title: Stress distribution in the mini-screw and alveolar bone during orthodontic treatment: a finite element study analysis.
abstract: OBJECTIVE
  This study aimed to investigate the reason for high failure rates of mini-screws during orthodontic treatment. We hypothesized that decreasing the length of the mini-screw outside the bone relative to the length inside the bone (outside/inside length ratio) and equalising it to the tooth crown/root ratio would lead to increased stability of the mini-screw against lateral loading when assessed using finite element analysis.
  
  
  METHODS
  We analysed stress distribution of mini-screws in the cortical and trabecular bone and von Mises stress levels when a 2-N force was applied to the head of four mini-screws of 6, 8, 10 and 12 mm in length. The direction of the force was perpendicular to the major axis of the screws.
  
  
  RESULTS
  Stress levels of screws in cortical bone increased in proportion to the length of mini-screws outside the bone. The length of mini-screws inside the bone did not affect stress levels in the cortical bone.
  
  
  CONCLUSIONS
  The results of this finite element analysis indicate that to stabilize the mini-screw, controlling the screw length outside the bone is more important than controlling the outside/inside length ratio.
authors.affiliation: Department of Orthodontics, Tokyo Dental College, Tokyo, Japan.
authors.forename: Shuntaro
authors.initials: S
authors.lastname: Kuroda
authors.affiliation: Department of Orthodontics, Tokyo Dental College, Tokyo, Japan.
authors.forename: Yasushi
authors.initials: Y
authors.lastname: Nishii
authors.affiliation: Oral Science Research Center, Tokyo Dental College.
authors.forename: Shigeru
authors.initials: S
authors.lastname: Okano
authors.affiliation: Department of Orthodontics, Tokyo Dental College, Tokyo, Japan.
authors.forename: Kenji
authors.initials: K
authors.lastname: Sueishi
mesh_terms: D000293:Adolescent; D000328:Adult; D000539:Alveolar Process / Q000502:physiology*; D001696:Biomechanical Phenomena; D015519:Bone Density / Q000502:physiology; D001863:Bone Screws; D003198:Computer Simulation; D003764:Dental Materials / Q000737:chemistry; D055119:Elastic Modulus; D020342:Finite Element Analysis; D006801:Humans; D021621:Imaging, Three-Dimensional / Q000379:methods; D008954:Models, Biological; D052280:Orthodontic Anchorage Procedures / Q000295:instrumentation*; D016382:Orthodontic Appliance Design; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry; D019228:Tooth Crown / Q000033:anatomy & histology; D014092:Tooth Root / Q000033:anatomy & histology; D055815:Young Adult
chemical_list: D003764:Dental Materials; D014025:Titanium
keywords: Orthodontic mini-screw; finite element analysis; outside/inside length ratio; stress distribution