Factors Affecting the Degree of Incisal Display in Smile

Objectives: The purpose of this study was to find the skeletal factors that affect the degree of incisal display in smile and rest posture and to analyze the correlation between the degree of incisal display and the skeletal factors.Methods: The subjects consisted of 78 young adults with the normal occlusion. They were taken with lateral cephalograms in smile and rest posture. 23 cephalometric variables regarding their skeletal characteristics were analyzed by a t-test to analyze gender difference. Pearson correlation analysis was then used to find the correlation between the degree of incisal display and the skeletal variables in smile and rest posture. Finally, the critical variables that could affect the degree of incisal display were suggested using a multiple linear regression analysis.Results:There was no statistically significant gender difference in the degree of incisal display in smile and at rest (p>0.05). A 3-variable model was constructed in rest posture by the multiple linear regression analysis: the interlabial gap, upper lip/lower lip ratio, U1 to S-N plane (R2=0.488).A 4-variable model was constructed in smile: the degree of lip elevation, anterior maxillary height, the upper lip length at rest, the interlabial gap (R2=0.759). Conclusion: This study suggests that the orthodontist need to make a treatment plan for the degree of incisal display in smile and at rest respectively, and which variables can affect the incisal display.