The distal radioulnar joint as a load-bearing mechanism--a biomechanical study.

PURPOSE A biomechanical study was performed to define the normal profiles of force transmission across the distal radioulnar joint (DRUJ) and their alteration as the result of surgical modification of the joint and its ligaments. METHODS Twelve cadaver arms were used in this investigation. A custom-made jig was designed to hold the arm and allow axial loading of the hand. Force transmitted across the DRUJ was measured with sensor film. The axial force transmitted through the ulna and radius was measured with strain gauges mounted to the surface of each bone and the strain in the dorsal and volar distal radioulnar ligaments (DRUL) was measured with differential variable reluctance transducers. Measurements of force and strain with the DRUJ in the surgically unmodified condition were performed in various positions of forearm rotation and with sequential loads. Subsequently, the same series of measurements was performed after DRUL division and after reconstruction of the ligament. The effect of excision of the ulna head (Darrach's procedure) was also studied. RESULTS Force varied significantly (p <.05) with the applied load and with forearm position, being greater in supination than pronation (sensor film data). Force transmitted through the ulna varied in an approximately reciprocal pattern with that of the radius. The greatest forces through the ulna occurred in supination and therefore mirrored the change in force transmission across the DRUJ. Between 32% and 34%, respectively, of the applied load (5-10 kg) was transmitted through the ulna and the remainder was transmitted through the radius (strain gauge data). Division of either the volar or the dorsal DRUL tended to increase the force transmitted across the DRUJ (p =.09), and reconstruction of the disrupted DRUL restored the force pattern toward the previous values (not significant) (differential variable reluctance transducer data). Excision of the ulna head disrupted the force transmission profiles, producing the biomechanical equivalent of a 1-bone forearm. Only 1.8% and 2.7% of the applied force (10 kg and 5 kg), respectively, was transmitted through the ulna (strain gauge data only). CONCLUSIONS The data show consistent and characteristic force transmission profiles across the DRUJ and through both ulna and radius. The joint is important in the transmission of load and its anatomic integrity should be respected in surgical procedures if normal biomechanics are to be preserved.