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). 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 ulna acts as the center point to the circle because when the arm is rotated the ulna does not move.
The radius is part of two joints: the elbow and the wrist. At the elbow, it joins with the capitulum of the humerus, and in a separate region, with the ulna at the radial notch. At the wrist, the radius forms a joint with the ulna bone.
The radius has an upper end, a shaft and a lower end.
GENERAL FEATURES
Upper End
Head is disc shaped and is covered with a 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 smooth; it is broad medially where it articulates with the radial notch of the ulna, narrow in the rest of its extent and then embraced by the annular ligament thus forming a superior radioulnar joint which provides supination and pronation movement.
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 with the socket
Tuberosity lies just below the medial part of the neck. It has a rough posterior and a smooth anterior part.
Shaft
The shaft of radius is prismoid in form, narrower above than below, and slightly curved, so as to be convex lateralward. It presents three borders and three surfaces.
Borders
1. Anterior or Volar border extends from the lower part of the tuberosity above to the anterior part of the base of the styloid process below and separates the volar from the lateral surface. 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 and lower part is crest like.
2. Posterior border begins above at the back of the neck and ends below at the posterior part of the base of the styloid process; it separates the posterior from the lateral surface. is indistinct above and below, but well-marked in the middle third of the bone.
3. Medial or Interosseous border is the sharpest of the three borders which extends from the radial tuberosity above to the posterior margin of the ulnar notch below. The interosseous membrane is attached to its lower 3/4th. In its lower part it forms the posterior margin of an elongated triangular area.
Surfaces
1. Anterior surface lies between the anterior and interosseous borders. A nutrient foramen opens in its upper part which is directed upwards. Nutrient artery is a branch of the anterior interosseous artery.
2. Posterior surface lies between posterior and interosseous borders.
3. 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.
ATTCHMENTS TO MUSCLES & LIGAMENTS AND JOINTS
The lateral ligaments of the elbow are collectively named the lateral collateral ligament complex, which is composed of four ligaments that are difficult to separate. First, the lateral collateral ligament (a.k.a. "radial collateral ligament") attaches from the lateral epicondyle of the humerus to the annular ligament. The lateral collateral ligament joins the radius to the humerus and protects the antebrachium against varus stress. The annular ligament attaches from anterior to the posterior radial notch of the ulna encircling the radial head allowing pronation and supination. The lateral ulnar collateral ligament attaches from the lateral epicondyle of the humerus to the ulnar supinator crest. The lateral ulnar collateral ligament helps protect the antebrachium against valgus stress. Finally, the accessory lateral collateral ligament attaches from the supinator crest to the inferior margin of the annular ligament, providing further stability to the joint.
Between the medial border of the radius and the lateral border of the ulna resides the interosseous membrane which divides the anterior and posterior compartments of the antebrachium. The interosseus membrane serves as an attachment site for muscle while allowing for force distribution across the forearm. When the forearm supinates, the interosseous membrane fibers become taught, stabilizing distal and proximal joints. Like the lateral collateral ligament complex, the composition of the interosseous membrane is of multiple ligaments that are difficult to separate. The five ligaments of the interosseous membrane are the proximal oblique cord, accessory band, central band, dorsal oblique accessory cord, and the distal oblique bundle.
The distal radioulnar articulation is composed of the palmar radioulnar ligament (volar radioulnar ligament) that attaches the anterior radius to the anterior ulna, the dorsal radioulnar ligament (posterior radioulnar ligament) which attaches the posterior radius to the posterior ulna, and the articular disc which lies between the distal ulna and radius. These structures allow for load distribution and allow the second pivot during pronation and supination of the forearm.
The distal radius is attached to the lunate bone via two ligaments, the long and short radiolunate ligaments. These ligaments prevent hyperextension of the lunate bone during wrist extension. The distal radius is attached to the scaphoid via the radial collateral ligament of the wrist, and the radioscaphocapitate ligament attaches the volar aspect of the radius to the scaphoid and capitate carpal bones. Lastly, the dorsal radiotriquetral ligament is a wide ligament, which connects the dorsal radius to the dorsal scaphoid, lunate, and triquetrum.
The muscles of the forearm divide into two compartments - posterior and anterior. The posterior compartment is composed primarily of muscles that allow for wrist extension, finger extension, and forearm supination. The anterior compartment contains mostly the muscles involved in wrist flexion, finger flexion, and forearm pronation.
The anterior compartment subdivides into the superficial, intermediate, and deep layers. The muscles of the superficial layer are the flexor carpi ulnaris, palmaris longus, flexor carpi radialis, and pronator teres. The intermediate layer contains a single muscle, the flexor digitorum superficialis. The deep anterior layer is composed of the flexor digitorum profundus, the flexor pollicis longus, and the pronator quadratus.
The posterior compartment of the forearm subdivides into superficial and deep layers. The muscles of the superficial layer are the brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, and anconeus muscles. The muscles of the posterior layer are supinator, abductor pollicis longus, extensor pollicis brevis, extensor pollicis longus, and extensor indicis muscles.
BLOOD SUPPLY, LYMPHATICS AND NERVES
The blood supply to the proximal radius is provided primarily by the radial and radial recurrent arteries. The radial recurrent artery travels proximally to anastomose with the radial collateral artery. Blood supply to the middle and distal radius is through the radial, posterior interosseous, and anterior interosseous arteries.
Lymph from the radius drains via the deep lymphatic vessels which follow the deep veins such as the brachial, ulnar, and radial veins. These vessels drain into the humeral axillary lymph nodes.
The nerves of the brachial plexus provide motor and sensory innervation to the antebrachium.
The radial nerve provides sensory innervation for the posterior forearm and motor innervation to the brachioradialis, extensor carpi radialis brevis, extensor carpi radialis longus, supinator, extensor carpi ulnaris, abductor pollicis longus, abductor pollicis brevis, extensor pollicis longus, extensor pollicis brevis, extensor indicis, extensor digitorum, and extensor digiti minimi muscles.
The medial and lateral antebrachial cutaneous nerves provide sensory innervation to the anteromedial and anterolateral forearm respectfully.
The musculocutaneous nerve is the source of motor innervation to the biceps brachii.
The median nerve provides motor innervation to the pronator teres, flexor carpi radialis, palmaris longus, flexor digitorum superficialis, flexor pollicis longus, pronator quadratus, and the lateral half of the flexor digitorum profundus muscles.
The ulnar nerve supplies motor innervation to the flexor carpi ulnaris and flexor digitorum profundus muscles.
EMBRYOLOGY AND OSSIFICATION
The development of the radius occurs through endochondral ossification and begins with lateral plate mesoderm - the origin of all long bones. Endochondral ossification is the replacement of hyaline cartilage with bone. Mesoderm-derived mesenchymal cells become chondrocytes, which proliferate rapidly and form a bone template.
Chondrocytes near the center of the model begin laying down collagen and fibronectin to the matrix which allows for calcification. Around week six of gestation, the calcification prevents nutrients from reaching the chondrocytes. The chondrocytes eventually undergo apoptosis. Blood vessels proliferate through the spaces vacated by the chondrocytes and eventually form the medullary cavity. Around the 12th week of gestation, osteoblasts create a thick area of compact bone in the diaphysis called the periosteal collar which becomes the site of primary ossification. Epiphyseal plates (growth plates) at either end of the bone contain proliferating chondrocytes, that continue to elongate the bone, until puberty. During puberty, sex hormones cause secondary ossification centers to form, and the epiphyseal plates ossify in a congruent fashion as the diaphysis.
Complied & Written by Dr. Palak Shah
