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The radius (plural: radii) is one of the two long bones present in the forearm, located laterally in the supinated anatomical position. It has a smaller proximal end and enlarges to a larger distal end (opposite to the ulna).
The radial shaft is thicker than the ulna and becomes thicker distally. It has a triangular cross section, with a lateral convexity, and a sharp medial (interosseous) border.
The distal radial shaft flares out to form the distal radius with a facet for articulation with most of the proximal carpal row. Laterally, there is the radial styloid and medially the ulnar notch. On the dorsal surface of the distal radius, Lister's tubercle (dorsal tubercle) separates the 2nd (extensor carpi radialis brevis and extensor carpi radialis longus) and 3rd (extensor pollicis longus) extensor compartments.
- radial head: capitellum of the humerus (elbow flexion and extension)
- radial head: radial notch of the proximal ulna (site of supination and pronation)
- annular ligament: radial head to radial notch of ulna
- volar/palmar radiocarpal ligament: non-articular volar surface of distal radius to the scaphoid, lunate and triquetrum
- dorsal radiocarpal ligament: non-articular dorsal surface of the distal radius to the scaphoid, lunate and triquetrum
- radial collateral ligament of wrist joint: apex of the radial styloid process to the scaphoid and trapezium
- radial collateral ligament of the elbow joint: anterior aspect of lateral epicondyle of the humerus to the annular ligament and supinator fascia
- interosseous membrane: forms syndesmotic joint with ulna along the medial border of the radius
Ossifies from 3 centers:
- body: appears in 8th week of fetal life
- proximal extremity: appears around 5 years of age and fuses with the body around 17-18 years of age
- distal extremity: appears around 9-26 months of life and fuses with the body around 20 years of age
- radioulnar synostosis
- short, hypoplastic or absent radius