Upper lip elevation in autonomous maximal smile: A three-dimensional facial imaging study

Three-dimensional (3D) facial images of autonomous maximal smile (AMS) hold unique value for generating orthodontic virtual patients. This study aimed to investigate upper lip elevation (ULE) in AMS using 3D facial images.

A total of 212 Chinese participants with average dental and gingival display in AMS were enrolled and categorized into four groups: male adults (MAdu), female adults (FAdu), male adolescents (MA), and female adolescents (FA). Three-dimensional facial images were taken to measure upper lip length (ULL), philtrum length (PL), and upper vermilion height (UVH) at repose, and to measure upper lip length in AMS (ULL@AMS), ULE, and upper lip elevation ratio (ULER) in AMS. Cephalometric measurements were performed to assess maxillary incisor exposure (U1-Stms) and upper lip thickness (Sn-thickness, Ls-thickness, and Stms-thickness). Statistical analyses were conducted using analysis of variance and Pearson's chi-square tests.

MAdu exhibited a significantly longer resting ULL (23.42 ± 2.23 mm) compared with FAdu (22.43 ± 1.83 mm), primarily due to their longer philtrum. ULL@AMS was 18.27 ± 2.20 mm in MAdu and 16.85 ± 2.36 mm in FAdu (P < 0.05); ULE was 5.15 ± 2.26 mm in MAdu and 5.58 ± 1.90 mm in FAdu (P > 0.05); ULER was 0.22 ± 0.09 in MAdu and 0.25 ± 0.08 in FAdu (P < 0.05). Both ULE and ULER were positively correlated with resting ULL and negatively correlated with Ls-thickness.

ULL, ULL@AMS and ULE in AMS were precisely measured using 3D imaging across different sex and age groups, offering preliminary normative references for related orthodontic treatment and esthetic procedures.