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 login to view 1 more bullet rupture login to view 4 more bullets subluxation and snapping can lead to ECU tendonitis Anatomy Extensor tendon compartments Compartment 1 (De Quervain's Tenosynovitis ) login to view 2 more bullets Compartment 2 (Intersection syndrome ) login to view 2 more bullets Compartment 3 login to view 1 more bullet Compartment 4 login to view 2 more bullets Compartment 5 (Vaughan-Jackson Syndrome ) login to view 1 more bullet Compartment 6 (Snapping ECU ) login to view 1 more bullet 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 login to view 3 more bullets ECU subluxates during ulnar deviation, supination, wrist flexion login to view 1 more bullet 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 login to view 4 more bullets Operative ECU subsheath reconstruction +- wrist arthroscopy login to view 6 more bullets