title: [Soft tissue changes of chin with different premolar extraction patterns].
abstract: OBJECTIVE
  The purpose of this retrospective cephalometric study was to compare the change of chin soft tissue in patients with Class I malocclusion after extraction of either first premolars or second premolars.
  
  
  METHODS
  The pretreatment and posttreatment records of 50 patients were used in the study. Twenty-five patients underwent extraction of first premolars and 25 patients underwent extraction of second premolars. All the patients were skeletal Class I and matched by the gender, age, facial divergence and crowding. The lateral cephalometric radiographs taken before and after treatment were traced and measured. The data was analyzed using SPSS 10.0 software package for Student's t test.
  
  
  RESULTS
  (1)The horizontal and vertical changes was both significant in the second extraction group compared with first extraction group, and the horizontal change was more apparent. But only vertical change was apparent in the first extraction group. (2)In the soft tissue measurements, the value of soft tissue facial angle and B'-Pos-FH in the first extraction group was less than those in the second extraction group significantly. After treatment all the patients had an increase in S-Ns-Pos angle, soft tissue facial angle, B'-Pos-FH and depth of mentolabial groove. At the same time, soft tissue convexity angle was diminished obviously. But the change was not significant between the two premolar extraction groups.
  
  
  CONCLUSIONS
  The growth pattern tends to vertical pattern in the first premolar extraction group and the horizontal growth of chin is more obvious in the second premolar extraction group. There is no significant difference of chin soft tissue changes between the two premolar extraction groups.
authors.affiliation: Department of Orthodontics, Wenzhou Medical College, China. shuofei1215@163.com
authors.forename: Shuo-fei
authors.initials: SF
authors.lastname: Zhang
authors.forename: Xin-ping
authors.initials: XP
authors.lastname: Lin
authors.forename: Min-ling
authors.initials: ML
authors.lastname: Zheng
mesh_terms: D001641:Bicuspid; D002508:Cephalometry; D002680:Chin; D005145:Face; D006801:Humans; D008310:Malocclusion; D008311:Malocclusion, Angle Class I; D012189:Retrospective Studies; D014081:Tooth Extraction