The Atlas and Axis
The vertebrae of the cervical spine and the base of the cranium interact to provide extensive three-dimensional placement of the head and neck, which is essential for optimal spatial orientation of the special senses (sight, hearing, smell, taste and balance).
The craniovertebral junction (CVJ) is the complex relationship between the occiput (the base of the cranium or skull), C1 and C2. This region houses vital neural and vascular structures while providing the majority of cranial flexion, extension, and axial rotation. In fact, the range of movement at the junction exceeds that of any other region of the vertebral column. An intricate combination of bony and ligamentous supports allows for stability despite the large range of movement, while highly specialized muscles control fine movements in the CVJ.
C1 is called the atlas and has no vertebral body or spinous process. It is essentially a bony ring that articulates with the occiput. The predominant movement between the occiput and the atlas is flexion and extension. In fact, it is suggested that this joint makes up about 50% of the flexion and extension range of movement in the cervical region (Bogduk and Mercer 2000).
C2 is called the axis. The axis also lacks a typical vertebral body but has a projecting process called the dens or odontoid process that forms a pivot for the atlas. The predominant movement of the C1–2 vertebral junction is rotation. Again, it is suggested that this joint makes up about 50% of the rotational range of movement in the cervical region (Bogduk and Mercer 2000). There is no intervertebral disc between the atlas and the axis, and this suggests that these vertebrae are not designed to bear weight greater than the head.
Reference:
Bogduk N. and Mercer S. (2000) ‘Biomechanics of the cervical spine. I: Normal kinematics.’ Clin Biomech 15, 633–648.
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