Evaluation of the ideal position of the maxillary incisor relative to upper lip thickness

Introduction

The anteroposterior position of the maxillary incisors has been studied since the beginning of cephalometric research. Most reports of ideal position have been based on measurements made on subjects with Class I occlusion without consideration of other facial variables. However, the ideal position is reflected in the soft tissues by the position of the lips. The purpose of this study was to consider the position of the maxillary incisor relative to other factors, including the thickness of the upper lip, and to evaluate its most aesthetic position using profile photographs.

Methods

The subjects in this study were 130 patients with Class I occlusion; their sex, age, thickness of the upper lip, and angular position of the maxillary incisor were recorded. From these 130 patients, 70 profile photographs that met the inclusion criteria were chosen for further evaluation. A survey was conducted among 60 lay people, 60 dentists, and 60 orthodontists. They were asked to classify the sets of photographs, from the most esthetic position of the upper lip to the least esthetic.

Results

For patients with thin lips, raters tended to prefer a more protruded position of the incisor than normal or between 8.0 mm and 11.5 mm in front of the Na-B bone and 4 mm in front of the Na-A bone. For patients with thick lips, the position of the incisor did not affect the perception of the profile.

Conclusions

A correct skeletal scheme or Class I occlusion does not necessarily result in ideal facial harmony. Therefore, it will be necessary to consider all the factors that come into play, especially the lips, which are a key element in facial esthetics.

Highlights

  • Raters preferred an advanced position of the incisor in patients with thin lips.

  • The position of the incisor had no impact on the profile in patients with thick lips.

  • A normal skeletal scheme does not necessarily mean ideal facial harmony.

  • Evaluation of the ideal position of the maxillary incisor relative to upper lip thickness.

The improvement of facial esthetics is a fundamental objective of orthodontic treatment. Soft tissue analysis is part of the judgment and diagnostic criteria that allow us to develop a treatment plan. One of the criteria for the success of this treatment is the final appearance and, therefore, the soft tissue arrangement. Individual traits and their harmony with each other must be identified before starting treatment, but this is not always easy because soft tissues do not always follow the underlying hard tissue. This issue is due to the great variability of the thicknesses of the cutaneous tissues. Thus, it would be better to analyze and understand the factors that affect it and on which we can act; 1 such factor is the maxillary incisor and the upper lip. The original position of the incisors, their inclination, as well as the soft tissue, specifically the lips, must be carefully evaluated. ,

The contour of the face should be considered as an important guide in the implementation of orthodontic treatment. This fact can be explained by the large individual variability in the thickness, length, and tone of the soft tissue covering the teeth and bones, and because the cephalometric analyses were performed on a population of patients who showed no skeletal disharmony. Therefore, it would be difficult to adapt these normal measures to patients who present some disharmony. This notion led us to undertake this investigation. Even if patients possess ideal or normal cephalometric values, it does not necessarily mean that the facial esthetics will be perfect.

Several studies have found ideal maxillary incisor positions that determine an aesthetically acceptable profile, but to the best of our knowledge, none have considered the individual variability of the thickness of the upper lip, which is an important factor in the determination of the protrusion of the lips.

Therefore, this study aims to determine, from profile photographs of patients with Class I occlusion, the ideal anteroposterior positions of the maxillary incisor while taking the variability of the thickness of the lip into consideration.

Material and methods

The inclusion criteria for the subjects were as follows: a white patient at the Faculty of Dental Medicine at Saint Joseph University in Beirut, Lebanon with skeletal Class I occlusion, aged >10 years, whose lateral cephalogram was taken at the radiology department of Saint Joseph University in Beirut and profile picture was taken within 1 month of the cephalogram. Cephalograms were taken by the same operator with the same machine, and the profile pictures were taken from a distance of 3 meters, with the patient looking straight into a mirror in a true vertical position. The exclusion criteria were the following: full beard or mustache, incisal implant, anterosuperior edentation, any cosmetic surgery or filling at the level of the upper lip, congenital anomalies, age <10 years, facial abnormalities, or facial surgery—especially at the level of the lips. This study was approved by the ethical committee of Saint Joseph University, Beirut.

Records were collected from the radiology site database and case presentations of the orthodontic department at Saint Joseph University from the past 10 years. Initially, 130 patients with skeletal Class I occlusion (60 male and 70 female subjects) were identified on the basis of 2 measurements, one angular (ANB between 0° and 4°) and the other linear (AO-BO between −2 mm and 2 mm).

