title: [Anatomical considerations for the placement of C₂ spinous process screws].
abstract: OBJECTIVE
  To quantitatively anatomically evaluate the C₂ spinous process, analyze the anatomical feasibility of the C₂ spinous process screws and its clinical significance.
  
  
  METHODS
  To dissect and evaluate 30 cervical cadaveric spines of C₂ which were taken to expose the lamina and spinous process. Anatomic quantitative evaluation of the C₂ spinous process included its height and width. Twenty cervical cadaveric spines of C₂ were chosen to the study of the placement of the C₂ spinous process screws. The starting point for the C₂ spinous process screw insertion was located at the base of the spinous process. After the entrance point of spinous process screws was determined, posterior C₂ spinous process screw implantation was performed bilaterally under direct visualization. On the morphologic CT scan, the width of C₂ spinous process base, the angle and length of the spinous process screw trajectory, and the distance between the tip of the screw and the spinal cord and the vertebral artery were measured.
  
  
  RESULTS
  The average height and width of the C₂ spinous process were (12.90 ± 1.30) mm and (18.86 ± 1.17) mm respectively. The C₂ spinous process screws were successfully placed without impingement the spinal cord or the vertebral artery and the breakage of the spinous process. On the CT scan, the average width of the base of C₂ spinous process was (20.7 ± 1.3) mm. The placed angles of the screws were 1.8° ± 1.0° in the axial plane. The distance between the tip of the screw and the spinal cord or the vertebral artery was (8.3 ± 2.6) mm and (20.2 ± 3.1) mm respectively. There were little differences between superior and inferior screws in the angle, the distance between the tip of the screw and the spinal cord or the vertebral artery, but without significance (P > 0.05). The average trajectory length of the C₂ spinous process screws was (19.7 ± 1.1) mm. The average trajectory length of the superior spinous process screws was shorter than that of inferior spinous process screws, with great differences (t = 3.566, P < 0.01).
  
  
  CONCLUSIONS
  There is the anatomic feasibility of the C₂ spinous process screw fixation which may afford an alternative to standard screw placement for axis fixation. The biomechanical study for the C₂ spinous process screw is also necessary.
authors.affiliation: Department of Orthopaedics, Ningbo Sixth People's Hospital, Ningbo 315040, China.
authors.forename: Guan-yi
authors.initials: GY
authors.lastname: Liu
authors.forename: Rong-ming
authors.initials: RM
authors.lastname: Xu
authors.forename: Wei-hu
authors.initials: WH
authors.lastname: Ma
authors.forename: Shao-hua
authors.initials: SH
authors.lastname: Sun
authors.forename: Jian-xiang
authors.initials: JX
authors.lastname: Feng
authors.forename: Yong
authors.initials: Y
authors.lastname: Hu
authors.forename: Liu-jun
authors.initials: LJ
authors.lastname: Zhao
authors.forename: Lei-jie
authors.initials: LJ
authors.lastname: Zhou
mesh_terms: D001368:Axis, Cervical Vertebra / Q000033:anatomy & histology* / Q000601:surgery; D001863:Bone Screws; D005593:Fracture Fixation, Internal / Q000379:methods*; D006801:Humans