Radius and Ulna: Anatomy, Joints, and Clinical Significance

Overview

The radius and ulna are the paired bones of the forearm, extending from the elbow to the wrist. They articulate at both the proximal and distal radioulnar joints, allowing the unique forearm movements of pronation and supination — rotation of the forearm around the longitudinal axis formed by the two radioulnar joints. This allows the palm to face upward (supination) or downward (pronation) without requiring shoulder rotation.

The Ulna

Key Features

• Located on the medial (pinky) side of the forearm
• Provides the primary articulation with the humerus at the elbow
• Olecranon process: posterior projection forming the tip of the elbow; insertion of triceps brachii
• Trochlear notch (semilunar notch): articulates with the trochlea of the humerus; shaped like a wrench
• Coronoid process: anterior projection below the trochlear notch; resists posterior displacement; insertion of brachialis
• Radial notch: on the lateral side of the coronoid process; articulates with the head of the radius (proximal radioulnar joint)
• Head of ulna: distal end; articulates with the ulnar notch of the radius (distal radioulnar joint) and the articular disc
• Styloid process of ulna: projects posteromedially; palpable at the wrist

Shaft

• Has three borders and three surfaces; interosseous border (lateral) gives attachment to interosseous membrane
• Subcutaneous (dorsal) border is palpable throughout its length

The Radius

Key Features

• Located on the lateral (thumb) side of the forearm
• Head of radius: disc-shaped; articulates with the capitulum of the humerus (radiohumeral joint) and the radial notch of the ulna (proximal radioulnar joint)
• Radial tuberosity: just distal to neck; insertion of biceps brachii
• Styloid process of radius: extends ~1 cm further distally than ulnar styloid; prevents excessive ulnar deviation at wrist
• Ulnar notch of radius: distal end; articulates with head of ulna (distal radioulnar joint)
• Carpal articular surface: articulates with scaphoid and lunate (proximal carpal row) — the radius bears 60% of wrist force, ulna 40% via the articular disc

Shaft

• Curved laterally (pronator quadratus flattens this bow during pronation)
• Interosseous border (medial) attaches to interosseous membrane

Interosseous Membrane

• A strong fibrous sheet connecting the interosseous borders of radius and ulna
• Fiber direction: generally from radius distally to ulna (transfers force from distal radius to proximal ulna → to humerus)
• Contains the central interosseous band (strongest portion)
• Divides forearm into anterior (flexor-pronator) and posterior (extensor-supinator) compartments

Proximal Radioulnar Joint (Elbow Level)

• Head of radius articulates in the radial notch of ulna
• Held by the annular ligament (encircles the radial head)
• Movements: pivoting of radial head during supination/pronation

Distal Radioulnar Joint (Wrist Level)

• Head of ulna articulates in the ulnar notch of radius
• Triangular fibrocartilage complex (TFCC): articular disc + meniscus homologue + ligaments; cushions and stabilizes the joint; frequently injured in wrist trauma

Pronation and Supination

• Supination (RULE: Biceps is the SUPER man): mainly by supinator and biceps brachii; rotates radius from crossed (pronated) to parallel (supinated) position over ulna
• Pronation: pronator teres (at elbow) + pronator quadratus (at wrist); rotates radius over the fixed ulna

During pronation, the radius crosses the ulna distally while the radial head remains in the proximal radioulnar joint.

Clinical Relevance

• Colles fracture: most common fracture in adults >50; fall on outstretched hand; extra-articular distal radius fracture with dorsal displacement → "dinner fork" deformity
• Smith fracture: reverse Colles; volar displacement of distal radius; fall on flexed wrist
• Monteggia fracture: fracture of proximal ulna + dislocation of radial head (remember: Monteggia = Ulnar fracture)
• Galeazzi fracture: fracture of distal radius + dislocation of distal radioulnar joint (remember: Galeazzi = Radius fracture, DRUJ dislocation)
• Pulled elbow (Nursemaid's elbow): subluxation of radial head from annular ligament; toddlers; caused by longitudinal traction on arm
• Essex-Lopresti injury: radial head fracture + disruption of interosseous membrane + DRUJ dislocation; longitudinal forearm instability

Study Points

• Identify the key bony landmarks of each bone and their muscle/ligament attachments.
• Explain the mechanism of forearm pronation and supination at the radioulnar joints.
• Describe the interosseous membrane and explain how it transmits forces in the forearm.
• Distinguish Monteggia from Galeazzi fracture-dislocations by mechanism and components.
• Explain why a radial head fracture might affect wrist function (Essex-Lopresti).