Extensor Digitorum Tendon to Middle Finger: Anatomy, Function, Imaging

Topic

Second extensor digitorum tendon (Extensor digitorum tendon to middle finger)

The second extensor digitorum tendon, also known as the extensor digitorum tendon to the middle finger, is one of the four tendons of the extensor digitorum (ED) muscle that extend the medial four digits. This tendon specifically serves the third digit (middle finger) and is a critical component of the dorsal digital extensor mechanism, responsible for coordinated extension of the metacarpophalangeal (MCP), proximal interphalangeal (PIP), and distal interphalangeal (DIP) joints.

It lies on the dorsal aspect of the hand and finger, superficial to the dorsal interossei and deep to the extensor retinaculum at the wrist. The tendon is part of the intricate extensor network that includes intertendinous connections (juncturae tendinum) and merges distally with the extensor expansion (dorsal digital aponeurosis).

Synonyms

Second extensor digitorum tendon
Extensor tendon to the third digit
Middle finger extensor tendon

Origin, Course, and Insertion

Origin: From the extensor digitorum muscle, arising from the lateral epicondyle of the humerus via the common extensor tendon.
Course:
- The tendon emerges on the dorsum of the forearm and passes beneath the extensor retinaculum within the fourth dorsal compartment of the wrist, along with the tendon to the index, ring, and little fingers.
- It continues distally over the dorsum of the hand, forming the second extensor slip directed toward the middle finger.
Insertion:
- The tendon divides into three slips that merge into the extensor expansion (dorsal digital hood) on the dorsum of the middle finger.
- The central slip inserts into the base of the middle phalanx, while the two lateral slips reunite and attach to the base of the distal phalanx.

Tendon Attachments

Proximally: Continuous with the extensor digitorum muscle belly.
Distally: Expands into the dorsal aponeurosis blending with tendons of lumbricals and interossei.
Connected to adjacent tendons (index and ring fingers) via intertendinous bands (juncturae tendinum), which help coordinate finger extension.
Covered by a thin synovial sheath as it passes beneath the extensor retinaculum.

Relations

Superficial: Skin, dorsal hand fascia, and extensor retinaculum.
Deep: Dorsal interosseous muscle of the middle finger and the metacarpal shaft.
Medial and lateral: Adjacent extensor tendons joined by juncturae tendinum.
Distal: Dorsal digital expansion and finger extensor mechanism.

Nerve Supply

Supplied by the posterior interosseous nerve, a branch of the radial nerve (C7–C8), which innervates the parent muscle (extensor digitorum).

Function

Extension of the middle finger: Extends the MCP, PIP, and DIP joints.
Assists wrist extension: Acts synergistically with other extensors to stabilize the wrist during finger extension.
Fine motor control: Provides independent extension of the middle finger, aided by intertendinous connections for coordinated movement.
Balance and posture: Helps maintain tension and finger alignment during grasp release.

Clinical Significance

Tendon rupture: May occur due to trauma, laceration, or rheumatoid arthritis; results in loss of active extension at affected joints.
Sagittal band injury: Causes extensor tendon subluxation or dislocation at the MCP joint.
Extensor hood tear: Disrupts extension mechanism of the middle finger.
Tenosynovitis: Inflammation of the tendon sheath due to repetitive strain or rheumatoid disease.
Surgical relevance: Important in tendon transfers, repair, and dorsal approach to MCP joint.

MRI Appearance

T1-weighted images:
- Normal tendon: low signal (dark linear band) throughout its course.
- Surrounding subcutaneous fat: bright, aiding delineation.
- Muscle belly: intermediate signal in the proximal forearm.
- Tendon tear or discontinuity: focal interruption with interposed high-signal gap (hematoma or granulation tissue).
T2-weighted images:
- Normal tendon: dark, may appear slightly thicker near MCP joint.
- Partial tear: focal bright hyperintense signal within tendon.
- Complete rupture: discontinuity with bright fluid gap.
- Tenosynovitis: hyperintense fluid surrounding the tendon sheath.
STIR:
- Normal tendon: intermediate-to-dark signal intensity.
- Pathology: bright hyperintense signal in cases of inflammation, fluid, or peritendinous edema.
- Highlights post-traumatic or infectious tenosynovitis.
Proton Density Fat-Saturated (PD FS):
- Normal: intermediate-to-dark, sharply marginated tendon profile.
- Partial tear or tendinitis: focal bright intratendinous signal with mild thickening.
- Complete rupture: bright fluid-filled defect with retracted tendon ends.
T1 Fat-Sat Post-Contrast:
- Normal tendon: minimal or no enhancement.
- Tendinitis or synovitis: diffuse or peritendinous enhancement.
- Post-surgical scar: enhancing peritendinous tissue without true discontinuity.
- Infection: intense, irregular enhancement with surrounding soft-tissue edema.

CT Appearance

Non-Contrast CT:

Tendon: fine linear soft-tissue density overlying metacarpal and phalanges.
Distinguishes tendon continuity and alignment with adjacent osseous landmarks.
Indirect signs of pathology: soft-tissue swelling, loss of tendon contour, or bony avulsion fragment.

Post-Contrast CT (standard):

Tendon sheath or peritendinous tissues may show enhancement in inflammation or infection.
Chronic tendinopathy: mild thickening or fibrosis without significant enhancement.
Useful for assessing calcifications, foreign bodies, or post-repair integrity when MRI is contraindicated.

MRI images

CT images

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