Extensor Carpi Ulnaris (ECU) tendinosis with interstitial tear

Extensor carpi ulnaris tendinosis is one of the common causes of ulnar sided wrist pain. Extensor carpi ulnaris passes through a fibro-osseous tunnel (the sixth extensor compartment), lying within a bony groove on the dorsal surface of ulna. ECU is maintained in this groove by the extensor retinaculum and the ECU subsheath. The ECU subsheath is not attached to the tendon.

The ECU originates from two heads, one from lateral epicondyle and other from the posterior middle third of ulna and distally it inserts into the posterior aspect of the base of fifth metacarpal.  

Racquet Sports and golf athletes are at an increased risk of ECU tendon pathology. In non-athletes rheumatoid arthritis is also to be excluded. The pathologies which can affect the ECU tendon includes tenosynovitis, tendinosis, tendon instability and rupture.

The PDFS axial and coronal image above shows mild thickening and intermediate signal intensity of the ECU tendon favoring tendinosis, with thickening and hyperintense signal intensity of the tendon sheath with associated mild soft tissue edema (suggesting tenosynovitis).

Serial axial sections of the wrist (clockwise) shows thickened ECU tendon with irregular shaped hyperintense intrinsic linear signal intensity favoring and interstitial tear.

No subluxation or dislocation of the ECU tendon is noted from the ulnar groove. The ECU subsheath and the extensor retinaculum appears intact. The orange thick arrow corresponds to the retinaculum and the thin green narrow corresponds to the ECU subsheath.

The above T2 axial image of the wrist is also showing the intact ECU subsheath (green narrow).

References: 

1. https://radiopaedia.org/articles/extensor-carpi-ulnaris-tendinopathy
2. https://radsource.us/extensor-carpi-ulnaris-subsheath-injury/