Abstract
Abstract Background
/purpose: Several factors such as identity, income, and age potentially associated with smile perceptions. This study aimed to identify the factors affecting the smile esthetic perception in different identities (layperson, general dentist and orthodontist) and to detect the extent of their association with smile perception.
Materials and methods
Extraoral photographs in frontal, lateral, and three-quarter views were shot and adjusted on Adobe Photoshop into 95 smile photographs with different smile patterns. Based on these photographs, the investigators were asked to fill the online questionnaire. Pearson chi-square test and logistic regression were used for statistical analyses.
Results
Identity, gender, age, and treatment experience were noted to affect smile esthetic perception. In addition, the perception of smile esthetics was significantly different among frontal, lateral, and three-quarters views regarding the arc ratio, most posterior teeth exposure, upper teeth exposure, and lower teeth exposure.
Conclusion
Identity, gender, age, and treatment experience influence the smile esthetics perception, with a significant difference in the results of the esthetic perception based on the 3 smile views. Of all demographic factors, identity had a strong relation to the perception of smile attractiveness. Nevertheless, additional studies are needed to realize how the demographic factors influence people's perception of smile esthetics, particularly in the three-quarter and lateral views.
Keywords: Affecting factors, Orthodontic, Perceptions, Smile esthetics
Introduction
The ideal orthodontic treatment goal is not only a proper facial appearance but also a harmonious and appealing smile.1,2 When orthodontists devise problem-oriented treatment plans, perception of smile esthetics were routinely considered to obtain a successful and esthetic treatment outcome. According to the past studies, there were many measurements have been used for quantitative and qualitative evaluation of smile perception.3, 4, 5, 6, 7, 8, 9, 10, 11, 12
Several factors potentially associated with smile perceptions, such as identity (layperson, general dentist and orthodontist), income, and age.13 For instance, dental professionals and laypeople (LP) differ in their preferences of smile esthetics.10,14 Johnston et al. stated that orthodontists were much more sensitive regarding the dental midline than LP without any dental background.6,15 Literatures have stated that three-quarter and lateral views should be included for evaluation of smile esthetics because various malocclusions may influence smile variables in frontal, lateral and three-quarter views.16, 17, 18 Also, Cheng has mentioned smile perspectives in three-quarter and lateral views might differ among different identities of people.19 Consequently, it is essential to define the affecting factors in perceptions of smile esthetics among them. To date, few studies have correlated smile esthetics based on three-quarter and lateral views with identity, gender, age, and treatment experience. The present study aims to analyze factors affecting perception of laypeople and dental professionals and to detect the extent of their association with smile perception.
Materials and methods
The cross-sectional study and its procedures were approved by the Institutional Review Board of Taipei Medical University (TMU-JIRB N201803029).
Questionnaires (refer to appendix 1. & 2.)
A two-part computer-based online questionnaire was used in our cross-sectional study.3,10,18,19 The first part comprised descriptive questions of individual demographic data, including 8 questions on identity (layperson, general dentist and orthodontist), gender, age group, orthodontic treatment experience, and income (Appendix 1.). The second part comprised 19 questions that were randomly arranged, multiple-five-choice questions regarding smile esthetic perceptions, as well as questions representing smile variables in frontal, three-quarter, and lateral views (Appendix 2.).
Variables and measurements
Five post-orthodontic patients from the orthodontic department in Taipei Medical University Hospital in Taiwan were randomly selected. These five patients were of skeletal class I, well-aligned and esthetically appealing occlusion, and aged 20–25 y/o. Further selection were made to pick up one of the best-appealing patient serving as standard model under consensus of the orthodontists from department of the Orthodontics in Taipei Medical University Hospital. Frontal, three-quarter, and lateral view photographs of the standard model's smile was obtained using single-lens reflex digital camera (700D; Canon, Tokyo, Japan) at a fixed 150 cm lens-object distance (See Appendix 3.). The frontal view photos were taken with the patient sitting in the natural head position looking at the center of the camera lens; the three-quarter view had the patient's left zygomatic soft tissue contour contacting with nose tip; the lateral view had the patient's head turning to her left side, with her eyebrows overlapping. Then, the preadjusted photographs in 3 different smile views were cropped to generate standard photographs (also see Appendix 3.) and following by digital alteration using Adobe Photoshop CS6 (Adobe Inc., San Jose, CA) and grouped into 19 sets, each set representing one smile variable in one smile view. The 19 sets of photographs were then randomly arranged in the questionnaire (see Appendix 2.), including 8 sets in the frontal view (arc ratio, most posterior upper teeth exposure, upper teeth exposure, lower teeth exposure, upper dental midline, buccal corridor, maxillary gingival display, cant), 4 sets in three-quarter view (arc ratio, visibility of most posterior upper teeth, upper teeth exposure, lower teeth exposure), and 7 sets in the lateral view (arc ratio, visibility of most posterior upper teeth, upper teeth exposure, lower teeth exposure, upper lip thickness, mouth angle, nasolabial angle). Each set contained 5 consecutive photographs to demonstrate the variation of the smile in continuous increments (refer to Appendix 2.). The range of increment was based on and further modified using the mean and standard deviation values from previous studies (see Table 1). The nose tip and chin button of the orthodontically-treated patient's face were shown in the online questionnaire (Appendix 3.) to define the midline, nasolabial angle, and reduce the possibility of misunderstanding by the investigators. Table 1 reveals the variables and their ranges and definitions.
