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Influence of lip form on the perceived ideal incisal show at rest

Influence of lip form on the perceived ideal incisal show at rest



American Journal of Orthodontics and Dentofacial Orthopedics, 2022-05-01, Volume 161, Issue 5, Pages e439-e445, Copyright © 2022


Introduction

A widely used guide of 2 mm incisal show at rest is used as an esthetic guide in treatment planning for many patients. However, this does not consider the lip form at rest. This study aimed to investigate the most esthetic incisal show at rest, at varying lip forms, perceived by dental professionals and laypeople.

Methods

Three volunteers of each gender and type of lip form (straight, moderate, and high) were recruited. For each volunteer, 6 images of their perioral region with a variable incisal display at rest were generated. A survey was conducted whereby respondents were asked to grade the attractiveness of each image using a numerical visual analog scale from 0-10.

Results

A survey sample of 107 respondents participated in the study (male-to-female ratio = 37 to 70; dental professionals-to-laypeople ratio = 50 to 57). Mann-Whitney U-test was used to analyze the differences in preferences between the various groups ( P <0.05). Both dental professionals and laypeople responded that 2 mm incisal show at rest is only suitable for patients with straight lip form. However, for patients with moderate or high lip forms, an incisal show of 4-5 mm was preferred. There was also no significant difference between the perceived ideal incisal show of dental professionals and laypeople, except for the ideal incisal show of models with straight lip form.

Conclusions

The guide of 2 mm incisal show at rest is appropriate only for patients with straight lip form. Patients with moderate or high lip form should have a greater incisal show. This finding was agreed on by both dental professionals and laypeople, regardless of their gender.

Highlights

  • The lip form must be considered when planning dental treatment for patients.

  • The common guide of 2 mm incisal show at rest is only suitable for patients with straight lip form.

  • Patients with moderate or high lip should be planned for 4 or 5 mm incisal show at rest, respectively.

  • These are generally well agreed by both dental professionals and laypeople.

The esthetic industry has been burgeoning in the past decade as the population became increasingly aware and conscious of facial esthetics. This may be partially attributed to the astronomical rise of media influence. Each part of the facial subunit contributes to the overall esthetic of an individual. As dental professionals, one of the main areas we focus on is the perioral tissues and dental esthetics in the lower facial third. The position of the maxillary incisors in relation to the upper lip line is an important consideration for esthetics in the lower facial third. This can be evaluated either at smile or rest. Both are affected by various factors such as tooth height, alveolar height, lip length, and muscular lip tone. , Although the morphology of the maxillary incisors can be altered by aging and disease processes, it is not as dynamic compared with the upper lip. The lip form or line, defined as the vertical position of the lower border of the upper lip, constantly changes in height and shape when we perform functions like eating, speaking, and smiling. The dynamic behavior of the lip form controls the number of teeth that can be seen (incisal show), and hence, it also plays an important role in the lower facial esthetics.

The amount of incisor shown during posed smiles has been well researched. In contrast, the incisal show at rest has received marginal attention in past literature. The greater focus on incisal show at smile is likely because most self-evaluation would occur through static photographs other than the mirror. Instinctively, most people would smile for their photographs, thus wanting an esthetic smile. This does not mean that the incisal show at rest is not equally important. A resting lip posture is what we would adopt most of the time subconsciously. In other words, we spend much less time smiling than we would at rest. The resting lip posture or incisal show at rest can be assessed by gently parting the patient’s lips or instructing the patient to enunciate certain words such as Mississippi or Emma. Among these, the lip position at rest was shown to be the most reproducible when the patients were instructed to say the word Emma.

Although scarce, available cross-sectional studies suggested that the mean value of incisal show at rest to be about 2-3 mm. , , Vig and Brundo surveyed subjects in California and. reported an average incisal show at rest to be 2.6 mm. However, when differentiated between the male and female subjects, the average incisal show at rest was 1.91 mm and 3.40 mm, respectively. In another study by Al Wazzan, an average of 2.88 mm incisal show at rest was measured in participants in Riyadh, Saudi Arabia. However, there was no significant difference between the incisal show at rest for male and female participants (2.66 mm and 2.91 mm, respectively). When compared across the different age groups, both studies also noted older participants had reduced incisal show at rest. It was also reported that a longer lip length is co-related with a reduced incisal show at rest and that the average incisal show at rest varies across different races. ,

