Tendon of flexor carpi radialis

The flexor carpi radialis (FCR) tendon is the distal, cord-like connective-tissue continuation of the flexor carpi radialis muscle—one of the superficial flexor muscles of the anterior forearm— that inserts primarily onto the base of the second metacarpal bone, and variably onto the base of the third metacarpal bone andanchors the muscle to the bony skeleton of the hand and transmits its contractile force to produce flexion and radial deviation (abduction) of the wrist.

Origin and course of the parent muscle-tendon unit

• The flexor carpi radialis muscle arises from the medial epicondyle of the humerus (via the common flexor origin), and its musculotendinous junction begins on average ~15 cm proximal to the radiocarpal joint, with the muscle fibers ending ~8 cm proximal to the wrist, so that the distal third of the unit is purely tendinous.

• At the wrist, the tendon lies superficial to the flexor retinaculum and outside the carpal tunnel, running lateral (radial) to the carpal tunnel contents and immediately adjacent to the palmar cutaneous branch of the median nerve and the radial artery.

Fibro-osseous tunnel

• Distally, the tendon enters its own fibro-osseous (osteofibrous) tunnel at the proximal border of the trapezium, separated from the carpal canal by a thick septum whose distal border acts as a pivot point for the flexor pollicis longus. Within this narrow tunnel the tendon occupies ~90% of the available space and contacts the roughened surface of the trapezium, making it susceptible to tendinopathy and rupture.

• The tendon undergoes a characteristic ~180° torsion of its fibers (half in the forearm, half within the wrist sheath), a feature relevant when it is harvested for ligamentoplasty.

Insertio

• The tendon inserts principally onto the base of the second metacarpal (~80%), with a smaller slip to the base of the third metacarpal (~20%) and an additional slip to the trapezial crest/tuberosity.

Investing sheath

• A synovial tendon sheath (Tendon sheath of flexor carpi radialis) envelops the tendon from origin to insertion; inflammation of this sheath (FCR tenosynovitis) or of the tendon itself (tendinopathy) is a recognized cause of volar radial wrist pain.

The primary functions of the flexor carpi radialis are wrist flexion and radial deviation (abduction). Minor functions include assisting in forearm pronation and contributing to the dynamic stabilization of the wrist during radial-ulnar deviation.