Radius

The radius lies lateral to the ulna and is the shorter of the two bones. It articulates above with the capitulum of the humerus, distally with the scaphoid and lunate bones of the proximal row of the carpus, and at each end with the ulna. It has a shaft and two ends, the inferior end being the larger.

The head is a thick disc with a concave superior surface for articulation with the capitulum. The outer, articular surface of the head is flattened, articulating inside a fibro-osseous ring formed by the radial notch of the ulna and the annular ligament. Below the head is the constricted neck, which slopes medially as it approaches the shaft. Where the shaft joins the neck it is round, but becomes triangular lower down. Together with the neck, the shaft has a slight medial convexity on its upper quarter, with a lateral convexity in its remaining lower part. The radial tuberosity lies anteromedially on the upper part of the shaft at the maximum convexity of the medial curve. The majority of the shaft presents three borders and three surfaces.

The interosseous border, to which the interosseous membrane attaches, is sharp and faces medially. It extends from just below the radial tuberosity to the medial side of the lower end of the radius, splitting into two ridges which become continuous with the anterior and posterior margins of the ulnar notch. The anterior and posterior borders pass obliquely downwards and latteraly from either side of the radial tuberosity to the roughened area for pronator teres lower down, while the posterior border becomes more rounded. These borders enclose the lateral, anterior and flatter posterior surfaces.

The inferior end of the radius is expanded having five distinct surfaces. The lateral surface, which extends down to the styloid process, has a shallow groove anteriorly for the tendons of abductor pollicis longus and extensor pollicis brevis. The medial surface forms the concave ulnar notch for articulation with the head of the ulna having a roughened triangular area supeiorly. The posterior surface is convex and grooved by tendons. The prominent ridge in the middle of this surface is the dorsal(Lister’s) tubercle. The lateral half of this surface continues down onto the styloid process. The anterior surface is smooth and curves forward to a distinct anterior margin. The distal articular surface is concave and extends onto the styloid process. It is divided by a ridge into two areas, a lateral triangular area for articulation with the scaphoid and a medial quadrilateral area for the lunate.

Palpation

The head of the radius can be palpated in a “dimple” on the posterolateral aspect of the elbow, particularly when the elbow joint is extended as it overhangs the capitulum; it can be felt to rotate during pronation and supination. The shaft of the radius can be palpated on the lateral side in the lower half of the forearm. Distally, on the posterior aspect, the dorsal tubercle can be identified above the wrist, as can the styloid process laterally inside the “anatomical snuff-box” between the extensor tendons of the thumb.

Ossification

A primary ossification centre appears in the shaft during the eighth week in utero, so that at birth only the head, inferior end and radial tuberosity are cartilaginous. The first secondary centre appears in the inferior end during the first year of life, fusing with the shaft between the ages of 20 and 22 years. The secondary centre for the head appears at about 6 years and fuses with the shaft between 15 and 17 years. A secondary centre usually appears in the radial tuberosity between 14 and 15 years, but soon fuses with the shaft.