The elbow joint is formed by the distal end of the humerus and the proximal ends of the radius and ulna. The superior radio-ulnar joint, between the head of the radius and the radial notch of the ulna and annular ligament, has a joint cavity continuous with that of the elbow joint. The inferior radioulnar joint, between the head of the ulna and the ulnar notch of the radius, has no communication with the wrist joint as a rule. The union by the interosseous membrane of the shafts of the radius and ulna is, by strict definition, a fibrous joint between the radius and ulna and is sometimes called the middle radio-ulnar joint.
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