Effect of mesiodistal angulation of the maxillary central incisors on esthetic perceptions of the smile in the frontal view

Introduction

The purpose of this study was to analyze the effect of mesiodistal angulation of the maxillary central incisors on the smiling esthetics in young adults.

Methods

Frontal smile photographs of a young man and a young woman were digitally modified to produce 28 smile images that were grouped into 4 series. These images were judged using visual analog scales by 52 orthodontists and 61 laypeople. The effects of the judges’ professions, subjects’ sexes, and photograph framings on the appreciation of smile esthetics with different mesiodistal angulations of the maxillary central incisors were evaluated. The data were analyzed with paired t tests, Dunnett t tests, and independent samples t tests; statistical significance was set at P <0.05.

Results

The images were ranked less attractive as the mesiodistal angulation of the maxillary central incisors increased during smiling. No relationship was found between the judges’ professions and the esthetic evaluations of incisal angulation, but the statistical analysis showed that both the subject’s sex and the photograph framing were significant variables ( P <0.05).

Conclusions

The mesiodistal angulation of the maxillary central incisors plays an essential role in smile esthetics in the frontal view. When formulating treatment plans, orthodontists should never underestimate the influence of mesiodistal angulation on smile attractiveness.

Highlights

  • Increased axial inclination of maxillary central incisors was judged to be less attractive.

  • The judges’ perception of the mesiodistal inclination of incisors showed a sex bias.

  • Evaluation of lower facial third photographs was more reliable than evaluation of the full face.

Smiling is the most common facial expression used to express friendliness, agreement, and appreciation. It is no surprise that improving smile attractiveness is a main motive for encouraging patients to seek oral care, and it is also a principal objective of orthodontic and prosthodontic treatment. A survey showed that 80% of adults seeking orthodontic care for themselves or their children are motivated by the desire to improve facial attractiveness, regardless of structural or functional considerations. However, the concept of esthetics is difficult to quantify because it is influenced by many factors, especially the principles of esthetics and the observations of groups who are considered to be esthetically privileged. Nevertheless, many investigators have tried to explore common values for facial esthetics to provide a basis for esthetic goals.

The position of the maxillary anterior teeth is a fundamental component of an attractive smile, determined through both common sense and professional evaluation. A previous study indicated that moving the maxillary anterior teeth could maximize the potential for improving a patient’s smile attractiveness. The effect of labiolingual and anteroposterior discrepancies in the maxillary incisors on the esthetics of the profile view of a smile has been intensively studied. Cao et al indicated that both maxillary incisor labiolingual inclination and anteroposterior position play essential roles in facial esthetics. Ghaleb et al further studied how the anteroposterior inclination of the maxillary incisors affects the esthetics of the profile view. In contrast to the labiolingual and anteroposterior inclinations, the effect of the mesiodistal angulation of the maxillary incisors on smile attractiveness is unclear. The mesiodistal axial angulation of anterior teeth is important in the 3-dimensional control of an ideal occlusion; therefore, it can influence the normal function of occlusion to a certain extent. The degree of incisor angulation in the mesiodistal plane determines the amount of mesiodistal space these teeth use and, therefore, affects the amount of mesiodistal space available for the posterior teeth. This has a considerable impact on the posterior occlusion.

Facial photographs are commonly used in sociopsychological studies, and judgments pertaining to photographs are shown to be fairly consistent with judgments pertaining to real people. Full facial photographs and lower facial third photographs are 2 commonly used framings. It appears that lower facial third photographs may allow easier control of background interferences (eg, hair contour and style, eye shape, skin tone, and complexion). Nevertheless, in real-life situations, people often observe a person as a whole rather than focusing on individual aspects. However, research comparing these 2 framings remains deficient.

Female beauty has taken a central place in art and culture for centuries, but more attention has been devoted to male attractiveness over the past few decades. The concept of esthetics may not be congruent between patients and dentists because the latter are professionally trained for esthetic appreciation. As Peck and Peck reported, “We orthodontists tend to forget that facial esthetics is a subject that interests all people everywhere, and the ultimate source of esthetic values should be the people and not just ourselves.” Therefore, the purposes of our study were to evaluate the effect of the mesiodistal angulation of the maxillary central incisors on the esthetic perception of the frontal smile view in young Chinese adults. The following factors were analyzed: (1) mesiodistal angulation of the maxillary central incisors, (2) photograph framing used for analysis, (3) judges’ professions, and (4) sexes of subjects.

