Flexor pollicis longus muscle

Origin and insertion

Flexor pollicis longus originates from a large area on the anterior surface of the radius, extending between the radial tuberosity superiorly and the attachment of pronator quadratus inferiorly. This origin point of flexor pollicis longus often also includes the adjacent anterior surface of the interosseous membrane and/or the lateral border of the coronoid process of ulna. Sometimes a small slip of the muscle originates from the medial surface of the coronoid process of ulna or the medial epicondyle of humerus. This accessory head of the flexor pollicis longus may be referred to as a 'Gantzer' muscle.

The muscle tapers into a flat tendon near the wrist and inserts into the base of the distal phalanx of the thumb on the palmar surface.

Relations

In the forearm, flexor pollicis longus is located lateral to flexor digitorum profundus. These two muscles form a groove in which the anterior interosseous nerve, artery and vein run over the anterior surface of the interosseous membrane. In the wrist, the tendinous sheath of flexor pollicis longus passes underneath the flexor retinaculum between opponens pollicis and the oblique head of adductor pollicis muscle.

Flexor pollicis longus is quite variable and can occasionally be blended with the flexor digitorum superficialis, flexor digitorum profundus or pronator teres. In a small part of the population the flexor pollicis longus can even be absent.

Innervation

Flexor pollicis longus receives nervous supply from the anterior interosseous branch of median nerve, derived from spinal roots C7 and C8. 

Blood supply

Flexor pollicis longus receives dual blood supply. Its medial part is supplied by the anterior interosseous artery, an indirect branch of the ulnar artery; the lateral part receives blood from the radial artery. Flexor pollicis longus may also receive contributions from the median artery if the muscle is well developed.

Function

The main function of the flexor pollicis longus is flexion of the thumb at the interphalangeal joint. Flexor pollicis longus is the only muscle that flexes the interphalangeal joint of the thumb, which makes it vital for activities that require hand gripping.

Flexor pollicis longus also flexes the thumb at the metacarpophalangeal joint and contributes to the flexion of the wrist. This muscle can be easily palpated in the lower third of the forearm when you flex your thumb.