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Common tendinous ring

The common tendinous ring (CTR), also known as the annulus of Zinn or common annular tendon, is a fibrous structure situated at the apex of the orbit that forms the shared origin for the four rectus extraocular muscles, enabling coordinated eye movements by anchoring these muscles to the orbital walls.^[1] Located at the convergence of the optic canal, superior orbital fissure, and anterior lateral sellar compartment, the CTR encircles the optic foramen and oculomotor foramen, blending seamlessly with the periosteum, periorbita, meningo-orbital band, optic nerve sheath, and epineurium of cranial nerves to provide structural stability at the orbital apex.^[1] Its anatomical configuration consists of a three-dimensional dual-ring assembly, including the posterior portions of the four rectus tendons (superior: approximately 5.92 mm; inferior: 8.66 mm; medial: 11.56 mm; lateral: 7.85 mm), two intermuscular tendinous connections (lateral: 4.04 mm; medial: 6.01 mm), and a singular common tendinous root (7.03 mm), all rooted at the infraoptic tubercle and lateral rectus spine.^[1] The superior, inferior, medial, and lateral rectus muscles originate directly from the CTR, with the superior oblique muscle attaching nearby above the optic canal; these attachments form the conical framework of the orbit, through which the optic nerve (II) and branches of cranial nerves III, IV, and VI pass via the central oculomotor foramen (transverse dimension: 6.24 mm; vertical: 3.74 mm) and optic foramen (transverse: 10.23 mm; vertical: 4.91 mm).^[1]^[3]^[4] No significant variations in size or left-right asymmetry have been observed in anatomical studies (P > 0.05).^[1] Clinically, the CTR is vital in neurosurgical and neuro-ophthalmological contexts, as its tendinous connections can be incised to improve surgical exposure during procedures like lateral transorbital or endoscopic endonasal approaches to the orbital apex, minimizing risks to adjacent neurovascular structures.^[1]

Anatomy

Location

The common tendinous ring, also known as the annulus of Zinn, is located at the apex of the orbit, where it encircles the optic canal and the medial portion of the superior orbital fissure.^[5]^[6] This positioning places it at the posterior convergence of the orbital walls, immediately surrounding the point where the optic nerve transitions from the intracranial to the intraconal space.^[7] It forms a fibrous ring around the optic nerve as it enters the orbit, providing a central anchor point amid the orbital apex's complex bony architecture.^[8] Posteriorly, the ring attaches to the lesser wing of the sphenoid bone superiorly and to the body of the sphenoid bone inferiorly, integrating with the margins of the optic canal for stability.^[7] Laterally, it extends to bridge the superior orbital fissure, attaching to a small tubercle on the greater wing of the sphenoid, thereby spanning key foramina essential for orbital communication.^[5] This precise spatial arrangement ensures the ring serves as a foundational element at the orbit's narrowest posterior region, approximately 40-45 mm from the orbital rim in adults.^[9]

Structure and attachments

The common tendinous ring, also known as the annulus of Zinn, is a thickened fibrous structure composed primarily of the proximal tendons of the four rectus extraocular muscles, interwoven with the periosteum of the sphenoid bone and elements of the optic nerve sheath. This tendinous complex forms a dense, annular band that encircles the optic canal and a portion of the superior orbital fissure at the orbital apex. Contributions from the medial rectus tendon are prominent in the inferior aspect, while a medial fibrous band connecting the superior and medial rectus tendons provides origin points for adjacent structures, including the superior oblique muscle.^[10]^[1] The ring serves as the primary origin for the four rectus muscles: the superior rectus arises from its superior portion, while the inferior, medial, and lateral rectus muscles originate from the inferior portion. These attachments are reinforced by tendinous connections, such as the lateral link between the superior rectus and lateral rectus (approximately 4 mm in length) and the medial link between the superior and medial rectus (about 6 mm), which stabilize the overall architecture. The superior oblique tendon, although passing through the trochlea anteriorly, indirectly contributes via its proximal fibrous extensions blending into the medial aspect of the ring. Additionally, the ring fuses with the periosteum at key points, including the infraoptic tubercle and lateral rectus spine, enhancing its anchorage to the orbital walls.^[1]^[11]^[10] Internally, the ring exhibits a partial septation through distinct fibrous strata: a superior stratum primarily associated with the superior rectus origin, forming a triangular configuration, and an inferior stratum giving rise to the medial, inferior, and lateral rectus muscles. This division creates a partial septum that delineates the pathways for ocular motor nerves, with the inferior stratum accommodating the bulkier medial rectus attachment. The overall structure tapers posteriorly, integrating seamlessly with bony landmarks to form a robust, Y- and V-shaped tendinous framework.^[10]^[1]

