Atlanto-occipital versus atlantoaxial joint
The head nods at the joint between the skull and C1, the first cervical vertebra (a bone of the neck), and turns mostly at the joint between C1 and C2.
Remember
Nodding is the skull tipping on C1 at the atlanto-occipital joint (between the skull and C1), whereas shaking the head for no is mostly C1 turning on C2 at the atlantoaxial joint (between C1 and C2).
Memory aid
nodding is tipping the skull on the atlas, whereas shaking "no" is turning the atlas on the axis
Use it to sort the two joints: the skull tips on C1 for a nod, and C1 turns on C2 for a shake of the head.
At the top of the spine, nodding and turning of the head happen at different joints. The atlas (C1) is the first cervical vertebra, a ring of bone that carries the skull. Below it, the axis (C2), the second cervical vertebra, has an upward peg of bone called the dens (also the odontoid process).
Nodding for yes is flexion and extension of the head, which means tipping it forwards and back. It happens mainly at the atlanto-occipital joint, where the upper surfaces of the atlas meet the occipital condyles, the two rounded knobs on the underside of the skull. A small amount of sideways tilting is possible there, but nodding is the main movement.
Turning the head from side to side, as in shaking it for no, happens mostly at the atlanto-axial joint. This is a pivot joint, which lets one bone rotate around a single axis: the atlas turns around the dens like a ring turning on a post. The transverse ligament of the atlas holds the dens in place, so the pivot stays stable.
In one study of healthy adults, about half of total head rotation came from the C1 to C2 level, more than from any single lower level, and other levels of the neck shared the rest. The lower cervical joints add to flexion and extension of the neck as a whole, but the characteristic nod belongs to the top of the spine.
Key facts
- Nodding
- flexion and extension of the head, mainly at the atlanto-occipital joint between the skull and C1.
- Shaking the head
- rotation, mostly at the atlantoaxial joint between C1 and C2.
- Atlas (C1)
- a ring of bone that carries the skull and turns around the dens.
- Dens
- an upward peg of bone on the axis (C2), also called the odontoid process, that the atlas turns around.
- Transverse ligament
- holds the dens in place and keeps the atlantoaxial joint stable.
- Share of rotation
- in one study of healthy adults, about half of total head rotation occurred between C1 and C2.
Exam traps
- The atlantoaxial joint is not the main site of nodding, because the atlas turns around the dens there, which gives a large share of the rotation of the neck, as when the head shakes for no.
- Head rotation takes place mainly at the atlantoaxial joint rather than the atlanto-occipital joint, which instead allows the characteristic nodding movement.
- Nodding is not mainly a movement of the atlantoaxial joint, because the dens, inside the ring of the atlas, forms a pivot, and the main job of that pivot is rotation rather than nodding.
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