The Hand Flexor Digitorum Superficialis Muscle (FDS), formerly known as the flexor digitorum sublimis, is the largest of the extrinsic flexors of the forearm. It forms the intermediate muscle layer between the superficial and deep muscle groups of the forearm.[rx] It contains four digital components, with a tendon that inserts onto each corresponding finger. The flexor digitorum superficialis is the primary flexor of the proximal interphalangeal (PIP) joints of the middle phalanges, but it also assists in flexion of the metacarpophalangeal (MCP) joints.
Flexor digitorum superficialis is an extrinsic flexor muscle of the fingers at the proximal interphalangeal joints.
It is in the anterior compartment of the forearm. It is sometimes considered to be the deepest part of the superficial layer of this compartment and sometimes considered to be a distinct, “intermediate layer” of this compartment.[rx] It is relatively common for the Flexor digitorum superficialis to be missing from the little finger, bilaterally and unilaterally, which can cause problems when diagnosing a little finger injury.[rx]
Structure of Hand Flexor Digitorum Superficialis Muscle
The flexor digitorum superficialis muscle has two origins. The humeroulnar head originates off the medial epicondyle of the humerus and the coronoid process of the ulna. The radial head originates on the upper half of the volar aspect of the radial shaft. The flexor digitorum superficialis courses along the volar aspect of the forearm, superficial to the flexor digitorum profundus and flexor pollicis longus muscles, and deep to the palmaris longus, flexor carpi radialis, flexor carpi ulnaris, and pronator teres.
As it courses down the forearm, the flexor digitorum superficialis separates into two planes of muscular fibers, superficial and deep:
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The superficial plane further divides to supply tendons for the middle and ring fingers
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The deep plane provides a muscular slip to join the part of the superficial plane that associates with the tendon of the ring finger and then divides to supply tendons for the index and little fingers
The four flexor digitorum superficialis tendons pass deep to the transverse carpal ligament, constituting four of the nine total tendons in the carpal tunnel[rx]. Continuing into the palm, the flexor digitorum superficialis tendinously slips insert onto the middle phalangeal bases of digits 2 through 5, on the volar surface of the hand. The flexor digitorum superficialis acts to flex the PIP joints of the fingers.[rx]
Flexor digitorum superficialis
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Origin: two heads; medial condyle of the humerus and radius
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Insertion: middle phalanges of digits 2-5
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Action: flexion of the metacarpophalangeal and proximal interphalangeal joints of digits 2-5
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Innervation: median nerve
Origin and Insertion of Hand Flexor Digitorum Superficialis Muscle
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 the 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.
Blood Supply of Hand Flexor Digitorum Superficialis Muscle
The primary blood supply to the flexor digitorum superficialis muscle is from the ulnar artery. Lymphatic drainage of the upper limb occurs via the axillary lymph nodes.
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.
Nerves Supply of Hand Flexor Digitorum Superficialis Muscle
The flexor digitorum superficialis muscle receives innervation by the median nerve (C7, C8, and T1).[rx]
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.
Function of Hand Flexor Digitorum Superficialis Muscle
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.
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