The flexor pollicis longus is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus. This muscle is unique to humans, being “either rudimentary or absent” in other primates.[rx]
The flexor pollicis longus (FPL) muscle is one of the three deep flexors of the volar compartment of the forearm. Originating on the volar surface of the proximal aspect of the radius, FPL courses along the radial aspect of the volar forearm before transitioning into a long tendinous slip that traverses deep to the transverse carpal ligament as a constituent of the carpal tunnel, and inserting distally on the base of the distal phalanx of the thumb.
Origin and Insertion of Flexor Pollicis Longus
It arises from the grooved anterior (side of palm) surface of the body of the radius, extending from immediately below the radial tuberosity and oblique line to within a short distance of the pronator quadratus muscle.[2] An occasionally present accessory long head of the flexor pollicis longus muscle is called ‘Gantzer’s muscle’. It may cause compression of the anterior interosseous nerve.
It arises also from the adjacent part of the interosseous membrane of the forearm, and generally by a fleshy slip from the medial border of the coronoid process of the ulna.[rx] In 40 percent of cases, it is also inserted from the medial epicondyle of the humerus, and in those cases a tendinous connection with the humeral head of the flexor digitorum superficialis is present.[rx]
The fibers end in a flattened tendon, which passes beneath the flexor retinaculum of the hand through the carpal tunnel. It is then lodged between the lateral head of the flexor pollicis brevis and the oblique part of the adductor pollicis, and, entering an osseoaponeurotic canal similar to those for the flexor tendons of the fingers, is inserted into the base of the distal phalanx of the thumb.[rx]
Anatomy of Flexor Pollicis Longus
The FPL muscle is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. Additionally, FPL has minor contributions in radial wrist deviation and wrist flexion.[rx]
FPL arises on the volar aspect of the radius and the adjacent interosseous membrane, just distal to the radial tuberosity. Its proximal attachment lies distal to the supinator muscle, as well as deep and radial to the radial head of the flexor digitorum superficialis (FDS). The muscle tapers into a long, flat tendon that courses superficial to the pronator quadratus muscle and directly lateral to the index finger tendon of FDS. The FPL tendon, now covered by the synovial sheath of the radial bursa, travels through the carpal tunnel passing deep to the transverse carpal ligament (flexor retinaculum)[rx]. FPL continues along the first metacarpal, passing between the two sesamoid bones which reside on the volar aspect of the thumb MCP joint. The FPL tendon and synovial sheath pass deep to the A1 pulley overlying the thumb MCP joint, the annular variable ligament and oblique ligament of the thumb’s proximal phalanx, and the A2 pulley overlying the IP joint before inserting at the volar base of the distal thumb phalanx.[rx]
Flexor pollicis longus
- Origin: radius and interosseous membrane
- Insertion: distal phalanx of thumb
- Action: flexion of the wrist and flexion of the MCP and IP joints of the digits
- Innervation: median nerve
Actions
- The flexor pollicis longus is a flexor of the phalanges of the thumb; when the thumb is fixed, it assists in flexing the wrist.[rx]flexible and strongest muscle in forearm
Nerve Supply of Flexor Pollicis Longus
- The flexor pollicis longus is supplied by the anterior interosseous(C8-T1) branch of the median nerve (C5-T1).[rx]
- Flexor pollicis longus receives nervous supply from the anterior interosseous branch of median nerve, derived from spinal roots C7 and C8.
Blood Supply of Flexor Pollicis Longus
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 of Flexor Pollicis Longus
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.
