title: Removal of fixation construct could mitigate adjacent segment stress after lumbosacral fusion: A finite element analysis.
abstract: BACKGROUND
  Combined usage of posterior lumbar interbody fusion and transpedicular fixation has been extensively used to treat the various lumbar degenerative disc diseases. The transpedicular fixator aims to increase stability and enhance the fusion rate. However, how the fused disc and bridged vertebrae respectively affect adjacent-segment diseases progression is not yet clear.
  
  
  METHODS
  Using a validated lumbosacral finite-element model, three variations at the L4-L5 segment were analyzed: 1) moderate disc degeneration, 2) instrumented with a stand-alone cage and pedicle screw fixators, and 3) with the cage only after fusion. The intersegmental angles, disc stresses, and facet loads were examined. Four motion tests, flexion, extension, bending, and twisting, were also simulated.
  
  
  RESULTS
  The adjacent-segment disease was more severe at the cephalic segment than the caudal segment. After solid fusion and fixation, the increase in intersegmental angles, disc stresses and facet loads of the adjacent segments were about 57.6%, 47.3%, and 59.6%, respectively. However, these changes were reduced to 30.1%, 22.7%, and 27.0% after removal of the fixators. This was attributed to the differences between the biomechanical characteristics of the fusion and fixation mechanisms.
  
  
  CONCLUSIONS
  Fixation superimposes a stiffer constraint on the mobility of the bridged segment than fusion. The current study suggested that the removal of spinal fixators after complete fusion could decrease the stress at adjacent segments. Through a minimally invasive procedure, we could reduce secondary damage to the paraspinal structures while removing the fixators, which is of utmost concern to surgeons.
authors.affiliation: Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan.
authors.forename: Yueh-Ying
authors.initials: YY
authors.lastname: Hsieh
authors.affiliation: Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan.
authors.forename: Chia-Hsien
authors.initials: CH
authors.lastname: Chen
authors.affiliation: Division of Neurosurgery, Department of Surgery, National Taiwan University Hospital, Taiwan; Institute of Biomedical Engineering, National Taiwan University, Taiwan.
authors.forename: Fon-Yih
authors.initials: FY
authors.lastname: Tsuang
authors.affiliation: Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan; Institute of Biomedical Engineering, National Taiwan University, Taiwan.
authors.forename: Lien-Chen
authors.initials: LC
authors.lastname: Wu
authors.affiliation: Graduate Institute of Biomedical Engineering, National Taiwan University of Science and Technology, Taiwan.
authors.forename: Shang-Chih
authors.initials: SC
authors.lastname: Lin
authors.affiliation: Department of Orthopaedics, Shuang Ho Hospital, Taipei Medical University, Taiwan; Department of Orthopaedics, School of Medicine, College of Medicine, Taipei Medical University, Taiwan. Electronic address: cjchiang@s.tmu.edu.tw.
authors.forename: Chang-Jung
authors.initials: CJ
authors.lastname: Chiang
mesh_terms: D001696:Biomechanical Phenomena; D020878:Device Removal; D018450:Disease Progression; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators; D055959:Intervertebral Disc Degeneration / Q000503:physiopathology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000601:surgery*; D065289:Pedicle Screws; D016059:Range of Motion, Articular; D012447:Sacrum / Q000601:surgery*; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*
keywords: Adjacent segment disease; Finite element; Interbody fusion; Spinal fixator