Lower lip inclination and chin prominence: interactive effects on facial aesthetics and implications for orthognathic treatment planning

Abstract Objective To assess how lower lip inclination and chin prominence interact to influence facial profile aesthetics and to establish evidence-based angular criteria for orthognathic treatment planning. Methods This study introduced a combined model using lower lip inclination angle (Li-Sbl) and lower lip-chin prominence angle (Lia-Pog′) to assess their interactive effects. Thirty standardized profile images combining three Li-Sbl angles (30°, 45°, 70°) with five Lia-Pog′ angles (0°–40°) were evaluated by 270 assessors (maxillofacial surgeons, orthodontists, skeletal Class II patients, laypeople) for aesthetic perceptions and surgical recommendations. Results Gender-specific optimal configurations were identified: females achieved highest scores (7.81–8.19) at Li-Sbl 45°/Lia-Pog′ 20°; males at Li-Sbl 45°/Lia-Pog′ 10°–20° or Li-Sbl 70°/Lia-Pog′ 0° (6.31–7.60). Significant Li-Sbl × Lia-Pog′ interactions demonstrated compensation mechanisms. Critically, Li-Sbl ≤ 30° yielded consistently low ratings (< 5.0) regardless of chin positioning. Evaluator background significantly influenced perceptions and recommendations (P < 0.05). Conclusions The Li-Sbl × Lia-Pog′ model provides the first quantitative framework for orthognathic treatment planning. It addresses two clinical decision points: identifying borderline cases requiring multidisciplinary consultation, and determining surgical approach where the Li-Sbl ≤ 30° threshold indicates isolated genioplasty is insufficient. We recommend gender-specific targets: Li-Sbl 45°/Lia-Pog′ 20° for females; Li-Sbl 45°/Lia-Pog′ 10°–20° or Li-Sbl 70°/Lia-Pog′ 0° for males. This framework enables evidence-based orthognathic surgical decision-making, reducing reliance on subjective assessment and improving treatment predictability.