Origin and Insertion
- Origin (O): Posterior surface of the ulna (ulna) and the interosseous membrane (membrana interossea). The origin site lies distal to the abductor pollicis longus muscle (m. abductor pollicis longus).
- Insertion (I): Base of the distal phalanx of the thumb.
Topography and Course of the Tendon
The muscle is located in the deep layer of the posterior forearm. Its tendon path features several important topographical characteristics:
- The tendon wraps around the dorsal tubercle of the radius (Lister's tubercle).
- It enters the hand through the 3rd compartment beneath the extensor retinaculum (retinaculum musculorum extensorum). Within this compartment, it is enclosed in its own synovial sheath (vagina tendinis m. extensoris pollicis longi).
- Emerging from the compartment, the tendon courses toward the dorsum of the thumb, turning sharply in a lateral direction.
- Crossing: On its way to the thumb, the tendon crosses superficial to the tendons of the extensor carpi radialis longus and brevis muscles (mm. extensores carpi radialis longus et brevis).
Function
- Extends the distal phalanx of the thumb.
- Assists in extending the first metacarpophalangeal and carpometacarpal joints of the thumb.
- Participates in extension of the wrist.
Blood Supply and Innervation
- Innervation: Supplied by branches of the radial nerve — n. radialis (spinal segments C7–C8).
- Blood supply: The muscle receives blood from the posterior interosseous artery (a. interossea posterior) and the radial artery (a. radialis).
Role in the Anatomical Snuffbox
The tendon of the m. extensor pollicis longus is a critical landmark on the radial side of the wrist. It forms the ulnar (medial) boundary of the anatomical snuffbox — a triangular depression visible when the thumb is strongly abducted.
The radial (lateral) boundary of this space is formed by the tendons of the m. abductor pollicis longus and m. extensor pollicis brevis. The floor of the snuffbox contains the radial artery (a. radialis), which then passes directly deep to the tendon of the extensor pollicis longus as it travels through the first interosseous space into the palm.
Differential Diagnosis: Extensor Hallucis Longus
In the lower limb, there is a similarly named muscle — the extensor hallucis longus (m. extensor hallucis longus). It originates from the anterior surface of the fibula, inserts into the distal phalanx of the great toe, extends it, and assists in dorsiflexion and supination of the foot. It is innervated by the deep fibular nerve (L4–S1).