Dr. Keerthan Shashidhar
Department of Orthodontics and Dentofacial Orthopedics, A B Shetty Memorial Institute of Dental Sciences, NITTE (Deemed to be University), Mangalore, Karnataka
India
 Source of Support: None, Conflict of Interest: None  | Check |
DOI: 10.4103/jioh.jioh_334_21
Aim: The aim of this review article is to discuss the current role of Clear Aligners in treating various malocclusions and the effectiveness of aligners when compared with braces using recent literature. Materials and Methods: Literatures pertaining to production of aligners, efficacy and treatment outcome of aligners, root resorption in aligners, periodontal health during aligner treatment, and comfort levels during aligner treatment were reviewed from PubMed, Scopus, and Google Scholar databases and included selected systematic reviews. Results: The literature cited during the initial days of aligners showed contrasting results when compared with articles cited recently. It was mainly observed in improvement in the scope of treating various malocclusions. However, the results show that aligners still do not possess the ability to create an extensive range of movements when compared with braces. Conclusion: Despite the claims, evidence for the effectiveness of clear aligners is generally lacking. In mild-to-moderate instances, shorter treatment duration and chair time appear to be the only substantial benefits of transparent aligners over conventional systems supported by current evidence. Advantages of clear aligner therapy were reported for better aesthetics, comfort at an early stage, easier oral hygiene maintenance, and improved periodontal health. Based on the available evidence, clear aligner therapy is definitely effective in managing minor malocclusion and shows acceptable results with moderate malocclusion. Keywords: Clear Aligner Appliances, Esthetics, Fixed Orthodontic Appliances, Malocclusion, Orthodontic Appliances, Removable Orthodontic Braces
How to cite this article:
Kanwal B, Shashidhar K, Kuttappa M N, Krishna Nayak U S, Shetty A, Mathew KA. Clear Aligners: Where are we today? A narrative review. J Int Oral Health 2022;14:222-9 |
How to cite this URL:
Kanwal B, Shashidhar K, Kuttappa M N, Krishna Nayak U S, Shetty A, Mathew KA. Clear Aligners: Where are we today? A narrative review. J Int Oral Health [serial online] 2022 [cited 2023 Oct 5];14:222-9. Available from: https://www.jioh.org/text.asp?2022/14/3/222/348420 |
| Introduction | |  |
Aligners are clear plastic alternatives for dental braces used to straighten teeth. With the increasing esthetic and comfort demands in patients seeking orthodontic treatment, clear aligner's technique (CAT) nowadays has become a popular alternative to traditional fixed orthodontic appliances in orthodontic society. Clear aligners were initially introduced to resolve mild-to-moderate dental crowding and close mild spacing. At present, there are numerous publications and experts’ experiences shown in more complex malocclusion such as extraction cases, which could be corrected with CAT.[1],[2],[3] Today, aligners are at a crossroads with fixed appliance therapy. What first started as a service provided by licensed orthodontists only, aligners are now being delivered by general practitioners as well. Today, companies such as Direct Smile provide aligners to individuals at their home’s doorstep without needing to visit a dental clinic. All this begs the question, “Do aligners work better in a particular set of malocclusions, or are they ready to overthrow the brackets from the throne of the orthodontic community, and will the need for orthodontists be obsolete shortly?” This comprehensive narrative review shall provide a bird’s eye view on various topics such as attachments in aligners, case selection, treatment outcome, comfort levels, apical root resorption, and periodontal health during aligner treatment that has all been based on current evidence.
| Materials and Methods | | |
A non-systematic narrative review was conducted with Boolean operators using the PubMed, Scopus, and Google Scholar databases [Figure 1]. Articles published until November 2021 were analyzed. The following search strategy and keywords related to the clear aligner therapy were used: “(Clear Aligners),” “(Clear aligner) AND(production),” “(Clear aligner attachments),” “(Case selection)AND(aligner treatment),” “(Efficacy of clear aligners),” “(Aligners vs. Braces),” “(Quality of life)AND(clear aligners),” “(Root resorption)AND(aligners),” “(Periodontal health)AND(aligner therapy),” “(Crowding)AND(aligners),” “(Spacing)AND(aligners),” “(Distalisation)AND (aligners),” “(Arch expansion) AND (aligners),” and “(Rotation) AND(aligners).” | Figure 1: PRISMA flowchart for selection of studies for the narrative review
Click here to view |
The data were obtained by the following PICO:
P: Subjects requiring orthodontic treatment who were subsequently treated by aligner therapy or fixed appliances.
