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The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. It is one of the three deep flexors of the volar compartment of the forearm and is classified as part of the extrinsic muscles of the hand. The FPL muscle is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. It is innervated by the median nerve via the anterior interosseous nerve (AIN) and receives its blood supply from the anterior interosseous artery, a branch of the ulnar artery.
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What You'll Learn
- The FPL is a deep muscle in the forearm and hand that flexes the thumb
- The FPL is one of three deep flexors of the volar compartment of the forearm
- The FPL muscle is innervated by the median nerve via the anterior interosseous nerve
- The FPL muscle is unique to humans and is either rudimentary or absent in other primates
- The FPL muscle is tested by holding the proximal phalanx of the thumb while flexing the distal phalanx against resistance
The FPL is a deep muscle in the forearm and hand that flexes the thumb
The flexor pollicis longus (FPL) is a muscle in the forearm and hand that is responsible for flexing the thumb. It is one of the three deep flexors of the volar compartment of the forearm, specifically located in the deep layer of the anterior or volar compartment of the forearm. The FPL is unique to humans, being either rudimentary or absent in other primates.
The FPL originates on the volar or anterior surface of the radius, which is the proximal aspect of the forearm bone, and from the adjacent interosseous membrane. It courses along the radial aspect of the volar forearm, which is the side of the forearm closest to the radius bone. The FPL then transitions into a long, flattened tendon that passes through the carpal tunnel, a passage in the wrist that allows tendons and nerves to travel from the forearm to the hand.
The FPL tendon travels through the carpal tunnel, passing deep to the transverse carpal ligament, also known as the flexor retinaculum. It continues along the first metacarpal bone of the thumb, passing between the two sesamoid bones located on the volar aspect of the thumb metacarpophalangeal (MCP) joint. The FPL tendon is covered and protected by a synovial sheath, which helps it glide smoothly through the carpal tunnel and over the various ligaments and bones of the thumb.
The FPL tendon eventually inserts at the volar or palmar base of the distal phalanx of the thumb, which is the bone closest to the tip of the thumb. By attaching to the thumb in this way, the FPL is able to flex the interphalangeal (IP) joint and metacarpophalangeal (MCP) joint of the thumb, allowing for movement and dexterity. The FPL also has minor contributions to radial wrist deviation and wrist flexion, assisting in the overall movement and function of the hand and wrist.
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The FPL is one of three deep flexors of the volar compartment of the forearm
The flexor pollicis longus (FPL) muscle is one of three deep flexors of the volar compartment of the forearm. The FPL is a long muscle located in the deep layer of the forearm's anterior compartment, alongside the flexor digitorum profundus and pronator quadratus muscles. It is classified as an extrinsic muscle of the hand due to its role in thumb movement.
The FPL originates from the mid-half of the anterior surface of the radius and the adjacent half of the interosseous membrane. It has also been found to originate from the medial epicondyle of the humerus, the coronoid process of the ulna, and the flexor digitorum superficialis. The FPL courses along the radial aspect of the volar forearm and transitions into a long tendinous slip that passes through the carpal tunnel, inserting distally on the base of the distal phalanx of the thumb.
The primary function of the FPL is to flex the thumb at its metacarpophalangeal (MCP) and interphalangeal (IP) joints. It also assists in radial wrist deviation and wrist flexion. The FPL is supplied by the anterior interosseous nerve (AIN), a branch of the median nerve. The AIN provides sensory innervation to the volar wrist capsule and arises from the anterolateral aspect of the median nerve, proximal to where it passes between the two heads of the pronator teres muscle.
The FPL is clinically significant due to its association with trigger thumb, which occurs when the FPL tendon cannot smoothly move through the flexor pulley system. This condition can be managed through conservative treatments such as activity modification, corticosteroid injections, NSAIDs, and splinting. In cases where conservative treatments are unsuccessful, surgical transection of the involved pulley can provide relief.
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The FPL muscle is innervated by the median nerve via the anterior interosseous nerve
The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. It is a deep muscle under the abductor pollicis brevis muscle. It is one of the extrinsic muscles of the hand. The FPL muscle is innervated by the median nerve via the anterior interosseous nerve.
