Lateral radiographs are taken of the cervical spine to show the relationships between atlas (C1) and axis (C2).
3 anteroposterior lines are drawn relative to the anterior and posterior arches of the atlas:
(1) upper: line along the superior surfaces of the anterior and posterior arches
(2) middle: line through the vertical midpoints of the anterior and posterior arches
(3) lower: line along the inferior surfaces of the anterior and posterior arches
On the axis the T points (most cranial bony parts or tips of the superior facets) are identified.
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Location of T Points Relative to the Atlantal AP Lines
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Grade of Dislocation
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below lower line
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I (normal)
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transected by lower line
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II
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transected by middle line
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III
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transected by upper line
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IV
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where:
• Grade I also includes slight thinning of the cartilage layers in the facet joints.
This method was found to be simple and easy to use. The methods of McRae and of McGregor may be difficult to perform if there is poor visualization of the apex of the dens. Redlund's method may be affected by variation in the height of the body of the axis.
If the method indicates vertical atlantoaxial dislocation, then it should be followed up by a method such as Redlund's that gives a more exact measurement.
