title: Effects of thread depth, taper shape, and taper length on the mechanical properties of mini-implants.
abstract: BACKGROUND
  The primary stability of a mini-implant is critical, since most orthodontic mini-implant failures occur at an early stage. As orthodontic mini-implants have restrictions in diameter and length, an optimal design of the shape is important for sufficient primary stability. The purpose of this study was to investigate the influence of various mini-implants design factors, including thread depth, degree of taper, and taper length on insertion torque, pullout strength, stiffness, and screw displacement before failure.
  
  
  METHODS
  Finite element analyses were conducted first for identification of optimal design parameters. Four types of mini-implants with different design parameters were then custom manufactured and tested mechanically. All mechanical tests were performed in artificial bone with homogenous density to remove the variability associated with bone.
  
  
  RESULTS
  Finite element results showed that, for mini-implants with a fixed external diameter of 2 mm, a thread length of 9.82 mm, and a pitch of 0.75 mm, those with greater thread depths, smaller taper degrees, and shorter taper lengths generated higher maximum stresses on the bone and thread elements. These mini-implants also had larger relative displacements. Maximum pullout resistance was attained with a core/external diameter ratio of 0.68. All mechanical results were compatible with the findings in the finite element analyses.
  
  
  CONCLUSIONS
  Modification of the mini-implant design can substantially affect the mechanical properties. The finite element method is an effective tool to identify optimal design parameters and allow for improved mini-implant designs.
authors.affiliation: Instructor, School of Dentistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Jenny Zwei-Chieng
authors.initials: JZ
authors.lastname: Chang
authors.affiliation: Associate professor, School of Dentistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Yi-Jane
authors.initials: YJ
authors.lastname: Chen
authors.affiliation: Postgraduate student, School of Dentistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Yuan-Yi
authors.initials: YY
authors.lastname: Tung
authors.affiliation: Postgraduate student, Mechanical Engineering Department, National Taipei University of Technology, Taipei, Taiwan.
authors.forename: Yu-Ying
authors.initials: YY
authors.lastname: Chiang
authors.affiliation: Instructor, School of Dentistry, National Taiwan University, Taipei, Taiwan. Electronic address: eddielai0715@yahoo.com.tw.
authors.forename: Eddie Hsiang-Hua
authors.initials: EH
authors.lastname: Lai
authors.affiliation: Professor, Mechanical Engineering Department, National Taipei University of Technology, Taipei, Taiwan. Electronic address: wpchen@ntut.edu.tw.
authors.forename: Weng-Pin
authors.initials: WP
authors.lastname: Chen
authors.affiliation: Professor, School of Dentistry, National Taiwan University, Taipei, Taiwan.
authors.forename: Chun-Pin
authors.initials: CP
authors.lastname: Lin
mesh_terms: D000497:Alloys; D001696:Biomechanical Phenomena; D015519:Bone Density / Q000502:physiology; D001842:Bone and Bones / Q000502:physiology; D017076:Computer-Aided Design; D003722:Dental Alloys / Q000737:chemistry; D015921:Dental Implants; D003799:Dental Stress Analysis / Q000295:instrumentation; D055119:Elastic Modulus; D004868:Equipment Failure; D020342:Finite Element Analysis; D006801:Humans; D052280:Orthodontic Anchorage Procedures / Q000295:instrumentation*; D016382:Orthodontic Appliance Design; D013314:Stress, Mechanical; D013499:Surface Properties; D014025:Titanium / Q000737:chemistry; D019415:Torque
chemical_list: D000497:Alloys; D003722:Dental Alloys; D015921:Dental Implants; C031462:titanium alloy (TiAl6V4); D014025:Titanium