The radius is a long bone located in the forearm. It is also known as the radial bone. The radius extends from the lateral side of the elbow to the thumb side of the wrist. It lies next to the ulna, which is the second bone of the forearm.
The radius articulates in several places, forming four joints:
- elbow joint, which is partly formed by an articulation between the head of the radius and the capitulum of the humerus;
- proximal radioulnar joint, which is 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: a proximal end, a shaft, and a distal end.
Proximal end of radius
The proximal end or extremity of the radius is the end of the bone that faces the humerus. Its significant features include the head of the radius, the neck, and the radial tuberosity.
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 the radial notch on the proximal end of the ulna.
The neck of the radius is a narrow region between the head and the radial tuberosity.
The radial tuberosity is also referred to as the bicipital tuberosity. It is an oval and convex bony projection just below the head of the radius. The biceps brachii muscle inserts here.
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 tendons originate and insert.
Laterally, two muscles of the forearm attach to the radial shaft:
- supinator - attaches to the lateral aspect of the radius, covering a significant part of it;
- pronator teres - attaches to the shaft below the attachment of the supinator, and it inserts on the pronator tuberosity, which is a rough area in the middle of the lateral surface.
Medially, origin sites of two other muscles are found, which are the following:
- flexor digitorum superficialis, and
- flexor pollicis longus - originates just below the origin site of the flexor digitorum superficialis;
Muscles that insert on the medial surface of the radial shaft are:
- pronator quadratus - covers the distal end of the medial surface of the shaft;
- brachioradialis - inserts slightly inferior to the pronator quadratus and on the opposite side of it.
The posterior surface of the radial shaft serves as an origin site for two other muscles:
- abductor pollicis longus - originates just below the posterior margin of the attachment of the supinator muscle;
- extensor pollicis longus - originates more distally.
Distal end of radius
The distal end or extremity of the radius is found on the side of the wrist. There are several essential 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.
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.