Flexor digitorum superficialis

The Flexor digitorum superficialis (FDS) is a muscle in the anterior (front) compartment of the forearm. It’s one of the most important muscles involved in flexing the fingers.

Location

The Flexor digitorum superficialis is located in the intermediate layer of the anterior compartment of the forearm, lying between the superficial and deep layers of flexor muscles.

Structure

• The Flexor digitorum superficialis muscle is unique because it has a bipennate structure, meaning its fibers are arranged like a feather along a central tendon. This structure allows the muscle to be more compact and generate more force.
• The muscle splits into four tendons at the wrist, which pass through the carpal tunnel. After passing the wrist, each tendon inserts into the fingers.
• Each tendon then splits into two parts to allow the passage of the Flexor digitorum profundus tendon, before inserting into the middle phalanges of the four fingers.

Origin

The muscle originates from two points:

• Humero-ulnar head: This larger part originates from the medial epicondyle of the humerus (part of the common flexor tendon) and the coronoid process of the ulna.
• Radial head: This smaller part originates from the upper part of the radius.

Insertion

The FDS inserts into the palmar surfaces of the middle phalanges of the second to fifth fingers. This allows it to flex the wrist and the middle phalanges of these fingers.

Nerve Supply

The FDS is innervated by the median nerve, which is one of the main nerves running through the arm into the hand. Specifically, it usually receives innervation from the anterior interosseous branch of the median nerve.

Function

The Flexor digitorum superficialis (FDS) has several important functions related to the movement of the fingers and wrist.

• Flexion of the Intermediate Phalanges: The primary function of the FDS is to flex the intermediate (middle) phalanges of the four fingers (index, middle, ring, and little fingers). When the FDS contracts, it pulls on the tendons that attach to the middle phalanges, causing these bones to move towards the palm, thus bending the fingers at the proximal interphalangeal joints (the middle joints of the fingers). This action is crucial for tasks such as gripping, typing, and holding objects.
• Flexion of the Proximal Phalanges: Although the FDS primarily flexes the intermediate phalanges, it also assists in flexing the proximal phalanges at the metacarpophalangeal joints (the knuckles). This is because the tendons of the FDS pass through the carpal tunnel and insert on the middle phalanges; when they contract, they also exert a pull on the bases of the proximal phalanges, contributing to finger curling.
• Flexion of the Wrist: The Flexor digitorum superficialis, along with other muscles in the forearm, assists in flexing the wrist. When the fingers are extended, contraction of the FDS can help bend the wrist towards the palm (palmar flexion). This is useful for stabilizing the wrist during activities that require forceful finger flexion, like gripping heavy objects.
• Assisting in Hand Functions: The FDS, by flexing the fingers and wrist, plays a crucial role in various hand functions such as grasping, holding, and manipulating objects. It works in coordination with other muscles in the hand and forearm to control the strength and precision of grip.
• Contribution to Tendon Action: The tendons of the Flexor digitorum superficialis pass under the flexor retinaculum (a strong fibrous band around the wrist) and through the carpal tunnel before inserting into the fingers. This anatomical setup allows the FDS to contribute to the overall smooth movement of the tendons, reducing friction and preventing tendon entrapment.
• Synergistic and Antagonistic Actions: The FDS works in concert with other flexor muscles like the Flexor digitorum profundus (FDP) for flexing the fingers, creating a coordinated movement. Additionally, it acts in opposition to the extensor muscles of the forearm and hand, which straighten the fingers and wrist, thereby providing a balanced muscular control for various manual activities.

Clinical Significance

The Flexor digitorum superficialis (FDS) muscle has significant clinical relevance, especially in the fields of orthopedics, sports medicine, and physical therapy.

• Tendon Injuries: The tendons of the FDS are prone to injuries due to their constant use and their passage through the carpal tunnel. Strains, tears, or inflammation of these tendons can lead to pain, swelling, and reduced functionality in the fingers and wrist. Such conditions might arise from repetitive strain injuries (RSIs), commonly seen in individuals who perform repetitive manual tasks, or from acute injuries in sports.
• Carpal Tunnel Syndrome (CTS): While the primary concern in carpal tunnel syndrome is the median nerve compression, the tendons of the FDS also pass through the carpal tunnel. They can contribute to the pressure within this space. In cases of CTS, symptoms such as pain, tingling, or numbness can be partly due to the increased pressure from inflamed or swollen FDS tendons.
• Trigger Finger (Stenosing Tenosynovitis): This condition is characterized by the narrowing of the sheath that surrounds the tendon of the FDS, leading to pain, stiffness, and a ‘locking’ sensation in the affected finger. It’s called “trigger finger” because the finger may suddenly snap straight, like pulling and releasing a trigger. The FDS tendon can be involved in this pathology, requiring interventions ranging from splinting and corticosteroid injections to surgical release.
• Dupuytren’s Contracture: Though primarily affecting the fascia of the palm, Dupuytren’s Contracture can lead to secondary effects on the FDS tendons due to the contraction and thickening of the palmar tissues. This can restrict finger motion, particularly affecting the bending and straightening of the fingers.
• Rheumatoid Arthritis (RA): RA is an autoimmune condition that can lead to inflammation of the synovial sheaths surrounding the FDS tendons, causing pain and impaired movement. Chronic inflammation can lead to tendon damage and deformity, significantly impacting hand function.
• Surgical Relevance: The FDS muscle and its tendons are important in various surgical procedures related to the hand and forearm. For example, in tendon transfer surgeries, portions of the FDS tendons might be used to replace or augment damaged tendons of other fingers or the thumb. Understanding the anatomy and function of the FDS is crucial for these procedures.
• Rehabilitation and Physical Therapy: After injury or surgery to the hand, wrist, or forearm, rehabilitation exercises often target the FDS to restore strength and flexibility. Knowledge of the FDS’s function is essential for designing effective rehabilitation programs.
• Diagnostic Procedures: The function of the FDS is often evaluated in clinical settings to diagnose the extent of an injury or the progress of a disease. Tests might include asking a patient to flex their finger at certain joints or resist against pressure to assess the integrity and strength of the FDS.