title: Tensile failure of C2 pedicles and of subsequent direct repair in a porcine model.
abstract: METHODS
  This repeated-measures biomechanical study evaluated the tensile force required to cause bipedicular (hangman's) fractures in isolated porcine C2 specimens, and the subsequent force to failure after direct fracture repair with bipedicular lag screws.
  
  
  OBJECTIVE
  To assess the pullout strength of direct lag screw fixation of hangman's fracture, relative to the strength of the intact specimens.
  
  
  BACKGROUND
  Clinical studies have reported successful treatment of hangman's fractures by direct screw repair followed by a collar after surgery. However, to the authors' best knowledge, there has not yet been a biomechanical analysis of the force required for screw pullout after direct repair.
  
  
  METHODS
  Of 60 mounted porcine C2 specimens subjected to tensile anteroposterior force, hangman's fracture occurred in 15 cases, 12 of which were fixed with bipedicular 4.0-mm lag screws. These fixed specimens were retested to failure in the same manner.
  
  
  RESULTS
  Most specimens had laminar fractures after the application of tensile force, with 15 of 60 (25%) showing bipedicular fractures. The force to bipedicular failure was 3259.1 +/- 148.5 N (mean +/- standard error of the mean). After screw fixation, the force to failure of the same specimens was 882.0 +/- 108.5 N (mean +/- standard error of the mean), or 27.3% of the intact bone.
  
  
  CONCLUSIONS
  The pullout strength was substantial (882 N), although the relative strength of fixation was only 27.3% of the fracture strength shown by the intact specimens. The findings are discussed in relation to previous studies. To the authors' best knowledge, this is the first study to examine screw pullout forces after direct repair of hangman's fracture.
authors.affiliation: Department of Orthopaedics, McMaster University, Hamilton, Ontario, Canada. mightysamster1@aol.com
authors.forename: Sam
authors.initials: S
authors.lastname: Kaspar
authors.forename: James P
authors.initials: JP
authors.lastname: Dickey
authors.forename: Jeff
authors.initials: J
authors.lastname: Perrier
authors.forename: Drew A
authors.initials: DA
authors.lastname: Bednar
mesh_terms: D000818:Animals; D001696:Biomechanical Phenomena / Q000379:methods / Q000706:statistics & numerical data; D001863:Bone Screws / Q000706:statistics & numerical data*; D002574:Cervical Vertebrae / Q000502:physiology* / Q000503:physiopathology; D019544:Equipment Failure Analysis / Q000706:statistics & numerical data; D066298:In Vitro Techniques; D016103:Spinal Fractures / Q000209:etiology* / Q000503:physiopathology / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013552:Swine