In the present study, in the five images evaluated, two angles were modified; the angle of the occlusal plane cant and the midline axial deviation. The upper half of the face was removed in the studied images because the upper half of the face might affect the observers’ evaluation of the smile's attractiveness inadvertently [7]. Also, in the changes made in the subjects’ images, the most pleasant (0º cant and 0º midline deviation) or the most unpleasant cases (4º cant and 10º midline deviation) were ignored. This reduced the number of images to five. Reducing the number of images prevents the observers from fatigue, reluctance, and a decrease in the accuracy of the real critique of the images [7]. The acceptable threshold is the least level of esthetics that is acceptable). Therefore, in the present study, the need for treatment was considered as a criterion for lack of harmony in terms of esthetics, which is not acceptable by the observer, and lower values (more proportionate cases) were acceptable to the observers, with no need for treatment for esthetics. The variables of this study exhibited normal distribution. An interval of one standard deviation was considered for confirming the beauty of the studied population. In other words, a photo was considered acceptable when at least 68% of the participants confirmed its beauty. In other words, a subject (i.e., a photo) was confirmed to need treatment when >32% of the participants reported the need for treatment. According to the results, photo 4 (having 2º cant and 5º axial deviation of the midline) in women and men can be considered as a threshold because if the mentioned angle changes in the previous photo (i.e., the third photo), the percentage of the treatment need will increase significantly.

The first photo (10º midline deviation) was reported to need treatment more than the fifth photo (4º cant). On the other hand, for the second photo, which had more cant than the fourth photo but less midline deviation, had a lower percentage participants reporting the need for treatment. It seems that the midline axial deviation has a more significant effect on treatment needs than the occlusal plane cant. Few studies have investigated the effect of maxillary midline axial angle on esthetic appearance. This inappropriate midline angle might not attract attention clinically due to the asymmetrical incisal edge abrasion [13]. However, Kokich et al. studied the angle of incisor crowns by changing the axial angle of the anterior teeth in the photograph of a smiling face and concluded that all the studied groups identified a 2-mm deviation in the midline axial angle [14]. A simple mathematical calculation shows that a 2-mm deviation is approximately equal to 11º of axial deviation, which almost corresponds to the threshold reported by previous studies. Therefore, it might be preferable in orthodontic esthetic treatments for the midline axial angle correction to take precedence over the occlusal plane angle.

In examining the cant of the occlusal plane alone, 0º cant is preferable. However, in the present study, the image with the lowest treatment need, i.e., the best image, had a 3º occlusal plane cant. Nonetheless, image 5, with a 4º cant and no midline deviation, needed treatment from the viewpoint of a high proportion of participants. Therefore, a combination of esthetic parameters might give rise to differences in acceptance than the results of accepting each criterion alone[15]. Therefore, perhaps it might be claimed that for correcting the cant, which is not accepted by the patient if a slight axial midline deviation is inevitably created, the aesthetic results will still be accepted by the patient. Kaya et al. showed that the participant’s perception of the most attractive curve of the smile arc varies with changes in the gingival display and vice versa; i.e., the perception of the most attractive and proportionate gingival display will vary with changes in the curve of the smile arc [7].

Most previous studies, such as that by Onschal et al., have assessed individuals’ overall views [16]. Syed et al. examined only female perspectives [17]. However, in the present study, to evaluate people’s views more accurately, both men and women were separately evaluated in four groups. In general, the treatment need percentage of female images was higher than male images, and it seems that the beauty of a woman’s smile is more important from both men’s and women’s perspectives. A study in 2003 by Thomas et al. showed that the gender of the images played a significant role in the attractiveness of the modified images, but the judges’ gender differences did not [13]. In the present study, the observers gave lower scores to the attractiveness of the females’ images, and the results showed a lower acceptance threshold of the females’ images than the males’ images. It seems that midline axial deviation is less tolerable in women than in men. A similar result was reported by Beyer et al. who mentioned that after the lateral deviation of the midline, a lower acceptance threshold was obtained in females’ images than in males’[18]. These results can be consistent with the idea that people generally think more critically about physical attractiveness in women. Another explanation is the attractiveness of the selected images themselves. Facial photographs are commonly used to evaluate individuals psychologically and socially and in other dental studies. It has been shown that judging by photographs seems relatively reasonable compared to the individual’s real appearance. However, the selected image of a man might seem more attractive to people than the image of a woman, which is one of the disadvantages of using facial photographs in orthodontic studies [13].

The female participants in this study had a more accurate and insightful view of the smile aesthetics than the male participants. Dunn et al. reported results similar to the present study, with women making more definitive judgments about dental esthetics than men [19]. Therefore, it is better to be more accurate and cautious in designing esthetic treatments for women than men for improving and correcting the smile line.

Conclusion

From the Zanjani people’s point of view, the esthetic threshold for combining the occlusal plane and the midline axial deviation is 2º of cant and 5º of midline deviation. In judging the beauty of a smile, the midline angle deviation is more important than the occlusal plane deviation. Women have a more accurate and insightful view of the beauty of a smile than men.

Limitations

The threshold for acceptability of axial deviation has been 10 in previous studies; however, opinions about beauty vary in different societies so at least 86% of the Zanjan people believed that this amount of midline axial deviation needs treatment. This means that the threshold for midline axial deviation might be<10 for the Zanjan people, which might be attributed to the more critical view of the Zanjan people about facial beauty. However, this different threshold affected the results of the present study, leading to the ambiguity of the combined threshold determination.

In this study, we studied the angular changes of cant and midline incrementally to reduce the number of images and consistent with other studies. Therefore, there are other angles between the two increments that have not been studied; for instance, 1.5º cant with the midline deviation angle of 6.5º might also not be considered “to need treatment” by the participants and be considered as a threshold that was not studied in this study.

The data reliability of such studies depend on time, cultural, and social changes, and might not be extended to other societies or times.

Suggestions

It is recommended that studies be conducted in Zanjan to determine the esthetics threshold for the various characteristics affect the smile.

It is recommended that these studies be repeated at regular intervals in other communities, too. However, other studies are necessary to determine this time interval.

Similar studies should be performed but with smaller increments.

Acknowledgments: We would like to appreciate Dr. Nima Motamed for undertaking statistical consultation of this research.

Conflict of interest: None

Financial support: None

Ethics statement: This article was extracted from the results of a thesis, for a degree in general dentistry. The Ethics Committee of Zanjan Medical Sciences University approved this study under the code IR.ZUMS.REC.1398.419.