Relations

To orbital structures

The common tendinous ring, also known as the annulus of Zinn, spans the medial third of the superior orbital fissure, dividing it into three sectors: a central (intra-annular) sector that transmits the superior and inferior divisions of cranial nerve III (oculomotor), cranial nerve VI (abducens), and the nasociliary branch of the ophthalmic division of cranial nerve V (trigeminal, V1); a lateral (extra-annular superior) sector through which cranial nerve IV (trochlear), the frontal nerve, and the lacrimal nerve (both branches of V1) and the superior ophthalmic vein pass; and an inferior sector through which the inferior ophthalmic vein passes.^[12] This division facilitates the organized passage of neurovascular elements into the orbit while maintaining structural integrity at the orbital apex.^[5] In close proximity to the optic canal, the ring directly encircles the optic nerve (cranial nerve II) at its entry into the orbit and lies adjacent to the anterior clinoid process of the sphenoid bone.^[5] This positioning ensures the tendinous ring surrounds the optic foramen, through which the optic nerve and ophthalmic artery enter the orbital cavity, integrating the visual pathway with the broader orbital framework.^[13] The ring further connects to the orbital walls by blending with the periosteum of the lesser wing of the sphenoid bone superiorly and the orbital plate of the greater wing inferiorly, often attaching to a specific tubercle on the greater wing for enhanced stability.^[5] These attachments anchor the ring within the sphenoid's orbital surfaces, contributing to the overall bony enclosure of the orbit.^[12]

To neurovascular elements

The common tendinous ring encircles the optic nerve (cranial nerve II) at the orbital apex, with the nerve piercing through the central optic foramen formed by the ring's posterior attachments around the optic canal.^[1] Accompanying the optic nerve through this foramen is the ophthalmic artery, which provides the primary arterial supply to the orbit and contributes to the circle of Zinn-Haller, an anastomotic arterial ring surrounding the optic nerve head within the sclera.^[7] This central passage positions the optic nerve and its vascular companion directly within the ring's fibrous boundaries, bordered anteriorly by the tendons of the superior and medial rectus muscles.^[1] The ring divides the superior orbital fissure into intra-annular and extra-annular compartments, directing the passage of several cranial nerves. Within the ring, through the oculomotor foramen—a narrow channel bordered by the superior and lateral rectus tendons—the oculomotor nerve (CN III) enters with its superior and inferior divisions, alongside the abducens nerve (CN VI) and the nasociliary nerve, a branch of the ophthalmic division of the trigeminal nerve (CN V1).^[1] Lateral to the ring, in the superior sector of the fissure above the superior rectus tendon, the trochlear nerve (CN IV), frontal nerve, and lacrimal nerve (both branches of CN V1) pass into the orbit, separated from the intra-annular structures by the ring's fibrous tissue.^[14] Vascular relations involve the superior ophthalmic vein, which courses through the superior sector of the superior orbital fissure outside the ring, parallel to the trochlear and frontal nerves.^[1] The inferior ophthalmic vein, originating from a plexus near the inferior rectus muscle, drains posteriorly through the inferior sector of the superior orbital fissure below the ring, connecting to the cavernous sinus either directly or via the superior ophthalmic vein.^[15] These neurovascular alignments highlight the ring's role in compartmentalizing orbital entry points, with the nasociliary nerve's medial passage within the ring facilitating its contributions to sensory innervation of the nasal cavity and anterior orbit.^[16]

Function

Support for extraocular muscles

The common tendinous ring, or annulus of Zinn, acts as a stable, centralized origin for the four rectus extraocular muscles—superior, inferior, medial, and lateral—located at the orbital apex surrounding the optic canal and a portion of the superior orbital fissure. This fibrous structure anchors the muscles firmly to the sphenoid bone, enabling coordinated and balanced traction on the eyeball to facilitate precise eye movements such as abduction, adduction, elevation, and depression. By providing a shared attachment point, the ring ensures that the rectus muscles can generate synchronized forces without independent shifting of their origins, which is crucial for maintaining ocular alignment during dynamic gazes.^[17] The tendinous composition of the ring contributes to effective muscle tension management by distributing contractile forces evenly across its fibrous framework, thereby minimizing localized stress and preventing slippage between the muscle bellies and their insertion on the sclera. This biomechanical distribution supports efficient force transmission, allowing the rectus muscles to exert pull on the globe with reduced risk of deformation or inefficient energy loss, as demonstrated in models of orbital mechanics where the ring's role stabilizes force vectors during contraction.^[18]^[19] Although the oblique extraocular muscles do not originate directly from the ring, the superior oblique arises from the periosteum of the sphenoid superomedial to it, positioning it for synergistic interaction with the rectus muscles to produce combined movements like depression and intorsion of the eye. This adjacent origin facilitates integrated actions, such as the superior oblique's primary role in intorsion and secondary depression when the eye is adducted, complementing the superior and medial rectus functions for overall ocular motility.^[20]

