The distal radioulnar joint is the lower of two articulations between the radius and ulna bones in the human forearm. As the radius and ulna run parallel, they meet where each bone broadens at its topmost and bottommost point — just below the elbow and just above the wrist, respectively. The upper junction is known as the proximal radioulnar joint, while the lower junction is known as the distal radioulnar joint, with distal referring to the structure farthest from the trunk of the body. Both articulations are referred to as a pivot joint, a type of synovial joint in which the articulating surfaces of the adjacent bones rotate past each other. Therefore, the specific movement allowed by the distal joint is pronation, or rotation of the forearm palm-down.
To understand the function of a pivot joint, one must first be familiar with its structure. A pivot joint is made up of a cylindrical bone — in this case the head of the ulna, which is shaped like the end of a baton — pressed up against another bone that has an attaching ring of ligament encircling the first bone. The proximal radioulnar joint features an actual ligament ring attached to the ulna, the annular ligament, through which the radius bone slips like a finger through a ring, and within this ring the bone can rotate either direction. As the rotation occurring at the distal radioulnar joint is a slightly different movement, however, the distal articulation is structured somewhat differently.
At the lower articulation of the radius and ulna, just above the wrist, the adjoining surfaces are the opposite of those on the proximal joint: the cylindrical head of the ulna, as opposed to that of the radius, articulates with a structure on the bottom of the radius called the ulnar notch. In this case the two are held together not by a ring-shaped ligament but by the volar and dorsal radioulnar ligaments, horizontal ligaments that connect the anterior and posterior aspects of the bones, respectively. The structure of the distal radioulnar joint allows for a palm-down rotation, or pronation.
During pronation, the pronator teres and pronator quadratus muscles, which are found in the forearm and which attach to the lateral or outside surface of the radius on the thumb side of the arm, contract and pull inward on the bone. This causes the radius to rotate past the ulna anteriorly, or to its front side, so that the two bones form an X. As the muscles relax, the bones rotate past each other again at the distal radioulnar joint and return to their neutral, parallel position.