Effect of maxillary incisors, lower lip, and gingival display relationship on smile attractiveness

Introduction

This study aimed to evaluate the effect of the relationship between maxillary incisors and lower lip in conjunction with the maxillary gingival display on perception of smile attractiveness.

Methods

The frontal intraoral photograph of ideally aligned teeth was modified using image processing software. Twenty different smiles were obtained with a combination of 10 different maxillary incisor edge—lower lip distances and 10 different maxillary gingival display amounts. Attractiveness of these 20 smiles was evaluated by orthodontists (n = 101), dentists (n = 107), and laypersons (n = 105). Two-way ANOVA was used to compare alterations on smile esthetics, and repeated measures ANOVA was used to evaluate the interaction between the variables. Bonferroni-adjusted pairwise comparisons were applied for determining the differences between the rater groups.

Results

Maxillary incisor edge—lower lip distance ( P < 0.01), maxillary gingival display amount ( P < 0.001), and rater group ( P < 0.01) had statistically significant influences on perception of smile attractiveness. Orthodontists prefer to see a minimal amount of mandibular incisor exposure and almost full crowns of maxillary central incisors. Dentists and laypersons are more tolerant of mandibular incisor exposure and maxillary central incisor coverage by the upper lip. None of the rater groups preferred the coverage of maxillary incisor edges or maxillary gingival display.

Conclusions

Elimination of maxillary gingival display helps to improve smile attractiveness, whereas the coverage of maxillary incisor edges has a negative influence.

Highlights

  • Exposure of upper incisor edges with lower incisors enhances smile attractiveness.

  • Maxillary gingival display negatively influences smile attractiveness.

  • Laypersons give the highest, and orthodontists give the lowest attractiveness scores.

Current dental literature emphasizes the importance of facial esthetics, which was shown to play an important role in the quality of life. Some studies revealed that facial attractiveness is the key to social success, and it is influenced by dental attractiveness. Another study reported that dental esthetics advances psychosocial well-being. In addition, the desire to have a beautiful, youthful, and healthy appearance increases in society, which causes esthetic considerations to become more critical in treatment plans. Thus, smile esthetics has become the primary focus of patients seeking orthodontic treatment. ,

An esthetic smile requires the interaction of various factors. Examination and improvement of the factors that constitute an esthetic smile are necessary for orthodontists because smile attractiveness is the parameter most patients use to appraise the success of orthodontic treatment. Therefore, the complicated relationship between smile components accompanied by occlusal functions must be taken into consideration to obtain ideal functional and esthetic results.

It is reported that the perception of esthetics is considerably affected by culture, education level, and social environment. , Therefore, differences may be expected in the judgment of orthodontic treatment results by patients and orthodontists. ,

Factors that contribute to smile esthetics such as buccal corridors, occlusal cant, gingival display, gingival height discrepancy, incisal edge discrepancy, smile arc, tooth size ratios, crown torques, and midline deviations were evaluated in previous studies. , Nevertheless, the interaction between most of these factors was not examined comprehensively. The purpose of this study was to evaluate the effect of the relationship between maxillary incisor edges and lower lip in conjunction with the maxillary gingival display on perception of smile attractiveness for orthodontists, dentists, and laypersons.

Material and methods

This study was approved by Başkent University Institutional Review Board and Ethics Committee and supported by Başkent University Research Fund (project no: D-KA14/14). Power analysis was performed to calculate the sample size required for the study. This analysis revealed that a sample size of 100 people per group was needed to detect statistically significant differences among the rater groups with a power of 99% and 0.05 significance level. One-way analysis of variance (ANOVA) was used for sample size estimation.

The frontal intraoral photograph of a female with ideally aligned teeth and the frontal extraoral photograph of the same person during posed smiles were obtained. The extraoral photograph was arranged such that all parts except for the lips and their surrounding area were excluded. The intraoral photograph was also arranged by isolating the maxillary teeth from the rest of the photograph so that they could be modified separately using the layer function of the image processing software (Adobe Photoshop, version 7.0; San Jose, Calif).

The intraoral photograph involving the teeth was combined with the extraoral photograph involving the lip frame by superimposition. The maxillary gingival display was 0 mm, and the incisal edges of maxillary central teeth were tangent to upper border of lower lip in the first superimposition. Then, 9 more superimpositions were obtained by moving the intraoral photograph upwards (+0.5, +1, +1.5, +2, +2.5 mm) and downwards (–0.5, –1, –1.5, –2 mm). Thus, 10 different smiles were obtained via moving the teeth within the lip frame in vertical direction gradually by 0.5-mm increments.

