Ligaments are tough, fibrous connective tissue that usually connects bone. The zona orbicularis is a ligament located on the neck of the femur, or thigh bone. It is an annular ligament, which means it encircles the neck of the femur. This ligament is alternatively known as the zonular band, ring ligament, and orbicular zone.
The zone orbicularis is made of the circular fibers of the hip joint's articular capsule, which is the ligamentous sac surrounding the joint of the hip. It has an irregular structure with varying thickness in the front and back of the hip joint. The articular capsule is loose and slender beneath the joint, and considerably thicker before the joint, where the highest level of resistance is needed. The articular capsule is made of longitudinal and circular fibers. Longitudinal fibers run along the length of the ligament, while circular fibers are rounded.
This bundle of fibers constitutes this ligament and is most prominent in the posterior (back) and lower part of the capsule. At this point, the fibers make a collar, or sling, around the neck of the femur. In the front part, the fibers attach to the inferior iliac spine, an area of the pelvis in the buttock region.
The zona orbicularis performs an important function by forming a locking ring around the neck of the femur. This helps to stabilize the hip in distraction, a position where the joint surfaces have space between them without any tearing in the ligaments or displacement.