Influence of upper lip curvature on smile attractiveness in patients with different degrees of gingival smiles: A cross-sectional study with opinions from oral health providers and laypersons


Although there is relative consensus about approaches to gingival smile management, there are still discrepancies as to whether a gingival smile is attractive or not. The purpose of this study was to quantify the influence of upper lip curvature shape and the amount of gingival display on the perception of smile attractiveness assessed by Peruvian orthodontists, dentists, and laypersons.


A frontal photograph was digitally altered to generate 3 types of upper lip curvature shapes (upward, straight, and downward) with 5 different levels of gingival smile exposure (0 mm, 2 mm, 3 mm, 4 mm, and 5 mm). Fifteen images were generated. Three groups of evaluators (50 dentists, 50 orthodontists, and 50 laypersons) assessed the images using a visual analog scale. One-way analysis of variance with Bonferroni post-hoc tests and multiple linear regressions were applied.


The downward lip curvature shape had a negative effect on the esthetic evaluation of the smile with gingival exposures of 3 mm or more for all the evaluating groups (approximately 20-30 points less than upward or straight smile curvature shapes, P <0.05). Laypeople gave higher scores of almost 10 points more than dentists and orthodontists when evaluating the upward lip curvature shape with 5 mm of gingival exposure during smile ( P = 0.029).


We found that upward or straight upper lip curvature shapes have a positive impact on perceived smile esthetics. In contrast, downward upper lip curvature shapes have a negative effect on perception when evaluating different degrees of gingival smiles.


  • The influence of the upper lip curvature shape on the perception was evaluated.

  • All patients presented a different level of gingival display.

  • Gingival display and upper lip curvature shape were combined for evaluation.

  • Downward upper lip curvature shape has a negative impact on perception.

  • There was a positive impact of the upward upper lip in gingival smiles.

Smile evaluation is an area of great interest in contemporary orthodontics. In the 1960s, some authors began to highlight the importance of conducting a frontal analysis of the lips regarding their morphology and relationship with the incisor teeth. , The diagnosis of facial soft tissues with emphasis on smile analysis has become fundamental in interdisciplinary case management. , In addition, it is essential to consider that orthodontic treatment may have a positive impact on the patient’s smile display and quality of life-related aspects. , In this sense, direct clinical measurements should be made of the soft tissues when smiling by positioning the patient frontally, , sitting on a chair or standing, , to obtain a more accurate picture of the best incisor position to maximize patients’ esthetic smiles. These measurements help to determine the dynamic relationship of the incisors with the soft tissues in which the lips play a controlling role by displaying or hiding the rest of the anterior oral structures when an individual smiles. ,

The amount of gingival exposure during a full smile has been traditionally classified into 3 groups: low, medium, or high, with the amount of incisor and gingival exposure differentiating one group from another. The clinical implications of gingival smiles have also previously been evaluated. Balance must be achieved among the amount of gingiva exposed in a smile, the length of the teeth, the lip, and the facial thirds. It is also necessary to define what is esthetically acceptable or not when evaluating a clinical procedure.

The upper lip curvature presents 3 different shapes during a smile: upward, straight, and downward, with the latter being qualified as the least esthetic , and the upward shape as the most esthetic. Before decision making related to orthodontic treatment, analysis of the amount of gingival exposure during a smile, teeth proportions, upper lip coverage, and facial thirds proportions, and other related measurements, should be considered. Moreover, it is necessary to understand what is and is not esthetically pleasing. In this regard, in patients with a high smile, a different esthetic standard may apply. Nonetheless, there is a lack of consensus on the maximum display of periodontal tissue that could be considered esthetically pleasing.

Independently of the multiple etiologic factors of the gingival smile described in the literature, , previously related research , did not simultaneously evaluate other components of the smile that may interact with the amount of gingival display. To the best of the authors’ knowledge, only 1 study has analyzed 2 combined esthetic components of the smile (amount of gingival display and smile arc) and reported that the component interactions of the smile might positively or negatively affect facial esthetics. No previous study has analyzed the upper lip curvature shape with simultaneous analysis of other smile components as an influencing smile factor.