In clinical practice, the incisal show at rest plays an important role in treatment planning. Sarver and Ackerman highlighted the importance of anterior tooth display during orthodontic treatment planning.. The position of the maxillary central incisors is key in achieving an esthetic outcome. , However, the position of the central incisors should be relative to a more stable reference point such as the lip position at rest rather than the lip position during smile as it is significantly dynamic. An incisal show at rest of 2 mm has served as a guide when planning for orthodontic therapy and orthognathic surgeries. , These can move the maxillary teeth in all 3 planes—anteroposterior, vertical, and sagittal. Although it is obvious how the vertical plane can influence the incisal show, it is also important to consider how an increase or decrease in the inclination of maxillary anterior teeth will result in a reduced or elongated incisal display, respectively.

Similarly, a 2 mm incisal show at rest has been taught in many dental institutes for determining the height of a complete maxillary denture during its fabrication process. Typically, the dental practitioner will adjust the maxillary occlusal wax rim such that it extends 2 mm below the relaxed lip during the treatment of the edentulous patient. This will translate to incisal show when the patient is wearing the final prosthesis.

Although 2 mm of incisal show at rest provides a useful reference, a standardized treatment may not always lead to the best outcome for all the patients. It has been shown that the morphology of the lip can have a huge influence on the perceived ideal incisal show at rest. This, in turn, is affected by the patient’s gender, age, and race. , With reference to cross-sectional studies by Vig and Brundo and Al Wazzan, patients with shorter lips have a longer incisal show at rest and vice versa. Kim et al defined 3 types of lip forms (straight, moderate, and high) on the basis of the distance between the highest portion of the lower vermillion border of the upper lip to a line drawn between the commissures. A straight, moderate, or high lip form will have a distance of 0-3 mm, 3-6 mm, and >6 mm, respectively. The authors concluded that as the lip form changed from straight to high, there was a preference for an increased incisal show at rest. This would suggest that the planned incisor position should vary according to each patient’s lip profile instead of a universal standard of 2 mm. However, insufficient studies are available to determine the ideal amount of incisal show for each lip form. Furthermore, esthetics is a very subjective matter, and the perceived ideal may vary between the clinician and the patient.

There are conflicting results between studies investigating the perception of smile esthetics between dental and nondental professionals. Caramello et al compared the perception of esthetics between orthodontists and laypersons with respect to varying amounts of incisal show at rest. The authors reported that both the orthodontists and laypersons perceived the most esthetic ideal show at rest to be 2.0-2.5 mm and 1.5-2.5 mm for the female and male gender, respectively. There was no significant difference in the perception between the orthodontists and laypersons. However, although this study provides insight into the ideal incisal show at rest, it does not account for the different lip forms.

The first objective of our study was to investigate the most esthetic or ideal incisal show at rest, at varying lip forms, perceived by both dental professionals and laypeople in an Asian population (Singapore). Secondly, this study aimed to determine if there was any difference in the perception between dental professionals and laypeople and between the male and female genders.

Material and methods

A cross-sectional study was conducted via an online electronic survey platform. This study was approved by our university Institutional Review Board before commencement. The required sample size was based on a prior study by Caramello et al. Therefore, a minimum of 48 respondents from each group (dental professional and laypeople) was required.

Six volunteers (aged 21-23 years) were recruited to fabricate the visual aids required for the survey (sample photographs). The volunteers were screened to exclude obvious facial or dental asymmetries, nonpristine dentition, periodontal diseases or perioral mucosa, or skin diseases. The distance from the highest aspect of the lower vermillion border of the upper lip to an imaginary line drawn between the commissures was used to classify the volunteers’ lip form into straight (0-3 mm), moderate (>3-6 mm), and high lip form (>6 mm), respectively. For each gender, a volunteer of each lip form, was selected (3 males and 3 females).

