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INTRODUCTION
The use of braces, ligatures, archwires, and other elements of conventional orthodontic treatment make dental hygiene difficult; this interferes with aesthetics and causes patient discomfort.123 With the significant recent improvements in computer-aided design/computer-aided manufacturing (CAD/CAM) and dental materials, we have seen an increase in the demand for plastic systems. Subsequently, plastic orthodontics, specially designed for adult patients, have been developed.
Proffit et al.4 proposed that the ideal orthodontic appliance should not interfere with occlusion or hygiene, and should not damage the oral tissues. It should be light but capable of withstanding masticatory forces, be firmly retained, apply a controlled force between visits, and allow good control of the anchorage.
In 1945, Kesling5 first introduced the use of multiple aligners to correct crowding. Later, Ponitz6 reported the use of a removable plastic retainer (Essix®; Dentsply, York, PA, USA); in the 1990s, Sheridan et al.7 popularized these retainers by combination with interproximal reduction (IPR).
In 1997, two students from Stanford University, Zia Chishti and Kelsey Wirth, together with a computer specialist, founded Align Technology in Palo Alto, CA, USA. After approval by the Food and Drug Administration, their technology (Invisalign®) was presented at the American Congress of Orthodontists in 1999; 2 years later, it was introduced in Europe.8
The primary focus of the Invisalign® system was initially to solve cases of low and moderate crowding and to close small spaces. However, its ongoing research and development has allowed treatment of more complex malocclusions. Currently, it is one of the most used systems among the aligners.
Invisalign® asserts that it can resolve, without the use of additional techniques, rotations of 40° in upper and lower central incisors, 45° in canines and premolars, 30° in lateral incisors, and 20° in molars. Extrusions and intrusions of 2.5 mm can be achieved in anterior teeth; radicular movements of 4 mm and 2 mm can be achieved in posterior teeth. However, few studies have been published to support the effectiveness and total correction that is asserted by proponents of these plastic systems.
Since Lagravère and Flores-Mir9 published the first systematic review in 2005, several authors have updated evidence on this subject.101112 Therefore, we investigated the available scientific evidence in the literature to assess whether the Invisalign® system exhibits similar effectiveness to that of conventional orthodontics. Therefore, we focused on dental changes and effectiveness, compared with fixed appliances.
MATERIALS AND METHODS
Protocol and registration
The protocol for this review was registered on the international prospective register of systematic reviews (www.crd.york.ac.uk/prospero, protocol number: CRD42018074337).
Eligibility criteria
The following inclusion and exclusion criteria were used:
Study design: meta-analysis, systematic reviews, randomized and non-randomized clinical trials, cohort studies, and control cases were included. Prospective, retrospective, and cross-sectional studies were reviewed. Participants: adult patients over 16 years old.
Participants: adult patients over 16 years old.
Intervention: articles that studied dental movement of cases treated with Invisalign® and Smartrack® (Align Technology, San Jose, CA, USA) material were included.
Exclusion criteria: articles older than 10 years, samples with adolescent patients, articles written in a language other than Spanish or English, in-vitro studies, surveys, engineering articles, author opinions, reviews of literature, letters to the editor, isolated cases, series of cases, surgical cases, or reports of patients with syndromes.
Results: studies were included that evaluated dental movement, superimposing virtual models or radiographs in 2 or 3 dimensions.
Information sources, search strategy, and study selection
A systematic search was performed of articles published between August 2007 and August 2017 in the following databases: MEDLINE, Embase, Cochrane Library, Web of Knowledge, Google Scholar, and LILACS. The search strategy comprised use of the following terms; “(Humans* OR adult* OR malocclusion* OR male* OR female*) AND (invisalign OR clear aligners OR aligners OR transparent aligners OR orthodontic appliances, removable*) AND (braces* OR orthodontic brackets* OR fixed appliances) AND (cephalometry* OR dental changes OR treatment outcome*).”
Additionally, a manual search was conducted in orthodontic journals of interest, such as the American Journal of Orthodontics, European Journal of Orthodontics, The Angle Orthodontist, Journal of Orthodontics, Journal of Clinical Orthodontics, and Journal of the World Federation of Orthodontists.
