Fourth Extensor Digitorum Tendon: Anatomy, Function, Imaging

Topic

Fourth extensor digitorum tendon (Extensor digitorum tendon to little finger)

The fourth extensor digitorum tendon is the terminal tendon of the lateral slip of the extensor digitorum muscle that inserts into the dorsum of the little finger (fifth digit). It is one of four extensor tendons derived from the extensor digitorum (ED), the primary extensor of the medial four digits.

This tendon lies on the dorsum of the hand, passes deep to the extensor retinaculum in the fifth dorsal compartment, and extends to the dorsal digital expansion of the little finger, where it joins with the extensor digiti minimi (EDM) tendon. The two often fuse distal to the metacarpophalangeal (MCP) joint, contributing to extension of the little finger’s proximal and distal phalanges.

Synonyms

Extensor digitorum tendon to fifth digit
Fourth tendon of extensor digitorum communis
Extensor digitorum proprius to little finger (variant term)

Origin, Course, and Insertion

Origin: Continuation of the extensor digitorum muscle arising from the lateral epicondyle of the humerus via the common extensor tendon.
Course:
- The fourth tendon arises from the extensor digitorum belly in the distal forearm.
- It passes beneath the extensor retinaculum, within the fifth dorsal wrist compartment, often sharing this tunnel with the tendon of the extensor digiti minimi.
- Distally, it continues along the dorsum of the hand and merges partially with the extensor digiti minimi tendon before inserting into the little finger.
Insertion:
- Inserts into the extensor expansion (dorsal aponeurosis) of the fifth digit, which attaches to the base of the distal phalanx via the extensor hood mechanism.

Tendinous Attachments and Connections

Sagittal bands: Anchor the tendon centrally over the MCP joint, preventing ulnar or radial displacement during extension.
Intertendinous connections (juncturae tendinum): Connect the fourth tendon with adjacent extensor tendons (commonly with the third and fifth), allowing coordinated finger extension.
Fusion: Frequently blends distally with the extensor digiti minimi tendon on the dorsum of the fifth digit.

Relations

Dorsally: Skin and dorsal hand fascia.
Ventrally: Dorsal interosseous muscles, joint capsule, and phalanges.
Medially: Extensor digiti minimi tendon (close relationship).
Laterally: Tendon of the third extensor digitorum (to the ring finger).
Proximally: Extensor retinaculum and common extensor origin.

Nerve Supply

Posterior interosseous nerve (branch of the radial nerve, root C7–C8) innervates the parent muscle (extensor digitorum), which gives rise to the tendon.

Function

Primary action: Extension of the little finger at the MCP, PIP, and DIP joints via the extensor hood.
Secondary role: Assists in wrist extension and ulnar deviation.
Synergistic action: Works with the extensor digiti minimi to provide full independent extension of the fifth digit.
Stabilization: Sagittal bands maintain the tendon’s midline position during finger motion.

Clinical Significance

Tendon rupture: May occur with trauma or rheumatoid arthritis, causing loss of active extension at the fifth MCP joint.
Sagittal band injury: Leads to tendon subluxation or snapping on the dorsal MCP joint.
Attritional wear: Seen in chronic inflammatory conditions (e.g., rheumatoid tenosynovitis).
Extensor lag: Incomplete extension of the little finger due to tendon adhesion, tear, or juncturae tendinum disruption.
Post-surgical considerations: Repair or grafting of this tendon is crucial in extensor tendon reconstruction for the ulnar digits.

MRI Appearance

T1-weighted images:
- Normal tendon: low signal (dark band) due to dense collagen composition.
- Muscle belly (in forearm): intermediate signal intensity.
- Surrounding fat and subcutaneous tissue: bright signal, providing contrast.
- Partial tear: focal thickening with intermediate signal.
- Complete rupture: tendon discontinuity with retraction and surrounding high-signal hematoma.
T2-weighted images:
- Normal tendon: low signal (dark) with smooth linear contour.
- Tendon sheath: thin rim of fluid may appear mildly hyperintense if physiologic.
- Tendinitis or partial tear: bright hyperintense signal within tendon substance or sheath.
- Complete tear: fluid-filled gap with proximal retraction.
STIR:
- Normal tendon: dark linear structure.
- Pathology (inflammation, tear, or tenosynovitis): bright hyperintense signal in tendon or peritendinous tissue.
- Best for detecting early tendon edema or sheath inflammation.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: dark, well-defined margins.
- Partial tear or tendinitis: bright hyperintense foci within tendon fibers.
- Tenosynovitis: fluid-filled sheath with surrounding bright peritendinous signal.
- Adhesions or fibrosis: heterogeneous intermediate signal along tendon course.
T1 Fat-Sat Post-Contrast:
- Normal tendon: minimal or no enhancement.
- Tendinitis or synovitis: shows peritendinous enhancement.
- Post-repair tendon: uniform mild enhancement due to granulation tissue.
- Chronic scarring: low-signal areas without enhancement.

CT Appearance

Non-Contrast CT:

Tendon appears as a thin, linear soft-tissue density dorsal to the metacarpals and phalanges.
Calcific tendinitis: may show punctate or linear hyperdensities.
Rupture or retraction: focal discontinuity or contour irregularity with adjacent soft-tissue swelling.

Post-Contrast CT (standard):

Enhances soft-tissue detail and inflammatory changes.
Tendinitis: mild peritendinous enhancement.
Tendon sheath effusion or abscess: appears as low-density area with rim enhancement.
Useful for evaluating foreign body, postoperative tendon graft integrity, or calcific deposits.

MRI images

CT images

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