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

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. 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. 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). 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.