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Second flexor digitorum superficialis tendon

The second flexor digitorum superficialis (FDS) tendon refers to the individual tendon slip of the FDS muscle that inserts into the middle phalanx of the middle finger (digit II). The FDS has four distinct tendons — one for each of the four fingers (index to little finger) — and the second tendon is dedicated to the middle finger, lying centrally in the flexor tendon hierarchy.

This tendon plays a crucial role in flexing the proximal interphalangeal (PIP) joint and assisting in metacarpophalangeal (MCP) joint flexion of the middle finger. It lies superficial to the flexor digitorum profundus (FDP) tendon within the carpal tunnel and digital sheath, where both function synergistically to enable smooth digital motion.

Synonyms

  • FDS tendon to middle finger

  • Second digital slip of the flexor digitorum superficialis

  • Middle finger flexor superficialis tendon

Origin, Course, and Insertion

  • Origin: The tendon arises from the central portion of the flexor digitorum superficialis muscle belly, which originates from the medial epicondyle of the humerus, ulnar collateral ligament, coronoid process of ulna, and anterior border of radius (via oblique line).

  • Course:

    • The tendon descends within the carpal tunnel, enclosed by the common flexor synovial sheath.

    • In the palm, it separates into individual slips for each finger; the second slip runs centrally toward the middle finger.

    • At the level of the proximal phalanx, it enters the fibrous digital sheath, remaining superficial to the FDP tendon.

  • Insertion:

    • At the base of the proximal phalanx, it divides into two slips (Camper’s chiasm) that split and curve around the FDP tendon.

    • The slips reunite dorsally and attach to the sides of the shaft of the middle phalanx, forming a tendinous arch through which the FDP tendon passes.

Relations

  • Anteriorly: Flexor sheath and palmar aponeurosis

  • Posteriorly: Flexor digitorum profundus tendon (passing through the split in FDS)

  • Laterally and medially: Fibrous digital sheath and vincula tendinea

  • Proximally: Common flexor sheath within the carpal tunnel

  • Distally: Middle phalanx insertion and continuation of FDP tendon toward distal phalanx

Nerve Supply

  • Supplied indirectly via its muscle belly by the median nerve (C7–T1) before tendon division

Function

  • Flexion of proximal interphalangeal joint (PIP): Primary action of the second FDS tendon on the middle finger

  • Flexion of metacarpophalangeal joint (MCP): Secondary contribution

  • Synergistic control: Coordinates with FDP for graded flexion and grip strength

  • Fine motor precision: Allows isolated flexion of the middle finger for typing, gripping, and instrumental manipulation

Clinical Significance

  • Tendon injuries: Commonly affected in hand lacerations, sports trauma, or zone II flexor injuries (“no-man’s land”)

  • Tenosynovitis: Inflammation of the digital sheath, causing pain, crepitus, and triggering

  • Adhesions: Scar formation between FDS and FDP reduces finger mobility post-surgery or trauma

  • Avulsion injuries: Sudden forceful extension may tear the tendon near its insertion

  • Imaging role: MRI crucial for localizing tears, partial ruptures, or adhesions within tendon sheath

MRI Appearance

  • T1-weighted images:

    • Tendon: Uniformly low signal (dark) linear structure within the digital sheath

    • Muscle belly (proximal): Intermediate signal intensity

    • Marrow of adjacent phalanges: Bright (fatty signal)

    • Surrounding fat planes: High signal providing clear delineation of the tendon

    • Tear or discontinuity: Focal gap or intratendinous intermediate hyperintensity

  • T2-weighted images:

    • Normal tendon: Low signal (dark), maintaining smooth linear contour

    • Inflammation or partial tear: Bright hyperintense areas within or around the tendon

    • Fluid within tendon sheath: Appears as hyperintense rim along tendon margins

    • Adhesions: Loss of clear peritendinous fat and irregular tendon border

  • STIR:

    • Normal tendon: Dark (low signal)

    • Pathologic conditions: Bright hyperintense regions indicating edema, tenosynovitis, or strain

    • Adjacent soft tissue edema or fluid: Bright signal outlining affected segment

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: Uniformly dark, sharply defined margins

    • Partial tear or tenosynovitis: Focal bright hyperintense signal with sheath distension

    • Adhesions or scarring: Irregular contour with patchy intermediate intensity

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: Minimal enhancement

    • Inflamed synovial sheath: Diffuse enhancement outlining tendon

    • Partial tears: Peripheral or focal enhancement adjacent to fluid-filled defect

    • Fibrotic adhesions: Mild peripheral enhancement with central low signal intensity

CT Appearance

Non-Contrast CT:

  • Tendon: Soft-tissue density band along volar aspect of the finger

  • Insertion: May show subtle indentation along middle phalanx

  • Pathology: Thickening, discontinuity, or retraction may be inferred indirectly

  • Calcific tendinitis: Appears as small hyperdense foci along tendon course

Post-Contrast CT (standard):

  • Tendon and sheath: Enhanced visualization of peritendinous tissues

  • Inflamed sheath: May show subtle rim enhancement

  • Tear or fluid collection: Appears as low-attenuation cleft or swelling around tendon

  • Useful for preoperative mapping of complex tendon injuries or adhesions when MRI is unavailable

MRI image

Second flexor digitorum superficialis tendon (middle) axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI image

Second flexor digitorum superficialis tendon (middle) axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT image

Second flexor digitorum superficialis tendon (middle) ct axial