Table 1.
Definitions of terms about smile variables.
| Smile variable | Range of values | Definition |
|---|---|---|
| Arc ratio3 | Flat, slight slope, parallel, reverse, and more reverse | Perpendicular distance of incisal edge of tooth 11 (FDI number) to a line connecting the cusp tips of the maxillary canine or distance between a tangent line of upper border of the lower lip and the maxillary intercanine line |
| Most posterior upper teeth exposure4 | Canine, 1st premolar, 2nd premolar, 1st molar, and 2 nd M | Most posterior upper teeth being seen in posed smile |
| Upper teeth exposure4 | 50%, 75%, 100%, 125%, and 150% | Stomion superiors to maxillary incisor edge or the clinical crown height of upper central incisor |
| Upper dental midline5,6 | 0, 1, 2, 3, and 4 mm | Distance from facial midline to the perpendicular line passing through the connectors between upper central incisors |
| Lower teeth exposure7 | 0%, 25%, 50%, 75%, 100% | Distance from the incisal edge of tooth 11 to upper border of the lower lip or the clinical crown height of lower central incisor |
| Buccal corridor4,8 | 0%, 5%, 10%, 15%, and 20% | Sum of width of the bilateral dark space or inner intercommissural width |
| Maxillary gingival display3,9 | 0, 1, 2, 3, and 4 mm | Vertical distance from zenith of upper central incisor to upper lip |
| Occlusal plane canting3,10 | 0, 3, 6, 9, and 12° | The minor angle formed by the intersection of occlusal plane and the horizontal line perpendicular to the facial midline |
| Upper lip thickness4 | 40%, 50%, 60%, 70%, and 80% | Ratio of “vertical distance from upper border to lower border of upper lip or upper incisal crown height” |
| Mouth angle11 | 20, 25, 30, 35, and 40° | The minor angle formed by the intersection of the tangent line of lower border of upper lip and the upper border of lower lip |
| Nasolabial angle12 | 80, 90, 100, and 110° | The minor angle formed by the intersection of the tangent line of lower border of nose and the philtrum when smiling |
Investigators
The investigators comprised the following 3 groups: layperson (LP), general dentists (GDs), and orthodontists (Orthos). The following was the selection criteria of LP, GDs and Orthos: the LP were randomly recruited from public areas, such as railway transit stations and department stores in Taiwan; GDs were randomly recruited from the annual conference meeting in Taiwan Dental Association; and the orthodontists were randomly recruited from the conference meeting in Taiwan Association of Orthodontics. Also, all the investigators (including LP, GDs, Orthos) had to be ≥ 20 years of age. The data were inputted through a computer-based online questionnaire given to the investigators privately by the authors at the reception area. These investigators were asked to rate the best-appealing photographs from each photo set of all smile variables, which contained 5 digitally altered photographs, randomly arranged and displayed on the personal computer screen.
Statistical analysis
The collected data were analyzed and categorized. Demographic data of the Investigator distribution and smile esthetic preference distribution were demonstrated with descriptive statistics. Pearson chi-square test was used to evaluate the difference of smile perception to each demographic variable. Multivariate logistic regression was used to evaluate the tendency of each demographic variable to smile perception in 3 smile views. The levels of reliability of the results were re-examined using intraclass correlation coefficient. The authors observed high levels of reliability. All analyses were conducted using the statistical software package SPSS (version 19; IBM, Chicago, IL, USA). Significance was set at P < 0.05.
Results
This cross-sectional study comprised 773 investigators, with 412 women and 361 men. Most of the investigators were aged 20–40 years. This study included 351 L P without orthodontic treatment experience, and 171 with treatment experience. The sample size of each income group was similar to the local population, and distributions of all demographic variables revealed significance (P < 0.05). The sample distribution of the investigators is presented in Table 2.
