One of the major reasons for seeking dental treatment is improving facial and dental attractiveness because a smile strongly affects facial esthetics in daily social interactions. Regarding orthodontic treatment, Ackerman et al stated that a posed frontal smile, which is voluntary and repeatable over time, should be recorded during routine data collection. However, in real life, smile attractiveness is not judged based on only the frontal view. Dierkes stated that a beautiful smile is produced through the harmony of the horizontal, vertical, and transverse components with face contours. Furthermore, Hulsey mentioned the importance of the anteroposterior dimension in malocclusions and stated that for a fundamental assessment of facial esthetics, information is needed concerning the relationship between teeth and the surrounding soft tissue in the frontal view. Therefore, dentists should evaluate smile composition in the frontal, three-quarter, and lateral views to determine smile improvement after treatment. However, limited studies have supported different preferences for the lateral and three-quarter smile views.

The perception of smile esthetics is subjective and influenced by a person’s experiences and social environment. Furthermore, preferences for an esthetic smile are different between dental professionals and the general population because of differing dental experiences. ^{, ,} Therefore, the attitudes and preferences of patients toward smile esthetics must be considered during orthodontic treatment planning. Orthodontists should also assess the different attributes of an esthetic smile, which vary from patient to patient. Kokich et al studied the perceptions of laypersons and dentists regarding variation in the appearance of the anterior teeth and the relationship of these teeth to the surrounding soft tissue. Their results demonstrated that orthodontists, general dentists, and laypersons perceive maxillary midline deviation and incisal plane cant discrepancies differently. Janson et al concluded that a small dental midline deviation of 2.2 mm is considered acceptable by both orthodontists and laypersons, whereas buccal corridor sizes and smile arc do not affect smile attractiveness. Parekh et al observed that both laypersons and orthodontists prefer a smile arc parallel to the lower lip and minimal buccal corridors, whereas smile arc flattening and excessive buccal corridors are perceived as considerably unattractive features. Taki et al concluded that Eastern Arabic orthodontists are more critical in analyzing and accepting discrepancies in smile arc and buccal corridors than dentists and laypersons. An et al reported that dentists are more perceptive in detecting changes in dental midline shifts and incisal plane canting than laypersons; furthermore, orthodontically treated laypersons are more critical than nontreated laypersons regarding incisal plane canting and dental midline shifts.

Various factors are associated with smile esthetics. Several studies have reported that demographic characteristics such as age, sex, and marital and economic status can affect dental esthetic perceptions. ^{, , ,} Therefore, these perceptions should be investigated in each country to aid dental esthetic treatment planning accordingly for patients. Nevertheless, few studies have evaluated esthetic smile preferences in the Thai population.

The purpose of this study was to analyze differences in esthetic smile preferences among Thai orthodontists, general dentists, orthodontic patients, and laypersons in terms of the three-quarter and lateral smile views for smile assessment in addition to the standard frontal smile and to determine the factors affecting esthetic smile preferences.

Material and methods

Ethical approval for this study was obtained from the Institutional Review Board of Taipei Medical University (TMU-JIRB No. N201803029). A computer-based questionnaire that linked questions to images was used in the survey, which comprised 2 parts. The first part was a descriptive survey regarding personal demographic characteristics, and the second part consisted of image-based multiple-choice questions on smile esthetics measurements in the three-quarter view. To ensure questionnaire reliability based on test-retest reliability after this procedure, the researcher administered the questionnaire to 10 participants per group. Seven days after giving their first response, participants were asked to recomplete the questionnaire. The reliability coefficient was used for evaluating questionnaire reliability. Intraclass correlation coefficients were used to evaluate intrarater agreement.

