Bone anatomy of the orbit
The four walls of the orbit (orbit) form a pyramid diverging anteriorly at an angle of about 45 degrees. The walls are formed by several cranial bones:
- Superior wall (roof): forms the floor of the frontal sinus. Consists of the frontal bone and the lesser wing of the sphenoid bone. At the anterior margin, there is an aperture for the passage of the supraorbital nerve (supraorbital nerve), a branch of V1 of the trigeminal nerve.
- Inferior wall (floor): serves as the roof of the maxillary sinus. Formed by the zygomatic, maxillary, and palatine bones. The wall is thin, which makes blow-out fractures (blow-out fracture) common here, as well as the potential spread of infection from the sinus. Includes the canal for the infraorbital nerve (infraorbital nerve, branch of V2).
- Medial wall: separates the orbit from the ethmoidal air cells. Consists of the maxillary, frontal, ethmoid, lacrimal, and sphenoid bones. Because of its fragility, it is termed the lamina papyracea (lamina papyracea).
- Lateral wall: separates the orbit from the temporal and middle cranial fossae. Formed by the frontal and zygomatic bones.
Note: The shape and structure of the bony orbit can be deformed in sinus diseases (e.g., silent sinus syndrome) or bone lesions (fibrous dysplasia).
Muscular apparatus and topographic compartments
Eye movements are provided by six extraocular muscles: four rectus and two oblique muscles.
The rectus muscles (superior, inferior, medial, lateral) originate at the orbital apex from the common tendinous annulus of Zinn (annulus of Zinn). Nearby runs the levator palpebrae superioris muscle. On cross-sectional imaging, it merges with the superior rectus muscle to form the superior muscular complex.
The superior oblique muscle runs to the superior margin, loops through the trochlea, and attaches to the globe. The inferior oblique originates from the anterior part of the inferior wall.
Orbital compartments: The rectus muscles with their fascias form the muscle cone (orbital cone). It divides the orbital space into three zones, which is crucial for locating the source of pathology:
- Extraconal space — outside the muscle cone.
- Conal space — the muscles themselves and their fascias.
- Intraconal space — inside the cone.
Globe and blood supply
The eyeball (globe) consists of three tunics:
- Outer layer: sclera (sclera) and cornea (cornea).
- Middle tunic: iris (iris) and ciliary body (ciliary body).
- Inner tunic: retina (retina).
The lens (lens) and iris divide the globe into the anterior chamber (containing aqueous humour) and the posterior chamber (filled with vitreous humour).
Blood supply derives from branches of the ophthalmic artery: frontal, lacrimal, and nasociliary. The optic nerve is nourished by the retinal artery (retinal artery), running directly inside the nerve. Venous drainage occurs via the superior and inferior ophthalmic veins.
Orbital nerves
The optic nerve (CN II) is divided into intraocular, intraorbital, intracanalicular, and intracranial segments. Histologically, it is an outgrowth of the brain, myelinated by oligodendrocytes (oligodendrocytes). Consequently, gliomas and meningiomas occur here, but schwannomas do not.
Motor innervation of the muscles is distributed as follows:
- Trochlear nerve (CN IV): innervates only the superior oblique muscle.
- Abducens nerve (CN VI): innervates the lateral rectus muscle.
- Oculomotor nerve (CN III): innervates all other extraocular muscles and the levator palpebrae superioris.
Briefly: answers to topical questions
What branches does the ophthalmic artery give off?
The ophthalmic artery gives off several branches supplying the orbit, the eyeball, and surrounding structures. These include:
- Retinal artery / central retinal artery (a. centralis retinae) — runs inside the optic nerve.
- Lacrimal artery (a. lacrimalis).
- Frontal artery (a. frontalis).
- Nasociliary artery (a. nasociliaris).
- Supraorbital artery (a. supraorbitalis).
- Ciliary arteries (aa. ciliares) — enter the eyeball.
- Supratrochlear artery (a. supratrochlearis) — terminal branch.
- Dorsal nasal artery (a. dorsalis nasi) — terminal branch.
- Anterior ethmoidal branches.
- Branches to the extraocular muscles.
- Anterior meningeal branches (r. meningeus anterior) — sometimes arise.
What structures pass through the superior orbital fissure?
The superior orbital fissure (fissura orbitalis superior) transmits cranial nerves innervating orbital structures, as well as blood vessels. These include:
- Oculomotor nerve (n. oculomotorius, CN III).
- Trochlear nerve (n. trochlearis, CN IV).
- Abducens nerve (n. abducens, CN VI).
- Ophthalmic nerve (n. ophthalmicus, first branch of CN V).
- Superior ophthalmic vein (v. ophthalmica superior).
- Anterior meningeal branches (r. meningeus anterior) from the ophthalmic artery (sometimes present).
Additionally, the pterygopalatine process of the buccal fat pad often extends into the cranial cavity through the inferomedial part of the fissure.
What constitutes the vascular tunic (middle layer) of the eyeball?
The vascular tunic (middle layer) of the eyeball (uvea) comprises three main anatomical structures. These include:
- Iris (iris).
- Ciliary body (ciliary body).
- Choroid (choroidea).
The choroid itself has a layered structure comprising the suprachoroidea, stroma of choroid, choriocapillaris, and basal complex (Bruch's membrane).
What structures pass through the common tendinous ring (annulus of Zinn)?
According to the cited sources, the optic nerve passes through/is located within the common tendinous ring (annulus of Zinn): the ring surrounds the optic nerve at the orbital apex.
Four extraocular rectus muscles originate from the annulus of Zinn:
- Superior rectus muscle.
- Inferior rectus muscle.
- Medial rectus muscle.
- Lateral rectus muscle.
What forms the lateral wall of the orbit?
The lateral wall of the orbit (paries lateralis) is formed by three bony structures. It includes:
- Zygomatic process of the frontal bone.
- Orbital surface of the greater wing of the sphenoid bone.
- Orbital surface of the zygomatic bone.
This wall separates the orbital cavity from the temporal fossa anteriorly and from the middle cranial fossa posteriorly.
What passes through the optic canal (canalis opticus)?
The optic canal (canalis opticus) transmits neural and vascular structures connecting the orbital cavity to the middle cranial fossa. These include:
- Optic nerve (n. opticus, cranial nerve II).
- Ophthalmic artery (a. ophthalmica), a branch of the internal carotid artery, located lateral to the optic nerve.
- Central retinal artery (a. centralis retinae) — a branch of the ophthalmic artery.
To remember the innervation: the VI nerve abducts the eye (lateral rectus), the IV nerve acts as a pulley (superior oblique), and the III nerve is the main oculomotor nerve (supplying all other muscles).
Class and exam questions
Why do optic nerve tumors differ from tumors of other cranial nerves?
The optic nerve is an extension of brain tissue. It is myelinated by oligodendrocytes, which is why gliomas occur here, whereas schwannomas are uncharacteristic.
What is the clinical danger of thinning of the orbital floor?
The inferior wall serves as the roof of the maxillary sinus. Its minimal thickness predisposes it to "blowout" fractures and the easy spread of severe infections from the sinus into the orbit.
What is the superior muscle complex on MRI?
This is the visual fusion of the superior rectus muscle and the levator palpebrae superioris muscle. On imaging, they are often difficult to distinguish from each other.
What else to study on this topic
- "Silent sinus" syndrome and the mechanisms of enophthalmos development
- Radiological pattern of orbital fibrous dysplasia ("ground-glass" appearance)
- Hemorrhagic cystic degeneration in bone pathologies of the orbit
- Differential diagnosis algorithm for intra- and extraconal lesions
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