Biomechanical evaluation of the dynamic radioulnar convergence after ulnar head resection, two soft tissue stabilization methods of the distal ulna and ulnar head prosthesis implantation

Introduction: The biomechanical performance of the Darrach procedure, its stabilizing methods and the effectiveness of an ulnar head prosthesis was evaluated in this experiment. Purpose: The purpose of this study was to: (1) analyze the dynamic effects of the resection of the distal ulna on radioulnar convergence; and (2) evaluate the mechanical performance of two soft tissue stabilizing procedures (pronator quadratus advancement flap and ECU/FCU tenodesis) for the unstable distal ulnar stump and the implantation of an ulnar head prosthesis (uHead) following a distal ulnar resection (Darrach procedure) on radioulnar convergence. Method: With a dynamic PC-controlled forearm simulator, cadaveric forearm rotation was actively and passively performed while simultaneously loading the relevant muscles. The resultant total forearm torque and the three-dimensional kinematics of the ulna, radius and third metacarpal were simultaneously recorded in seven fresh-frozen cadaver upper extremities. Results: The resection of the distal ulna created an extreme instability of the forearm with movement of the radius toward the ulna (0.92-0.38 cm compared to the intact state) in each particular loading condition. The implantation of the ulnar head prosthesis effectively restored the stability of the DRUJ by simulating the geometry of the ulnar head. There were significantly better results after the implantation of the prosthesis compared with the Darrach and the soft tissue stabilization procedures. Conclusion: This study provides laboratory validity to the option of implanting an ulnar head endoprosthesis as an attempt to stabilize the distal forearm with instability after Darrach resection in lieu of performing soft tissue stabilization techniques.