Radius !

The radius is the forearm bone on the thumb side; it turns around the ulna to flip the palm and carries most of the wrist's load.

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What it is

The forearm bone on the thumb side; it widens at the wrist.

What it does

Rolls around the ulna to turn the palm up and down and carries most of the wrist's load.

Key facts

Joints with: Humerus (capitulum), ulna (proximal and distal radio-ulnar joints), scaphoid and lunate
Landmarks: Head, neck, radial tuberosity, interosseous border, ulnar notch, dorsal (Lister's) tubercle, styloid process

Relations

Above: Radial head against the capitulum, encircled by the annular ligament; the radial nerve divides in front of the elbow into the posterior interosseous nerve and the superficial radial nerve
In front: Distal biceps tendon to the radial tuberosity, with the bicipitoradial bursa; the supinator wraps the neck with the posterior interosseous nerve passing through it; FPL and pronator quadratus on the front of the shaft; the radial artery lies on the distal radius
Behind: Lister's tubercle separates the second extensor compartment (ECRL, ECRB) from the third (EPL), whose tendon turns around it; APL and EPB cross the back of the distal third
Medial: Ulna, joined by the interosseous membrane; the ulnar notch forms the distal radio-ulnar joint, stabilised by the TFCC
Lateral: Brachioradialis to the base of the styloid process; the first compartment tendons (APL, EPB) over the styloid; the superficial radial nerve crosses it under the skin; pronator teres inserts on the rough middle of the lateral surface, below the supinator, which wraps the upper third

How to find it

The radial head is felt just distal to the lateral epicondyle, turning under the thumb in pronation and supination. At the wrist, Lister's tubercle is felt on the back, in line with the index–middle finger cleft, and the styloid process on the thumb side, about 1 cm more distal than the ulnar styloid; a displaced distal radius fracture brings the two level.

On imaging

PA and lateral wrist views measure inclination, height and palmar tilt; a step of about 2 mm or more in the joint surface is significant, and CT maps intra-articular fractures. At the elbow, the radiocapitellar line (through the radial neck and head) should cross the capitulum on every view; if it does not, the radial head is dislocated.

Common problems

Distal radius fracture (Colles fracture): the most common arm fracture, usually from a fall on an outstretched hand.
Radial head fracture at the elbow, from a fall.
A step in the wrist joint surface after a fracture can lead to arthritis.

How it is diagnosed

X-rays measure the tilt, length and joint surface; CT if the joint is involved. Check of the median nerve and the circulation.

Treatment

Closed reduction (straightening without an incision) and a cast for stable fractures.
Surgery with a plate and screws for unstable or displaced fractures.
Hand therapy afterward to regain motion and grip.

Clinical notes

The EPL tendon can rupture weeks after a distal radius fracture, typically an undisplaced one, where it turns around Lister's tubercle
Acute carpal tunnel syndrome after a displaced distal radius fracture: the median nerve is checked before and after reduction
Pulled elbow in toddlers: the radial head slips partly out of the annular ligament after a sudden pull on the hand
The superficial radial nerve is at risk with wrist splints, casts and approaches to the radial styloid

Anatomical variants

Congenital radio-ulnar synostosis, usually proximal, which fixes the forearm in pronation
Radial longitudinal deficiency (hypoplastic or absent radius, radial club hand), which can occur with VACTERL association and other syndromes

Development

The radial head is the second elbow centre to appear (around age 3–5, the R of CRITOE). The distal physis provides most of the radius's growth in length, and Salter–Harris type II injuries of it are among the most common physeal fractures in children.

Memory aid

"MUGR": Monteggia = Ulna fracture, Galeazzi = Radius fracture; in each, the joint at the other end of the forearm is dislocated.

Test yourself

Think of the answer, then tap the question to see it.

How do Colles and Smith fractures of the distal radius differ?

Colles has dorsal tilt of the fragment (dinner-fork deformity); Smith has palmar tilt (garden-spade deformity).

What is a Galeazzi fracture of the radius?

A fracture of the distal third of the radial shaft with disruption of the distal radio-ulnar joint.

What are the usual normal X-ray values for the distal radius?

Radial inclination about 22°, radial height about 11 mm and palmar tilt about 11°.

Which tendon can rupture weeks after a distal radius fracture, and where?

The extensor pollicis longus tendon, where it turns around Lister's tubercle, typically after an undisplaced fracture.

What is the Essex-Lopresti injury of the radius?

A radial head fracture with a tear of the interosseous membrane and distal radio-ulnar joint injury, letting the radius migrate proximally.

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Educational information, not medical advice. For any symptom, see a health professional.

Its names

Official Latin (TA2): radius TA2 1210
Official Spanish (DeCS): Radio
English (TA2): Radius

Sources: Terminologia Anatomica (TA2); Anatomy, Shoulder and Upper Limb, Forearm Radius; Anatomy, Shoulder and Upper Limb, Forearm Bones; Anatomía humana (4.ª ed.), tomo 1, cap. 54: Huesos del miembro superior.

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Representative model, for educational purposes only.

Radius highlighted on the Durgical 3D model