Anatomy and Physiology of the Forearm: Bones, Muscles, Vessels, and Nerves

The Forearm

Overview

The forearm is the part of the upper limb between the elbow joint and the wrist joint. It contains two long bones, the radius and ulna, and is divided into anterior (flexor) and posterior (extensor) compartments. The forearm is essential for movements of the wrist, hand, and fingers, and for pronation and supination of the hand.

• Proximal and distal relationships: The forearm connects the elbow to the wrist.

• Compartments: Divided into anterior (flexor) and posterior (extensor) compartments.

• Key functions: Movement of the wrist, hand, and fingers; pronation and supination.

Bones of the Forearm

Radius

The radius is located on the lateral (thumb) side of the forearm. It is involved in both flexion/extension and rotation (pronation/supination) of the forearm.

• Shaft and distal end: Narrow proximally, broad distally. Has three borders (anterior, posterior, interosseous) and three surfaces (anterior, posterior, lateral).

• Anterior border: Begins at the medial side, continues to the radial tuberosity.

• Posterior border: Separates posterior and lateral surfaces.

• Interosseous border: Sharp, for attachment of the interosseous membrane.

• Dorsal tubercle: Prominent on the distal end, acts as a pulley for the extensor pollicis longus tendon.

• Distal features: Styloid process, ulnar notch.

Ulna

The ulna is located on the medial (little finger) side of the forearm. It is longer than the radius and forms the elbow joint proximally.

• Shaft and distal end: Three borders (anterior, posterior, interosseous) and three surfaces (anterior, posterior, medial).

• Distal features: Ulnar styloid process, head of ulna.

Joints of the Forearm

Distal Radio-Ulnar Joint

The distal radio-ulnar joint is a pivot-type synovial joint between the head of the ulna and the ulnar notch of the radius. It allows pronation and supination of the forearm.

• Articular disc: Separates the joint from the wrist joint.

• Ligaments: Anular ligament, interosseous membrane.

Interosseous Membrane

The interosseous membrane is a fibrous sheet connecting the radius and ulna along their interosseous borders. It stabilizes the bones and provides attachment for muscles.

• Function: Transmits forces, separates compartments, provides muscle attachment.

Pronation and Supination

Pronation and supination are rotational movements of the forearm. Pronation turns the palm downward, while supination turns it upward.

• Pronation: Radius crosses over the ulna.

• Supination: Radius and ulna are parallel.

• Key muscles: Pronator teres, pronator quadratus (pronation); biceps brachii, supinator (supination).

Muscles of the Forearm

Anterior Compartment (Flexors)

The anterior compartment contains muscles that flex the wrist and fingers and pronate the forearm. It is organized into superficial, intermediate, and deep layers.

• Superficial layer: Flexor carpi ulnaris, palmaris longus, flexor carpi radialis, pronator teres.

• Intermediate layer: Flexor digitorum superficialis.

• Deep layer: Flexor digitorum profundus, flexor pollicis longus, pronator quadratus.

Muscle	Origin	Insertion	Innervation	Function
Flexor carpi ulnaris	Humeral head: medial epicondyle; Ulnar head: olecranon	Pisiform, hamate, base of 5th metacarpal	Ulnar nerve (C7, C8)	Flexes and adducts wrist
Palmaris longus	Medial epicondyle	Palmar aponeurosis	Median nerve (C7, C8)	Flexes wrist joint
Flexor carpi radialis	Medial epicondyle	Base of 2nd and 3rd metacarpals	Median nerve (C6, C7)	Flexes and abducts wrist
Pronator teres	Medial epicondyle, coronoid process	Lateral surface of radius	Median nerve (C6, C7)	Pronation

Posterior Compartment (Extensors)

The posterior compartment contains muscles that extend the wrist and fingers and supinate the forearm. It is organized into superficial and deep layers.

• Superficial layer: Brachioradialis, extensor carpi radialis longus and brevis, extensor digitorum, extensor digiti minimi, extensor carpi ulnaris, anconeus.

• Deep layer: Supinator, abductor pollicis longus, extensor pollicis brevis and longus, extensor indicis.

Muscle	Origin	Insertion	Innervation	Function
Brachioradialis	Lateral supracondylar ridge	Lateral surface of distal radius	Radial nerve (C5, C6)	Accessory flexor of elbow
Extensor carpi radialis longus	Lateral supracondylar ridge	Base of 2nd metacarpal	Radial nerve (C6, C7)	Extends and abducts wrist
Extensor digitorum	Lateral epicondyle	Extensor expansions of fingers	Posterior interosseous nerve (C7, C8)	Extends fingers
Extensor carpi ulnaris	Lateral epicondyle, ulna	Base of 5th metacarpal	Posterior interosseous nerve (C7, C8)	Extends and adducts wrist

Vessels of the Forearm

Arteries

The forearm is supplied by the radial and ulnar arteries, which branch from the brachial artery.

• Radial artery: Runs along the lateral aspect, supplies lateral forearm and hand.

• Ulnar artery: Runs along the medial aspect, supplies medial forearm and hand.

• Common interosseous artery: Branches into anterior and posterior interosseous arteries, supplying deep structures.

Veins

Deep veins accompany arteries and drain into brachial veins. Superficial veins are subcutaneous and visible.

Nerves of the Forearm

Median Nerve

The median nerve innervates most muscles of the anterior compartment, except the flexor carpi ulnaris and part of the flexor digitorum profundus (ulnar nerve).

• Anterior interosseous nerve: Branches from the median nerve, supplies deep muscles.

• Palmar branch: Supplies skin of the palm.

Ulnar Nerve

The ulnar nerve supplies the flexor carpi ulnaris and the medial half of the flexor digitorum profundus. It also provides sensory branches to the hand.

• Muscular branches: To flexor carpi ulnaris and medial flexor digitorum profundus.

• Palmar and dorsal branches: Sensory to hand.

Radial Nerve

The radial nerve innervates all muscles of the posterior compartment. It divides into superficial (sensory) and deep (motor) branches.

• Deep branch: Motor to extensor muscles.

• Superficial branch: Sensory to dorsum of hand.

Clinical Relevance

• Fractures: Common at the distal radius and ulna; may cause displacement and affect joint function.

• Nerve injuries: Median, ulnar, or radial nerve damage can cause motor and sensory deficits in the forearm and hand.

• Compartment syndrome: Increased pressure in forearm compartments can compromise blood flow and nerve function.

Summary Table: Key Muscles of the Forearm

Muscle	Compartment	Function	Innervation
Flexor carpi ulnaris	Anterior	Flexes/adducts wrist	Ulnar nerve
Flexor carpi radialis	Anterior	Flexes/abducts wrist	Median nerve
Pronator teres	Anterior	Pronation	Median nerve
Extensor carpi radialis longus	Posterior	Extends/abducts wrist	Radial nerve
Extensor digitorum	Posterior	Extends fingers	Radial nerve
Supinator	Posterior	Supination	Radial nerve

Key Equations and Concepts

• Force transmission: The interosseous membrane transmits force from the radius to the ulna during weight-bearing activities.

• Pronation/Supination axis: Rotation occurs around the longitudinal axis of the forearm.

$\text{Pronation/Supination Angle} = \theta$

Additional info:

Some anatomical details and clinical correlations have been expanded for clarity and completeness.