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Radiographs for Atalanto Axial Rotatory Fixation

- Lateral View:
    - key element is whether there is anterolithesis, since this will dramatically narrow the SAC w/ any degree of rotation;
    - ADI:
           - it may be difficult to measure the ADI due to lateral mass rotation;
           - in children upto 3.5 mm may be normal;
           - ADI: 3.5 to 5 mm, transverse ligament is insufficient; (this is a type II injury);
           - ADI: > 5 mm:
                  - indicates failure of the alar ligaments;
                  - consistent w/ type III rotatory subluxation;

- Open Mouth View:
     - dens is laterally displaced towards one side w/ asymmetry of lateral masses;
           - ie, the dens is closer to one lateral mass than the other;
     - one lateral mass is rotated anteriorly, appearing wider and rotated toward midline;
           - the opposite lateral mass, appears father from midline;
           - joint space of lateral mass rotated posteriorly appears to be overlapping the one below;
     - position of C2 spinous process:
           - in rotaroy fixation, the patient's chin and spinous process will be on the same side of the midline and the C2 spinous process tilts  
                 toward the side of facet displacement;

- Radiographic Differential Dx:
     - asymmetry of the lateral masses can be a normal finding if the patient's head is turned while the radiograph is being taken;
           - w/ rotatory fixation, however, attempts to gently rotate the head to the opposite side will not change the asymmetric relationship
                 of the dens to the lateral masses;
     - torticollis
           - note that a right sided torticollic tilt, will cause a left sided shift of the lateral mass and spinous process;
           - in contrast, rotatory fixation involves ipsilateral rotation and contralateral tilt;
                 - anatomically, if the patient's head is turned to the right, then the right lateral mass is rotated posteriorly;
           - finally, unlike rotatory fixation, true torticollis will show reversal of the rotation of C1 on C2, as the head is turn to the opposite side;

- Flouroscopy
    - cineroentogenograms are most useful to demonstrate the fixed relationship of the atlas to lateral masses as the head is turned to the 
           opposite side;

- CT scan:
    - obtain thin section CT thru upper C spine inorder to evaluate eccentric position of dens & rotation of the atlas w/ respect to the axis;
    - failure of C1 to reposition on a left and right rotation CT scan indicates a fixed deformity;
    - turn neck in contralateral direction as far as voluntarily possible; those with transient torticollis will show a reduction or reversal of the
          rotation of C1 on C2, while those with atlantoaxial fixation will not;
    - lateral masses of C1 are displaced
    - one anteriorly and the other posteriorly w/ respect to C2;
    - ref:
          - Dynamic computerized tomography of the occiput-atlas-axis complex in trauma patients with odontoid lateral mass asymmetry.
          - Atlanto-axial instability and spinal cord compression in children--diagnosis by computerized tomography.
          - Pitfalls in the CT diagnosis of atlantoaxial rotatory subluxation

Problems in the radiographic diagnosis of atlanto-axial rotation deformity. Klein DM, Kuhn JP. Conc Pediatr Neurosurg. 1985;5:26-33.