title: Finite element analysis of mono- and bicortical mini-implant stability.
abstract: OBJECTIVE
  Loosening and loss rates of monocortical mini-implants are relatively high, therefore the following null hypothesis was tested: 'The local bone stress in mono and bicortically-anchored mini-implants is identical'.
  
  
  METHODS
  Anisotropic Finite Element Method (FEM) models of the mandibular bone, including teeth, periodontal ligaments, orthodontic braces, and mini-implants of varying length, were created. The morphology was based on the Computed Tomography data of an anatomical preparation. All mini-implants with varying insertion depths (monocortical short, monocortical long, bicortical) were typically loaded, and the induced effective stress was calculated in the cervical area of the cortical bone. The obtained values were subsequently analysed descriptively and exploratively using the SPSS 19.0 software.
  
  
  RESULTS
  The null hypothesis was rejected, since the stress values of each anchorage type differed significantly (Kruskal-Wallis Test, P < 0.001). Therefore, the lowest effective stress values were induced in bicortical anchorage (mean = 0.65MPa, SD = 0.06MPa) and the highest were induced in monocortical (short) anchorage of the mini-implants (mean = 1.79MPa, SD = 0.29MPa). The Spearman rank correlation was 0.821 (P < 0.001).
  
  
  CONCLUSIONS
  The deeper the mini-implant was anchored, the lower were the effective stress values in the cervical region of the cortical bone. Bicortical implant anchorage is biomechanically more favourable than monocortical anchorage; therefore, bicortical anchorage should be especially considered in challenging clinical situations requiring heavy anchorage.
authors.affiliation: Department of Orthodontics, University of Munich, Bavaria, Germany cholberg@med.lmu.de.
authors.forename: Christof
authors.initials: C
authors.lastname: Holberg
authors.affiliation: Department of Orthodontics, University of Munich, Bavaria, Germany.
authors.forename: Philipp
authors.initials: P
authors.lastname: Winterhalder
authors.affiliation: Department of Orthodontics, University of Munich, Bavaria, Germany.
authors.forename: Ingrid
authors.initials: I
authors.lastname: Rudzki-Janson
authors.affiliation: Department of Orthodontics, University of Munich, Bavaria, Germany.
authors.forename: Andrea
authors.initials: A
authors.lastname: Wichelhaus
mesh_terms: D000293:Adolescent; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D003758:Dental Implantation, Endosseous / Q000379:methods*; D015921:Dental Implants; D003799:Dental Stress Analysis / Q000379:methods; D020342:Finite Element Analysis; D006801:Humans; D007091:Image Processing, Computer-Assisted / Q000379:methods; D008297:Male; D008334:Mandible / Q000033:anatomy & histology / Q000601:surgery*; D008904:Miniaturization; D008954:Models, Biological; D052280:Orthodontic Anchorage Procedures / Q000295:instrumentation*; D016382:Orthodontic Appliance Design; D013314:Stress, Mechanical; D013499:Surface Properties; D014057:Tomography, X-Ray Computed / Q000379:methods
chemical_list: D015921:Dental Implants