title: Biomechanical evaluation of a dynamic pedicle screw fixation device.
abstract: BACKGROUND
  Recent innovations in dynamic devices have promised a reduction in stress shielding, protection of adjacent segment degeneration, and decreased implant failure. However, there have been few studies comparing the biomechanical properties of a rigid device in comparison to a dynamic posterior fixation device. The purpose of this study was to compare the immediate stability of a new dynamic pedicle screw fixation device with an equivalent rigid device.
  
  
  METHODS
  Six thoracolumbar cadaver spines (T10-L4) were fixed in a biomechanical testing frame. Pure moments of 10Nm were loaded in six directions: flexion, extension, right and left lateral bending, and right and left axial rotation. For each spine, four different stages were tested: intact, destabilization of the middle segment, fixation with the dynamic device, and fixation with the rigid device. Ranges of motion were measured using stereophotogrammetry. The specimens with each device were then subjected to flexion-compression loading for five cycles on a MTS 858 Universal Testing Machine. The average stiffness of the last three cycles was recorded.
  
  
  RESULTS
  Both dynamic and rigid devices were found to provide stability for the injured segment in flexion-extension and lateral bending. In axial rotation, the devices could restore the stability to levels similar to those in an intact spine. Results also indicated a slight increase in range of motion in flexion-extension and significant reduction in stiffness of flexion-compression with the dynamic device (P < 0.01), in comparison to the rigid device.
  
  
  CONCLUSIONS
  The dynamic device offers a system that may alter favorably the movement and load transmission of a spinal motion segment without sacrificing construct stability.
authors.affiliation: Zhejiang Spine Research Center, Department of Orthopaedic surgery, Second Affiliated Hospital of Wenzhou Medical College, China. spinexu@126.com
authors.forename: Hua-Zi
authors.initials: HZ
authors.lastname: Xu
authors.forename: Xiang-Yang
authors.initials: XY
authors.lastname: Wang
authors.forename: Yong-Long
authors.initials: YL
authors.lastname: Chi
authors.forename: Qing-An
authors.initials: QA
authors.lastname: Zhu
authors.forename: Yan
authors.initials: Y
authors.lastname: Lin
authors.forename: Qi-Shan
authors.initials: QS
authors.lastname: Huang
authors.forename: Li-Yang
authors.initials: LY
authors.lastname: Dai
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena / Q000379:methods*; D001863:Bone Screws; D002102:Cadaver; D019245:Compressive Strength; D019544:Equipment Failure Analysis; D005592:Fracture Fixation / Q000295:instrumentation*; D006801:Humans; D066298:In Vitro Techniques; D007593:Joint Instability / Q000503:physiopathology* / Q000601:surgery; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D008297:Male; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation*; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000503:physiopathology* / Q000601:surgery; D016896:Treatment Outcome