Radius is a long bone, a bone with a shaft and expanded ends, on the thumb side of the forearm. It lies beside the ulna between the elbow and wrist, contributes to both joints, and rotates around the ulna to change the direction the palm faces.
Where is the radius in the forearm?
In anatomical position—the body upright with the palms facing forwards—the radius is lateral to the ulna, meaning farther from the body's midline. The left radius and right radius occupy corresponding positions in their respective forearms. Each radius has a relatively small upper end near the elbow and a broad lower end near the wrist.
The humerus, the bone of the upper arm, lies above the radius. The ulna is the other forearm bone and lies on the little-finger side in anatomical position. The carpal bones, the small bones forming the wrist, lie below the radius.
Its shaft is separated from the ulnar shaft by a space crossed by the interosseous membrane, a strong sheet of connective tissue joining the bones.
What are the main parts of the radius?
The proximal end is the end nearer the trunk; the distal end is farther from the trunk. Between them lies the shaft, the elongated central portion. The radius widens towards its distal end, which supports the wrist.
| Region | Main landmarks | Anatomical role |
|---|---|---|
| Proximal end | Head, neck and radial tuberosity | Meets the humerus and ulna and receives the biceps tendon |
| Shaft | Anterior, posterior and lateral surfaces; interosseous border | Provides muscle attachments and anchors the interosseous membrane |
| Distal end | Ulnar notch, radial styloid process and dorsal tubercle | Meets the ulna and wrist bones and guides nearby tendons |
The shaft has anterior, or front-facing, posterior, or back-facing, and lateral surfaces. Its interosseous border is the sharp edge facing the ulna, where the interosseous membrane attaches. The shaft's gentle curve helps accommodate forearm rotation.
What landmarks are found at the elbow end?
The radial head is the disc-shaped proximal end. Its shallow upper depression meets the capitulum, the rounded surface on the lateral side of the lower humerus. The smooth circumference of the head meets the radial notch, a concave surface on the ulna.
The radial neck is the narrowed region immediately below the head. Below the neck, the radial tuberosity is a rough prominence on the medial side, towards the body's midline. It receives the tendon of biceps brachii, the muscle on the front of the upper arm; a tendon is the strong connective tissue that transmits a muscle's pull.
The annular ligament surrounds the radial head and holds it against the radial notch of the ulna. A ligament is a strong connective-tissue band that stabilises a joint. This arrangement retains the head while allowing it to turn.
What landmarks are found at the wrist end?
The distal radius has a broad lower surface that meets the scaphoid and lunate, carpal bones on the thumb side and towards the centre of the wrist. These contact surfaces are covered by articular cartilage, the smooth tissue covering bone ends within a joint.
The ulnar notch lies on the medial side of the distal radius and receives the head of the ulna. The radial styloid process is the pointed projection on the lateral side. The dorsal tubercle is a prominence on the back of the distal radius.
Grooves on the posterior distal radius guide tendons travelling towards the hand. The tendon of extensor pollicis longus, a muscle that straightens the thumb, changes direction around the dorsal tubercle.
Which joints does the radius form?
The four joints listed below are synovial joints: joints with a fluid-containing cavity between cartilage-covered articular surfaces. The interosseous membrane also forms a fibrous joint, or syndesmosis, between the shafts of the radius and ulna, often called the middle radioulnar joint.
- Humeroradial joint: the radial head meets the humeral capitulum at the elbow, contributing to bending and straightening.
- Proximal radioulnar joint: the radial head turns against the radial notch of the ulna within the annular ligament.
- Distal radioulnar joint: the ulnar notch of the radius moves around the head of the ulna during forearm rotation.
- Radiocarpal joint: the distal radius meets the scaphoid and lunate at the wrist. An articular disc, a plate of fibrous cartilage, separates the distal ulna from the carpal bones.
What does the radius do, and which muscles attach to it?
Pronation turns the palm backwards from anatomical position; supination turns it forwards again. During pronation, the radius crosses in front of the ulna. During supination, the bones return to a parallel arrangement.
Several muscles pull on the radius to produce these movements or bend the elbow:
- Biceps brachii attaches to the radial tuberosity and contributes to supination and elbow bending.
- Supinator attaches around the proximal radius and produces supination.
- Pronator teres attaches to the lateral mid-shaft and produces pronation.
- Pronator quadratus attaches to the anterior distal radius and produces pronation.
- Brachioradialis attaches near the radial styloid process and bends the elbow.
The radius also receives forces from the hand at the wrist. The interosseous membrane helps distribute these forces between the radius and ulna.
How can you find the radius in the 3D model?
In the 3D model next to this article, select a radius, isolate it, then rotate and zoom to view it from different directions. You can also hide or fade other structures and switch layers to adjust the view while keeping your selected structure isolated.
To explore the model in another way, cut a section along the sagittal, coronal or transverse plane. You can also pull parts apart, then rotate and zoom to view the selected structures from a different direction.