A proper diagnosis must consider the components of the smile, such as the upper lip in combination with other components of the smile, such as high gingival exposure. Although clinicians are able to determine the etiology of the gingival smile and select the best way to manage it, the final esthetic display determines the actual need for treatment. Orthodontists must consider that the clinical decision for treating a patient with gingival exposure when smiling is a matter of personal esthetic consideration by the patient and professional, and the upper lip curvature shape can directly influence this perception. For these reasons, the purpose of this study was to compare different levels of a gingival smile with 3 upper lip morphologic contours when smiling to assess the effect of these combined variables on the perception of esthetics by 3 evaluating groups (dentists, orthodontists, and laypeople).

Material and methods

This cross-sectional study was approved by the ethics and research committee of the Universidad Científica del Sur, Lima, Perú.

The sample size calculation was determined using a formula to compare 2 means (the mean score of visual analog scale [VAS] in patients with an upward upper lip curvature shape with 5 mm of gingival exposure, which is the highest exposure with the most significant clinical impact according to orthodontists and laypeople) on the basis of the following criteria: alpha 95%, beta 80%, and difference of 12 points in the mean VAS score (these data were obtained from a previous pilot study in which orthodontists considered a high smile as being of 5 mm with an upward upper lip curvature shape of 36 ± 20 vs an upward upper lip curvature shape of 48 ± 20 determined by laypeople). Although the minimum sample size required was 44 people per group ( ), 50 people per group were finally included for this investigation.

The sample for this study consisted of a total of 150 people distributed into 3 groups: 50 dentists, 50 orthodontists, and 50 laypeople, who evaluated perceived smile esthetics using a VAS after viewing a set of 15 photographs (see details further below).

The inclusion criteria for the 3 groups were: (1) dentists: licensed dentists with active clinical practices in dentistry from 3 private universities from San Marcos University, Cayetano Heredia University, and Científica del Sur University with authorization to practice in Lima; (2) orthodontists: orthodontists with a degree from the same universities described before and members of the Peruvian Society of Orthodontists; and (3) laypeople: adults with no studies in dentistry randomly recruited from a neighborhood in Lima, Perú.

The dentist and orthodontist groups were also asked about years of clinical experience in their areas, and the layperson group was asked about previous orthodontic treatment because this variable could modify the main outcome.

A patient with a gingival, albeit harmonious, smile of proper proportions with characteristics of high smile esthetics was selected by the principal investigator (S.V.M.) and 2 dentists specializing in dental esthetics. Teeth were in optimal alignment (no crowding, no irregularity, good consonant smile, and proper proportions). A photograph was taken 1 meter away from the subject with a Nikon 7100 DSRL camera, Micro-Nikkor AF-S VR 105 mm f2.8G lens, and studio lights. In addition, an intraoral photograph was taken to edit the images in Photoshop (Adobe, San Jose, Calif, 2017) to provide more intraoral information.

Three upper lip display variations at full smile (upward, straight, and downward) were combined with 5 different levels of elevation of the same lip (0 mm, 2 mm, 3 mm, 4 mm, and 5 mm), thus obtaining 15 images cropped to only represent the lower facial third ( Fig 1 ).

Fig 1
All the images were presented to the rater groups in randomized order.

The Photoshop expert was instructed to alter the gingival exposure by moving the philtrum moved upwards and downwards and to change the upper lip morphology by moving the commissures moved upwards and downwards. Further digital arrangements were made to help the image look more realistic and symmetrical.

The images were printed in a 5 × 7-in format and were randomly allocated in a photograph album. The data collection document with VAS scores was numbered from 1 to 100, with the extreme anchors of 0 as very unattractive and 100 as very attractive .

The participants reviewed all the images first and then selected the photograph they considered most attractive and rated it first. This approach provided a general attractiveness parameter with which the remaining images could be compared as many times as the evaluators considered necessary. A total of 150 interviews were completed.

The principal investigator (S.V.M.) and a trained collaborator took measurements of the data collected using a digital 150 mm caliper (Uberman). Two months later, 15 random assessments were selected from each group to repeat the measurements and apply reliability tests achieving an intraclass correlation coefficient greater than 0.90 for all groups.

Statistical analyses

The data analysis was performed using SPSS software (version 25.0; IBM, Armonk, NY). The data distribution was confirmed with the Shapiro-Wilk test. The initial sample characteristics were compared among groups using the chi-square (sex) and Tukey tests (age). Comparisons among the groups were made by 1-way analysis of variance with the Bonferroni post-hoc test to compare the outcome variables among the 3 groups. Finally, multiple linear regression was carried out to evaluate the influence of the predictive variables on the preference of the photographs evaluated (VAS). The level of significance was established as a P value <0.05 for all tests.


