The radius is a long bone located in the forearm. It is also known as the radial bone. It extends from the lateral side of the elbow to the thumb side of the wrist, lying next to the ulna, which is the second bone of the forearm.

The radius articulates in several places, forming the four following joints:

Elbow joint - partly formed by an articulation between the head of the radius and the capitulum of the humerus;
Proximal radioulnar joint - formed by an articulation between the head of the radius and the radial notch of the ulna;
Distal radioulnar joint - an articulation between the ulnar notch of the radius and the head of the ulna;
Wrist joint - an articulation between the distal end of the radius and the carpal bones.

As a long bone, the radius consists of three main parts: proximal end, shaft, and distal end.

Proximal end of radius

The proximal end (proximal extremity) of the radius is the end of the bone facing the humerus. Its major features include the following:

The head of the radius is a disc shaped structure that has a concave surface for articulation with the capitulum of the humerus. This articulation forms a part of the elbow joint. Medially, the head of the radius also articulates with a notch on the proximal end of the ulna, known as the radial notch.

The neck of the radius is a narrow region between the head of the radius and the radial tuberosity.

Just below the neck of the radius, is the radial tuberosity, also known as the bicipital tuberosity. It is an oval and convex bony projection. The biceps brachii muscle inserts onto the radial tuberosity.

Shaft of radius

The shaft of the radius or radial shaft is the long wide middle part of the bone. It is convex on the lateral side and enlarges towards the wrist. The radial shaft has three borders (anterior, posterior, and interosseous) and three surfaces (anterior, posterior, and lateral).

Several important anatomical landmarks are found on the shaft of the radius, where muscle tendons originate and insert.

Laterally, two muscles attach to the radial shaft:

Supinator - attaches to the lateral aspect of the radius, covering a major part of it (also inserting on the posterior and anterior surfaces);
Pronator teres - also attaches to the lateral and posterior surfaces of the radial shaft below the attachment of the supinator and inserts on the pronator tuberosity, which is a distinct rough area in the middle of the lateral surface.

Muscles that insert on the anterior surface of the radial shaft are:

Pronator quadratus - covers the distal end of the anterior surface of the shaft;
Brachioradialis - inserts slightly inferior to the pronator quadratus and on the opposite side of it.

Medially, origin sites of two other muscles are found (on the anterior surface of the radial shaft), which are the following:

Flexor digitorum superficialis - partly attaches to the proximal part of the radial shaft (on the anterior border of the radius);
Flexor pollicis longus - originates just below the origin site of the flexor digitorum superficialis (on the anterior surface).

The posterior surface of the radial shaft serves as an attachment site for several muscles:

Abductor pollicis longus - originates just below the posterior margin of the attachment of the supinator muscle;
Supinator and pronator teres - partly insert into the posterior surface of the radial shaft, as mentioned before.

Distal end of radius

The distal end or extremity of the radius is found on the side of the wrist. There are several important landmarks found on the distal end, including the dorsal tubercle and the ulnar notch. The lateral side of the distal end projects distally as the styloid process of the radius (or radial styloid process).

The dorsal tubercle is a bony projection on the posterior aspect of the distal end of the radius. It lies between the grooves for the tendons of the extensor carpi radialis longus and brevis, as well as the tendon of the extensor pollicis longus.
The ulnar notch is a concavity in the medial surface of the distal end of the radius. It articulates with the head of the ulna, forming the distal radioulnar joint.
The radial styloid process is a bony projection on the lateral side of the radius, at its distal end. Its base serves for the attachment of the brachioradialis muscle. And its apex is where the radial carpal collateral ligament of the wrist attaches.

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Radius