title: Intervertebral disc replacement maintains cervical spine kinetics.
abstract: METHODS
  An in vitro biomechanical study of C4-C5 intervertebral disc replacement using a cadaveric model.
  
  
  OBJECTIVE
  To investigate the degree of motion afforded by a ball-and-socket cervical intervertebral disc prosthesis design.
  
  
  BACKGROUND
  Intervertebral disc prostheses designs attempt to restore or maintain cervical disc motion after anterior cervical discectomy and reduce the likelihood of accelerated degeneration in adjacent discs by maintaining normal motion at the affected disc level. Surprisingly, the actual kinetic and biomechanical effects that cervical disc arthroplasty imparts on the spine have not been widely reported. Accordingly, we investigated what effect implanting a cervical disc prosthesis has on the range of motion at the affected level as well as how it changes the coupled motion patterns at the level of implantation.
  
  
  METHODS
  Six fresh-frozen human cadaveric cervical spines (C2-C7) were used in this study. We evaluated two different spinal conditions: intact and after disc replacement at C4-C5. Compression (using the follower load concept) and pure moment loading were applied to the specimen. Range of motion was measured using an optical tracking system. Statistical differences between the intact and replaced condition range of motion was determined using analysis of variance with post hoc comparisons (alpha = 0.05).
  
  
  RESULTS
  The data indicate that the intervertebral disc prosthesis approximated the intact motion in all three rotation planes at the affected level. Finally, changes in cervical coupled rotations, specifically lateral bending during axial rotation loading and axial rotation during lateral bending loading, were not statistically significant between the two tested conditions.
  
  
  CONCLUSIONS
  Our data demonstrate that a ball-and-socket design can replicate physiologic motion at the affected and adjacent levels. More importantly, the data indicate that motion coupling, which is most dramatic in the cervical spine and plays an important biomechanical role, is maintained.
authors.affiliation: Orthopaedic Biomechanics Laboratory, San Francisco General Hospital, and the Department of Orthopaedic Surgery, University of California, San Francisco, San Francisco, CA 94110, USA. cputtlitz@orthosurg.ucsf.edu
authors.forename: Christian M
authors.initials: CM
authors.lastname: Puttlitz
authors.forename: Marc Antoine
authors.initials: MA
authors.lastname: Rousseau
authors.forename: Zheng
authors.initials: Z
authors.lastname: Xu
authors.forename: Serena
authors.initials: S
authors.lastname: Hu
authors.forename: Bobby K-B
authors.initials: BK
authors.lastname: Tay
authors.forename: Jeffrey C
authors.initials: JC
authors.lastname: Lotz
mesh_terms: D019643:Arthroplasty, Replacement / Q000379:methods*; D001696:Biomechanical Phenomena; D002102:Cadaver; D002574:Cervical Vertebrae / Q000601:surgery*; D066298:In Vitro Techniques; D007403:Intervertebral Disc / Q000601:surgery*; D011474:Prosthesis Design; D016059:Range of Motion, Articular; D012399:Rotation