Table 2.
Summary table of sample distribution of the raters.
| Characteristic | Sample (%) |
X2 | Intragroup P | |||
|---|---|---|---|---|---|---|
| All sample |
LP |
GD |
Ortho |
|||
| N = 773 | N = 522 | N = 188 | N = 63 | |||
| Gender | 8.75 | 0.01 | ||||
| Men | 361 (46.7%) | 225 (43.1%) | 104 (55.3%) | 32 (50.8%) | ||
| Women | 412 (53.3%) | 297 (56.9%) | 84 (44.7%) | 31 (49.2%) | ||
| Age | 117 | <0.001 | ||||
| 20-30 y | 448 (58.0%) | 361 (69.2%) | 81 (43.1%) | 6 (9.52%) | ||
| 31-40 y | 241 (31.2%) | 107 (20.5%) | 93 (49.5%) | 41 (65.1%) | ||
| >40 y | 84 (10.9%) | 54 (10.3%) | 14 (7.45%) | 16 (25.4%) | ||
| Treatment | 309 | <0.001 | ||||
| w/o T | 351 (45.4%) | 351 (67.2%) | ||||
| T | 171 (22.1%) | 171 (32.8%) | ||||
| T | 251 (32.5%) | 188 (100%) | 63 (100%) | |||
| Salary | 595 | <0.001 | ||||
| <20 T | 197 (25.5%) | 197 (37.7%) | ||||
| 20 T–40 T | 148 (19.1%) | 148 (28.4%) | ||||
| 40 T–60 T | 95 (12.3%) | 95 (18.2%) | ||||
| 60 T–80 T | 36 (4.66%) | 36 (6.90%) | ||||
| >80 T | 46 (5.95%) | 46 (8.81%) | ||||
| >80 T | 251 (32.5%) | 188 (100%) | 63 (100%) | |||
LP: laypeople, GD: general dentist, Ortho: orthodontist, w/o T: without treatment, T: with treatment.
The results of the most favorable smile esthetics in the 3 smile views were compared (Table 3). The differences in perception of the 3 smile views were evaluated. The distributions of these 4 smile variables exhibited a significant difference regarding the 3 smile views (P < 0.001).
Table 3.
Frequency distribution of raters’ preference of smile variable in the frontal, three-quarter, and lateral smile views.
| Arc ratio | Flat | Sloped | Parallel | Reverse | More reverse | X2 | Intragroup P |
|---|---|---|---|---|---|---|---|
| frontal | 46 (5.95%) | 264 (34.2%) | 405 (52.4%) | 30 (3.88%) | 28 (3.62%) | 653 | <0.001 |
| Three-quarter | 71 (9.18%) | 164 (21.2%) | 324 (41.9%) | 142 (18.4%) | 72 (9.31%) | ||
| lateral | 238 (30.8%) | 215 (27.8%) | 222 (28.7%) | 47 (6.08%) | 51 (6.60%) | ||
| Most posterior upper teeth exposure | canine | 1st premolar | 2nd premolar | 1st molar | 2 nd M | ||
| frontal | 90 (11.6%) | 124 (16.0%) | 258 (33.4%) | 165 (21.3%) | 136 (17.6%) | 146 | <0.001 |
| three-quarter | 175 (22.6%) | 116 (15.0%) | 259 (33.5%) | 170 (22.0%) | 53 (6.86%) | ||
| lateral | 34 (4.40%) | 97 (12.5%) | 156 (20.2%) | 275 (35.6%) | 211 (27.3%) | ||
| Upper teeth exposure | 0.5 | 0.75 | 1 | 1.25 | 1.5 | ||
| frontal | 28 (3.62%) | 98 (12.7%) | 86 (11.1%) | 500 (64.7%) | 61 (7.89%) | 897 | <0001 |
| three-quarter | 30 (3.88%) | 149 (19.3%) | 243 (31.4%) | 313 (40.5%) | 38 (4.92%) | ||
| lateral | 29 (3.75%) | 121 (15.7%) | 406 (52.5%) | 164 (21.2%) | 53 (6.86%) | ||
| Lower teeth exposure | 0 | 0.25 | 0.5 | 0.75 | 1 | ||