The volunteers were instructed to enunciate Emma to attain a reproducible resting posture. Each volunteer was photographed with their lip position at rest, from the philtrum to above the soft-tissue menton (iPhone XR; Apple Inc, Cupertino, Calif). The images were subsequently modified using Photoshop (version CS5.1; Adobe Systems Inc, San Jose, Calif) to remove inconsistencies across subjects such as skin tone. A standard image of the maxillary dentition was added to all images. The amount of maxillary incisal show varied across images by adjusting the position of the standard dentition image ( Fig ). The upper and lower interval limits for each lip form were based on the results of a prior study. The maximum vertical space between the upper and lower lips of the volunteers. Six intervals of 1 mm were created for both the moderate and high lip form. However, 6 intervals of 0.5 mm were created for the straight lip form because of the shorter vertical space available. As such, 36 images are created from the 6 initial photographs of the volunteers. The increment intervals, upper and lower limits of incisal display for each lip form are stated in Table I .

The distance between the highest aspect of the lower vermillion border of the upper lip (green dotted line) to an imaginary line drawn between the commissures (red line) was used to classify the lip forms (straight, moderate, and high). Photoset (A-F) shows varying incisal show at rest at an increasing interval of 0.5 mm.
Fig
The distance between the highest aspect of the lower vermillion border of the upper lip (green dotted line) to an imaginary line drawn between the commissures (red line) was used to classify the lip forms (straight, moderate, and high). Photoset (A-F) shows varying incisal show at rest at an increasing interval of 0.5 mm.

Table I
Increment intervals for incisal show at rest for high, moderate, and straight lip forms
High lip form (1 mm increments) Moderate lip form (1 mm increments) Straight lip form (0.5 mm increments)
Incisal show at rest 2 mm 1 mm 0 mm
3 mm 2 mm 0.5 mm
4 mm 3 mm 1.0 mm
5 mm 4 mm 1.5 mm
6 mm 5 mm 2.0 mm
7 mm 6 mm 2.5 mm

The following demographic information was collected from the electronic survey questionnaire; age, gender, race, and occupation (dental professional or nondental professional). The survey respondents were instructed to rate each photograph using a numerical visual analog scale from 0 to 10. A score of 0 was defined as very unattractive, whereas a score of 10 was defined as very attractive. A score of 5 was defined as average. A total of 36 photographs were included in each survey (6 photographs for each gender and each type of lip form). All photographs were randomized for every respondent, and backward navigation was disabled to prevent comparisons with previous photographs. Although there was no time limit, the respondents were encouraged to grade the photographs on the basis of their impression at first glance. The mean ideal incisal show for each group was calculated first by tabulating each patient’s highest-scoring incisal show, followed by taking the mean of the total score.

The reliability of this method was determined retrospectively after the main study was conducted with 22 respondents (11 laypeople and 11 dental professionals). The latter consists of 7 general dentists and 4 dental specialists. The dental specialists (consisting of 2 orthodontists and 2 oral and maxillofacial surgeons) were asked to participate survey process twice. The respondents were asked to complete the questionnaires twice with at least 2 weeks of washout period in between each. The photographs of each group (each lip form and gender) were ranked 1-6 according to the score given. Intraclass correlation coefficient test showed interexaminer reliability of 0.80 and 0.87 for dental professionals and laypeople, respectively. In addition, the intraexaminer reliability was 0.94 and 0.92 for dental professionals and laypeople, respectively. The responses from the reliability tests were not included as part of the main study.

Statistical analysis

Analysis was done using SPSS (version 25.0; IBM, Armonk, NY) to evaluate the following: Preference of incisal show at rest for each lip form and whether there is a difference in preference between dental professionals and nondental professionals (laypeople), and, between the male and female respondents. The Shapiro-Wilk test was used to test the normality of the results. As all the results do not follow a normal distribution, a nonparametric test (Mann-Whitney U-test) was used to analyze the differences in perception between the various groups. The result is statistically significant when the P <0.05.

Results

The online electronic survey was conducted from April 2020 to July 2020 (4 months). The following information was extracted from the response.

There was a total of 107 respondents to our survey, out of which 35% were male (n = 37) and 65% were female (n = 70). Among the male respondents, 94% were Chinese (n = 36), whereas the remaining 6% were Eurasian or Malay (n = 1 for each). Seventy-three percent of the male respondents were aged 21-30 years (n = 27), 19% were aged 31-40 years (n = 7), and the remaining 8% were aged 51-60 years (n = 3). For the female respondents, 93% were Chinese (n = 65), whereas the remaining 7% were Indians or identified themselves as other (n = 2 for each). In terms of the age of female respondents, 91% were aged 21-30 years (n = 64), 5% were aged 31-40 years (n = 4), and 4% were aged 51-60 years (n = 3). A detailed breakdown of the demographics is shown in Table II .

