Quantitative Structural Modeling of Nasolabial Angle Expression: Multivariate Cephalometric Analysis of Soft-Tissue and Skeletal Contributions

Background: The nasolabial angle is widely used in orthodontic diagnosis, but its composite geometric nature complicates interpretation. Whether it primarily reflects skeletal or soft-tissue influences remains unclear, as no previous study has quantitatively partitioned its angular components using multivariable regression in a large adult sample. Objective: The objective of this study is to quantify the relative angular contributions of skeletal classification and soft-tissue parameters to NLA morphology using a multivariate cephalometric model. Methods: A retrospective analysis of 504 adult lateral cephalograms was conducted. Subjects were categorized by sagittal (Class I, II, or III) and vertical skeletal patterns (hyperdivergent, normodivergent, and hypodivergent). Measured variables included the NLA, nasal inclination (N/FH), upper lip inclination (L/FH), upper lip thickness, and maxillary incisor inclination. Correlation and multivariable regression analyses were performed. Results: Although the NLA differed across sagittal classes (p = 0.001; η2 = 0.047), upper lip inclination demonstrated the strongest association with the NLA (r = 0.766; R2 = 0.588). In multivariable analysis, L/FH exhibited the largest standardized contribution (β = 0.752, p < 0.001), while vertical pattern and lip thickness were not independently associated. Class II showed a modest independent association (β = 0.083, p = 0.006). Conclusions: Nasolabial angle variation largely reflects upper lip orientation rather than skeletal classification alone. These findings support component-based interpretation, emphasizing separate evaluation of lip inclination during esthetic orthodontic assessment.