title: The effects of rigid spinal instrumentation and solid bony fusion on spinal kinematics. A posterolateral spinal arthrodesis model.
abstract: METHODS
  Spinal kinematics after the implementation of rigid spinal instrumentation or the achievement of a solid fusion was studied using a sheep posterolateral spinal arthrodesis model.
  
  
  OBJECTIVE
  To investigate the effects of rigid spinal instrumentation or solid fusion on spinal kinematic parameters.
  
  
  BACKGROUND
  Numerous studies have attempted to define spinal instability in terms of kinematics. Recent in vitro studies have documented the neutral zone, or a measure of spinal laxity, as more sensitive to spinal instability than the range of motion.
  
  
  METHODS
  Seven skeletally mature sheep underwent a single-level posterolateral lumbar arthrodesis using autologous bone graft augmented with transpedicular screw fixation. The animals were killed 4 months after surgery. The identical surgical procedures were performed in seven sheep cadaveric spines, which served as acute postoperative controls. Each functional spinal unit was tested biomechanically before and after hardware removal. The experimental control groups consisted of destabilized spines and spines that underwent transpedicular screw fixation alone, whereas the fusion groups included spines that underwent posterolateral fusion alone or posterolateral fusion with instrumentation.
  
  
  RESULTS
  Rigid instrumentation and solid fusion significantly decreased the neutral zone and range of motion in all testing modes. In axial rotation and lateral bending, solid fusion reduced the range of motion significantly more than transpedicular screw fixation alone. However, in all testing modes, the neutral zones showed no statistical difference between transpedicular screw fixation alone and fusion groups.
  
  
  CONCLUSIONS
  The range of motion was an equivalent or better indicator of fixation or fusion stability compared with the neutral zone. Moreover, the immediate postoperative fixation stability, even if using transpedicular screw fixation, was less than the stability present after a solid fusion.
authors.affiliation: Orthopaedic Biomechanics Laboratory, Union Memorial Hospital, Baltimore, Maryland, USA.
authors.forename: M
authors.initials: M
authors.lastname: Kanayama
authors.forename: B W
authors.initials: BW
authors.lastname: Cunningham
authors.forename: J C
authors.initials: JC
authors.lastname: Weis
authors.forename: L M
authors.initials: LM
authors.lastname: Parker
authors.forename: K
authors.initials: K
authors.lastname: Kaneda
authors.forename: P C
authors.initials: PC
authors.lastname: McAfee
mesh_terms: D000818:Animals; D007593:Joint Instability / Q000601:surgery; D007596:Joints / Q000502:physiology; D011859:Radiography; D016059:Range of Motion, Articular; D012680:Sensitivity and Specificity; D012756:Sheep; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013131:Spine / Q000000981:diagnostic imaging / Q000502:physiology*; D016474:Weight-Bearing