title: Pretension effects of the Dynesys cord on the tissue responses and screw-spacer behaviors of the lumbosacral construct with hybrid fixation.
abstract: METHODS
  The pretension of the Dynesys cord was varied to evaluate its effects on both tissue responses and screw-spacer behaviors by the finite-element method.
  
  
  OBJECTIVE
  This study aimed to provide detailed information about the motion-preserving and load-shielding mechanisms of the Dynesys screw-spacer joint.
  
  
  BACKGROUND
  Intuitively, higher cord pretension aims to ensure the occurrence of screw-spacer contact, thus making the spacer the transmitter of the vertebral loads. However, detailed investigations of the cord-pretension effects have not yet been carried out. METHODS.: Using a validated lumbosacral model, the moderately degenerative L4-L5 segment was instrumented by a static fixator and the Dynesys fixator was further used to bridge a mildly degenerative L3-L4 segment. The pre-tended cord was modeled as an elastic spring with 0- and 300-N pretensions. The disc range-of-motion, disc stress, facet force, bone-screw stress, and screw-spacer force were chosen as comparison indices. RESULTS.: At the transition and adjacent segments, the range-of-motion differences between the 2 pretensions were 7.7% and 2.0% on average, respectively. The mechanical differences at the transition and adjacent segments were 9.0% and 5.2% (disc stress) and 9.4% and 9.1% (facet force), respectively. The results indicated that the cord pretension has a minor effect on the adjacent segments in comparison with the transition segment. However, the stress at the screw hub and force of the screw-spacer contact of the 300-N pretension were increased by 33.7% and 316.5% on average than without pretension, respectively.
  
  
  CONCLUSIONS
  The moment arm from the screw-cord center to the fulcrum is significantly less than that of vertebral loads. This leads to the minor effect of increasing the cord pretension on the responses of the adjacent segments. However, the cord pretension can significantly affect both screw-spacer force and bone-screw stress.
  
  
  METHODS
  4.
authors.affiliation: Graduate Institute of Mechanical and Electrical Engineering, National Taipei University of Technology, Taipei, Taiwan.
authors.forename: Chen-Ying
authors.initials: CY
authors.lastname: Chien
authors.forename: Ching-Hong
authors.initials: CH
authors.lastname: Tan
authors.forename: Tsai-Hsiung
authors.initials: TH
authors.lastname: Lu
authors.forename: Shang-Chih
authors.initials: SC
authors.lastname: Lin
authors.forename: Wen-Hsien
authors.initials: WH
authors.lastname: Chuang
authors.forename: Ming-Chieh
authors.initials: MC
authors.lastname: Chiang
authors.forename: Yuan-Ping
authors.initials: YP
authors.lastname: Luh
authors.forename: Yeung-Jen
authors.initials: YJ
authors.lastname: Chen
mesh_terms: D001696:Biomechanical Phenomena; D001863:Bone Screws; D004548:Elasticity; D006801:Humans; D016268:Internal Fixators; D007403:Intervertebral Disc / Q000503:physiopathology; D008159:Lumbar Vertebrae / Q000473:pathology / Q000503:physiopathology / Q000601:surgery*; D008161:Lumbosacral Region; D008953:Models, Anatomic; D019637:Orthopedic Procedures / Q000295:instrumentation* / Q000379:methods; D019736:Prostheses and Implants; D016059:Range of Motion, Articular; D013314:Stress, Mechanical; D021801:Zygapophyseal Joint / Q000503:physiopathology