The flexor pollicis longus (FPL) is an extrinsic muscle of the hand, originating in the forearm to control the flexing of the thumb. It is the only muscle that flexes the final joint of the thumb, the interphalangeal joint, an action required for gripping and fine motor tasks. The FPL is also unique to humans, as in other primates it is either absent or rudimentary.
Anatomical Pathway of the Flexor Pollicis Longus
The FPL originates from a wide area on the anterior surface of the radius, extending from just below the radial tuberosity down toward the pronator quadratus muscle. It also arises from the adjacent interosseous membrane, a fibrous sheet that connects the radius to the ulna. This muscle is situated in the deep compartment of the forearm, beneath more superficial muscles.
From its origin, the muscle fibers form a flattened tendon that travels down the forearm. This tendon passes through the carpal tunnel, a narrow passageway in the wrist formed by bones and ligaments. Within the carpal tunnel, the FPL tendon is the most radial structure, meaning it is closest to the thumb side, and is enclosed in its own synovial sheath to help it glide smoothly.
After exiting the carpal tunnel, the tendon runs along the palmar aspect of the thumb. It is positioned between parts of the opponens pollicis and adductor pollicis muscles. The tendon inserts onto the base of the distal phalanx, which is the last bone at the tip of the thumb.
Key Structures for Cadaveric Identification
During a cadaveric dissection, identifying the FPL requires locating it relative to its neighboring structures. The FPL lies in the deep flexor compartment of the forearm alongside the flexor digitorum profundus (FDP). The FPL is positioned lateral to the FDP, meaning it is on the side closer to the thumb. More superficial is the flexor digitorum superficialis (FDS), which covers both deep muscles.
A reliable method for confirming the FPL’s identity is to locate the neurovascular bundle that runs between it and the FDP. The anterior interosseous nerve and the anterior interosseous artery and vein are situated on the interosseous membrane, nestled in the groove between these two muscles. Exposing this bundle provides a clear landmark that separates the FPL from the FDP.
Near the wrist, the FPL tendon can be distinguished as it enters the carpal tunnel. Following the prominent flexor carpi radialis (FCR) tendon is a good approach. The FPL tendon is located immediately deep and slightly to the thumb side of the FCR tendon, which helps confirm its identity before it enters the hand.
Anatomical Variations Observed in Dissection
Anatomical structures are not always uniform, and the FPL is subject to several common variations. The most notable is the Linburg-Comstock anomaly, which is a tendinous connection between the FPL and the flexor digitorum profundus (FDP) tendon of the index finger. This interconnection can result in the inability to flex the tip of the thumb without also unintentionally flexing the tip of the index finger.
The prevalence of the Linburg-Comstock anomaly is high, present in over 40% of individuals, with about half of those cases occurring bilaterally. While most people with this connection are unaware of it, it can sometimes cause pain or a feeling of tightness in the forearm or palm with repetitive hand use.
Another frequent variation is an accessory head of the FPL, sometimes called Gantzer’s muscle. This accessory muscle belly can originate from the medial epicondyle of the humerus or the coronoid process of the ulna, and it merges with the main FPL muscle belly or its tendon. This variation is observed in a substantial number of cadavers, with some studies reporting a prevalence of around 48%.
Clinical Relevance Informed by Cadaveric Anatomy
Understanding the FPL’s anatomical course has direct applications in medicine. For surgeons repairing a lacerated or ruptured FPL tendon, knowing its exact path through the carpal tunnel and its relation to other tendons is necessary for a successful outcome. Ruptures can occur after certain wrist fracture repairs, particularly if a surgical plate irritates the tendon where it passes over the distal radius.
The distinct synovial sheath that surrounds the FPL tendon is also clinically important. This sheath can become inflamed and thickened, a condition known as stenosing tenosynovitis, or “trigger thumb.” This causes the tendon to catch as it tries to glide, resulting in a painful clicking or locking of the thumb. Treatment often involves a surgical release of the constricted pulley that holds the tendon down.
The FPL’s close proximity to the median nerve within the carpal tunnel is a consideration during carpal tunnel release surgery. Surgeons must be aware of the FPL tendon’s position to avoid injuring it while decompressing the nerve. The presence of an accessory head (Gantzer’s muscle) can also be relevant, as it may compress the anterior interosseous nerve, leading to a specific type of nerve palsy.