Several studies have assessed smile attractiveness and self-satisfaction regarding smiles. Some studies assessed one criterion of the smile and compared it within their populations [4, 13, 14], while others assessed three or more criteria of the smile and measured the esthetic perception for each criterion [4, 6, 15, 16].
An esthetic smile is the result of the interaction of different smile components, including a balance between the teeth and soft tissues [17]. Variables that influence the attractiveness of a smile include the buccal corridor, gingival display, occlusal cant, dental midline, and presence of a midline diastema. A minimal buccal corridor is a critical smile feature, excessive gingival display does not appear to be well tolerated by raters, and maxillary midline deviations can upset the balance of an otherwise esthetic smile [10, 15, 18]. Similarly, the presence of midline diastema produces an unattractive smile. Rodrigues et al. reported that large midline diastemas negatively influence smile esthetics, while a midline diastema ≤ 1.5 mm was regarded as attractive [19]. Kokich et al. reported that an occlusal cant was detrimental to smile esthetics [15]. In addition, the location, shape, and contour of the gingiva in the maxillary anterior region also affects smile esthetics [20]. The amount of vertical dental and gingival exposure during smiling is a characteristic of interest in smile esthetics [21]. A gingival smile occurs due to a combination of different variables, such as maxillary vertical excess, high muscular ability to elevate the superior lip when smiling, increased inter-labial spacing during resting, and increased overjet and overbite. Some studies have reported that variables such as upper lip length, clinical crown length, and angles of the mandibular and palatal planes do not contribute to a gingival smile [9, 22]. In contrast, other studies suggest that a short upper lip and clinical crown length may contribute to gingival exposure [4, 13].
Kokich et al. were the first to systematically quantify orthodontists’ and laypersons’ perceptions of smiles using static photos of incrementally adjusted posed smiles. They evaluated eight esthetic criteria, including perception of the amount of gingival exposure, using smile photographs that were intentionally modified using a computer [15]. Variations between the distance from the upper lip to the upper incisors (gingival margin) were introduced, generating five types of images: 2 mm of incisor coverage by the lips, lips touching the gingival margin of the incisors (0 mm gingival exposure), and 2, 4, and 6 mm of gingival exposure. Orthodontists, laypersons, and general dentists evaluated the images. In this study, laypersons and general dentists considered gingival exposure ≤ 4 mm as acceptable; however, orthodontists considered exposure > 2 mm to be unesthetic. In another study, one male and one female full-face photograph were modified to create seven photos of each. Smile attractiveness was rated for each photo, and attractiveness was compared between photos. Although raters who had received previous orthodontic treatment were more sensitive, 0–2 mm of gingival exposure was within the acceptable range among raters [13]. Studies in Japan and Jordan have shown that approximately the same range of gingival display, that is, ≤ 3 mm, is acceptable in these populations too. [14, 23].
Population-based studies on smile perception have been conducted in different regions of Saudi Arabia. One study measured the perception of altered smile features and found that smiles with gingival display ≥ 1 mm were not perceived as attractive [16]. Other studies compared the perception of dental students and laypeople and reported that students had a higher perception of dental esthetics [6, 22]. However, most previous studies have focused on groups of smile features, including the buccal corridor, teeth width, and facial profile.
In this study, the attractiveness of different levels of gingival display ranging from 4 mm of gingival coverage of the upper central incisors (− 4 mm) to 4 mm of gingival display on photographs of male and female smiles was compared using VAS. In 2004, Flynn et al. concluded that the VAS is a valid instrument with high reliability for evaluating smile perception [24]. Another study by Couper et al. (2006) found that although VAS requires a longer completion time, it provides better measurements in web-based surveys compared with numeric input or radio buttons [25].
The present study included two groups of evaluators: Saudi Arabian dental professionals from various dental disciplines and Saudi Arabian laypeople. Among all smile photographs used in this study, dental professionals rated the female smile photo with 1 mm gingival coverage as the most attractive smile, followed by the female smile photograph with 2 mm gingival coverage, which was rated as the most attractive smile by the Saudi Arabian laypeople. Smile photos with no gingival exposure (0 mm) were rated as attractive by dental professionals, but laypeople found them unattractive. Smiles with gingival exposure ≥ 1 mm were considered “non-attractive” by 86.5% of dental professionals and 81.1% of laypeople. For male smile photographs, most dental professionals and laypeople considered the 1-mm LLL as the most attractive smile, followed by 3- and 2-mm of LLLs, respectively. The smile photo with 4 mm gingival exposure was rated as the least attractive male smile by 90.6% and 89.1% of dental professionals and laypeople, respectively. Regarding attractiveness, a significant difference was observed between male and female evaluators for the 4 mm and 2 mm LLLs of both male and female smiles (both p ≤ 0.0001).
Our findings are consistent with those of several previous studies. Loi et al. reported that in an ideal smile, there is no gingival exposure, and the upper lip rests near the gingival margin of the upper central incisors or partially covers the teeth [14]. Kokich et al., reported similar results: laypersons, clinicians, and orthodontists found the smile to be more esthetic when the upper lip rested on the gingival margin (0 mm) or covered the incisor crowns by 1–2 mm [15]. Dutra et al. found that female and male smiles with 4 mm gingival exposure or coverage were the least esthetic according to orthodontists, clinicians, and laypersons. In contrast, regarding the male smile, laypersons considered the most esthetic smile as the one with the upper lip at the level of the gingival margin of the maxillary incisors (0 mm), while orthodontists and clinicians considered attractive smiles as those with the upper lip resting at the gingival margin (0 mm) or covering the maxillary incisors by 2 mm, which is comparable to our results. In a comparison between dental students and laypeople, the lowest threshold for gingival margin height was evident [6]. Furthermore, Althagafi et al. showed that senior dental students were more sensitive to esthetic alterations [22], which suggests that dental knowledge might affect the esthetic perception. Laypeople considered 1 mm of gingival display as unattractive, which supports our findings [22]. Talic et al. studied different facial features and their effects on smile attractiveness. Regarding gingival display, smiles with gingival display > 2 mm were not pleasant for participants with a dental background, while laypeople were less sensitive to the change [16]. These results are in contrast to our finding that laypeople find lesser gingival display attractive.
In this study, the smile with 4 mm gingival exposure was the least attractive smile on photographs of both sexes, which is consistent with the findings of a previous study in which 3 and 4 mm gingival exposures were progressively related to less attractive smiles [13].
Attractiveness has been suggested to influence social interaction. In this study, 76.4% of laypersons responded that the impact of an attractive smile on social acceptance was high. This was in agreement with the findings of several previous studies on the psychological impact of different smile esthetic features and the importance of an attractive smile on social acceptance [26–28].
Esthetic treatment is a sensitive area, and global preference is not an indicator of population preference, as there is a shift toward gingiva-covering smiles. Additionally, smile perception is personal and can vary between individuals [10, 27, 29].
Our study had some limitations. First, participants rated their preference for photos showing only the smile. Other studies reported different results with photos revealing more facial features; however, the photographs used were limited to the mouth to reduce the effect of confounders (Kokich et al., 2006; Martin et al., 2007; Ioi et al., 2009). Second, raters were mainly from the central region of Saudi Arabia, and smile perception may vary between Saudi Arabian regions.