Ultrasonographic Evaluation of Extensor Carpi Ulnaris Tendon Subluxation at the Ulnar Groove

Subluxation of the extensor carpi ulnaris (ECU) tendon resulting from dysfunction of the ECU subsheath is frequently observed in clinical settings. However, there is a lack of information on how the distal extension of the ECU subsheath and ulnar groove morphology contribute to tendon stability. This study aimed to investigate the ECU tendon kinematics at the ulnar groove in intact and in subsheath-sectioned cadaver wrists, and examine the correlation between tendon subluxation and ulnar groove morphology during forearm and wrist motion.

Eight human cadaveric upper extremities were fixed to a testing jig, allowing free wrist and forearm motion. An ultrasonographic probe was fixed to the ulna and placed in the ulnar groove, and images were obtained at three forearm and four wrist positions. The groove depth and the ECU subluxation distance were measured sonographically, and the distances were compared between different forearm and wrist positions (supination/pronation, flexion/extension, and radial/ulnar deviation). The ECU tendon stabilizing structures were sequentially sectioned, and the subluxation distance was compared between the intact state and these soft tissue-sectioned wrists.

Significant tendon subluxation occurred in the subsheath-intact wrists with a shallow groove in forearm supination, and wrist flexion and ulnar deviation. Changes in subluxation distance during pronation-supination motion were significantly correlated with the ulnar groove depth. Subsheath sectioning alone did not significantly increase tendon subluxation; however, further sectioning of the distal extension of the ECU subsheath significantly increased the subluxation distance in forearm pronation.

The ECU tendon moved in the ulnar direction with forearm supination, wrist flexion, and wrist ulnar deviation. A shallower groove was associated with greater ECU tendon subluxation. Complete ECU subsheath sectioning, including the distal extension, resulted in tendon subluxation.

A shallow ulnar groove and dysfunction of the distal extension of the ECU subsheath should be considered when examining ECU subluxation or dislocation.