The radius is one of the two bones of the forearm, lying lateral to the ulna. It articulates with the humerus at the elbow and the carpal bones at the wrist, and also has two points of articulation with the ulna.
Anterior and posterior views of the radius
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The head of the radius can be felt in a depression lateral to the olecranon on the posterior surface of the elbow, notably rotationg on pronation and supination of the forearm. The distal half to one third of the radius is palpable, particularly the posterior aspect. The dorsal tubercle may be felt posteriorly. The radial styloid is easily visible and palpable.
The head of the radius is disc-shaped, with a shallow cup on its proximal surface to accommodate the humeral capitulum. Both this surface and the rim are articular surfaces, the rim articulating with the ulnar radial notch at the proximal radioulnar joint. The neck is the contracted area immediately distal to the head, which overhangs it more laterally than medially. The radial tuberosity (bicipital tuberosity) lies distal to the neck on the medial side; the oblique line runs distally and laterally from this.
The shaft is triangular in cross-section, being sharpest on the medial border. The anterior border is most defined at either end, whilst the posterior border is sharpest in the middle third. The nutrient foramen lies in the middle of the anterior surface, its canal being sloped proximally. Anterior and posterior oblique lines run across the shaft in its upper half.
The distal end of the radius is four-sided. The styloid process (radial styloid) is a lateral projection from its surface. The carpal articular surface is smooth, divided into lateral and medial portions by a central ridge. The medial surface is the ulnar notch, which is smooth to allow articulation at the distal radioulnar joint. There is a dorsal tubercle on the posterior surface, as well as several grooves for the long extensor tendons.
- Abductor pollicis longus arises from the middle third of the posterior surface
- Extensor pollicis brevis arises from the lower third of the posterior surface
- Flexor digitorum superficialis has three heads, the radial head arising from the anterior oblique line
- Flexor pollicis longus arises from a large area of the anterior surface immediately distal to the anterior oblique line.
- Supinator inserts into the neck and shaft, between the two oblique lines
- Biceps brachii inserts into the radial tuberosity
- Pronator teres inserts half way down the lateral surface
- Pronator quadratus inserts into the lower quarter of the anterior surface
- Brachioradialis inserts into the base of the styloid process.
In addition the triangular articular disc of the distal radioulnar joint is attached to a smooth ridge distal to the ulnar notch.
- The humeroradial joint at the elbow
- The radiocarpal joint at the wrist
- The proximal and distal radioulnar joints
- The radial artery passes over the origin of flexor pollicis longus to lie on the anterior surface of the radius in its distal third, where it is easily palpable
- The superficial radial nerve runs alongside the radial artery
- The annular ligament surrounds the radial head
- A bursa lies beneath the biceps tendon nearing its attachment to the radial tuberosity
The radius ossifies from three centres, one in the shaft and one at either end. The distal epiphysis fuses between fourteen and seventeen years, the proximal between seventeen and ninteen.