The Radius or Radial Bone is the horizontal long bone of the forearm. It is one of the two bones of the forearm, the other being the ulna. The radius is shorter than the ulna and is prism-shaped long bone. The radius is homologous to the medial bone of the leg, tibia.
The radius is a long bone and is made up of three parts which are named: upper end, shaft, and lower end.
The upper end provides head, neck, and radial tuberosity. The head is disc shaped and articulates above along with the capitulum of humerus. Below the head is the neck, which is the constricted part. The radial tuberosity is just below the medial portion of the neck.
- The head of the radius bone is formed like a disc and in living it is covered by an articular hyaline cartilage.
- It articulates superiorly along with capitulum to create humero-radial articulation.
The circumference of head is smooth and articulates medially along with the radial notch of ulna, rest of it is surrounded by the annular ligament.
- The neck of the radius bone is the constricted part just below the head and is embraced by the lower part of annular ligament.
The quadrate ligament is connected to the medial side of the neck.
- Biceps tendon is inserted to radial tuberosity's rough, posterior part.
A small synovial bursa covers its smooth anterior part and separates it from the biceps tendon.
The long shaft of the radius provides a lateral convexity extending between the upper and lower ends. It broadens towards the distal end and is concave anteriorly in its distal part. Its sharpest interosseous border is located on the medial side.
The shaft of the radius has three borders and three surfaces.
- The anterior border of the shaft of the radius starts off below the anterolateral part of radial tuberosity and runs downwards and laterally to the styloid process.
- The upper part of this border is referred to as anterior oblique line and lower part forms the sharp lateral border of the anterior surface.
Its anterior oblique line provides origin to radial head of flexor digitorum superficial (FDS).
- The posterior border of the shaft of the radius is well-defined only in its middle third of the shaft.
- Above it runs upwards and medially to the radial tuberosity and makes the posterior oblique line.
Medial (interosseous) border
- The medial border of the shaft of the radius is the sharpest border.
- It extends above up to radial tuberosity and below its lower part forms the posterior margin of the small triangular area on the medial side of the lower end of the bone.
- Interosseous membrane is connected to its lower three- fourth.
- The anterior surface of the shaft of the radius is concave and is located between anterior and interosseous borders.
- Flexor pollicis longus emerges from its upper two-fourth.
- Pronator quadratus is inserted on its lower one-fourth.
Nutrient foramen appears a little above the middle of this surface in its upper part. The nutrient canal is directed upwards. Nutrient artery for radius is a branch from anterior interosseous artery.
- The posterior surface of the shaft of the radius is located between the interosseous and posterior borders.
- Abductor pollicis longus (APL) comes up from the middle one-third of this surface.
Extensor pollicis brevis (EPB) comes up from lower part of this surface.
- The lateral surface of the shaft of the radius is located between anterior and posterior borders.
- Supinator is inserted on the widened upper one-third of this surface.
Pronator teres is inserted on the rough area in the most convex middle part of this surface.
The lower end of the radius is the widest part and provides five surfaces. The lateral surface projects distally as the styloid process. The dorsal surface provides a palpable dorsal tubercle (Listers tubercle), which is limited medially by an oblique groove.
The lower end is the widest portion of the bone and has five surfaces.
Anterior surface: The anterior surface of the lower end of the radius provides a thick ridge, which provides connection to palmar radio-carpal ligament of wrist joint.
Posterior surface: The posterior surface of the lower end of the radius provides the dorsal tubercle of Lister lateral to the groove for the tendon of extensor pollicis longus. It also provides grooves for other extensor tendons. The groove lateral to the Lister's tubercle is traversed by tendons of extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB). Through the groove medial to groove for extensor pollicis longus passes tendons of extensor digitorum and extensor indicis.
Medial surface: The medial surface of the lower end of the radius provides the ulnar notch for articulation with the head of ulna. Articular disc of inferior radio-ulnar joint is connected to the lower margin of ulnar notch.
Lateral surface: The lateral surface of the lower end of the radius projects downward as the styloid process and is associated to tendons of adductor pollicis longus and extensor pollicis brevis. The brachioradialis is inserted to the base of styloid process and radial collateral ligament of wrist joint is connected to the tip of styloid process.
Inferior surface: The inferior (distal) surface of the lower end of the radius provides a lateral triangular area for articulation along with the scaphoid and a medial quadrangular area for articulation with the lateral components of the lunate.
Anatomical Position And Side Determination
The side of radius can be identified by keeping the bone vertically in such a manner that:
- The narrow disc-shaped end ( head) is directed upwards.
- The sharpest border (interosseous border) of the shaft is kept medially.
- The styloid process at the lower end is directed laterally and prominent tubercle (Listers tubercle) at lower end faces dorsally.
The convexity of shaft faces laterally, and concave anterior surface of shaft faces anteriorly.
In the image below pink color represents origin and blue color represents insertion.
Muscles that originates from the radius bone are:
- Flexor pollicis longus muscle
- Flexor digitorum superficialis muscle (radial head)
- Abductor pollicis longus muscle
- Extensor pollicis brevis muscle
Muscles that inserts into the radius bone are:
- Biceps brachi
- Supinator muscle
- Pronator teres muscle
- Pronator quadratus
The radius ossifies from the following three centers:
1. One primary center shows up in the mid-shaft during 8th week of 1UL.
2. Two secondary centers, one for each end:
- Center for lower end shows up at the age of first year.
- Center for upper end shows up during fifth year.
The upper epiphysis merges at the age of 12 years. The lower epiphysis merges at the age of 20th year.
Fracture of radius : The radius is a weight-bearing bone of the forearm; for this reason fractures of radius are more frequent than ulna.
- In fracture shaft of radius, with fracture line below the insertion of biceps and above the insertion of pronator teres the upper fragment is supinated by supinator and lower fragment is pronated by the pronator teres.
- In fracture at the distal end of radius (Colles' fracture) the distal fragment is displaced backwards and upwards. The opposite of Colles' fracture is referred to as Smith's fracture.
Fracture of styloid process of radius is termed 'Chauffeur's fracture'.
Madelung deformity: It is a congenital defect of radius which provides the following clinical features: The anterior bowing of distal end of radius, as a result of an abnormal growth of distal epiphysis. It occurs between 10 and 14 years of age. There is premature disappearance of distal epiphyseal line. There may be subluxation or dislocation of distal end of ulna, because of defective development of distal radial epiphysis.