title: Influence of thread design on pedicle screw fixation. Laboratory investigation.
abstract: OBJECTIVE
  The authors conducted a study to determine the thread properties that provide optimal screw fixation in cancellous bone, when screws of the same external screw diameter are used.
  
  
  METHODS
  Three compliance engineering-certified screws in clinical use, all of the same external diameter and length, were compared in an axial pullout experiment with respect to advantageous thread properties. As test material, standardized Sawbone blocks with 3 different densities (0.12, 0.16, and 0.32 g/cm3) were used.
  
  
  RESULTS
  Screw thread Type 1, whose flank overlap area (FOA; 261 mm2) results from narrowing the conical core in the thread area, showed significantly better holding strength than the other types. Screw thread Type 2 (FOA 326 mm2) with a conical but thicker core and a smaller thread pitch was found to be the only one without increase of pull-out forces when test materials density changed from 0.12 to 0.16 g/cm3. A screw tested as control, with a constant (cylindrical) core diameter (Type 3; FOA 206 mm2), had the same thread pitch as Type 1 but without the compressive effect on the surrounding bur hole wall material. Nevertheless, it showed higher pullout forces in the 0.16-g/cm3 material than screw Type 2.
  
  
  CONCLUSIONS
  By reducing the core diameter of a screw toward the tip, while maintaining a constant nominal (external) diameter, one achieves frictional connection due to compression of surrounding material. In addition, the FOA is increased, which, in summary, leads to better fixation, as shown by screw Type 1.
authors.affiliation: Department for Spine Surgery, Paracelsus Medical University Hospital, Salzburg, Austria. m.krenn@salk.at
authors.forename: Michael Hannes
authors.initials: MH
authors.lastname: Krenn
authors.forename: Wolfgang Peter
authors.initials: WP
authors.lastname: Piotrowski
authors.forename: Rainer
authors.initials: R
authors.lastname: Penzkofer
authors.forename: Peter
authors.initials: P
authors.lastname: Augat
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D004867:Equipment Design; D013123:Spinal Fusion / Q000295:instrumentation*