HYPOPLASTIC POSTERIOR ARCH OF ATLAS WITH FLOATING POSTERIOR TUBERCLE

AP and Lateral Radiographs of cervical spine showing absent posterior arch of atlas behind the lateral masses with free floating posterior tubercle. The lateral masses are well seen in AP radiograph.



Embryology of atlas

·        It has three primary ossification centers in the embryological period - an anterior center for formation of the anterior tubercle and two lateral centers from which the lateral masses and posterior arch form.

·        In 7th week of gestation, the lateral centers extend dorsally to form the posterior arch.

·        An additional center may be present posteriorly in the midline, forming the posterior tubercle of the C1 in second year of life in about 2% of the population.

·        During maturation, the anterior arch of the C1 derives from usually one midline, but occasionally two or more ossification centers are seen radiologically in the first year of life.

·        Ossification of the posterior arch usually proceeds perichondrally from two centers in the lateral masses, towards the midline ,and fusion occurs between the 3rd and 5th years of life.

·        Incomplete fusion of the two hemiarches may be normal in the first 5-10 years. However, incomplete fusion may persist in 3 to 5% of patients.

·        The anterior center usually fuses with the two lateral centers between 5 and 9 years age.

·        In rare cases, a separate ossification center forms in the midline and fuses secondarily with the lateral masses forming the posterior arch.

·        A posterior midline ossified tubercle develops if this center fails to fuse with the hemiarches.

·        At least two different anomalies can develop during the ossification:

1)   median clefts of the posterior arch of C1

2)   varying degrees of posterior arch dysplasia

Currarino’s Morphologic Classification for C1 Hypoplasia

          A   Failure of posterior midline fusion of the two hemiarches

          B   Unilateral defect

          C   Bilateral defects

          D   Absence of posterior arch, with persistent posterior tubercle

          E   Absence of the entire arch, including the tubercle

Reference : Congenital Hypoplasia of the Posterior Arch of the Atlas, Turkish Neurosurgery 2011, Vol: 21, No: 1, 97-103

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