Flexor Pollicis Longus Muscle Anatomy

The Flexor Pollicis Longus Muscle is one of three deep muscles in the forearm, alongside the flexor digitorum profundus and the pronator quadratus muscles. It’s also called the long flexor of the thumb (L. pollex, thumb).

Flexor Pollicis Longus Muscle Anatomy

The flexor pollicis longus arises on the volar aspect of the radius and the adjacent interosseous membrane, just distal to the radial tuberosity. It lies lateral to the flexor digitorum profundus, where it clothes the anterior aspect of the radius distal to the attachment of the supinator.

The flat flexor pollicis longus tendon passes deep to the flexor retinaculum of the hand through the carpal tunnel, enveloped in its own synovial tendinous sheath of the flexor pollicis longus on the lateral side of the common flexor sheath, inserting at the volar base of the distal thumb phalanx.

See Also: Forearm Muscles Anatomy & Function

The flexor pollicis longus is innervated by the anterior interosseous nerve a branch of the median nerve (C8 and T1), it takes its blood supply from the anterior interosseous artery a branch of the common interosseous artery, a short arterial trunk that arises from the ulnar artery in the cubital region of the forearm.

The flexor pollicis longus primarily flexes the distal phalanx of the thumb at the interphalangeal joint and, secondarily, the proximal phalanx and 1st metacarpal at the metacarpophalangeal and carpometacarpal joints, respectively. The flexor pollicis longus is the only muscle that flexes the interphalangeal joint of the thumb. It also may assist in flexion of the wrist joint.

An accessory flexor pollicis longus muscle head may originate from either the flexor digitorum superficialis muscle, coronoid process of the ulna, or medial epicondyle of the humerus. The accessory head subsequently inserts into the ulnar aspect of the flexor pollicis longus or Flexor Digitorum Profundus muscles.

The Linburg-Comstock anomaly is a tendinous interconnection between flexor pollicis longus and the flexor digitorum profundus of the second digit. The presence of this tendon slip results in the inability to independently flex the thumb IP joint and the index finger distal interphalangeal joint. However, this anomaly does not typically lead to the development of any symptoms.

Surgical Considerations for Flexor Tendon Injuries

Flexor tendon lacerations, including those of the flexor pollicis longus (flexor pollicis longus), are typically caused by sharp objects. Primary tendon repair involves reconnecting the severed ends. Flexor tendon injuries are categorized into zones for treatment purposes:

• Thumb Zones:
• T1: FPL insertion to A2 pulley
• T2: A2 pulley to A1 pulley
• T3: A1 pulley to carpometacarpal joint

Damage to the flexor pollicis longus tendon can be partial or complete. Treatment varies:

• Nonoperative Management: For partial lacerations involving less than 50-60% of the tendon.
• Surgical Repair: Required if more than 75-80% of the tendon is lacerated, or more than 50-60% with triggering.

For zone I injuries, primary tendon repair is possible if the distal stump is longer than 1 cm. If shorter, the proximal tendon must be fixed to the bone. Zones II to V injuries are managed with end-to-end repair using multiple suture techniques to ensure strength. Postoperative splinting and early mobilization improve outcomes.

Mannerfelt’s Lesion: This specific flexor pollicis longus rupture within the carpal tunnel results from chronic abrasion, commonly due to rheumatoid arthritis. It leads to passive thumb flexion without active IP joint flexion. Early diagnosis allows for tendon grafting; otherwise, a tendon transfer is necessary.

Clinical Significance

Trigger Thumb: This condition occurs when a pulley constricts the flexor pollicis longus tendon, typically due to tenosynovitis of the A1 pulley. Treatment includes activity modification, corticosteroid injections, NSAIDs, and splinting. Surgery is an option if conservative treatments fail.

Volkmann’s Contracture: A severe condition resulting from untreated acute compartment syndrome, often due to fractures or crush injuries, leading to muscle and nerve ischemia. The flexor digitorum profundus and flexor pollicis longus muscles are first affected, causing a classic flexed deformity. Early diagnosis using the “Six Ps” (pain, paresthesia, paresis, pallor, pulselessness, poikilothermia) and prompt fasciotomy are crucial to prevent irreversible damage. Treatment varies by severity and includes stretching, splinting, tendon lengthening, muscle slides, and free muscle transfers.

To test the flexor pollicis longus, the proximal phalanx of the thumb is held and the distal phalanx is flexed against resistance.

References & More

1. Clinically Oriented Anatomy – 8th Edition
2. Clinical Anatomy by Regions, Richard S. Snell.
3. Benson DC, Miao KH, Varacallo M. Anatomy, Shoulder and Upper Limb, Hand Flexor Pollicis Longus Muscle. [Updated 2023 Jul 24]. In: StatPearls [Internet]. Treasure Island (FL): StatPearls Publishing; 2024 Jan-. Available from: Pubmed