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Zona orbicularis ligament

The zona orbicularis is a band of circular fibers within the hip joint capsule that encircles the femoral neck. It is an essential component of the hip capsule, contributing significantly to joint stability by forming a collar-like structure that holds the femoral head within the acetabulum. The fibers blend with longitudinal capsular fibers and are reinforced by the iliofemoral ligament.

Clinically, the zona orbicularis is important in hip stability, surgical approaches to the hip, and in imaging where it may be visualized as part of the joint capsule. It plays a role in the screw-home mechanism of the hip, preventing distraction of the femoral head from the acetabulum.

Synonyms

  • Annular ligament of the hip

  • Circular fibers of hip capsule

  • Orbicular zone of hip capsule

Structure and Relations

  • Structure:

    • Circular bundle of collagen fibers forming a ring around the femoral neck

    • Blends with longitudinal fibers of the hip joint capsule

    • Tightens when the hip is extended, increasing stability

  • Relations:

    • Encloses the femoral neck within the joint capsule

    • Reinforced anteriorly by the iliofemoral ligament

    • Lies deep to rectus femoris and iliopsoas tendons anteriorly

    • Related posteriorly to the ischiofemoral ligament

Nerve Supply

  • Articular branches from the femoral nerve, obturator nerve, and sciatic nerve supply the capsule, including the zona orbicularis

Arterial Supply

  • Capsular branches from the medial circumflex femoral artery

  • Contributions from lateral circumflex femoral artery and obturator artery

Venous Drainage

  • Capsular veins draining into the medial and lateral circumflex femoral veins → femoral vein

Function

  • Provides circumferential reinforcement to the hip joint capsule

  • Prevents inferior and anterior displacement of femoral head

  • Tightens in extension to increase hip stability

  • Important in resisting joint distraction forces

Clinical Significance

  • Contributes to hip stability in normal gait and weight-bearing

  • May be disrupted in traumatic hip dislocation

  • Surgical landmark in hip arthroscopy and open hip procedures

  • Weakening may predispose to hip instability or subluxation

  • Important in understanding mechanics of hip replacement and ligament-preserving surgeries

MRI Appearance

T1-weighted images:

  • Appears as a low-signal intensity ring encircling the femoral neck

  • Surrounded by bright signal of pericapsular fat

T2-weighted images:

  • Fibrous ligament: low-signal intensity

  • Joint effusion or synovitis highlights the ligament against bright fluid

STIR (Short Tau Inversion Recovery):

  • Ligament remains dark

  • Adjacent inflammation, edema, or fluid collections appear bright, improving visualization

T1 Fat-Sat Post-Contrast:

  • Normal ligament shows little or no enhancement

  • Inflamed or injured capsule/ligament shows linear or focal enhancement

CT Appearance

Non-Contrast CT:

  • Not directly visualized due to fine fibrous structure

  • Capsule inferred as thin soft tissue density around femoral neck

  • Joint effusion or calcifications may outline it

Post-Contrast CT:

  • Ligament itself does not enhance

  • Adjacent capsule may enhance slightly in cases of inflammation or tumor infiltration

  • Effusion or synovitis appears as enhancing fluid/tissue within joint space

MRI image

Zona orbicularis ligament MRI coronal anatomy image-img-00000-00000

MRI image

Zona orbicularis ligament MRI coronal anatomy image-img-00000-00000_00001

MRI image

Zona orbicularis ligament MRI coronal anatomy image-img-00000-00000_00002

MRI image

Zona orbicularis ligament MRI sagittal anatomy image-img-00000-00000

CT image

Zona orbicularis ligament CT axial anatomy image-img-00000-00000