Ulnar shortening effect on distal radioulnar joint stability: a biomechanical study.

Purpose: The ulnar-shortening procedure has been adopted widely to reduce pressure between the ulna and ulnar carpus in ulnocarpal abutment syndrome. The hammock-like structure of the triangular fibrocartilage complex (TFCC), which supports and connects the ulnocarpal and distal radioulnar joint (DRUJ), variably is torn in this condition. The degree to which the torn TFCC may be tensioned to restabilize the DRUJ with ulnar recession is uncertain. This study examined changes in the stabilizing effect of the ulnar-shortening procedure in several TFCC tear conditions.

Methods: Six fresh-frozen cadaver arms amputated at the midportion of the humerus were used. The skin, muscles, and capsuloligamentous structures below the elbow all were preserved. The ulna and humerus were affixed firmly to a custom mount that allowed 60 degrees of forearm rotation. An external fixator was attached to the distal ulna leaving space for a 10-mm resection of the ulna to allow progressive shortening. The radius was attached to a materials testing machine. The load-displacement curves were obtained while translating the distal radius dorsally or palmarly with respect to the ulna at 1.25 mm/s. Stiffness in dorsopalmar displacement was recorded at 1-mm intervals through 6 mm of length. These measurements then were compared with controls (0 mm shortening of the intact specimens) at 60 degrees pronation, neutral position, and 60 degrees supination. The tests then were repeated after sectioning either the dorsal or palmar portion of the radioulnar ligament (RUL) and then after complete sectioning of the RUL. Each portion was sectioned at its attachment to the ulnar fovea.

Results: The stiffness of the DRUJ increased significantly in all 3 rotatory positions after shortening the ulna. A shortening of 6 mm resulted in a 26% to 44% increase in DRUJ stiffness. The stiffness decreased after partial sectioning of the RUL but increased with further ulnar shortening in all 3 positions. The DRUJ stiffness with the partially sectioned RUL after a shortening of 3 to 6 mm was as large as that of the intact specimens. The stiffness of the DRUJ after the complete section of the RUL was significantly smaller than that of the intact specimens even after shortening of 6 mm.

Conclusions: The ulnar-shortening procedure can stabilize the DRUJ by increasing intrastructural tension of the TFCC, only when the RUL is attached totally or partially to the ulnar fovea. If the RUL is avulsed completely then stability of the DRUJ no longer is obtained by the ulnar-shortening procedure.