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