INTRODUCTION

The importance of an attractive smile in dentofacial esthetics and the implication that any alteration can provoke anxiety both socially and psychologically has been much debated. Studies indicate that individuals with an attractive facial appearance are more socially acceptable, more desirable as friends and partners, and are able to enter the labor market more easily.^1,2 Consequently, there are more patients of different ages and social backgrounds with a desire to improve their image and the smile is commonly the main target.

The level of esthetic perception among the dentofacial features, especially the smile, is perceived in different ways; dental professionals, especially orthodontists, are much more critically perceptive than laypersons.³ This is important, since the esthetic objectives of professionals involved in the treatment are not always in line with the goals of the patients.

Although there are various features in the literature used to describe smile esthetics, upper central incisors are the most prominent. The vertical positioning,^3–5 the width/height ratio,^3,6 and the presence of gingival and incisal asymmetries between them^3–5,7 are the main characteristics that determine an attractive smile. Among these, symmetry between the edges of the upper central incisors is perhaps the most important since even slight asymmetries (0.5 mm) can be detected by orthodontists and laypersons.^5,7

The studies of smile esthetic perception found in the literature normally use digital simulations in photographs in a close-up smile view^5,7–9 or full-face view.^5,10,11 Although photography is the most common tool used to evaluate the various variables that might influence smile esthetics, video shooting allows a dynamic recording of the whole process because it is a more natural and reliable methodological tool.^12–14

The aim of this study was to investigate the influence of asymmetries of the maxillary central incisors on dentofacial esthetics among orthodontists, prosthodontists, and laypersons, using a novel methodology with a video analysis.

MATERIALS AND METHODS

This study was approved by the Research Ethics Committee of the Dental School of the Federal University of Bahia, registered under No. 1,155,661. All the participants of this study signed a free and informed consent form.

A pilot study with 15 subjects in each group was carried out to calculate the number of evaluators needed using the software R (www.r-project.org, version 3.3.2). The calculation was based on an alpha significance level of 0.05 and a sample effect of 0.80, and with the goal to achieve a power of 80%.^{2,4,5,7,8,10,11} The result showed that 46 individuals were needed for each of the three groups of evaluators.

Full-face films of a 52-year-old afro-descendant woman with extensive incisal wear in the esthetic zone and a midline diastema of 1.5 mm were made for this study. Despite these dental alterations in the maxillary anterior region, she had good alignment of the upper and lower dental arches as well as a harmonic face (Figure 1). It is important to clarify that the extensive incisal wear in the anterior maxillary teeth was the main feature in subject selection. This condition allowed performance of an ideal mockup, with the ability to make progressive tooth reductions without injuring healthy dental tissue.

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A diagnostic mockup of the esthetic zone was made with bisacrylic resin, in color A2 (3M ESPE, St Paul, MN, USA) (Figure 2). The mockup was made considering some characteristics of an attractive smile such as: adequate width/height proportion in the esthetic zone, convexity of the smile arc, a 2 mm step between the central and lateral incisors, no gingival exposure, and a progressive increase of the incisal embrasures from the incisors to the canines.^6,15–18

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The facial filming was obtained frontally with the individual standing with the Frankfort horizontal plane and the bipupilar line parallel to the ground and the median sagittal plane in the direction of the operator. The digital photographic equipment used was a Canon EOS Rebel T5i (Canon Inc., Taiwan, China), with an LED flashlight (Neewer 160 Led CN-160 Dimmable Ultra High Power Panel Digital Camera; Neewer, Shenzhen, China) attached to a tripod. The distance between the individual and the camera was standardized at 0.90 m.¹⁴ under the same lighting conditions and resolution in full HD (1,080i) for all films.

The individual was instructed to repeat a sentence that caused different amounts of the upper anterior teeth to be exposed during filming.¹⁴ The films began with the patient in a neutral rest position and ended in a posed smile position. Each film lasted 7 seconds. When evaluated by the examiners, the soundtrack was turned off to not cause any interference with the esthetic evaluation.

