title: Could junctional problems at the end of a long construct be addressed by providing a graduated reduction in stiffness? A biomechanical investigation.
abstract: METHODS
  The effect of long, rigid fixation on adjacent level hypermobility was investigated in a human cadaver model with and without a transitional posterior dynamic stabilization (PDS) device placed at the last caudal level.
  
  
  OBJECTIVE
  To evaluate if PDS devices are useful in the setting of spinal deformities to restore increased adjacent level motions, which occur in long constructs. The hypothesis is that load-sharing benefits of these devices will be most suitable in long constructs and may reduce thoracolumbar junctional effects. The PDS device evaluated has a compressive spacer and flexion-dampening bumper.
  
  
  BACKGROUND
  Mechanical factors such as excessive mobility, increased disc height due to instrumentation, and abnormal loading are thought to accentuate distal level problems, which occur in extended instrumentation. Specifically adjacent level degeneration and distal junctional kyphosis are known to occur in these cases.
  
  
  METHODS
  Seven cadaver spines were tested from T7 to L3. Long instrumentation was applied in 2 rigid groups, R1: Rigid (T8-L2) and R2: Rigid (T8-L1), and PDS to the last caudal level of each, RP1: Rigid (T8-L1) + PDS (L1-L2), and RP2: Rigid (T8-T12) + PDS (T12-L1). Range of motion was evaluated at surgical and distal adjacent levels after displacement controlled loading in a spine tester.
  
  
  RESULTS
  Distal adjacent level motion was increased after 5- and 6-level rigid fixation in flexion-extension, lateral bending, and axial rotation. Most of the increases were seen in axial rotation and lateral bending. Replacing the last caudal instrumented level with the PDS test device was able to alleviate hypermobile conditions of the adjacent noninstrumented level, closer to intact (24%, 12% reduction in RP2, RP1, respectively).
  
  
  CONCLUSIONS
  Reduction of hypermobility caused by extended arthrodesis may represent a new and ideally suited function for PDS devices. Mechanically, the devices were seen to kinematically restore abnormal distal motion, especially with placement of the PDS at the thoracolumbar junction.
authors.affiliation: Center for Advanced Spine Technologies, Cincinnati, OH, USA.
authors.forename: Atiq
authors.initials: A
authors.lastname: Durrani
authors.forename: Viral
authors.initials: V
authors.lastname: Jain
authors.forename: Rasesh
authors.initials: R
authors.lastname: Desai
authors.forename: Brandon
authors.initials: B
authors.lastname: Bucklen
authors.forename: Aditya
authors.initials: A
authors.lastname: Ingalhalikar
authors.forename: Aditya
authors.initials: A
authors.lastname: Muzumdar
authors.forename: Mark
authors.initials: M
authors.lastname: Moldavsky
authors.forename: Saif
authors.initials: S
authors.lastname: Khalil
mesh_terms: D001696:Biomechanical Phenomena; D002102:Cadaver; D006801:Humans; D020007:Implants, Experimental; D007403:Intervertebral Disc / Q000502:physiology; D007593:Joint Instability; D008159:Lumbar Vertebrae / Q000502:physiology*; D008953:Models, Anatomic; D019736:Prostheses and Implants; D016059:Range of Motion, Articular / Q000502:physiology; D012600:Scoliosis; D013123:Spinal Fusion / Q000295:instrumentation* / Q000379:methods; D013314:Stress, Mechanical; D013904:Thoracic Vertebrae / Q000502:physiology*; D016474:Weight-Bearing; D021801:Zygapophyseal Joint / Q000502:physiology*