title: [Factors influencing the changes of lower third molars' angulation and space/crown width ratio by orthodontic treatment].
abstract: OBJECTIVE
  To investigate the factors which influence the changes of lower third molars' angulation and space/crown width ratio by orthodontic treatment.
  
  
  METHODS
  Pretreatment and posttreatment panoramic radiographs of 120 orthodontically treated cases were taken. The lower third molars' angulation and space/crown width ratio were examined and the relative factors on changes were analyzed.
  
  
  RESULTS
  (1)The changes of the lower third molars' angulation and space/crown width ratio after orthodontic treatment were quite different among the cases. (2)The factors which influenced these changes mainly included extraction, mandibular plane angle, angulation and space/crown width ratio of lower third molar before orthodontic treatment. (3) Extraction was the most significantly variable associated with lower third molars' upright (angulation <30 degree) after orthodontic treatment.
  
  
  CONCLUSIONS
  Orthodontic extraction is helpful to upright the lower third molars and to increase the space for third molars; and the pre-treatment lower third molars' angulation and space/crown width ratio are important variables for the changes after treatment; cases with lower mandibular plane angle are apt to greater possibility for the lower third molars' upright.
authors.affiliation: Department of Orthodontics, Peking University School, Beijing 100081, China. xiaotonglee@hotmail.com
authors.forename: Xiao-tong
authors.initials: XT
authors.lastname: Li
authors.forename: Dong-mei
authors.initials: DM
authors.lastname: Zhang
authors.forename: Yan-qi
authors.initials: YQ
authors.lastname: Yang
mesh_terms: D002648:Child; D005260:Female; D006801:Humans; D008297:Male; D008310:Malocclusion / Q000628:therapy*; D008334:Mandible; D008964:Molar, Third / Q000254:growth & development* / Q000502:physiology; D019516:Orthodontic Space Closure; D009971:Orthodontics, Corrective / Q000379:methods*; D014078:Tooth Eruption; D014081:Tooth Extraction; D055815:Young Adult