Table II
Demographic profile of respondents (n = 107)
Variables n
Total
Laypeople 57
Dentists 50
Gender
Males 37
Females 70
Laypeople 57
Males 21
Females 36
Dentists 50
Male 16
Female 34
Age
21-30 y 91
31-40 y 11
41-50 y 0
51-60 y 5
Race
Chinese 101
Malay 1
Indian 2
Eurasian 1
Others 2

When segregated by their professions, 53% were laypeople (n = 57), whereas 47% were dentists (n = 50). Among the laypeople, 37% were males (n = 21) and 63% were females (n = 36). Among the dentists, 32% were males (n = 16), and 6% were females (n = 34). Only 28% of the respondents are certified specialists (8 orthodontists, 5 oral and maxillofacial surgeons, and 1 periodontist), whereas the remaining 72% were general practitioners (n = 36). No difference in the perception of the ideal incisal show at rest was elicited in this study. However, the sample sizes between groups were unequal and therefore may not be conclusive.

The perceived ideal incisal show at rest by dental professionals and laypeople for each gender was summarized in Table III The results suggest no significant difference between the perceived ideal incisal show for high and moderate lip forms between dental professionals and laypeople ( P >0.05). However, for both males and females with straight lip form, there was a significant difference between the perceived ideal incisal show by dental professionals and laypeople for males with a straight lip form ( P = <0.0001) and females with a straight lip form ( P = 0.007). For males with straight lip form, dental professionals preferred an incisal display of 2.5 mm, whereas laypeople preferred 2.0 mm. For females with a straight lip form, dental professionals preferred an incisal display of 2.0 mm, whereas laypeople preferred 1.5 mm instead.

Table III
Comparisons between the dental professionals and laypeople
Gender (model) Lip form Incisal show at rest deemed to be the most esthetic (in mm) Statistical significance, P value
Dental professionals Laypeople
Male High 5 5 0.472
Moderate 4 4 0.884
Straight 2.5 2.0 <0.0001
Female High 5 5 0.236
Moderate 4 4 0.094
Straight 2 1.5 0.007
Note. Statistical analysis was conducted using the Mann-Whitney U-test.

Statistically significant difference ( P <0.05).

Responses by dental professionals were subdivided on the basis of their gender, male or female, and compared ( Table IV ). The results showed no statistically significant difference between a female dentist’s perception and a male dentist’s perception of the ideal incisal show across the 3 lip forms ( P >0.05). Responses by laypeople were subdivided on the basis of their gender, male or female ( Table V ). Similarly, the results showed no statistically significant difference between female and male laypeople’s perceptions of the ideal incisal show for all different lip forms ( P >0.05).

Table IV
Comparisons between male and female dental professionals
Gender (model) Lip form Incisal show at rest deemed to be the most esthetic (in mm) Statistical significance, P value
Male dental professionals Female dental professionals
Male High 5 5 0.995
Moderate 4 4 0.793
Straight 2.5 2.5 0.731
Female High 5 5 0.174
Moderate 4 4 0.946
Straight 2 2 0.836
Note. Statistical analysis was conducted using the Mann-Whitney U-test.

Table V
Comparisons between male and female laypeople
Gender (model) Lip form Incisal show at rest deemed to be the most esthetic (in mm) Statistical significance, P value
Male laypeople Female laypeople
Male High 5 5 0.963
Moderate 4 4 0.738
Straight 2.0 2.0 0.409
Female High 5 5 0.865
Moderate 4 4 0.761
Straight 1.5 1.5 0.137
Note. Statistical analysis was conducted using the Mann-Whitney U-test.

Discussion

The widely quoted saying of “Beauty is the eye of the beholder” rightly describes the subjective nature of esthetics. References and guides attempt to create a more scientific and objective approach to esthetics. A 2 mm incisal show at rest is commonly used as a guide for orthodontics, restorative dentistry, and maxillofacial surgical procedures. However, this guide may have neglected the effect of lip morphology on the esthetic evaluation of the lower facial third. The upper lip length directly impacts the amount of incisal show at rest, and the difference in lip length will, in turn, affect how much incisal show at rest would be perceived as ideal by the respondents.

