Effects of retraction of anterior teeth on horizontal and vertical lip positions in Japanese adults with the bimaxillary dentoalveolar protrusion

The purposes of this study were to evaluate the upper and lower lip changes after orthodontic treatment of bimaxillary protrusion in adult Japanese. We also intended to predict the upper and lower lip positions from the changes of the maxillary and mandibular incisor positions in both the horizontal and vertical planes. Pre-treatment and post-treatment cephalograms of 38 patients (6 males and 32 females, aged 24.2 ± 2.9 and 22.8 ± 4.1 years, respectively) who had four first premolars extracted, were taken with lips in their voluntary relaxed position, traced and compared between them. Stepwise multiple regression analysis revealed that a 1 mm retraction and a 1 mm intrusion of the maxillary incisor cervical point would produce a 0.22 mm retraction of the upper lip, a 1 mm retraction of the mandibular incisor tip would produce 0.76 mm retraction of the lower lip, and a 1 mm mandibular incisor tip retraction would produce a 0.50 mm retraction of Stomion. Moreover, a 1 mm maxillary incisor tip intrusion would produce a 0.56 mm upward movement of the upper lip and a 1.00 mm upward movement of the lower lip. The predictability of this study may be beneficial for orthodontists in planning and discussing treatment plans and options with patients.