These 130 patients were divided according to several factors. The first distribution was made according to sex. Subsequently, 3 new groups were formed according to age: 10-13 years, 14-18 years, and >18 years. This distribution was made on the basis of studies performed on the thickness of the lip that concluded that the age of 14 years is critical because after this age, the thickness of the lip stops increasing. The thickness was measured on the profile cephalograms taken previously, and they were divided into 2 groups according to this variable: thin lip or normal-sized lip and thick lip. To calculate the thickness of the lip, we based our study on the 2014 article by Bergman et al, which is the most recently published article and followed patients between the ages of 6 and 18 years. The thickness of the upper lip was measured from the skin vermilion point to the inside of the lip where the maxillary incisor rests. ( Fig 1 )

Fig 1
Measurement of the thickness of the lip.

We used the anatomical points method demonstrated by Hwang because it was reliable and calculated the intraclass correlation coefficient, which was 0.92. A final distribution was made according to the angulation of the maxillary incisor, and 2 groups were then formed: those presenting a normal angulation to the Frankfort plane between 102° and 112° and those presenting an accentuated angulation of the incisors >112°. We chose these measurements because they are often used in our orthodontic practice.

Although facial esthetics are not fully evaluated by a single method of analysis, the profile view provides necessary information for diagnosis and treatment planning. Profiles are evaluated on photographs or lateral cephalograms. Color photographs have been shown to provide more facial details and are more realistic than silhouettes or drawings. For this study, 70 profile pictures were chosen from the 130 patients. The 60 not selected were excluded because the photographs were unclear or had similarities with other photographs regarding the position of the incisor. To reduce bias that could be encountered during the judgment, the photographs were cropped, leaving a frame extending from the suborbital point to below point B. Thus, the hair, eyes, upper part of the nose, and chin were eliminated to increase the focus at the level of the upper lip. This was also done because the chin has a considerable impact on people’s opinions concerning profile pictures. In total, 20 groups were formed according to the many distributions made. Four female groups could not be completed because of a lack of patients presenting specific measurements ( Table I ).

Table I
Distribution of the sample collected
Age Male subjects Female subjects
10-13 years Set 1: Thin lips with normal position of the incisors
Set 2: Thin lips with proclined incisors
Set 3: Thick lips with normal position of the incisors Set 13: Thick lips with normal position of the incisors
Set 4: Thick lips with proclined incisors Set 14: Thick lips with proclined incisors
14-18 years Set 5: Thin lips with normal position of the incisors Set 15: Thin lips with normal position of the incisors
Set 6: Thin lips with proclined incisors Set 16: Thin lips with proclined incisors
Set 7: Thick lips with normal position of the incisors Set 17: Thick lips with normal position of the incisors
Set 8: Thick lips with proclined incisors Set 18: Thick lips with proclined incisors
>18 years Set 9: Thin lips with normal position of the incisors Set 19: Thin lips with normal position of the incisors
Set 10: Thin lips with proclined incisors Set 20: Thin lips with proclined incisors
Set 11: Thick lips with normal position of the incisors
Set 12: Thick lips with proclined incisors

Several studies have shown that the treatment goals desired by patients depend on their social and cultural backgrounds. If certain criteria require the opinion of the orthodontist, the evaluation of the beauty or attractiveness of the face should be left to the general public, as evidenced by several studies by orthodontists and psychologists.

Orthodontists, then, need to understand the preferences and trends of the public to compare them with the perspectives and norms on which they are normally based. Therefore, this evaluation was made in the form of a survey among 60 laypeople, 60 dentists, and 60 orthodontists, by asking them to classify in each set, the photographs from the most esthetic position to the least esthetic position of the upper lip of the subjects presented. After obtaining informed consent from all the patients involved, the survey was circulated ( Figs 2 and 3 ). After 1 month, the same survey was sent to 20% of the participants (ie, 36 people) to assess the reproducibility and reliability of the results. In addition, it was sent to another 20% of the participants after the pictures were standardized and made unicolor ( Fig 4 ), to see if the results might change according to skin tone.

Fig 2
Profile photographs of patients from sets 1 to 10.

Fig 3
Profile photographs of patients from sets 11 to 20.

Fig 4
Example of the changes made to the skin tone (A) for the male subjects and (B) for the female subjects.