The median nerve has two major branches in the forearm: the anterior interosseous nerve and the palmar cutaneous nerve. The anterior interosseous nerve supplies the deep muscles in the anterior forearm, while the palmar cutaneous nerve innervates the skin of the lateral palm. The median nerve also innervates muscles in the superficial and intermediate layers of the forearm.
The FPL muscle is located in the deep layer of the anterior compartment of the forearm, along with the flexor digitorum profundus and pronator quadratus muscles. It is one of the two muscles that make up the third layer of the anterior compartment of the forearm. The FPL muscle is unique to humans, being either rudimentary or absent in other primates.
The FPL muscle plays a crucial role in thumb movement and is responsible for flexing the interphalangeal joint of the distal phalanx of the thumb. It also serves as an accessory flexor for the metacarpophalangeal joint of the proximal phalanx and the carpometacarpal joints of the first metacarpal. Additionally, the FPL may assist in wrist joint flexion.
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The FPL muscle is unique to humans and is either rudimentary or absent in other primates
The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. It is one of the two muscles that make up the third layer of the anterior compartment of the forearm, along with the flexor digitorum profundus muscle. It is a deep muscle under the abductor pollicis brevis muscle. It passes through the carpal tunnel and is classified as one of the extrinsic muscles of the hand.
Modern humans are unique among hominids in having an FPL muscle belly that is separate from that of the flexor digitorum profundus (FDP). While the FPL is not a separate muscle belly in great apes, a distinct tendon from the FDP belly might be present. In some individuals, this tendon acts more like a ligament, restricting the extension of the interphalangeal joint of the thumb.
In orangutans, there is a tendon similar in insertion and function to the FPL in humans, but it has an intrinsic origin on the oblique head of the adductor pollicis. Lesser apes (gibbons) and Old World monkeys (baboons) share an extrinsic FPL muscle tendon with humans. In most lesser apes, the FPL belly is separate from the FDP belly, but in baboons, the FPL tendon bifurcates from the FDP tendon at the wrist within the carpal tunnel.
The FPL is an extrinsic thumb flexor that is either rudimentary or absent in great apes. Previous researchers have linked the origin of a well-developed FPL muscle to the acquisition of precision grasping and stone tool-making capabilities in early hominids. Modern humans have accumulated more muscle character state changes than chimpanzees, and both have accumulated more changes than gorillas.
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The FPL muscle is tested by holding the proximal phalanx of the thumb while flexing the distal phalanx against resistance
The flexor pollicis longus (FPL) is a muscle in the forearm and hand that flexes the thumb. It is one of the three deep flexors of the volar compartment of the forearm. It is also one of the two muscles that make up the third layer of the anterior compartment of the forearm, along with the flexor digitorum profundus muscle. The FPL is a long muscle located at the deep layer with flexor digitorum profundus and pronator quadratus in the anterior compartment of the forearm. It is also one of the extrinsic muscles of the hand.
The FPL muscle originates on the volar surface of the proximal aspect of the radius and courses through the carpal tunnel, crossing three joints of the hand to attach at the base of the distal phalanx of the thumb. It is innervated by the median nerve via the anterior interosseous nerve and receives blood supply from a branch of the ulnar artery.
The FPL is primarily responsible for flexing the interphalangeal (IP) joint of the distal phalanx of the thumb. It also serves as an accessory flexor of the metacarpophalangeal (MCP) joint of the proximal phalanx and the carpometacarpal joint of the first metacarpal. Additionally, the FPL contributes to radial wrist deviation and wrist flexion.
The FPL muscle is susceptible to various injuries, such as tendon lacerations or ruptures, which can impact its ability to flex the thumb and cause pain or discomfort. Treatment options for FPL injuries may include primary tendon repair or fixing the proximal tendon to cancellous bone to facilitate healing, depending on the extent and location of the injury.
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Frequently asked questions
The flexor pollicis longus (FPL) muscle is a long muscle located at the deep layer of the anterior compartment of the forearm. It is one of the three deep flexors of the volar compartment of the forearm.
The FPL muscle is the primary flexor of the thumb, providing flexion at its metacarpophalangeal (MCP) joint and interphalangeal (IP) joint. It also serves as an accessory flexor of the carpometacarpal joint of the first metacarpal and may assist in wrist joint flexion.
The FPL muscle is one of the extrinsic muscles of the hand, and its function is seen in thumb movement.
