Analysis Between The Retraction Of Incisors And Soft Tissue Profile Changes In Patients With Bimaxillary Protrusion

Objective:To study the effect of anterior teeth retraction on the soft tissue profile of patients with bimaxillary protrusion and to obtain an accurate linear regression model,providing a reference for the treatment plan for orthodontists.Methods:Forty-eight patients with the extraction treatment of four premolars who attended the orthodontic department of Jilin University Stomatology Hospital from February 2012 to April 2019 were selected as study subjects,and coordinate axes were established: the horizontal reference plane was registered on Sella(S)and oriented 7°inferior to the sell nation(S-N)line as the X-axis,and the perpendicular line through S as the Y-axis(positive values are 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:1.At the end of the orthodontic treatment,the differences between the upper and lower lips and the nasolabial angle are all statistically significant before and after the orthodontic treatment,while the differences between the chin labial angle before and after the orthodontic treatment are not statistically significant.2.Multivariate stepwise regression analysis:(1)retraction of the upper incisor has the greatest effect on both the upper lip(Beta=0.509)and lower lip(Beta=0.635)retraction;(2)the ratio of between the retraction of the upper incisor and upper lip is about 2.2:1,the ratio of between the retraction of upper incisor and lower lip is about 2:1;(3)The vertical change of the lower incisor has the greatest effect on the vertical change of the upper lip(Beta=0.367)and the lower lip(Beta=0.567).(4)The reduction of the mandibular plane angle cause an anterior shift of the chin soft tissue.Conclusions:Extraction treatment 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 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.