Forearm Mechanics and its Implication on the Distal Radioulnar Joint

Abstract

The dexterity of the upper limb is made possible, in part, by the ability to rotate the hand. That rotation is achieved within the forearm. Relatively little is understood of either the kinematics or kinetics of forearm rotation, nor is distal radioulnar joint contact properly understood. As a consequence, the outcomes when treating disorders and injuries of the forearm are often inconsistent, particularly as they relate to distal radioulnar joint health. The goal of this research was to investigate forearm kinematics and the contributions various muscles make to forearm rotation. The results of these analyses were combined with a detailed continuum model to examine distal radioulnar joint contact throughout the forearm range of motion and under simulated clinical conditions. The relationship between ulna and radius motion and the mechanical axis of forearm rotation was complex. Based on the results of this analysis, radius motion is taskindependent, constrained about a fixed axis of rotation. Conversely, ulna motion is taskdependent and unconstrained. Ulna motion makes task-specific radius rotation possible. The contributions of individual muscles to pronation and supination were not straightforward, with many muscles active for both movement directions. Muscle physiological cross-sectional areas were found to be poorly predicted by both cadaveric data and in vivo volume fractions. Peak distal radioulnar joint contact pressure occurred in mid-supination, with a second peak in mid-pronation. Lowest contact pressure was observed in full pronation. Ulnar shortening led to increased joint contact at the distal radioulnar joint, while lengthening the ulna decreased the load at the joint. Predicted contact at the distal radioulnar joint was most sensitive to dorsal rotation of the distal radius, while radial and ulnar rotation did not significantly affect joint contact pressure. In general, optimal joint contact was achieved with the subject’s normal distal radius orientation. The results of the contact simulations did demonstrate the role articular cartilage plays in mitigating changes in joint contact force in order to maintain a more stable joint contact pressure.