The upper cervical spine is a complex anatomical structure. In this section, we are interested in the Atlanto-Occipital joint and Atlanto-Axial joint. Let’s look at each bone segment individually.
Base of the skull
The base of the skull is made up primarily of the occipital bone. There is a large opening on the floor of the occipital bone called the Foramen Magnum where the brain and spinal cord pass through. On the anterior-lateral portions around the Foramen Magnum there are two convex condyles that allow the articulation of the head to the cervical spine. These two condyles fit the opposing convex articulation of Atlas. The motion that occurs at this joint is primarily flexion and extension of the head, as in nodding your head “yes”.
Atlas
The Atlas is the first vertebra and is essentially the seat of the skull. It has the name Atlas as to refer to the myological god who held the world on his shoulders. In a literal sense, the Atlas vertebra holds up the head. This vertebra is a ring shaped, 2 ounce bone that resembles two wedges between two circular surfaces. This is a unique bone in the body. The stability of atlas relies on the tendons, ligaments, and muscles. There are no bony stops or facets that guide or limit the motion of this vertebra.
Atlas articulates with the skull and the 2nd vertebra, Axis. The motion afforded with the skull is flexion and extension of the head. With Axis, the motion is rotation left and right, as in shaking your head “no”.
Due to the nature of the this bone being wedged-shaped and having no bony stops, there is an incredible amount of motion the joint complex can move through. The trade-off is that there is less structural stability. The atlas vertebra is able to slip in many directions if there is a large enough force to move it. In the case of a whiplash injury, the 12 lbs head is forced through and often beyond full range of motion. This will cause the atlas to shift out of it’s original position.
Axis
Axis is the second vertebra in the neck. It has a peg-like structure that articulates with the inner ring of atlas and will allow for atlas to rotate around. Axis has flat superior facets that also articulate with atlas and provide a base for atlas to rotate. The underside of Axis is similar to a typical vertebra including slanted facets that limit rotation motion.
