title: On the understanding of clinical instability.
abstract: METHODS
  Three-dimensional flexibility changes due to the application of an external fixator at C4-C5 were studied in cervical spine specimens.
  
  
  OBJECTIVE
  To evaluate the biomechanical effects of applying a cervical external fixator to a patient using an in vitro model.
  
  
  BACKGROUND
  There is controversy regarding the relationship between the changes in spinal motion and clinical instability.
  
  
  METHODS
  Using fresh cadaveric C4-C7 specimens, multidirectional flexibility was measured at all vertebral levels, before and after the fixator application at C4-C5, C5-C6, and C4-C6.
  
  
  RESULTS
  The average ranges of motion for flexion, extension, lateral bending, and axial rotation were 8.3 degrees, 7.2 degrees, 5.3 degrees, and 5.6 degrees, which decreased by 40%, 27%, 32%, and 58%, respectively, because of the fixator application. The corresponding neutral zones were 3.4 degrees, 3.4 degrees, 3.0 degrees, and 2.0 degrees, which decreased by 76%, 76%, 54% and 69%, respectively. The decreases with the fixation at C4-C5 were similar to those for fixation at C5-C6.
  
  
  CONCLUSIONS
  This in vitro study documented that the application of an external fixator to the cervical spine decreases the intervertebral motion in general, and decreases flexion, extension and torsional neutral zones in particular. The findings help explain the clinical instability of the spine and support the hypothesis that the neutral zone is more closely associated with the clinical instability than is the range of motion.
authors.affiliation: Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, New Haven, Connecticut.
authors.forename: M M
authors.initials: MM
authors.lastname: Panjabi
authors.forename: C
authors.initials: C
authors.lastname: Lydon
authors.forename: A
authors.initials: A
authors.lastname: Vasavada
authors.forename: D
authors.initials: D
authors.lastname: Grob
authors.forename: J J
authors.initials: JJ
authors.lastname: Crisco
authors.forename: J
authors.initials: J
authors.lastname: Dvorak
mesh_terms: D000328:Adult; D001696:Biomechanical Phenomena; D002574:Cervical Vertebrae / Q000601:surgery*; D016267:External Fixators; D005260:Female; D006801:Humans; D007593:Joint Instability / Q000601:surgery / Q000628:therapy; D008297:Male; D008875:Middle Aged; D010146:Pain / Q000175:diagnosis; D059408:Pain Management