Abductor pollicis longus muscle

Origin and insertion

Abductor pollicis longus originates from the posterior surface of the proximal half of the radius, ulna and intervening interosseous membrane. The site of origin is located just inferior to the insertion of the anconeus muscle. From here, the muscle courses inferolaterally towards the radial side of the hand, becoming more superficial as it enters the distal third of the forearm.

Just proximal to the wrist, the muscle gives off a narrow tendon. This tendon passes through a groove carved on the lateral surface of the distal end of the radius, where it is joined by the tendon of extensor pollicis brevis muscle.

Abductor pollicis longus tendon then passes beneath the extensor retinaculum. In doing so, it traverses the first dorsal (extensor) compartment of the retinacular space. Once inside the dorsum of the hand, the tendon splits into two slips that insert onto the base of first metacarpal and trapezium bones, respectively. Some fibers may also insert into surrounding structures; opponens pollicis, abductor pollicis brevis and the fascia of thenar eminence.

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Relations

The muscle belly of abductor pollicis longus lies in the distal half of the posterior forearm. It is found deep to extensor digitorum and lateral to extensor pollicis longus muscle, while its tendon is located lateral to the tendon of extensor pollicis brevis. The posterior interosseous nerve and artery course between the abductor pollicis longus and extensor digitorum, passing over abductor pollicis longus’ superficial surface.

The tendon of abductor pollicis longus forms the lateral border of the anatomical snuffbox. This is a triangular space found at the base of the thumb, distal and posterior to the styloid process of radius. The anatomical snuffbox is traversed by the neurovascular structures of the hand; the superficial branch of the radial nerve, the radial artery and cephalic vein. The margins of this space are completed by extensor pollicis longus (medial) and brevis (lateral), while the floor is comprised by the scaphoid and trapezium bones.

Innervation

Abductor pollicis longus is innervated by the posterior interosseous nerve (C7, C8), which is a continuation of the deep branch of the radial nerve. The radial nerve is a branch of the posterior cord of the brachial plexus.

Blood supply

Blood supply to the abductor pollicis longus muscle comes from the interosseous branches of the ulnar artery;

• The proximal part of the muscle is supplied by the lateral branch of the posterior interosseous artery.
• The distal part is vascularised by a perforating branch of the anterior interosseous artery.

Function

Acting alone or with abductor pollicis brevis, abductor pollicis longus pulls the thumb away from the palm. More specifically, it produces (mid-) extension and abduction of the thumb at the first metacarpophalangeal joint. This action is seen in activities such as bowling and shoveling.

Working together with the long and short extensors of the thumb, the muscle also helps to fully extend the thumb at the metacarpophalangeal joint. This action is important for loosening the hand grip, for example, when letting go of objects previously being held. Abductor pollicis longus also helps to abduct the hand (radial deviation) at the radiocarpal joint.