A radioulnar joint is one of two articulations between the radius and ulna bones in the human forearm. As these two bones run parallel, they meet the proximal and at the distal radioulnar joint. Both articulations are a type of synovial joint known as a pivot joint, which means that the articulating surfaces of the adjacent bones may rotate past each other. The specific movement allowed by these joints, then, is rotation of the forearm.
At the proximal radioulnar joint, the adjoining surfaces are the head of the radius bone along its or inner side and the radial notch of the ulna, an oblong depression near the top of the bone alongside the semiulnar notch, which is where the humerus meets the ulna to form the elbow joint. As a pivot joint comprises a cylindrical bone — in this case the head of the radius — with another bone and an attaching ring of ligament rotating around it, the proximal radioulnar joint features a ligament connected to the ulna known as the annular ligament. Like a ring on a finger, the annular ligament encircles the head of the radius and attaches to the ulna; the part of the ulna that articulates with the head of the radius inside the ring is the radial notch.
The movement allowed at the proximal radioulnar joint is supination, or rotation of the forearm palm-up. To make this movement possible, the muscles responsible, the biceps brachii in the upper arm and the supinator in the forearm, attach to the radius and pull on it during contraction. This causes rotation of the radius within the annular ligament of the ulna. The same rotation occurs in the opposite direction during the motion of pronation, or rotation of the forearm palm-down, but most of the action occurs at the distal radioulnar joint.
At the lower articulation of the radius and ulna, the distal radioulnar joint, the adjoining surfaces are the opposite of those on the proximal joint: the cylindrical head of the ulna meets with the ulnar notch on the bottom of the radius. 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 this articulation allows for both pronation and supination, but pronation involves more substantial movement at the distal joint.
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, contract and pull on the bone. This causes the radius to rotate past the ulna 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.