Material and methods

Frontal smiling photographs of an 18-year-old woman and a 20-year-old man were selected based on the following selection criteria: permanent dentition without retained deciduous teeth, Class I occlusion, hard and soft tissues within normal limits as determined by cephalometric analysis software (Winceph version 7.0; Rise, Sendai, Japan), harmonious high smile, and normal overjet and overbite. Informed consent was obtained from the subjects for the use of their photographs. Frontal photographs were taken with a digital camera (D100; Nikon, Tokyo, Japan) in the “esthetic position” as recommended by Bass.

The photographs were altered using image editing software (Adobe Photoshop Creative Suite 4, version 11.0; Adobe, San Jose, Calif). The facial axis of the clinical crown was deemed the rotation axis whose rotation center is the intersection point between the facial axis of the clinical crown and the marginal gingiva. The sagittal plane was used as the reference plane. The angulation of the facial axis of the clinical crown relative to the reference plane was altered; this caused the maxillary central incisors to incline mesially and distally by up to 15°. The distal angulation was designated as positive, and the mesial angulation as negative ( Fig 1 ). In a preliminary study, the acceptable axis angulation extent was judged by 10 orthodontists and 10 laypeople. The results showed that it was unacceptable for all judges when the angulation of the facial axis of the clinical crown extended beyond −6° and +8°. Importantly, increasing the mesiodistal angulation required more space, which was not available clinically; therefore these angulations of the teeth were excluded from the experiment. Finally, 7 images were obtained: 2 mesially inclined images (−2°, −4°), 4 distally inclined images (+2°, +4°, +6°, and +8°), and 1 unchanged image (0°). Artistic editing was used to maintain a natural appearance when necessary. All images were cropped into full-face or lower facial third photographs of the same size. A condensed version of the altered photographs is shown in Figure 2 . Digitized images were calibrated to control the background noise using a method that was suggested to be a successful way to study variations in the assessment of facial esthetics.

Fig 1
Diagram of the axial inclinations of the maxillary central incisors, which were inclined mesially by 2° and 4° (labeled −2° and −4°) and distally by 2°, 4°, 6°, and 8° (labeled +2°, +4°, +6°, and +8°); 0° was the undeviated control.

Fig 2
Axial angulations of the maxillary central incisors in the female and male models: A, male model; B, female model. A 1 and B 1 show lower facial third photographs; A 2 and B 2 are the full-face photographs. The maxillary central incisors were inclined mesially by 2° and 4° (labeled −2° and −4°) and distally by 2°, 4°, 6°, and 8° (labeled +2°,+4°, +6°, and +8°); 0° was the undeviated control.

After the alterations, 2 series comprising 28 smiling photographs were arranged according to a random number table. A professional panel (52 persons) and a nonprofessional panel (61 persons) were selected to evaluate these photographs. Each photograph was fixed with a 100-mm visual analog scale. The visual analog scale was anchored by the descriptors “least attractive” (0) and “most attractive” (100). This method has been endorsed by many investigators for use in attractiveness ratings because of its simplicity and rapidity. Scores ≤60 were rated as unacceptable, those 61 to 80 were acceptable, and >80 were considered attractive. To simulate social distance, the evaluators were asked to maintain a distance of 60 to 80 cm from the images. To prevent any interference of the evaluators’ visual memory, there was a 5-second time gap between each image, and each photograph was evaluated twice by each evaluator, with a minimum interval of 2 weeks, and the mean score was recorded.

Statistical analysis

SPSS software (version 17.0; SPSS, Chicago, Ill) was used to analyze the data. The means of the raw data were determined to assess whether there were specific preferences for smiling frontal photographs. All altered pictures were divided into the following 4 groups: lower facial third photographs, full-face photographs, male subject, and female subject. The paired t test was used for determining the significance of the judges’ professions (professional vs lay), photograph framings (lower facial third vs full-face photographs), and sex of the subject (male vs female). The Dunnett t test and the independent samples t test were used to analyze the differences between the varied incisor angulations (7 variations). The level of significance was set at P <0.05 for all statistical tests, unless otherwise stated.

