Extensor Pollicis Longus: Origin, Insertion, Nerve, Action & The "Pulley" Muscle

The Extensor Pollicis Longus (EPL) is the longer and larger of the two thumb extensors. It takes a unique, winding path around the radius, using a bony prominence as a pulley to change its direction. It provides the final "flick" of extension at the tip of the thumb.

[Image of Extensor Pollicis Longus muscle anatomy]

Quick Anatomy Snapshot

Origin (Proximal)	Posterior surface of the middle third of the Ulna and the Interosseous Membrane.
Insertion (Distal)	Dorsal surface of the base of the Distal Phalanx of the Thumb (1st digit).
Nerve Supply	Posterior Interosseous Nerve (Deep branch of Radial Nerve) - C7, C8.
Blood Supply	Posterior Interosseous Artery.
Primary Actions	Thumb Extension: Extends the IP (tip), MCP, and CMC joints. Adduction: Assists in bringing the extended thumb back toward the index finger.

Deep Dive: The Anatomical Pulley

The EPL is famous for its relationship with Lister's Tubercle, a small bony bump on the dorsal aspect of the distal radius.

1. The 45-Degree Turn

The EPL originates on the Ulna but must reach the Thumb (radial side). To do this, it travels through the 3rd Dorsal Compartment and hooks around Lister's Tubercle. This tubercle acts as a pulley, changing the tendon's direction by about 45 degrees towards the thumb.

2. The Anatomical Snuffbox

The EPL forms the Medial (Ulnar) Border of the anatomical snuffbox. It is the single, distinct tendon on the back of the thumb side of the wrist, separated from the APL and EPB by the snuffbox depression.

Physio Corner: Clinical Relevance

💪 Functional Fact: The EPL is the only muscle capable of hyperextending the Interphalangeal (IP) joint of the thumb. This action is essential for the "Thumbs Up" sign.

Palpation

Ask the patient to lift their thumb tip off the table while keeping the palm flat. The EPL tendon will pop up clearly on the ulnar side of the snuffbox. You can trace it all the way to the thumbnail.

⚠️ Clinical Pathology: Drummer Boy's Palsy / EPL Rupture
Spontaneous rupture of the EPL tendon is a known complication of distal radius fractures (Colles' fracture). The tendon rubs against the rough bone or callous near Lister's tubercle and snaps, sometimes weeks after the initial injury. The patient suddenly loses the ability to extend the thumb tip.

Manual Muscle Testing (MMT)

To isolate the EPL, we must test the distal phalanx.

Testing Tip: Stabilize the proximal phalanx firmly. If you don't, the EPB and APL might move the whole thumb, masking weakness at the tip.

Step-by-Step Procedure (Oxford Scale)

Grade	Patient Action & Resistance
Grade 3, 4, 5 (Against Resistance)	Position: Forearm in neutral (mid-position), wrist neutral. Action: Patient extends the IP joint (lifts the thumb tip). Resistance: Applied to the dorsal aspect of the Distal Phalanx, pushing down into flexion. Grade 3: Full range against gravity. Grade 4/5: Holds against resistance.
Grade 2 (Gravity Eliminated)	Position: Forearm pronated (palm down) on table. Action: Patient slides the thumb tip laterally into extension. Result: Full range of motion.
Grade 0, 1 (Palpation)	Action: Palpate the ulnar border of the snuffbox. Cue: "Try to straighten the tip of your thumb." Grade 1: Tendon tension felt. Grade 0: No activity.

Frequently Asked Questions

Why is it called "Longus"?

Because its tendon travels all the way to the Distal Phalanx (tip) of the thumb. The "Brevis" (short) muscle stops at the Proximal Phalanx.

Does EPL help with adduction?

Yes. Because of its oblique angle around Lister's tubercle, as it extends the thumb, it also pulls it slightly towards the index finger (adduction) and supination.

Can you transplant another tendon to replace EPL?

Yes. A common surgery for EPL rupture is the EIP Transfer, where the Extensor Indicis Proprius (from the index finger) is rerouted to power the thumb.

Test Your Knowledge: EPL Quiz

1. Which bony landmark acts as a pulley for the EPL tendon?

2. Where does the EPL insert?

3. Which nerve supplies the Extensor Pollicis Longus?

4. The EPL tendon travels through which dorsal compartment?

5. The EPL forms which border of the Anatomical Snuffbox?

6. Where does the EPL originate?

7. Which muscle is commonly used to repair a ruptured EPL?

8. What is the unique action of EPL at the IP joint?

9. Spontaneous rupture of the EPL is associated with fractures of the:

10. Besides extension, the EPL assists in:

References

• Moore, K. L., Dalley, A. F., & Agur, A. M. R. (2018). Clinically Oriented Anatomy. 8th ed. Philadelphia: Wolters Kluwer.
• Standring, S. (2016). Gray's Anatomy: The Anatomical Basis of Clinical Practice. 41st ed. Elsevier.
• Magee, D. J. (2014). Orthopedic Physical Assessment. 6th ed. Elsevier.