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:

1. Frontal bone
2. Lacrimal bone
3. Ethmoid bone
4. Maxillary bone
5. Zygomatic bone
6. Palatine bone
7. Sphenoid bone

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

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.

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.

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.

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.

Foramina in the orbit

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

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.

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’

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

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.

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.

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.

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.

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.

• 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.

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

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.

• 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.

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

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.

• 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