The exposure of the incisors varied during the sentence depending on the words being spoken; for example: lips at rest, no exposure; on pronouncing the syllable “ma” in the word “ema,” there was low exposure of the upper incisor; on pronouncing the syllable “Cze” in the word Czechoslovakia, there was high exposure of the upper and lower incisors; and, finally, the posed smile exposed the upper incisors in full¹⁴ (Figure 3).

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The first film was made using the bisacrylic resin mockup, with symmetry between the upper central incisal edges, and was named the control film. Then, the upper right central incisor underwent four 0.5 mm reductions along the incisal edge until reaching an asymmetry of 2.0 mm with its homologue. The step-by-step changes in this mock-up yielded another four films: asymmetry of 0.5 mm, 1.0 mm, 1.5 mm, and 2.0 mm, as shown in Figure 4.

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The films were edited in Quicktime Pro (Apple, Cupertino, CA) to remove excesses at the beginning and end of the film and standardize the brightness, contrast, and color. After editing, all films had, as the upper limit, the region just above the eyebrows and, as the lower limit, the tip of the chin. To guarantee the calibration and magnification of the images, the width of the right upper central incisor, previously measured in the mouth, was used. This was recorded and transferred to the tablet screen to maintain a 1:1 ratio.

A total of 138 examiners evaluated the images: 46 orthodontists (22 men, 24 women), 46 prosthodontists (25 men, 21 women), and 46 laypersons (20 men, 26 women) with a college education but no dental background.^4,5,7,10 The films were watched by the examiners in a random order as determined by Random software (random.org).

The films were copied into Keynote presentation software (version 7.2, Apple) and shown to the examiners on a tablet (iPad Pro 9.7-inch, Apple) in a quiet setting where only the examiner and the researcher were present. Prior to showing the films, the examiners were instructed by the researcher to evaluate the dentofacial esthetics of the individual in each film. Additionally, they were advised that they would be allowed to watch each film only once.

The evaluators positioned themselves comfortably 1 meter from the tablet with the bipupilar line parallel to the screen. The researcher showed each video individually. The examiners were given 5 seconds after the film was shown to evaluate and then the researcher would automatically switch to the next film.

The level of attractiveness given for each film was recorded on a form with visual analog scales, one for each film.^9,19,20–22 The scale ranged from “very unattractive” on the far left to “very attractive” on the far right. A 10 cm dotted line with a center mark joined the two ends. The examiners were instructed to mark their perception of the dentofacial esthetics on the dotted line for each of the five films. The distance between the mark made by the examiner and the leftmost point was measured with an electronic digital caliper (Starrett, Suzhou, China).

To evaluate the reliability of the method, 2 months after the initial evaluation, 15 evaluators from each group were randomly selected and requested to evaluate the films once more. The difference this time was that two films were identical. The intraclass test was used to compare the scores of these films to determine the intra-evaluator agreement. The concordance index was high, showing a coefficient greater than or equal to 0.70 for the three groups of evaluators: 0.81 for the orthodontists, 0.70 for the prosthodontists, and 0.71 for the laypersons.

The data were statistically analyzed with software R (www.r-project.org, version 3.3.2). Initially, the Shapiro-Wilk test was applied to verify the distribution normality of the sample. The analysis of variance test and Tukey's posttest were then used to compare the groups of evaluators and to compare the perceptions of the different groups of examiners. The level of significance was established at 5%.

RESULTS

The highest scores from all three examiner groups were for the film without any asymmetry and the one with only 0.5 mm asymmetry, with no significant difference between them. The films with the lowest scores were those with asymmetry of 1.5 and 2.0 mm for all groups (Table 1).

Table 1 Evaluation of Asymmetry Perception Between the Upper Central Incisors by Orthodontists, Laypersons and Prosthodontists

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The comparison of the scores of the three groups showed that the laypersons gave higher scores than the orthodontists for the films with the greater asymmetries (1.5 and 2.0 mm) (Table 1).

DISCUSSION

Upper central incisors are of great importance when evaluating smile esthetics. Various studies have been carried out to establish smile esthetics parameters. However, the methodology employed in all of these studies involved the use of photographs as the main tool of the evaluation process.^{3–5,7,10,23–25} Unique from previous studies, the present work used video analysis to investigate smile esthetics perception.

