An esthetic reshapement of the canine plays an important role in a successful canine substitution treatment. This study investigated the influence of the canine edge width and height on dental esthetics and found that the esthetically most pleasant canine edge widths were 50% and 62.5% of the central incisor width and the esthetically most pleasant canine edge heights were 0 mm and − 0.5 mm incisal to the central incisor edge. There was an interaction between the width and height of the canine edge on the dental esthetics, with the 62.5% width and − 0.5 mm height considered to be the esthetically most pleasant canine shape for the canine substitution treatment by all observers in the study.
The shape of maxillary central incisor is generally used as the reference for reshaping the canines during canine substitution treatment [10, 18]. The most esthetic width ratio of the maxillary lateral and central incisors has been considered to vary from 0.62:1 to 0.72:1 [28,29,30,31]. The canine edge width of 62.5% of the central incisor width in the study, which was ranked as top 1, was also the closest ratio to the naturally esthetic standard ratio [11, 24].
The canine shapes with relatively pronounced cusps (i.e., 0%–12.5% width and 0.5 mm height; 25%–37.5%width and 0 mm height) were preferred by a considerable number of observers, who regarded them as a symbol of youth and vitality in the study. This is different from the findings of other studies, in which they demonstrated that the esthetic-reshaped canine shares a similar contour with a natural lateral incisor [24, 32] and the deviation may be due to the morphological distinction between the canine and lateral incisor.
The canine edge height has been found to play an important role in the smile arc [33] and gingival margin [34, 35]. The gingival height of canine in our study was relatively more incisal compared with a previous study [34]. It should be noted that the height of gingival margin was digitally altered in the same amount as canine did in the study, but the actual gingival change is usually less than the tooth height change in the real tooth movement due to a delayed or limited periodontal remodeling. Considering that the soft tissues such as gingiva and lips may potentially affect the observers’ grades and rankings on the dental esthetics, the images used in the current study only included the upper dentition [19].
One of the limitations of the study is that the evaluation of dental esthetics was focused on the canine edge (width and height) and was performed on two-dimensional images displaying the upper dentition only. The other canine shape parameters, such as clinical crown width and height, were not assessed in the study. Though we found that the dental esthetics of canine following the canine substitution treatment were influenced by not only the independent parameters of the canine (i.e., edge width and height) but also the different collocations of these two parameters, the other factors, including crown torque [36, 37], enamel color [20, 38, 39], gingival margin [34, 35], and lip position [5], can also affect dental esthetics during the canine substitution treatment. Furthermore, it is important to note that the natural tooth morphology of the anterior teeth can also vary among different ages and genders [40].
Oberservers of different genders and professions may have different expectations in canine substitution treatment cases. In the study, canine shapes of 50–75% edge width and − 0.5–0 mm edge height are widely preferred by all observers. The orthodontists had a greater consistency in grading and ranking the images used in the study than did the laypersons, which was in agreement with the previous studies [20, 41]. This may be because the orthodontists were trained and able to perceive a minor difference in teeth morphology [14, 41]. Additionally, in terms of the pronounced canine cusp (e.g., 0–25% width and 0.5 mm height), the number of male orthodontists showed more preference than did the female orthodontists. In comparison with the orthodontist, the layperson seemed to have less tolerance on the canine shapes with the edge width of 25 to 37.5% and the edge height of 0 mm.
It is also important to note that there were a number of patients and orthodontists who considered a relatively pronounced canine cusp (e.g., 0 to 12.5% width and 0 to 0.5 mm height) to be esthetically attractive. Considering that the canine shape with a relatively pronounced cusp involves relatively less enamel grinding, a customized patient-centered preference of the canine shape (instead of the natural width ratio of the lateral and central incisors) should be applied in practice when treating a patient with canine substitution. Therefore, the involvement of patients in the clinical decisionmaking for the canine reshaping is essential for a successful canine substitution treatment [26, 42].
Another clinical application of the study findings is in the Digital Smile Design (DSD) technology during the Invisalign treatment [43, 44]. A virtual simulation of the tooth movement and shape alteration for patients who need canine substitution would enhance patients’ understanding of different treatment options as well as the communication between patients and clinicians.
Last but not the least, to clinically achieve the most attractive effects, the upper canine replacing the lateral incisor may also receive reshaping with resin, a veneer, or even a crown. Resin restoration can make the canine edge look more similar to the lateral incisor, which however may suffer from discoloration and risk of fall-off. A veneer can reshape not only the edge, but also the labial face of the canine, which is usually more convex compared to a lateral incisor. A crown, with more grinding of the dentin and usually necessitating root canal therapy, can change not only the shape, but also the size of the canine. However, many patients may not choose any of these three approaches, due to the adverse impacts on function (risks when biting hard food with the front teeth) and health (more grinding and even devitalization) of the tooth, and additional costs as well. Therefore, just grinding the canine cusp is an approach of great importance for such cases.