The 4 images that were obtained by moving the intraoral photograph downwards were modified again by elongating crown lengths of maxillary teeth in gingival direction gradually by 0.5-mm increments. Thus, 10 more different smiles were obtained via arranging new combinations of maxillary incisor edge—lower lip distance and maxillary gingival display amount by elongating the maxillary tooth crowns. Hence, a total of 20 images with different smiles were obtained by changing the relationship between the teeth and lips ( Fig 1 ). The changes were realized by altering the position of teeth in 10 smile images, which represents intrusion or extrusion of maxillary teeth by orthodontic mechanics. On the other hand, the changes were realized by altering the length of teeth in 10 smile images, which represents either lengthening of maxillary tooth crowns at gingival margins by periodontal surgery or lengthening of maxillary tooth crowns at incisal edges by restorative dentistry.

Fig 1
The 20 different smiles evaluated in the study.

For measuring 0.5-mm increments, the intraoral photographs were calibrated by calculating the ratio of the physical incisogingival length of the right central incisor in millimeters to its length in the original intraoral photograph in pixels. Positive values were allocated when maxillary incisor edges were exposed and when the upper lip covered the maxillary central incisors. Negative values were allocated when lower lip covered maxillary incisor edges, and when the upper lip did not cover maxillary central incisors.

These color images in 50 × 100 mm standard size were randomly positioned and printed on a questionnaire. A visual analog scale that consisted of a 100-mm bar was also placed at the bottom of each image for subjective esthetic evaluation, and the questionnaires composed of 20 images and were evaluated by 3 rater groups. The first group involved orthodontists (n = 101), the second group involved dentists (n = 107), and the third group involved laypersons (n = 105). All 313 raters were Caucasians. The orthodontists included in the study had completed at least 2 years in residency with a maximum of 30 years in practice, whereas the dentists were at least 2 years postgraduate with a maximum of 34 years in practice. The laypersons included in the study were selected among patients sitting in the waiting area of other dental clinics who had not received orthodontic treatment.

The raters were asked to score the attractiveness of each smile image separately using the visual analog scale graded from unattractive to attractive by placing a vertical mark on the scale. They were instructed not to go back to compare the images with each other. All raters completed scoring the 20 images within 1 session of 3-4 minutes in a well-illuminated room, under the researchers’ care. The researchers gave the necessary instructions to the raters, controlled the scoring time, and checked if there were missing scores for any of the smile images, but did not guide, help, or interfere in the scoring. The scores given to the smile images by the raters were measured manually by the same examiner (HT) with a digital caliper. Thus, scores ranging from 0 to 100 were obtained, with the scores “0” being very unattractive and “100” being very attractive.

Statistical analysis

The data analysis was performed by SPSS for Windows version 20 (SPSS Inc, Chicago, Ill). The data were shown as mean ± standard deviation. A 2-way ANOVA was used to compare alterations on smile esthetics. A repeated-measures ANOVA was used to evaluate the interaction between maxillary incisor edge—lower lip distance, maxillary gingival display amount, age, gender, and rater group. The Bonferroni adjustment for pairwise comparisons was applied for determining the differences between rater groups for both maxillary incisor edge—lower lip distance and maxillary gingival display amount measurements. A P value of <0.05 was considered statistically significant.

Twenty-seven randomly selected raters from each group (at least 25% of the raters in each group) were asked to score the same 20 images once more to determine the intrarater reliability in judging the smiles. Intraclass correlation coefficients (with 95% confidence intervals) calculated for each image ranged between 0.79 and 0.87 for orthodontists, 0.78 to 0.83 for dentists and 0.73 to 0.87 for laypersons. Hence, the judgments of all 3 groups were found to be reliable.

Results

Demographic distributions of the rater groups are presented in Table I .

Table I
Demographic distributions of the rater groups
Variables Orthodontists Dentists Laypersons
Number 101 107 105
Age, year 30.9 ± 6.4 31.0 ± 7.8 33.6 ± 8.7
Gender (F/M) 57/44 76/31 60/45

Note: Values are mean ± standard deviation.
F, Female; M, Male.

Table II indicates the source of variation for smile attractiveness scores among variables such as upper incisor edge—lower lip distance, maxillary gingival display amount, age, gender, and rater group. The interaction between these variables is also presented. Maxillary incisor edge—lower lip distance ( P <0.01), maxillary gingival display amount ( P <0.001), and rater group ( P <0.01) exhibited a statistically significant influence on the perception of smile attractiveness. Age and gender did not have a statistically significant effect on perception of smile attractiveness. No interaction was observed between the variables.