The advantage of this method over photographs was that it enabled examiners to carry out a dynamic evaluation of the smile, considering the interaction between teeth and lips, and during facial movements. Dynamic smile registration through video was previously shown to provide more reliable and informative data to the clinician.^26,27

Another aspect of the present study was that it used a widely used resource in prosthodontics: a mockup.^28,29 This important diagnostic tool allowed clinical simulation of the asymmetries between upper central incisors. The significant incisal wear of the upper central incisors in the individual under investigation was a prime motive to use a mockup. With this tool, it was possible to simulate different levels of asymmetries between the centrals by wearing down one of the acrylic teeth without touching healthy dental structure. Such a procedure would be impossible in an individual with healthy upper incisors.

The results demonstrated that among orthodontists, prosthodontists, and laypersons, there was a tolerance for asymmetries of up to 0.5 mm between the central incisor edges, whereas from 1.0 mm on, the perception of esthetics was decreased. The least attractive films were the ones with wear of 1.5 mm and 2.0 mm. In the study by Ribeiro et al.⁵ where the authors analyzed the smile of two adult males, one white and one afro-descendant, aged between 25 and 30 years old, there was no tolerance for asymmetries between the central incisor edges; the worst scores were for asymmetries between 1 and 1.5 mm.⁵ Similar results were found by Machado et al.⁷ These authors analyzed the smile of two adult women, one white, and the other afro-descendant, aged between 20 and 30 years old.⁷ The difference in tolerances for incisal edge asymmetries between the previous literature^5,7 compared with the 0.5 mm tolerance found in the current study may be explained by the different methodologies used. Previous studies^5,7 used photographs while the current evaluation used video. The dynamic movement of the lips, eyes, as well as the presence of other elements such as the nose and hair could possibly distract the attention of the examiner from some specific details of the teeth; however, in a photo analysis, small differences might be more evident.

From a clinical standpoint, the video analysis showed that the need to establish symmetry between the upper central incisors is of fundamental importance. It reinforced the dominance of these teeth in defining smile esthetics. However, to obtain such symmetry between the upper centrals, a thorough clinical evaluation is required to recognize the presence of incisal edge wear and or the details of gingival design. Ultimately, multidisciplinary treatment might be necessary including restorative procedures and/or orthodontics to achieve optimum symmetry between the upper central incisors.^3,4,7,21

The examiners consisted of orthodontists, prosthodontists, and laypersons. Orthodontists are cited in previous studies as considered more thorough in detecting alterations.^3,9,19,22,23 The prosthodontists were included since, along with orthodontists, they are the specialists who deal more directly with esthetic rehabilitation. The laypersons represented the universe of people without specific technical knowledge; however, they were perhaps the most important group since their opinion is the guiding force for change in the standards of esthetic evaluation.³⁰

The results showed that orthodontists were more critical in their evaluation for the assessments of the least attractive smiles.^{4,5,7,10,11,25} Another interesting aspect was that there were no statistically significant differences in ratings by the three groups for the most attractive smiles.^5,7,10 It can be hypothesized that an ideal smile arrangement can easily be recognized as attractive by any group of raters.⁷

It is necessary to bear in mind that the results found in this research regarding the impact of the asymmetries between the central incisor edges should be approached with care. This is because it was a study that used films from only one 52-year-old female individual, only three groups of examiners and the values found were converted into averages. Also the findings here should be considered much more as a guideline, rather than something that should be imposed rigidly on individual patients. On the other hand, the opinion and participation of patients in esthetic planning is fundamental, considering that the concepts of esthetics are subjective and subject to individual interpretation.

The development of future studies with this novel methodology may provide further details regarding the parameters used in the evaluation of smile esthetics, supporting standards that are already established or establishing new frontiers in the field of dentofacial esthetics.

CONCLUSIONS

• The presence of an incisal edge asymmetry as little as 1.0 mm between the maxillary central incisors negatively influenced the perception of dentofacial esthetics as evaluated by orthodontists, prosthodontists, and laypersons.

• In general, there were no statistically significant differences between the three groups of evaluators, except for the films showing greater asymmetry (1.5 and 2.0 mm), where the laypersons gave higher scores than orthodontists.