Does variation in lower lip position and lower facial height affect the perception of smile attractiveness? : A quantitative analysis.

PURPOSE: This cross-sectional image-based study evaluated how variations in lower facial height (LFH) and lower lip position (LLP) affect smile attractiveness as perceived by orthodontists, dentists, and laypeople. METHODS: A frontal extraoral photograph of a volunteer was digitally modified to generate 15 images with three LFH levels (decreased, normal, increased) and five LLPs (ranging from +2 mm to -2 mm). In all, 178 evaluators rated images on a 10-point scale. Data was analyzed using mixed factorial analysis of covariance (ANCOVA) with Bonferroni adjustments. Clinical relevance was assessed using a 15% threshold of the visual analog scale (VAS), corresponding to a 1.5-point difference. RESULTS: Among the covariates, only age influenced ratings (p = 0.026), with older evaluators tending to provide slightly higher attractiveness scores. LFH showed a significant main effect (p = 0.022), where decreased LFH was rated less attractive than normal or increased heights. LLP also affected ratings (p < 0.001), with higher scores observed as lower incisor exposure increased. A significant LLP × evaluator group interaction was observed (p < 0.001), indicating that dentists rated 2 mm of mandibular incisor exposure higher than orthodontists (p = 0.030). No significant main effect was found for profession (p > 0.05), nor were the LFH × LLP, LFH × evaluator group, and the three-way LFH × LLP × evaluator group interactions significant (p > 0.05). CONCLUSION: Within the limitations of this study, both LFH and LLP influenced smile attractiveness perception. Inferior LLPs with slight mandibular incisor exposure tended to receive the highest ratings, while superior LLPs covering the maxillary incisor edges were rated least attractive, a preference largely consistent across LFH variations and evaluator groups.