title: [Effects of different premolar extraction on lower third molar eruption].
abstract: OBJECTIVE
  To analyze the effect of different premolar extraction on the position of the mandibular third molar during orthodontic treatment.
  
  
  METHODS
  Cephalometric radiographs were taken before and after 2 years of treatment in 28 female patients selected from Department of Orthodontic, Ninth People's Hospital, School of Medicine, Shanghai Jiao Tong University. Treatment for 12 patients in group I included extraction of 4 second premolars,the other 16 patients in group II were treated with extraction of 4 first premolars. Modified sectional arch wire technique was used to redistribute the spaces.Change of the mandibular third molar position was valued by tracing. The data was analyzed using SAS6.12 software package for student's t test.
  
  
  RESULTS
  The vertical change in the third molar position in group I and II was (4.58+/-1.85)mm and (0.21+/-1.11)mm (P<0.01), the horizontal change was (2.95+/-2.55)mm and (-0.03+/-1.74)mm (P<0.01), the mesio-angulation was decreased (-10.12+/-8.27) degrees and (-5.06+/-3.60) degrees (P<0.05),and the angle to the second molar decreased (-15.58+/-9.65) degrees and (-4.21+/-3.68) degrees (P<0.001), respectively.
  
  
  CONCLUSIONS
  Second premolar extraction may increase the rates of movement of the third molar in horizontal and vertical direction and decrease the mesio-angulation and the angle to the second molar than first premolar extraction.
authors.affiliation: Department of Dentistry, Zhongshan Hospital, Fudan University, Shanghai 200032, China.
authors.forename: Xue-hua
authors.initials: XH
authors.lastname: Guo
authors.forename: Yu-fen
authors.initials: YF
authors.lastname: Qian
authors.forename: Qi-ping
authors.initials: QP
authors.lastname: Feng
mesh_terms: D001641:Bicuspid; D005260:Female; D006801:Humans; D008964:Molar, Third; D009971:Orthodontics, Corrective; D014078:Tooth Eruption; D014081:Tooth Extraction; D014085:Tooth Migration