Flexor pollicis longus

At a glance

English

Flexor pollicis longus

Latin (TA2)

Flexor longus pollicis

System

Muscle

Region

Upper limb

Side

Paired, left and right

Arises from

The radius

About the flexor pollicis longus

Flexor pollicis longus lies in the deep layer of the front of the forearm, on the radius and beside flexor digitorum profundus. It is the only muscle that can bend the last joint of the thumb, and it is a feature of the human hand that supports a precise pinch.

It arises from the anterior surface of the radius, from below the radial tuberosity down to the upper border of pronator quadratus, and from the adjoining interosseous membrane. A small extra head often comes from the ulnar coronoid process or from the medial epicondyle. The fibres converge on a flat tendon that passes through the carpal tunnel, lying on the radial side and wrapped in its own synovial sheath, the radial bursa. In the palm it runs between the two heads of flexor pollicis brevis and the sesamoid bones of the thumb, then in a fibrous sheath to the front of the thumb tip bone at its base.

The muscle bends the thumb at its interphalangeal joint and, as it continues to act, the metacarpophalangeal and carpometacarpal joints. It is essential for gripping small objects between thumb and index tip and for a firm grasp. It also helps flex the wrist a little. The median nerve serves it through its anterior interosseous branch, and the artery of the same name brings its blood.

Damage to the anterior interosseous nerve leaves the patient unable to make a round O with thumb and index, because this muscle and the index part of the deep flexor fail. Infection in the radial bursa can spread up into the forearm or across to the ulnar bursa.

Attachments, action and supply

Origin

Anterior surface of the radius between the radial tuberosity and pronator quadratus, the interosseous membrane and often a small slip from the ulna or humerus.

Insertion

Front of the thumb tip bone, at its base.

Action

Bends the thumb interphalangeal joint, then its metacarpophalangeal and carpometacarpal joints; assists wrist flexion.

Nerve supply

Median nerve, through its anterior interosseous branch (C7, C8).

Blood supply

Anterior interosseous artery.