The Extensor Carpi Ulnaris
The ECU arises from a common extensor origin on the lateral epicondyle of the humerus. It is inserted into the posterior surface of the base of the 5th metacarpal bone. The ECU tendon lies in the sixth dorsal extensor compartment of the wrist. The ECU tendon has its own fibro-osseous tunnel near the wrist. The ECU is the only wrist extensor that lies within its own fibro-osseous tunnel. The ECU tendon passes through a groove on the ulnar side of the distal ulna which is covered by an annular ligament. The ECU acts to extend and ulnar deviate the hand at the wrist. It helps the ECRL and the ECRB in the extension of the wrist. The ECU is innervated by the posterior interosseous nerve.
There are two conditions that are clinically important and related to the ECU:
- The recurrent subluxation of the ECU tendon
a. The tendon dislocates with supination and relocates with pronation
b. Subluxation of the tendon is secondary to rupture of the ECU sheath and it usually subluxes in a volar to ulnar direction.
2. Rupture of the ECU sheath
Recurrent dislocation will have a painful snap over the ulnar-dorsal aspect of the wrist, especially during forearm rotation. With supination and ulnar deviation, the tendon leaves the sheath and the groove. The patient will have pain with an audible “snap” and this will cause tendonitis. With supination, the tendon dislocates. With pronation, the tendon relocates. The ECU is more palpable with the patient’s wrist extended and ulnarly deviated. This condition can be confused with recurrent subluxation of the distal radioulnar joint. The extensor carpi ulnaris subsheath is critical to the extensor carpi ulnaris tendon stability and is part of the triangular fibrocartilage complex (TFCC). In ECU subluxation, MRI may show tendonitis, tear or may show a Triangular Fibrocartilage Complex (TFCC) tear. This diagnosis is usually a clinical one. Attention should be given to the possibility of a TFCC tear which occurs in about 50% of the cases as shown by arthroscopy. The distal radioulnar joint stabilizers are the volar and dorsal radioulnar ligaments. This joint is most stable in supination. In case of irreducible dislocation of the DRUJ by closed means, the ECU may block the reduction, especially in Galeazzi fracture. Check for palpable ECU and an empty tendon sulcus. Open surgical reduction is needed.