| frontal | 131 (16.9%) | 90 (11.6%) | 458 (59.2%) | 59 (7.63%) | 35 (4.53%) | 852 | <0.001 |
| three-quarter | 228 (29.5%) | 336 (43.5%) | 114 (14.7%) | 67 (8.67%) | 28 (3.62%) | ||
| lateral | 87 (11.3%) | 363 (47.0%) | 206 (26.6%) | 89 (11.5%) | 28 (3.62%) | ||
| Upper dental midline | 0 | 1 mm | 2 mm | 3 mm | 4 mm | ||
| frontal | 495 (64.0%) | 175 (22.6%) | 23 (2.98%) | 52 (6.73%) | 28 (3.62) | ||
| Buccal corridor | 0 | 0.05 | 0.1 | 0.15 | 0.2 | ||
| frontal | 204 (26.4%) | 359 (46.4%) | 130 (16.8%) | 54 (6.99%) | 26 (3.36%) | ||
| Maxillary gingival display | 0 | 1 mm | 2 mm | 3 mm | 4 mm | ||
| frontal | 526 (68.0%) | 159 (20.6%) | 36 (4.66%) | 30 (3.88%) | 22 (2.85%) | ||
| Cant | 0 | 3 deg | 6 deg | 9 deg | 12 deg | ||
| frontal | 570 (73.7%) | 176 (22.8%) | 21 (2.72%) | 1 (0.13%) | 5 (0.65%) | ||
| Upper lip thickness | 0.4 | 0.5 | 0.6 | 0.7 | 0.8 | ||
| lateral | 132 (17.1%) | 198 (25.6%) | 176 (22.8%) | 162 (21.0%) | 105 (13.6%) | ||
| Mouth angle | 20 deg | 25 deg | 30 deg | 35 deg | 40 deg | ||
| lateral | 165 (21.3%) | 73 (9.44%) | 204 (26.4%) | 61 (7.89%) | 270 (34.9%) | ||
| Nasolabial angle | 70 deg | 80 deg | 90 deg | 100 deg | 110 deg | ||
| lateral | 201 (26.0%) | 224 (29.0%) | 77 (9.96%) | 211 (27.3%) | 60 (7.76%) |
The demographic factors affecting all smile variables were further surveyed using the running chi-square statistics, with identity, gender, age, and treatment experience. All revealed significant differences (Table 4, Table 5, Table 6, Table 7).
Table 4.
Difference in the most preferred photograph for each identity person (orthodontist, GD, and laypeople) by using chi-square test.
| Side | Smile Variables | X2 | P | Power |
|---|---|---|---|---|
| Frontal view | ||||
| Arc ratio | 89.2 | <0.001∗ | 1.00 | |
| Most posterior upper teeth expsoure | 116 | <0.001∗ | 1.00 | |
| Upper teeth exposure | 64.7 | <0.001∗ | 1.00 | |
| Upper dental midline | 44.8 | <0.001∗ | 1.00 | |
| Lower teeth exposure | 19.7 | 0.01∗ | 0.91 | |
| Buccal corridor | 26.4 | <0.001∗ | 0.98 | |
| Maxillary gingival display | 24.9 | 0.01∗ | 0.97 | |
| Cant | 52.9 | <0.001∗ | 1.00 | |
| Three-quarter view | ||||
| Arc ratio | 48 | <0.001∗ | 1.00 | |
| Most posterior upper teeth expsoure | 28.5 | <0.001∗ | 0.99 | |
| Lower teeth exposure | 10.8 | 0.22 | 0.63 | |
| Upper teeth exposure | 54.3 | <0.001∗ | 1.00 | |
| Lateral view | ||||
| Arc ratio | 33.2 | <0.001∗ | 0.99 | |
| Most posterior upper teeth exposure | 35.8 | <0.001∗ | 1.00 | |
| Lower teeth exposure | 13.9 | 0.09 | 0.76 | |
| Upper lip thickness | 11.5 | 0.18 | 0.66 | |
| Mouth angle | 28.5 | <0.001∗ | 0.98 | |
| Nasolabial angle | 57 | <0.001∗ | 1.00 | |
| Upper teeth exposure | 69.3 | <0.001∗ | 1.00 | |
∗P < 0.05 was set as statistically significant.
Table 5.
