Muscles
Superficial Muscles
Four of these muscles (extensor carpi radialis brevis (ECRB), extensor digitorum communis (EDC), extensor digiti minimi (EDM), and extensor carpi ulnaris (ECU)) arise from the common extensor origin at the lateral epicondyle of the humerus.
Brachioradialis (BR)
Origin: Proximal two-thirds of lateral supracondylar ridge of the humerus
Insertion: Base of the radial styloid
Nerve Supply: Radial nerve (C5, 6)
Action: Flexion of the elbow wrist. Most powerful when the forearm is semipronated
Related Structures: Brachioradialis forms the lateral border of the cubital fossa. The radial nerve and radial artery lie deep to the muscle and its tendon. Distally, the tendon lies deep to abductor pollicis longus and extensor pollicis brevis.
Extensor carpi radialis longus (ECRL)
Origin: Distal third of the lateral supracondylar ridge of the humerus
Insertion: Base of the second metacarpal
Nerve Supply: Radial nerve (C6, 7)
Action: Extension and abduction of the wrist. Assists in flexion of the elbow, and as a synergist during finger flexion
Related Structures: Similar to brachioradialis, distally it lies deep to abductor pollicis longus and extensor pollicis longus/brevis
Extensor carpi radialis brevis (ECRB)
Origin: Common extensor origin (lateral epicondyle of humerus)
Insertion: Base of third metacarpal
Nerve Supply: Branch from posterior interosseous nerve (C7, 8)
Action: Extension and abduction of the wrist
Related Structures: Tendon distally lies deep to abductor pollicis longus and extensor pollicis longus/brevis.
Extensor carpi ulnaris (ECU)
Origin: Common extensor origin (lateral epicondyle of humerus)
Insertion: Base of the fifth metacarpal
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extension and adduction of the wrist
Related Structures: Tendon lies adjacent to the ulnar styloid as it passes deep to the extensor retinaculum
Extensor digiti minimi (EDM)
Origin: Common extensor origin (lateral epicondyle of humerus)
Insertion: Extensor expansion of the fifth digit
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extension of the 5th digit and wrist
Related Structures: Arises in common with extensor digitorum
Extensor digitorum communis (EDC)
Origin: Common extensor origin (lateral epicondyle of humerus)
Insertion: Extensor expansions of medial four digits
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extension of the medial four digits at the metacarpophalangeal joints, extension of the wrist
Related Structures: Four tendons pass together deep to the extensor retinaculum, superficial to the tendon of extensor indicis
Anconeus
Origin: Posterior surface of the lateral epicondyle of the humerus
Insertion: Lateral surface of olecranon and superior part of posterior surface of ulna
Nerve Supply: Radial nerve (C7, 8)
Action: Assists triceps in elbow extension and abducts ulna during pronation
Related Structures: Lies superficial to supinator at the proximal part of the posterior compartment of the forearm
Deep Muscles
Supinator
Origin: Lateral epicondyle of humerus, radial collateral and annular ligaments, supinator crest and fossa of ulna
Insertion: Lateral surface of the radius
Nerve Supply: Posterior interosseous nerve (C6, 7)
Action: Supination of the forearm; note biceps brachii is the prime mover unless the elbow is completely extended
Related Structures: The posterior interosseous nerve passes between the superficial and deep layers of supinator as is exits the cubital fossa to enter the posterior forearm
Abductor pollicis longus (APL)
Origin: Posterior surfaces of radius, ulna and interosseous membrane
Insertion: Base of first metacarpal and trapezium
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extension of the thumb at the carpometacarpal joint. It may assist in abduction and flexion of the wrist
Related Structures: The posterior interosseous nerve and artery pass over the muscle belly superficially after exiting supinator
Extensor pollicis longus (EPL)
Origin: Posterior surface of middle third of ulna and interosseous membrane
Insertion: Base of distal phalanx of thumb
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extends the terminal phalanx of the thumb, assists in extension and abduction of the wrist
Related Structures: Long tendon passes around Lister’s tubercule and forms the ulnar boundary of the anatomical snuffbox
Extensor pollicis brevis (EPB)
Origin: Posterior surface of radius and interosseous membrane
Insertion: Base of proximal phalanx of thumb
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extends the carpometacarpal and metacarpophalangeal joints of the thumb. Prevents flexion of the metacarpophalangeal joint when terminal phalanx is flexed
Related Structures: The tendon forms the radial boundary of the anatomical snuffbox
Extensor indicis proprius (EIP)
Origin: Posterior surface of the ulna
Insertion: Extensor expansion of the index finger
Nerve Supply: Posterior interosseous nerve (C7, 8)
Action: Extension of the index finger
Related Structures: Runs deep to and shares a synovial sheath with extensor digitorum
Anatomical snuffbox
- The anatomical snuffbox lies between the EPL tendon on the medial side and the EPB + APL tendons on the lateral side.
- The cutaneous branches of the radial nerve cross these tendons superficially.
- The cephalic vein originates at the roof of the snuffbox, while the radial artery is palpable as it runs on its floor
- Bony prominences present within the snuffbox include the radial styloid, scaphoid, trapezium, and base of the thumb metacarpal from proximal to distal
Nerves
Posterior interosseous nerve
Course: Enters the posterior compartment after branching off the radial nerve by passing between the superficial and deep parts of the supinator muscle. Passes superficial to abductor pollicis longus at its origin before passing superficial to the interosseous membrane and deep to the deep muscles of the extensor compartment up to the wrist joint.
Innervation: Supinator, Extensor digitorum, Extensor carpi ulnaris, Extensor carpi radialis brevis, Extensor pollicis longus, Extensor pollicis brevis, Extensor digiti minimi, Extensor indicis, Abductor pollicis longus
Vessels
Posterior interosseous artery
Course: Enters the extensor compartment by passing between the radius and ulna proximal to the interosseous membrane and distal to the oblique cord of the forearm. It then follows the course of the posterior interosseous nerve to supply muscles of the posterior forearm.
Anterior interosseous artery
Course: Enters the extensor compartment by piercing the interosseous membrane proximal to the proximal edge of pronator quadratus. It then enters the wrist and forms the dorsal carpal arch.
References
Sinnatamby, Chummy S. (2011). Last’s anatomy : regional and applied (12th). New York: Churchill Livingstone/Elsevier.
Authors
Fergus Scott, medical student MD2, Western Health 2024
Matthew Sun, orthopaedic surgical HMO, Western Health 2024