Correlation Between The Changes Of Incisors Inclination And Protrusion After Extraction

Bimaxillary dental protrusion is one of the most common malocclusion withnormal position of upper and lower jaw, apparent inclined incisors, less or slightlycrowded dentition and protruded lips. Orthodontic compensatory treatment for suchpatients often requires premolars extracted. Then contracting incisors to close thespace, along with the lips closed,to achieve a good profile. Extraction causes thechanges of incisors inclination and protrusion. The correlation between their changesis of great importance,which can make the treatment objective visuable and helpclinicians figure out appropriate treatment plans when combined to the VTO analysisin clinic cases.Objective:The cephalograms of40female bimaxillary dental protrusion cases were tracedand measured,both before and after treatment.The changes of incisors inclination andprotrusion and the correlation between them can be concluded. Formulas would befigure out to quantify their relationship. Thus to provide a new reference of theincisors objective position in VTO static analysis.Methods:1.40cases of female patients with bimaxillary dental protrusion,older than16years old(including16years old) before treatment,were selected from Department ofOrthodontics,Hospital of Stomatology,JiLin University.The mean age was20.5yearsold.2.A coordinate system was established on cephalometric radiographs before andafter treatment.The reference line x was assigned to be the SN line,minus7°。Thereference line y was assigned to be a constructed line perpendicular to the x referenceline,via S point.6hard tissue landmarks were selected.The main measurement itemsincluded the angle between axis of maxillary central incisor and the SN line(U1-SN °),the angle between axis of maxillary central incisor and the NA line (U1-NA°), theangle between axis of mandibular central incisor and the MP line(L1-MP°),the anglebetween axis of mandibular central incisor and the NB line(L1-NB°), the horizontalordinate of upper and lower central incisor crown and root apical point(U1c-Xmm,U1r-Xmm, L1c-Xmm, L1r-Xmm), the distance from upper and lower central incisorcrown and root apical point to the NA and NB line(U1c-NAmm, U1r-NAmm,L1c-NBmm, L1r-NBmm).3.Paired t-tests were used to compare the differences between above items beforeand after treatment. Linear regression analysis were made and the regressionequations were obtained.Results:1.Statistical changes were found between all the measurement items before andafter treatment.The inclination and protrusion of upper and lower incisors weresignificantly reduced.2.MP-SN°was slightly increased by0.45±1.28°(p=0.034<0.05) aftertreatment.3.Changes of incisor inclination and protrution was significantly correlated.Theregression equations were as follows:①U1-SN°=0.608+2.627×U1c-Xmm-2.507×U1r-Xmm（R2=0.893）②U1-NA°=0.086+2.636×U1c-NAmm-2.145×U1r-NAmm（R2=0.861）③L1-MP°=2.458+2.644×L1c-Xmm-2.273×L1r-Xmm（R2=0.623）④L1-NB°=0.977+3.208×L1c-NBmm-2.741×L1r-NBmm（R2=0.836）.