Insight into clear aligner therapy protocols and preferences among members of the American Association of Orthodontists in the United States and Canada

Esthetics is a major concern for orthodontic patients. The introduction of Invisalign (Align Technology, San Jose, Calif) in 1997 was a major leap in the esthetic world of orthodontics.¹  A series of preformed thermoplastic splints are worn by patients and sequentially changed according to the treatment planned to move teeth in three dimensions.²  Some advantages offered by clear aligner treatment (CAT) include that it is an esthetic alternative to braces and the ability to remove the aligners to eat and access the facial and lingual surfaces of teeth for appropriate oral hygiene practices.²  On the other hand, the strict requirement of patient compliance and difficulties controlling tooth movement in three different planes of space are some of the limitations with CAT.² 

CAT includes a wide range of appliances that differ in modes of action and methods of construction in the treatment of various malocclusions.³  There are significant differences in the clear aligner systems available in today's market. The first clear aligner system was used to treat minor malocclusions/irregularities; however, a new generation of clear aligner systems target more complex malocclusions that require comprehensive orthodontic treatment.²  In addition, some current aligner systems are targeted directly to consumers without the direct supervision of trained professionals.⁴ 

This huge variety in the systems available has led clinicians planning to employ CAT to rely primarily on weak forms of published evidence (low-level, poorly designed studies) or otherwise their own clinical experience.⁵  Consensus is lacking among clinicians in their choice of aligner system, malocclusions that can be treated successfully with CAT, treatment strategies and protocols employed, and the choice of retainer afterward.

This is the first validated online survey to investigate aligner treatment protocols and preferences among orthodontists in the United States and Canada. It also assessed factors influencing clinician choices with CAT, treatment protocols, and malocclusions targeted with CAT. Therefore, the primary objective of this survey was to investigate CAT applications and practices among orthodontists in Canada and the United States. A secondary objective was to determine the level of confidence of orthodontists in treating different types of malocclusions with CAT.

This cross-sectional study was approved by the institutional review board at the University of Connecticut Health (21X-090-2). An intensive literature review was conducted to identify the most common protocols, preferences, and factors related to clinician choices in CAT.

A preliminary electronic survey was developed specifically for this project using Google (Mountain View, Calif) Forms online survey tool. It included three sections and 48 questions that were evaluated for content validity by 14 consultant orthodontists (Appendix 1). In addition, the questions were rated on a three-point scale. The content validity ratio was calculated for each question according to the method by Lawshe.⁶  Nine questions were not significant at the critical level; therefore, they were eliminated from the final version.

The final version of the questionnaire consisted of three sections and 39 questions. Section 1 included questions related to demographics and clinical experience with CAT. Section 2 had questions related to patient selection, satisfaction, and demands. In addition, section 2 assessed clinician confidence in treating various malocclusions. Section 3 had questions related to treatment protocols and adjustments used by clinicians (Appendix 1). The questionnaire was reviewed by the American Association of Orthodontists (AAO) Survey Review Committee. Once approved, the AAO Partners in Research Program distributed the survey via e-mail to active members in the United States and Canada. To increase the response rate, the e-mail was distributed twice with a reminder e-mail to randomly selected population samples. The e-mail was sent to two different groups of randomly selected active members (n = 2116 and 2100). Participation in the survey was voluntary, and the average time to complete the survey was 10 minutes.

Demographics and practice experience as well as responses to survey questions were summarized using frequencies and percentages. Fisher's exact tests were used to test the associations between confidence in using clear aligners to manage a particular malocclusion (confident/very confident vs not confident). In addition, grouped bar plots were used to highlight the significant associations. P values smaller than .05 were considered statistically significant. All statistical analyses were performed in R version 4.1.1 (R Foundation for Statistical Computing, Vienna, Austria).

The survey was distributed to 4216 active members of the AAO. A total of 160 providers completed the survey. The response rate was 3.8%.

Table 1 describes the sample characteristics and basic information regarding clear aligner experiences. CAT was used by 65.00% of respondents. It constituted more than 20% of practice for 47% of respondents. Of the respondents, 56.25% had been using it for more than 10 years. Invisalign was the most popular type (81.25%).

Table 2 summarizes questions related to patient selection, satisfaction, and demands for CAT. Of the orthodontists, 97.5% primarily used CAT to treat adult patients. In addition, 63.75% used CAT for patients between 31 and 45 years old. Clear aligners were equally used for both sexes by 63.75% of orthodontist respondents. Interestingly, 97.5% of orthodontists believed that esthetic concern was the primary reason for seeking CAT. In addition, only 61% of orthodontists reported that their patients were always content with the results achieved.

Table 3 describes respondent confidence in treating malocclusions and rankings of tooth movements achieved with aligners. Tipping movement was ranked as the easiest (1.79 ± 1.35). Extrusion (4.34 ± 1.53) and root movement (4.31 ± 1.27) were ranked as the most difficult tooth movements with CAT. Approximately 98.8% of the orthodontists were confident in treating mild/less severe malocclusions with CAT. Of the orthodontists, 82.5% used CAT for crowding, 96.9% for spacing, and 85% for anterior crossbite. However, only 10% of orthodontists reported that they used CAT for impaction, 22.5% for facial asymmetry, and 30.6% for severe skeletal discrepancies.