Difference in the most preferred photograph for both gender by using chi-square test.
| Side | Smile Variables | X2 | P | Power |
|---|---|---|---|---|
| Frontal view | ||||
| Arc ratio | 5.11 | 0.28 | 0.4 | |
| Most posterior upper teeth exposure | 11.7 | 0.02∗ | 0.79 | |
| Upper teeth exposure | 5.86 | 0.21 | 0.46 | |
| Upper dental midline | 1.78 | 0.78 | 0.16 | |
| Lower teeth exposure | 12.8 | 0.01∗ | 0.83 | |
| Buccal corridor | 9.33 | 0.05 | 0.68 | |
| Maxillary gingival display | 19.3 | <0.001∗ | 0.96 | |
| Cant | 2.52 | 0.64 | 0.21 | |
| Three-quarter view | ||||
| Arc ratio | 13.2 | 0.01∗ | 0.84 | |
| Most posterior upper teeth expsoure | 6.52 | 0.16 | 0.51 | |
| Lower teeth exposure | 21.5 | 0.001∗ | 0.97 | |
| Upper teeth exposure | 4.35 | 0.36 | 0.35 | |
| Lateral view | ||||
| Arc ratio | 10.3 | 0.04∗ | 0.73 | |
| Most posterior upper teeth exposure | 3.91 | 0.42 | 0.31 | |
| Lower teeth exposure | 13 | 0.01∗ | 0.84 | |
| Upper lip thickness | 2.63 | 0.62 | 0.22 | |
| Mouth angle | 16.1 | 0.00∗ | 0.91 | |
| Nasolabial angle | 2.88 | 0.58 | 0.24 | |
| Upper teeth exposure | 2.36 | 0.67 | 0.2 | |
∗P < 0.05 was set as statistically significant.
Table 6.
Difference in the most preferred photograph for each age group (20–30, 30–40, and >40 y) by using chi-square test.
| Side | Smile Variables | X2 | P | Power |
|---|---|---|---|---|
| Frontal view | ||||
| Arc ratio | 23.5 | 0.00∗ | 0.96 | |
| Most posterior upper teeth exposure | 57.3 | <0.001∗ | 1.00 | |
| Upper teeth exposure | 21.7 | 0.01∗ | 0.94 | |
| Upper dental midline | 12.4 | 0.13 | 0.70 | |
| Lower teeth exposure | 7.46 | 0.49 | 0.44 | |
| Buccal corridor | 12.7 | 0.12 | 0.71 | |
| Maxillary gingival display | 7.24 | 0.51 | 0.43 | |
| Cant | 12.1 | 0.15 | 0.69 | |
| Three-quarter view | ||||
| Arc ratio | 5.48 | 0.7 | 0.33 | |
| Most posterior upper teeth expsoure | 11.5 | 0.18 | 0.66 | |
| Lower teeth exposure | 5.03 | 0.75 | 0.3 | |
| Upper teeth exposure | 21.2 | 0.01∗ | 0.93 | |
| Lateral view | ||||
| Arc ratio | 10.1 | 0.26 | 0.59 | |
| Most posterior upper teeth exposure | 15.6 | 0.05∗ | 0.82 | |
| Lower teeth exposure | 11.2 | 0.19 | 0.65 | |
| Upper lip thickness | 2.97 | 0.94 | 0.18 | |
| Mouth angle | 10.7 | 0.22 | 0.63 | |
| Nasolabial angle | 17.6 | 0.02∗ | 0.87 | |
| Upper teeth exposure | 4.48 | 0.81 | 0.27 | |
∗P < 0.05 was set as statistically significant.
Table 7.
Difference in the most preferred photograph between orthodontically treated and untreated groups by using chi-square test.
| View | Smile Variables | X2 | P | Power |
|---|---|---|---|---|
| Frontal view | ||||
| Arc ratio | 4.93 | 0.29 | 0.39 | |
| Most posterior upper teeth exposure | 10.3 | 0.04∗ | 0.7 | |
| Upper teeth exposure | 4.9 | 0.3 | 0.39 | |
| Upper dental midline | 15.5 | 0.00∗ | 0.90 | |
| Lower teeth exposure | 4.06 | 0.4 | 0.32 | |
| Buccal corridor | 3.56 | 0.47 | 0.29 | |
| Maxillary gingival display | 2.65 | 0.62 | 0.22 | |
| Cant | 19 | <0.001∗ | 0.95 | |
| Three-quarter view | ||||
| Arc ratio | 2.94 | 0.57 | 0.24 | |
| Most posterior upper teeth expsoure | 1.26 | 0.87 | 0.12 | |
| Lower teeth exposure | 9.14 | 0.06 | 0.67 | |
| Upper teeth exposure | 10.5 | 0.03∗ | 0.74 | |
| Lateral view | ||||
| Arc ratio | 4.96 | 0.29 | 0.39 | |
| Most posterior upper teeth exposure | 7.32 | 0.12 | 0.56 | |
| Lower teeth exposure | 3.02 | 0.56 | 0.25 | |
| Upper lip thickness | 4.08 | 0.4 | 0.33 | |
| Mouth angle | 3.8 | 0.43 | 0.3 | |
| Nasolabial angle | 8.54 | 0.07 | 0.64 | |
| Upper teeth exposure | 9.81 | 0.04∗ | 0.71 | |
∗P < 0.05 was set as statistically significant.
