Anatomy · Bones

Radius

The thumb-side forearm bone

Upper limb · forearm

The radius is the lateral (thumb-side) bone of the forearm, running parallel to the ulna. It is narrow at the elbow, where its disc-shaped head pivots against the humerus and ulna, and broad at the wrist, where it forms most of the wrist joint. In pronation and supination it rotates around the ulna.

How to spot them

▸Shorter forearm bone on the thumb side, with a small disc-shaped head at the elbow
▸Widens towards the wrist, ending in a broad surface for the scaphoid and lunate and a pointed styloid process
▸A dorsal tubercle (Lister's tubercle) is palpable on the back of its lower end
▸The radial tuberosity below the neck is where the biceps tendon inserts

What to know

The shorter and wider-at-the-wrist bone of the forearm, on the thumb side

Radius — narrow at the elbow, broad at the wrist.

A disc-shaped head at the elbow lets it spin against the humerus and the other forearm bone, turning the palm up or down

Radius — its head pivots in the radial notch of the ulna.

A fall on an outstretched hand breaks its lower end, with the hand displaced backwards in a 'dinner fork' shape, a fracture named for an Irish surgeon

Radius — Colles' fracture, after Abraham Colles, 1814.

The pulse is traditionally taken at the wrist over the lower end of this bone

Radius — the radial artery lies on its front surface.

A pull on a small child's hand can slip its head out of a ring of ligament, causing 'pulled elbow'

Radius — radial head subluxation, or nursemaid's elbow.

A small bump on the back of its lower end acts as a pulley for the tendon that extends the thumb

Radius — Lister's (dorsal) tubercle, for extensor pollicis longus.

Together with the scaphoid and lunate it forms the wrist joint, while the other forearm bone is kept from touching the wrist bones by a cartilage disc

Radius — it forms the radiocarpal joint.

Key ideas

In pronation the lower end of the radius rotates around the ulna, crossing over it; this is the only major rotation of the forearm.
Its styloid process projects further than the ulna's, limiting sideways movement of the hand at the wrist.

Quick-fire facts

•It is the lateral bone of the forearm in the anatomical position, on the thumb side.
•Unlike the ulna it is narrow at the elbow and broad at the wrist.
•The radial tuberosity, just below the neck, is where the biceps brachii tendon inserts, so biceps is the chief supinator of the forearm.
•A distal radius fracture is one of the most common fractures in adults; Colles' fracture, described in 1814 by Abraham Colles of Dublin, has the fragment tilted dorsally.
•A Smith's fracture is the reverse, with the fragment displaced towards the palm, after a fall on a flexed wrist.
•Lister's tubercle is named after Joseph Lister, the pioneer of antisepsis, who described it in the 1800s.
•The name is Latin for 'spoke' or 'ray', of a wheel.
•It articulates with the humerus (capitulum), the ulna (twice), and the scaphoid and lunate.

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Sources

OpenStax — Anatomy and Physiology 2e — 8.2 Bones of the Upper Limb
TeachMeAnatomy — Radius — Bones of the Upper Limb