An eye-tracking and visual analogue scale attractiveness evaluation of black space between the maxillary central incisors

ABSTRACT Objective: To study the influence of black space between the maxillary central incisors on the aesthetic visual perception of the face, via eye-tracking and visual analogue scale (VAS). Methods: Black space between the central incisors was created, for both sexes, as follows: control, 1-mm black space, 2-mm black space and 3-mm black space. Ninety raters participated in this study, divided into three groups: 30 laypeople, 30 nonorthodontists, and 30 orthodontists. After the visual calibration of each observer, eight photographs were presented in the Ogama® software concomitant with the use of the hardware The Eye Tribe®. Ogama generated information depending on the eye-tracking of each rater, regarding the time until the first fixation, time of fixation, heatmap, scanpath, and total time of fixation, to evaluate the areas deemed to be of interest according to the raters. Later on, the VAS was used, where each rater evaluated the images in an album on a scale of zero to 10 points. Results: The eyes and mouth were the areas more often noticed by the raters according to the heatmaps, while no significant difference was observed in time until the first fixation between the three groups of raters (p> 0.05). However, regarding the time of fixation on the mouth, a significant difference was observed (p< 0.05) when comparing the three groups. Conclusion: Black space has a negative effect on the aesthetic perception of the face. The amount of attention on the mouth is correspondent to the size of the black space.


INTRODUCTION
Orthodontic treatment is focused on enhancing the facial aesthetics, function, and general appearance. Importantly, even the smallest details can affect the smile attractiveness.
Both the smile and facial attractiveness constitute important aesthetic and social factors, 1 because attractiveness increases the social interactions and develops the personality. 1,2 The seek for facial perfection augments the necessity of studying aesthetic perceptions, and the use of digital programs helps to achieve more satisfactory results for patients. 3,4 In health sciences, the construction of the values and meaning of the corporal aesthetic is receiving increased interest, influencing individual's identity construction and self-perception. 5 Knowledge of the patient's psychological, anatomical, and functional needs can lead to better detection of changes or defects that may alter the aesthetic perceptions of the smile and identify the existing problems, in order to improve the aesthetic outcomes of orthodontic treatment and increase the quality of life of the patients. 5,6 Aesthetic perception is related to educational, cultural, socioeconomical and emotional contexts. Importantly, studies have shown that perception differs between orthodontists, nonorthodontists (i.e., dentists), and laypeople. 7 Al-lahham A, Souza PHC, Miyoshi CS, Ignácio SA, Meira TM, Tanaka OM -An eye-tracking and visual analogue scale attractiveness evaluation of black space between the maxillary central incisors Eye-tracking has been used in visual perception investigations for a long time. 8 The technique has been continuously refined since the introduction of the first eye-tracking machine. 9 Black space is also known by the terms "black triangle" or "gap" and can be the result of: inclination of the maxillary central incisors in the mesial or distal direction; bone loss; triangular formation of the maxillary central incisors; or lesions associated with plaque, trauma, or tooth loss. 9-11 The existence of these spaces can alter the smile aesthetic, although the degree of impact depends on the self-evaluation of the patient. 9,11 A study demonstrated that in 98% of cases the interdental papilla is considered complete when the distance between the alveolar crest and the area of contact between the maxillary central incisors is equal to or less than 5 mm. With a distance of 6 mm, the papilla is considered complete in 56% of cases and this number decreases in the case of a 7-mm distance (27% of the cases). 12 Previous studies with an objective of establishing a relation between the black space and smile attractiveness have confirmed that the black space has a negative repercussion on the dental aesthetic. 13 In addition, young people are more capable of detecting the black space, ranking the smile as less attractive when greater black space is present. 7 Al-lahham A, Souza PHC, Miyoshi CS, Ignácio SA, Meira TM, Tanaka OM -An eye-tracking and visual analogue scale attractiveness evaluation of black space between the maxillary central incisors Eye-tracking is a trusted technique used to study aesthetic visual perception. 14 Attractive judgment using a visual analogue scale (VAS) is also a simple and objective method for evaluating aesthetic perceptions and for helping to compare results between groups of raters. 15 Therefore, the aim of the present study was to evaluate the effects of different sizes and magnitudes of black space between the maxillary central incisors in both sexes, with regard to aesthetic perception, by using an attractive assessment VAS and eye-tracking technique.