Identity exhibited a strong influence on all smile variables regardless of the smile views (Table 4), barring the lower teeth exposure in the lateral and three-quarter views, and upper lip thickness in the lateral view. Notably, the lower teeth exposure demonstrated a significant difference related to identity in the frontal view.
Gender exerted some influence on the smile variables (Table 5). It revealed a significant difference in the three-quarter and lateral views related to the arc ratio. Regarding the most posterior upper teeth exposure, it showed gender-based significant difference in the frontal view. For lower teeth exposure, it showed gender-based significant difference in the 3 smile views. In addition, the maxillary gingival display (in frontal view) and mouth angle (in lateral view) demonstrated gender-based significant difference.
Furthermore, age showed some influence on the smile variables (Table 6). An age-based significant difference was noted regarding the frontal view. Regarding the most posterior upper teeth exposure, an age-based statistical difference related to the frontal and lateral views was noted. For upper teeth exposure, an age-based significant difference was noted regarding the frontal and three-quarter views. In addition, the nasolabial angle in the lateral view exhibited an age-based significant difference. The rest of the factors did not exhibit a significant difference by age.
Orthodontic treatment experience had some influence on the smile variables (Table 7). No treatment experienced-based significant difference was observed related to arc ratio and lower tooth exposure in the 3 smile views. A significant difference based on treatment experience was noted regarding most posterior upper teeth exposure in the frontal view, but none in the three-quarter and lateral views. By contrast, no treatment experience-based significant difference was noted regarding upper teeth exposure in the frontal view, but observed in the three-quarter and lateral views. Moreover, significant difference based on treatment experience was observed related to occlusal canting. The rest of the factors had no significant difference based on treatment experience.
The associations between smile variables of the 3 smiles views and demographic factors were evaluated (Table 8, Table 9).
Table 8.
Correlation between each demographic factors and all smile variables in frontal, three-quarter, and lateral views by using multivariate logistic regression.
| Characteristic |
Arc ratio |
Most post upper teeth exposure |
Upper teeth exposure |
Lower teeth exposure |
|||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| View | frontal | 3/4 | lateral | frontal | 3/4 | lateral | frontal | 3/4 | lateral | frontal | 3/4 | lateral | |
| Gender | Men | ||||||||||||
| Women | −0.09 | −0.39∗∗ | 0.07 | 0.16 | 0.03 | 0.08 | −0.20∗ | −0.31∗ | −0.15 | 0.18 | −0.18∗ | −0.23∗ | |
| Age (y/o) | 20–30 | ||||||||||||
| 31–40 | 0.08 | −0.11 | −0.10 | −0.24 | −0.08 | −0.01 | −0.16 | −0.01 | 0.11 | 0.01 | −0.15 | 0.16 | |
| >40 | −0.22 | −0.66∗ | 0.09 | −0.45 | 0.17 | 0.13 | −0.11 | −0.29 | 0.20 | −0.05 | −0.17 | 0.17 | |
| Identity | LP | ||||||||||||
| GD | 0.06 | −0.11 | −0.11∗∗ | −0.20∗ | 0.06 | −0.27∗∗ | 0.25∗∗ | 0.25 ∗∗ | −0.50∗∗ | 0.06 | −0.2 ∗ | 0.00 | |
| Ortho | 0.04 | −0.14 | −0.04∗∗ | −0.11 | 0.04 | 0.07∗∗ | 0.37∗∗ | 0.37∗∗ | 0.11∗∗ | 0.04 | −0.11 | −0.03 | |
| Treatment | w/o T | ||||||||||||
| T | −0.09 | 0.09 | 0.20∗ | −0.13 | −0.05 | −0.35∗∗ | −0.26∗ | 0.23 | 0.02 | −0.16 | 0.08 | −0.06 | |
| Salary | <20 T | ||||||||||||
| 20–40 T | 0.16 | −0.14 | 0.1 | 0.21 | −0.08 | 0.09 | −0.11 | 0.02 | −0.04 | 0.16 | −0.26∗ | 0.08 | |
| 40–60 T | 0.11 | −0.24 | −0.33∗ | 0.19 | 0.13 | 0.24 | −0.15 | 0.23 | −0.18 | 0.39∗ | −0.03 | −0.14 | |
| 60–80 T | 0.43∗ | 0 | −0.19 | 0.4 | −0.61∗ | 0.32 | 0.21 | 0.25 | 0.14 | 0.27 | 0.15 | 0 | |
| >80 T | 0.25 | 0.14 | 0.01 | 0.55∗ | 0.04 | −0.21 | −0.15 | 0.18 | −0.08 | 0.19 | −0.23 | 0.13 | |
Most post teeth exposure: most posterior teeth exposure, LP: laypeople, GD: general dentist, Ortho: orthodontist, w/o T: without treatment, T: treated.
