Bones of the orbit
By definition, the orbit (bony orbit or orbital cavity) is a skeletal cavity comprised of seven bones situated within the skull. The cavity surrounds and provides mechanical protection for the eye and soft tissue structures related to it.
The bones that make up the orbit contain several foramina and fissures through which important neurovascular structures (such as the optic nerve (CN II)) pass through on their way from the brain to the eye and face and vice versa.
Maxilla, Frontal bone, Zygomatic bone, Ethmoid bone, Lacrimal bone, Sphenoid bone, and Palatine bone
Mnemonic: Many Friendly Zebras Enjoy Lazy Summer Picnic
|Base and apex||
Apex: optic foramen
Orbital margin (rim):
- supra-orbital margin: frontal bone
- medial margin: frontal process of the maxilla
- infra-orbital margin: zygomatic process of the maxilla and the zygomatic bone
- lateral margin: zygomatic process of the frontal bone and the zygomatic bone and its frontal process
Roof (superior): orbital part of the frontal bone, lesser wing of the sphenoid bone
Medial: orbital plate of the ethmoid bone, lacrimal bone, frontal process of the maxilla, greater wing of the sphenoid bone
Floor (inferior): orbital surface of the maxilla, zygomatic bone, palatine bone
Lateral: zygomatic bone, sphenoid bone
|Articulations (sutures)||Frontomaxillary, zygomaticomaxillary, frontozygomatic (or zygomaticofrontal), fronto-ethmoidal, sphenofrontal (or frontosphenoidal), sphenozygomatic, frontolacrimal, spheno-ethmoidal, lacrimomaxillary|
|Landmarks and openings||Optic foramen (canal), lacrimal fossa, lacrimal groove, anterior and posterior ethmoidal foramina, trochlea, superior and inferior orbital fissures|
|Content of the orbit||Eyeballs, orbital fascia, orbital fat, extraocular muscles, neurovasculature, lacrimal apparatus|
|Clinical relations||Orbital fractures, inflammatory and neoplastic processes|
This article will discuss the bones of the orbit, their articulations, and the most important anatomical landmarks. Finally, a few words about the most common pathological conditions related to the orbit will be discussed.
- Landmarks and openings
- Contents of the orbit
- Clinical relations
The orbit appears as a quadrangular pyramidal cavern in the upper face. It is made up of four facial bones and three cranial bones: maxilla, zygomatic bone, lacrimal bone, palatine bone, frontal bone, ethmoid bone, and sphenoid bone.
The base of this pyramid opens anteriorly onto the face, while the apex is pointed posteromedially towards the center of the skull. The walls that complete this pyramid are the medial, lateral, roof (superior), and floor (inferior) walls. The bones of the orbit are lined with periosteum called periorbita.
Base and apex
The apex of the orbit is the optic foramen (canal), bound medially by the body of the sphenoid bone and laterally by the lesser wing of the sphenoid bone. It is medial to the superior orbital fissure. The optic foramen provides passage for the optic nerve (CN II) and ophthalmic artery to exit the skull.
The base of the orbit is called the orbital margin or orbital rim. Its function is to protect the contents of the orbit and to provide an attachment site for the orbital septum. The margin has a quadrangular shape, with the following borders:
- Supra-orbital margin – the frontal bone
- Medial margin – the frontal process of the maxilla
- Infra-orbital margin – the zygomatic process of the maxilla, and the zygomatic bone
- Lateral margin – the zygomatic process of the frontal bone, the zygomatic bone, and the frontal process of the zygomatic bone
The roof, or superior wall, of the orbit separates the orbit from the anterior cranial fossa. The largest contributor of the roof is the orbital part of the frontal bone, while the small posterior portion is completed by the lesser wing of the sphenoid bone. The only important landmark of this wall is the anterolaterally located depression called the lacrimal fossa, intended for hosting the orbital part of the lacrimal gland.
The medial wall is built out of four bones: the maxilla, lacrimal bone, ethmoid bone, and sphenoid bone. A useful mnemonic to remember them is, “My Little Eye Sits in the orbit.”
The majority of the wall is made out of the orbital plate of the ethmoid bone, which is located posterosuperiorly on the wall. This is the part of the ethmoid bone that contains ethmoidal cells and is very thin. Anterior to the ethmoid bone is the lacrimal bone, and anterior to the lacrimal bone is the frontal process of the maxilla. The lacrimal bone and the frontal process of the maxilla build the lacrimal groove which contains the lacrimal sac. Posterior to the ethmoid bone, the medial wall is completed by a small portion of the greater wing of the sphenoid bone that forms the medial part of the optic foramen (canal).
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Besides the lacrimal groove, important landmarks are the anterior and posterior ethmoidal foramina and the trochlea. The ethmoidal foramina are found at the junction between the roof and the medial wall as a part of the fronto-ethmoidal suture, and serve as the passageways for the anterior and posterior ethmoidal nerves and vessels. The trochlea is the only cartilage found in the orbit. It is a pulley-like structure in the orbit through which the tendon of the superior oblique muscle passes. Fun fact: the word trochlea comes from the Greek word for pulley.
The floor, or inferior wall, separates the orbit and the maxillary sinus. It is formed by three bones: maxilla, zygomatic bone, and palatine bone. The orbital surface of the maxilla makes up most of it while small portions of the zygomatic and palatine bones make up the rest.
The most important landmark of the floor is the inferior orbital fissure. It separates the floor from the lateral wall, and provides passage for several neurovascular structures. It begins posteriorly and then courses along the lateral border of the floor. The part of the fissure that goes through the floor of the orbit is anteriorly bordered by the zygomatic bone, posteriorly by the orbital process of the palatine bone, and medially by the maxilla.