Results

The judges who participated in the survey consisted of 52 orthodontists or senior orthodontic residents and 61 laypeople. The analysis showed that the mesiodistal angulation of the maxillary central incisors had an apparent influence on smile attractiveness in the frontal view. Attractiveness scores for the male subject, from high to low, of both image framings ranked as follows: 0°, −2°, +2°, −4°, +4°, +6°, and +8°. For the female group, the scores between the 2 photograph framings were slightly different. In the lower facial third photographs group, the ranking from high to low was 0°, −2°, +2°, −4°, +4°, +6°, and +8°, whereas in the full-face photograph group it was 0°, +2°, −2°, −4°, +4°, +6°, and +8°. The most attractive image was the undeviated control (0°), and the least attractive image was the +8° photograph in all 4 groups ( Table I ). We compared the attractiveness scores between the undeviated control (0°) and other angulations of the maxillary central incisors ( Table II ) and showed that there were significant differences between the undeviated control and the following angulations: −4°, +4°, +6°, and +8° in the lower facial third photographs of the man; −4°, +4°, +6°, and +8° in the full-face photographs of the woman; +4°, +6°, and +8° in the full-face photographs of the man; and all angulations, except −2°, in the lower facial third photographs of the woman.

Table I
Comparison of attractiveness scores rated by the professional panel vs the lay panel for mesiodistal angulation of maxillary central incisors
Photograph Angulation Professional panel (n = 52) Lay panel (n = 61) P
Mean SD Mean SD
Male, lower facial third 80.03 11.263 79.94 10.391 0.965
−4° 74.34 10.745 73.63 10.648 0.726
−2° 77.75 10.183 78.54 8.084 0.655
+2° 79.39 10.246 76.23 9.093 0.088
+4° 61.56 17.308 65.54 12.379 0.169
+6° 66.93 14.779 68.63 9.909 0.482
+8° 63.80 13.208 65.75 11.218 0.405
Male, full face 78.13 11.351 80.17 8.784 0.284
−4° 75.92 11.691 75.29 8.716 0.744
−2° 77.69 11.201 79.58 7.536 0.290
+2° 77.15 12.854 78.77 8.471 0.424
+4° 72.13 11.874 72.42 8.781 0.625
+6° 67.98 13.373 71.92 9.478 0.884
+8° 67.07 14.120 68.23 10.487 0.078
Female, lower facial third 79.05 11.685 80.46 10.818 0.509
−4° 74.02 11.744 73.81 10.419 0.921
−2° 76.69 11.568 77.69 9.298 0.616
+2° 76.26 15.102 73.85 9.801 0.325
+4° 70.33 15.135 72.33 9.833 0.401
+6° 68.84 14.405 70.92 9.231 0.371
+8° 65.00 15.460 65.54 10.408 0.826
Female, full face 79.69 11.810 80.31 8.540 0.748
−4° 76.05 12.577 75.35 10.062 0.746
−2° 76.70 10.671 76.73 9.408 0.989
+2° 77.62 10.407 77.13 7.470 0.773
+4° 74.80 12.153 74.31 9.574 0.813
+6° 73.89 11.746 73.29 8.180 0.752
+8° 69.56 15.176 71.13 10.888 0.533
P values were obtained by independent samples t tests.

Table II
Comparison of attractiveness scores rated by the undeviated control (0°) vs other degrees of maxillary central incisor angulation
Dependent variable Angulation Control Mean difference (Angulation–Control) SE P
Male, lower facial third −4° −5.973 1.559 0.001†
−2° −1.876 1.559 0.682
+2° −2.053 1.559 0.598
+4° −16.602 1.559 0.000†
+6° −12.274 1.559 0.000†
+8° −15.292 1.559 0.000†
Male, full face −4° −3.442 1.455 0.085
−2° −0.513 1.455 0.999
+2° −1.177 1.455 0.921
+4° −11.469 1.455 0.000†
+6° −6.805 1.455 0.000†
+8° −9.274 1.455 0.000†
Female, lower facial third −4° −5.779 1.610 0.002†
−2° −2.549 1.610 0.410
+2° −4.549 1.610 0.025*
+4° −9.903 1.610 0.000†
+6° −8.451 1.610 0.000†
+8° −14.451 1.610 0.000†
Female, full face −4° −4.248 1.446 0.018*
−2° −3.257 1.446 0.111
+2° −2.575 1.446 0.293
+4° −5.398 1.446 0.001†
+6° −6.363 1.446 0.000†
+8° −9.690 1.446 0.000†
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Apr 6, 2017 | Posted by in Orthodontics | Comments Off on Effect of mesiodistal angulation of the maxillary central incisors on esthetic perceptions of the smile in the frontal view
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