Introduction
The purpose of this study was to evaluate the influence of the size of buccal corridors on the assessment of attractive smile esthetics by Japanese and Korean orthodontists and orthodontic patients.
Methods
Buccal corridors were modified digitally from 0% to 25% compared with the inner intercommissural width. Using a visual analog scale, 41 Japanese and 25 Korean orthodontists, and 96 Japanese and 72 Korean orthodontic patients (15-29 years old) rated the attractiveness of 6 smiles with altered buccal corridors. The influence of the size of the buccal corridor on smile esthetics was assessed.
Results
Although there was no significant sex difference in judging the effects of buccal corridors for the Korean and Japanese orthodontists, significant sex differences were shown for the Japanese orthodontic patients. There were significant differences in the 6 levels of median esthetic scores for all groups: 0%, 5%, and 10% buccal corridors in the smiles tended to be preferred to the other smiles, to become clinically significant for all groups. The median esthetic score decreased to become clinically significant from 10% to 15% buccal corridors for all groups.
Conclusions
Japanese and Korean orthodontists and orthodontic patients prefer narrow or medium buccal corridors to broad buccal corridors.
The smile plays an important role in facial expression. It influences a person’s perceived attractiveness and is the cornerstone of social interaction. People mainly focus on another person’s eyes and mouth during interpersonal interactions. Goldstein stated that the smile ranks second only to the eyes as the most important feature in facial attractiveness. On the other hand, Havens et al reported that tooth alignment is a more important factor than the eyes for evaluating facial esthetics. Therefore, contemporary orthodontists must consider esthetic smiles by managing the dentition and soft tissues. In clinical orthodontics, patient-driven esthetic diagnosis and treatment planning have become important. Thus, smile analysis has become an essential element of diagnosis and treatment planning.
Buccal corridors are the spaces between the facial surfaces of the maxillary teeth and the inner commissures. There seems to be a difference of opinion among investigators about the esthetic value of buccal corridors. Several researchers have reported that buccal corridor minimization is a critical smile feature, whereas others concluded that buccal corridors have little impact on smile esthetics. The preference of the amount of buccal corridors might be influenced by the regional differences of raters, who have a variety of cultural and ethnic backgrounds. A few researchers have stated that demographic, cultural, and ethnic backgrounds affect a layperson’s perceptions of esthetic and smile characteristics. However, little is known about how cultural and ethnic differences influence esthetic perceptions.
Koreans and Japanese stem from the same racial group and have similar cultural backgrounds. Moreover, at the present time, young adults experience greater exposure to mass media through television, movies, the Internet, and travel. The perception of a pleasing facial profile might differ between the 2 countries or be converging on a more internationally pleasing one. In light of these facts, in this study, we aimed to evaluate the influence of the size of buccal corridors on smile esthetics as assessed by Japanese and Korean orthodontists as well as orthodontic patients.
It is important to evaluate the perceptions of dentofacial esthetics by laypeople or orthodontic patients when an orthodontic treatment plan is determined. In general, orthodontists tend to be more critical of dental esthetics than do laypeople when a dental disharmony is small. On the other hand, Phillips et al reported that trainee orthodontists gave higher scores when rating overall dentofacial attractiveness than did dental undergraduates and laypeople. In our previous studies investigating attractive anteroposterior lip positions in Japanese subjects, both the orthodontists and the orthodontic patients showed similar tendencies in their preferences. As of yet, no report has compared the preferences for buccal corridors between orthodontists and orthodontic patients for Japanese and Korean people.
The overall purpose of this study was to identify the threshold where the size of the buccal corridors begins to diminish dentofacial esthetics in the smiles as assessed by Japanese and Korean orthodontists and orthodontic patients. The hypotheses were that (1) the amounts of buccal corridor affect the assessments of smile esthetics, (2) there are no significant differences in the assessment of smile esthetics between Japanese and Korean people, (3) there are no significant differences in the assessment of smile esthetics between orthodontists and orthodontic patients, and (4) a distinct threshold would exist when the effects of the buccal corridors on the smile are evaluated.
Material and methods
This study was performed in accordance with the guidelines of the Helsinki Declaration revised in Seoul (2008). The research protocol was approved by the institutional review board for clinical research of Kyushu University, Fukuoka, Japan.
A sample size calculation was undertaken by using nQuery Adviser (version 6.01; Statistical Solutions, Cork, Ireland). According to our pilot study, the effect size was estimated at 0.95. On the basis of a significance level of alpha 0.05, the sample size was calculated to achieve 80% power. The sample size calculation showed that 21 subjects for each group were necessary.
One frontal intraoral photograph of a woman with ideally aligned teeth and 1 extraoral photograph of a woman displaying esthetically smiling lips were obtained from different persons. These ideally aligned teeth and lips were combined to form a standard composite smile with all teeth shown to the first molars. The lower lip coincided with the curvature of the incisal edges of the maxillary incisors and canines so that the smile arc was consonant. These images were modified (CS4; Adobe Photoshop, San Jose, Calif) to create bilaterally symmetrical teeth and lips. The amount of buccal corridor was calculated as the difference between the inner intercommissural width and the visible maxillary dentition width divided by the inner intercommissural width. The ratio was reported as a percentage. As the visible maxillary dentition width increases, the buccal corridor decreases, and it would result in narrow buccal corridors. Six sizes of buccal corridors were created: narrow (0%), medium-narrow (5%), medium (10%), medium-broad (15%), broad (20%), and extrabroad (25%). Six images were arranged in the order of the amount of buccal corridor spaces and displayed on size A-4 paper ( Fig 1 ).
