title: Effect of miniscrew angulation on anchorage resistance.
abstract: BACKGROUND
  Even though the use of titanium miniscrews to provide orthodontic anchorage has become increasingly popular, there is no universally accepted screw-placement protocol. Variables include the presence or absence of a pilot hole, placement through attached or unattached soft tissue, and angle of placement. The purpose of this in-vitro study was to test the hypothesis that screw angulation affects screw-anchorage resistance.
  
  
  METHODS
  Three-dimensional finite element models were created to represent screw-placement orientations of 30°, 60°, and 90°, while the screw was displaced to 0.6 mm at a distance of 2.0 mm from the bone surface. In a parallel cadaver study, 96 titanium alloy screws were placed into 24 hemi-sected maxillary and 24 hemi-sected mandibular specimens between the first and second premolars. The specimens were randomly and evenly divided into 3 groups according to screw angulation (relative to the bone surface): 90° vs 30° screw pairs, 90° vs 60° screw pairs, and 30° vs 60° screw pairs. All screws were subjected to increasing forces parallel to the occlusal plane, pulling mesially until the miniscrews were displaced by 0.6 mm. A paired-samples t test was used to assess the significance of differences between 2 samples consisting of matched pairs of subjects, with matched pairs of subjects including 2 measurements taken on the same subject. One-way analysis of variance (ANOVA) with the post-hoc Tukey studentized range test was conducted to determine whether there were significant differences, and the order of those differences, in anchorage resistance values among the 3 screw angulations at maxillary and mandibular sites.
  
  
  RESULTS
  The finite element analysis showed that 90° screw placement provided greater anchorage resistance than 60° and 30° placements. In the cadaver study, although the maximum anchorage resistance provided by screws placed at 90° to the cadaver bone surface exceeded, on average, the anchorage resistance of the screws placed at 60°, which likewise exceeded the anchorage resistance of screws placed at 30°, these differences were not statistically significant.
  
  
  CONCLUSIONS
  Placing orthodontic miniscrews at angles less than 90° to the alveolar process bone surface does not offer force anchorage resistance advantages.
authors.affiliation: Department of Orthopaedics and Sports Medicine, University of Washington, Seattle, USA.
authors.forename: Niles
authors.initials: N
authors.lastname: Woodall
authors.forename: Srinivas C
authors.initials: SC
authors.lastname: Tadepalli
authors.forename: Fang
authors.initials: F
authors.lastname: Qian
authors.forename: Nicole M
authors.initials: NM
authors.lastname: Grosland
authors.forename: Steve D
authors.initials: SD
authors.lastname: Marshall
authors.forename: Thomas E
authors.initials: TE
authors.lastname: Southard
mesh_terms: D000704:Analysis of Variance; D001863:Bone Screws; D002102:Cadaver; D003758:Dental Implantation, Endosseous / Q000379:methods*; D003799:Dental Stress Analysis / Q000379:methods; D055119:Elastic Modulus; D020342:Finite Element Analysis; D006801:Humans; D008334:Mandible / Q000601:surgery; D008437:Maxilla / Q000601:surgery; D008904:Miniaturization; D052280:Orthodontic Anchorage Procedures / Q000295:instrumentation*; D018709:Statistics, Nonparametric