The smile is a significant part of facial esthetics and requires the teeth to be in harmony with the lips, gingiva, and other tissues of the face. The harmonious smile is not only characterized by the form, contour, position, and color of the teeth but also by the health of the periodontal tissues, their marginal position, as well as their visibility and the dento-labio-gingival relationship (1). Additionally, the degree of anterior tooth exposure and the placement of the upper lip in a smile is dependent on the demographic parameters of age and sex, and should be considered individually for each patient (2).

Smile line: The relationship between the upper lip and the exposed anterior teeth and gingiva when smiling, and determines how much of the periodontium is visible (3). It is an important component of the dentolabial-gingival relationship and is affected by the mobility of the lips, lip length, clinical crown height, muscular activity and underlying skeletal relationships. Evaluation of natural and forced smiles has shown that the amount of gingival and tooth exposure can differ depending on the type of smile that is being made, making it important to standardise smile conditions when assessing the smile clinically and photographically (4). In addition, age and sex may affect the appearance of incisors because of changes in soft tissue and lip position over time, which can change the amount of maxillary anterior teeth that are displayed when speaking and smiling (5).

Another factor of smile line position is lip morphology. A cross-sectional evaluation of lip length and smile line has shown that these are related and can differ between population groups, which may offer an anatomical explanation for the differences in gingival exposure when smiling (6). The visibility of the interdental papillae in the anterior maxilla is another factor that affects smile esthetics. The concept of an interdental smile line has been introduced in clinical evaluation of papilla display, which shows that the esthetic evaluation should not only include the height of the upper lip but also the appearance of the gingival tissues between the adjacent anterior teeth (7). These factors are clinically significant as they can lead to too much or too little gingival display, which can change the harmony of an otherwise well-aligned dentition.

Another aspect of smile harmony is the position of the maxillary anterior teeth with respect to the facial midline. Relative anterior dental landmark deviations, even as small as 1 mm, can impact esthetic perception, but this acceptable deviation may differ from one observer to another (8). Likewise, the anteroposterior position of the maxillary central incisors is important for the esthetic appearance of the smiling facial profile and must be considered in conjunction with both the dental and soft-tissue reference lines (9). Ethnic, professional, gender and social background may also affect how people perceive a dental midline deviation, indicating that smile attractiveness is influenced by the characteristics and expectations of the observer as well as the physical features of the smile (10).

Tjan et al. divided the smile line into three types, and these are still the most popular types. A low smile line is defined as less than 75% of the clinical crown height of the maxillary incisors, an average smile line is defined as 75–100% of the clinical crown height of the maxillary incisors, and a high smile line is defined as 2 mm of exposure of the maxillary gingiva (11). The 454 young participants in the study had the average smile line as the most common smile line (68.9%), and the authors found that patients with a gummy smile need the most esthetic attention (3).

Increased gingival display at smile has been reported in females compared to males by several researchers, perhaps due to the longer upper lip in males (3). Tjan et al. reported that 14% of women had a high smile line, while 7% of men had a high smile line, and 30% of men had a low smile line, compared to 12% of women (11). In a Nepalese population, Sapkota et al. found the same, in that the high smile line was present approximately twice as often in females, and the low smile line was present approximately twice as often in males (4, 12).

The smile line is also affected by age. In 2/3 (66.7%) of the participants older than 51 years, Sapkota et al. reported a low smile line, while high smile lines were mostly reported in younger participants (4). Similarly, Jensen et al. found that the smile line lowered significantly with age (13), and Dong et al. noted that smiling exposure of the maxillary central incisors decreased with age (14). Drummond and Capelli observed that the maxillary display of the central incisor at rest, speech, and smile significantly decreases with age, especially among males (5).

Ethnic differences have also been outlined. The average smile line was the most common smile line among Malay participants, Chinese participants, and Indian participants in a Malaysian population, but the high smile line was the most common smile line in the Chinese participants (43.0%) (6). Smile lines were reported to be low in 49.5% of Indians by Sepolia et al. (15) and higher in Han-Chinese than in Caucasians by Hu et al. (16) and were attributed to differences in musculoskeletal and facial profile. To the best of our knowledge, this is the first study to report on the prevalence of smile line types among the ethnic groups of the Pakistani population.

