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The orbit

Summary
Bones of the orbit
The walls of the orbit
Superior wall (roof)
Inferior wall (floor)
Medial wall
Lateral wall
Foramina in the orbit
Optic canal
Superior orbital fissure
Structures inside the common tendinous ring
Structures outside the common tendinous ring
Inferior orbital fissure
Nasolacrimal duct
Infraorbital foramen
Supraorbital foramen
Ethmoidal foramina
Radiology in Focus
Computed Tomography (CT)
Magnetic Resonance Imaging (MRI)
3. Ultrasound
Author(s)

Summary

The orbit is a bony structure which contains the eyeball and openings for the passage of the optic nerve (CN 2), vessels and lymphatics. It is composed of seven bones that form four walls.

Bones of the orbit

The orbit is formed by seven bones. These are:

Frontal bone
Lacrimal bone
Ethmoid bone
Maxillary bone
Zygomatic bone
Palatine bone
Sphenoid bone

The diagram below shows the bones that are involved in forming the orbital cavity in which the eyeball is placed:

Diagram showing the bones of the orbit.

The walls of the orbit

The orbit has four walls:

Superior wall (roof)
Inferior wall (floor)
Medial wall
Lateral wall

The bones involved in the walls of the orbit are the: frontal, maxillary, ethmoid, sphenoid, lacrimal and zygomatic bones.

We will now explore each wall of the orbit separately.

Superior wall (roof)

The superior wall of the orbit involves the lesser wing of the sphenoid bone and orbital portion of the frontal bone.

Bones involved	Memory aid
Frontal, Sphenoid (lesser wing)	‘FRONT LESS’

The superior wall (roof) of the orbit.

Inferior wall (floor)

The inferior wall of the orbit involves the orbital surfaces of the maxilla, zygomatic and palatine bones.

It is the thickest wall of the orbit and supports the weight of the eye.

Bones involved	Memory aid
Maxillary, Zygomatic, Palatine	‘MY ZIPPED PANTS’

The inferior wall (floor) of the orbit.

Medial wall

The medial wall of the orbit involves the orbital plate of the ethmoid bone, body of the sphenoid bone, frontal bone, lacrimal bone and the maxilla. Note that the body of the sphenoid bone is not visible in the diagram below.

Bones involved	Memory aid
Sphenoid, Maxillary, Ethmoid, Lacrimal	‘SMEL’

The medial wall of the orbit.

Lateral wall

The lateral wall of the orbit involves the greater wing of the sphenoid, orbital plate of the frontal bone and the frontal process of the zygomatic bone.

It is the strongest wall of the orbit.

Bones involved	Memory aid
Sphenoid bone (greater wing), Zygomatic bone	‘GREATER Z’

The lateral wall of the orbit.

Foramina in the orbit

The orbit contains several important foramina and fissures that facilitate the passage of nerves and blood vessels.

Important foramina in the orbit.

Optic canal

The optic canal runs obliquely through the lesser wing of the sphenoid bone.

It contains:

Optic nerve (CN 2)
Ophthalmic artery

Superior orbital fissure

The superior orbital fissure is situated between the greater and lesser wings of the sphenoid bone. It allows the passage of several cranial nerves (which we will explore below) and the ophthalmic vein.

There is a common tendinous ring (also known as the Annulus of Zinn) which surrounds the optic nerve at its entrance at the apex of the orbit. This ring separates some structures.

Diagram showing the contents of the optic canal, surrounded by the common tendinous ring (Annulus of Zinn).

Structures inside the common tendinous ring

Superior division of the oculomotor nerve (CN 3)
Nasociliary nerve (from the ophthalmic division of CN 5)
Inferior division of the oculomotor nerve (CN 3)
Abducens nerve (CN 6)

Memory aid: ‘Several Orbital Nerves In One Annulus’

Diagram showing structures inside the common tendinous ring (Annulus of Zinn).

Structures outside the common tendinous ring

Lacrimal nerve (from the ophthalmic division of CN 5)
Frontal nerve (from the ophthalmic division of CN 5)
Trochlear nerve (from CN 4)
Superior ophthalmic vein

Memory aid: ‘LFTs’

Inferior orbital fissure

This is formed by the greater wing of the sphenoid bone superiorly and the palatine and maxillary bone inferiorly.

It contains:

Ganglionic branches (from pterygopalantine ganglion to maxillary nerve)
Infraorbital nerve (from the maxillary division of CN 5)
Zygomatic nerve (from the maxillary division of CN 5)
Infraorbital artery and vein
Inferior ophthalmic vein

Diagram showing structures of the inferior orbital fissure.

Nasolacrimal duct

Formed by the lacrimal bone, maxilla and inferior nasal concha. It connects the lacrimal sac and the inferior nasal meatus.