The intraclass correlation coefficient values ranged from 0.90-1.00 in the 3 evaluator groups. The error method was less than 1 mm on the 100 mm VAS scale.

Table I shows that there was no significant difference in the years of clinical experience between orthodontists and dentists. However, on average, the orthodontists were older (37.70) than the dentists (32.68). Sex distribution among the 3 groups was similar ( Table II ).

Table I
Age and clinical experience in all groups
Rater group n Variable Mean SD
Dentist 50 Age 32.68 a 6.99
Clinical experience 8.90 6.37
Orthodontist 50 Age 37.70 b 7.36
Clinical experience 7.50 6.44
Laypeople 50 Age 36.62 a,b 10.59
Clinical experience

Note. Different letters (a,b) indicate significant differences between means as determined using the Tukey test.
SD , standard deviation.

P <0.05.

Table II
Demographic distribution of rater groups
Specialty Total
Dentist Orthodontist Laypeople
Male 21 17 21 59
Female 29 33 29 91
Total 50 50 50 150

Note. P = 0.640 (chi-square test).

Table III shows the comparison of esthetic perception for the 3 upper lip curvature shapes at full smile at 5 different gingival smile heights by the 3 evaluation groups. There were no statistically significant differences between upward and straight lip morphologies for all rater groups at the highest gingival exposure levels (3 mm, 4 mm, and 5 mm), whereas the downward lip had a lower esthetic perception in comparison ( P < 0.05 for all comparisons). Upward labial curvature with gingival exposure of 0 mm was given the highest score (>75) by the 3 groups of evaluators followed by a straight smile (score of approximately 67), with downward labial curvature receiving the lowest score (≤40) ( P <0.05) at all heights of gingival exposure.

Table III
Comparison of esthetic perception (VAS) between different upper lip curvature shapes while smiling at different levels of gingival exposure for each rater group—rater group as differentiator
Gingival exposure Group Upper lip curvature shape n Mean (0-100) SD
0 mm Dentists Upward 50 77.74 a 18.05
Straight 50 68.60 b 22.73
Downward 50 29.95 c 18.58
Orthodontists Upward 50 78.92 a 17.41
Straight 50 66.01 b 21.61
Downward 50 31.69 c 21.79
Laypeople Upward 50 77.03 a 17.69
Straight 50 68.44 b 20.87
Downward 50 38.82 c 24.55
2 mm Dentists Upward 50 59.47 a 22.25
Straight 50 64.82 a 17.76
Downward 50 23.47 b 18.24
Orthodontists Upward 50 62.64 a 23.58
Straight 50 64.71 a 19.06
Downward 50 21.31 b 18.21
Laypeople Upward 50 67.72 a 22.60
Straight 50 59.59 b 18.78
Downward 50 27.06 c 23.74
3 mm Dentists Upward 50 50.83 a 44.28
Straight 50 46.63 a 42.08
Downward 50 25.77 b 27.83
Orthodontists Upward 50 54.69 a 43.57
Straight 50 52.63 a 43.25
Downward 50 26.19 b 26.70
Laypeople Upward 50 57.49 a 48.04
Straight 50 53.79 a 46.40
Downward 50 27.50 b 30.99
4 mm Dentists Upward 50 44.28 a 18.01
Straight 50 42.08 a 16.56
Downward 50 27.83 b 17.73
Orthodontists Upward 50 43.57 a 21.12
Straight 50 43.25 a 22.65
Downward 50 26.70 b 20.89
Laypeople Upward 50 48.04 a 21.65
Straight 50 46.40 a 22.35
Downward 50 30.99 b 21.45
5 mm Dentists Upward 50 35.84 a 19.75
Straight 50 37.71 a 20.44
Downward 50 17.36 b 15.41
Orthodontists Upward 50 36.41 a 24.33
Straight 50 40.14 a 20.05
Downward 50 15.16 b 15.55
Laypeople Upward 50 46.67 a 22.75
Straight 50 45.43 a 24.49
Downward 50 18.05 b 15.85
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Apr 19, 2021 | Posted by in Orthodontics | Comments Off on Influence of upper lip curvature on smile attractiveness in patients with different degrees of gingival smiles: A cross-sectional study with opinions from oral health providers and laypersons
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