Extensor carpi ulnaris subluxation or dislocation? Ultrasound measurement of tendon excursion and normal values.

Dislocation or subluxation of the extensor carpi ulnaris (ECU) tendon is one of the differential diagnoses of ulnar-sided wrist pain. No data exists on the degree of subluxation occurring in normal wrists. Retrospective review of surgical cases revealed two patients with dorsal and four with volar instability. Magnetic resonance imaging and ultrasound were not always predictive of the direction of instability. A standardised ultrasound technique was used to evaluate ECU subluxation in 20 asymptomatic wrists. In normal subjects, as a percentage of the width of the osseous groove, the volar edge of extensor carpi ulnaris may displace by up to 40% beyond the volar lip of the osseous groove with wrist flexion and the dorsal edge by 33% beyond the dorsal lip with wrist extension. The effect of gender on normal range and the diagnostic use of the standardised ultrasound technique need further evaluation.