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Third extensor digitorum tendon (Extensor digitorum tendon to ring finger)

The third extensor digitorum tendon refers to the tendon of the extensor digitorum muscle that inserts into the ring finger (fourth digit). It is one of the four main tendons of the extensor digitorum (ED) arising from the lateral epicondyle of the humerus, passing along the dorsum of the hand and inserting onto the dorsal digital expansion of the ring finger.

This tendon is part of the common extensor system of the hand, contributing to coordinated finger extension along with the extensor indicis, extensor digiti minimi, and lumbrical/interosseous muscles. It plays a critical role in extension at the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints of the ring finger.

Synonyms

  • Extensor digitorum tendon to the ring finger

  • Fourth digit extensor tendon

  • EDC tendon to the ring finger

Origin, Course, and Insertion

Origin: Arises indirectly from the common extensor digitorum muscle, which originates from the lateral epicondyle of the humerus via the common extensor tendon and adjacent deep fascia of the forearm

Course:

  • Travels distally in the posterior compartment of the forearm, deep to the extensor retinaculum within the fourth dorsal compartment, accompanied by other EDC tendons and the extensor indicis tendon

  • Over the dorsum of the hand, the tendons diverge toward their respective digits; the third tendon continues centrally toward the ring finger

  • At the dorsum of the proximal phalanx, the tendon expands into the dorsal digital expansion (extensor hood)

Insertion:

  • Divides into a central slip, which inserts into the base of the middle phalanx, and two lateral slips, which unite and attach to the base of the distal phalanx

  • This arrangement enables simultaneous extension at the MCP, PIP, and DIP joints of the ring finger

Relations

  • Proximally: Deep to the extensor retinaculum in the fourth dorsal compartment

  • Distally: Overlying the dorsal surface of the proximal and middle phalanges

  • Medially: Adjacent to the extensor tendon to the little finger (EDM)

  • Laterally: Adjacent to the extensor tendon to the middle finger

  • Deep surface: In contact with the dorsal capsule of MCP and PIP joints

  • Superficial surface: Covered by dorsal hand skin and subcutaneous fascia

Tendinous Connections

  • The juncturae tendinum (intertendinous connections) link the third EDC tendon to neighboring tendons, especially those to the middle and little fingers

  • These intertendinous connections coordinate finger extension and distribute extensor forces

  • The sagittal bands and extensor hood stabilize the tendon over the MCP joint, preventing lateral displacement

Nerve Supply

  • Supplied by the posterior interosseous nerve, a branch of the radial nerve (C7–C8)

Function

  • Extension of the ring finger: Extends the MCP, PIP, and DIP joints through the extensor mechanism

  • Assists wrist extension: When acting with other EDC tendons

  • Coordinated finger motion: Works with interossei and lumbricals for fine motor control

  • Dynamic stabilization: Maintains alignment of the extensor hood during flexion and extension cycles

Clinical Significance

  • Tendon rupture or laceration: May occur due to trauma or degenerative conditions (e.g., rheumatoid arthritis) leading to loss of active extension at the ring finger

  • Sagittal band injury: Can cause extensor tendon subluxation or dislocation over MCP joint

  • Extensor tendinitis (tenosynovitis): Common in repetitive extension or gripping activities

  • Juncturae tendinum injury: May affect coordination between ring and adjacent finger movements

  • Mallet finger deformity: Distal attachment disruption leading to loss of DIP extension

  • Surgical relevance: Frequently encountered in dorsal hand surgery, tendon repair, and extensor hood reconstruction

MRI Appearance

  • T1-weighted images:

    • Tendon: Uniformly low signal (dark linear structure) extending from the dorsal hand to the distal phalanx

    • Muscle belly (proximal forearm): Intermediate signal intensity

    • Peritendinous fat: Bright signal outlining tendon contours

    • Tear or rupture: Focal discontinuity or gap with intermediate-to-bright signal replacing tendon fibers

  • T2-weighted images:

    • Normal tendon: Low signal (dark), sharply marginated

    • Partial tear or tendinitis: Focal hyperintense areas within tendon substance

    • Complete tear: Gap filled with bright fluid signal

    • Surrounding edema or inflammation: Hyperintense soft-tissue signal around tendon sheath

  • STIR:

    • Normal tendon: Dark flow void appearance

    • Pathology: Bright hyperintense regions indicate peritendinous fluid, synovial inflammation, or partial tear

    • Useful for early detection of tenosynovitis or small fiber injury

  • Proton Density Fat-Saturated (PD FS):

    • Normal tendon: Uniformly dark signal

    • Pathologic: Bright linear or focal hyperintensity at insertion or within tendon fibers (indicative of microtears or edema)

    • Excellent for assessing sagittal band integrity and tendon continuity

  • T1 Fat-Sat Post-Contrast:

    • Normal tendon: Minimal enhancement

    • Tendinitis or synovitis: Peritendinous enhancement

    • Chronic degeneration: Peripheral enhancement with central low signal (fibrosis)

CT Appearance

Non-Contrast CT:

  • Tendon: Seen as a fine soft-tissue band with slightly lower attenuation than bone, coursing dorsally along the phalanges

  • Bone landmarks: MCP, PIP, and DIP joints well visualized for tendon alignment reference

  • Pathology:

    • Calcific deposits in chronic tendinopathy may appear hyperdense

    • Avulsion fractures at tendon insertion detected clearly

    • Subtle cortical irregularities at enthesis (attachment site) identifiable

Post-Contrast CT (standard):

  • Normal tendon: Mild, uniform enhancement due to vascularized synovium around sheath

  • Inflammation: Enhanced peritendinous soft tissue in tenosynovitis or post-traumatic changes

  • Tear or rupture: Focal contrast pooling in tendon gap

MRI images

Third extensor digitorum tendon (Extensor digitorum tendon to ring finger) axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000

MRI images

Third extensor digitorum tendon (Extensor digitorum tendon to ring finger) axial cross sectional anatomy 3T MRI AI enhanced radiology image-img-00000-00000_00001

CT images

Third extensor digitorum tendon (Extensor digitorum tendon to ring finger) ct axial