After analysis of the results, we found that 3 measurements determine the ideal position of the maxillary incisor with regard to the position of the upper lip. The lines and angles of reference that are almost constant give us the best chance to determine treatment objectives that can be used as guidelines during our treatment. For example, the N-A line has proven to be constant because, according to the longitudinal studies, the SNA angle changes by only 1°, on average, over 5 years. The chosen reference lines in our study are Na-A bone, Na-B bone, and Na-B soft tissue. We did not choose Pog to avoid being influenced by the chin. The distance between each incisor tip and these 3 lines was measured.

Statistical analysis

SPSS statistical software (version 24; SPSS for Windows, Chicago, Ill) was used for statistical analysis of data. The significance threshold used corresponds to P value ≤0.05. chi-square and Fisher exact tests were used to compare percentages of responses between laypeople, dentists, and orthodontists. Kappa test was used to determine the intraexaminer reliability and the reproducibility of the results when the skin tones are changed.

Results

A high level of agreement was noted with kappa coefficients between 0.73 and 0.82 with a P <0.001; this was measured by the kappa test, which was used to measure rater agreement for each set of pictures. The detailed results of each set are found in Tables II and III . Table IV shows which photograph of each set got the highest percentage given in total by the laypeople, dentists, and orthodontists.

Table II
Detailed results showing the percentages of the number of times each picture was chosen in each set
Set Photograph Laypeople Dentists Orthodontists Total P value
1 1 21 (35.0) 10 (16.7) 13 (21.7) 44 (24.4) 0.019
2 24 (40.0) 38 (63.3) 40 (66.7) 102 (56.7)
3 15 (25.0) 12 (20.0) 7 (11.7) 34 (18.9)
2 1 2 (3.3) 3 (5.0) 1 (1.7) 6 (3.3) 0.752
2 1 (1.7) 3 (5.0) 4 (6.7) 8 (4.4)
3 24 (40.0) 23 (38.3) 27 (45.0) 74 (41.1)
4 33 (55.0) 31 (51.7) 28 (46.6) 92 (51.1)
3 1 15 (25.0) 15 (25.0) 21 (35.0) 51 (28.3) 0.373
2 45 (75.0) 45 (75.0) 39 (65.0) 129 (71.7)
4 1 0 (.0) 4 (6.7) 2 (3.3) 6 (3.3) 0.008
2 7 (11.7) 9 (15.0) 2 (3.3) 18 (10.0)
3 24 (40.0) 29 (48.3) 39 (65.0) 92 (51.1)
4 29 (48.3) 18 (30.0) 17 (28.3) 64 (35.6)
5 1 37 (61.7) 45 (75.0) 46 (76.7) 128 (71.1) 0.135
2 23 (38.3) 15 (25.0) 14 (23.3) 52 (28.9)
6 1 45 (75.0) 48 (80.0) 43 (71.7) 136 (75.6) 0.659
2 5 (8.3) 5 (8.3) 8 (13.3) 18 (10.0)
3 6 (10.0) 2 (3.3) 6 (10.0) 14 (7.8)
4 4 (6.7) 5 (8.3) 3 (5.0) 12 (6.7)
7 1 16 (26.7) 9 (15.0) 7 (11.7) 32 (17.8) 0.184
2 27 (45.0) 25 (41.7) 25 (41.7) 77 (42.8)
3 13 (21.7) 16 (26.7) 21 (35.0) 50 (27.8)
4 4 (6.7) 10 (16.7) 7 (11.7) 21 (11.7)
8 1 7 (11.7) 7 (11.7) 6 (10.0) 20 (11.1) 0.046
2 13 (21.7) 20 (33.3) 28 (46.7) 61 (33.9)
3 8 (13.3) 4 (6.7) 5 (8.3) 17 (9.4)
4 32 (53.3) 29 (48.3) 21 (35.0) 82 (45.6)
9 1 18 (30.0) 9 (15.0) 15 (25.0) 42 (23.3) 0.141
2 42 (70.0) 51 (85.0) 45 (75.0) 138 (76.7)
10 1 27 (45.0) 37 (61.7) 44 (73.3) 108 (60.0) 0.006
2 33 (55.0) 23 (38.3) 16 (26.7) 72 (40.0)
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Aug 20, 2020 | Posted by in Orthodontics | Comments Off on Evaluation of the ideal position of the maxillary incisor relative to upper lip thickness
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