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Several subtypes of atlanto-occipital dislocation are known. One suggested categorization scheme includes anterior, vertical and posterior of the head relative to the spine. Other variants with lateral or rotatory displacement have been described, as well as mixed types.
The posterior atlantooccipital membrane (posterior atlantooccipital ligament) is a broad but thin membrane [1] [2]: 99 extending between the posterior margin of the foramen magnum above, and posterior arch of atlas (first cervical vertebra) below. It forms the floor of the suboccipital triangle.
The atlanto-occipital joint (Articulatio atlantooccipitalis) is an articulation between the atlas bone and the occipital bone. It consists of a pair of condyloid joints. It is a synovial joint .
Craniocervical instability (CCI) is a medical condition characterized by excessive movement of the vertebra at the atlanto-occipital joint and the atlanto-axial joint located between the skull and the top two vertebra, known as C1 and C2.
The clivus is an important landmark for checking for anatomical atlanto-occipital alignment. When viewed on a lateral C-spine radiograph, the clivus forms a line which, if extended, is known as Wackenheim's clivus line. Wackenheim's clivus line should pass through the dens of the axis or be tangential to it. [9]
A widening of the atlanto-axial joint, as measured between the posterior surface of the anterior arch of atlas and the front of the odontoid process, indicates an injury to the transverse atlantal ligament. [6] Normally, this atlanto-dental distance is less than 2 mm, sometimes a maximum of 3 mm is accepted in men and 2.5 mm in women. [6]
When the occipital bone and the atlas (C1) are fused together in a condition called atlanto-occipital assimilation, it causes improper functioning of the cervical spinal nerves due to the vascular compression. Surgical procedure can decompress the nerves and reduce symptoms.
The under surface of the jugular process is rough, and gives attachment to the Rectus capitis lateralis muscle and the lateral atlanto-occipital ligament; from this surface an eminence, the paramastoid process, sometimes projects downward, and may be of sufficient length to reach, and articulate with, the transverse process of the atlas.