title: Pullout resistance of thoracic extrapedicular screws used as a salvage procedure.
abstract: BACKGROUND
  Extrapedicular screws are placed more laterally than intrapedicular screws and pass through the transverse process or rib head before entering the vertebral body. These screws are sometimes placed to salvage failed pedicle screws, but the change in pullout resistance of extrapedicular screws after salvage has not been quantified.
  
  
  OBJECTIVE
  To quantify the pullout resistance of thoracic extrapedicular screws compared with intrapedicular screws and the pullout resistance of newly inserted screws compared with extrapedicular screws used as salvage for failed intrapedicular screws.
  
  
  METHODS
  In vitro paired comparison of screw pullout resistance in isolated thoracic vertebrae.
  
  
  METHODS
  Tapered monoaxial pedicle screws were inserted in the left or right pedicle of 11 human cadaveric thoracic vertebrae. An extrapedicular screw was inserted on the contralateral side. Both screws were pulled out axially at 0.5 mm/s using a servohydraulic test frame while applied load was recorded. Then a fresh extrapedicular screw was inserted as a salvage screw on the intrapedicular screw side and pulled out.
  
  
  RESULTS
  In uncompromised vertebrae, the pullout strength of extrapedicular screws was 80+/-32% of that of intrapedicular screws (p=.073, repeated-measures one-way analysis of variance/Tukey). Salvage screws restored pullout strength to 65+/-30% of that of intrapedicular screws (p=.003).
  
  
  CONCLUSIONS
  Extrapedicular screws provided comparable but slightly lower pullout resistance to intrapedicular screws in uncompromised vertebrae. They are therefore a feasible salvage technique when a compromised pedicle precludes reinsertion of an intrapedicular screw, but the salvage screw is significantly weaker than the original screw.
authors.affiliation: Spinal Biomechanics, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, 350 W. Thomas Road, Phoenix, AZ 85013, USA.
authors.forename: K Zafer
authors.initials: KZ
authors.lastname: Yüksel
authors.forename: Mark S
authors.initials: MS
authors.lastname: Adams
authors.forename: Robert H
authors.initials: RH
authors.lastname: Chamberlain
authors.forename: Mark
authors.initials: M
authors.lastname: Potocnjak
authors.forename: Sung Chan
authors.initials: SC
authors.lastname: Park
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D001863:Bone Screws; D002102:Cadaver; D006801:Humans; D008297:Male; D008422:Materials Testing; D013123:Spinal Fusion / Q000295:instrumentation*; D013904:Thoracic Vertebrae / Q000601:surgery*