Musculoskeletal System
Radial head and forearm rotation
The head of the radius spins against the ulna, so the radius turns around it for pronation (palm down) and supination (palm up).
Remember
The radius is the mobile forearm bone: its head spins against the ulna, so pronation and supination are the radius turning around the ulna.
Pronation (turning the palm down) and supination (turning the palm up) are the two turning movements of the forearm. They happen when the radius, the bone on the thumb side of the forearm, swivels around the ulna at the radioulnar joints. The proximal radioulnar joint is near the elbow, and the distal radioulnar joint is near the wrist.
At the elbow end, the radial head is the pivot. It forms a disk-like cap at the top of the radius. Its upper surface meets the capitulum (the small rounded surface at the front of the lower humerus). Its rim meets the radial notch (the surface on the upper ulna that receives it).
The head spins within the annular ligament, a collar that holds it against the ulna. Both radioulnar joints are pivot joints, meaning joints in which one bone rotates around an axis, like a wheel turning on its axle. At the distal joint, the ulnar notch (a small hollow on the lower end of the radius) slides around the rounded head of the ulna.
The wrist is a separate joint, where the radius meets the scaphoid and lunate (two of the carpal bones of the hand). The elbow hinge is different again: the ulna forms it with the humerus, whereas the radius is the mobile bone that turns around the ulna.
Radial head fractures characteristically follow a fall onto an outstretched hand, when the head is forced into the capitulum. Fractures of the proximal radius carry a recognised risk of lost forearm rotation.
Memory aid
the radius as a wheel turning around the ulna, with the radial head as the hub spinning inside its collar
Picture the radius as the wheel and the radial head as the hub that spins in its collar.
Key facts
- Moving bone
- the radius turns around the ulna, giving pronation and supination.
- Proximal joint
- the head of the radius spins within the annular ligament against the radial notch of the ulna.
- Distal joint
- the ulnar notch of the radius slides over the head of the ulna.
- Joint type
- both radioulnar joints are pivot joints.
- Elbow hinge
- the ulna forms it with the humerus, whereas the radius is the mobile bone that turns around the ulna.
- Radial head fracture
- it characteristically follows a fall onto an outstretched hand, when the head is forced into the capitulum.
Exam traps
- The radial notch belongs to the ulna and receives the radial head, so it is the partner of the part that rotates around the ulna.
- The ulnar notch of the radius is on the lower end of the radius and meets the head of the ulna at the distal joint, so it does not meet the humerus and is not on the proximal radius.
- The radial tuberosity is the bony projection where biceps inserts, so it is a muscle attachment and not a joint surface, and it does not meet the capitulum or the ulna.
- The interosseous membrane is a sheet of connective tissue joining the two shafts (the long middle parts of the bones), but it is flexible, so the radius still pivots around the ulna and the two bones do not move as one block.
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