This wall is formed by the zygomatic bone anteriorly and the greater wing of the sphenoid bone posteriorly. It is the thickest and strongest wall of the orbit, which posteriorly separates the orbit from the middle cranial and temporal fossae.
The most prominent landmark of the lateral wall is the superior orbital fissure, found between the greater and lesser wing of the sphenoid bone. The greater wing of the sphenoid bone also provides the lateral border of the inferior orbital fissure.
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Articulations of the orbit are not often discussed. Rather than purely memorizing their names, let’s explain the logic here. As we know, the articulations of the bones of the skull are immobile fibrous joints called sutures.
Sutures are named after the bones that they connect, so let’s look at the orbit and name them one by one, just like they do in anatomical textbooks. All bones of the orbit articulate among themselves, except for the sphenoid bone and maxilla. There is no suture between them, as they are separated by the inferior orbital fissure. Note that the first three of the listed sutures are most commonly described within the textbooks.
|Frontomaxillary suture||Frontal bone → frontal process of the maxilla|
|Zygomaticomaxillary suture||Zygomatic bone → maxilla|
|Frontozygomatic (zygomaticofrontal) suture||Frontal bone → zygomatic bone|
|Fronto-ethmoidal suture||Frontal bone → ethmoid bone|
|Sphenofrontal (frontosphenoidal) suture||Sphenoid bone → frontal bone|
|Sphenozygomatic suture||Greater wing of the sphenoid bone → zygomatic bone|
|Frontolacrimal suture||Frontal bone → lacrimal bone|
|Spheno-ethmoidal suture||Lesser wing of the sphenoid bone → ethmoid bone|
|Lacrimomaxillary suture||Lacrimal bone → maxilla|
Landmarks and openings
For the sake of effective memorizing, let’s list all the landmarks and opening in one place:
- Optic foramen (canal) – located at the apex, the opening to the optic foramen is bordered by the body of the sphenoid bone and the lesser wing of the sphenoid bone. It transmits the optic nerve and ophthalmic artery.
- Lacrimal fossa – located anterolaterally on the orbital roof and hosts the lacrimal gland.
- Lacrimal groove – placed on the medial orbital wall between the lacrimal bone and maxilla. It contains the lacrimal sac.
- Anterior ethmoidal foramen – found at the junction between the superior and medial orbital walls. It transmits the anterior ethmoidal vein, artery, and nerve.
- Posterior ethmoidal foramen – also found at the junction between the superior and medial orbital walls. It transmits the posterior ethmoidal vein, artery, and nerve.
- Trochlea – a pulley-like structure at the orbital roof through which the tendon of the superior oblique muscle passes. It is the only cartilage found in the orbit.
- Superior orbital fissure – located between the greater and lesser wings of the sphenoid bone. It transmits the superior ophthalmic vein, a branch of the inferior ophthalmic vein, superior and inferior branches of the oculomotor nerve (CN III), lacrimal, frontal, and nasociliary branches of the ophthalmic division of the trigeminal nerve (CN V1), trochlear (CN IV) and abducens (CN VI) nerves.
- Inferior orbital fissure – on the lateral border of the orbital floor. It transmits the inferior ophthalmic veins, infraorbital artery, zygomatic and infraorbital branches of the maxillary division of the trigeminal nerve (CN V2), and orbital branches of the pterygopalatine ganglion. The infraorbital artery and infraorbital nerve then go on and exit the skull through the infraorbital foramen. Remember these structures by using the mnemonic "Zoom In the Orbit" (Zygomatic nerve and vessels, Infraorbital nerve and vessels, Orbital branches of the pterygopalatine ganglion)
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Contents of the orbit
Most of the space in the orbital cavity is occupied by the eyeballs and orbital fascia; the rest of it is filled with orbital fat which stabilizes the eye and extraocular muscles. Extraocular muscles insert into the eyeball and enable movements of the eye and the superior eyelid. There are six: superior, inferior, medial, and lateral rectus muscles and the superior and inferior oblique muscles.The rest of the soft tissue content that fills the orbit are the neurovascular structures that communicate with the contents of the orbit through foramina and are listed above. The lacrimal apparatus is also found within the orbit.
The most common clinical conditions related to the orbit are fractures. Any of the walls can be affected, but most commonly it’s the floor, followed by the medial wall (because of the fragility of thin ethmoidal cells). When the orbital floor is affected, the inferior rectus muscle is often dragged into the fracture line which results in an inability to move the eyeball upwards in the affected eye (known as upward gaze diplopia).
On the other hand, the ethmoid bone and its labyrinth are usually affected in medial wall fractures. This usually results with creating continuity (craniosinus fistula) between the ethmoid paranasal sinuses and the orbit and is clearly visible in radiographs. The craniosinus fistula enables leakage of the cerebrospinal fluid from the cranium through the nose which leads to a drop in intracranial pressure (hypotension) and manifests as headache, nausea, vomiting, and difficulty concentrating. In addition to this, bone fragments can physically damage the eye and cause blindness and nasal deformity.
Inflammatory and neoplastic processes
When it comes to the contents of the orbit, any inflammatory processes such as conjunctivitis, or even neoplastic processes, like chorodial melanoma, that affect the eye or its accessory structures show a tendency to spread into the cranium through orbital openings as they provide a direct communication between the orbit and cranial fossae. Depending on the nature of the process, it may result either with inflammation of the meninges (meningitis), or with creation of metastatic masses (cancer) within the brain tissue.