Radioulnar Joints Anatomy
The radius and ulna articulate at the proximal and distal radioulnar joints, which are synovial. At the proximal joint, the head of the radius articulates with the radial notch of the ulna (Fig. 3.88). The head and neck are encircled by the anular ligament, which attaches to the anterior and posterior margins of the notch on the ulna (Fig. 3.92) and blends with the capsule and radial collateral ligament of the elbow. Thus the cavity of the proximal joint is continuous with that of the elbow.
The distal radioulnar joint occurs between the head of the ulna and the ulnar notch of the radius (Fig. 3.93). An articular disc (a triangular cartilage) attaches the ulnar styloid process to the distal end of the radius and separates the cavity of the distal joint from that of the wrist.
The anular ligament of the proximal joint and the articular disc of the distal joint prevent separation of the radius and ulna, yet allow the radius to rotate freely. In addition, the shafts of the bones are joined by the interosseous membrane, whose fibres incline downwards and medially from the interosseous border of the radius to that of the ulna. The membrane may be regarded as a fibrous radioulnar joint.
The movements of supination and pronation occur through approximately 180° at the radioulnar joints. During pronation the radius rotates across the ulna and twists the forearm and hand so that the palm faces posteriorly. Supination returns the limb to the anatomical position. The axis of the movement passes through the head of the radius and the styloid process of the ulna. Supination is the more powerful movement and is produced by biceps and supinator, although biceps is ineffective when the elbow is fully extended. Pronation is produced by pronator teres and pronator quadratus. Also, when the elbow is flexed, brachioradialis rotates the forearm and returns the limb to the midposition from the extremes of supination or pronation. The head of the radius can be felt rotating about 2 cm distal to the lateral epicondyle during these movements.
In a fall onto the hand, the interosseous membrane may transmit force from the radius to the ulna, protecting the radial head from compression against the capitulum of the humerus. However, since the membrane does not oppose distraction, a sudden tug on the hand may dislocate the radial head downwards from within the anular ligament of the proximal radioulnar joint.