All PhD Theses

P.A.A.M. van der Geld

Smiling and speaking in a dental perspective. A dental and psychological approach to the smile line, tooth display, and smile attractiveness.


A scientific essay in Medical Sciences

DOCTORAL THESIS defended in public on 15th of September 2009


Chapter 1 describes the smile from a psychological and dental perspective. From a psychological point of view, smiling and laughing can be considered as part of expressive behavior. Next to speech and facial expression, physical appearance influences the interpersonal communication. The oral area becomes a centre of attention when people speak, smile, and display their feelings. As a result of this, the oral area plays an important part in facial attractiveness. Moreover a bright smile is associated with sympathy, extraversion, intelligence, and attractiveness. A number of psychological theories predict that attractiveness of an individual has a significant influence on the judgments and treatments by others and on the behavior and traits of the individual itself. Metaanalysis of facial attractiveness studies has shown that attractive children and adults exhibited more positive behaviors and traits than unattractive persons. A study also suggests a substantial relation between smile attractiveness and a positive psychological state, but until now there is no convincing evidence for this indication. Furthermore some studies show that other’s perception of attractiveness can diverge with self-perception. Moreover there are indications for discrepancies between the dental professional norms and the patients’ satisfaction regarding the esthetics of their dental appearance. The question remains how patients judge their smiles and dental appearance in everyday life and which esthetic aspects are meaningful in the self-perception of smile attractiveness. From a dental point of view a balanced lip-tooth relationship forms the basis of an attractive smile. The elevation of the lips during smiling determines the display of the teeth and thus the height of the smile line. Consequently the smile line can guide the choice and realization of a dental therapy from an esthetic point of view. A smile line which displays only a portion of the teeth is often associated with aging. However, in case of a high smile line, the effects of tooth loss or esthetic complications of the teeth or gums are more exposed. Attention for the smile line is therefore essential in dentistry and dental esthetic diagnostics. Yet attention for the smile line deserves some remarks. The contemporary methods are not or insufficiently designed to determine the smile line adequately. These methods are mainly designed for analysis of the posed social smile and not for spontaneous ‘Duchenne’ smiling. Furthermore empirical data concerning tooth display during spontaneous smiling and speaking is rather limited. Besides, data is lacking concerning the effects of maturation and aging on the height of the smile line and tooth display.

Chapter 2 concerns the development of a method to measure tooth display and lip-line height during smiling and speaking. Tooth display and lip position in smiling are important aspects of the esthetic analysis in the practice of orthodontics and dentofacial surgery. The spontaneous smile and speech are considered valuable diagnostic criteria in addition to the posed social smile. The digital videographic measurement method was developed to measure tooth display in both smile types and in speech. In this chapter the digital videographic measurement method was applied in clinical practice and its reliability established. The faces of 20 subjects, while watching a video film, were individually filmed with a digital video camera. Spontaneous smiles were elicited by a video film with practical jokes. Posed social smiles and speech records were also included. The recordings of smiling and speech were transferred to a computer and analyzed on videoframe level. The dynamics of the spontaneous smile were captured twice and two independent raters were involved in the measurements. The statistical analysis of the reliability of the digital videographic measurement method was established by means of the generalizability theory. It incorporated rater, replication, and selection facets. The generalizability coefficients ranged from .99 for the anterior teeth to .80 for the posterior teeth. Inaccuracies were mainly associated with rater and selection facets. The replication facet was a minor source of inaccuracy. This digital videographic measurement method is a reliable approach to determine tooth display and lip position in spontaneous and posed smiling, and speaking. Application of the method (watching the comical video film included) is warranted especially when obtaining an emotional smile is difficult, such as cleft lip and palate or disfigured patients.

Chapter 3 presents the study to self-perception of smile attractiveness. The role of smile line and other aspects of smile-esthetics on the selfperception were determined and their influence on personality traits was investigated. In this study, the subjects were requested to assess their smile attractiveness by means of a patient-specific questionnaire. On the left page a spontaneous smiling photograph of the subject was printed. On the right page, subjects judged the general attractiveness of and satisfaction with the smile. Furthermore they assessed the contribution to the smile attractiveness of size and color of teeth, position and visibility of teeth, the upper lip position and the visibility of gums. In addition the objective smile line height of each subject was measured using a digital videographic method for smile analysis. The personality of the subjects was assessed with the Dutch Personality Index (NPV). The smile judgment questionnaire achieved high reliability (Cronbach’s α = .77). The results showed that size of teeth, visibility of teeth, and upper lip position were critical factors in self-perception of smile attractiveness (social dimension). Color of teeth and gingival display were critical factors in satisfaction with smile appearance (individual dimension). Subjects, smiling with their teeth entirely displayed and some gingival display (two to four millimeters),  perceived their smile line as most esthetic. However, smiles with disproportional gingival display were judged negatively. This negative perception correlated with the personality characteristics of neuroticism and selfesteem. A positive perception of visibility and position of teeth correlated with dominance. The results of this research underpin the psychosocial and dental importance of an attractive smile.

