title: Design characteristics, primary stability and risk of fracture of orthodontic mini-implants: pilot scan electron microscope and mechanical studies.
abstract: OBJECTIVE
  Orthodontic mini-implants (OMIs) are increasingly used in orthodontics but can fail for various reasons. This study investigates the effects of OMI design characteristics on the mechanical properties in artificial bone.
  
  
  METHODS
  Twelve self-drilling OMIs (2 small, 6 medium, 4 large) from 8 manufacturers were tested for their primary stability in simulated medium-high cancellous bone and the risk to fracture in high-density methacrylate blocks. For the assessments of the maximum insertion torque (IT) and torsional fracture (TF) 5 of each OMI were used and for the pull-out strength (POS) 10. The OMIs were inserted with a torque screwdriver (12 sec/360°) until the bottom at 8 mm depth was reached. OMI designs were analyzed with a scan electron microscope (SEM).
  
  
  RESULTS
  SEM images revealed a great variation in product refinement. In the whole sample, a cylindrical OMI shape was associated with higher POS (p<0.001) but lower IT (p=0.002) values. The outer and inner OMI diameters were design characteristics well correlated with POS, IT and TF values (ranging from 0.601 to 0.961). Greater thread depth was related to greater POS values (r= 0.628), although OMIs with similar POS values may have different IT values. Thread depth and pitch had some impact on POS. TF depended mainly on the OMI inner (r= 0.961) and outer diameters (r=0.892). A thread depth to outer diameter ratio close to 40% increased TF risk.
  
  
  CONCLUSIONS
  Although at the same insertion depth the OMI outer and inner diameters are the most important factors for primary stability, other OMI design characteristics (cylindrical vs. conical, thread design) may significantly affect primary stability and torsional fracture. This needs to be considered when selecting the appropriate OMI for the desired orthodontic procedures.
authors.affiliation: Universitat Internacional de Catalunya, Campus Sant Cugat, Josep Trueta s/n, 08195, St. Cugat del Vallès, Barcelona, Spain, apuigdollersp@gmail.com.
authors.forename: André
authors.initials: A
authors.lastname: Walter
authors.forename: Heinz
authors.initials: H
authors.lastname: Winsauer
authors.forename: Jordi
authors.initials: J
authors.lastname: Marcé-Nogué
authors.forename: Sergi
authors.initials: S
authors.lastname: Mojal
authors.forename: Andreu
authors.initials: A
authors.lastname: Puigdollers
mesh_terms: D003799:Dental Stress Analysis; D008422:Materials Testing; D008855:Microscopy, Electron, Scanning; D052280:Orthodontic Anchorage Procedures / Q000295:instrumentation*; D011474:Prosthesis Design; D011475:Prosthesis Failure; D012307:Risk Factors; D054159:Torsion, Mechanical