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Common tendinous ring (Annulus of zinn)

The common tendinous ring (Annulus of Zinn) is a fibrous ring located at the apex of the orbit. It serves as the major attachment site for the four rectus muscles of the eye and acts as an important anatomical landmark for both clinicians and radiologists. The ring surrounds the optic canal and part of the superior orbital fissure, playing a critical role in the structural and functional integrity of the orbital apex.

Synonyms

  • Annulus of Zinn

  • Annular tendon

  • Common annular tendon

  • Zinn's ring

Function

  • Serves as the origin for the four rectus muscles: superior, inferior, medial, and lateral rectus muscles.

  • Acts as a point of passage for several important neurovascular structures (e.g., optic nerve, oculomotor nerve, nasociliary nerve, abducens nerve, ophthalmic artery).

  • Provides structural support to the orbital apex.

MRI Appearance

  • T1-weighted images:
    The common tendinous ring appears as a low signal intensity structure at the orbital apex, blending with the surrounding connective tissue and extraocular muscles.

  • T2-weighted images:
    The ring maintains a low signal intensity. Adjacent orbital fat appears bright, providing some contrast to visualize the ring in high-resolution studies.

  • STIR (Short Tau Inversion Recovery):
    The tendinous ring remains hypointense, while edema or inflammation in adjacent tissues will show high signal, aiding in the identification of pathology around the ring.

CT Appearance

  • The common tendinous ring itself is not distinctly visible on standard CT due to its small size and soft tissue density.

  • Indirect visualization is possible as a dense soft tissue band at the orbital apex, where the rectus muscles converge.

  • Contrast-enhanced CT may help in differentiating the ring from adjacent structures, but visualization is still limited.

MRI images

Common tendinous ring orbits mri image