The anular ligament is a strong band of fibers, which encircles the head of the radius (bone)

Radius (bone)

The radius is one of the two large bones of the forearm, the other being the ulna. It extends from the lateral side of the elbow to the thumb side of the wrist and runs parallel to the ulna, which exceeds it in length and size. It is a long bone, prism-shaped and slightly curved longitudinally...

, and retains it in contact with the radial notch

Radial notch

The radial notch of the ulna is a narrow, oblong, articular depression on the lateral side of the coronoid process; it receives the circumferential articular surface of the head of the radius....

 of the ulna

Ulna

The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form and runs parallel to the radius, which is shorter and smaller. In anatomical position The ulna is one of the two long bones in the forearm, the other being the radius. It is prismatic in form...

.

Per Terminologia Anatomica

Terminologia Anatomica

Terminologia Anatomica is the international standard on human anatomic terminology. It was developed by the Federative Committee on Anatomical Terminology and the International Federation of Associations of Anatomists and was released in 1998. It supersedes the previous standard, Nomina...

, the spelling is "anular", but the spelling "annular" is frequently encountered.

Anatomy

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

Radial collateral ligament (elbow)

The radial collateral ligament is a short and narrow fibrous band, less distinct than the ulnar collateral, attached, above, to a depression below the lateral epicondyle of the humerus; below, to the annular ligament, some of its most posterior fibers passing over that ligament, to be inserted...

, 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.

The annular ligament provides stability to the radius against distal dislocation.

Clinical significance

Children who have not finished fusing their proximal radial epiphyseal plate may suffer dislocations of this joint. This frequently happens when parents sharply jerk their children by their arms, e.g. the act of grabbing a child away from traffic.