It drains tears from the lacrimal sac into the nasal cavity. It plays a vital role in tear drainage and maintaining ocular surface health.

Nasolacrimal duct anatomy. Image source.

Infraorbital foramen

This is situated in the maxillary bone, in the lower part of the orbit. It contains the infraorbital nerves (from the maxillary division of CN 5) as well as vessels. It is important for facial sensation.

Diagram showing the infraorbital foramen. Image source.

Supraorbital foramen

The supraorbital foramen is situated in the frontal bone, in the upper part of the orbit. It contains the supraorbital nerves (from the ophthalmic division of CN 5) as well as vessels. It is significant for sensation in the forehead region.

Supraorbital foramen. Image source.

Ethmoidal foramina

This is situated in the ethmoid bone, lateral to the olfactory groove. There are anterior and posterior ethmoidal foramina.

The anterior ethmoidal foramina contains the anterior ethmoid nerves (from the ophthalmic division of CN 5) as well as vessels.

The posterior ethmoidal foramina contains the posterior ethmoid nerves (from the ophthalmic division of CN 5) as well as vessels.

Ethmoidal foramen. Image source.

Radiology in Focus

Imaging plays a crucial role in the evaluation of orbital anatomy and pathology. Understanding the bony structure of the orbit and its associated foramina is essential for interpreting radiological findings. The following imaging modalities are commonly used in clinical practice to assess orbital conditions:

Computed Tomography (CT)

CT scans are the primary imaging modality for evaluating the orbit due to their excellent ability to delineate bony structures. They are particularly useful in the following clinical scenarios:

Trauma: CT can provide detailed images of the orbital walls and can identify any displacement of bone fragments. The scan below shows an orbital fracture.

$CT showing inferior orbital wall fracture with rectus muscle entrapment. Case courtesy of David Cuete,$

CT showing inferior orbital wall fracture with rectus muscle entrapment. Case courtesy of David Cuete, Radiopaedia.org. From the case rID: 27376.

Tumours: CT can help identify orbital masses, such as lymphomas, meningiomas, or metastatic lesions. It provides information on the extent of the mass and its relationship with surrounding structures.

CT showing bilateral intraocular calcified masses, larger on the left. This is a bilateral retinoblastoma. Case courtesy of Frank Gaillard, Radiopaedia.org. From the case rID: 9764.

Infections: in cases of orbital cellulitis or abscess, CT can reveal the extent of the infection and any associated complications, such as sinus involvement.

CT showing left periorbital cellulitis. Case courtesy of Bruno Di Muzio, Radiopaedia.org. From the case rID: 14848.

Magnetic Resonance Imaging (MRI)

MRI is particularly useful for evaluating soft tissue structures within the orbit. It is often used in the following situations:

Optic Nerve Assessment: MRI is the preferred modality for assessing optic nerve pathology, such as optic neuritis or compressive lesions. It provides detailed images of the optic nerve and surrounding soft tissues.

MRI of optic neuritis. The left optic nerve shows mild enlargement with slightly hyperintense T2 and mild post-contrast enhancement. Case courtesy of Mohammad A. ElBeialy, Radiopaedia.org. From the case rID: 38584.

Soft Tissue Tumours: MRI is superior for characterising soft tissue masses, including those arising from the lacrimal gland or extraocular muscles. It helps differentiate between benign and malignant lesions based on their signal characteristics.

MRI of lacrimal gland lymphoma. Case courtesy of Ahmed Abdrabou, Radiopaedia.org. From the case rID: 39101.

Inflammatory Conditions: MRI is useful in evaluating conditions such as thyroid eye disease (Graves' disease), where it can show muscle enlargement and fat expansion.

MRI of thyroid orbitopathy showing bilateral intraocular muscle enlargement. Case courtesy of Ian Bickle, Radiopaedia.org. From the case rID: 85413.

3. Ultrasound

Ultrasound can be used as an adjunctive tool, particularly in paediatric patients or when radiation exposure is a concern. It is helpful for:

Assessing the Globe: Ultrasound can evaluate the integrity of the globe and detect conditions such as retinal detachment or vitreous haemorrhage.

Ultrasound of retinal detachment. Case courtesy of G Balachandran, Radiopaedia.org. From the case rID: 5695.

Guiding Procedures: Ultrasound can assist in guiding fine-needle aspiration biopsies of orbital masses or cysts.

Author(s)

Dr Abhiyan Bhandari

Abhiyan is the Co-Founder and Radiology & Imaging Lead of Ophtnotes. He is a doctor who graduated from UCL Medical School in London. He scored in the top 10% of candidates who sat the Duke Elder examination and runs ophthalmology and Duke Elder revision sessions aimed at medical students. He also runs a YouTube channel aimed at medical students, covering topics ranging from study tips, productivity and vlogs of his journey through medical school.

Published: 20/2/22

Last updated: 23/1/25