Photographs of a posed smile of an Asian woman in frontal ( Fig 1 ), three-quarter ( Fig 2 ), and lateral ( Fig 3 ) smile views were digitally altered using Adobe Photoshop CS6 (Adobe Systems, San Jose, Calif). The number of smile variables in frontal, three-quarter, and lateral smile views was 8, 4, and 7, respectively. Table I shows the definitions and range of values of the smile esthetic variables. The 19 sets of manipulated smile photographs were used for evaluation, with each set containing 5 photographs that were digitally altered according to the range of values. These ranges of values were based on mean and standard deviation values obtained in previous studies to reflect smile esthetic discrepancy. However, data for some smile variables were lacking, especially in the three-quarter and lateral views; therefore, the corresponding measurements were selected after consulting clinically experienced orthodontists. Other facial structures—such as the nose, cheeks, and chin—were excluded from the smile photograph set to minimize distractions and the number of confounding variables, except for the maxillary midline in the frontal view and nasolabial angle in the lateral view, which were retained in the nose tip to represent the facial midline and to construct the angle, respectively.

Smile esthetic variables in the frontal view: A, arc ratio = a (lower border of the maxillary incisor perpendicular to intercanine connecting line)/a’ (upper border of the lower lip perpendicular to intercanine connecting line in the lower lip); B, buccal corridor; C, most posterior maxillary teeth visible; D, maxillary dental midline; E, maxillary teeth exposure; F, mandibular teeth exposure (%) = f (height of the visible mandibular central incisor on smiling)/f’ (height of the mandibular central incisor measured from maxillary incisal edge to the gingival margin of the mandibular incisors) × 100; G, maxillary gingival display; H, cant.

Smile esthetic variables in the three-quarter view (45° from the frontal view): A, arc ratio = a (lower border of the maxillary incisor perpendicular to intercanine connecting line)/a’ (upper border of the lower lip perpendicular to intercanine connecting line in the lower lip); B, most posterior maxillary teeth visible; C, maxillary teeth exposure; D, mandibular teeth exposure (%) = d (height of the visible mandibular central incisor on smiling)/d’ (height of the mandibular central incisor measured from maxillary incisal edge to the gingival margin of the mandibular incisors) × 100.

Smile esthetic variables in the lateral view (90° from the frontal view): A, arc ratio = a (lower border of the maxillary incisor perpendicular to intercanine connecting line)/a’ (upper border of the lower lip perpendicular to intercanine connecting line in the lower lip); B, most posterior maxillary teeth visible; C, mandibular teeth exposure (%) = c (height of visible mandibular central incisor on smiling)/c’ (height of the mandibular central incisor measured from maxillary incisal edge to the gingival margin of the mandibular incisors) × 100; D, upper lip thickness; E, mouth angle; F, nasolabial angle; G, maxillary teeth exposure.

Table I

Definitions and range of values of smile esthetic variables

Smile variable	Range of values	Definition
Arc ratio ∗ , † , ‡	−1, −0.5, 0, 0.5, and 1	The perpendicular distance from the incisal edge of maxillary central incisors to a line connecting the cusp tips of the maxillary canine divided by the distance between a tangent line of the upper border of the lower lip and the maxillary intercanine line
Most posterior maxillary teeth visible ∗ , † , ‡	Canine (3), first premolar (4), second premolar (5), first molar (6), and second molar (7)	The most posterior maxillary teeth exposed on both sides while smiling
Maxillary midline ∗	0, 1, 2, 3, and 4 mm	Maxillary dental midline (measured between central incisors) compared with the facial midline (a line representing the midline of the face as defined by the center of the philtrum and nasal tip)
Maxillary teeth exposure ∗ , † , ‡	50%, 75%, 100%, 125%, and 150%	Height of visible maxillary central incisor on smiling divided by the actual height of maxillary central incisor
Mandibular teeth exposure ∗ , † , ‡	0%, 25%, 50%, 75%, and 100%	Height of visible mandibular central incisor while smiling divided by the actual height of mandibular central incisor (measured from the maxillary incisal edge to gingival margin of mandibular incisors)
Buccal corridor ∗	0%, 5%, 10%, 15%, and 20%	The horizontal distance of the total dark space on both sides of the mouth divided by the total smile width
Maxillary gingival display ∗	0, 1, 2, 3, and 4 mm	Height of visible gingiva between the lower border of the upper lip and gingival zenith of the maxillary central incisors
Cant ∗	0°, 3°, 6°, 9°, and 12°	Amount of rotation in the maxillary teeth from the horizontal plane through the middle of the maxillary central incisors
Upper lip thickness ‡	4, 5, 6, 7, and 8 mm	The vertical distance from the most superior point of the cupid’s bow to the most inferior portion of the tubercle of the upper lip
Mouth angle ‡	20°, 25°, 30°, 35°, and 40°	The angle formed by the most inferior border of the upper lip and most superior border of the lower lip on smiling
Nasolabial angle ‡	70°, 80°, 90°, 100°, and 110°	The angle between columella of the nose and anterior surface of the upper lip

