2.1 Anatomy
The craniocervical junction anomalies are classified as congenital,
developmental,
and acquired.
Many congenital anomalies have been described in this region.
In this review,
we present the anomalies as those related to the occipital,
atlas and axis bones.
Fig. 1
The craniocervical junction(CCJ) is composed of three bone structures: atlas(C1),
axis(C2) and the occiput.
These structures are approximated at two major joints,
the atlantooccipital and atlantoaxial joints.
The atlantoaxial joints enable head rotation around the dens axis,
whereas the atlantooccipital joints primarily allow flexion,
extension,
and lateral bending.
The CCJ flexible and complex anatomy also makes it the weakest region of the entire vertebral column.
The recognition of the anatomy and the anomalies of this region will facilitate the understanding of the clinic of patients with suboccipital and neck pain.
2.2 Normal lines and angles Fig. 2
Chamberlain line was first described in plain radiograph,
extends from the posterior margin of the hard palate to the posterior margin of the foramen magnum (Opisthion) [1].
McGregor argued that opisthion could not be clearly distinguished on plain radiographs [2].
McGregor took the posterior lowest point of the occipital bone for his measurements instead of the opisthion.
We prefer to use the chamberlain line for evaluating basal invagination daily routine imaging.
Fig. 3
Wackenheim clivus baseline is constructed by drawing a line along the clivus and extending it inferiorly into the upper cervical spinal canal.
The basal angle was described on plain radiographs by McGregor argued that the basal angle is not significant in terms of basilar invagination.
He suggested that a part of the skull base shows its position relative to the other [2]. The basal angle formed by the intersection of nasion–midsella and midsella–basion tangents.
Basal angle measurements above 150° support platybasia [3].
Recent studies also described a modified angle measurement method for T1 MR images suggesting a line drawn parallel to the anterior cranial fossa and a line from the dorsum sella following the clivus posterior corner [4].
The Welcher basal angle is formed at the intersection of the nasion-tuberculum sella line and the tuberculum sella-basion line.
We prefer to make measurements with this method in our daily routine.
Measuring 140 ° above the welcher angle is significant in terms of platybasia [5].
The clivus-canal angle is the angle formed by Wackenheim’s line and a line constructed along the posterior margin of the dens and axis body.
In basiocciput hypoplasia and condyle hypoplasia cases the angle narrows.
Severe pressure symptoms may occur in patients whose angle has narrowed considerably.
The atlanto-occipital joint axis angle is formed at the intersection of tangents drawn parallel to the atlanto-occipital joints.
It is normal to be between 124-127 °.
In the condylar hypoplasia,
the joint space is flattened and the angle increases [6]. Fig. 4