Esthetic perception of clear aligner therapy attachments using eye-tracking technology


The purpose of this research was to assess and compare esthetic perceptions of clear aligner therapy with attachments and esthetic brackets by measuring differences in eye fixations using eye-tracking technology.


The sample involved 250 adult subjects. The subjects gave verbal consent, then viewed photographs showing 4 variations of orthodontic appliances: clear aligner control with minimal attachments, clear aligner with anterior and posterior attachments, esthetic brackets, and clear aligner with posterior attachments. Images were displayed for 6 seconds each on a computer monitor. Location and time to first fixation, total fixation duration, and total visit count and duration for each type of appliance were measured. Subjects were then asked to complete an online survey.


Participants spent the least amount of time looking at the photograph of the control, followed by those of the ceramic brackets, posterior attachments, and anterior and/or posterior attachments. The anterior and/or posterior image had the least number of visits but garnered the longest visit duration (1.32 visits averaging 0.74 seconds per visit). This was followed by the images of the posterior attachments (1.40 visits, 0.70 seconds per visit), ceramic brackets (1.43 visits, 0.65 seconds per visit), and minimal attachments control (1.45 visits, 0.61 seconds per visit). The hierarchy of most preferred appliances across all 250 respondents was as follows: minimal attachments control, ceramic brackets, posterior attachments, and anterior and/or posterior attachments. Overall, 88.4% of subjects would compromise appliance esthetics during treatment for a better outcome (n = 221).


Eye-tracking data show that time to the first fixation was negatively correlated with its survey ranking and that an increase in attachments led to an increase in total fixation duration. There is a general desire for clear aligners without attachments and ceramic brackets over clear aligners with multiple attachments. Survey data suggest that although respondents viewed appliance esthetics as highly important, nearly all would compromise appliance esthetics during treatment if it resulted in a better outcome.


  • Eye-tracking data suggest that subjects tend to spend more time on attachments.

  • Clear aligners with minimal attachments or ceramic brackets are preferred.

  • Clear aligners with multiple attachments are less acceptable.

In recent years, orthodontic clinicians have seen an increase in adults seeking orthodontic care, resulting in a record high of nearly 1.5 million adult patients in the United States and Canada. This shift in the market has resulted in orthodontic treatment that has continually evolved in response to available technology to meet the desires of the adult consumer. Patient demands for esthetic treatment outcomes have grown to include esthetic appliances during treatment. This demand has driven manufacturers to develop systems designed to appeal to the patient, with an underlying goal of reducing appliance visibility.

Orthodontic patients and practitioners currently have a slew of esthetic alternatives to traditional braces that reduce the visibility of appliances, including options such as ceramic brackets, lingual appliances, and clear aligners.

Previous studies had shown that nearly two-thirds of young adults would reject orthodontic treatment if it involved being treated with visible appliances. Visible options were not only seen as being less attractive but also led to the assumption of the wearer as having less favorable traits, including decreased intellectual ability. In this way, clear aligners have quickly become synonymous with esthetics for most patients, and this is how the product is marketed nowadays .

Although aligners assumed to be the most esthetic treatment among the choices, it has not been fully studied, and its attractive aspects are still controversial. To date, little research has been done to assess the patients’ perception of how they view the appearance of esthetic appliance systems, such as clear aligners with auxiliaries attached and their desired treatment preferences. The question remains how one objectively and subjectively perceives even the most esthetic appliances on the market.

Material and methods

For the present study, 250 subjects over the age of 18 years were randomly recruited from the Saint Louis University campus in St Louis, Missouri. The sample consisted of 158 females and 92 males. The only exclusion criteria were subjects with visual or cognitive impairments. Participants were not required to be in orthodontic treatment, nor were they required to have ever had any treatment in the past.

Images of the clear aligner and esthetic appliances were taken with a Canon EOS Rebel T5 EF-S camera (Canon, Huntington, NY) by a single photographer in the same location to ensure for analogous lighting conditions and positioning of each photograph.

One photograph of each of the following was taken on a live model: clear aligner control with minimal attachments (on maxillary second premolars) (Align Technology Inc, San Jose, Calif), clear aligner with anterior and posterior attachments, ceramic brackets with 0.016-in nickel-titanium wire and clear ligature ties (AO Radiance Plus; American Orthodontics, Sheboygan, Wis), and clear aligner with posterior attachments (from maxillary canines to second premolars). For the images depicting clear aligners with attachments, aligners containing attachments were fabricated, filled with shade A2 Filtek Supreme Ultra by 3M (St. Paul, Minn), and worn by the model. Attachments were not bonded to the model. See Figure 1 for a list of visible attachment types per photograph.

Fig 1
Lists of visible attachments used per photograph as displayed on Invisalign ClinCheck software.

The photographs were merged into two 2 × 2 composite images. The first photograph consisted of the following: (1) clear aligner control with minimal attachments, (2) clear aligner with anterior and posterior attachments, (3) ceramic esthetic brackets, and (4) clear aligner with posterior attachments ( Fig 2 ).

Fig 2
Composite image of appliances. A, Clear aligner control with minimal attachments; B, clear aligner with anterior and posterior attachments; C, ceramic brackets; D, clear aligner with posterior attachments.

