Tendon of extensor pollicis brevis

The tendon of extensor pollicis brevis (EPB) forms in the distal third of the posterior forearm, as the muscle belly tapers and extends obliquely toward the lateral aspect of the wrist.

Course in Relation to Other Muscles

• As the EPB tendon descends, it passes superficial (posterior) to the tendons of the extensor carpi radialis longus (ECRL) and extensor carpi radialis brevis (ECRB).

• These radial wrist extensors form a deep muscular plane over which the EPB and its companion, the abductor pollicis longus (APL), glide as they approach the wrist.

• Together, the APL and EPB tendons converge toward the lateral (radial) side of the wrist.

Passage Beneath the Extensor Retinaculum

• Just proximal to the wrist, the EPB tendon enters a fibrous tunnel within the first dorsal compartment of the extensor retinaculum, alongside the APL tendon.

• This tunnel lies lateral to the radial styloid process, and is lined by a common synovial sheath shared by both tendons. Within this compartment, the tendons are held close to the radius to prevent bowstringing during thumb movements.

Formation of the Anatomical Snuffbox Boundary

• Distal to the extensor retinaculum, the EPB tendon emerges on the lateral side of the dorsum of the wrist and continues toward the thumb.

• As it crosses obliquely over the radial aspect of the wrist and metacarpal, it forms the lateral boundary of the anatomical snuffbox.

Termination

The tendon of EPB passes distally to insert into the base of the proximal phalanx of the thumb (dorsal surface), where it acts to extend the metacarpophalangeal joint and assist in thumb abduction at the carpometacarpal joint.

Clinical Note

Inflammation of the synovial sheath enclosing the APL and EPB tendons beneath the extensor retinaculum results in De Quervain’s tenosynovitis, producing pain and swelling along the lateral wrist and base of the thumb, particularly on resisted thumb extension or abduction.