Flexor Pollicis Longus Muscle

Anatomy and Location

In the forearm, the flexor pollicis longus is located in the deep (third) layer of the anterior compartment. It lies laterally, whereas the deep finger flexor (m. flexor digitorum profundus) lies medial to it.

• Origin (Origo): Anterior surface of the radius and the interosseous membrane (membrana interossea) between the junction of the upper and middle thirds of the forearm.
• Insertion (Insertio): Base of the distal phalanx of the thumb.

Tendon Topography in the Hand

The muscle tendon is enclosed in a synovial sheath and follows this specific topographic pathway:

1. The proximal blind end of the sheath is located in the pronator quadratus space (Pirogov-Parona space), approximately 2 cm proximal to the flexor retinaculum (retinaculum mm. flexorum).
2. The tendon passes through the carpal tunnel.
3. It enters the proximal part of the midpalmar space.
4. It pierces the lateral intermuscular septum.
5. It runs through the thenar fascia and subcutaneous tissue between the heads of the short thumb flexor (m. flexor pollicis brevis).
6. It inserts onto the base of the distal phalanx.

The medial aspect of the thenar fascial space communicates laterally with the synovial sheath of the m. flexor pollicis longus tendon.

Blood Supply and Innervation

• Innervation: Median nerve (n. medianus), specifically via the anterior interosseous nerve (segments C6–C8).
• Blood Supply: Anterior interosseous artery (a. interossea anterior), radial artery (a. radialis), and ulnar artery (a. ulnaris). In the forearm, the anterior interosseous artery lies on the interosseous membrane deep to m. flexor pollicis longus and m. flexor digitorum profundus. In the hand, the principal artery of the thumb (a. princeps pollicis) runs along the tendon between m. adductor pollicis and m. flexor pollicis brevis.

Function

The primary action of the muscle is flexion of the distal phalanx of the thumb. It also assists in flexing the entire thumb and the wrist joint.

Clinical Distinction: Flexor Hallucis Longus

In lower limb anatomy, there is a corresponding muscle called the flexor hallucis longus (m. flexor hallucis longus).

• Location: Deep posterior compartment of the leg; it lies laterally, close to the fibula, originating from the posterior surface of the fibula and the interosseous membrane.
• Topography: Its tendon passes through a synovial sheath in the medial retromalleolar region.
• Function: Flexes the great toe, assists in plantarflexion and stabilization of the ankle and foot, and supports the medial longitudinal arch.
• Innervation and Blood Supply: Tibial nerve (n. tibialis, L4–S2), posterior tibial and fibular (peroneal) arteries.