Pivot joint and head rotation
A pivot joint is a peg turning in a ring and allows rotation only, such as turning the head from side to side.
Remember
The head turns from side to side mainly at the atlantoaxial joint, a pivot joint between the atlas (C1, a ring of bone under the skull) and the axis (C2, the second vertebra). The atlas turns around the dens, the peg-like upward projection of the axis.
Memory aid
nodding "yes" happens at the top joint, and shaking "no" happens one level down, at the peg and ring.
Use it to place the two head movements: the top joint under the skull nods, and the peg and ring one level down turns the head.
A pivot joint is a peg turning in a ring. The peg is a rounded piece of bone, and the ring is made partly of another bone and partly of a ligament (a tough band joining bones). The peg turns within the ring, so the movement is rotation around one line, called an axis. Pivot joints are uniaxial, which means they move around one axis only.
The classic example is the atlantoaxial joint in the neck. Here the atlas (C1) forms the ring, and the dens (the peg-like upward projection of the axis, C2) is the peg. A ligament holds the peg in place, and the atlas rotates on top of the axis. The head turns to the right or left, as in shaking the head to say 'no'. Ligaments and muscles of the spine limit how far the joint turns, which helps keep it stable.
Do not confuse it with nodding. Nodding 'yes' happens at the atlanto-occipital joint between the skull and the atlas, a condyloid joint (an oval ball in an oval socket) that mainly allows nodding. Shaking 'no' happens one level down, at the peg and ring.
In the forearm, the turning happens at the radioulnar joint. At the proximal radioulnar joint, the head of the radius turns in a ligament ring, which makes it a second pivot joint.
Key facts
- Structure
- a rounded peg of bone turns inside a ring made partly of bone and partly of ligament.
- Movement
- rotation around one axis only, which is why a pivot joint is called uniaxial.
- Classic example
- the atlantoaxial joint, where the atlas (C1) is the ring and the dens of the axis (C2) is the peg.
- Head movement
- turning the head from side to side (the 'no' movement) happens mainly at the atlantoaxial pivot joint.
- Nodding
- nodding 'yes' is flexion and extension at the atlanto-occipital joint between the skull and the atlas, rather than at the pivot joint.
- Second example
- the proximal radioulnar joint in the forearm, where the head of the radius turns in a ligament ring.
- Stability
- ligaments and muscles of the spine limit how far the atlantoaxial joint turns, which helps keep it stable.
Exam traps
- The humeroulnar joint of the elbow is a hinge, with a cylindrical surface in a trough that allows flexion and extension, so it is not a pivot joint. The forearm turns at the radioulnar joint, which is the pivot joint in that region.
- The hip allows rotation, but it is a ball-and-socket joint (a rounded head sitting in a socket) that moves around many axes rather than rotating around one, so it is not a pivot joint.
- Flexion and extension of the head, as in nodding 'yes', happen at the atlanto-occipital joint between the skull and the atlas, so they are not the movement of the atlantoaxial pivot joint, which turns the head from side to side.
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