Tables 4 and 5 depict treatment protocols and adjustments of cases treated with CAT. A total of 26.87% of orthodontists said that they avoided extraction with CAT, but 58.75% reported extracting single teeth: a mandibular incisor. Elastics were the most common auxiliaries (92.50%) used by orthodontists for treating various malocclusions. Other adjuncts included inter-radicular buccal temporary anchorage devices (TADs) used by 50% of the orthodontists. Interestingly, 43.75% of the orthodontists used limited treatment with braces, whereas 70% of the orthodontists use sectional fixed appliances when treating with CAT.

Importantly, 58.12% recommended aligners to be changed weekly. Of the orthodontist respondents, 83.75% requested overcorrection in their digital set up, especially for tooth rotations (78.12%). Refinements were needed most of the time (8.44 ± 1.77 on a scale from 1 [rarely] to 10 [100% of the time]), as predicted vs achieved outcomes were substantially different.

Interestingly, 56.25% of orthodontists agreed that fixed braces had better three-dimensional (3D) control of tooth movement and resulted in better occlusal contacts. In addition, 79.38% of respondents did not use any adjuncts to accelerate tooth movement. Notably, 59% of respondents were using in-house aligners for limited treatment (39.86%), relapse treatment (40.56%), or minor adjustments (44.76%). Of the orthodontists, 83.12% preferred Essix retainers following CAT.

Appendix 2 shows the association between confidence in treating severe malocclusions and the aligner system used. The significant associations are highlighted in Figure 1. Orthodontists who were using CAT for severe malocclusions were more likely to use Invisalign and less likely to use 3M (St Paul, Minn) than those who were not confident (Appendix 2, Figure 1).

• Increased overjet more than 8 mm: 90% of confident respondents used Invisalign to treat severe overjet compared with 76% of nonconfident respondents who used Invisalign to treat severe overjet; the difference between the groups was statistically significant (P = .022).

• Posterior crossbite: 89% of confident respondents used Invisalign to treat posterior crossbite compared with 75% of the nonconfident respondents who used Invisalign to treat posterior crossbite cases; the difference between the groups was statistically significant (P = .040). On the other hand, 4% of confident respondents used 3M for posterior crossbite treatment, whereas 15% of nonconfident respondents used 3M for posterior crossbite; the difference between the groups was statistically significant (P = .012).

• Anterior openbite: 88.5% of confident respondents used Invisalign to treat anterior openbite cases compared with 48.3% of nonconfident respondents who used it for the same purpose; differences between the groups were statistically significant (P < .001). On the other hand, 8% of confident respondents used 3M to treat openbite malocclusion, whereas 28% of nonconfident respondents used 3M aligners for the same purpose; the difference between the groups was significant (P = .006).

The primary aim of this cross-sectional study was to provide an overview of how clinicians perceive CAT and delve into common clinical practices related to its use. The strengths of this study were the systematic method used to create a validated survey instrument and the random sample selected by the AAO Partners in Research Program for its distribution.

With the increased number of adults seeking orthodontic treatment, the demand for esthetic appliances has increased significantly.³  This study showed that esthetic concern is perceived as the primary reason for patients seeking CAT for orthodontic treatment. It was previously reported that the motivation for patients requesting aligners for treatment is primarily esthetic.²  The survey further showed that patients were generally satisfied with the outcomes with CAT. In addition, studies assessing patient satisfaction with CAT showed that Invisalign was associated with improved oral health and greater satisfaction compared with fixed appliances.^7,8 

Tipping movement was ranked as the easiest and extrusion and root movement were ranked as the most difficult to achieve. Aligners being fabricated from thermoplastic materials do not offer the flexibility to control force systems delivered to teeth,⁹  in contrast with fixed appliances in which force delivery is controlled by altering wires and brackets.¹⁰  Mechanical properties of aligner materials can change after thermoforming or exposure to the oral environment, which affects force delivery and final outcomes.¹¹  It was shown that the properties of polymers hinder their ability to deliver an appropriate load system (moment of a couple) for precise control of root movement.¹²  With CAT, forces can be easily applied away from the center of resistance, creating a moment of a force that produces a “tipping” movement,¹³  which is the default movement achieved with CAT.³  Therefore, malocclusions that can be treated by tipping of teeth (crowding, spacing, crossbites, openbites) are more easily corrected by CAT.^12,14  It was reported that CAT works better for intrusion rather than extrusion.¹⁴  Therefore, extrusion of teeth has been described as one of the least predictable movements achieved with CAT in the literature and in this study.³ 

