Introduction Overuse of wrist can lead to spectrum of ECU tendonitis and instability 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 Risks tennis golf 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 (Vaughn-Jackson Syndrome ) EDM Compartment 6 (Snapping ECU ) ECU ECU tendon ECU subsheath is part of the TFCC that is most critical to ECU stability 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 extension and supination of the wrist elicit a painful snap ECU tendon reduces with pronation Imaging Radiographs unremarkable Ultrasound can dynamically assess ECU stability MRI can show tendonitis, TFCC pathology, or degenerative tears of ECU 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