Extensor Pollicis Longus Tendon: Anatomy, Function, Imaging

Topic

Extensor pollicis longus tendon

The extensor pollicis longus (EPL) tendon is a strong, cord-like structure in the posterior forearm that extends from the EPL muscle to the distal phalanx of the thumb. It passes obliquely across the dorsal wrist, changing direction around the Lister’s tubercle of the radius before coursing toward the thumb. This unique pulley action increases the leverage of the tendon, allowing powerful thumb extension and control.

The tendon forms the medial border of the anatomical snuffbox, and its integrity is essential for thumb function. It is clinically significant because of its vulnerability to rupture after distal radius fractures, rheumatoid tenosynovitis, or attrition against the dorsal radial cortex.

Synonyms

Tendon of the long extensor of the thumb
EPL tendon

Origin, Course, and Insertion

Origin (muscle belly): Middle third of the posterior surface of the ulna and adjacent interosseous membrane.
Course:
- The tendon emerges from beneath the extensor retinaculum in the third dorsal compartment of the wrist.
- It passes obliquely around the Lister’s tubercle (dorsal radial tubercle), which acts as a pulley, redirecting its course toward the thumb.
- Crosses the wrist joint dorsally and proceeds toward the thumb dorsum.
Insertion:
- Dorsal base of the distal phalanx of the thumb.
- Sometimes sends minor slips to the dorsal base of the proximal phalanx or extensor hood.

Tendon Attachments

Enclosed in a synovial sheath under the extensor retinaculum (third compartment).
Glides over Lister’s tubercle, which maintains tendon alignment and direction.
Distally blends with the extensor expansion of the thumb.

Relations

Proximally: Extensor indicis and extensor digitorum tendons (laterally).
At the wrist: Curves around Lister’s tubercle of the radius.
Distally: Lies superficial to the dorsal capsule of the thumb joints.
Medially (ulnarly): Anatomical snuffbox and radial artery (deep to it).
Laterally (radially): Extensor pollicis brevis and abductor pollicis longus tendons.

Nerve Supply

Posterior interosseous nerve (branch of the radial nerve, roots C7–C8).

Function

Extension of the thumb: Extends both proximal and distal phalanges at interphalangeal and metacarpophalangeal joints.
Assist in wrist extension: Acts as a secondary wrist extensor during thumb extension.
Precision control: Stabilizes the thumb during pinching, writing, and gripping.
Anatomical landmark: Defines the ulnar border of the anatomical snuffbox.

Clinical Significance

Tendon rupture: Common after distal radius fracture or chronic attrition over Lister’s tubercle.
Tendinopathy and tenosynovitis: Associated with overuse, rheumatoid arthritis, or inflammatory disorders.
Trigger thumb: Thickening or adhesions cause painful tendon gliding restriction.
Surgical importance: EPL tendon often used for tendon transfer procedures (e.g., for extensor pollicis brevis or extensor indicis substitution).
Postoperative imaging: MRI or ultrasound used to evaluate tendon repair integrity or graft placement.

MRI Appearance

T1-weighted images:
- Tendon: Low signal (dark), linear structure following dorsal wrist contour.
- Muscle belly: Intermediate signal intensity.
- Fat planes: Bright, providing contrast around tendon.
- Pathology: Partial tears show focal intermediate signal; complete rupture shows tendon discontinuity with proximal retraction.
T2-weighted images:
- Normal tendon: Low signal (dark).
- Muscle: Intermediate signal, slightly darker than on T1.
- Pathology: Tendinopathy shows localized bright signal within tendon or sheath; fluid or inflammation appears hyperintense.
STIR:
- Normal tendon: Intermediate-to-dark flow void.
- Pathology: Bright hyperintensity around tendon sheath indicating edema, tenosynovitis, or postoperative inflammation.
- Sensitive for detecting small tears and peritendinous edema.
Proton Density Fat-Saturated (PD FS):
- Normal tendon: Homogeneous dark signal.
- Abnormal tendon: Bright focal areas within tendon indicating degeneration or partial tear.
- Tenosynovitis: Fluid-filled bright sheath surrounding dark tendon.
T1 Fat-Sat Post-Contrast:
- Normal tendon: No significant enhancement.
- Inflamed or reparative tissue: Peripheral or diffuse enhancement along sheath.
- Post-surgical repair: Linear enhancement at repair site consistent with granulation or scar tissue.

CT Appearance

Non-Contrast CT:

Tendon not directly seen unless calcified; appears as a fine soft-tissue density dorsal to radius and thumb.
Lister’s tubercle provides key bony landmark for tendon course.
Indirect findings: soft-tissue thickening, calcific deposits, or step-off deformity after fracture.

Post-Contrast CT (standard):

Enhancing tendon sheath indicates inflammation or synovitis.
Post-traumatic or postoperative changes show mild soft-tissue enhancement.
Useful for identifying osseous impingement, hardware irritation, or tendon entrapment following wrist fractures.

MRI images