Earlier, CAT relied on a shape-molding effect to achieve desired results.²  Auxiliary elements (attachments, elastics, power ridges, bite ramps, TADs, etc) were added to improve treatment efficacy and range of movement.¹⁵  Attachments help improve the predictability of specific tooth movements such as rotations and extrusion, and their tactical arrangement and morphology is predicted to enhance force delivery.¹⁴  Interproximal reduction, the main mechanism to create space, is virtually planned and adjusted with the new 3D treatment simulation. Extraction to resolve severe arch length discrepancy seemed to be avoided by some respondents because of its unpredictable outcome with CAT,¹⁶  whereas others reported the possibility of extracting a single tooth (mandibular incisor). In challenging cases, integration of elastics aids in addressing severe sagittal and transverse discrepancies. In addition, in the current study, orthodontists preferred adjuncts such as buccal mini-implants or limited or sectional fixed braces over extraction to resolve these severe discrepancies.¹⁵ 

Unlike comprehensive treatment with braces, treatment outcome with CAT relies on the clinicians' experience with the virtual 3D set up software in addition to coming up with a comprehensive treatment plan for their patients.¹⁷  It was shown that patients' choice for CAT was primarily based on the providers' sound clinical judgment and proper case diagnosis (Table 2).

Orthodontists who responded to the survey preferred aligners to be changed every 7 days. Al-Nadawi et al. found that 2-week changes were more accurate for some posterior movements, but this was not clinically significant, and a 7-day regimen is an acceptable protocol.¹⁸  In addition, the current study indicated that regardless of whether overcorrection was prescribed for rotated teeth, refinements were needed as the predicted and achieved outcomes did not match. A previous study showed that the outcome achieved with CAT was 50% for the majority of tooth movements. ¹⁴  In addition, rotations of more than 15 degrees constitute a challenge to treat with CAT.¹⁹  Because of the differences between predicted and achieved outcomes, orthodontists resort to refinements to achieve satisfactory outcomes.¹⁴ 

More than 50% of the orthodontists agreed that braces provided better 3D control of tooth movement. Despite the advantages CAT offers in terms of esthetics, fewer emergencies, comfort, and periodontal health,²  it still lacks efficiency in treating moderate to complex malocclusions. Buschang et al. recommended CAT for simple malocclusions.⁵  It was indicated that aligners were not as efficient as braces in torque control, producing adequate occlusal contacts and posttreatment retention.¹⁴  Shortened treatment duration and significantly reduced chair time in mild cases appeared to be the only significant advantages of CAT over conventional systems.²⁰ 

Adjunct therapies for accelerating orthodontic tooth movement are not popular with CAT (79.38% not using adjuncts). This study showed vibration as the most commonly used modality for acceleration with aligners (12.50%), although the literature is controversial regarding the effects of vibration on the rate of tooth movement.²¹  In this survey, advocates of in-house aligners (n = 59) used them for minor adjustments (44.76%) and the treatment of relapse malocclusion (40.56%) or limited treatment (39.86%). Unlike other aligner systems, they are economical and do not require an outside service, duration of treatment is very fast, and the orthodontist has full control over the workflow.²²  With the emphasis on improving treatment efficacy with regard to monitoring patients and reducing clinical and financial burdens associated with an increased number of appointments, the popularity of dental monitoring (DM) is increasing among practitioners. This study showed that 20.62% of orthodontists used DM with CAT. Hansa et al. indicated that the number of appointments were reduced by 3.5 visits with DM with Invisalign, and aligner tracking was improved in the DM group.²³  However, DM is expensive and may increase the financial economic burden on patients.

Essix retainers were the most popular retainers following CAT and were used by 83.12% of the orthodontists. Essix retainers offer several benefits: they are easy and quick to fabricate, they provide an opportunity for better oral hygiene maintenance, and they are strong and flexible enough to act as a positioner or nightguard.²⁴ 

The popular Invisalign system has evolved through the years. Malocclusions that were impossible to manage with Invisalign a decade ago can now be treated with increased patient satisfaction. Also, efficacy of the treatment was enhanced by the addition of auxiliaries and, recently, with the introduction of optimized attachments that are placed automatically by the manufacturer's software to provide better control of tooth movement. The 3M Clarity aligners, developed by Dr Warshawski, are considered unique because their improvement and accuracy relies primarily on prospective analysis of preliminary data related to their use.²⁵  Data regarding the efficiency of CAT systems is still deficient. However, both systems seem to be commonly used among clinicians.

The limitations of this cross-sectional survey study included the low response rate despite two rounds of distribution. This may have been attributed to the length of the survey and no incentives for the respondents. Also, it must be noted that most e-mail message-based surveys with high response rates were done before 1995 when e-mail messages were a novel tool; rates have decreased as novelty has diminished.^26,27  On the other hand, the mean age of the respondents was 52.56 ± 11.94 years old; therefore, reported CAT practices and protocols might have been different if this survey was distributed to residents or new graduates.

• Invisalign is the most popular clear aligner system, and clinicians seem to be confident using it.

• Providers are aware of the pitfalls of CAT; they find it challenging to perform root movement and extrusion with aligners. They seem confident treating mild to moderate malocclusions with clear aligners and avoid complex patients with impactions or severe skeletal problems.

• Extraction is often avoided; integration of elastics, TADs, expanders, or sectional fixed appliances is preferred to resolve severe discrepancies.

• Providers prefer the 1-week change protocol, and refinements are often required to reach desired goals.

• In-house aligners and dental monitoring are starting to gain popularity with CAT.

Appendixes 1 and 2 with supplemental data are available online.