A cross-sectional study was carried out to find the conjugated effect of the smile arc and buccal corridors on attractiveness of face, as evaluated by orthodontists, general dentist and laypersons. After obtaining the ethical approval from the ethical committee of the institution, study was carried out in the Department of Orthodontics and Dentofacial Orthopaedics, Bagalkot, Karnataka, India. The sample consisted of a male and a female subject who satisfied the eligibility criteria. Those subjects between the age of 18 and 25 years, with ideally aligned teeth and no history of orthodontic treatment and with ideal smile arc and minimal buccal corridors and who were willing to participate and gave a written consent as a participant, were selected for the study. The photographs were shot with Nikon D-40 digital camera (Nikon corporation, Japan) with Lens ED-18-55mm. Standardization of both the photographs were taken in auto mode of the camera with uniform distance from the subject. The captured images were transferred to a personal computer and image processing software (Adobe Photoshop 7.0) was used to digitally alter the images.

Source of data: Two subjects (one male & one female) were selected from the regional population fulfilling the criteria of an ideal smile arc and ideal buccal corridor space. Smile view photographs of these subjects were taken. Two photographs were modified by using Adobe Photoshop 7.0 to create combination of three smile arc variance (flat, ideal, excessive) and three buccal corridor variations (none, ideal, excessive) respectively, thus, producing nine male & nine female images [Table/Fig-1,2]. These 18 images of the modified smiles were made and shown to 25 orthodontists, 25 general dentists & 25 laypersons. The altered images were arranged randomly for slide show using Microsoft Power Point 2007 and presented to evaluators comprising of 25 orthodontists, 25 dentists and 25 laypersons in separate sessions, (each image shown for 10 seconds) and the evaluators were not informed about the alterations made in the images [Table/Fig-3].

[Table/Fig-1]:

Showing altered images with flat smile arc and normal buccal corridors space of female and male respectively.

[Table/Fig-2]:

Showing altered images with excessive smile arc and buccal corridors space.

[Table/Fig-3]:

presentation of slides to evaluators.

Evaluators were provided with a rating sheet and asked to score the attractiveness of each image on a numerical scale of 1 to 10, with 1 being the least attractive and 10 the most attractive. The rating sheets from these evaluators were collected and data was statistically analysed using One-way ANOVA and regression analysis.

Mean, standard deviation and analysis of variance test (ANOVA) revealed that for all photographs, there was statistically highly significant difference among the laypersons, dentists and orthodontists except for ideal SA(smile arc), excessive BC (buccal corridors) {F} image in which no statistically significant difference was found [Table/Fig-4]. Correlation coefficient was done to find the correlation between the smile arc and buccal corridor scores with the ratings of smile arc (-0.9292, -0.5448 and -0.0316 for orthodontists, dentists and laypersons) and buccal corridor (-0.8857,-0.3499,-0.0988 for orthodontists, dentists and laypersons) on facial attractiveness with orthodontist showing the highest, i.e., as the smile arc steepened and buccal corridor increased, the ratings decreased among all the three groups [Table/Fig-5]. Regression analysis done to know the combined influence of the smile arc and buccal corridor on the evaluators in each group, depicted negative correlation between the facial attractiveness and the smile arc and buccal corridor, i.e., as the smile arc steepened and buccal corridors increased, the ratings decreased across all the three groups. (Orthodontists- reg coefficient of - 5.5018, -2.0361, Dentists – reg coefficient of -8.2447,-4.9806 and laypersons – reg coefficient of -2.7531, -3.3418). The correlation was statistically significant for the orthodontist group) ‘p’ value of 0.006-SA and 0.1450-BC) and dentist group (p-value of 0.0042-SA and 0.0638-BC). But the correlation was not statistically significant for laypersons (p-value of 0.5381 for smile arc and 0.4508 for buccal corridor) [Table/Fig-6,7 and 8].

Smile is one of the most primordial forms of facial expression and plays a vital role in enhancing the personal attractiveness. In recent times, due to ever advancing technology, ways to achieve an ideal and alluring smile has undergone paradigm shift. Various parameters like: the smile arc, dental alignment, tooth colour and shape, incisal edge regularity, amount of incisor and gum display, and buccal corridor has been identified as important components of smile [4–6]. The smile arc is defined as the relationship of the contour of the incisal edges of the maxillary anterior teeth relative to the curvature of lower lip during a social smile [7].

On the basis of this relationship, smile lines have been delineated into three types. Consonant smile arc- has the curvature of incisal edges of the maxillary anterior teeth parallel to the upper border of the lower lip [8]. Straight smile arc is that in which the incisal edges of the maxillary anterior teeth are in a straight line to the upper border of the lower lip. Reverse or non-consonant smile arc is the one in which the incisal edges of the maxillary anterior teeth are curved in reverse to the upper border of the lower lip [9,10]. Reverse smile arc occurs when the central incisors are shorter than the canines along the incisal plane which can be due to occlusal malfunction or loss of vertical dimension [11].

