Abstract: Objective: To study the effect of anterior tooth retraction on soft tissue profile of patients with bimaxillary protrusion and to obtain an accurate linear regression model. Methods: Forty-eight patients extracted four premolars who attended the orthodontic department of Jilin University Stomatology Hospital from February 2012 to April 2019 were selected. The coordinate axes were established, the horizontal reference plane was registered on sella (S) and oriented 7° inferior to the sellanasion (S-N) line as the X-axis, and the perpendicular line through S as the Y-axis (positive values were taken on the right side of the Y-axis and below the X-axis). Cephalometric parameters of related soft and hard tissues were measured and multiple stepwise regression analysis was performed to obtain an accurate linear relationship model between the changes of soft and hard tissues. Results: At the end of the orthodontic treatment, the differences between the upper and lower lips and the nasolabial angle were statistically significant before and after the orthodontic treatment, while the differences between the chinlabial angle before and after the orthodontic treatment were not statistically significant. The sagittal changes of the pogonion of soft tissue, gnathion of soft tissue and menton of soft tissue were all weakly negatively correlated with the occlusal plane angle, the sagittal changes of the pogonion of soft tissue and gnathion of soft tissue were all moderately negatively correlated with the MP-SN angle, the sagittal changes of the menton of soft tissue was negatively correlated with the MP-SN angle. Multivariate stepwise regression analysis: retraction of upper incisor incisal had the greatest effect on both upper lip (Beta=0.509) and lower lip (Beta=0.635) recovery, and the ratio of between the retraction of upper incisor incisal and upper lip was about 2.2∶1.The vertical change of the incisor of the lower incisor had the greatest effect on the vertical change of the upper lip (Beta=0.367) and the lower lip (Beta=0.567). The reduction of the mandibular plane angle caused an anterior shift of the chin soft tissue. Conclusion: Extraction can effectively improve the convex facial profile. In contrast to the lower incisors, the retraction of the upper incisors has the greatest effect on the changes of the lower lip, and the change of the incisal of the upper incisors is the key to the change of the soft tissue of the lip. The reverse rotation of the mandibular plane can mask the underdevelopment of the chin, but extraction alone does not significantly affect the change of the chin.

Key words: soft tissue profile, tooth movement, tooth extraction, bimaxillary protrusion