I: Being a narrative review there was no intervention planned.
C: While some studies did not require comparison, other studies that were included saw the comparison between aligner therapy and fixed appliance therapy.
O: The outcome was to assess the efficacy of aligner therapy in treating malocclusions and how far they have come since they were first introduced.
All the papers considered underwent an initial analysis and were selected accordingly to the following inclusion criteria: The literature was limited to full-text English language and peer-reviewed journals. Certain systematic reviews that were directly relevant to the study aims were included. Studies regarding one or multiple of the following parameters, clear aligners, fixed appliances, pain, root resorption, tooth movement, quality of life, periodontal health, were included in the study.
Exclusion criteria: Letters to the editors and studies that were published beyond November 2021 were excluded. Studies that were not yet published but fit into the time period were excluded as well.
Article selection was done by two authors to broaden the area of search. Once the articles were screened and selected, all authors systematically reviewed the articles. Sixty-one articles relevant to the study were reviewed.
| Results | | |
A large selection of literature was available for each of the headings outlined below. However, there is not enough evidence to say that aligners work better than braces. Shorter treatment duration and chair time appear to be the only substantial benefits of transparent aligners over conventional systems that are supported by current evidence. The advantages of CAT according to current evidence include better aesthetics, better comfort, better hygiene maintenance, and improved periodontal health. Although CAT could achieve comparable outcomes with that of fixed appliance therapy in mild–moderate malocclusions, they are still lacking in treating complex malocclusions and in achieving good occlusal contacts.
| Discussion | | |
The history of clear aligner treatment
Clear aligners are based on a concept that has been around since the 1940s. Kesling[4] invented a tooth-positioning device in 1945 to simulate the movement needed during orthodontic treatment. Years later, Nahoum[5] in the late 1950s developed the vacuum-formed dental contour appliance often termed as the “Invisibles.” In 1971, Ponitz[6] of Ann Arbor, MI, USA introduced the so-called “invisible” retainers. He claimed that these appliances had limited tooth movement. Invisible retainers were also mentioned by McNamara et al.[7] as a way to produce minor tooth movement. Later, Sheridan et al.[8] developed a method including interproximal tooth reduction and progressive alignment using clear Essix appliances, and Hilliard and Sheridan[9] refined those approaches with a set of unique thermoforming pliers designed to improve certain movements. Taking the principles of Kesling, Nahoum, Raintree Essix, and others, and even further using CAD-CAM, a series of customized clear appliances, called “aligners,” are generated.
Clear aligner production process
An intra-oral scan or polyvinyl siloxane impression is needed. Although a bite registration is helpful, it is not always necessary. The intra-oral scans are used to create an accurate 3D digital model. At the digital treatment facility, the teeth are digitally sectioned, the dental arches are related to each other, tooth movement is staged based on doctor instructions, and the treatment plan is placed online for the doctor to review. Following approval, the digital models are sent to a casting plant, where a stereolithographic model for each stage is made. Each model is covered in a clear plastic aligner. Finally, a set of aligners are delivered to the dentist.
Materials used in aligners
Thermoforming clear aligner uses a variety of thermoplastic materials, including polyvinyl chloride, polyurethane, polyethylene terephthalate, and polyethylene terephthalate glycol. For each aligner to be thermoformed and finally trimmed, a physical model (created using 3D printing, stereolithography, or material jetting) is required.
Attachments in aligners
Attachments are small tooth-colored shapes that are attached to the teeth before or during the aligner therapy. They are the means through which orthodontic forces are transmitted from the aligner to the teeth.
