title: Mechanical stability of a novel screw design after repeated insertion: can the double-thread screw serve as a back up?
abstract: BACKGROUND
  We investigated mechanical pull-out behavior and tightening torque of a novel dual-core pedicle "6T screw" (6T). The aim of this study was to test if these changes in screw geometry are increasing the strength of the pedicle screw fixation after repeated insertion.
  
  
  METHODS
  Three different types of pedicle screws were inserted in rigid foam blocks. Tightening torque and pull-out strength were measured during two repetitive insertions of a standard 6.5×45-mm conical screw. The third insertion into the pilot hole was performed using either standard 6.5×45-mm or 7.2×45-mm conical screws or the novel 6.5×45-mm (6T) screw. Additionally, we performed a surface analysis to investigate the bone/screw interface.
  
  
  RESULTS
  The maximal tightening torque at the third insertion of the novel 6T screw was 194% higher compared to the standard 6.5×45-mm conical screw and 135% higher compared to the standard 7.2×45-mm conical screw. The pull-out strength of the 6T screw showed no significant changes, and surface analysis revealed a compression of the screw-foam interface due to the different internal diameters.
  
  
  CONCLUSIONS
  The modified geometrical design of the 6T screw seems to have no statistically significant effect on the pull-out strength, although it achieved a higher tightening torque. This might be due to the different pitch angle cutting a new thread into the material and also to the enlarged inner diameter.
authors.affiliation: Department of Neurosurgery, University Hospital Erlangen, Erlangen, Germany - kurt.wiendieck@gmail.com.
authors.forename: Kurt
authors.initials: K
authors.lastname: Wiendieck
authors.affiliation: Department of Psychiatry and Psychotherapy, University Hospital Erlangen, Erlangen, Germany.
authors.forename: Helge
authors.initials: H
authors.lastname: Müller
authors.affiliation: Department of Neurosurgery, University Hospital Erlangen, Erlangen, Germany.
authors.forename: Michael
authors.initials: M
authors.lastname: Buchfelder
authors.affiliation: Department of Neurosurgery, University Hospital Erlangen, Erlangen, Germany.
authors.forename: Björn
authors.initials: B
authors.lastname: Sommer
mesh_terms: D001696:Biomechanical Phenomena; D000071556:Cancellous Bone / Q000601:surgery*; D004867:Equipment Design; D006801:Humans; D008422:Materials Testing; D065289:Pedicle Screws; D019415:Torque