In order to provide reference for orthodontic treatment, the three-dimensional characteristics of facial soft tissue of adolescents with skeletal classⅠ were compared between extraction and non-extraction cases based on 3dMD^? 3D facial imaging system.

A total of 50 adolescent patients with skeletal class Ⅰ treated at the Department of Orthodontics of Hospital of Stomatology of Sun Yat-sen University from 2016 to 2019 were selected, including 25 patients in the extraction group and 25 patients in the non-extraction group as the control group. 3dMD^? was used to collect the 3D facial image data before and after orthodontic treatment, 3dMD^? vultus software was used to measure line distance, angle and volume, and the differences between the two groups were analyzed by SPSS 20.0 software.

After orthodontic treatment, the changes in the distance of lower facial height of extraction group and non-extraction group were (0.28 ± 2.28) and (1.85 ± 2.18) mm respectively (t = -2.238, P = 0.031) ; the changes in mandibular height, chin height and lip width of extraction group were (-0.52 ± 2.31) , (-1.14 ± 2.64) and (-1.33 ± 3.62) mm respectively, and those of non-extraction group were (1.64 ± 2.42) , (0.93 ± 2.51) and (1.40 ± 2.16) mm respectively (t_{mandibular height} = -2.878, P_{mandibular height} = 0.007; t_{chin height} = -2.506, P_{chin height} = 0.017; t_{lip width} = -2.942, P_{lip width} = 0.006) . In terms of angle, the changes in nasolip angle of extraction group and non-extraction group were 4.91° ± 3.12° and 1.50° ± 4.26° respectively (t = 2.692, P = 0.011) ; the changes in chin-lip angle of extraction group and non-extraction group were -0.24° ± 6.83° and 4.82° ± 6.50° respectively (t = -2.373, P = 0.023) ; the changes in lip angle of extraction group and non-extraction group were -16.80° ± 12.65° and -1.05° ± 11.24° respectively (t = -4.171, P<0.001) . In terms of volume, the changes in lip of extraction group and non-extraction group were (-0.66 ± 1.24) and (0.46 ± 1.43) cm³ respectively (t = -2.584, P = 0.014) ; the changes in chin of extraction group and non-extraction group were (-0.16 ± 0.94) and (0.65 ± 0.91) cm³ respectively (t = -2.766, P = 0.009) .

(1) There was no significant difference between extraction and non-extraction in the changes of horizontal direction of 1/3 below the face, but there was difference in the vertical direction of 1/3 below the face. And non-extraction was more likely to lead to the increase in face height than extraction. (2) It can effectively improve the shape of chin and lip in adolescent patients with poor lip seal after extraction correction. (3) The patients with thin face and protruding cheekbones should be treated carefully by extraction.