The role of lip thickness in upper lip response to sagittal change of incisor position.

OBJECTIVES: To investigate the association between upper lip thickness and the amount of upper lip repositioning upon retraction of maxillary incisors. SETTING AND SAMPLE POPULATION: Pre- and post-treatment lateral cephalograms of 101 patients were reviewed. All subjects were treated with fixed orthodontic appliances including extraction of two maxillary premolars. Only subjects without change in lip thickness between pre- and post-treatment cephalograms were included. MATERIAL AND METHODS: Digital software measurements for lip thickness, pre- and post-treatment upper lip position were performed. Appropriate regression models were developed to explore the correlation between maxillary incisor retraction and lip retraction when controlling for lip thickness and other confounding factors. The level of statistical significance was set at 0.05 for all analyses. RESULTS: A significant correlation was found between change in maxillary incisor position following premolars extraction and change in upper lip position (r = 0.95, P < 0.001). The average ratio between maxillary incisor retraction and upper lip repositioning was 1.43:1. Upper lip thickness was not significantly associated with this ratio (r = 0.003, P > 0.05). CONCLUSION: Although there is a highly significant correlation between maxillary incisor retraction and upper lip repositioning, lip thickness is not significantly associated with the amount of repositioning.