Flexor digitorum superficialis

The flexor digitorum superficialis, formerly known as the flexor digitorum sublimis, is a large, superficial to intermediate muscle of the anterior compartment of the forearm. It is the primary flexor of the proximal interphalangeal (PIP) joints of the fingers and also contributes to metacarpophalangeal (MCP) and wrist flexion.

Origin

The flexor digitorum superficialis typically arises from two heads:

• The humeroulnar head originates broadly from the medial epicondyle of the humerus (via the common flexor tendon), the ulnar collateral ligament, and the coronoid process of the ulna.

• The radial head arises from the superior half of the anterior border of the radius, specifically from the anterior oblique line.

Course and Tendons

The muscle forms a broad belly that courses down the forearm, positioned deep to the most superficial flexors (pronator teres, flexor carpi radialis, palmaris longus, and flexor carpi ulnaris) and superficial to the flexor digitorum profundus and flexor pollicis longus. In the distal forearm, the flexor digitorum superficialis belly divides into four distinct tendons. These four tendons, along with five other tendons, pass deep to the transverse carpal ligament, entering the hand through the carpal tunnel.

As each flexor digitorum superficialis tendon continues into the palm and approaches the corresponding proximal phalanx, it uniquely splits (bifurcates) to allow the passage of the flexor digitorum profundus tendon through its opening, before reuniting distal to the profundus tendon.

Insertion

The flexor digitorum superficialis tendons are inserted onto the sides of the shafts of the middle phalanges of the index, middle, ring, and little fingers.

Innervation

The flexor digitorum superficialis is innervated by the median nerve. The median nerve enters the forearm by passing between the humero-ulnar and radial heads of the flexor digitorum superficialis muscle, and then continues its distal course lying deep to the muscle belly.

Actions

The flexor digitorum superficialis is a key flexor, primarily flexing the proximal interphalangeal (PIP) joints of the medial four digits. It also assists in the flexion of the metacarpophalangeal (MCP) joints and contributes to wrist flexion. It can flex each finger it serves independently.