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Flexor digitorum superficialis muscle

The flexor digitorum superficialis (FDS) is a large, flat muscle located in the superficial layer of the anterior compartment of the forearm. It is the largest and most powerful superficial flexor of the fingers, responsible for flexing the proximal interphalangeal (PIP) joints of the second to fifth digits and assisting in wrist and metacarpophalangeal joint flexion.

It lies deep to the pronator teres and palmaris longus, but superficial to the flexor digitorum profundus (FDP) and flexor pollicis longus (FPL). The FDS has a humeroulnar head and a radial head, forming a fibrous arch through which the median nerve and ulnar artery pass — a key anatomical relationship of clinical importance.

Synonyms

  • Superficial digital flexor

  • Sublimis muscle

Origin, Course, and Insertion

  • Origin:

    • Humeroulnar head: Medial epicondyle of the humerus via the common flexor tendon, ulnar collateral ligament, and coronoid process of the ulna

    • Radial head: Upper two-thirds of the anterior border of the radius

  • Course:

    • Muscle fibers form a large belly in the forearm that gives rise to four tendons passing deep to the flexor retinaculum through the carpal tunnel

    • In the palm, each tendon splits into two slips that diverge around the flexor digitorum profundus (FDP) tendon and reunite distally

  • Insertion:

    • Each tendon inserts on the sides of the middle phalanx of the second to fifth fingers

Tendon Attachments

  • Four tendons enclosed within a common synovial sheath in the carpal tunnel

  • Split tendons (Camper’s chiasm) allow passage of FDP tendons beneath

  • Attached firmly to the middle phalanges, enabling PIP joint flexion without affecting DIP joints

Relations

  • Superficially: Palmaris longus and pronator teres

  • Deeply: Flexor digitorum profundus and flexor pollicis longus

  • Laterally: Flexor carpi radialis and radial artery

  • Medially: Flexor carpi ulnaris and ulnar artery

  • Posteriorly: Median nerve and ulnar artery pass deep to its fibrous arch

Nerve Supply

  • Median nerve (C7–T1) — enters the muscle between the humeroulnar and radial heads

  • Motor innervation is typically from the C8–T1 fibers, providing fine motor control for finger flexion

Arterial Supply

  • Ulnar artery — primary source via its muscular branches

  • Radial artery — contributes through radial recurrent and anterior ulnar recurrent arteries

  • Common interosseous artery — minor deep supply to proximal fibers

Venous Drainage

  • Venae comitantes accompanying the ulnar and radial arteries

  • Drain into brachial veins, eventually reaching the axillary vein

Function

  • Flexion of fingers: Flexes the proximal interphalangeal (PIP) joints of digits II–V

  • Assists in wrist flexion: Synergist with flexor carpi radialis and flexor carpi ulnaris

  • Flexion at metacarpophalangeal joints: Aids finger curling movements during grip

  • Precision grip control: Provides independent finger flexion for fine motor coordination

Clinical Significance

  • Median nerve entrapment: Compression under FDS fibrous arch may cause forearm pain and paresthesia (pronator syndrome)

  • Tendinitis or partial tear: Common in athletes or manual laborers due to repetitive wrist/finger flexion

  • FDS function test: Isolated by blocking other digits and testing independent finger flexion

  • Trauma: Lacerations or tendon ruptures impair finger flexion and hand strength

  • Surgical importance: Serves as donor muscle in tendon transfer procedures for nerve injury or reconstruction

MRI Appearance

  • T1-weighted images:

    • Muscle belly: intermediate signal intensity, homogeneous appearance

    • Tendons: low signal (dark bands) extending through forearm and carpal tunnel

    • Fat planes between muscles: bright, clearly delineating compartments

    • Chronic atrophy: muscle replaced by bright fatty signal

  • T2-weighted images:

    • Normal muscle: intermediate-to-low signal, slightly darker than T1

    • Tendons: dark linear structures with smooth margins

    • Pathology: bright hyperintense foci in muscle or tendon (tendinitis, tear, or edema)

    • Surrounding fascia: may show high signal if inflamed or thickened

  • STIR:

    • Normal muscle: intermediate-to-dark signal intensity

    • Pathologic: bright hyperintense signal in muscle belly or myotendinous junction (edema, strain, or compartment inflammation)

  • Proton Density Fat-Saturated (PD FS):

    • Normal muscle: intermediate-to-dark, homogeneous

    • Pathologic: bright hyperintense areas in tendon or peritendinous region (tendinitis or fluid collection)

    • Excellent for subtle fiber injury or tenosynovitis detection

  • T1 Fat-Sat Post-Contrast:

    • Normal: uniform mild enhancement

    • Inflamed tendon sheath: shows linear or peritendinous enhancement

    • Partial tear or synovitis: patchy enhancement at tendon margins

    • Chronic tendinopathy: minimal or peripheral enhancement around fibrotic core

CT Appearance

Non-Contrast CT:

  • Muscle belly: soft-tissue density, well defined between surrounding fascia

  • Tendons: fine dense bands extending distally toward the wrist

  • Chronic changes: calcification or ossification may be visible in chronic tendinitis or trauma

  • Provides clear depiction of muscle bulk, fascial boundaries, and tendon continuity

Post-Contrast CT (standard):

  • Normal muscle: homogeneous enhancement

  • Inflamed or injured regions: focal or diffuse hyperenhancement

  • Detects subtle tenosynovitis, hematoma, or soft-tissue thickening

  • Useful for evaluating postoperative or traumatic conditions when MRI is unavailable

MRI images

Flexor digitorum superficialis axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000

MRI images

Flexor digitorum superficialis axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00001

MRI images

Flexor digitorum superficialis axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00002

MRI images

Flexor digitorum superficialis axial cross sectional anatomy 3T MRI AI enhanced radiology image -img-00000-00000_00003

CT images

Flexor Digitorum Superficialis Muscle ct axial

MRI image of wrist

Flexor digitorum superficialis muscle axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

CT image of wrist

Flexor digitorum superficialis ct axial