Occipitocervical vertical distraction injuries: anatomical biomechanical, and 3-tesla magnetic resonance imaging investigation.

Study Design: 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.
Summary of Background Data: 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.
Conclusion: 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.