title: Effect of spacer diameter of the Dynesys dynamic stabilization system on the biomechanics of the lumbar spine: a finite element analysis.
abstract: METHODS
  A finite element analysis to simulate the behavior of lumbar spines implanted with a posterior dynamic neutralization system, Dynesys, under displacement-controlled loading.
  
  
  OBJECTIVE
  To investigate whether Dynesys spacers with different diameters would alter the distribution of range of motion, disk stress, and facet contact force at the Dynesys bridging level and the cranial adjacent level.
  
  
  BACKGROUND
  The Dynesys system is designed to preserve intersegmental motion and reduce loading at adjacent levels, but clinical reports do not support these claims. This system has been shown to be almost as stiff as rigid fixation, which acts to hinder intersegmental motion. Few studies have investigated methods of reducing this stiffness.
  
  
  METHODS
  In the finite element study, a previously validated lumbar spine model was used. Five Dynesys constructs with different spacer diameters (0.8, 0.9, 1.0, 1.1, and 1.2 times the original standard size) were implanted into the spine model and bore 4 displacement-controlled loading cases: flexion, extension, torsion, and lateral bending. Resultant range of motions (ROMs), disk stress, and facet contact forces at the bridged level and the cranial adjacent level were compared with the results of a spine model without Dynesys implantation.
  
  
  RESULTS
  The results of ROMs, disk stress, and facet contact forces at the bridged levels were all less than those in the intact spine, except for contact forces at the left facet under lateral bending, facet contact forces at the right facet under torsion, and disk stress under torsion. The results of ROMs, disk stress, and facet contact forces at the cranial adjacent levels were all higher than those in the intact spine.
  
  
  CONCLUSIONS
  The results of the present study show that changing the diameter of the spacers will alter the stiffness of the Dynesys construct. Dynesys constructs with larger diameters behave stiffer under flexion but behave softer under extension, torsion, and lateral bending. Changing the diameter of the Dynesys spacers does not significantly influence the load distribution at adjacent levels.
authors: Orthopaedic Biomechanics Laboratory, Insitute of Biomedical Engineering, National Yang-Ming University, Taipei, Taiwan.
authors: Shih-Liang
authors: SL
authors: Shih
authors: Chen-Sheng
authors: CS
authors: Chen
authors: Hung-Ming
authors: HM
authors: Lin
authors: Li-Ying
authors: LY
authors: Huang
authors: Chien-Lin
authors: CL
authors: Liu
authors: Chang-Hung
authors: CH
authors: Huang
authors: Cheng-Kung
authors: CK
authors: Cheng
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology; D017586:Diskectomy / Q000295:instrumentation / Q000379:methods; D020342:Finite Element Analysis / Q000592:standards*; D006801:Humans; D016268:Internal Fixators / Q000592:standards*; D007403:Intervertebral Disc / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery; D007405:Intervertebral Disc Displacement / Q000503:physiopathology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000033:anatomy & histology / Q000502:physiology / Q000601:surgery*; D008422:Materials Testing / Q000379:methods*; D008953:Models, Anatomic; D009068:Movement / Q000502:physiology; D011183:Postoperative Complications / Q000517:prevention & control; D019736:Prostheses and Implants / Q000592:standards*; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods; D013314:Stress, Mechanical; D016474:Weight-Bearing / Q000502:physiology