Hence smile evaluation is a multidimensional process, which includes tooth proportions and tooth position, the visibility of the gums, the movement of the lips, facial relationships, and demographic characteristics. In recent years, the posed smile has also been evaluated and several smile-related characteristics have been shown to work together to enhance esthetic appearance, not as individual features (17). As part of this concept, the smile line type is a very helpful clinical parameter, as it is directly related to the amount of maxillary anterior tooth and gingival display during smiling. It is essential to know its distribution and its association with demographic parameters for the planning of treatment in restorative dentistry, prosthodontics, periodontology and orthodontics. Thus, it is clinically relevant to investigate the correlation between smile line type in the maxillary anterior dentition and other variables like age, gender and ethnicity for personalized esthetic diagnosis and treatment planning.

Objective: To evaluate the association of smile line type in the maxillary anterior dentition with age, gender, and ethnicity among dental outpatients.

Null hypothesis (H0): There will be no relationship between smile line in the maxillary anterior dentition with respect to age, gender, and ethnicity.

Alternative hypothesis (Ha): There will be a relationship between smile line in the maxillary anterior dentition with respect to age, gender, and ethnicity.

Operational Definitions

Smile line type was defined according to Tjan et al. (11) as follows:

Low smile line: Less than 75% of the clinical crown height of the maxillary incisors is visible.

Average smile line: 75–100% of the clinical crown height of the maxillary incisors is visible.

High smile line: 2 mm of maxillary gingival exposure is visible.

Study Design: Analytical cross-sectional study Study Setting: The study was conducted at the Dental Out Patient Departments of Dow University of Health Sciences, Karachi. Duration of the Study: The duration of the study was 12 months. Study Population: The young adult population of Karachi attending the Dental OPD. Inclusion Criteria: Participants aged 18–30 years with complete maxillary anterior dentition, without periodontal or gingival problems or rehabilitation that would compromise a good tissue-to-tooth relationship, without interdental spacing, rotation, or crowding (Grade 1 of the Index of Orthodontic Treatment Need), and with no history of orthodontic treatment. Exclusion Criteria: Participants with gingival changes or dental abnormalities of the maxillary anterior teeth, visible loss of tooth structure from fracture, attrition, caries, or restorations, facial or dental deformities that may impact the smile, or crowns, bridges, or other restorations in the anterior region of the maxilla. Sample Size: The sample size was determined using OpenEpi version 3, comparing two means. The mean ± SD canine width of males (8.07 ± 0.57) and females (7.82 ± 0.65) was taken from a previous study (18). The sample size calculated was 178 females and 178 males (total 356) with a power of 90% and a 99% confidence interval. In order to improve the accuracy of the results and narrow the margin of error, the sample size was expanded to 210 males and 210 females, for a total of 420 participants. Sampling Technique: Convenience sampling was employed, which is a non-probability sampling method. Subjects were recruited until the sample size was obtained, based on the inclusion and exclusion criteria. Ethical Considerations: The study was approved by the Institutional Review Board of Dow University of Health Sciences (IRB-1001/DUHS/Approval/2018/52). All participants signed informed consent forms after a thorough explanation of the study and confidentiality of participation. Data Collection Procedure: After informed consent, 420 participants (210 males and 210 females) aged 18–30 years were selected on the basis of the eligibility criteria. All photographs were saved to a personal computer with an administrator password known only to the principal investigator, and backed up on a hard disk. Camera Positioning and Settings: A digital single-lens reflex (DSLR) camera (Canon EOS 1300D) equipped with a 105 mm macro lens (Sigma EX 105 mm f/2.8 DG) and a macro ring flash (Yongnuo YN14EX) was used for frontal photographs of the participant, taken while seated upright. The camera was set on a tripod at 12 o'clock, and a distance from the camera focal plane to the horizontal plane between the canines of the participant was established and marked with a measuring tape for replication and to prevent barrel distortion (19). The camera settings were based on the Canon dental photography guideline for intra-oral photography (19): magnification 1:3, shutter speed 1/200 s, aperture f/22, ISO 100, and half flash power for the ring flash. Participant Positioning: The height of the chair was set to the participant's height to ensure that the field was at the level of the lower face from the nose to the chin and extended horizontally to the labial commissures. The inter-canine plane was aligned parallel to the horizontal grid of the camera, and the sagittal plane was obtained by centering the dental midline to prevent head movement. The labial commissures, the tips of the canines, and the dental midline were verified on the camera grid for standardization of the participants' position. All photographs were captured in the smiling view (Figures 1 and 2). Analysis of the Photographs: The photograph of each participant was analyzed in Adobe Photoshop version 7 (Adobe Systems Inc.). The smile line was classified as low, average, and high based on the classification of Tjan et al. (11), and all were noted on a structured proforma by the principal investigator. For inter-rater reliability, ten photographs were randomly selected and re-evaluated by two blind investigators of the Department of Operative Dentistry. The intraclass correlation coefficient was computed in SPSS, and there was 99% agreement between the principal investigator and the blinded investigators. Study Variables: Smile line type (low, average, or high) was the dependent variable. Age group (18-21, 22-25 and 26-30 years), gender and ethnicity (Pathan, Punjabi, Sindhi, Urdu-speaking and others) were the independent variables. Statistical Analysis: The Statistical Package for the Social Sciences (SPSS) version 25 (SPSS Inc., Chicago, Illinois, USA) was used to analyze the data. Frequencies and percentages were calculated for the categorical variables. A chi-square test was used to evaluate the association of smile line type with age group, gender, and ethnicity, and a p-value <0.05 was used as a threshold for statistical significance.

Figure 1. Photograph in the smiling view showing an average smile line in a female participant.

Figure 2. Photograph in the smiling view showing a low smile line in a male participant.

The sample size for analysis was 420, comprising 210 males and 210 females. The distribution of smile line types revealed that the most common smile line type was the average smile line, with 283 (67.4%) participants. The low smile line was the next most common with 100 (23.8%) participants, while the high smile line was the least common with 37 (8.8%) participants. The overall distribution of smile line types is illustrated in Figure 3.

Figure 3. Distribution of smile line types in the total sample (n=420).

When considering gender, the average smile line was the most common smile line for both the males and females. Of the 210 male participants, 141 (67.1%) had an average smile line, 60 (28.6%) a low smile line, and only 9 (4.3%) a high smile line. Among 210 females, 142 (67.6%) had an average smile line, 40 (19.0%) had a low smile line, and 28 (13.3%) demonstrated a high smile line. Therefore, the incidence of low smile lines was higher in males and high smile lines in females.  (Table 1; Figure 4).

Table 1. Distribution of smile line type according to gender

Percentages are row percentages.

Figure 4. Distribution of smile line types according to gender.

Analysis by age showed that the average smile line was the most prevalent one for all three age groups (Table 2; Figure 5). Among participants aged 18–21 years, 88 (57.5%) had an average smile line, compared with 134 (73.2%) participants aged 22–25 years and 61 (72.6%) aged 26–30 years. The frequency of the high smile line gradually reduced with age groups, from 13.7% for 18-21 years to 7.7% for 22-25 years and 2.4% for 26-30 years.

Table 2. Distribution of smile line type according to age group

Percentages are row percentages.

Figure 5. Distribution of smile line types according to age group.

As far as ethnicity is concerned, average smile line was the most common smile line in all ethnic groups. Urdu speakers were the largest ethnic group with 180 participants (70.9% within the group) having an average smile line, followed by 46 with low smile line and 28 with high smile line. The distributions of the Pathan, Punjabi, Sindhi and other ethnic groups are given in Table 3 and Figure 6.

Table 3. Distribution of smile line type according to ethnicity

Percentages are row percentages.