The smile raters were 41 Japanese orthodontists (18 men, 23 women; ages, 31.5 ± 7.2 years); 96 Japanese orthodontic patients (36 males, 60 females; age range, 15-29 years; SD, 21.5 ± 3.8 years) from Kyushu University Hospital in Fukuoka, Japan; 25 Korean orthodontists (12 men, 13 women; ages, 27.9 ± 2.3 years); and 72 Korean orthodontic patients (33 males, 39 females; age range, 15-29 years; SD, 22.2 ± 3.2 years) from Pusan National University Dental Hospital in Pusan, Korea. The determination of the subjective esthetic value of each smile was accomplished by using a visual analog scale. This rating scale was designed for minimal constraints and the most freedom to express a personal response style. The visual analog scale was 50 mm long, and the raters used their own esthetic values to rank each smile from “least attractive” to “most attractive.” An esthetic score was obtained by multiplying the distance between the least attractive (0) and the hash mark by 2. The esthetic scores were distributed from 0 to 100, with 0 the minimum and 100 the maximum esthetic values.
Ten randomly selected raters from each of the Japanese and Korean orthodontists and orthodontic patients were asked to evaluate 6 images twice at least 2 weeks later to determine reliability. Intraclass correlation coefficients were used for determining intrarater agreement. High levels of reliability were found, since all intraclass correlation coefficients were greater than or equal to 0.7.
Statistical analysis
To compare the distributions of the median scores between the male and female raters of each rater group, the Mann-Whitney U test was used. Differences in the median esthetic scores in the 6 levels of buccal corridor were analyzed by using the Kruskal-Wallis test. The minimum level of statistical significance was set at P <0.05.
Visual analog scales have been used for pain research, and generally a minimum clinically significant difference ranges from 9% to 13% of the scale. Parekh et al applied a 15% visual analog scale difference as a clinically significant difference to differentiate the smile esthetics. We also used a 15% visual analog scale difference to determine the clinical significance of the esthetic scores.
Results
There was no significant difference in judging the effects of buccal corridors on smile attractiveness between the men and women for both the Japanese and Korean orthodontists. Therefore, the pooled data of male and female raters for Japanese and Korean orthodontists were used for the following analysis. The median values and ranges of the esthetic scores of each buccal corridor space for the Japanese and Korean orthodontists are shown in the Table and Figure 2 . For the Japanese orthodontists, the median esthetic scores increased gradually from 0% to 10% buccal corridors and then decreased to become clinically significant (15% visual analog scale score difference) from 10% to 25% buccal corridors. For the Korean orthodontists, the median esthetic scores increased to become clinically significant from 0% to 5% buccal corridors and then decreased from 5% to 25% buccal corridors; particularly, the scores decreased clinically significantly from 10% to 25% buccal corridor. Moreover, no clinically significant difference was observed in the median visual analog scale scores between the Japanese and Korean orthodontists for each level of buccal corridor.
| Raters | Buccal corridor space level | ||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| 0% | 5% | 10% | 15% | 20% | 25% | Kruskal-Wallis test | |||||||||||||
| Median | P25 | P75 | Median | P25 | P75 | Median | P25 | P75 | Median | P25 | P75 | Median | P25 | P75 | Median | P25 | P75 | ||
| Japanese orthodontists | 74.5 | 53.1 | 87.8 | 78.7 | 57.7 | 88.7 | 82.0 | 43.1 | 96.2 | 44.7 | 22.5 | 57.2 | 23.3 | 9.8 | 32.1 | 6.0 | 2.3 | 11.6 | <0.0001 |
| Korean orthodontists | 63.4 | 33.9 | 81.7 | 89.6 | 69.2 | 99.4 | 74.1 | 61.4 | 90.2 | 55.7 | 34.8 | 70.1 | 25.6 | 9.8 | 41.2 | 3.2 | 0.0 | 6.3 | <0.0001 |
| Japanese orthodontic patients | |||||||||||||||||||
| Male | 73.3 | 64.2 | 86.0 | 82.2 | 62.4 | 94.0 | 75.6 | 52.4 | 88.3 | 52.0 | 33.5 | 75.3 | 35.1 | 22.0 | 57.1 | 18.7 | 7.3 | 50.7 | <0.0001 |
| Female | 82.1 | 59.2 | 95.4 | 78.6 | 61.1 | 88.6 | 71.4 | 52.1 | 84.2 | 41.1 | 26.3 | 54.2 | 26.3 | 11.3 | 46.3 | 8.8 | 0.0 | 29.9 | <0.0001 |
| Korean orthodontic patients | |||||||||||||||||||
| Male | 79.2 | 50.5 | 91.6 | 86.1 | 71.2 | 93.3 | 73.1 | 52.4 | 83.2 | 40.9 | 24.6 | 58.1 | 27.7 | 13.2 | 43.4 | 11.6 | 4.2 | 24.4 | <0.0001 |
| Female | 76.3 | 42.4 | 93.1 | 79.5 | 51.1 | 93.0 | 70.4 | 56.1 | 83.9 | 53.1 | 29.2 | 79.5 | 21.4 | 13.8 | 37.8 | 6.2 | 1.8 | 12.0 | <0.0001 |
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