Upper limb

Tendon sheath of fourth extensor compartment (ED and EI)

Vagina tendinum musculorum extensoris digitorum et extensoris indicis

Origin

Fourth dorsal compartment on the posterior distal radius beneath the extensor retinaculum

Insertion

Encloses the four tendons of extensor digitorum and the tendon of extensor indicis over the dorsal midcarpal region

Function

Facilitates frictionless excursion of the digital extensor tendons across the wrist joint

Innervation

Posterior interosseous nerve (C7–C8)

Movement & chain

Continuous excursion during finger extension and metacarpophalangeal motion

Palpation

Palpable in the center of the dorsal wrist; presents prominent swelling in inflammatory tenosynovitis

Clinical

Dorsal tenosynovitis in rheumatoid arthritis; untreated pannus can cause sequential extensor tendon ruptures (Vaughan-Jackson syndrome)

Segmental innervation

C7–C8 roots

In clinical practice

Early diagnostic ultrasound for inflammatory synovial hypertrophy; surgical synovectomy to preserve tendon continuity

Region in the model

Upper limb