title: The biomechanical effect of pedicle screws' insertion angle and position on the superior adjacent segment in 1 segment lumbar fusion.
abstract: METHODS
  A finite element analysis.
  
  
  OBJECTIVE
  To investigate the association between the position of an inserted pedicle screw and the corresponding facet contact force or intradiscal pressure.
  
  
  BACKGROUND
  Although superior facet joint violation by pedicle screws is not an uncommon occurrence in instrumented lumbar fusion surgery, its actual biomechanical significance is not well understood. Furthermore, the association between the position of the pedicle screw and the stress on the corresponding disc/facet joint has yet to be investigated.
  
  
  METHODS
  According to the positions of pedicle screws in L4 of the L4-L5 lumbar fusion, 4 L4-L5 fusion models were simulated. These models included the violation of both L3-L4 superior facet joints by pedicle screws (facet joint violation [FV] model), the nonencroachment of both L3-L4 superior facet joints by pedicle screws (facet joint preservation [FP] model), and the removal state of pedicle screws in the FV model (removal of violated pedicle screws [rFV] model). The facet joint contact [FC] model represented the scenario in which the pedicle screws did not encroach upon either facet joint but were inserted close to the L3-L4 facet joint surface. Moreover, the uninstrumented fusion [UF] model represented the uninstrumented L4-L5 fusion. In each scenario, the intradiscal pressures and facet contact forces at the L2-L3 and L3-L4 segments were analyzed under extension and torsion moments.
  
  
  RESULTS
  The FV model yielded the greatest increases in facet contact force and intradiscal pressure at the L3-L4 segment under extension and torsion moments. Following the FV model, the increases in intradiscal pressure and facet contact force were the second highest in the FC model followed by the FP model. Furthermore, the rFV model represented prominent reductions of previously increased facet contact force and intradiscal pressure at the L3-L4 segment.
  
  
  CONCLUSIONS
  In models of 1-segment lumbar fusion surgery, the positions of pedicle screws were closely linked with corresponding disc stresses and facet contact forces. However, even in cases of facet violation by pedicle screws, removal of the pedicle screw after fusion completion can reduce facet contract forces and disc stresses under both extension and torsional moments.
authors.affiliation: Spine Center and Department of Orthopaedic Surgery, Seoul National University College of Medicine and Seoul National University Bundang Hospital, Seoul, Korea.
authors.forename: Ho-Joong
authors.initials: HJ
authors.lastname: Kim
authors.forename: Heoung-Jae
authors.initials: HJ
authors.lastname: Chun
authors.forename: Kyoung-Tak
authors.initials: KT
authors.lastname: Kang
authors.forename: Seong-Hwan
authors.initials: SH
authors.lastname: Moon
authors.forename: Hak-Sun
authors.initials: HS
authors.lastname: Kim
authors.forename: Jin-Oh
authors.initials: JO
authors.lastname: Park
authors.forename: Eun-Su
authors.initials: ES
authors.lastname: Moon
authors.forename: Bo-Ram
authors.initials: BR
authors.lastname: Kim
authors.forename: Joon-Seok
authors.initials: JS
authors.lastname: Sohn
authors.forename: Yu-Na
authors.initials: YN
authors.lastname: Ko
authors.forename: Hwan-Mo
authors.initials: HM
authors.lastname: Lee
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D020878:Device Removal; D006801:Humans; D007403:Intervertebral Disc / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000601:surgery; D008953:Models, Anatomic; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D021801:Zygapophyseal Joint / Q000503:physiopathology / Q000601:surgery