OVERVIEW
Key Points:
• Extends the thumb at interphalangeal joint.
• Extrinsic extensor muscle.
• Located in the third extensor/dorsal compartment.
EXAMINATION
Muscle Function:
Primary:
• Extends the interphalangeal joint of the thumb.
• Assists in extension of the metacarpal-phalangeal and carpal-metacarpal joints.
Secondary:
• Assists in abduction and extension of the wrist.
Palpation: Palpate the tendon on the ulnar aspect of the anatomic snuffbox over the dorsal aspect of the hand. The tendons of the extensor pollicis brevis and abductor pollicis longus form the radial border of the anatomic snuffbox.
Strength Testing:
• Anti-gravity Test: Position ““ the thumb metacarpal-phalangeal joint in extension. Stabilize ““ the hand. Resist ““ thumb interphalangeal joint extension by applying pressure over the dorsal distal phalanx (in the direction of flexion). Watch for retropulsion of the thumb.
• Alternate Anti-gravity Test: Position ““ hand on table with palm down. Resist ““ retropulsion of the thumb off the table.
• Or, place the subjects thumb in extension and tell them to stay in that position while the examiner tries to flex the thumb at the interphalangeal joint.
Possible Substitutions: Extensor pollicis brevis ““ make sure to assess interphalangeal joint extension and/or retropulsion.
Relevant Anatomy: Extensor pollicis longus is located in the third extensor/dorsal compartment.
Innervation:
• Roots: C7, C8.
• Nerve: Radial nerve, posterior interosseous nerve.
• Innervation Route: C7, C8 → radial nerve → posterior interosseous nerve → extensor pollicis longus branch.
Origin: Dorsal and radial aspect of ulna, distal to origin of abductor pollicis longus; interosseous membrane.
Insertion: Dorsal base of distal phalanx of thumb.