title: Cervical spine instability following axial compression injury: a biomechanical study.
abstract: BACKGROUND
  Axial compression injuries of the cervical spine occur during contact sports, automobile collisions, and falls. The objective of this study was to use flexibility tests to determine biomechanical instability of the cervical spine due to simulated axial compression injuries.
  
  
  OBJECTIVE
  We hypothesized that the axial compression injuries cause severe biomechanical instability throughout the cervical spine.
  
  
  METHODS
  The injuries were simulated using 2.4m/s head-first impacts of a cadaveric cervical spine model (n=10) mounted horizontally to a torso-equivalent mass on a sled and carrying a surrogate head in protruded posture. Intact and post-impact flexibility tests were performed up to 1.5, 3, and 1.5 Nm in flexion-extension, axial torque, and lateral bending, respectively. Instability parameters of range of motion (RoM) and neutral zone (NZ) were determined for injured spinal levels and statistically compared (P<0.05) between intact and post-impact.
  
  
  RESULTS
  The sagittal instability parameters indicated extension-compression injuries at the upper and middle cervical spine and flexion-compression injuries at the lower cervical spine. Increases in extension RoM were 14.9° at the upper cervical spine and 24.9° (P<0.05) at the middle cervical spine and in flexion RoM at C7/T1 were 25.6°. RoM and NZ increases in axial rotation and lateral bending were nearly symmetric among left and right.
  
  
  CONCLUSIONS
  Multidirectional instability of the upper cervical spine caused by atlas and dens fractures was evidenced by increases between 36% and 53% in RoM and NZ due to the impacts. The sagittal RoM of injured spinal levels of the middle and lower cervical spine exceeded a proposed threshold for clinical instability by between 67% and 114%. The instability documented throughout the cervical spine was consistent with clinical observations of cord injuries and paralysis in patients.
  
  
  METHODS
  Level IV, controlled laboratory investigation.
authors.affiliation: Biomechanics Research Laboratory, Department of Orthopaedics and Rehabilitation, Yale University School of Medicine, 333, Cedar Street, P.O. Box 208071, New Haven, CT 06520-8071, USA. Electronic address: paul.ivancic@yale.edu.
authors.forename: P C
authors.initials: PC
authors.lastname: Ivancic
mesh_terms: D000368:Aged; D000369:Aged, 80 and over; D001368:Axis, Cervical Vertebra / Q000293:injuries*; D001696:Biomechanical Phenomena; D005260:Female; D050815:Fractures, Compression / Q000150:complications*; D006801:Humans; D007593:Joint Instability / Q000209:etiology*; D008297:Male
keywords: Axial compression; Biomechanics; Cervical spine injury; Head-first impact; Instability