Division of superior orbital fissure

The common tendinous ring, also known as the annulus of Zinn, functionally divides the superior orbital fissure into distinct sectors, facilitating organized passage of neurovascular structures from the cranial cavity into the orbit. The ring creates a superior sector above the oculomotor foramen, through which the trochlear nerve (CN IV), frontal nerve, lacrimal nerve, and superior ophthalmic vein pass. The central sector, known as the oculomotor foramen and enclosed by the ring, transmits the oculomotor nerve (CN III, superior and inferior branches), nasociliary nerve (a branch of the ophthalmic division of the trigeminal nerve, V1), and abducens nerve (CN VI). The inferior sector, below the oculomotor foramen, and the lateral sector are typically sealed by fat, orbitalis muscle, connective tissue, and a dense meningo-orbital band, containing no significant neurovascular structures.^[21] The inferior ophthalmic vein may variably pass through the superior orbital fissure inferior to the ring or primarily via the inferior orbital fissure, while the recurrent meningeal artery is not consistently noted in these sectors.^[12] This compartmentalization ensures that critical neural and vascular elements are routed through separate pathways within the fissure, which measures approximately 20-22 mm in length and is located at the orbital apex between the greater and lesser wings of the sphenoid bone.^[22] The structural arrangement provided by the ring promotes functional isolation between motor innervation and major vascular conduits, minimizing potential interference and optimizing the efficiency of orbital blood supply and nerve distribution. For instance, the separation shields the motor nerves (CN III, IV, VI) and select sensory fibers from direct adjacency to larger venous drainage, reducing risks of compression or aberrant signaling during physiological movements.^[21] This separation is evolutionarily advantageous for precise coordination of extraocular function and venous return, as the superior ophthalmic vein primarily drains the anterior orbit.^[22] Furthermore, the ring's strategic positioning at the convergence of the superior orbital fissure and optic canal extends its influence over the broader transition from intracranial to orbital spaces. By encircling the optic foramen medially and bridging the anterior ridge of the optic strut, the ring's tendinous components—particularly the superior and medial rectus tendons—demarcate boundaries that guide the optic nerve, ophthalmic artery, and associated dural reflections into the orbit without crossover into fissure pathways.^[21] This integrated architecture maintains compartmental integrity across the orbital apex, supporting unimpeded axonal and vascular flow while preserving the structural stability of the sphenoid bone's posterior orbit.^[22]

Clinical significance

Pathologies

The common tendinous ring, or annulus of Zinn, is implicated in orbital apex syndrome, a condition arising from compression or inflammation at the orbital apex that affects structures passing through or near the ring, including the optic nerve (CN II) and cranial nerves III, IV, V1, and VI. This leads to acute vision loss due to optic neuropathy, complete ophthalmoplegia, ptosis, and sensory deficits in the V1 distribution.^[23] Common etiologies include tumors, infections, or trauma compressing the ring and adjacent fissures.^[24] Tolosa-Hunt syndrome represents an idiopathic granulomatous inflammation involving the cavernous sinus, superior orbital fissure, and orbital apex around the common tendinous ring, resulting in painful ophthalmoplegia with involvement of CN III, IV, VI, and occasionally CN II or V1. Patients typically experience severe, unilateral periorbital pain followed by restricted eye movements and potential pupillary abnormalities, with rapid response to corticosteroids distinguishing it from other causes.^[23] The inflammation may extend to the tendinous ring attachments, exacerbating motility deficits.^[24] Congenital anomalies of the common tendinous ring are rare but can include agenesis, hypoplasia, or accessory muscular slips originating from or attached to the ring, disrupting normal extraocular muscle origins and leading to strabismus or variants of Duane retraction syndrome. In Duane syndrome, anomalous fibrous bands from the ring to the lateral rectus or globe may cause limited abduction, globe retraction on adduction, and synergistic movements due to aberrant innervation.^[25] Abnormal insertions or accessory muscular slips originating from or attached to the ring have been associated with vertical or horizontal deviations, often requiring surgical exploration for correction.^[25]

