title: Influence of Dynesys system screw profile on adjacent segment and screw.
abstract: METHODS
  Displacement-controlled finite element analysis was used to evaluate the mechanical behavior of the lumbar spine after insertion of the Dynesys dynamic stabilization system.
  
  
  OBJECTIVE
  This study aimed to investigate whether different depths of screw placement of Dynesys would affect load sharing of screw, range of motion (ROM), annulus stress, and facet contact force.
  
  
  BACKGROUND
  In clinical follow-up, a high rate of screw complications and adjacent segment disease were found after using Dynesys. The pedicle screw in the Dynesys system is not so easy to implant into the standard position and causes the screw to protrude more prominently from the pedicle. Little is known about how the biomechanical effects are influenced by the Dynesys screw profile.
  
  
  METHODS
  The Dynesys was implanted in a 3-dimensional, nonlinear, finite element model of the L1 to L5 lumbar spine. Different depths of screw position were modified in this model by 5 and 10 mm out of the pedicle. The model was loaded to 150 N preload and controlled the same ROMs by 20, 15, 8, and 20 degrees in flexion, extension, torsion, and lateral bending, respectively. Resultant ROM, annulus stress, and facet contact force were analyzed at the surgical and adjacent level.
  
  
  RESULTS
  Under flexion, extension, and lateral bending, the Dynesys provided sufficient stability at the surgical level, but increased the ROM at the adjacent level. Under flexion and lateral bending, the Dynesys alleviated annulus stress at the surgical level, but increased annulus stress at the adjacent level. Under extension, the Dynesys decreased facet loading at the surgical level but increased facet loading at the adjacent level.
  
  
  CONCLUSIONS
  This study found that the Dynesys system was able to restore spinal stability and alleviate loading on disc and facet at the surgical level, but greater ROM, annulus stress, and facet loading were found at the adjacent level. In addition, profile of the screw placement caused only a minor influence on the ROM, annulus stress, and facet loading, but the screw stress was noticeably increased.
authors.affiliation: Faculty of Medicine, National Yang-Ming University, Taipei, Taiwan.
authors.forename: Chien-Lin
authors.initials: CL
authors.lastname: Liu
authors.forename: Zheng-Cheng
authors.initials: ZC
authors.lastname: Zhong
authors.forename: Shih-Liang
authors.initials: SL
authors.lastname: Shih
authors.forename: Chinghua
authors.initials: C
authors.lastname: Hung
authors.forename: Yong-Eng
authors.initials: YE
authors.lastname: Lee
authors.forename: Chen-Sheng
authors.initials: CS
authors.lastname: Chen
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws; D003198:Computer Simulation; D020342:Finite Element Analysis; D006801:Humans; D016268:Internal Fixators; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery; D008953:Models, Anatomic; D016059:Range of Motion, Articular; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods*; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology