title: The effect of vertical bracket positioning on torque and the resultant stress in the periodontal ligament--a finite element study.
abstract: BACKGROUND
  The ideal built-in tip and torque values of the straight wire appliance reduce the need for wire bending and hence reduce chair time. The vertical position of the bracket on the tooth surface can alter the torque exerted on the tooth. This is a result of the altered surface curvature observed at each vertical position. To further clarify the role of vertical bracket positioning on the applied torque and the resultant stresses in the periodontal ligament (PDL), we designed a mandibular first premolar using finite element modeling.
  
  
  METHODS
  Cone beam computed tomography of 52 patients (83 lower first premolars) was selected to be included in the study. Curvature was measured for points along the labial surface with increasing distances (0.5 mm increments) from the cusp tip by calculating the angle between tangents drawn from these points and the axis joining the cusp tip and the root apex. The mean values for each distance were calculated, and a finite element model was designed incorporating these mean values. The resultant stress and hydrostatic pressure in the PDL were calculated using finite element analysis.
  
  
  RESULTS
  The labial surface of the mandibular first premolar demonstrated a 26.39° change from 2.5 to 6 mm from the cusp tip. The maximum Von-Mises stress and hydrostatic pressure in the PDL were observed at the root apex for all of the bracket positions, and these values demonstrated, respectively, a change of up to 0.059 and 0.186 MPa between two successive points.
  
  
  CONCLUSIONS
  It can be concluded that the variation in the vertical position of the bracket can have an important effect on the torque and subsequently on the stresses and pressures in the PDL.
authors.affiliation: Student Research Commitee, Department of Orthodontics, Orthodontics Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, 71956-15878, Iran. sadra.sardarian@yahoo.com.
authors.forename: Ahmadreza
authors.initials: A
authors.lastname: Sardarian
authors.affiliation: Department of Orthodontics, Orthodontics Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, 71956-15878, Iran. smomenidanaei@yahoo.com.
authors.forename: Shahla Momeni
authors.initials: SM
authors.lastname: Danaei
authors.affiliation: Department of Oral and Maxillofacial Radiology, Biomaterial Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, 71956-15878, Iran. shoalehshahidi@yahoo.com.
authors.forename: Shoaleh
authors.initials: S
authors.lastname: Shahidi
authors.affiliation: Student Research Commitee, Department of Orthodontics, Orthodontics Research Center, School of Dentistry, Shiraz University of Medical Sciences, Shiraz, 71956-15878, Iran. sahar_ghodsi@gmail.com.
authors.forename: Sahar Ghodsi
authors.initials: SG
authors.lastname: Boushehri
authors.affiliation: Department of Orthodontics, Dental Research Center, School of Dentistry, Tehran University of Medical Sciences, North Kargar Street, Tehran, 14399-55991, Iran. gueramya@yahoo.com.
authors.forename: Allahyar
authors.initials: A
authors.lastname: Geramy
mesh_terms: D000293:Adolescent; D000328:Adult; D001641:Bicuspid / Q000502:physiology; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D054893:Cone-Beam Computed Tomography / Q000379:methods; D020342:Finite Element Analysis; D006801:Humans; D006874:Hydrostatic Pressure; D008334:Mandible / Q000502:physiology; D008954:Models, Biological; D016910:Orthodontic Brackets; D009969:Orthodontic Wires; D010513:Periodontal Ligament / Q000502:physiology*; D013314:Stress, Mechanical; D019227:Tooth Apex / Q000502:physiology; D019228:Tooth Crown / Q000502:physiology; D019415:Torque; D055815:Young Adult