The Flexor Digitorum Superficialis (FDS) is a powerful muscle located in the anterior compartment of the forearm that plays a major role in hand function. It is classified as an extrinsic flexor, meaning its muscle belly resides in the forearm, but its tendons extend into the hand to act on the fingers. Knowledge of the FDS’s precise nerve supply helps in diagnosing the location and severity of nerve injuries in the upper limb.
Function and Location of the Flexor Digitorum Superficialis
The primary action of the Flexor Digitorum Superficialis is the flexion of the four medial fingers (index, middle, ring, and little fingers). It bends the proximal interphalangeal (PIP) joints, which are the middle joints of these fingers. This action is essential for grasping and manipulating objects with the hand.
The muscle originates from two distinct points in the forearm: a humeroulnar head and a radial head. The humeroulnar head attaches broadly to the medial epicondyle of the humerus, the coronoid process of the ulna, and the ulnar collateral ligament. The radial head arises from the superior anterior border of the radius bone.
The FDS muscle belly is situated in the intermediate layer of the anterior forearm. It forms four long tendons that travel into the wrist, passing through the carpal tunnel. These tendons insert onto the sides of the middle phalanges of the four fingers.
The FDS tendons have a unique anatomical arrangement where they split just before their insertion to allow the deeper Flexor Digitorum Profundus tendons to pass through them. Beyond flexing the PIP joints, the muscle also assists in flexing the metacarpophalangeal (MCP) joints (the knuckles). It also contributes weakly to wrist flexion.
The Specific Nerve Supply
The Flexor Digitorum Superficialis muscle is innervated by the Median Nerve. This nerve originates from the lateral and medial cords of the brachial plexus, carrying nerve fibers from spinal roots C7, C8, and T1. The Median Nerve provides motor control to most of the muscles in the anterior forearm, and the FDS is one of its primary targets.
The Median Nerve branches that supply the FDS typically arise high in the forearm, often immediately after the nerve passes through the two heads of the muscle. The nerve enters the forearm by passing between the humeroulnar and radial heads of the FDS muscle. This close anatomical relationship means the FDS receives its motor input early in the nerve’s course down the arm.
The FDS is distinguished from its deeper counterpart, the Flexor Digitorum Profundus, which receives a dual nerve supply from both the Median and Ulnar nerves. Because its function depends solely on the Median Nerve, the FDS is a reliable indicator of Median Nerve integrity.
The motor branches travel directly from the main trunk of the Median Nerve to the muscle belly in the proximal third of the forearm. This high point of branching is important for localizing nerve injuries. If the nerve is damaged before this branching point, the FDS will be paralyzed.
Clinical Assessment and Injury
Knowledge of the FDS innervation is important in clinical practice, especially when evaluating suspected Median Nerve damage. The function of the FDS is tested using a specific maneuver to isolate its action, commonly known as the FDS test.
To perform the FDS test, the patient flexes a single finger at the PIP joint against resistance. The examiner holds the other three fingers fully extended. Holding the adjacent fingers straight helps to inactivate the deeper Flexor Digitorum Profundus muscle.
Paralysis of the FDS, resulting in the inability to flex the PIP joints, indicates a high Median Nerve lesion. This means the injury occurred high up in the arm or forearm. Conditions like a supracondylar fracture of the humerus or Pronator Teres Syndrome can compress the nerve proximal to where the FDS branches arise.
If a patient shows a loss of FDS function, it suggests the nerve damage is at or above the elbow, or in the proximal forearm. Conversely, if FDS function is preserved but other Median Nerve-innervated muscles in the hand are weak, it indicates a more distal lesion. This includes Carpal Tunnel Syndrome, which affects the nerve near the wrist.