BASILAR INVAGINATION

Axial T2 image through the skull base revealed odontoid process projecting anterior to medulla oblongata. Sagittal T1 image showed superior ascent of dens into the foramen magnum and causing smooth indentation and displacement of craniovertebral junction. Also note occipitalization of atlas.



Radiological criteria for Basilar Invagination:

1.Ranawat's line:

·        center of C2 pedicle to a line connecting the anterior and posterior C1 arches

·        normal measurement in men is 17 mm, whereas in women it is 15 mm

·        distance of < 13 mm is consistent with impaction

·        less than 7 mm is associated with medullary compression on MR image

2.McRae's line:

·        defines the opening of the foramen magnum

·        the tip of the dens may protrude slightly above this line, but if the dens is below this line then impaction is not present

3.McGregor's line:

·        line drawn from the posterior edge of the hard palate to the caudal posterior occipit curve

·        cranial settling is present when the tip of dens is more than 4.5 mm above this line

·        this measurement can be difficult when there is dens errosion

·        problem w/ this measurement is that the hard palate position may vary w/ mid facial anomalies

4.Chamberlain's line:

·        line from dorsal margin of hard palate to the posterior edge of the foramen magnum

·        this line is often hard to visualize on standard radiographs

·        if dens is > 6 mm above this line, consistent w/ impaction

      Reference : Wheeless' Textbook of Orthopaedics