Starred mark: statistical significance (P < 0.05) and correlation were revealed ∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001.
Table 9.
Correlation between each demographic factors and all smile variables in frontal, three-quarter, and lateral views by using multivariate logistic regression (cont'd).
| Characteristic | UDM |
BC |
MGD |
Cant |
ULT |
MA |
NLA |
|
|---|---|---|---|---|---|---|---|---|
| frontal | frontal | frontal | frontal | lateral | lateral | Lateral | ||
| Gender | Men | |||||||
| Women | −0.02 | 0.09 | −0.24 | 0.16∗ | 0.12 | −0.28∗∗ | −0.07 | |
| Age (y/o) | 20–30 | |||||||
| 31–40 | −0.15 | 0.07 | −0.19 | 0.11 | −0.16 | −0.30 | −0.11∗ | |
| >40 | −0.34 | −0.56∗ | −0.10 | −0.01 | 0.04 | −0.14 | 0.01 | |
| Identity | LP | |||||||
| GD | −0.11 | −0.20∗ | 0.25∗∗ | −0.11∗∗∗ | 0.20 | 0.15∗∗ | 0.74∗∗ | |
| Ortho | −0.14 | −0.11 | 0.37∗∗ | −0.14∗∗∗ | −0.08 | 0.04∗∗ | 0.17∗∗ | |
| Treatment | w/o T | |||||||
| T | −0.20∗ | 0.14 | −0.12 | −0.14 | 0.10 | −0.10 | −0.22∗∗∗ | |
| Salary | <20 T | |||||||
| 20–40 T | −0.34∗∗ | −0.01 | −0.13 | 0 | −0.13 | −0.07 | −0.12 | |
| 40–60 T | −0.33∗ | −0.29 | −0.08 | 0.15 | 0.06 | 0.05 | 0.13 | |
| 60–80 T | 0.01 | 0.07 | 0.01 | −0.15 | −0.47∗ | −0.15 | −0.16 | |
| >80 T | −0.09 | 0.07 | −0.16 | −0.13 | −0.23 | 0.14 | −0.01 | |
UDM: upper dental midline, BC: buccal corridor, MGD: maxillary gingival display, ULT: upper lip thickness, MA: mouth angle, NLA: nasolabial angle, LP: laypeople, GD: general dentist, Ortho: orthodontist, w/o T: without treatment, T: treated.
Starred mark: statistical significance (P < 0.05) and correlation were revealed ∗P < 0.05, ∗∗P < 0.01, ∗∗∗P < 0.001.
Discussion
The perception of smile esthetics has been a widely discussed topic among people. The orthodontists not only focus on maximizing the intercuspation but also seek to obtain the best harmony of dentition, lips, nose, and eyes.9 To ascertain the factors affecting the smile esthetic perception of LP and dental professionals, a dozen studies and systemic reviews have already investigated the frontal smile esthetics.3,5,10,14,15,20, 21, 22, 23, 24, 25, 26, 27, 28, 29 However, not many articles have been published regarding the three-quarter and lateral views.19,30 Therefore, this study focused on the factors affecting the smile attractiveness perception in the frontal view and compared it with those of the three-quarter and lateral views. Notably, a five-choice questionnaire was adopted to simplify the study method, reducing the possibility of investigators’ confusion and improving the reliability of the result. The online questionnaire was used in a public area, such that the sample characteristics would be similar to that of the general population.3,10
Furthermore, to determine which of the demographic factors affect the smile esthetic perceptions, the chi-square statistics were used, and the results revealed that identity (LP and dental professionals), gender (men and women), age, and treatment experience were the factors affecting smile esthetic perceptions. Among these 4 factors, identity played a critical role on smile esthetic perception because dental professionals could detect even minor discrepancies compared with the LP.5 Moreover, GDs were noted to seek a harmonious smile arc in the lateral view, less posterior teeth exposure in the frontal and lateral views, more upper teeth exposure in the frontal and three-quarter views, less upper teeth exposure in the lateral view, and less lower teeth exposure. By contrast, the orthodontists preferred a flat smile arc in the lateral view; more posterior teeth exposure in the lateral view; and more upper teeth exposure in the frontal, three-quarter, and lateral views. This finding was similar to that of Kokich who noted that orthodontists could detect even minor midline discrepancies and incisal plane canting compared with the LP, thereby stressing on achieving a harmonious smile arc.5
Regarding gender as a factor, the results revealed that women preferred a parallel smile arc, less lower incisor and gingival displays than men. This result was similar to that of Hulsey who determined that a smile index of 1.00–1.25 or perfect harmony between the incisal edges and both the edges of the upper and lower lips were more attractive than the others.15 Moreover, according to Machado, women preferred a more curved smile arc than men.23 Nevertheless, compared with the results in the frontal view, significant difference was only observed in the three-quarter and lateral views. Despite the scarcity of articles investigating three-quarter and lateral views, this article implies that women tend to accept the image of commercial smile, wherein the edges of the upper anterior teeth are aligned with (or parallel to) the upper border of the lower lip in the three-quarter and lateral views.