In addition, studies that were not yet published, listed in the national clinical trials database ClinicalTrials.gov (www.clinicaltrials.gov) and the National Research Register (www.controlled-trials.com), were queried with the terms “clear aligner” or “Invisalign.”
RESULTS
Determination of relevance, validation, and data extraction
An independent search was performed by two of the authors (L.G.L. and E.P.) and existing disagreements were discussed with the third author (J.B.G.). The researchers were not blinded with respect to search results or authors.
Data items and collection
The following information was determined for all articles: year of publication, names of the authors, study design, number and groupings of participants, type of intervention, comparative groups, and results obtained (Table 1).
Risk of bias and quality assessment in the studies
All studies were analyzed and attempts were made to identify existing biases14 (Table 2) with the exclusion of the systematic review article by Rossini et al.10 All studies with a score lower than 5 were discarded (Kravitz et al.,15 2009; Kassas et al.,16 2013; Zhang et al.,17 2015; Ravera et al.,18 2016; Grünheid et al.,19 2017). Therefore, 15 articles remained for analysis, together with the systematic review published by Rossini et al.10 in 2015.
Results of individual studies and additional analyses
Dentoalveolar changes
This was the subject analyzed most frequently in the included studies.
- Transversal movements:
• Intercanine, interpremolar and intermolar width: several authors2122 demostrated that Invisalign® increases dentoalveolar width when the crowding is mild or severe. This increase was significantly lower when compared with treatments involving self-ligating brackets,23 and was significantly higher when compared with conventional treatments.22
- Rotations:
• Canines: IPR favored precision movement, especially with respect to maxillary and mandibular canines.24 The predicted rotation was 11.8°, but only 35.8% was expressed.
• Premolars: the accuracy of the correction was significantly reduced when it reached values > 15°. It is not recommended to correct > 1.5° per aligner.25
- Vertical movements: Krieger et al.26 stated that vertical movements were more difficult to accomplish than transverse or sagittal movements. They found that overbite was the most difficult parameter to predict and correct. Gu et al.27 agreed with this statement. Rossini et al.10 reported 0.72 mm of true intrusion. In general, 1.5 mm can easily be resolved.28
• Open bite: this is primarily corrected by extrusion of incisors without changes in the mandibular plane.28
• Deep bite: its rectification results from proclination of the lower incisors, minimum intrusion of upper incisors, and 0.5 mm extrusion of molars with a 0.5° opening of the mandibular plane,28 similar to those reported by Rossini et al.10
- Sagittal movements:
• Anterior: most treatments that do not require extractions use IPR and protrude the incisor to reach a correct alignment of the anterior sector.26
• Posterior: the effectiveness of molar distalization does not increase if we incorporate an attachment and its accuracy movement rounds 87.65%.25
• Overjet: total correction of overjet with Invisalign® is not reported in all cases.27
Accuracy of movement
The accuracy of movement among the studies was 55% to 72%, and was reportedly dependent on whether the aligner was changed weekly or biweekly.29 Chisari et al.30 revealed accuracy of 57%. In contrast, Kravitz et al.24 obtained an accuracy of 35.8% in a sample where only canines were assessed. Simon et al.25 reported an average accuracy of 59.3% for anterior torque movements (50.3%), premolar derotation (39.95%), and molar distalization (87.65%). Houle et al.31 achieved good transverse expansions regarding Clincheck® (Align Technology) prediction of 72.8% for the maxillary arch and 87.7% for the mandibular arch, with statistically significant results. However, in recent studies, they did not find clinically significant differences between what was expected and what was achieved.2631
Cephalometric changes
There were two studies2132 that analyzed the results of treatment by lateral cephalometric radiography. They observed that when crowding was > 6 mm, the incisor tended to procline and protrude.21 Invisalign® produces less average proclination than that observed with fixed appliances in moderate crowding: 3.4° ± 3.2° and 5.3° ± 4.3°, respectively32; this difference was not statistically significant.
Cone-beam computed tomography (CBCT)
Only two studies evaluated patients treated with Invisalign® using images with CBCT.1929 They demonstrated that uncontrolled dental inclination occurred with aligners1929 and asserted that brackets provided superior root control.19 In contrast, a systematic review in 201510 asserted 17° of root control, with greater control in lateral incisors, compared with canines or central incisors, as evaluated by dental cast.
