The lower bow-shaped curve as a novel reference frame to determine the lateral limit of the maxillary anterior arch for smile esthetics

Introduction: Smile analysis in horizontally angled views is indispensable for esthetic assessment and could reveal teeth-to-lip disharmony, which might escape discovery in the frontal and profile views. However, evidence is lacking on where the anterior teeth should be positioned for esthetics in angled smiles. Methods: Based on 3-dimensional facial image processing and geometric analysis, the lip edges were projected to the horizontal plane, and the horizontal teeth-to-lip relation was simplified and represented by the distances from dental landmarks to lower bow-shaped curves (LBSC), with the distance from facial-axis (FA) point of the canine to LBSC (FA-tangent line [TL] distance) identified as the key parameter. Using photographic modification and esthetic assessment, the effect of FA-TL distances on the attractiveness of 45 degrees angled smiles was tested, with esthetic ranges identified. A simplified method was developed to obtain the estimative LBSC and FA-TL distances using 2-dimensional photographs and geometric analysis to facilitate clinical application. Results: The FA-TL distance remarkably affected the esthetics of 45 degrees angled smiles. Smiles were attractive when the FA-TL distance ranged from -1.0 to 1.5 mm perceived by orthodontists and from -1.5 to 1.5 mm perceived by laypersons. The 2-dimensional photograph-derived estimative FA-TL distance was not significantly different from that obtained in a 3-dimensional image, validating the simplified method. Conclusions: The LBSC could serve as a reference frame to determine the lateral limit of the maxillary anterior arch for the esthetics of 45 degrees angled smiles. The FA-TL distance, which represented the spatial relation of the maxillary canine with the lower lip, was an esthetically essential parameter. For females aged 20-30 years, the FA point of the maxillary canines should be positioned no more than 1.5 mm labial or lingual to the LBSC.

Automated summary

This study identified the key parameter for the positioning of anterior teeth in 45 degrees angled smiles through 3-dimensional facial image processing and geometric analysis. It provides an important reference for esthetic assessment and establishes esthetic ranges for different observers.