The zona orbicularis is a ligament located on the neck of the femur bone. It is an annular ligament that is made of the circular fibers of the hip joint's articular capsule. This ligament is alternatively known as the zonular band, ring ligament, and orbicular zone. 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. The bundle of circular fibers constitutes this ligament, and these are found in abundance in the posterior and lower part of the capsule. At this point, the fibers make a collar or sling around the neck of the femur. In the anterior part, the fibers are attached to the inferior iliac spine. The zona orbicularis performs an important biomechanical function by forming a locking ring around the neck of the femur. This makes it an important structure that helps to stabilize the hip in distraction.
Written and medically reviewed by the Healthline Editorial Team
In Depth: Zona orbicularis