title: The biomechanics of a multilevel lumbar spine hybrid using nucleus replacement in conjunction with fusion.
abstract: BACKGROUND
  Degenerative disc disease is commonly a multilevel pathology with varying deterioration severity. The use of fusion on multiple levels can significantly affect functionality and has been linked to persistent adjacent disc degeneration. A hybrid approach of fusion and nucleus replacement (NR) has been suggested as a solution for mildly degenerated yet painful levels adjacent to fusion.
  
  
  OBJECTIVE
  To compare the biomechanical metrics of different hybrid implant constructs, hypothesizing that an NR+fusion hybrid would be similar to a single-level fusion and perform more naturally compared with a two-level fusion.
  
  
  METHODS
  A cadaveric in vitro repeated-measures study was performed to evaluate a multilevel lumbar NR+fusion hybrid.
  
  
  METHODS
  Eight cadaveric spines (L3-S1) were tested in a Spine Kinetic Simulator (Instron, Norwood, MA, USA). Pure moments of 8 Nm were applied in flexion/extension, lateral bending, and axial rotation as well as compression loading. Specimens were tested intact; fused (using transforaminal lumbar interbody fusion instrumentation with posterior rods) at L5-S1; with a nuclectomy at L4-L5 including fusion at L5-S1; with NR at L4-L5 including fusion at L5-S1; and finally with a two-level fusion spanning L4-S1. Repeated-measures analysis of variance and corrected t tests were used to statistically compare outcomes.
  
  
  RESULTS
  The NR+fusion hybrid and single-level fusion exhibited no statistical differences for range of motion (ROM), stiffness, neutral zone, and intradiscal pressure in all loading directions. Compared with two-level fusion, the hybrid affords the construct 41.9% more ROM on average. Two-level fusion stiffness was statistically higher than all other constructs and resulted in significantly lower ROM in flexion, extension, and lateral bending. The hybrid construct produced approximately half of the L3-L4 adjacent-level pressures as the two-level fusion case while generating similar pressures to the single-level fusion case.
  
  
  CONCLUSIONS
  These data portend more natural functional outcomes and fewer adjacent disc complications for a multilevel NR+fusion hybrid compared with the classical two-level fusion.
authors: Medical Devices Center, University of Minnesota, 111 Church St SE, Minneapolis, MN 55455, USA.
authors: Michael C
authors: MC
authors: Dahl
authors: Arin M
authors: AM
authors: Ellingson
authors: Hitesh P
authors: HP
authors: Mehta
authors: Justin H
authors: JH
authors: Huelman
authors: David J
authors: DJ
authors: Nuckley
mesh_terms: D001696:Biomechanical Phenomena / Q000502:physiology; D001863:Bone Screws; D006801:Humans; D055959:Intervertebral Disc Degeneration / Q000503:physiopathology / Q000601:surgery*; D008159:Lumbar Vertebrae / Q000502:physiology / Q000601:surgery*; D016059:Range of Motion, Articular / Q000502:physiology*; D013123:Spinal Fusion / Q000295:instrumentation / Q000379:methods*