Abstract
Understanding age-related changes in the position of the upper lip relative to the esthetic plane (UL-EP) and lower lips relative to the esthetic plane (LL-EP) is crucial for optimizing aesthetic outcomes in orthodontic and orthognathic treatment. Previous studies often lack comprehensive adjustments for confounding factors and ignore the nonlinear relationships. This study addresses these issues by investigating age-related changes in upper and lower lip positions relative to the E-line. This cross-sectional study analyzed the lip positions of 2961 individuals (Age: 21.45 ± 7.43). Data were adjusted for confounding factors, including gender, dental, skeletal, and soft tissue parameters. Statistical methods, including Restricted Cubic Splines (RCS), multivariate regression, interaction analysis, and mediation analysis, were employed to identify age-related changes and their interactions. The findings indicate a significant decrease in both UL-EP and LL-EP distances with increasing age, following non-linear patterns of rapid decline, slight increase, and gradual decrease. Interaction analysis revealed significant interactions between age and variables such as gender, ANB, SN-MP, L1-MP, and overjet. The UL-EP decreases most rapidly in males with a Class III hypodivergent skeletal pattern and lingual inclination of lower incisors. In contrast, the LL-EP decreases most rapidly in patients with a Class III skeletal pattern and normal overjet. Mediation analysis suggests that age influences lip position through factors such as overjet, ANB, L1-MP, and Prn-Sn. These findings underscore the multi-phase, non-linear changes in lip position with age and highlight the critical role of age-specific considerations in orthodontic and orthognathic treatment planning. By identifying precisely when, where, and for whom these changes occur, clinicians can facilitate more individualized and effective outcomes.