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Radius

Medically Reviewed by Anatomy Team

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The radius is one of the two long bones located in the forearm, alongside the ulna. It is located on the lateral (thumb) side of the forearm, in contrast to the ulna, which is situated on the medial (pinky) side. The radius plays a key role in the movement of the forearm and wrist. It is slightly shorter than the ulna but is more involved in the movement of the wrist, particularly during actions such as rotation of the forearm (pronation and supination).

Location

The radius is located on the lateral side of the forearm, extending from the elbow to the wrist. It articulates with the humerus at the elbow joint and with the ulna and carpal bones at the wrist.[6] The head of the radius is near the elbow, and it gradually tapers toward the wrist, where it forms part of the wrist joint.

Structure and Anatomy

The radius is a long bone in the forearm, located on the lateral side (thumb side) of the body. It works closely with the ulna to allow for the complex movements of the wrist, forearm, and elbow. Here is a detailed breakdown of the anatomy and structure of the radius:

Proximal End of the Radius

The proximal end of the radius is located near the elbow, where it forms important articulations with the humerus and the ulna.

Head of the Radius

  • The head is a circular, disc-shaped structure that articulates with the capitulum of the humerus at the elbow joint. It allows for the rotation of the forearm, enabling movements such as pronation and supination.
  • The superior surface of the head also articulates with the radial notch of the ulna, forming the proximal radioulnar joint.

Neck of the Radius

The neck is a narrow section just below the head of the radius, acting as a transition between the head and the shaft. It is often a site of injury or fracture, especially in cases of trauma.

Radial Tuberosity

Located just below the neck, on the medial side of the radius, the radial tuberosity is a roughened area that serves as the attachment point for the biceps brachii tendon.[4] This attachment is important for flexion of the elbow and supination of the forearm.

Shaft of the Radius

The shaft of the radius is the long, cylindrical portion of the bone that extends between the proximal and distal ends. The shaft has several important features:

Anterior Surface

The anterior surface of the radius is smooth and provides attachment points for muscles that control wrist and forearm movements.

Posterior Surface

The posterior surface is more pronounced and offers attachments for muscles such as the extensor muscles of the forearm, which contribute to wrist extension and finger movement.

Medial and Lateral Borders

The lateral border is more prominent and serves as the attachment point for ligaments and tendons, contributing to the stability of the distal radioulnar joint. The medial border is less pronounced and contributes to the smooth transition toward the distal end of the radius.

Interosseous Border

Along the length of the radius, on the medial side, there is an interosseous border, which faces the ulna. This border provides attachment for the interosseous membrane — a dense fibrous tissue structure that connects the radius and ulna along their lengths, helping to stabilize the bones.

Distal End of the Radius

The distal end of the radius is located near the wrist and plays a major role in the formation of the wrist joint.[1]

Styloid Process of the Radius

The styloid process is a prominent, cone-shaped bony projection located at the distal end of the radius, on the lateral side of the wrist. It serves as an attachment point for the radial collateral ligament and provides stability to the wrist joint.

Distal Articular Surface

The distal end of the radius features a broad, flat articular surface that articulates with the scaphoid and lunate bones of the wrist. This articulation is critical for wrist motion and contributes to flexion, extension, and other movements of the wrist.

Ulnar Notch

On the medial side of the distal radius, the ulnar notch is a depression that accommodates the head of the ulna. It forms part of the distal radioulnar joint, which allows for the rotation of the radius and ulna during pronation and supination of the forearm.

Articulations

The radius forms several important articulations with neighboring bones:

  • Proximal Articulation:
    The head of the radius articulates with the capitulum of the humerus at the elbow joint, allowing for the rotation of the forearm. It also articulates with the radial notch of the ulna, forming the proximal radioulnar joint.
  • Distal Articulation:
    The distal radius articulates with the scaphoid and lunate bones of the wrist, forming the radiocarpal joint.[8] Additionally, the ulnar notch on the distal radius forms the distal radioulnar joint with the head of the ulna.

Muscle Attachments

The radius serves as an attachment point for numerous muscles involved in wrist, forearm, and hand movements:

  • Biceps Brachii: The biceps tendon attaches to the radial tuberosity for elbow flexion and supination.
  • Forearm Flexors: Muscles like the flexor carpi radialis and flexor pollicis longus attach to the radius, contributing to wrist and finger flexion.
  • Forearm Extensors: Muscles such as the extensor carpi radialis longus and extensor carpi radialis brevis attach to the radius for wrist extension.
  • Pronators and Supinators: Muscles like the pronator teres and supinator muscles attach to the radius, facilitating pronation and supination movements of the forearm.

Vascular and Nerve Supply

The radius is supplied by several arteries and nerves:

  • Radial Artery: The radial artery runs along the lateral side of the forearm, supplying blood to the radius, hand, and forearm muscles.
  • Nerve Supply: The radial nerve, which runs along the posterior surface of the radius, innervates muscles responsible for extension at the wrist and elbow, as well as sensation in the lateral hand.

