Radius

The radius is the lateral bone of the forearm, on the thumb side, narrow at the elbow and broad at the wrist.

At a glance

English: Radius
Latin (TA2): Radius
System: Bone
Region: Upper limb
Side: Paired, left and right
Muscles on it: Flexor pollicis longus

About the radius

The radius is the lateral of the two forearm bones, on the thumb side when the palm faces forwards. It is short and slim at the elbow and broad at the wrist, the reverse of the ulna beside it. Because it turns around the ulna, it is the bone that lets the hand rotate from palm-up to palm-down.

At its upper end is a small disc-shaped head. Its shallow concave top rests on the humeral capitulum, and its smooth rim spins inside the annular ligament against a small notch on the ulna. Below the head a narrow neck leads to the radial tuberosity, a rough knob on the medial side where the biceps tendon inserts. The shaft curves gently outwards and carries a sharp interosseous border facing the ulna; the interosseous membrane spans the gap between the two bones.

The lower end is the widest part of the bone. Its distal surface forms the main joint with the carpus, taking the scaphoid and lunate. On its medial side the ulnar notch receives the head of the ulna at the distal radio-ulnar joint, and on its lateral side the radial styloid process projects downwards and can be felt at the wrist. The dorsal surface is grooved for the extensor tendons, with the dorsal tubercle (Lister's tubercle) acting as a pulley for extensor pollicis longus.

Muscles attached along the bone include supinator and the radial head of flexor digitorum superficialis near the top, flexor pollicis longus and abductor pollicis longus on the front and back of the shaft, pronator teres at the point of greatest lateral curve, pronator quadratus near the wrist and brachioradialis at the base of the styloid. Distal radius fractures are among the most frequent in adults after a fall on the outstretched hand: in a Colles fracture the fragment tips backwards, in a Smith fracture forwards, and a Barton fracture enters the wrist joint. A Galeazzi injury combines a shaft fracture with a disrupted distal radio-ulnar joint, and an Essex-Lopresti injury pairs a radial head fracture with tearing of the interosseous membrane. Radial aplasia is a congenital absence or shortening of the bone.

Function and supply

Function: Carries the hand through pronation and supination and transmits most of the force from the hand to the forearm at the wrist.
Nerve supply: The posterior interosseous branch of the radial nerve winds around the neck within supinator; the superficial radial nerve crosses the styloid region.
Blood supply: A nutrient artery from the anterior interosseous artery; the distal end also receives branches of the radial artery and the palmar and dorsal carpal arches.

Parts of the radius

Each number is marked on the model. Press one, here or on the model, to see it named in place.

Anterior border of radiusMargo anterior radiiThe front edge of the shaft, running from below the radial tuberosity down to the front of the styloid. Its upper part forms the oblique line, from which the radial head of flexor digitorum superficialis arises.
Anterior surface of radiusFacies anterior radiiThe front face of the shaft, between the anterior and interosseous borders. Its upper part is hollowed and gives origin to flexor pollicis longus, while its flat lower part receives pronator quadratus.
Articular circumference of head of radiusCircumferentia articularis capitis radiiThe smooth rim around the radial head. Its wider inner part turns against a small notch on the side of the ulna, and the rest spins inside the annular ligament during pronation and supination.
Articular facet of head of radiusFovea articularis capitis radiiThe shallow cup on the top of the radial head. It meets the humeral capitulum and glides on it as the elbow bends and the forearm rotates.
Body of radiusCorpus radiiThe shaft, three-sided in section, slimmer above and wider below. It bows outwards, and this curve is what lets it swing round the ulna when the hand turns over.
Carpal articular surfaceFacies articularis carpeaThe smooth, concave lower end of the radius that forms the main part of the wrist joint. A low ridge divides it into a lateral area for the scaphoid and a medial area for the lunate.
Dorsal radial tubercleTuberculum dorsale radiiA small ridge on the back of the lower radius, also called Lister's tubercle, easily felt at the back of the wrist. The tendon of extensor pollicis longus hooks round its inner side to change direction towards the thumb.
Grooves for extensor tendonsSulci tendinum extensorumShallow channels on the back of the lower radius, separated by low ridges. Extensor tendons to the wrist, thumb and fingers run in them, held down by the extensor retinaculum.
Head of radiusCaput radiiThe short, disc-shaped top of the radius, with a cupped upper face and a smooth rim. It is the part that turns at the elbow and can be felt rotating just below the lateral epicondyle.
Interosseous border of radiusMargo interosseus radiiThe sharp inner edge of the shaft, facing the ulna. The interosseous membrane that binds the two forearm bones attaches along it, and below it divides to enclose the ulnar notch.
Lateral surface of radiusFacies lateralis radiiThe outer face of the shaft, convex along its length. Supinator wraps its upper part, and pronator teres inserts about its middle on the pronator tuberosity.
Neck of radiusCollum radiiThe narrow, smooth bar of bone just below the head. Part of supinator attaches to its back, and in children a fall on the hand often breaks the radius here.
Posterior border of radiusMargo posterior radiiThe back edge of the shaft, which starts behind the neck and fades out near the dorsal tubercle at the wrist. It is the boundary between the back and outer faces and is sharpest in the middle third.
Posterior surface of radiusFacies posterior radiiThe back face of the shaft. Supinator covers its upper part, and abductor pollicis longus and extensor pollicis brevis arise from its middle.
Pronator tuberosityTuberositas pronatoriaA rough oval area on the outer side of the shaft at about its midpoint, at the peak of its curve. Pronator teres inserts here, giving the muscle a long lever to roll the radius over.
Radial styloid processProcessus styloideus radiiThe pointed downward projection on the outer side of the lower end, reaching further than the ulnar styloid. Brachioradialis inserts near its base, and it can be felt at the side of the wrist.
Radial tuberosityTuberositas radiiA rough oval knob on the inner side of the shaft just below the neck. The biceps brachii tendon inserts on its back part, so biceps both flexes the elbow and turns the palm up.
Suprastyloid crestCrista suprastyloideaA ridge on the outer side of the lower radius, a little above the styloid. Brachioradialis inserts along it.
Ulnar notchIncisura ulnarisA shallow, concave joint surface on the inner side of the lower end. It receives the head of the ulna at the distal radio-ulnar joint, so the radius can turn round it.

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At a glance

About the radius

Function and supply

Related structures

See the radius where it really sits, with every layer of the body around it.