Summary Snapping ECU is a clinical condition characterized by pain over the ulnar wrist caused by instability and tendonitis of the ECU tendon secondary overuse. Diagnosis is made with clinical examination with palpation of the ECU tendon and noting a painful snap while moving the wrist from pronation to supination. Treatment is usually rest and wrist splinting. In refractory cases ECU subsheath reconstruction may be considered. Epidemiology Incidence common with sporting activities Risk factors tennis golf rupby Etiology Pathoanatomy ECU subluxation is secondary to attenuation or rupture of the ECU subsheath (6th dorsal compartment) attenuation remains intact but is stripped at ulnar/palmar attachment to produce a false pouch that the ECU tendon can subluxate/dislocate into rupture ulnar sided ECU subsheath tears ECU subluxates on supination, and reduces on pronation radial sided ECU subsheath tears ECU subluxates on supination, and lies on top of the torn subsheath on pronation subluxation and snapping can lead to ECU tendonitis Anatomy Extensor tendon compartments Compartment 1 (De Quervain's Tenosynovitis ) APL EBP Compartment 2 (Intersection syndrome ) ECRL ECRB Compartment 3 EPL Compartment 4 EIP EDC Compartment 5 (Vaughan-Jackson Syndrome ) EDM Compartment 6 (Snapping ECU ) ECU ECU tendon ECU tendon inserts onto the 5th metacarpal base ECU subsheath is part of the TFCC that is most critical to ECU stability ECU lies dorsal during supination and translates ulnar during pronation in pronation, the ECU tendon exits the wrist in a straight direction. in supination, the tendon exits the sixth compartment at an angle of approximately 30° tension on the ECU retinaculum and subsheath is therefore greater during activities involving supination ECU subluxates during ulnar deviation, supination, wrist flexion this position has the greatest angulation of the ECU tendon with respect to the ulna Presentation Symptoms pain and snapping over dorsal ulnar wrist Physical exam flexion and supination of the wrist elicit a painful snap ECU tendon reduces with pronation Pain at ulnar wrist with ECU synergy test may be indicative of tendonitis Imaging Radiographs unremarkable Ultrasound can dynamically assess ECU stability MRI can show tendonitis, TFCC pathology, or degenerative tears of ECU Differential TFCC tear Hook of hamate fracture Ulnar styloid fracture Ulnar styloid impaction syndrome Ulnar tunnel syndrome Pisotriquetral arthritis Diagnosis Clinical diagnosis is made with careful history and physical examination Treatment Nonoperative wrist splint or long arm cast indications first line of treatment technique arm immobilized in pronation and slight radial deviation Operative ECU subsheath reconstruction +- wrist arthroscopy indications if nonoperative management fails technique direct repair in acute cases chronic cases may require a extensor retinaculum flap for ECU subsheath reconstruction wrist arthroscopy shows concurrent TFCC tears in 50% of cases