Radioulnar Load-Sharing in the Forearm. A Study in Cadavera*

Custom-designed miniature load-cells were inserted into the distal end of the ulna and the proximal end of the radius in ten fresh-frozen forearms from cadavera. The forces transmitted through the bones at these sites were measured under 134 newtons of constant axial load that was applied through the metacarpals as the forearm was rotated from 60 degrees of supination to 60 degrees of pronation. The simultaneous measurements of these forces allowed the calculation of radioulnar load-sharing at the wrist and the elbow as well as the calculation of the amount of force that was transferred from the radius to the ulna through the interosseous membrane. With the elbow in valgus alignment (that is, with contact between the radial head and the capitellum), the main pathway for load transmission through the forearm was direct axial loading of the radius; measurements from both load-cells were unaffected by the angle of elbow flexion. When the forearm was in neutral rotation, the mean force in the distal end of the ulna averaged 2.8 per cent of the load applied to the wrist and the mean force in the proximal end of the ulna averaged 11.8 per cent; this indicated that only a small amount of tension developed in the interosseous membrane. With the elbow in varus alignment (that is, with no contact between the radial head and the capitellum), load was transmitted through the forearm by a transfer of force from the radius to the ulna through the interosseous membrane. When the forearm was in neutral rotation, the force in the distal end of the ulna averaged 7.0 per cent of the load applied to the wrist and the force in the proximal end of the ulna averaged 93.0 per cent; the force through the interosseous membrane decreased with supination of the forearm. Testing with the elbow in valgus alignment and shortening of the distal end of the radius in two-millimeter increments produced corresponding increases in force in the distal end of the ulna and decreases in force in the radial head. The forces through the interosseous membrane remained low after each amount of radial shortening. CLINICAL RELEVANCE: Transfer of load from the wrist, through the radius and the ulna, to the elbow is a complex event that depends on the position of the forearm, anatomy of the wrist, and soft-tissue linkages between the radius and the ulna. Varus-valgus alignment of the elbow influences the basic mechanism of force transmission through the forearm. The interosseous membrane plays a minimum role in load transmission with the elbow in valgus alignment. Dynamic gripping activities that include varus stress to the elbow would be expected to develop force in the interosseous membrane. Radioulnar load-sharing at the wrist and the elbow changed significantly when the distal end of the radius was shortened by as little as two millimeters (p < 0.05). With the elbow in valgus alignment, four millimeters of shortening was required to approximately balance the radioulnar loads at the wrist.