The extensor carpi ulnaris tendon (ECU) originates as two heads which attach to the lateral epicondyle and the middle third of the posterior ulna. It has a single distal insertion upon the posterior aspect of the base of the fifth metacarpal. The ECU functions to extend and adduct the hand, and is important in the ability to ulnar deviate the hand.

A unique anatomical characteristic of the ECU is the fibro-osseous tunnel which stabilizes the tendon at the level of the distal ulna. This fibro-osseous tunnel is formed by the distal ulna and a 1.5 to 2cm in length band of connective tissue referred to as the ECU subsheath. The subsheath lies deep to the extensor retinaculum, which itself does not attach to or stabilize the ECU tendon. The ECU, its subsheath, and the extensor retinaculum are readily seen using MRI.

Tenosynovitis and tendinosis of the ECU are not uncommon, with these abnormalities being a frequent early finding in patients with rheumatoid arthritis. In athletes, the ECU is the second most common site of wrist tendinopathy, typically associated with rowing, racquet sports, and golf. In such patients, chronic stress upon the tendon results in inflammation of its synovial lining, causing tenosynovitis. Over time, stress may also lead to tendon degeneration and altered collagen content, resulting in tendinosis with or without partial tears.