Figure 6. Distribution of smile line types according to ethnicity.

Chi-square was used to examine the association between age group, gender, and ethnicity with smile line type. At a significance level of p<0.05, statistically significant associations were found between smile line type and age group (p=0.004), gender (p=0.001), and ethnicity (p=0.027) (Table 4). Therefore, there was a significant difference in the distribution of smile line type for all three demographic factors measured in this study.

Table 4. Association of smile line type with age group, gender and ethnicity

*Chi-square test; level of significance p<0.05.

This present analytical cross sectional study aimed to analyze the relationship between smile line type in the maxillary anterior dentition and age, gender, and ethnicity. The results showed that the average smile line was the most common smile line in the study population with about two-thirds of the study population showing this smile line. Low smile line was second most common, and high smile line was relatively rare. This is similar to the findings of Tjan et al., who reported that the most common smile line was the average smile line (68.9%) (11), and in a Malaysian population in which the majority of the subjects presented with the average smile line (6). Based on these observations, the average smile line was suggested to be a frequent dentolabial relationship in young adults and thus could be used as a reference for esthetic evaluation and treatment planning. In the present study, a statistically significant difference was found between the smile line type and gender. Low smile lines were more common in males, whereas high smile lines were more common in females, but the average smile line was the most common smile line in both males and females. In particular, there were 60 males with low smile line and 40 females with low smile line, and 28 females with high smile line and 9 males with high smile line. The same has been reported by Sapkota et al., with females having a higher percentage of high smile lines, and males having a higher percentage of low smile lines (4). Similarly, Tjan et al. and Peck et al. reported that the high smile line was more prevalent in females and the low smile line in males (11, 12). The gender related difference could be explained by the variation in upper lip length, lip mobility, muscular activity and facial soft tissue characteristics. Drummond and Capelli also found that the display of incisors during smiling is different between sexes and age groups, as females tend to have more incisor exposure in the maxilla than males (5). The relationship between gender and smile line is of great clinical importance. A high smile line exposes more of the gingival tissues and thus makes gingival symmetry, gingival margins, interdental papillae, crown length and restorative margins more visible. A low smile line, on the other hand, may be hiding a lot of maxillary anterior teeth and gingiva. Hence, when considering esthetic procedures such as anterior restoration, veneer, orthodontic movement, and implant supported restoration, the patient's smile line should be considered. Hochman et al. highlighted the visibility of the interdental papillae during a smile and showed that it is an important element of smile esthetics, not a secondary feature (7). Smile line type was also significantly related to age. The average smile line was more common in all age groups in the present study, but the number of high smile lines decreased with age. Twenty-one participants (18-21 years) had a high smile line, 14 (22-25 years) participants had a high smile line, and only 2 (26-30 years) participants had a high smile line. The decreasing trend indicates that age-related changes in soft tissues may affect the amount of maxillary tooth and gingival display even in a relatively young adult population. This finding is consistent with earlier studies which showed that the visibility of the maxillary incisors diminished with age due to the changes in lip length, muscle tone, and soft-tissue elasticity (5). Similarly, Sapkota et al. found that participants older than 51 years (66.7%) had the lowest smile line, while the highest smile line was seen in younger participants (4). Desai et al. investigated 261 smiles in five age groups and reported that the smile line was lower in older age groups (20), while Jensen et al. and Dong et al. reported that the height of the smile line decreased with age (13, 14) and that fewer incisors were exposed with increasing age. This age-related trend is further underlined by the correlation between lip features and smile line. Al-Juboori et al. studied the correlation between lip length and smile line and showed that the smile line exposure of the teeth and gums during smiling could be affected by the difference in the dimensions of the upper lip (6). As the age increases, the length and position of the upper lip may change gradually, which can gradually decrease the display of the upper central incisors. Jensen et al. suggested three explanations for this age-related change: (1) a reduction in the height of the face with age, (2) a change in the dimensions of the upper lip with age, and (3) gingival recession associated with periodontal disease, which lowers the lip line with age, and (4) a progressive decrease in soft-tissue elasticity