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Fourth flexor digitorum superficialis tendon

The fourth flexor digitorum superficialis (FDS) tendon is one of the four tendinous divisions of the FDS muscle, which lies in the superficial layer of the anterior forearm. This particular tendon serves the little finger, providing flexion at the proximal interphalangeal (PIP) joint and contributing to finger grip strength.

The FDS muscle forms four tendons that pass through the carpal tunnel, enclosed in a common synovial sheath. At the level of the palm, these tendons diverge to reach the four medial digits. The fourth tendon lies most medially and superficially, heading toward the fifth digit (little finger). Though occasionally smaller or thinner than the others, it remains functionally vital for coordinated digital flexion.

Synonyms

  • FDS tendon to the little finger

  • Fourth tendon of flexor digitorum superficialis

  • Superficial flexor tendon of digit V

Origin, Course, and Insertion

  • Origin (muscle): Common flexor origin from the medial epicondyle of the humerus, coronoid process of ulna, and upper two-thirds of the radius (via the FDS muscle).

  • Course (tendon):

    • The fourth tendon descends within the carpal tunnel, deep to the flexor retinaculum and medial to the flexor digitorum profundus tendons.

    • In the palm, it travels toward the little finger, ensheathed in the ulnar bursa (common flexor sheath).

    • Near the base of the proximal phalanx, the tendon bifurcates into two slips, forming the camper’s chiasm, through which the profundus tendon passes.

  • Insertion: The two slips reunite on the sides of the shaft of the middle phalanx of the little finger, inserting into its anterior aspect.

Relations

  • Superficially: Palmar aponeurosis and fibrous digital sheath

  • Deeply: Flexor digitorum profundus tendon (to same finger)

  • Medially: Tendon to ring finger

  • Laterally: Tendon sheath and lumbrical origin

  • Anteriorly (distal): Digital sheath of the little finger

  • Posteriorly: Proximal phalanx and vincula system (vascular support structure)

Tendon Attachments

  • Proximal: Arises from the muscle belly of the FDS in the forearm

  • Distal: Inserts onto the sides of the middle phalanx of the little finger

  • Vincula: Attached to the phalanges via vincula brevia and longa, providing vascular and connective support

  • Sheath: Enclosed in a synovial sheath continuous with the common flexor sheath in the carpal tunnel

Function

  • Flexion at the PIP joint: Primary flexor of the middle phalanx of the little finger

  • Assists MCP joint flexion: Acts with flexor digitorum profundus and lumbricals

  • Grip strength: Stabilizes and powers fine finger movements and grasp

  • Independent finger control: Allows isolated flexion of the fifth digit (though less independent than other digits due to tendinous interconnections)

Clinical Significance

  • Tendon injury: Lacerations or ruptures cause loss of PIP flexion in the little finger

  • Avulsion: Rare, but may occur with forceful hyperextension or trauma

  • Trigger finger: Tenosynovitis or nodular thickening causing catching sensation at A1 pulley

  • Adhesions or synovitis: Common after carpal tunnel surgery or flexor zone injuries

  • Variant anatomy: In some individuals, the FDS tendon to the little finger may be absent or fused with the ring finger tendon

  • Surgical relevance: Important in flexor tendon repair, grafting, and tendon transfer procedures

MRI Appearance

  • T1-weighted images:

    • Normal tendon: Uniform low signal (dark) due to dense collagen fibers

    • Surrounding fat planes: Bright, providing contrast for tendon delineation

    • Muscle belly (in forearm): Intermediate signal intensity

    • Tear or laceration: Discontinuity or focal intermediate-to-bright signal at site of injury

  • T2-weighted images:

    • Normal tendon: Low signal (dark line)

    • Muscle belly: Intermediate signal, darker than on T1

    • Pathology: Partial tear or tenosynovitis shows bright hyperintense signal within tendon or sheath

    • Peritendinous fluid: Appears bright and may outline tendon margins

  • STIR:

    • Normal tendon: Intermediate-to-dark signal

    • Inflammation or strain: Bright hyperintensity surrounding the tendon (synovial fluid or edema)

    • Highly sensitive for early tenosynovitis, strain, or postoperative scarring

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: Intermediate-to-dark signal, homogeneous in contour

    • Pathology: Bright signal areas correspond to edema, small partial tears, or sheath effusion

    • Excellent for evaluating trigger finger, partial ruptures, and adhesions

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: Mild peripheral enhancement from vascular sheath

    • Tendinitis or synovitis: Focal or diffuse enhancement of the sheath

    • Complete tear: Non-enhancing tendon gap with enhancement of granulation tissue at ends

    • Useful for assessing healing, scar tissue, or infection

CT Appearance

Non-Contrast CT:

  • Tendon: Soft-tissue density, linear structure with smooth contour

  • Bony landmarks: Middle phalanx base and flexor pulley system visible

  • Pathology: Thickening, calcific deposits, or partial discontinuity in trauma or chronic strain

  • Fracture correlation: Helpful in identifying associated avulsion fractures or bony entrapment

Post-Contrast CT (standard):

  • Normal tendon: Minimal homogeneous enhancement

  • Inflamed tendon sheath or synovitis: Localized or circumferential enhancement

  • Chronic injury: Fibrotic tendon thickening with irregular margins

  • Surgical follow-up: Distinguishes recurrent inflammation from postoperative scar tissue

MRI image

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

MRI image

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

CT image

Fourth flexor digitorum superficialis tendon (little) ct axial