The 2 composite images were imported into Tobii Lab Pro eye-tracking software (Tobii Pro Lab, version 1.4; Tobii Technology AB, Danderyd, Sweden) installed on a 21.5-in iMac (Apple, Cupertino, CA) running Windows 10 Education (version 1803; Microsoft Corporation, Redmond, Wash). The desktop was equipped with Tobii eye-tracking hardware (Tobii X2-60, Danderyd, Sweden). Each appliance was given its area of interest (AOI) to allow the software to isolate data for later analysis ( Fig 3 ). The demarcated areas were grouped into a single AOI per appliance type.

Fig 3
Tobii Pro Lab AOI. AOIs were demarcated, and anterior and posterior AOIs were grouped. Each appliance type had one total AOI.

The study protocol, recruiting statement, participant consent, appliance photographs, and study survey were approved by the Saint Louis University Institutional Review Board (IRB protocol no. 29146) before initiation of the study.

Potential adult subjects were randomly recruited from the Saint Louis University campus and given a brief description of the study, stating that the purpose was to assess esthetic orthodontic appliances. All subjects were seated at eye-level, approximately 60-65 cm (23-26-in) in front of a computer equipped with a Tobii X2-60 eye-tracking device and calibrated to the eye-tracking device by following a moving ball across the screen. Calibration was repeated until satisfactory, aiming for estimated accuracy and precision below 1°. On successful calibration, the composite image showing the 4 appliance variations was displayed on the screen for 6 seconds, according to a previous eye-tracking study. Data including location and time of first fixation, total fixation duration, total visit count, and total duration were collected for each AOI. Fixations were measured as any instance; the eye remained stagnant for 80 milliseconds or greater. After viewing the image, the participant was asked to complete an online survey ( Fig 4 ).

Fig 4
Sample survey as viewed online.

Statistical analysis

Statistical analysis was performed using SPSS software (version 24.0; SPSS, Chicago, IL). Descriptive statistics were calculated for each appliance variation. Descriptive statistics and 1-way ANOVAs between age, gender, and ethnicities were used to analyze data. In variables in which the same distribution was nonparametric, the independent sample Kruskal-Wallis tests or 1-way ANOVA on ranks was used. The null hypothesis is that there is no significant difference in how clear aligner therapy attachments and esthetic brackets are viewed using eye-tracking. The alternative hypothesis is that there is a significant difference in how clear aligner therapy attachments and esthetic brackets are viewed using eye-tracking.


When analyzing the location of the first AOI fixation, participants tended to look at the photograph in the top left corner first and continued in a clockwise pattern. When averaging the 2 photographs to offset this tendency, it took participants 1.55 seconds to fixate on the minimal attachment control after the initial composite image was shown on the screen. This image was the first that participants fixated on the screen. Participants looked at the ceramic brackets second, taking an average of 1.84 seconds before fixating on this image ( Table I ). Participants looked at the anterior and/or posterior attachments third, spending an average of 2.09 seconds before fixating on this image, and lastly, posterior attachments (2.22 seconds to the first fixation). Although there were no group differences by gender or ethnicity in terms of what they looked at first and the time it took to the first fixation, there was a group difference by age. The viewing patterns of participants differed depending on their age group ( P = 0.019) for the image with posterior attachments ( Table II ). Post-hoc tests did not reveal any significant difference between age groups.

Table I
Descriptive statistics for eye-tracking metrics for all participants
Metric Average SD
Time to first fixation, s
Control 1.55 1.40
Ant and/or Post attachments 2.09 1.30
Ceramic brackets 1.84 1.46
Post attachments 2.22 1.45
Time to first duration, s
Control 0.73 0.68
Ant and/or Post attachments 0.81 0.70
Ceramic brackets 0.80 0.62
Post attachments 0.81 0.65
Time to first duration, s
Control 1.45 0.88
Ant and/or Post attachments 1.32 0.85
Ceramic brackets 1.43 0.88
Post attachments 1.40 0.81
Time to visit duration, s
Control 0.61 0.48
Ant and/or Post attachments 0.74 0.49
Ceramic brackets 0.65 0.45
Post attachments 0.70 0.46

SD , standard deviation; Ant , anterior; Post , posterior.

Table II
ANOVA comparing time to the first fixation across age groups
Metric Sum of squares df Mean square F P
Between groups 16,291, 964.158 6 2,715,327.360 1.383 0.220
Within groups 814,523, 717.150 415 1,962,707.752
Total 830,815,681.308 421
Ant and/or Post attachments
Between groups 10,780,181.860 6 1,796,696.977 1.051 0.392
Within groups 665,122,451.867 389 1,709,826.354
Total 675,902,633.727 395
Ceramic brackets
Between groups 8,161,267.777 6 1,360,211.296 0.634 0.703
Within groups 860,814,612.495 401 2,146,669.857
Total 868,975,880.272 407
Post attachments
Between groups 31,781,012.863 6 5,296,835.477 2.559 0.019
Within groups 850,653,825.319 411 2,069,717.337
Total 882,434,838.182 417
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Oct 7, 2020 | Posted by in Orthodontics | Comments Off on Esthetic perception of clear aligner therapy attachments using eye-tracking technology
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