Mechanism of action
The most basic function is retention. The second function of attachments is to assist movement by providing an active surface for pushing the teeth. The aligner applies pressure to a specified surface on each attachment, whereas the non-active surface is relieved to avoid any interference.
Force and couples to generate moments can be applied to move teeth depending on the design of the attachment relative to the seating of an aligner. The type of tooth movement is directly related to the moment-to-force ratio (M: F) for tipping, translation, and root torque. Anchorage is taxed significantly more by bodily tooth movement (translation) and especially root torque than by tipping movements. Because the surrounding material applies loads to the teeth, aligners can achieve a variety of tooth movements without the use of attachments. Attachments are rarely required for incisor rotation or tipping the crowns of teeth. Without attachments, complex tooth movement and rotation (other than incisors) are difficult to achieve.
Enhanced treatment features
Invisalign is currently the leading supplier of orthodontic aligners. The reason they have been so successful is because of the constant evolution of their attachments. Henceforth, we shall be discussing the various attachments that have been used by Invisalign.
Ellipsoid, rectangular, and rectangular beveled are the principal attachments. Although the beveled attachments are useful for modest orthodontic correction, they are not compatible with the complex movement required for comprehensive orthodontics. As a result, smart force features were developed.
An optimized rotation attachment for bicuspids (previously only available for cuspids), a new power ridge (TM) feature for lower anterior (previously only available for the upper arch), and a lingual power ridge feature for upper anteriors are among the new smart force features in Invisalign G3. The G3 concept[10] for attachment-mediated tooth movement was designed for a broader range of applications, including rotations and torque control.[10],[11] Inter-arch elastics are frequently used to provide anchorage control in the treatment of class II/III malocclusions. Precision cuts, which are doctor-prescribed pre-cuts in the aligners that allow for the usage of elastics, are a new feature in Invisalign G3. Invisalign G4 was designed to improve clinical outcomes when treating anterior open bites by using a new multi-tooth approach for the open-bite treatment and improved optimized extrusion attachments. Additionally, new optimized multi-plane movement features are intended to increase control of upper laterals that are undergoing extrusion, rotation, and/or crown tip. New optimized root control attachments in Invisalign G4 enable greater mesiodistal root control for canines and central incisors. With the advent of clinical innovations, Invisalign further evolved its smart force attachments into G5, G6, and G7 generations. On the posterior teeth, dome-shaped fifth-generation (G5) and seventh-generation (G7) attachments, which are unique to Invisalign, can be used. Premolars are designated as G5s, whereas molars as G7s, but there is no design difference. These attachments are particularly used for deep bite correction. The new optimized retraction attachment and optimized anchorage attachments in InvisalignG6 for first premolar extraction are designed to improve clinical outcomes in severe crowding or bi-maxillary procedures that require extraction and are planned for maximum anchorage. The latest series of biomechanical innovations for Invisalign treatment is Invisalign G8 with Smart Force aligner activation. The solution aims to improve cases of deep bite, crowding, and crossbite. The aligner activation function improves these cases by assisting in anterior intrusion for deep bite cases and posterior arch expansion for crowding and crossbite cases. The latest series of biomechanical innovations for Invisalign treatment is Invisalign G8 with Smart Force aligner activation. The solution aims to improve cases of deep bite, crowding, and crossbite. The aligner activation function improves these cases by assisting in anterior intrusion for deep bite cases and posterior arch expansion for crowding and cross bite cases.
Conventional attachments, unlike optimized attachments, are not exclusive to Invisalign and are used by other companies that manufacture transparent aligners or software for creating in-office aligners with 3D printers. For rotations of canines and premolars, as well as extrusion of incisors and canines,[12] standard attachment methods may be equally as successful as Invisalign’s custom-optimized attachments. Although the accuracy of orthodontic tooth movements with Invisalign has been studied, the effectiveness of different attachment types, as well as other aligner variables, has not.
