Effects of retraction of anterior teeth and initial soft tissue variables on lip changes in Japanese adults

The aim of this study was to examine the effects of retraction of anterior teeth and the initial soft tissue profile variables on upper and lower lip changes in Japanese adults. Pre- and post-treatment cephalometric radiographs of 33 Japanese female adults (aged 23.0 ± 5.0 years), with a Class II division 1 malocclusion, who had two or four first premolars extracted, were evaluated. Lateral cephalograms taken with the lips in a voluntary relaxed position were traced and compared. Nine linear and eight angular measurements were constructed for the hard tissue measurements and 16 linear and 2 angular for the soft tissue measurements. Stepwise multiple regression analysis revealed that the horizontal upper lip position could be explained by the position of the maxillary incisor cervical point and the occlusal plane to SN with a 54 per cent contribution ratio, and horizontal lower lip position, maxillary incisor tip position, initial interlabial gap, and aesthetic line to the tip of lower lip with a 51 per cent contribution ratio. All parameters employed explained the horizontal position of the upper and lower lip with higher than 96 per cent confidence. While the horizontal positions could not be predicted by a limited number of parameters, the vertical positions of lips could be explained by three parameters with higher than 62 per cent confidence. The predictability found in this study could be beneficial for orthodontists in treatment planning.