American Board of Orthodontics (ABO)
Only two articles have compared the results of treatment with Invisalign® and treatment with fixed appliances using the objective classification system of the ABO.3334 The trial by Li et al.34 consists of a much larger sample (182 patients), using a randomized and prospective approach. Conversely, Kuncio et al.33 used retrospective analysis. Both treatment modalities provided significant resolution, fulfilling all ABO objectives. However, buccolingual inclination and occlusal contacts were inferior with Invisalign®, which conflicts with the findings of Rossini et al.10 In addition, relapse is greater in the maxillary arch than in the mandible in patients treated with plastic systems, 3 years following completion of orthodontic treatment.
Treatment duration
The durations of treatment according to the degree of crowding were: 13.4 ± 5.28 months (mild), 15.93 ± 5.17 months (moderate), and 17.92 ± 4.07 months (severe).21 Krieger et al.26 reported 13 months of treatment to correct moderate crowding. An randomized controlled trial found that the treatment time is longer with Invisalign® than with brackets.34 In contrast, Gu et al.27 reported a significantly shorter treatment time with Invisalign® than with fixed appliances. Notably, Pavoni et al.23 achieved a treatment duration of 21.6 months in both groups.
DISCUSSION
Although the level of evidence was high because we incorporated two A-grade studies, we found 7 studies with moderate evidence and 5 with low evidence; these incorporated biases into the results of this review. Unlike the systematic review published by Rossini et al.10 in 2015, the current study evaluates the treatment of more complex malocclusions treated with transparent systems. They only included 11 studies, whereas we initially began with 20. Our results are thus more precise and involve additional aspects.
Alteration of the intercanine and intermolar widths is primarily included in treatments without extractions; because it favors alignment and has an aesthetic effect on the smile,35 the orthodontist can control this through Clincheck®. It is important to note that the control of these buccolingual inclinations is greater with brackets.34
There has been no study regarding retroclination with Invisalign® treatment. However, the least predictable movements are rotation25 and vertical movements.26 For rotational movements, IPR and no more than 1.5° rotation per aligner are recommended. Additional techniques should be incorporated when corrections greater than 15° are required.2425 In contrast, vertical problems are solved exclusively by anterior extrusion or intrusion movements, with minimum change in the posterior area.28 Thus, the incorporation of attachments is recommended to improve results.
Recently, Align Technology has indicated that weekly changes of aligners can be made; however, we recommend that this must be individualized, depending on the complexity of the case and the degree of desired movement. In addition, the tooth requires a period of adaptation to recover from the force exerted,30 in order to stabilize movement29 and subsequently help retention.33
The precision of movement and correction of dental parameters has improved exponentially in recent years, reaching values of 70% to 80%. This change is due to the continuous research performed by Align Technology and the new products that have been released. The lack of consensus among results is likely because many of the articles published do not include new technologies launched after G5 in 2014.36 However, it has been proven that brackets remain more accurate than plastic systems.232734
The great majority of authors24252831 recommend overcorrecting with Invisalign® because the movement is not total and there is little root control, which produces uncontrolled tipping of the tooth and can affect relapse.222329 However, good root parallelization has been confirmed when Invisalign® is used in treatments with extractions.34
Many variables influence dental movement, but very few studies have analyzed these parameters in treatments with plastic systems. In those studies, only age and sex have been consistently related to orthodontic tooth movement influences.30
CONCLUSION
• Invisalign® and fixed appliances are able to alter intercanine, interpremolar, and intermolar width in the presence of crowding. Moreover, incisors tend to procline and protrude when crowding is > 6 mm.
• Vertical movement and derotation are difficult movements to accomplish with aligners. And IPR is recommended, especially in canines.
• It is not necessary to incorporate an attachment when molar distalization is required in Invisalign® treatment.
The expression of the programmed movement is not fully accomplished with Invisalign®.
• Sex and age affect tooth movement in both modalities.
• There is better root control with fixed appliances.
• The majority of tooth movement occurs during the first week with plastic systems.
• Buccolingual inclination and occlusal contacts are worse with Invisalign®.
Although this is a more complete systematic review than that presented in 2015,10 further studies with good methodological design are needed to confirm some of the aspects addressed in this review. Although it is possible to treat complex malocclusions with plastic systems, the results are less accurate than those achieved with fixed appliances.
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