Logan Steele
The radius is one of the two bones that make up the forearm, the other being the ulna. The radius is a long horizontal bone and is smaller than the ulna. It has three main parts: a proximal end, shaft, and distal end. The proximal end has a head that articulates with the distal humerus and the proximal ulna, while the distal end articulates with the head of the ulna and carpal bones at the wrist. The shaft of the radius bone has three borders and three surfaces, and multiple muscles attach to it, including the biceps brachii, pronator teres, pronator quadratus, supinator, and extrinsic hand muscles.
| Characteristics | Values |
|---|---|
| Muscle insertions | Supinator muscle, Pronator teres muscle, Abductor pollicis longus, Extensor pollicis brevis muscle, Flexor digitorum superficialis muscle, Flexor pollicis longus muscle, Pronator quadratus muscle, Brachioradialis muscle, Biceps brachii muscle |
| Description | The radius is one of the two long bones in the forearm, the other being the ulna. It is a horizontal bone with three borders and three surfaces. |
| Proximal end | Radial head, neck, and tuberosity |
| Distal end | Joins with the head of the ulna and carpal bones at the wrist |
| Shaft | Connected to the ulna by the interosseous membrane |
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What You'll Learn
The biceps brachii muscle
The biceps brachii is a two-joint muscle, with both heads enforcing opposite movements in the shoulder joint. The long head pulls the arm away from the trunk (abduction) and turns it inwards (inward rotation), while the short head pulls the arm back towards the trunk (adduction). When both heads contract simultaneously, it leads to a bending of the arm (flexion). In the elbow joint, the muscle bends the forearm (flexion) and rotates it outwards (supination). The supination is most powerful when the elbow is flexed.
The biceps brachii also has several weaker functions at the glenohumeral joint (shoulder joint). It assists in forward flexion of the shoulder joint (bringing the arm forward and upwards) and may contribute to abduction (bringing the arm out to the side) when the arm is externally or laterally rotated. Additionally, the short head of the biceps brachii assists with horizontal adduction (bringing the arm across the body) when the arm is internally or medially rotated.
The distal insertion point of the biceps brachii has been a topic of controversy in recent years due to its importance in distal biceps reconstruction. The muscle inserts on the radial tuberosity of the upper extremity of the radius bone. The radial tuberosity is located beneath the neck of the radius, on its medial side, and its surface is divided into a posterior, rough portion for the insertion of the biceps tendon, and an anterior, smooth portion with a bursa interposed between the tendon and the bone.
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The supinator muscle
The main function of the supinator is to supinate the forearm, working with the biceps brachii for powerful supination. It is the most active muscle in forearm supination during unresisted supination, while the biceps become increasingly active with heavy loading. Supination strength decreases by 64% if the supinator is disabled, for example, by injury. The supinator always acts together with the biceps, except when the elbow joint is extended.
In rats, the supinator muscle originates from the ulna and winds around the lateral side of the forearm before inserting onto the radius. The normal position of the rat forearm is pronation, with the palm of the forepaw resting on the ground, and the radius is habitually crossed over the ulna.
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The pronator teres muscle
The pronator teres is a long, round muscle located mainly in the forearm. It is a fusiform muscle and belongs to the group of superficial flexors of the forearm, along with flexor carpi radialis, palmaris longus, flexor digitorum superficialis, and flexor carpi ulnaris muscles. It is the most lateral muscle of this group. The pronator teres has two heads, named after their origin sites: the humeral head (superficial head) and the ulnar head (deep head). The humeral head originates from the medial supracondylar ridge of the humerus, located superior to the medial epicondyle of the humerus and inferior to the attachment of the brachialis muscle. The ulnar head originates from the coronoid process of the ulna, between the attachments of the brachialis and flexor digitorum superficialis muscles, and superior to the origin of the flexor pollicis longus muscle.
The two heads of the pronator teres muscle run inferolaterally, coursing under the brachioradialis muscle. They eventually fuse into a single muscle belly that inserts via a flat tendon onto the lateral surface of the radius, specifically at the rough area at the middle of its shaft called the pronator tuberosity. The median nerve runs between the two heads of the muscle, and it is separated from the ulnar artery by the ulnar head. The pronator teres is innervated by the median nerve and nerve roots C6 and C7.
The main action of the pronator teres is the pronation of the forearm, which is an exclusive upper limb movement. The muscle pulls the radius medially, causing its head to rotate around the proximal part of the ulna at the proximal radioulnar joint. This action also rotates the palm of the hand, bringing it into a position to face the ground. Since it crosses the elbow joint, the pronator teres also assists in the flexion of the forearm.
Pronator teres syndrome is a condition that affects the median nerve distribution, causing numbness with repetitive pronation/supination of the forearm. Early fatigue of the forearm muscles can also be seen with repetitive stressful motion, especially pronation.
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The abductor pollicis longus muscle
The abductor pollicis longus (APL) is an extrinsic muscle of the hand. Its primary function is to abduct the thumb at the carpometacarpal joint, moving the thumb anteriorly. It also assists in extending and rotating the thumb, abducting the wrist, and flexing the hand. The APL is one of the five deep extensors in the forearm, along with the supinator, extensor pollicis brevis, extensor pollicis longus, and extensor indicis.
The APL arises from the lateral part of the dorsal surface of the ulna, the interosseous membrane, and the middle third of the dorsal surface of the radius. The muscle belly of the APL lies in the distal half of the posterior forearm, just below the supinator, and gives off a tendon just proximal to the wrist. This tendon passes through a groove on the lateral side of the distal end of the radius, accompanied by the tendon of the extensor pollicis brevis.
The APL tendon then passes beneath the extensor retinaculum, traversing the first dorsal compartment of the retinacular space. Within the hand, the tendon travels inferolaterally to its insertion site. The tendon splits into two slips that insert onto the base of the first metacarpal and trapezium bones, respectively. Some fibres may also insert into surrounding structures, such as the opponens pollicis, abductor pollicis brevis, and the fascia of the thenar eminence.
The APL is innervated by the posterior interosseous nerve, a continuation of the deep branch of the radial nerve after it passes through the supinator muscle. The posterior interosseous nerve is derived from spinal segments C7 and C8. The APL is supplied by the posterior interosseous artery. An accessory APL tendon is present in more than 80% of people, and a separate muscle belly is found in 20% of people.
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The extensor pollicis brevis muscle
The main action of the extensor pollicis brevis muscle is to extend the thumb on the carpometacarpal and metacarpophalangeal joints, together with its long counterpart, the extensor pollicis longus muscle. The tendon of the extensor pollicis brevis passes through the same groove on the lateral side of the lower end of the radius as the abductor pollicis longus, to be inserted into the base of the first phalanx of the thumb.
The anatomical snuffbox is an important clinical area as it is the most common spot for the palpation of the radial pulse. The extensor pollicis brevis muscle is closely connected with the abductor pollicis longus, and both muscles extend and abduct the thumb at the carpometacarpal joint.
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Frequently asked questions
The radius is one of the two long bones that make up the forearm, the other being the ulna.
The radius is located in the lateral forearm when in the anatomical position.
Multiple muscles attach to the radius, including the pronator teres, pronator quadratus, supinator, and extrinsic hand muscles.
The radius, along with the ulna, enables pronation and supination of the forearm and hand, and provides surfaces for the attachment of upper limb muscles that control movements of the upper limb.
Yes, fractures of the distal radius, known as Colles fractures, are common and can be caused by falling on an outstretched hand. Radial dysplasia or radial club hand can also occur in paediatric patients as a congenital condition, resulting in a shortening or complete lack of the radius in the forearm.