This study was approved by the Committee of Human Ethics and
Research of Pontifícia Universidade Católica do Paraná (# 2.235.302).
Photos of individuals of both sexes were used, excluding those with characteristics that alter the visual attention such as beards, tattoos, exaggerated makeup or exotic hairstyles.
Facial and intraoral photographs were obtained using a Canon XT camera (Canon Inc., Tokyo, Japan), 50mm Sigma macro lens and Sigma flash. All photographs were obtained in a proper studio, with a white background.
High-resolution photographs were selected by three experienced orthodontists. The aspects of normality regarding symmetry, volume and color were observed. The images were edited Subsequently, the individuals' actual smiles were excluded and another smile with better occlusion was inserted, to reduce the bias of the visual attention, because the original smiles were not symmetrical and the malocclusion could alter the visual attention and reduce the aesthetic perception. The aim of this study was to evaluate only the impact of the black spaces and its effect on the aesthetic perception. 14 The black space was copied from a real photograph of a patient with black space, and then readapted to the study images. 13 Black space between the central incisors was created, for both sexes, as follows:, Control, 1-mm black space, 2-mm black space and 3-mm black space (Fig 1).  After eye-tracking test, the same images were presented printed and organized in the same order, so that the raters could assess the attractiveness according to the VAS from zero to 10 points, with closer to zero points being less attractive, and closer to 10 points being more attractive.

STATISTICAL ANALYSIS
The obtained results from eye-tracking and VAS were tabulated  were female and 48% were male (n = 43).

RESULTS
The results of the eye-tracking showed that the mouth, right eye, and left eye were the most captured areas.
In relation to the time to first fixation and number of fixations of the mouth, dentists and orthodontists presented lower values than laypeople (p > 0.05) and there was no difference between nonorthodontists and orthodontists (p > 0.05) (Tables 1 and 2).
In relation to attractive judgment based on the VAS, it was observed that the aesthetic perception decreased as the magnitude of the black space increased and that the highest notes were given for the images without any smile problems (Table 3). Regarding the results of the heatmaps, it was shown that laypeople looked more to the eyes, in comparison to nonorthodontists and orthodontists, while the concentration of the gaze on the mouth increased as black spaces of 1, 2, and 3 mm appeared (Figs 3 and 4).

Al-lahham
For nonorthodontists and orthodontists, the heatmaps demonstrated that the concentration on the mouth was greater than for laypeople, regardless of the magnification of black spaces in both sexes, mainly for orthodontists (Figs 3 and 4).
The scanpaths and the heatmaps revealed that the rater eye focus was correspondent to the size of the black space, especially for orthodontists. Upon evaluating the scanpaths of the control images, the eyes and the mouth were deemed to be the regions with greater focus.     However, the concentration toward the mouth was increased as black spaces were inserted in the photographs, suggesting the worsening of the aesthetics, confirmed by the evaluations through the VAS. Other studies that used eye-tracking also observed the same pattern of visualization between the eyes and mouth, and an increased concentration on the mouth region when black spaces were added to the smile. 14,21 Besides that, it was also demonstrated that, in relation to time to first fixation in the mouth, there was a statistically significant difference between laypeople and nonorthodontists and between laypeople and orthodontists. As the area of concern for these professionals is the mouth, it is natural that they looked faster to the smile. However, other studies using eye-tracking have confirmed that time to first fixation may not be a very reliable measure. 14,22,23 Al-lahham A, Souza PHC, Miyoshi CS, Ignácio SA, Meira TM, Tanaka OM -An eye-tracking and visual analogue scale attractiveness evaluation of black space between the maxillary central incisors

Al-lahham
In the present study, orthodontists showed a higher attention to the mouth in the presence or absence of black space, regardless of rater sex, and were also more critical in the evaluation through VAS. These results are in agreement with other studies of perception, in which orthodontists perceived defects of a smaller magnitude when compared to other groups of raters. 11,24,25 The present results can be expected then, since these professionals are trained to diagnose several types of occlusal abnormalities involving aesthetics and function.
A perfect smile has a positive repercussion on the dental aesthetics. 26 Comparing the three groups, it can be seen with the The raters focused the eyes more than the mouth in both sexes in the control images. However for the images with 1mm of black space, the focus shifted to the mouth for the male patient.
Separately, in images of 2 mm and 3 mm of black space, the concentration of the raters was on the mouth in both sexes.