Case selection and treatment outcome
The evolution of clear aligners since their introduction into the orthodontic fraternity has been remarkable. What once started as a treatment option for mild crowding/spacing[13] is now being used to treat even the most complex of malocclusions.[14] But how effective is the treatment outcome? Under this section, various types of tooth movements/malocclusion traits that have been proven to be treated effectively and not so effectively by clear aligners have been discussed. A summarized version of the effective/ineffective tooth movements and effective/ineffective treatment outcomes has been presented in tabular form as well [Table 1] and [Table 2]. | Table 1: Clinical conditions treated effectively/ineffectively by aligners
Click here to view |
Crowding (non-extraction)
When aligners were first introduced, Joffe[15] in his classic study laid down guidelines that said that crowding of 5 mm and above would be difficult to treat. However, as the attachments and treatment mechanics have evolved, it can be said that mild crowding (1–3 mm) and moderate crowding (4–6 mm) can be effectively treated by clear aligners (non-extraction).[19],[20]
Duncan et al.[20] stated that the buccal arch expansion and interproximal reduction dominated the relief in crowding while treating their cases without extracting premolars.
Severe crowding/bimaxillary cases (extraction)
Joffe[15] stated that skeletal anteroposterior discrepancies of over 2 mm are difficult to be treated by clear aligners. He also stated that more than 5 mm of crowding and closure of extraction spaces is difficult with clear aligners. This holds today as well. Studies done by Dai et al.,[21] Djeu et al.,[22] Li et al.,[23] Grünheid et al.,[24] and Gu et al.[25] prove that clear aligners are less effective in bodily tooth movement in extraction space closure because of less torque control which in turn led to poor root and contact point approximation. A study done by Baldwin et al.[26] showed that the closure of extraction space occurred due to tipping of adjacent teeth and failure of bodily movement of the teeth, thereby supporting the findings of the above-mentioned authors.
Maxillary molar distalization
Studies by Ravera et al.,[30] Simon et al.,[30] and Caruso et al.[31] prove that aligners are effective in achieving efficient maxillary molar distalization of 2.25–3 mm without causing any significant extrusion/tipping of the molars. What makes them more efficient is that they did not affect the vertical skeletal relation of the patients since there was no extrusion.
Rotations
Joffe[15] stated that clear aligners were ineffective in correcting severe rotations (more than 20°). This holds today as well. Aligners are not clinically effective in rotational corrections, according to studies by Djeu et al.,[22] Kravitz et al.,[34] Nguyen and Chen,[35] and Simon et al.,[30] with the predictability of 39% for canines/premolars and 60% for incisors. A recent study by Lambardo et al.[36] shows that the rotational predictability of lower molars and premolars was found to be the highest being 85.4 and 82.7, respectively. This was followed by incisors (61.5%) and the least was found to be for the lower canines (54.2%).
Interproximal reduction and staging reduction to 1.5° per aligner are two methods for increasing predictability.[22],[30],[36]
Arch expansion
Aligners have proven to be effective in arch expansion. But one has to keep in mind that the expansion is the dental expansion and not skeletal. As Houle et al.[27] state in their study, the expansion observed was dental expansion which meant that the expansion was caused by dental tipping. Their study also showed that the expansion was less accurate in the posterior region when compared with the anterior region. Morales-Burruezo et al.[28] found that the expansion was more successful in the premolar region and less effective in the canine and second molar regions in a recent study.
Extrusion/intrusion
Joffe[15] had stated that extrusive movements were a limitation of Invisalign. Studies done by Kravitz et al.[16] and Rossini et al.[37] have also stated that the least accurate movement in their study was extrusion of incisors. Nguyen and Chen[35] also showed that the incisors and anteriors had the least accuracy for extrusion (26% and 29%, respectively).
