Chin changes in bimaxillary protrusion:A comparison of the effects of tooth extraction in the adults and the juveniles

Objective The aim of the study was to evaluate the chin changes in adults and juveniles with bimaxillary protrusion after extraction of the four first premolars.Methods 30 patients(15 adults and 15 juveniles) with bimaxillary protrusion after extraction of the 4 first premolars were choosen.The chin morphology changes of the two groups before and after teeth extraction were investigated with cephalometric analysis.Data were analyzed by t-test.Results There were significant increases in Proptosis degree of chin,the depth of mentolabial furrow and B'Pos/FH,while decreasing in the tilt angle of chin.No significant difference was found in the thickness of chin at B,P and root tip,the height of chin,the fossa of chin,the curve of chin and the thickness of chin soft tissue.More than 1.67 degree increase of the tilt angle of chin and 0.04 mm thickness increase of the thickness of chin soft tissue were observed in the juvenile group than in the adult.Conclusion Orthodontic treatment with the first premolar extraction can improve the morphology of the chin and the relationship of lips to chin in the patients with bimaxillary protrusion.The changes are more significant in juveniles than the adults.