Study Design: This is a cross-sectional analysis of resonance magnetic images of 111 patients with
craniocervical malformations and those of normal subjects.
Objective: To test the hypothesis that atlas assimilation is associated with basilar invagination
(BI) and atlas’s anterior arch assimilation is associated with craniocervical instability
and type I BI.
Summary of Background Data: Atlas assimilation is the most common malformation in the craniocervical junction.
This condition has been associated with craniocervical instability and BI in isolated
cases.
Methods: We evaluated midline Magnetic Resonance Images (MRIs) (and/or CT scans) from patients
with craniocervical junction malformation and normal subjects. The patients were separated
into 3 groups: Chiari type I malformation, BI type I, and type II. The atlas assimilations
were classified according to their embryological origins as follows: posterior, anterior,
and both arches assimilation.
Results: We studied the craniometric values of 111 subjects, 78 with craniocervical junction
malformation and 33 without malformations. Of the 78 malformations, 51 patients had
Chiari type I and 27 had BI, of whom 10 presented with type I and 17 with type II
BI. In the Chiari group, 41 showed no assimilation of the atlas. In the type I BI
group, all patients presented with anterior arch assimilation, either in isolation
or associated with assimilation of the posterior arch. 63% of the patients with type
II BI presented with posterior arch assimilation, either in isolation or associated
with anterior arch assimilation. In the control group, no patients had atlas assimilation.
