Changes Of Lip Contour After Extraction And Correlation Between Hard And Soft Tissue Changes

Extraction of four premolars are often prescribed for bimaxillary dentalprotrusion patients,thereby the retraction of upper and lower lips,a more pleasingprofile can be achieved.Changes of hard and soft tissues after extraction has beenstudied by some researchers,the rule underneath their changes and the correlationbetween them is of great importance,which can help clinicians figure out a morereasonable treatment plan and achieve a better outcome.Objective:The cephalograms of39female bimaxillary dental protrusion cases were traced and measured,both before and after treatment.The changes of hard and soft tissues and the correlation between them can be concluded,an predicting equation of soft tissue changes based on hard tissue changes can be deducted.This research will provide reliable evidence on how to predict soft tissue changes of bimaxillary dental protrusion cases after extraction of premolars,as well as the option of extraction site and anchorage.Methods:The samples were selected from Department of Orthodontics,Hospital ofStomatology,JiLin University.The inclusion criteria were as follows:female patientswho had four premolars extracted,older than15years at the initiation oftreatment(including15years old),classⅠmolar relationship before treatment,mild tomorderate crowding,no general disease,no craniofacial deformity or congenitalmissing teeth，no history of orthognathic surgery and facial plastics.The mean agebefore treatment is21.50±4.01ys,the mean treatment duration is18.56±4.23ms.Thereference line x was assigned to be the sella-nasion line,minus7o。The reference line y was assigned to be a constructed line perpendicular to the x referenceline,continuning through sella.6hard tissue landmarks and7soft tissue landmarkswere selected and19mearsurements were constructed,including17linearmearsurements and2angular mearsurements.Paired t-tests were used to compareddifferences between viaribles before and after treatment. Pearson’correlation wereused to analyze the correlation exsiting between hard and soft tissues.Stepwisemultiple regression analysis was used to predict future soft tissue changes dependenton hard tissues.Results:1:Statistical changes were found between linear mearurements before and aftertreatment,both horizontally and vertically.The protrusion of both lips are decreasedobviously,nasal-labial angle was decreased statistically,change of mento-labial anglewas insignificant,a more pleasing profile was achieved.The length and thickness ofboth lips weret insignificant.2:The correlation between hard and soft tissue were strong both horizontally andvertically,especially horizontally.3:1mm retraction of maxillary incisor cervical point will produce0.643mm retractionof the most anterior point of upper lip(U-Lip) to y-line.1mm retraction of lowerincisor tip point and1mm retraction of lower incisor cervical point will produce0.889mm retraction of most anterior point of lower lip point(L-Lip).1mm intrusion ofupper incisor tip point will produce0.503mm upward movement of upper lip.1mmintrusion of lower incisor cervical point will produce0.376mm downwardmovement of lower lip.