The zona orbicularis or annular ligament is a ligament

Ligament

In anatomy, the term ligament is used to denote any of three types of structures. Most commonly, it refers to fibrous tissue that connects bones to other bones and is also known as articular ligament, articular larua, fibrous ligament, or true ligament.Ligament can also refer to:* Peritoneal...

 on the neck

Femur neck

The femur neck or neck of the femur is a flattened pyramidal process of bone, connecting the femoral head with the femoral shaft, and forming with the latter a wide angle opening medialward.- Angle of inclination :...

 of the femur

Femur

The femur , or thigh bone, is the most proximal bone of the leg in tetrapod vertebrates capable of walking or jumping, such as most land mammals, birds, many reptiles such as lizards, and amphibians such as frogs. In vertebrates with four legs such as dogs and horses, the femur is found only in...

 formed by the circular fibers of the articular capsule

Capsule of hip joint

The articular capsule is strong and dense.Above, it is attached to the margin of the acetabulum 5 to 6 mm. beyond the labrum behind; but in front, it is attached to the outer margin of the labrum, and, opposite to the notch where the margin of the cavity is deficient, it is connected to the...

 of the hip joint. It is also known as the orbicular zone, ring ligament, and zonular band.

Structure

The articular capsule is much thicker above and in front of the joint, where the greatest amount of resistance is required, and thin and loose behind and below the joint. The capsule consists of two sets of fibers, circular and longitudinal. The circular fibers, the zona orbicularis, are most abundant at the lower and back part of the capsule where they form a sling or collar around the femoral neck. Anteriorly they blend with the deep surface of the iliofemoral ligament

Iliofemoral ligament

The iliofemoral ligament is a ligament of the hip joint which extends from the ilium to the femur in front of the joint. It is also referred to as the Y-ligament or the ligament of Bigelow, and any combinations of these names....

, and gain an attachment to the anterior inferior iliac spine.
Gray's Anatomy (1918), 7a3

Function

The zona orbicularis and proximal hip joint capsule are poorly understood, but recent studies seem to confirm that the proximal to middle part of the articular capsule, including the zona orbicularis, acts biomechanically as a locking ring wrapped around the femoral neck and thus is a key structure for hip stability in distraction.