Kravitz et al.[16] also stated that aligners were effective at achieving mild degrees of maxillary incisor intrusion. Nguyen and Chen[35] also showed that the accuracy of achieving intrusion in the incisors and anteriors was 85% and 79%, respectively. On the contrary, a study done by Charalampakis et al.[38] stated that the most difficult movement to achieve was intrusion of teeth. They said that all the teeth that were supposed to intrude eventually ended up extruding and that this could have been due to the superimposition method used by them or due to intrusion of the molars, which make the incisors appear extruded.[38]
However, a recent article by the Angle society of Europe[29] helped form a viewpoint on the clinical consensus of aligners. They chose the study done by Kravitz et al.[16] since they felt it had a low-to-moderate risk of bias and therefore have stuck to the opinion that extrusions are the least accurate movement and that the incisors can be effectively intruded to a mild degree.
Spacing
Joffe[15] and Ali and Miethke[17] have stated that a mild spacing of up to 5 mm can be effectively treated by clear aligners. However, spacing of more than 5 mm, although treatable, is more difficult due to poor translatory movements through the aligners. Kravitz et al.[16] and Clements et al.[18] also stated that mandibular anterior space closure can be effectively treated by aligners.
Reviewing the efficacy of clear aligners compared with conventional fixed appliances
Although fixed orthodontic appliances are still extensively used today, the introduction of removable clear aligners has undeniably transformed the discipline of orthodontics in recent years. Clear aligners have been shown in studies to have an advantage in segmented tooth movement and a shorter treatment time.[39],[40] They were effective in controlling posterior buccolingual inclination and upper molar bodily movements of about 1.5 mm[37],[40] However, the study found that aligner cases are more prone to relapse, which could be explained by the tipping mechanism of aligners as opposed to the bodily movement of fixed appliances.[40]
In a study by Papageorgiou et al.,[41] aligner orthodontic therapy was associated with a lower treatment outcome in adult patients when compared with fixed appliances. The current evidence did not support the clinical use of aligners as a treatment modality that was as successful as the gold standard of braces, according to the study.
Comparing patient comfort levels between various fixed appliances and clear aligners
During orthodontic treatment, patients experience pain and discomfort to varying degrees. Clear aligners have been shown to cause less pain during the initial stages of treatment than fixed orthodontic appliances, among other advantages.[41] During the initial few days of treatment, patients wearing clear aligners appeared to have less pain, anxiety, and a higher oral health-related quality of life than those wearing fixed appliances.[42],[43]
According to a study conducted by Almasoud,[44] patients treated with Invisalign aligners used fewer analgesics than those treated with passive self-ligating fixed appliances. During the first week, a larger percentage of patients in the fixed-appliance group reported taking analgesics for dental pain.[45]
When compared with patients treated with labial and lingual appliances, aligner patients have the highest oral quality of life scores.[45],[46] When compared with conventional and conventional low-friction brackets, lingual brackets, and aligner, Antonio-Zancajo et al.[47] found that patients in the lingual orthodontic group experienced less pain at all times.
Zhang et al.[48] concluded that the effect of CAT on overall dental health-related quality of life when compared with fixed appliance therapy was still equivocal due to the current lack of data. To draw more reliable conclusions on the effect of CAT and fixed appliance therapy on dental health-related quality of life, further high-quality clinical trials utilizing validated oral health-related quality of life instruments are needed.
Reviewing the extent of external apical root resorption with clear aligners
Root resorption in orthodontics is a common occurrence. However, the amount of root resorption that occurs in orthodontic treatment is of prime importance. Many studies have shown that the amount of external root resorption seen in patients treated with aligners is much lower than that seen in individuals treated with fixed appliance therapy.[49],[50],[51]
Studies conducted independently to assess the root resorption in CAT show that clear aligners cause mild-to-moderate root resorption[52],[53] with the upper anteriors having a higher resorption rate than the lower anteriors.[53],[54] According to a systematic review conducted by Elhaddaoui et al.,[55] the maxillary incisors were the teeth with the highest amount of root resorption.
Periodontal health during clear aligners treatment
As bands, brackets, elastics, and ligature wires support the growth of microbial flora and food deposits, fixed orthodontic braces may hinder plaque clearance and damage gingival health.[56] When orthodontic patients do not exercise proper dental care, gingivitis can develop.[57] Plaque collection surrounding orthodontic braces can lead to periodontal disease and dental caries over time.
