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Flexor Pollicis Longus Muscle Anatomy

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Origin
  • Anterior surface of the radius
  • Adjacent interosseous membrane of the forearm
Insertion
  • Palmar aspect of the base of the distal phalanx of the thumb
Action
  • Flexes the thumb
  • Very weakly assists in flexing the wrist
Innervation
  • Anterior interosseous nerve (C7, C8)
Blood Supply
  • Anterior interosseous artery
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Location & Overview

The flexor pollicis longus muscle is located in the anterior part of the forearm and is one of the three deep flexor muscles found in the volar compartment of the forearm. The other two muscles in this compartment are the flexor digitorum profundus and pronator quadratus. This muscle lies deep to the radial head of the flexor digitorum superficialis muscle and is distal to the supinator muscle. Its primary function is to flex the thumb, making it a key muscle for gripping. Despite being located in the forearm, the flexor pollicis longus is classified as one of the extrinsic muscles of the hand.[1,2,3]

In Latin, ‘pollicis’ means ‘of the thumb’, ‘flexor’ means ‘a muscle that flexes a joint’, and ‘longus’ means ‘long’. Translated into English, flexor pollicis longus is ‘long flexor muscle of the thumb’.

Anatomical variation may occur in the form of an accessory head of flexor pollicis longus, often referred to as Gantzer’s muscle. A meta-analysis of cadaveric studies reported a pooled prevalence of approximately 48% for the accessory head of flexor pollicis longus, although estimates varied between studies, and the unit of analysis was upper limbs rather than individuals. This variant may be clinically relevant because of its anatomical relationship to the anterior interosseous nerve.[4]

flexor pollicis longus muscle
Here we can see the flexor pollicis longus muscle from multiple views.
superficial view of the flexor pollicis longus
Here we can see the flexor pollicis longus muscle. We can see a superficial view with the flexor pollicis longus highlighted in green and an isolated view of the muscle by itself.

Origin & Insertion

The flexor pollicis longus originates on the anterior surface of the radius (also known as the volar aspect), just distal to the radial tuberosity. It also originates from the adjacent interosseous membrane. The interosseous membrane is a strong fibrous sheet that connects the radius and ulna, which run parallel to each other in the forearm. The flexor pollicis longus also passes deep to the flexor retinaculum, also known as the transverse carpal ligament. The muscle then proceeds toward its insertion on the palmar aspect of the base of the distal phalanx of the thumb.[1,2,3]

flexor pollicis longus origin: anterior surface of the radius
Here we can see the first origin point of the flexor pollicis longus highlighted in red. The origin is on the anterior surface of the radius.
flexor pollicis longus origin: interosseous membrane of forearm
Here we can see the interosseous membrane of the forearm, which is an origin of the flexor pollicis longus.
flexor pollicis longus insertion: palmar aspect of the base of the distal phalanx of the thumb
Here we can see the insertion of the flexor pollicis longus highlighted in blue. The insertion is on the palmar aspect of the base of the distal phalanx of the thumb.
Bones and joints of the thumb
Here we can see the different bones and joints in the thumb, color-coded according to their respective locations.

Action & Function

The flexor pollicis longus muscle is the primary flexor of the thumb. It provides flexion at both the metacarpophalangeal and interphalangeal joints. In addition to thumb flexion, it can support other muscles by contributing weak flexion of the wrist joint.[1,2]

Image of a hand demonstrating the movement of thumb flexion.
This image demonstrates thumb flexion, which is the action of bending the thumb inward toward the palm of the hand. The left side of the image depicts thumb flexion, while the right side shows thumb extension. Thumb extension is the opposite action of thumb flexion. The flexor pollicis longus muscle is the primary flexor of the thumb.
Image shows an arm demonstrating the movement of wrist flexion.
This image shows wrist flexion, which is the movement of bending the wrist toward the underside of the forearm (the anterior surface of the forearm). The opposite of wrist flexion is wrist extension, which involves extending the wrist in the direction of the top of the forearm (the posterior surface of the forearm). The flexor pollicis longus can contribute weakly to wrist flexion.

Innervation

The flexor pollicis longus is innervated by the anterior interosseous nerve. The anterior interosseous nerve is a branch of the median nerve. The median nerve passes between the two heads of the pronator teres muscle before giving rise to the anterior interosseous nerve.[1,2]

anterior interosseous nerve and median nerve
Pictured here, we can see the anterior interosseous nerve, which is a branch of the median nerve. The median nerve is highlighted in red on the right side.

Blood Supply

Blood is supplied to the flexor pollicis longus by the anterior interosseous artery. The anterior interosseous artery branches from the common interosseous artery, which branches from the ulnar artery. Branches of the median artery also provide blood to the pre-digital portion of the flexor pollicis longus. The digital portion of the tendon receives its blood supply from the vincula V1 and V2. V1 originates from either the princeps pollicis artery or both digital arteries. V2 originates from both digital arteries.[1,2]

References

  1. [1] Moore KL, Agur AMR, Dalley AF. Clinically Oriented Anatomy. 8th ed. Philadelphia: Lippincott Williams & Wilkins; 2017
  2. [2] Benson DC, Miao KH, Varacallo M. Anatomy, Shoulder and Upper Limb, Hand Flexor Pollicis Longus Muscle. (Updated 2021 Jul 26). In: StatPearls (Internet). Treasure Island (FL): StatPearls Publishing; 2022 Jan-. Available from: https://www.ncbi.nlm.nih.gov/books/NBK538490/
  3. [3] Standring S. (2015). Gray’s Anatomy: The Anatomical Basis of Clinical Practice, 41st Edn. Amsterdam: Elsevier.
  4. [4] Asghar, A., Jha, R. K., Patra, A., Chaudhary, B., & Singh, B. (2022). The prevalence and distribution of the variants of Gantzer’s muscle: a meta-analysis of cadaveric studies. Anat Cell Biol, 55(1), 3-13. https://doi.org/10.5115/acb.21.141

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