 

∗ Variables in frontal view.

† Variables in three-quarter view.

‡ Variables in lateral view.

The evaluator participants were categorized into 4 groups: orthodontists; general dentists; orthodontic patients who were receiving or had completed orthodontic treatment and were recruited from dental clinics in Thailand; and laypersons who had never received orthodontic treatment, reported no background of orthodontic treatment, and were recruited from public areas, such as mass rapid transit stations and shopping malls in Thailand. Furthermore, the evaluators were categorized into 2 groups: professionals (orthodontists and general dentists) and nonprofessionals (orthodontic patients and laypersons). The inclusion criteria for all the groups were age ≥ 20 years and willingness to participate in this study. Data were collected through a computer-based questionnaire distributed personally to the participants by the researcher. The participants were asked to examine and select only the most preferred photograph from each set of photographs with different smile esthetic variable values; each set comprised 5 altered photographs that were randomly arranged and displayed on the computer screen.

Results

In total, 1040 Thai people living in Thailand participated in the study. Professionals included orthodontists (n = 61; mean age, 33.2 ± 5.4 years) and general dentists (n = 180; mean age, 29.5 ± 3.6 years). Nonprofessionals group included orthodontic patients (n = 378; mean age, 32.0 ± 7.3 years) and laypersons (n = 421; mean age, 35.2 ± 9.5 years). The questionnaire reliability was high, with the intraclass correlation coefficient values for orthodontists, general dentists, orthodontic patients, and laypersons being 0.955, 0.910, 0.867, and 0.71, respectively.

Table II shows the comparisons in the most preferred smile photographs between men and women. Of the 19 smile esthetic variables, significant differences were found in 17 variables, except the upper lip thickness and most posterior maxillary teeth visible in the three-quarter view.

Table II

Differences in the most preferred smile photograph for each variable between men and women

Views	Smile variables	P value
Frontal view	Arc ratio	0.000 ∗
	Most posterior maxillary teeth visible	0.035 ∗
	Maxillary teeth exposure	0.002 ∗
	Mandibular teeth exposure	0.000 ∗
	Maxillary dental midline	0.000 ∗
	Buccal corridor	0.000 ∗
	Maxillary gingival display	0.000 ∗
	Cant	0.004 ∗
Three-quarter view	Arc ratio	0.000 ∗
	Most posterior maxillary teeth visible	0.556
	Maxillary teeth exposure	0.000 ∗
	Mandibular teeth exposure	0.000 ∗
Lateral view	Arc ratio	0.000 ∗
	Most posterior maxillary teeth visible	0.000 ∗
	Maxillary teeth exposure	0.000 ∗
	Mandibular teeth exposure	0.000 ∗
	Upper lip thickness	0.403
	Mouth angle	0.000 ∗
	Nasolabial angle	0.026 ∗

 

∗ Significance as determined by the chi-square test ( P < 0.05).

