title: Occipitocervical vertical distraction injuries: anatomical biomechanical, and 3-tesla magnetic resonance imaging investigation.
abstract: METHODS
  Biomechanical load-to-failure findings correlated with anatomic dissection measurements and intact (prefailure) 3-Tesla (3-T) magnetic resonance images (MRI).
  
  
  OBJECTIVE
  To better understand why the same distractive force to the head can result in occipitoatlantal dislocation (OAD) in some individuals and atlantoaxial dislocation (AAD) in others.
  
  
  BACKGROUND
  Distraction injuries to the cranio-vertebral junction have been studied biomechanically but have not been studied relative to ligamentous anatomic variations. We theorized that morphologic variations in the ligaments should influence the injury pattern during axial distraction.
  
  
  METHODS
  After obtaining 3-T MRI scans, 10 occiput-C2 specimens were loaded to failure under axial tension. Direct anatomic measurements were also obtained from the distracted and injured specimens.
  
  
  RESULTS
  AAD was observed in 7 specimens (mean force +/- standard deviation 1229 +/- 181 N) at a significantly higher magnitude than OAD, which was observed in 3 specimens (823 +/- 127 N; P = 0.009, nonpaired t test). In OAD specimens, the superior cruciate ligament (SCL), which was smaller than the inferior cruciate ligament (ICL), failed. The apical ligament was unidentifiable in these 3 specimens. In 5 of the 7 AAD specimens, the ICL ruptured and was smaller than the SCL. In the remaining 2 specimens, both SCL and ICL ruptured. The apical ligament, which ruptured, was identifiable in all 7 specimens.
  
  
  CONCLUSIONS
  Axial distraction across the cranio-vertebral junction can produce either OAD or AAD. The SCL and ICL dimensions, alar ligament orientations, and apical ligament presence may affect the injury site. Visualization with 3-T MRI allows better understanding of the injury mechanism and location, which is important clinically in selecting single- or multilevel fixation.
authors.affiliation: Spinal Biomechanics Research Laboratory, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, AZ 85013, USA.
authors.forename: K Zafer
authors.initials: KZ
authors.lastname: Yüksel
authors.forename: Murvet
authors.initials: M
authors.lastname: Yüksel
authors.forename: L Fernando
authors.initials: LF
authors.lastname: Gonzalez
authors.forename: Seungwon
authors.initials: S
authors.lastname: Baek
authors.forename: Joseph E
authors.initials: JE
authors.lastname: Heiserman
authors.forename: Volker K H
authors.initials: VK
authors.lastname: Sonntag
authors.forename: Neil R
authors.initials: NR
authors.lastname: Crawford