Reverse smile profile was less aesthetic than parallel and straight smiles. Since the smile arc depends upon occlusal plane inclination and second order crown angulations in the upper anterior teeth, there are some constraints to the achievement of this ideal smile arc on every patient. A feasible objective is to prevent a flat or reverse smile arc and to obtain some degree of curvature that resembles, one found in the lower lip [12].

Hulsey, the first to quantify the smile arc, disclosed that orthodontically treated patients had lower smile ratios (i.e., flatter smile arcs) than untreated patients [1]. Another conceit-the buccal corridor spaces, given by Frush and Fisher [10] emerged during the 50’s in concern with ensuring natural looking dentures. This component of smile aesthetics, also known as lateral dark space, lateral negative space or “shadow tunnel,” consists of the existing dynamic space that appears, when a person smiles, between the labial surface of maxillary posterior teeth and the inner mucosa of the soft tissues that form the corners of the mouth and the cheeks. This space arises from the dark background of the mouth, and depends on the shape and width of the upper dental arch and the facial muscles responsible for the breadth of the smile [13]. Inspite of little information on the ideal buccal corridor size, available in the literature, the scientific studies that addressed this issue have revealed controversial disclosures [14]. On the contrary, Hulsey found that buccal corridor spaces did not contribute significantly to smile aesthetics [1]. Some of the parameters that have been studied by various authors have been mentioned in the [15–17] [Table/Fig-9] much disparity, as revealed by past research [13,18,19]. Since, there are very few Indian studies reported in such context; the mentioned study was undertaken to evaluate such a response from general dentists, orthodontist and lay persons. The subjects between the ages of 18 to 25 years were chosen so that the nature’s compensation in the development of soft tissue could be studied.

The main criterion for selecting the samples was that they should have acceptable smiles and not have undergone any kind of orthodontic treatment, so that none of the naturally compensating smile parameters would have been altered. Digital photographs of the samples were modified using the commercially available Adobe Photoshop software-Version 7, as this software offers a free hand manipulation of the facial features in a very simple way and has been built in tools to match the colour and tone of the tooth, gingiva, mucosa, lip and skin [3,20,21].

Regression analysis showed that the correlation was negative to the facial attractiveness across all the three groups. As the smile arc and buccal corridor increased, the scores decreased. The correlation was statistically significant for the orthodontist group and dentist group. But the correlation was not statistically significant for laypersons. These findings suggest that orthodontists and dentists were more precise in judging the smile arc and buccal corridors as compared to laypersons. The difference in opinions could again be attributed to the professional training of the vocations involved in assessing a face. Synonymous results were obtained for evaluation of buccal corridor in the study by Sabrina Elisa Zange et al., and Huma Kiania et al., also revealed same results between orthodontist and laypersons [22,23]. While a study by Farhan Zaib et al., revealed contrast results to the mentioned study with regard to buccal corridor [18].

With relation to smile aesthetics, no difference in rating was found between the orthodontist and laypersons [24]. This study found that the highest ratings were given to that image where in lower lip was just touching and parallel to the incisal edges of upper incisors i.e., ideal SA by orthodontists, dentists and laypersons. This is in agreement with the previous studies [1,8,25–27]. But some studies concluded that ‘the patients whose lower lips touched or did not touch the incisal edges had a higher aesthetic score than those whose incisal edges were slightly covered’, was not in accordance with our findings [28,29], because the samples where lower lip was just touching and parallel to the incisal edges of upper incisors i.e., ideal SA were given highest rating by orthodontists, dentists and layperson. All the evaluators in the present study agreed that minimal buccal corridors are essential for a pleasing smile. This observation was in concordance with the research conducted by Moore, Parekh SM, Fields HW, Beck M & Rosenstiel S and Dunn et al., [3,13,30]. However, studies done by Jeryl English, Krishnan V, Daniel S, Lazer D and Asok A were not in congruence with the present study [31].

Clinical implications: The concept of smile analysis should be incorporated in orthodontic treatment planning. It is therefore emphasized that all the above discussed elements of smile analysis should be considered as reference points for beginning aesthetic evaluation, treatment planning and subsequent orthodontic treatment. The findings of these studies should be applied with caution, taking into account, in particular, the individual characteristics of each patient and their aesthetic expectations.

This information becomes a vital tool for orthodontist, as it will aid in a more coherent decision on the necessity of treatment of buccal corridor and smile arc along with the patient’s complaint for misalignment.

Future implications: Ever increasing technology may create software in future, which will create the ideal smile image of an individual and also will provide the orthodontist and the dentist - the mechanisms and restorative treatment to achieve the same.

Vitally, all the three groups tended to agree that, as the smile arc and buccal corridor increased, the facial attractiveness decreased. Orthodontists were more precise in discerning the smile arc and buccal corridor compared to dentists and laymen. Thus, it can be concluded that everyone has got different perceptions and it varies from professional to individual. Hence, a detailed clinical examination of smile parameters should be included in the orthodontic protocol before planning any orthodontic treatment.