title: The quantitative assessment of risk factors to overstress at adjacent segments after lumbar fusion: removal of posterior ligaments and pedicle screws.
abstract: METHODS
  Finite element method.
  
  
  OBJECTIVE
  To investigate the changes in the disc stress and range of motion (ROM) at adjacent segments after lumbar fusion based on whether or not pedicle screws are removed and whether or not the continuity of the proximal posterior ligament complex (PLC) is preserved.
  
  
  BACKGROUND
  The ablation of proximal PLC continuity and the presence of pedicle screws have been reported to affect the biomechanics at adjacent segments after lumbar fusion. However, there have been few studies regarding the quantitative assessment of their contribution to overstress at adjacent segments after lumbar fusion.
  
  
  METHODS
  In the validated intact lumbar finite element model (L2-L5), four types of L3-L4 fusion models were simulated. These models included the preservation of the PLC continuity with pedicle screws (Pp WiP), the preservation of PLC continuity without pedicle screws (Pp WoP), the sacrifice of PLC with pedicle screws (Sp WiP), and the sacrifice of PLC without pedicle screws (Sp WoP). In each scenario, the ROM, maximal von Mises stress of discs, and the facet joint contract force at adjacent segments were analyzed. RESULTS.: Among the four models, the Sp WiP yielded the greatest increase in the ROM and the maximal von Mises stress of the disc at adjacent segments under four moments. Following the SP WiP, the order of increase of the ROM and the disc stress was Pp WiP, Sp WoP, and Pp WoP. Furthermore, the increase of ROM and disc stress at the proximal adjacent segment was more than at the distal adjacent segment under all four moments in each model. The facet joint contact was also most increased in the Sp WiP under extension and torsion moment.
  
  
  CONCLUSIONS
  The current study suggests that the preservation of the PLC continuity or the removal of pedicle screws after complete fusion could decrease the stress at adjacent segments, and their combination could act synergistically.
authors.affiliation: Spine Center, Yonse sarang Hospital, Seoul, Korea.
authors.forename: Ho-Joong
authors.initials: HJ
authors.lastname: Kim
authors.forename: Kyoung-Tak
authors.initials: KT
authors.lastname: Kang
authors.forename: Seong-Hwan
authors.initials: SH
authors.lastname: Moon
authors.forename: Heoung-Jae
authors.initials: HJ
authors.lastname: Chun
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: Hwan-Mo
authors.initials: HM
authors.lastname: Lee
mesh_terms: D001863:Bone Screws; D006801:Humans; D017846:Longitudinal Ligaments / Q000502:physiology*; D008159:Lumbar Vertebrae / Q000502:physiology*; D008297:Male; D008875:Middle Aged; D008954:Models, Biological; D016059:Range of Motion, Articular / Q000502:physiology; D012307:Risk Factors; D013123:Spinal Fusion / Q000295:instrumentation* / Q000592:standards; D013314:Stress, Mechanical