Imaging and surgery

High-resolution computed tomography (CT) is particularly effective for visualizing the common tendinous ring at the orbital apex, providing detailed assessment of its bony attachments to the sphenoid bone and relations to adjacent foramina such as the optic canal and superior orbital fissure.^[26] Thin-section axial CT with multiplanar reconstructions excels at depicting osseous structures, including potential fractures or erosions that may involve the ring, while contrast-enhanced sequences can highlight associated soft tissue changes.^[26] Magnetic resonance imaging (MRI) complements CT by offering superior soft tissue contrast for evaluating the tendinous ring and its muscular origins, especially in detecting inflammation, nerve compression, or neoplastic involvement at the orbital apex.^[27] On T1-weighted and T2-weighted sequences, the ring appears as a low-signal fibrous structure within hyperintense orbital fat, with coronal and parasagittal views best delineating its position relative to the optic nerve and extraocular muscles; gadolinium-enhanced T1-weighted imaging further identifies periring enhancement indicative of pathology.^[27] In surgical contexts, the common tendinous ring is accessed during orbital decompression procedures for conditions like orbital apex syndrome, where combined endoscopic endonasal and transorbital approaches allow precise fenestration and relief of compression around the ring to restore visual function.^[28] These techniques, often employed in dysthyroid optic neuropathy, involve incising the periorbita near the ring to decompress the optic nerve and surrounding structures, yielding significant improvements in visual acuity and proptosis reduction.^[28] For tumor resection at the orbital apex, the ring guides dissection to preserve extraocular muscle integrity, with transsphenoidal approaches carrying a risk of inadvertent damage due to the structure's proximity to the sphenoid sinus and optic canal.^[29] Intraoperatively, the common tendinous ring serves as a critical landmark in strabismus surgery, facilitating identification and reattachment of extraocular muscle tendons to their precise origins, thereby minimizing postoperative misalignment.^[30] Its fibrous composition and fixed position relative to the orbital apex provide reliable orientation during deep dissection, particularly for rectus muscle adjustments.^[31]

History and eponymy

Historical discovery

The anatomical structure known as the common tendinous ring, or annulus of Zinn, was first described in the early 18th century by Italian anatomist Antonio Maria Valsalva (1666–1723). In lectures delivered on 17 June 1715 and 6 December 1716 at the Institute of Sciences in Bologna, Valsalva identified a fibrous annular structure surrounding the optic nerve at the orbital apex, which he termed the "annulum nervi optici moderatorem" (regulator annulus of the optic nerve). He proposed that this ring regulated the transmission of nervous impulses and served as a common origin for the extraocular rectus muscles, based on detailed dissections of human cadavers. These findings were published posthumously in 1740 as part of his Dissertatio anatomica prima and Dissertatio anatomica altera within the collected Opera omnia, edited by Giovanni Battista Morgagni.^[32] The structure received its most influential and detailed characterization in 1755 from German anatomist and botanist Johann Gottfried Zinn (1727–1759). In his seminal work Descriptio anatomica oculi humani iconibus illustrata, Zinn provided a precise anatomical account in Chapter VIII, entitled "De ligamento communi" (on the common tendon), derived from meticulous dissections of the human orbit. He depicted the ring as a robust fibrous band encircling the optic nerve and superior orbital fissure, from which the four rectus extraocular muscles arise, emphasizing its role in anchoring these muscles at the orbital apex. Zinn's illustrations and observations marked a significant advancement in understanding the ring's form and attachments, influencing subsequent anatomical studies.^[32]^[33]

Eponym and nomenclature

The common tendinous ring is most widely recognized by its eponym, the annulus of Zinn, named in honor of the German anatomist and botanist Johann Gottfried Zinn, who provided the detailed description in his 1755 anatomical treatise Descriptio anatomica oculi humani iconibus illustrata.^[32] This eponym reflects Zinn's dissection and illustration of the fibrous ring at the orbital apex, establishing it as a key point of origin for the extraocular rectus muscles.^[33] There is a historical debate on attribution, as Antonio Maria Valsalva described the structure nearly 40 years earlier; a 2014 proposal suggested renaming it the "annulus of Valsalva-Zinn" to acknowledge both contributors, though the name "annulus of Zinn" remains in standard use.^[32] In standardized anatomical terminology, the structure is designated as the "common tendinous ring," a term adopted in the inaugural Terminologia Anatomica published in 1998 by the Federative Committee on Anatomical Terminology (FCAT) under the International Federation of Associations of Anatomists (IFAA).^[34] The corresponding Latin official term is anulus tendineus communis, emphasizing its role as a shared tendinous origin.^[34] Common synonyms include annular tendon, Zinn's ring, and Zinn's ligament, which highlight its fibrous, ring-like configuration and historical naming, though these are less formal than the TA designations. It is essential to distinguish the annulus of Zinn from the zonule of Zinn, another eponymous structure described by the same anatomist; the latter refers to the ciliary zonule, a system of fibers suspending the lens within the eye, unrelated to the orbital apex. Nomenclature evolved further with the second edition of Terminologia Anatomica in 2019, developed by the Federative International Programme for Anatomical Terminology (FIPAT), which retained anulus tendineus communis while reinforcing its anatomical relation to the optic canal and superior orbital fissure for precise international usage.^[34] This update underscores the ring's encirclement of the optic nerve at the canal's entrance, promoting consistency in clinical and educational contexts.

References

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