According to a systematic review by Rossini et al.[2] and a meta-analysis by Jiang et al.,[58] patients treated with aligners had improved periodontal health than those treated with conventional fixed appliances. When CAT was compared with fixed appliances, patients in the aligner group showed a significant improvement in periodontal health indexes. Clear aligners are better for periodontal health than fixed appliances, and they may be advised for individuals who are at high risk of gingivitis.
When compared with patients receiving orthodontic treatment with fixed orthodontic appliances, patients using the Invisalign® System had better periodontal health in the short term. Invisalign® should be considered as a first treatment option in individuals at risk of developing periodontal disease, according to this 3-month clinical trial.[59]
Despite the claims, evidence for the effectiveness of clear aligners is generally lacking. The skills of an orthodontist are extremely important to have an understanding of how teeth move. A licensed Orthodontist has the advantage of knowing how to treat complex malocclusions that (based on current evidence) cannot be efficiently treated by aligners. It is our professional opinion that malocclusions should ideally be treated by a licensed Orthodontist because he/she has developed the intricate skills of treating malocclusions through years of education. Clear Aligners may currently be the “IN” thing in Orthodontics, but they have a lot to prove to replace braces as a whole. If history has proven anything, it is that evolution always wins. The evolution of Invisalign’s attachments serves as a reminder as to how far the aligners have come in treating malocclusion. Recently, Align Technology (Invisalign) has also introduced a mandibular advancement device. According to a recent study by Caruso et al.,[60] Invisalign’s Mandibular advancement device was efficient in correcting the patient’s class 2 skeletal relation.
Although this review tried to concise the data as well as we could possibly do, it should be noted that there are many articles that provide contrasting opinions. Being a narrative review, the risk of bias was considerably high. It is beyond the scope of this article to include all the studies on clear aligners. A systematic review on each of the topics mentioned earlier would bring a clearer picture of the efficiency of aligners. As years go by, new attachments will help overcome the shortcomings of the aligners. Until then, fixed appliance therapy will continue to have an edge over the CAT.
| Conclusion | | |
In mild-to-moderate malocclusions, shorter treatment duration and chair time appear to be the only substantial benefits of transparent aligners over conventional systems that are supported by current evidence.
Advantages of CAT were reported for better aesthetics, comfort at an early stage, easier oral hygiene maintenance, and improved periodontal health.
Based on the available evidence, CAT is effective in managing minor malocclusion and can also bring about acceptable results in moderate malocclusion.
CAT could achieve comparable treatment outcomes with that of the fixed orthodontic appliance in non-growing patients with mild malocclusion. However, the fixed appliances are more effective in great improvement, including adequate occlusal contacts than CAT.
CAT is more effective in the control of incisor intrusion than extrusion.
The Aligners perform poorly in correcting rotations, particularly rotations of the lower canine and premolar.
Arch expansion with CAT is primarily achieved by crown tipping and exhibits less accuracy on second molars.
There is no concrete consensus about the efficacy of CAT for complex malocclusion, e.g., extraction.
Acknowledgements
We would like to thank Dr. Tarona Azem Subba for providing support in gathering data for this study and for her timely help in editing the manuscript.
Financial support and sponsorship
Nil.
Conflicts of interest
The authors declare that they have no conflicts of interest.
Authors’ contribution
BK: data curation, investigation, methodology, writing—original draft; KS: conceptualization, data curation, investigation, methodology, project administration, resources, software, supervision, validation, visualization, writing—original draft, writing—review and editing; MNK: methodology, project administration, supervision, validation, writing—review and editing; USKN: investigation, project administration, resources, supervision, writing—review and editing; AS: data curation, resources, writing—original draft; KAM: data curation, methodology, writing—review and editing.
Ethical approval and Institutional Review Board statement
Not applicable.
Declaration of patient consent
Not applicable.
Date availability statement
Not applicable.
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[Figure 1]
[Table 1], [Table 2]
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