|Position of radius (shown in red).|
|Right hand, palm down (left) and palm up (right). Radius is #1|
|Gray's||subject #52 219|
The radius or radial bone is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally. The radius articulates with the capitulum of the humerus, the radial notch and the head of the ulna. The corresponding bone in the lower leg is the tibia.
The word radius is Latin for "ray". In the context of the radius bone, a ray can be thought of rotating around an axis line extending diagonally from center of capitulum to the center of distal ulna. While the ulna is the major contributor to the elbow joint, the radius primarily contributes to the wrist joint. 
The radius is named so because the radius (bone) acts like the radius (of a circle). The ulna acts as the center point to the circle because when the arm is rotated the ulna does not move. The radius (bone) acts like the radius (of a circle) because it rotates around the ulna and the far end (where it joins to the bones of the hand), known as the styloid process of the radius, is the distance from the ulna (center of the circle) to the edge of the radius (the circle).
The radius has a body and two extremities. The upper extremity of the radius consists of a somewhat cylindrical head articulating with the ulna and the humerus, a neck, and a double tuberosity. The body of the radius is self-explanatory, and the lower extremity of the radius is roughly quadrilateral in shape, with articular surfaces for the ulna, scaphoid and lunate bones. The distal end of the radius forms a palpable point called the styloid process. Along with the proximal and distal radioulnar articulations, an interosseous membrane originates medially along the length of the body of the radius to attach the radius to the ulna.
Muscle attachments 
The biceps muscle inserts on the radial tuberosity of the upper extremity of the bone. The upper third of the body of the bone attaches to the supinator, the flexor digitorum superficialis, and the flexor pollicis longus muscles. The middle third of the body attaches to the extensor ossis metacarpi pollicis, extensor primi internodii pollicis, and the pronator teres muscles. The lower quarter of the body attaches to the pronator quadratus muscle and the tendon of the supinator longus.
The long narrow medullary cavity is enclosed in a strong wall of compact bone. It is thickest along the interosseous border and thinnest at the extremities, save over the cup-shaped articular surface (fovea) of the head.
The trabeculae of the spongy tissue are somewhat arched at the upper end and pass upward from the compact layer of the shaft to the fovea capituli (the humerus's cup-shaped articulatory notch); they are crossed by others parallel to the surface of the fovea. The arrangement at the lower end is somewhat similar. It is missing in radial aplasia.
Radial aplasia refers to the congenital absence of the radius.
Specific fracture types of the radius include:
- Essex-Lopresti fracture - a fracture of the radial head with concomitant dislocation of the distal radio-ulnar joint with disruption of the interosseous membrane.
- Distal radius fracture
- Galeazzi fracture - a fracture of the radius with dislocation of the distal radioulnar joint
- Colles' fracture - a distal fracture of the radius with dorsal (posterior) displacement of the wrist and hand
- Smith's fracture - a distal fracture of the radius with volar (ventral) displacement of the wrist and hand
- Barton's fracture - an intra-articular fracture of the distal radius with dislocation of the radiocarpal joint.
In other animals 
In four-legged animals, the radius is the main load-bearing bone of the lower forelimb. Its structure is similar in most terrestrial tetrapods, but it may be fused with the ulna in some mammals (such as horses) and reduced or modified in animals with flippers or vestigial forelimbs.
See also 
- Marieb, E., R.N., Ph.D; Mallatt, J., Ph.D. & Wilhelm, P., Ph.D. (2008), Human Anatomy (5th ed.), San Francisco, CA: Pearson Benjamin Cummings, p. 188
- Clemente, Carmine D. (2007), Anatomy: A Regional Atlas of the Human Body (5th ed.), Philadelphia, PA: Lippincott Williams & Wilkins
- Essex Lopresti fracture at Wheeless' Textbook of Orthopaedics online
- Romer, Alfred Sherwood; Parsons, Thomas S. (1977). The Vertebrate Body. Philadelphia, PA: Holt-Saunders International. p. 199. ISBN 0-03-910284-X.
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