Internal Craniocervical Ligaments

The broad, strong tectorial membrane lies within the vertebral canal. It prolongs the posterior longitudinal ligament upward from the posterior surface of the body of the axis to the anterior and anterolateral margins of the foramen magnum, where it blends with the dura mater. It covers the dens and its ligaments and gives added protection to the junctional area between the medulla oblongata and spinal cord.

The median atlantoaxial pivot joint lies between the dens of the axis and the ring formed by the anterior arch and transverse ligament of the atlas (see Plate 3-2). Two small synovial cavities surrounded by thin articular capsules are present between the dens and the anterior arch in front, and the transverse ligament of the atlas behind.

The transverse ligament of the atlas is a strong band passing horizontally behind the dens and attached on each side to a tubercle on the medial side of the lateral mass of the atlas. From its midpoint, bands pass vertically upward and downward to become fixed, respectively, to the basilar part of the occipital bone between the tectorial membrane and the apical ligament of the dens and to the posterior surface of the body of the axis: the superior and inferior longitudinal fascicles. These transverse and vertical bands together form the cruciform ligament.

The apical ligament is a slender cord connecting the apex of the dens to the anterior midpoint of the foramen magnum, lying between the anterior atlanto-occipital membrane and the upper limb of the cruciform ligament.

The alar ligaments are two fibrous bands stretching upward and outward from the superolateral aspects of the dens to the medial sides of the occipital condyles. They check excessive rotation at the median atlanto-occipital joint.

Lateral atlantoaxial joints are formed between the almost-flat inferior articular facets on the lateral masses of the atlas and the superior articular facets of the axis. They are synovial joints with thin, loose articular capsules. An accessory ligament extends from near the base of the dens to the lateral mass of the atlas, close to the attachment of the transverse ligament. It assists the alar ligaments in restricting atlantoaxial rotation.

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