The anular ligament (orbicular ligament) is a strong band of fibers that encircles the head of the radius and retains it in contact with the radial notch of the ulna. According to Terminologia Anatomica, the spelling is "anular", but the spelling "annular" is frequently encountered.
Anterior aspect of the capsule of the elbow joint (distended). The anular ligament (spelled annular here) is visible above the radius.
The anular ligament is attached by both its ends to the anterior and posterior margins of the radial notch of the ulna, together with which it forms the articular surface that surrounds the head and neck of the radius. The ligament is strong and well defined, yet its flexibility permits the slightly oval head of the radius to rotate freely during pronation and supination.
The head of the radius is wider than the bone's neck, and, because the anular ligament embraces both, the radial head is "trapped" inside the ligament, which thus acts to prevent distal displacement of the radius.
Superiorly, the ligament is supported by attachments to the radial collateral ligament and the fibrous capsule of the elbow joint. Inferiorly, a few fibers attached to the neck of the radius support a fold of the synovial membrane without interfering with the movements at the joint.
The fibrocartilage on the upper part of the ligament is continuous with the hyaline cartilage of the radial notch. At the posterior attachment, the ligament widens to reach above and below the radial notch.
A thickened band that extends from the inferior border of the anular ligament below the radial notch to the neck of the radius is known as the quadrate ligament