with age due to changes in the metabolism of the connective tissue (13). This is especially significant in the restorative and prosthodontic treatment where an esthetically pleasing display of incisors for one age group may be inappropriate for another. This means that treatment planning should take into consideration the patient's age and not the same amount of display of maxillary anterior teeth for everyone. Smile line type was also a significant variable in this study that was linked to ethnicity. Average smile line was the most prevalent pattern for all ethnic groups, but there were differences among ethnic groups in the relative distribution of low and high smile lines. The largest number of participants and the highest number of average smile lines were from the Urdu speaking participants. The p value for the chi-square analysis was 0.027, indicating that there was a statistically significant relationship between ethnicity and smile line type. Al-Juboori et al. also noted variations in the smile line between Malay, Chinese and Indian populations, indicating that ethnicity and anatomical features may affect the position of the upper lip and gingival exposure (6). These differences can be due to various factors like craniofacial morphology, lip anatomy, tooth dimensions, and skeletal relationships. The present findings could not be compared with national norms because no previous study has described the distribution of smile line types among the ethnic groups of the Pakistani population. Most Indians have a low smile line (49.5%), and this was attributed to the cultural habit of not smiling openly (15), while Hu et al. reported that the smile line was higher in Han Chinese than in Caucasians due to musculoskeletal differences in the anatomy and a more convex facial profile (16). There is more general evidence that the perception and anatomy of the smile may vary between populations, which supports the importance of ethnicity in the smile analysis. Ethnic and social backgrounds have been shown to affect perception of upper dental midline deviations, suggesting that there is biological variation and observer-related variation in esthetic judgment (10). Thus, smile analysis should consider dentofacial characteristics of the patient, not just pre-determined esthetic norms. The significant associations of smile line type with age, gender and ethnicity highlight the multi-factorial nature of smile esthetics. Smile appearance is not only dependent on tooth size and alignment, but on the position of the lips, gingival display, facial symmetry, dental midline and the three-dimensional position of the maxillary anterior teeth. Wang et al. emphasized the significance of dental midline relationships in smile attractiveness (8), and Li et al. showed the importance of the position of the maxillary central incisor in smile profile esthetics (9). More recently, Zhong et al. pointed out that the attractiveness of a posed smile is not determined by any single smile feature, but by a combination of features (17). The results of this study suggest that smile line assessment should be a part of the normal esthetic evaluation. The significant associations found with gender, age and ethnicity emphasize the importance of patient-specific treatment planning in orthodontics, restorative dentistry, prosthodontics and periodontology. Limitations: The age range was limited to 18-30 years, and the ethnic groups were not of equal size, as the participants were recruited by convenience sampling, and Urdu speakers were more numerous. The results are not applicable to older age groups, and comparisons between ethnic groups should be made with caution. It is suggested that larger and more ethnically diverse samples are used from several centers. Strengths: The sample was balanced for gender (210 males and 210 females) and was calculated to give adequate statistical power. Photographs were taken in a standardised way, with computer-assisted analysis, which provides reproducibility and magnification of images, and interrater reliability was 99%. Clinical Importance: The results may be used to inform the planning of the esthetic anterior region in the oral rehabilitation process and may also be clinically utilized in restorative dentistry, prosthodontic rehabilitation, and periodontal surgery.

The present study showed that the average smile line type was the most common smile line type among the study participants, followed by the low smile line, and the high smile line was the least common smile line type among the study participants. The smile line type was statistically significantly associated with age, gender and ethnicity. High smile lines were more frequently seen in females while low smile lines were more frequently seen in males. There was also a decrease in the frequency of high smile lines with age, reflecting age-related changes in maxillary anterior tooth and gingival display. The average smile line was the most common in all ethnic groups, but there were differences in the distribution. The results highlight the need to take into account dentofacial and demographic features when assessing and planning esthetic treatment. In the context of orthodontics, restorative, prosthodontic and periodontal treatments involving the maxillae anterior region, smile line assessment can therefore be helpful as a clinical guide.