The anular ligament of radius is a strong band of fibers, which encircles the head of the radius, and retains it in contact with the radial notch of the ulna. It forms about four-fifths of the osseo-fibrous ring, and is attached to the anterior and posterior margins of the radial notch; a few of its lower fibers are continued around below the cavity and form at this level a complete fibrous ring. Its upper border blends with the anterior and posterior ligaments of the elbow, while from its lower border a thin loose membrane passes to be attached to the neck of the radius; a thickened band which extends from the inferior border of the annular ligament below the radial notch to the neck of the radius is known as the quadrate ligament. The superficial surface of the annular ligament is strengthened by the radial collateral ligament of the elbow, and affords origin to part of the Supinator. Its deep surface is smooth, and lined by synovial membrane, which is continuous with that of the elbow-joint.