Tables III–V showed the results of the most preferred smile photographs from the frontal, three-quarter, and lateral view, respectively. According to Kruskal-Wallis tests, the preferences of orthodontists, general dentists, orthodontic patients, and laypersons were considerably different regarding the most posterior maxillary teeth visible, mandibular teeth exposure, maxillary dental midline, and cant in the frontal view ( Table III ). No significant differences were found among 4 groups in the three-quarter view ( Table IV ). Significant differences were found in the variables of arc ratio, maxillary teeth exposure, and mouth angle in the lateral view ( Table V ). Further post-hoc tests provided detailed paired comparisons among the 4 groups. According to post-hoc tests, the 2 professional groups did not have significantly different preferences for any variable. Similarly, no significant differences were found between the 2 nonprofessional groups. All the significant differences noted when paired comparisons were performed between 1 professional group and 1 nonprofessional group.

Table III

Results of percentage of the most preferred smile photographs of 8 variables in the frontal view

Frontal view variables	Most favored smile photographs					P value †	P value ‡
Arc ratio	−1	−0.5	0	0.5	1	0.275	0.005 ∗
O	0.00	0.00	0.00	41.00	59.00 §
G	0.00	0.60	0.60	54.40 §	44.40
P	0.50	1.60	3.20	48.40 §	46.30
L	0.70	1.20	6.90	53.40 §	37.80
Most posterior maxillary teeth visible	3	4	5	6	7	0.002 ∗	0.000 ∗
O	0.00	24.60	54.10 §	4.90	16.40	O ¶ P
G	1.70	43.30 §	41.10	6.10	7.80	O ¶ L
P	6.30	44.20 §	27.20	7.70	14.60
L	9.00	44.40 §	20.20	9.30	17.10
Maxillary teeth exposure	50%	75%	100%	125%	150%	0.294	0.000 ∗
O	0.00	0.00	90.20 §	9.80	0.00
G	0.00	0.00	97.80 §	2.20	0.00
P	0.80	7.10	78.30 §	11.90	1.90
L	1.00	10.00	73.60 §	12.60	2.90
Mandibular teeth exposure	0%	25%	50%	75%	100%	0.008 ∗	0.000 ∗
O	18.00	67.20 §	9.80	3.30	1.60	G ¶ P
G	13.90	68.30 §	12.80	4.40	0.60
P	25.90	45.50 §	14.80	8.70	5.00
L	20.40	43.00 §	16.20	12.10	8.30
Maxillary dental midline	0 mm	1 mm	2 mm	3 mm	4 mm	0.000 ∗	0.000 ∗
O	73.80 §	19.70	3.30	3.30	0.00	G ¶ P
G	75.60 §	18.90	1.70	2.20	1.70	G ¶ L
P	46.30 §	24.90	10.10	8.50	10.30
L	41.10 §	27.30	8.10	12.60	10.90
Buccal corridor	0%	5%	10%	15%	20%	0.602	0.003 ∗
O	54.10 §	26.20	18.00	0.00	1.60
G	38.30 §	25.60	30.00	3.90	2.20
P	51.60 §	18.30	19.80	7.10	3.20
L	53.40 §	18.30	21.10	4.00	3.10
Maxillary gingival display	0 mm	1 mm	2 mm	3 mm	4 mm	0.334	0.000 ∗
O	59.00 §	31.10	9.80	0.00	0.00
G	70.00 §	23.30	4.40	1.10	1.10
P	71.20 §	13.80	8.50	4.20	2.40
L	68.20 §	16.20	7.60	4.30	3.80
Cant	0°	3°	6°	9°	12°	0.000 ∗	0.000 ∗
O	93.40 §	4.90	1.60	0.00	0.00	O ¶ P
G	91.70 §	8.30	0.00	0.00	0.00	O ¶ L
P	65.90 §	31.70	1.90	0.30	0.30	G ¶ P
L	64.60 §	33.00	1.20	0.70	0.50	G ¶ L

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