title: Effect of Device Rigidity and Physiological Loading on Spinal Kinematics after Dynamic Stabilization : An In-Vitro Biomechanical Study.
abstract: OBJECTIVE
  To investigate the effects of posterior implant rigidity on spinal kinematics at adjacent levels by utilizing a cadaveric spine model with simulated physiological loading.
  
  
  METHODS
  Five human lumbar spinal specimens (L3 to S1) were obtained and checked for abnormalities. The fresh specimens were stripped of muscle tissue, with care taken to preserve the spinal ligaments and facet joints. Pedicle screws were implanted in the L4 and L5 vertebrae of each specimen. Specimens were tested under 0 N and 400 N axial loading. Five different posterior rods of various elastic moduli (intact, rubber, low-density polyethylene, aluminum, and titanium) were tested. Segmental range of motion (ROM), center of rotation (COR) and intervertebral disc pressure were investigated.
  
  
  RESULTS
  As the rigidity of the posterior rods increased, both the segmental ROM and disc pressure at L4-5 decreased, while those values increased at adjacent levels. Implant stiffness saturation was evident, as the ROM and disc pressure were only marginally increased beyond an implant stiffness of aluminum. Since the disc pressures of adjacent levels were increased by the axial loading, it was shown that the rigidity of the implants influenced the load sharing between the implant and the spinal column. The segmental CORs at the adjacent disc levels translated anteriorly and inferiorly as rigidity of the device increased.
  
  
  CONCLUSIONS
  These biomechanical findings indicate that the rigidity of the dynamic stabilization implant and physiological loading play significant roles on spinal kinematics at adjacent disc levels, and will aid in further device development.
authors.affiliation: Department of Neurosurgery, Baylor College of Medicine, Houston, TX, USA.
authors.forename: Kwonsoo
authors.initials: K
authors.lastname: Chun
authors.affiliation: Department of Medical Biotechnology, Dongguk University, Seoul, Korea.
authors.forename: Inchul
authors.initials: I
authors.lastname: Yang
authors.affiliation: Department of Neurosurgery, Baylor College of Medicine, Houston, TX, USA.
authors.forename: Namhoon
authors.initials: N
authors.lastname: Kim
authors.affiliation: Department of Neurosurgery, Ewha Womans University College of Medicine, Seoul, Korea.
authors.forename: Dosang
authors.initials: D
authors.lastname: Cho
keywords: Axial loading; Cadaveric study; Dynamic stabilization; Implant rigidity; Robotic testing; Spinal kinematics