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Fourth flexor digitorum profundus tendon

The fourth flexor digitorum profundus (FDP) tendon is the terminal tendon of the medial portion of the flexor digitorum profundus muscle, which specifically inserts into the distal phalanx of the little finger. It lies deep within the anterior compartment of the forearm and passes through the carpal tunnel before entering the fibro-osseous digital sheath of the fifth digit.

This tendon is responsible for flexion of the distal interphalangeal (DIP) joint of the little finger and contributes to overall finger flexion and grip strength. It works in close coordination with the flexor digitorum superficialis (FDS) tendon, which flexes the proximal interphalangeal (PIP) joint.

The FDP tendon of the little finger is typically supplied by the ulnar half of the flexor digitorum profundus muscle, which is innervated by the ulnar nerve. Anatomical variations are common, including fusion with the ring finger FDP tendon or partial intertendinous connections within the palm.

Synonyms

  • FDP tendon to the fifth digit

  • Deep flexor tendon of the little finger

  • Terminal tendon of the flexor digitorum profundus (ulnar part)

Origin, Course, and Insertion

  • Origin: From the medial and anterior surfaces of the ulna and the adjacent interosseous membrane as part of the ulnar (medial) half of the flexor digitorum profundus muscle.

  • Course:

    • The tendon descends deep to the flexor digitorum superficialis in the forearm.

    • It passes beneath the flexor retinaculum and through the carpal tunnel within the common flexor sheath.

    • In the palm, it runs deep to the lumbrical muscles and continues into the fibro-osseous digital canal of the little finger.

    • It passes through the split of the flexor digitorum superficialis tendon (Camper’s chiasm) at the level of the proximal phalanx.

  • Insertion: Attaches to the palmar surface of the base of the distal phalanx of the fifth digit (little finger).

Relations

  • Proximal forearm: Deep to flexor digitorum superficialis, adjacent to the ulna.

  • At wrist: Lies within the carpal tunnel, medial to the FDP tendons of the index, middle, and ring fingers.

  • In the palm: Deep to the lumbrical muscle of the fifth finger and surrounded by the synovial sheath of the ulnar bursa.

  • In the digit: Deep to the flexor digitorum superficialis tendon within the fibrous flexor sheath of the little finger.

  • Distally: Crosses the DIP joint to insert on the distal phalanx.

Tendon Attachments and Sheaths

  • Enclosed within a synovial sheath continuous with the ulnar bursa of the hand.

  • Supported by annular (A2, A4) and cruciform (C1, C2, C3) pulleys that maintain tendon alignment during flexion.

  • Associated with vincula longa and brevia, small synovial folds carrying blood vessels from the periosteum to the tendon surface.

  • At the insertion, merges with the periosteum of the distal phalanx.

Function

  • Primary flexor of the distal interphalangeal (DIP) joint of the little finger.

  • Assists in flexion of the proximal interphalangeal (PIP) and metacarpophalangeal (MCP) joints when acting synergistically with the FDS and intrinsic hand muscles.

  • Contributes to overall grip strength, fine digital control, and power grasp.

  • Important in precision grip and hand dexterity, particularly in ulnar-side activities like cupping or grasping cylindrical objects.

Clinical Significance

  • Tendon injury: Lacerations in flexor zones I and II (“no man’s land”) can lead to loss of DIP flexion in the little finger.

  • Avulsion (Jersey finger): FDP tendon may avulse from the distal phalanx during forced extension.

  • Adhesions or tenosynovitis: Common in repetitive use or after repair; limits smooth tendon gliding.

  • Rupture in rheumatoid arthritis: May occur due to attrition over bony spurs or the hook of hamate.

  • Surgical importance: FDP tendon repairs require precise tensioning for functional restoration.

  • Imaging relevance: MRI crucial for evaluating partial tears, complete ruptures, adhesions, and synovial sheath inflammation.

MRI Appearance

  • T1-weighted images:

    • Tendon: Low signal (dark), homogeneous, well-defined borders.

    • Muscle belly (in forearm): Intermediate signal intensity.

    • Fat planes: Bright, separating tendon and adjacent structures.

    • Pathology:

      • Partial tear: focal thinning or discontinuity with intermediate signal within tendon.

      • Complete rupture: retracted tendon end with bright interposed fluid or granulation tissue.

  • T2-weighted images:

    • Tendon: Low signal intensity, darker than on T1.

    • Normal sheath fluid: Minimal or absent.

    • Tendinitis: Focal or diffuse hyperintense signal within or around the tendon.

    • Tenosynovitis: Bright hyperintense fluid surrounding tendon within sheath.

  • STIR:

    • Normal tendon: Intermediate-to-dark signal.

    • Pathology: Hyperintense signal in tendon or peritendinous space indicating inflammation, edema, or partial tear.

    • Excellent for detecting early inflammatory changes and postoperative adhesions.

  • Proton Density Fat-Saturated (PD FS):

    • Normal: Intermediate-to-dark signal in muscle and tendon; smooth, uniform tendon contour.

    • Pathologic: Focal bright signal indicates tendinosis or partial tear.

    • Adhesions or sheath thickening: Appear as irregular peritendinous hyperintensity.

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: Minimal or no enhancement.

    • Inflammation or postoperative repair: Enhancing peritendinous soft tissue or synovial lining.

    • Tear: Peripheral or nodular enhancement around tendon defect, granulation tissue, or retraction zone.

CT Appearance

Non-Contrast CT:

  • Tendon: Seen as a soft-tissue density band within the flexor sheath; low attenuation relative to muscle.

  • Bone insertions: Distal phalanx attachment clearly seen; avulsion fractures appear as small cortical fragments at insertion site.

  • Chronic tendinopathy: May show mild thickening or calcific densities along tendon course.

Post-Contrast CT (standard):

  • Tendon and sheath: Mild enhancement of synovial sheath or peritendinous tissues in tenosynovitis or post-trauma.

  • Complete rupture: Contrast may outline retracted tendon stump or enhance surrounding scar tissue.

  • Utility: Helpful when MRI is contraindicated, especially for evaluating calcific tendinopathy or post-surgical integrity.

MRI image

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

MRI image

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

CT image

Fourth flexor digitorum profundus tendon (little) ct image