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Extensor Pollicis Brevis (EPB)

Key Points:

• Extends the thumb at metacarpal-phalangeal joint.
• Extrinsic extensor muscle.
• Located in the first extensor/dorsal compartment.

Muscle Function:
• Extends the metacarpal-phalangeal joint of the thumb.
• Extends and abducts the carpal-metacarpal joint of the thumb
• Assists with radial deviation of the wrist.
Palpation: Palpate the tendon on the radial aspect of the anatomic snuffbox over the dorsal aspect of the hand. It lies on the ulnar side of the abductor pollicis longus tendon. The extensor pollicis longus forms the ulnar border of the anatomic snuffbox.
Strength Testing: Position – the thumb interphalangeal joint in slight flexion. Stabilize – the thumb by holding it proximal to the metacarpal-phalangeal joint. Resist – thumb extension by applying force against the dorsal proximal phalanx (in the direction of flexion).
Possible Substitutions: Extensor pollicis longus – if the interphalangeal joint is not kept in flexion.
Relevant Anatomy: Extensor pollicis brevis is located in the first extensor/dorsal compartment and has the most distal muscle bell in the first extensor compartment.
• Roots:  C7, C8.
• Nerve: Radial nerve, posterior interosseous nerve.
• Innervation Route: C7, C8 → radial nerve → posterior interosseous nerve → extensor pollicis brevis branch.
Origin: Distal and dorsal aspect of radius, distal to the origin of abductor pollicis longus; interosseous membrane.
Insertion: Dorsal ridge over dorsal base of proximal phalanx of thumb. Extensor pollicis brevis lies radial to extensor pollicis longus.