Stress in the human elbow joint. II. Proximal radio-ulnar joint.

The proximal radio-ulnar joint has two different types of articular surfaces. The contact area between the articulating surfaces is at its greatest when the joint is in a middle position. The highest density of osseous material is found on the ulnar side of the head of the radius. The subchondral bone beneath the radial notch of the ulna has a higher density at the edges than in the middle. In photoelastic experiments the intensity and configuration of isochromatics correspond to the material density of the osseous tissue of ulna and radius. The analysis of spongiosa near the joint shows bands of compressive cancellous trabeculae radiating axially into the subchondral cortex, crossed at right angles by bands of tensile cancellous trabeculae. These findings are in agreement with the course of the trajectories found in photoelastic experiments. The composition of the annular ligament is variable. The palmar and dorsal parts are made up of firm connective tissue; opposite the radial notch of the ulna, however, cartilage cells are embedded in the ligament. The histological composition of the annular ligament and the cancellous architecture in the radius and the ulna are in agreement with Pauwels' (1963) hypothesis that part of the compressive force in the elbow joint is transferred to the proximal radio-ulnar joint via the annular ligament. The distribution of the material in the subchondral bony tissue indicates that there is only partial contact between the radial head circumference and the radial notch of the ulna in all positions of the joint.