Table II
Source of variation for perception of smile attractiveness regarding repeated measures ANOVA results
Source of variation P value
Upper incisor edge—lower lip distance 0.002**
Maxillary gingival display amount <0.001***
Age 0.086
Gender 0.775
Rater group 0.003**
Maxillary incisor edge – lower lip distance × Age 0.910
Maxillary incisor edge – lower lip distance × Gender 0.448
Maxillary incisor edge – lower lip distance × Rater group 0.128
Maxillary gingival display amount × Age 0.504
Maxillary gingival display amount × Gender 0.530
Maxillary gingival display amount × Rater group 0.069
Maxillary incisor edge – lower lip distance × Maxillary gingival display amount 0.203

** P < 0.01.
*** P < 0.001.

Table III shows the smile attractiveness score rankings of each rater group from the highest to the lowest. The highest scores were obtained at +0.5 mm maxillary incisor edge—lower lip distance and at +0.5 mm maxillary central incisor coverage by upper lip among orthodontists and dentists ( Fig 2 ), whereas at +2 mm maxillary incisor edge—lower lip distance and at +2 mm upper central incisor coverage by upper lip among laypersons ( Fig 3 ). The lowest scores were obtained at –2 mm maxillary incisor edge coverage by the lower lip and –2 mm maxillary gingival display among all rater groups ( Fig 4 ). The highest smile attractiveness scores were given by laypersons, and the lowest scores are given by orthodontists.

Table III
Smile attractiveness score rankings within the rater groups
Orthodontists Dentists Laypersons
Smile Mean SD Smile Mean SD Smile Mean SD
Lip +0.5, Gin +0.5 62.9 2.0 Lip +0.5, Gin +0.5 66.3 1.8 Lip +2.0, Gin +2.0 68.1 2.2
Lip 0.0, Gin 0.0 58.0 2.0 Lip +2.0, Gin +2.0 65.6 2.4 Lip +1.5, Gin +1.5 67.0 2.1
Lip +1.0, Gin +1.0 57.5 2.1 Lip +1.5, Gin +1.5 65.1 2.0 Lip +1.0, Gin +1.0 66.5 2.0
Lip –0.5, Gin 0.0 55.0 1.8 Lip +1.0, Gin +1.0 65.0 1.9 Lip +2.5, Gin +2.5 66.4 2.4
Lip +1.5, Gin +1.5 50.9 2.0 Lip 0.0, Gin 0.0 59.6 1.7 Lip +0.5, Gin +0.5 65.5 2.0
Lip –2.0, Gin 0.0 50.8 1.8 Lip +2.5, Gin +2.5 58.9 2.2 Lip –0.5, Gin 0.0 62.2 2.1
Lip –1.0, Gin 0.0 49.4 1.7 Lip –0.5, Gin –0.5 57.6 1.9 Lip –1.0, Gin –0.5 60.8 2.1
Lip –1.5, Gin 0.0 48.7 2.0 Lip –1.0, Gin 0.0 55.1 1.8 Lip –2.0, Gin 0.0 60.6 2.3
Lip –0.5, Gin –0.5 48.0 1.7 Lip –0.5, Gin 0.0 55.0 1.7 Lip 0.0, Gin 0.0 59.3 2.1
Lip +2.0, Gin +2.0 47.3 2.0 Lip –1.5, Gin 0.0 54.3 2.0 Lip –1.5, Gin 0.0 59.2 2.3
Lip –1.5, Gin –0.5 46.7 1.8 Lip –2.0, Gin 0.0 54.0 1.9 Lip –1.0, Gin 0.0 59.0 2.2
Lip –1.0, Gin –0.5 46.3 1.8 Lip –1.5, Gin –0.5 53.6 1.8 Lip –2.0, Gin –0.5 58.7 2.1
Lip –2.0, Gin –0.5 45.0 1.7 Lip –2.0, Gin –0.5 51.5 1.8 Lip –1.5, Gin –1.0 55.9 2.1
Lip +2.5, Gin +2.5 43.9 2.0 Lip –1.0, Gin –0.5 50.6 1.6 Lip –1.5, Gin –0.5 55.8 2.2
Lip –1.5, Gin –1.0 40.7 1.8 Lip –2.0, Gin –1.0 46.6 1.8 Lip –0.5, Gin –0.5 55.1 2.2
Lip –2.0, Gin –1.0 39.4 1.7 Lip –1.5, Gin –1.0 45.9 1.8 Lip –2.0, Gin –1.0 53.9 2.1
Lip –1.0, Gin –1.0 36.9 1.9 Lip –1.0, Gin –1.0 44.0 2.0 Lip –1.0, Gin –1.0 53.0 2.2
Lip –2.0, Gin –1.5 32.9 1.6 Lip –1.5, Gin –1.5 36.8 1.6 Lip –2.0, Gin –1.5 52.5 2.2
Lip –1.5, Gin –1.5 32.5 1.7 Lip –2.0, Gin –1.5 35.7 1.7 Lip –1.5, Gin –1.5 49.4 2.2
Lip –2.0, Gin –2.0 22.7 1.2 Lip –2.0, Gin –2.0 31.1 1.6 Lip –2.0, Gin –2.0 46.8 2.2
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Mar 9, 2020 | Posted by in Orthodontics | Comments Off on Effect of maxillary incisors, lower lip, and gingival display relationship on smile attractiveness
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