Previous work [14, 15] has shown that the anterior teeth play an important role in smile aesthetics. Previous studies of lateral incisors indicated that the width ratio of lateral incisors to central incisors should correspond to the golden ratio [1, 16]. However, many studies later reported that the golden ratio of lateral incisors to central incisors has negligible effect on smile aesthetics . For example, one work  showed that lateral incisors with a width of 67-72% of the central incisors and a length 1.5 mm shorter than central incisors were considered attractive. Another study  demonstrated that most people preferred short, broad lateral incisors, and they could tolerate a small mesial inclination of lateral incisors. A third study  compared different aesthetic changes due to different teeth inclinations and suggested that slight symmetrical mesial inclination of both lateral incisors could make a smile more attractive.
The studies mentioned above included only tooth changes in a 2D plane, but inclination and displacement of teeth in 3D planes are quite common in orthodontic cases. One study  studied different anteroposterior positions of the maxillary central incisors, showing that raters were more tolerant of labial protrusion of the maxillary central incisors than of lingual retrusion. Another study  found that labial inclination and lingual retrusion of maxillary central incisors were relatively unacceptable. A third study  examined the labiolingual inclination and anteroposterior position of maxillary incisors in three different facial patterns, showing that different facial patterns led to different aesthetic criterion; nevertheless, consistently with previous studies, raters showed less tolerance for the labiolingual movement of maxillary central incisors than for their labiolingual inclination.
The Little's Irregularity Index (LII), the sum of the distances between the four anterior teeth’s anatomic contact points, is an important index of dentition crowding . However, studies have suggested that the LII index lacks reproducibility [23, 24]. Scanning models may be more reliable than the LII index . In the present study, the Sirona intra-oral scanner was used to obtain the volunteer’s digital dentition models. and iOrtho7.0 software was used to change the position of maxillary lateral incisors. The 2D photos were created based on the visual effect of the 3D models, so that the changes taking place in 3D direction were transformed into 2D images. As orthodontists face more cases of 3D malformation of teeth or dentition in the clinics, our research method may provide references for future research. The angle used in taking the photographs should be matched with the angle to view the dental cast in iOrtho7.0 software, which should be explored in future research.
Our study also confirmed that VAS can be used for the analysis of various factors affecting smile aesthetics due to its convenience and repeatability . Some studies asked participants to rank photos in the order of attractiveness to find which photo was the most or the least attractive . Other studies combined ranking orders and VAS scores together and evaluated their consistency . We did not ask the raters to rank the photos in order of preference, since it seemed likely that greater amounts of labial or lingual movement would be judged correspondingly less acceptable.
The rater’s sex may be one of the factors influencing perception of smile aesthetics , but some studies [1, 26] have shown that there are no significant differences between male and female raters, which is consistent with our study. Studies have shown that the sex of the subject whose smile is under evaluation could also influence the perceived smile aesthetics [27, 28]. In our study, only one photograph of a front view of a man’s smile was used, and only the part of the mouth was retained to minimize the effects of the subject’s gender. Therefore, future studies are needed to assess potential differences in how a man or woman’s smile aesthetics depends on the labiolingual position of the maxillary lateral incisors. Future work should also explore perception of smile aesthetics based on lateral or 3/4-lateral views, and not only the frontal view in the present study. Such work should also examine how smile aesthetics change when more than one tooth movement is involved, which is the more frequent situation in the clinic.
Our study indicated that orthodontists and laypersons awarded the highest score to the control group (0 mm), and they gave lower scores as the moving distance of maxillary lateral incisors increased. Compared with laypersons, orthodontists had a lower tolerance for labial movement of lateral incisors, and they assigned marginally lower scores when there was lingual movement of maxillary lateral incisors. This indicates that orthodontists had higher aesthetic standards, and were more sensitive than laypersons when the lateral incisors were moved labially. Laypersons and orthodontists alike were less tolerant of labial movement than lingual movement over the same distance. This is in contrast with a study , which reported the lingual movement and labial inclination of the maxillary central incisors were less acceptable. The divergence of the conclusions may result from the fact that different labiolingual position of the maxillary central incisors tends to change the position of the entire dentition, and the receding dentition could make the smile look less full , and people value fuller smiles. Our study was based on the position of the maxillary lateral incisors, which changed the position independently of the entire dentition. Moreover, a front-view picture was used in our research instead of a profile picture as in that previous study , which can also explain the different observations.
In our study, orthodontists did not find a difference between smiles at 0 mm and -0.5 mm, while laypersons did. This was contrary to previous work where orthodontists were more likely to distinguish between subtle changes in dentition [1-3]. Therefore, before orthodontic treatment, communication and discussion between orthodontists and patients is needed to achieve better therapeutic and aesthetic outcomes.
Our study presents several limitations. First, we found that, when we changed the position of lateral incisors in the 3D model after fixing the adjacent teeth, the space between the lateral incisor and its adjacent teeth was magnified on the photographs of +1.0 mm and +1.5 mm, which was somewhat different from the actual situation. In future studies, we will seek a better balance between controlling a single variable and simulating the clinical situation, which may make the results more useful. Second, the effect of rater age on their scoring was not considered when designing this study, so we did not recruit sufficient, similar numbers of raters across relevant age groups. Orthodontists are generally older than orthodontic patients, as was the case in our study, so future work should examine the potential influence of age on perception of smile aesthetics.