Flexor digitorum superficialis muscle

Origin and insertion

Flexor digitorum superficialis arises at two origin points/heads:

• Humeroulnar head: as the name suggests, this head originates from both the medial epicondyle of humerus via the common flexor tendon, as well as the medial margin of the coronoid process of ulna. Some fibers also originate from the anterior band of ulnar collateral ligament of the elbow joint.
• Radial head: arises from the upper two-thirds of the anterior border of the radius. It has a long linear origin extending from the radial tuberosity to the insertion of pronator teres in the distal forearm.

From these origin points, the muscular heads of the flexor digitorum superficialis courses distally across the anterior forearm. Usually, the muscle is arranged into superficial and deep layers, or strata. About halfway along the forearm, each layer gives off two tendons. The tendons of the superficial layer are directed to digits 3 and 4, whereas the tendons of the deep layer insert into digits 2 and 5. Additionally, the deep layer gives off a small slip that joins the tendon of the superficial layer directed to the digit 2. At the wrist joint, the tendons pass deep to the flexor retinaculum through the carpal tunnel, after which they diverge into two pairs.  

As the tendons of flexor digitorum superficialis enter the hand, they course superficial to the corresponding flexor digitorum profundus tendon. Upon passing the metacarpophalangeal joint, the two tendons enter the digital flexor sheath. Before reaching the proximal interphalangeal joint, each tendon of flexor digitorum superficialis bifurcates to pass around the corresponding tendon of flexor digitorum profundus.

An easy way to remember this little fact is to keep in mind the following mnemonic.

• Superficialis Splits in two, to Permit Profundus Passing through

The split tendon of flexor digitorum superficialis reunites deep to the tendon of flexor digitorum profundus, around which it forms a loop. The flexor digitorum superficialis tendons then insert into the palmar surface of the base of the middle phalanx.

Relations

Flexor digitorum superficialis lies in the anterior compartment of the forearm lying superficial to flexor digitorum profundus and flexor pollicis longus, and deep to pronator teres, palmaris longus, flexor carpi radialis and flexor carpi ulnaris muscles. The two heads of flexor digitorum superficialis form a muscular arch, through which the median nerve and ulnar artery pass. 

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Innervation

Flexor digitorum superficialis is innervated by muscular branches of the median nerve, derived from roots C8 and T1 that arises from the medial and lateral cords of the brachial plexus. The skin that overlies the muscle is supplied by roots C6-8 and T1.

Blood supply

The primary arterial blood supply to the flexor digitorum superficialis is derived from the ulnar artery and its anterior recurrent branch. In addition to branches of the ulnar artery, the anterior and lateral surfaces of the muscle are supplied by branches of the radial artery; and its posterior surface also receives branches from the median artery.

Function

The main function of flexor digitorum superficialis is flexion of the digits 2-5 at the proximal interphalangeal and metacarpophalangeal joints. Unlike the flexor digitorum profundus, flexor digitorum superficialis has independent muscle slips for all four digits. This allows it to flex the digits individually at their proximal interphalangeal joints. In addition, flexor digitorum superficialis aids the aids flexion of the wrist.

Test your newly acquired knowledge on the flexor digitorum superficialis and other flexors of the forearm with our quiz!