The study described in chapter 4 investigates tooth display and the position of the lips during spontaneous smiling and the posed (social) smile. The differences were studied between the posed smiling record, traditionally produced for orthodontic diagnosis, and the spontaneous (Duchenne) smile of joy, determined with the digital videographic measurement method. The study focused on differences in tooth display, lip-line height, and smile width. The faces of 122 male subjects were each filmed during spontaneous smiling. Spontaneous smiles were elicited through the subjects watching a comical movie. Additionally a posed smile record was made of each subject. The smiles were analyzed and measured by means of the digital videographic measurement method. Measurements were performed of the maxillary and mandibular lip-line heights, the display of the teeth, and the smile width. The maxillary lip-line heights during spontaneous smiling were significantly higher than during posed smiling. Compared to spontaneous smiling, tooth display in the (pre)molar area during posed smiling decreased by up to 30%. Furthermore smile width decreased significantly. During posed smiling, also mandibular lip-line heights changed and the teeth were more covered by the lower lip than during spontaneous smiling. From a diagnostic point of view, spontaneous smiling records next to posed smiling records are indicated in dental practice. This requires a digital videographic method because of the dynamic nature of spontaneous smiling. In this way a more realistic diagnosis from lip-line height, smile arc, buccal corridors, and plane of occlusion is obtained.

The aims of the study, presented in chapter 5, were to analyze lip-line heights and age effects in an adult male population during spontaneous smiling, speech, and tooth display in the natural rest position. Furthermore, it was determined whether lip-line height follows a consistent pattern during these different functions. The sample consisted of 122 randomly selected male subjects from three age cohorts (20–25 years, 35–40 years, and 50–55 years). The lip-line heights were measured with the digital videographic method for smile analysis. The maxillary lip-line heights during spontaneous smiling were generally higher in the premolar area than at the anterior teeth. The esthetic zone in 75 per cent of the participants included all maxillary teeth up to the first molar. Significant correlations confirmed that the maxillary lip-line height of a person follows a coherent pattern during spontaneous smiling, speech, and tooth display in the natural rest position. In older subjects, maxillary lip-line heights decreased significantly in all situations. Lip-line heights during spontaneous smiling were reduced by approximately 2 mm. The mandibular lip-line heights also changed significantly in older subjects. During spontaneous smiling the mandibular teeth were displayed less, but tooth display in the rest position increased significantly. Upper lip length increased significantly by almost 4 mm in older subjects, whereas upper lip elevation did not change significantly. The significant increasing lip coverage of the maxillary teeth indicates that the effects of age should be included in orthodontic treatment planning.

In chapter 6 a study is described in which exact measuring of the smile line-height is compared with visual estimation. By means of the digital videographic measurement method, spontaneous smiling can be captured and analyzed, and the smile line height can be measured. However, exact measurements of the smile line and tooth display might not be required for all soft tissue analyses. Furthermore quantitative measurements are relative time consuming and therefore less suitable for routine diagnostic screening of the smile line. Next to quantitative measurements, another method to determine smile line height is to use visual estimation. Visual estimation is performed using a standard categorization and is referred to as the semi-quantitative method. The semi-quantitative estimation can be based on a 3-grade scale (a low-, average-, and high smile line) or a 4-grade scale (a low-, average-, high-, or gummy smile line). The reliability of the semi-quantitative estimation with either the 3- and 4-grade scale estimation was established in a sample of 122 subjects. Two raters were included and a replication measurement was performed. In addition quantitative measurements of the smile line height and tooth display were performed with the digital measurement method, which had previously been tested and found reliable. Inter- and intra-examiner reliability was high with median kappa values from 0.79 to 0.88. Agreement of 3-grade scale estimation with quantitative measurement showed higher median kappa values (0.76) than the agreement of the 4-grade scale estimation with quantitative measurement (0,66). Consequently the estimation of a low-, average-, and high smile line for each tooth showed high reliability close to quantitative measurements. Smile line analysis can be performed reliably with a semiquantitative 3-grade scale visual estimation. An exception is the specific category of 'gummy smile patients' for whom measuring of the height of the smile line is needed for a more comprehensive diagnosis.  

In chapter 7 the overall results of the study are discussed. Further study topics include the patients’ perception of oral esthetics, oral esthetics as part of facial attractiveness, and in the context of body perception of an individual also in interaction with others. Furthermore longitudinal studies are needed into aging processes and their influence on smile line heights and tooth display. The fast progressing technology for capturing emotions, such as face detection technology, can offer an important contribution to future research of the smile line.