Several studies have already investigated the influence of age on lip-to-tooth relationship. In this study, the influence of age on perceptions of smile esthetics was analyzed, and the results revealed that the influence of the frontal view was limited to the smile arc, most posterior upper teeth exposure, and upper teeth exposure. This implied that as people age, their perception of esthetic smile arc, total number of exposed teeth, and upper incisor exposure in the frontal view would undergo a change, similar to what Desai has mentioned about esthetic perception and perspective of people would alter through the process of aging.21 Moreover, this survey revealed that the elderly population tend to accept a flat smile arc, with decreased upper and lower teeth and reduced nasolabial angle demonstrated in both frontal and three-quarter view, although the results only revealed minor significance (P = 0.01–0.05). Although the age factor did not show significance among the three views of photographs, careful interpretation in terms of the age factor could be of clinical value.
The influence of orthodontic treatment experience on smile esthetic perception was also investigated. The results revealed that treatment experience might influence smile esthetic perceptions in the frontal and lateral views, wherein LP with treatment experience preferred slightly decreased maxillary tooth exposure, consonant or flat smile arc, and were relatively intolerant to midline discrepancy and occlusal plane canting. The results revealing significance in the frontal and lateral views might suggest orthodontic patients preferred an occlusion with no occlusal plane canting, dental midline coincident with facial midline, decreased gingival display, prominent buccal corridor in the frontal view, a consonant smile arc, more upper lip thickness, as well as a slightly more acute nasolabial angle in the lateral view. Based on these findings, orthodontic treatments should also focus on maintaining the upper lip support while optimizing the occlusion.
Limitations in this cross-sectional study would be the random sampling method. The sample distribution of the investigators based on gender, age, treatment experience, and income all revealed statistical difference (P < 0.01), which is not in normal distribution. Noticeably, the group of the orthodontists are mostly >30 y/o, and around 25% of the orthodontists are >40 y/o. As to the LP and GD group, only 7–10% of them were >40 y/o. The group of the age >40 y/o were not further divided in a 10-y/o interval because in this study, there were only few layperson investigators with age >40 y/o. Future studies should include more layperson investigators >40 y/o of age in order to provide enough statistical power. Although this article represented the esthetic perspective among different populations in Taiwan, the results might differ from other races and cultures. Owing to the fact that a smile is dynamic and we people live in a three-dimensional world, to evaluate a smile more comprehensively, the three-quarter and lateral views photographs prior to orthodontic treatment planning is highly recommended. Future studies should focus on three-dimensional animated facial scanning to better determine affecting factors of smile esthetics.
In this study, identity, gender, age, and treatment experience influenced the smile esthetic perceptions, and the esthetic perception results revealed significance in frontal, lateral, and three-quarter views. Identity was strongly associated with the perception of smile attractiveness. Women, young adults, orthodontists, and orthodontically treated patients preferred a harmonious smile arc in the three-quarter view, less upper and lower incisor exposure in the frontal and three-quarter views, and smaller mouth angle in the lateral view. Further studies ascertaining how these demographic factors influence people's perception, especially in the three-quarter and lateral views are needed.
Declaration of competing interest
The authors declare no competing interests respective to the research and publication of this article.
Acknowledgements
The work was supported by the School of Dentistry, College of Oral Medicine, Taipei Medical University, Taiwan.
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