Abductor Pollicis Longus

The abductor pollicis longus (APL) is a deep muscle in the posterior compartment of the forearm. It is primarily responsible for abduction of the thumb at the carpometacarpal joint and plays a key role in thumb positioning for gripping and precision movements. It often shares a close anatomical relationship with the extensor pollicis brevis, with which it passes through the first dorsal compartment of the wrist.

Location

The APL is located in the deep layer of the posterior forearm, lying lateral to the extensor pollicis longus and often partially overlapping the extensor pollicis brevis. Its tendon travels obliquely from the forearm to the base of the first metacarpal, forming part of the lateral border of the anatomical snuffbox.

Structure

The muscle has a long, fleshy belly that gives rise to a strong tendon. It frequently divides into multiple slips before inserting.

• Origin: Posterior surfaces of the middle third of the radius and ulna, and the adjacent interosseous membrane.
• Insertion: Lateral side of the base of the first metacarpal; may also send slips to the trapezium or abductor pollicis brevis.
• Tendon Path: Passes under the extensor retinaculum through the first dorsal compartment, along with the extensor pollicis brevis tendon.

Function

The abductor pollicis longus contributes to:

• Thumb Abduction: Abducts the thumb at the carpometacarpal joint, moving it away from the palm in the sagittal plane.
• Thumb Extension Assistance: Aids in extension at the carpometacarpal joint.
• Radial Deviation: Assists in abduction of the wrist (radial deviation) when acting with extensor carpi radialis longus and brevis.

Innervation

Innervated by the posterior interosseous nerve (C7–C8), a branch of the radial nerve.

Blood Supply

• Posterior interosseous artery: The main blood supply.

Relations

The APL lies deep to the extensor digitorum and superficial to the radius. Its tendon is lateral to that of the extensor pollicis brevis at the wrist, and both together define the lateral boundary of the anatomical snuffbox. The radial artery passes deep within this space.

Surface Anatomy

The APL tendon becomes visible on the dorsolateral wrist during thumb abduction or resistance. It contributes to the prominence on the lateral side of the anatomical snuffbox.

Development

The APL arises from the dorsal muscle mass of the forearm during embryonic development. Like other extensors and abductors of the thumb, it is innervated by the radial nerve derivatives.

Anatomical Variations

• Frequently has two or more tendinous slips inserting into various locations on the base of the first metacarpal or trapezium.
• May fuse partially with the extensor pollicis brevis.
• Accessory tendons are common and may complicate surgical decompression in De Quervain’s syndrome.

Clinical Significance

• De Quervain’s Tenosynovitis: Inflammation of the synovial sheath enclosing the APL and EPB tendons leads to pain on the radial side of the wrist, especially with thumb movement or gripping.
• Finkelstein's Test: A clinical maneuver used to diagnose De Quervain’s involves ulnar deviation of the wrist with the thumb grasped in a fist, stretching the APL and EPB tendons.
• Tendon Entrapment: The APL tendon may become entrapped or thickened, contributing to restricted thumb motion or pain.

Comparative Anatomy

In many quadrupeds, the APL functions as an abductor of the first digit or pollex but lacks the refined role seen in humans. In humans, it has evolved to support the high degree of thumb mobility necessary for opposable grip and tool use.