Bone Composition

Like other long bones, the radius is composed of:

  • Compact Bone: Dense outer layer, providing strength and support.
  • Spongy Bone (Cancellous Bone): Found primarily at the ends (proximal and distal ends), housing bone marrow and providing structural support while reducing the bone’s weight.

Function of the Radius

The radius is a vital bone in the forearm that plays a critical role in the movement, stability, and strength of the upper limb.

Facilitating Pronation and Supination of the Forearm

  • Pronation: The radius allows the forearm to rotate so that the palm of the hand faces downward. During this movement, the radius crosses over the ulna at the wrist.
  • Supination: The radius also enables the opposite movement, where the palm faces upward and the radius returns to a parallel position with the ulna.[7]

These movements are essential for everyday actions like turning a doorknob, using a screwdriver, or performing fine motor tasks.

Contributing to Wrist and Hand Movements

The radius plays a key role in the mobility of the wrist and hand, allowing for a wide range of movements.

  • Wrist Flexion and Extension: The radius is involved in wrist flexion (bending the wrist forward) and extension (straightening the wrist), movements controlled by muscles like the flexor carpi radialis and extensor carpi radialis.
  • Radial and Ulnar Deviation: The radius facilitates radial deviation (movement of the wrist toward the thumb) and ulnar deviation (movement toward the pinky), which are important for various hand functions such as gripping and fine motor activities.

Transmitting Forces from the Hand to the Elbow

The radius helps transmit mechanical forces from the hand through to the elbow. This function is especially important during weight-bearing activities, such as pushing or lifting. The radius assists in evenly distributing these forces, maintaining upper limb functionality.

Providing Muscle Attachments for Forearm and Hand Movement

The radius serves as a central attachment point for muscles involved in forearm, wrist, and hand movements:

  • Biceps Brachii: The biceps tendon attaches to the radial tuberosity of the radius for flexion of the elbow and supination of the forearm.[5]
  • Forearm Flexors: Muscles like the flexor carpi radialis and flexor pollicis longus attach to the radius, allowing for wrist and finger flexion.
  • Forearm Extensors: Muscles like the extensor carpi radialis longus and extensor carpi radialis brevis attach to the radius, enabling wrist extension.
  • Supinators and Pronators: The supinator and pronator teres muscles attach to the radius, helping with forearm rotation.

Providing Structural Support for the Forearm

The radius provides the necessary structural support for the forearm and the wrist joint. It works with the ulna to stabilize the forearm during various activities, ensuring the proper functioning of both joints in tandem.

Clinical significance

The clinical significance of the radius is most evident in its vulnerability to fractures, particularly in the distal part. Colles’ fracture, a common injury resulting from a fall onto an outstretched hand, is a fracture of the distal radius that can lead to deformities like the “dinner fork” shape. It often causes pain, swelling, and a limited range of motion in the wrist.[3] If not properly treated, such fractures may result in long-term complications, including decreased grip strength and impaired wrist function. Similarly, Smith’s fractures, where the distal radius fractures in the opposite direction, can cause significant wrist instability and require surgical intervention.

The radius is also crucial in the forearm’s rotational movements (pronation and supination), so injuries like distal radioulnar joint dislocations or fractures along the radial shaft can impair these movements. In cases of compartment syndrome, which may result from severe trauma or fractures, pressure builds up in the forearm, risking permanent damage to muscles, nerves, and the radius itself if not treated promptly. Additionally, degenerative conditions such as osteoarthritis may affect the wrist joint, particularly at the distal radius, leading to chronic pain and limited functionality.[2] Therefore, proper diagnosis and treatment of radius-related injuries are essential for maintaining upper limb functionality.

References

  1. Marieb, E. N., & Wilhelm, P. B. (2019). Human Anatomy (9th ed.). Pearson. ISBN 9780135168059.
  2. Drake, R. L., Vogl, W., & Mitchell, A. W. M. (2020). Gray’s Anatomy for Students (4th ed.). Elsevier. ISBN 9780323393041.
  3. Palastanga, N., & Soames, R. (2018). Anatomy and Human Movement: Structure and Function (7th ed.). Elsevier Health Sciences. ISBN 9780702072260.
  4. Flatt, A. E. (2007). The Care of the Skeletal System (2nd ed.). Charles C Thomas. ISBN 9780398077428.
  5. Martini, F. H., Nath, J. L., & Bartholomew, E. F. (2017). Fundamentals of Anatomy & Physiology (10th ed.). Pearson. ISBN 9780321909077.
  6. Sinnatamby, C. S. (2020). Last’s Anatomy: Regional and Applied (13th ed.). Elsevier. ISBN 9780702077050.
  7. Gilroy, A. M., MacPherson, B. R., & Ross, L. M. (2021). Atlas of Anatomy (4th ed.). Thieme. ISBN 9781684202034.
  8. McMinn, R. M. H., Hutchings, R. T., & Abrahams, P. H. (2013). McMinn and Abrahams’ Clinical Atlas of Human Anatomy (7th ed.). Elsevier. ISBN 9780702051319.