Extensor Pollicis Longus

The extensor pollicis longus (EPL) is a deep extensor muscle of the posterior forearm. It is responsible for extending the thumb at the interphalangeal, metacarpophalangeal, and carpometacarpal joints. This muscle plays a crucial role in thumb mobility, enabling precise movements required for pinching, grasping, and manipulating objects.

Location

The EPL originates from the mid-posterior surface of the ulna and interosseous membrane. It travels obliquely across the wrist, passing medial to Lister's tubercle of the radius, and inserts into the distal phalanx of the thumb. It forms the medial boundary of the anatomical snuffbox.

Structure

The extensor pollicis longus has a fusiform muscle belly and a long tendon that changes direction as it passes the dorsal wrist.

• Origin: Posterior surface of the middle third of the ulna and adjacent interosseous membrane.
• Insertion: Dorsal base of the distal phalanx of the thumb.
• Tendon Path: Passes through the third dorsal compartment of the extensor retinaculum and changes direction around Lister's tubercle (dorsal radial tubercle), acting as a pulley.

Function

The extensor pollicis longus performs:

• Extension of the Thumb: Extends the distal phalanx at the interphalangeal joint.
• Metacarpophalangeal and Carpometacarpal Extension: Also assists in extending these joints of the thumb.
• Wrist Movement: Aids in weak extension and radial abduction of the wrist when acting with other muscles.

Innervation

The EPL is innervated by the posterior interosseous nerve (C7–C8), the terminal continuation of the deep branch of the radial nerve.

Blood Supply

• Posterior interosseous artery: Primary blood supply.

Relations

The EPL is located deep to the extensor digitorum proximally. As the tendon passes the wrist, it crosses obliquely over the dorsal radius and becomes superficial. It forms the medial boundary of the anatomical snuffbox, with the extensor pollicis brevis and abductor pollicis longus forming the lateral boundary. The radial artery passes deep within this snuffbox.

Surface Anatomy

The tendon of the EPL is visible and palpable when the thumb is extended. It becomes prominent along the dorsum of the hand and forms the distinct medial border of the anatomical snuffbox, especially during active thumb extension.

Development

The extensor pollicis longus develops from the deep extensor muscle group of the dorsal forearm during limb bud development. Its innervation by the radial nerve reflects its dorsal origin.

Anatomical Variations

• Variations in tendon thickness and insertion may occur.
• Accessory slips or interconnections with the extensor pollicis brevis or abductor pollicis longus are occasionally present.

Clinical Significance

• Tendon Rupture: The EPL tendon is vulnerable to rupture, especially after distal radius fractures or repetitive strain. The rupture may be spontaneous or associated with conditions like rheumatoid arthritis.
• Surgical Repair or Grafting: In cases of rupture, tendon transfers (e.g., from extensor indicis) may be used to restore thumb extension.
• Anatomical Snuffbox Tenderness: Tenderness along the EPL tendon may indicate pathology in the underlying scaphoid or soft tissue inflammation.

Comparative Anatomy

In many non-human primates, thumb movement is less developed and lacks the degree of extension enabled by the EPL. In humans, this muscle has evolved to enable independent and precise extension of the thumb, essential for fine motor skills such as writing and tool use.