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An annular ligament is a ligament linking the radius bone to the ulna bone at the proximal radioulnar joint in the human forearm. Circular in shape, this ligament wraps around the head of the radius, which runs parallel to the ulna, and attaches to the ulna bone. This shape is what allows the radius to rotate back and forth alongside the ulna and therefore permits pronation and supination of the forearm, or palm-down and palm-up rotation. The annular ligament also holds the radius in place against the ulna.
Beginning and ending on the radial notch of the ulna, an oblong depression near the top of the bone on its inside surface, the annular ligament is found just below the elbow joint. A portion of the radius bone articulates with the radial notch between the margins where the ligament begins and ends to form the proximal radioulnar joint, a type of synovial joint known as a pivot joint. Pivot joints are made up of a cylindrical bone — in this case the head of the radius — with another bone and an attaching ring of ligament rotating around it. This particular joint is distinguished by the presence of the annular ligament.
Like a ring on a finger, the annular ligament encircles the head of the radius and attaches to the ulna. The movement allowed by this articulation 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. A similar 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, where the radius and ulna meet just above the wrist.
The annular ligament is also what keeps the radius in place against the ulna. As such, the most common injury to this connective tissue and the joint it envelops is a downward dislocation of the head of the radius from the ligament. Also known as nursemaid’s elbow, it is most often seen in small children and is typically a consequence of a forceful yanking of the arm, as in an adult tugging on or lifting a child by the wrist. The result is that the radius is pulled out of place, an injury that is usually treated non-surgically by a physician who can relocate the radius within the ligament ring.