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- Discussion:
    - flexion-extension laterals;
          - prior to intubation for elective surgery, all patients at risk should have flexion extension laterals of cervical  spine r/o
                atlantoaxial instability;

- Radiographs:
    - atlantodens interval (ADI) needs to be evaluated;
          - instability is present when a 3.5 mm ADI difference on flex/ext views;
          - 7 mm difference may imply disruption of the alar ligaments;
          - difference of > 9 mm is associated with an increase in neurologic injury and will usually require posterior fusion and wiring;
    - space available for cord (SAC):
          - posterior space of < 13 mm is contraindication to elective surgery until C-spine is stabilized first;

- Radiographic Evaluation:
    - Cross Table Lateral:
    - McGregor's line:
         - hard palate-posteror occipit curve;
         - dx is based on tip of dens being > 8 mm above this line in men and > 10 mm above the line in women;
    - Ranawat's line:
         - center of C2 pedicle to the C1 arch;
         - nl is > 17 mm and < 13 mm is consistent with impaction;
         
    - Chamberlain's line:
         - anterior foramen to the top of the C1 arch;
         - if the dens is > 6 mm above this line, consistent w/ impaction