Radius – the lateral bone of the forearm (Features & Ossification)

Radius – the lateral bone of the forearm (Features & Ossification)

The radius is the lateral bone of the forearm, and is homologous with the tibia of the lower limb. It has an upper end, a lower end and a shaft.

General Features:

Upper End :

1. The head is disc-shaped and is covered with hyaline cartilage. It has a superior concave surface which articulates with the capitulum of the humerus at the elbow joint.

The circumference of the head is also articular. It fits into a socket formed by the radial notch of the ulna and the annular ligament, thus forming the superior radioulnar joint.

2. The neck is enclosed by the narrow lower margin of the annular ligament. The head and neck are free from capsular attachment and can rotate freely within the socket.

3. The tuberosity lies just below the medial part of the neck. It has a rough posterior part and a smooth anterior part.

Shaft:

It has three borders and three surfaces.

Borders :

1. The anterior border extends from the anterior margin of the radial tuberosity to the styloid process. It is oblique in the upper half of the shaft, and vertical in the lower half. The oblique part is called the anterior oblique line. The lower vertical part is crest-like.

2. The posterior border is the mirror image of the anterior border, but is clearly defined only in its middle one-third. The upper oblique part is known as the posterior oblique line.

3. The medial or interosseous border is the sharpest of the three borders. It extends from the radial tuberosity above to the posterior margin of the ulnar notch below. The interosseous membrane is attached to its lower three-fourths. In its lower part, it forms the posterior margin of an elongated triangular area.

Surfaces :

1. The anterior surface lies between the anterior and interosseous borders. A nutrient foramen opens in its upper part, and is directed upwards. The nutrient artery is a branch of the anterior interosseous artery.

2. The posterior surface lies between the posterior and interosseous borders.

3. The lateral surface lies between the anterior and posterior borders.

Lower End:

The lower end is the widest part of the bone. It has 5 surfaces.

1. The anterior surface is in the form of a thick prominent ridge. The radial artery is palpated against this surface.

2. The posterior surface presents four grooves for the extensor tendons. The dorsal tubercle (of Lister) lies lateral to an oblique groove.

3. The medial surface is occupied by the ulnar notch for the head of the ulna.

4. The lateral surface is prolonged downwards to form the styloid process.

5. The inferior surface bears a triangular area for the scaphoid bone, and a medial quadrangular area for the lunate bone. This surface takes part in forming the wrist joint.

Side Determination:

The smaller circular upper end is concave followed by a constricted neck. Just below the medial aspect of neck are the radial tuberoses.

The wider lower end is thick with a pointed styloid process on its lateral aspect and a prominent dorsal tubercle on its posterior surface. Medial or interosseous border is thin and sharp.

Particular Features :

1. The biceps brachii is inserted into the rough posterior part of the radial tuberosity. The anterior part of the tuberosity is covered by a bursa.

2. The supinator is inserted into the upper part of the lateral surface.

3. The pronator teres is inserted into the middle of the lateral surface.

4. The brachioradialis is inserted into the lowest part of the lateral surface just above the styloid process.

5. The radial head of the flexor digitorum superficialis takes origin from the anterior oblique line and the upper part of anterior border.

6. The flexor pollicis longus takes origin from the upper two-thirds of the anterior surface.

7. The pronator quadratus is inserted into the lower part of the anterior surface and into the triangular area on the medial side of the lower end. The radial artery is palpated as “radial pulse” as it lies medial to the sharp anterior border of radius, lateral to the tendon of flexor carpi radialis.

8. The abductor pollicis longus and the extensor pollicis breuis arise from the posterior surface.

9. The quadrate ligament is attached to the medial part of the neck.

10. The oblique cord is attached on the medial side just below the radial tuberosity.

11. The articular capsule of the wrist joint is attached to the anterior and posterior margins of the inferior articular surface.

12. The articular disc of the inferior radioulnar joint is attached to the lower border of the ulnar notch.

13. The extensor retinaculum is attached to the lower part of the anterior border.

14. The interosseous membrane is attached to the lower three-fourths of the interosseous border.

Ossification :

The shaft ossifies from a primary centre which appears during the 8th week of development.

The lower end ossifies from a secondary centre which appears during the first year and fuses at 20 years; it is the growing end of the bone. The upper end (head) ossifies from a secondary centre which appears during the 4th year and fuses at 18 years.