title: Hybrid dynamic stabilization: a biomechanical assessment of adjacent and supraadjacent levels of the lumbar spine.
abstract: OBJECTIVE
  The object of this study was to evaluate the effect of hybrid dynamic stabilization on adjacent levels of the lumbar spine.
  
  
  METHODS
  Seven human spine specimens from T-12 to the sacrum were used. The following conditions were implemented: 1) intact spine; 2) fusion of L4-5 with bilateral pedicle screws and titanium rods; and 3) supplementation of the L4-5 fusion with pedicle screw dynamic stabilization constructs at L3-4, with the purpose of protecting the L3-4 level from excessive range of motion (ROM) and to create a smoother motion transition to the rest of the lumbar spine. An industrial robot was used to apply continuous pure moment (± 2 Nm) in flexion-extension with and without a follower load, lateral bending, and axial rotation. Intersegmental rotations of the fused, dynamically stabilized, and adjacent levels were measured and compared.
  
  
  RESULTS
  In flexion-extension only, the rigid instrumentation at L4-5 caused a 78% decrease in the segment's ROM when compared with the intact specimen. To compensate, it caused an increase in motion at adjacent levels L1-2 (45.6%) and L2-3 (23.2%) only. The placement of the dynamic construct at L3-4 decreased the operated level's ROM by 80.4% (similar stability as the fusion at L4-5), when compared with the intact specimen, and caused a significant increase in motion at all tested adjacent levels. In flexion-extension with a follower load, instrumentation at L4-5 affected only a subadjacent level, L5-sacrum (52.0%), while causing a reduction in motion at the operated level (L4-5, -76.4%). The dynamic construct caused a significant increase in motion at the adjacent levels T12-L1 (44.9%), L1-2 (57.3%), and L5-sacrum (83.9%), while motion at the operated level (L3-4) was reduced by 76.7%. In lateral bending, instrumentation at L4-5 increased motion at only T12-L1 (22.8%). The dynamic construct at L3-4 caused an increase in motion at T12-L1 (69.9%), L1-2 (59.4%), L2-3 (44.7%), and L5-sacrum (43.7%). In axial rotation, only the placement of the dynamic construct at L3-4 caused a significant increase in motion of the adjacent levels L2-3 (25.1%) and L5-sacrum (31.4%).
  
  
  CONCLUSIONS
  The dynamic stabilization system displayed stability characteristics similar to a solid, all-metal construct. Its addition of the supraadjacent level (L3-4) to the fusion (L4-5) did protect the adjacent level from excessive motion. However, it essentially transformed a 1-level lumbar fusion into a 2-level lumbar fusion, with exponential transfer of motion to the fewer remaining discs.
authors: Spine Research Lab, Lutheran Hospital, Cleveland Clinic, Cleveland, Ohio 44195, USA.
authors: Prasath
authors: P
authors: Mageswaran
authors: Fernando
authors: F
authors: Techy
authors: Robb W
authors: RW
authors: Colbrunn
authors: Tara F
authors: TF
authors: Bonner
authors: Robert F
authors: RF
authors: McLain
mesh_terms: D000328:Adult; D000368:Aged; D001696:Biomechanical Phenomena; D001858:Bone Nails; D001863:Bone Screws; D005260:Female; D006801:Humans; D055959:Intervertebral Disc Degeneration / Q000503:physiopathology* / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000503:physiopathology* / Q000601:surgery*; D008297:Male; D008875:Middle Aged; D011183:Postoperative Complications / Q000503:physiopathology*; D016059:Range of Motion, Articular / Q000502:physiology; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*; D016474:Weight-Bearing / Q000502:physiology