Radius

1. Overview

The radius is one of the two long bones of the forearm, the other being the ulna. Located on the lateral (thumb) side of the forearm, the radius plays a critical role in forearm rotation and wrist articulation. It is essential for pronation and supination, allowing the hand to rotate palm-up or palm-down. The radius also contributes to load-bearing and mobility of the upper limb.

2. Location

The radius is positioned laterally in the forearm, parallel to the ulna:

• Proximally: Articulates with the capitulum of the humerus and the radial notch of the ulna at the elbow joint.

• Distally: Articulates with the carpal bones (scaphoid and lunate) and the ulnar head at the wrist joint.

• Lateral to: The ulna in anatomical position (thumb side).

3. Structure

The radius is a long bone with three key regions:

• Proximal end:

  • Head: Disc-shaped; articulates with the humerus and ulna.

  • Neck: Narrow region below the head.

  • Radial tuberosity: A rough prominence below the neck; insertion site for the biceps brachii tendon.

• Shaft (body):

  • Triangular in cross-section with anterior, posterior, and lateral surfaces.

  • Interosseous border on the medial aspect provides attachment for the interosseous membrane, connecting to the ulna.

• Distal end:

  • Broad and flat.

  • Styloid process: Projects laterally and can be palpated near the wrist.

  • Ulnar notch: Articulates with the ulna.

  • Carpal articular surface: Articulates with the scaphoid and lunate bones of the wrist.

4. Function

The radius serves several critical functions in the upper limb:

• Forearm rotation: Rotates over the ulna during pronation and supination, enabling turning of the palm.

• Elbow joint motion: Participates in flexion and extension by articulating with the humerus.

• Wrist articulation: Transmits forces between the hand and forearm; forms the major connection between forearm and carpal bones.

• Muscle attachment: Provides insertion points for muscles such as biceps brachii, supinator, pronator teres, and others.

5. Physiological role(s)

While passive structurally, the radius contributes to various physiological and functional roles:

• Force transmission: Bears and transmits load from the hand to the elbow, especially during weight-bearing or impact activities.

• Fine motor control: Enables wrist mobility necessary for gripping, writing, and tool manipulation.

• Proprioception: Contains sensory receptors that provide feedback about limb position and movement.

• Joint alignment: Helps maintain congruent positioning of the wrist and elbow joints through ligament and muscle interactions.

6. Clinical Significance

The radius is involved in many orthopedic and sports-related injuries:

• Colles’ fracture:

  • Fracture of the distal radius with dorsal displacement; common in falls on an outstretched hand (FOOSH), especially in the elderly.

• Smith’s fracture:

  • Fracture of the distal radius with volar (palmar) displacement; typically from a fall onto a flexed wrist.

• Galeazzi fracture-dislocation:

  • Fracture of the distal third of the radius with dislocation of the distal radioulnar joint.

• Radial head fractures:

  • Common in elbow trauma; may limit pronation and supination and require surgical fixation depending on severity.

• Osteomyelitis or tumors:

  • Infectious or neoplastic conditions may affect the radius, though less common than trauma-related issues.

• Growth plate injuries:

  • In children, distal radial growth plate (physis) injuries must be monitored to prevent future deformity.