Zona orbicularis

At a glance

English

Zona orbicularis

Latin (TA2)

Zona orbicularis

System

Joint

Region

Lower limb

Side

Paired, left and right

About the zona orbicularis

The zona orbicularis is the band of circular fibres in the deep layer of the hip joint capsule. It is also called the orbicular zone, the annular or ring ligament, or the zonular band. Unlike the iliofemoral, pubofemoral and ischiofemoral ligaments, which are thickenings on the outside of the capsule, it lies on the inner side, wrapped round the neck of the femur at its narrowest point, just beyond the femoral head.

The hip capsule has two sets of fibres. The longitudinal fibres pass from the pelvis to the femur and are thickest over the upper and front aspect, where most strain falls; behind and below, the capsule is thin and loose. The circular fibres of the zona orbicularis are, in contrast, best developed in the lower, posterior capsule, where they loop round the neck as a sling. At the front the ring merges with the underside of the iliofemoral ligament, and through it gains a hold on the anterior inferior iliac spine. The outer ligaments partly radiate into the ring and cover it.

Because the femoral head is wider than the ring, the zona orbicularis behaves like a buttonhole or locking collar. Studies suggest that it is one of the main structures resisting distraction of the hip, helping to keep the head seated in the acetabulum when the leg is pulled. Contraction of iliopsoas, which has fibres blending with the front of the capsule, is said to tighten it. It also divides the joint cavity into a central part around the head and a peripheral part along the neck, a pattern that can be seen during hip arthroscopy.

Its precise role is still under study. Because it helps hold the head in place, damage to it during injury or surgery may add to instability of the hip.

Attachments and function

Formed by

Arises from the deep circular fibres of the hip capsule, with an anterior attachment via the iliofemoral ligament on the anterior inferior iliac spine.

Joins

Forms a closed ring around the narrowest part of the femoral neck; it has no bony insertion on the femur.

Movements

Acts as a locking collar round the femoral neck, resisting distraction of the hip and helping to keep the femoral head in the acetabulum.

Nerve supply

Shares the nerve supply of the hip capsule, from branches of the femoral, obturator and sciatic nerves.

Blood supply

Shares the supply of the hip capsule from the medial and lateral circumflex femoral arteries.