Analysis of dynamic distal radioulnar convergence after ulnar head resection and endoprosthesis implantation.

The most common method to treat the arthralgic distal radioulnar joint is resection of the entire ulnar head (Darrach procedure). Pain and weak grip strength usually manifest complications related to instability of the distal forearm. In an attempt to mechanically stabilize the distal forearm after ulnar head resection, an endoprosthesis was developed to replace the ulnar head after Darrach resection. The goals of this study were to evaluate the dynamic effects of the Darrach procedure on radioulnar convergence and the mechanical efficacy of implantation of an ulnar head endoprosthesis after Darrach resection. Cadaver forearms were rotated actively and passively and relevant muscles were loaded simultaneously with a dynamic PC-controlled forearm simulator. Resultant total forearm torque and 3-dimensional kinematics of the ulna, radius, and third metacarpal were recorded simultaneously in 7 fresh-frozen cadaver upper extremities. Comparisons were made between the intact state, after Darrach resection, and after implantation of ulnar head endoprosthesis. The Darrach resection created substantial forearm instability with movement of the radius ulnarly (0.92-0.38 cm vs intact state) and anteroposterior translation in each loading condition. Implantation of the ulnar head endoprosthesis effectively restored distal radioulnar joint stability by simulating the geometry of the ulnar head, further stabilized by attaching the triangular fibrocartilage complex. These laboratory data provide validity to implanting an ulnar head endoprosthesis to stabilize the distal forearm after Darrach resection.