The findings of this study suggest that the 2 mm guide is only acceptable for patients with a straight lip form. For patients with moderate or high lip form, a greater incisal show at rest may be more appropriate. One reason why the 2 mm guide is still widely applied might be because dentures are typically fabricated for elderly patients. With age, the upper lip lengthens and is more likely to present with a straight lip profile. , In contrast, younger patients tend to present with a shorter lip length, higher lip form, and increased incisal show at rest. , This coincides with the preference for a greater incisal show at rest for the models with moderate and high lip forms. Similar findings in the American population were reported by Kim et al. Based on the findings of our study, patients with moderate and high lip form should have about 4 mm and 5 mm incisal show at rest, respectively.

It has been reported that different thresholds on maxillary anterior teeth esthetics exist between dental professionals and laypeople in studies evaluating the perception of smiles. , Therefore, the other intention of this study was to investigate whether the ideal incisal show at rest perceived by dental professionals is different from the laypeople. The findings from this study suggest there is no difference between dental professionals and laypeople regarding their perception of the ideal incisal show in subjects with high or moderate lip forms. However, for subjects with straight lip forms, dental professionals prefer a greater amount of incisal show at rest than laypeople. Although deemed statistically significant, the differences of 0.5 mm may not be clinically discernable. Kim et al also reported that dental professionals preferred longer incisal displayed on female models than male models.. However, similarly, the differences were <0.5 mm, and its clinical significance remains to be determined. The gender of the respondents, be it the dental professionals or the laypeople, also did not appear to influence the perception of the ideal incisal show at rest.

There are some limitations to this study. First, two thirds of respondents were female, and 90% were Chinese and aged 21-30 years. Unequal respondents between gender appear to be prevalent for most survey studies regardless of whether it was done electronically or not. This would mean that there is a potential for the study to be representative only for females, rather than the general population. The Mann-Whitney test allows for statistical analysis between groups of unequal sample sizes. Based on the tests, there were no significant differences between the response of the male and female respondents. Therefore, the results from this study may be appropriate to be extrapolated to the general population (for both genders). This would mean that the findings of studies will be biased toward a younger Chinese population.

Despite the limited variability in the age of our respondents, a prior study suggested that age may not affect the respondent’s view on the incisal show. Second, despite the effort to minimize variability in our sample models, there are still minor inconsistencies between models, influencing the respondent’s perception. For instance, the lip color and presence of facial hair may be confounding factors. Third, this study did not investigate how the incisal show at rest with various lip forms correlates with smile esthetics. Future studies can incorporate a concomitant assessment of perceived smile esthetic in relation to incisal show at rest, upper lip smile line, and lower lip curve. Finally, the reliability tests were added subsequently and were not conducted simultaneously as the initial investigation. Because the respondents’ identities were not collected as part of the survey questionnaires, a separate invitation was sent out to invite previous respondents to participate and repeat the survey once more. Because of the time-lapse between the 2 surveys, the validity of the reliability tests may be affected. Therefore, the results of the study should be interpreted with caution because of the retrospective nature of the reliability test that was not incorporated as part of the main study.

Despite the drawbacks and limitations, the findings from this study are still pertinent for orthodontic treatment planning. Although comprehensive hard and soft-tissue analysis is a standard process in orthodontic diagnosis, there may not be sufficient emphasis on lip morphology its relation to anterior tooth display. When planning for the position of the maxillary incisors, there should not be a blind pursuit of a 2 mm incisal show at rest. The clinician should assess the adjacent perioral structures and customize the ideal incisal show for the patient. For patients with moderate or high lip forms, increased incisal show (ie, 4-5 mm) should be prescribed to achieve a more esthetic outcome. This is especially crucial when planning for orthognathic surgeries as a maxillary impaction to achieve 2 mm incisal show at rest should be avoided in these patients. Patients with moderate or high lip forms who wish to reduce their gummy smiles should be assessed whether adjunctive therapies such as lip lengthening surgery, crown lengthening, or botulinum toxin injection can be administered.

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