M01.02.009 The Bony Orbit

The bony orbits, or eye sockets, are bilateral and symmetrical cavities in the head that enclose the eyeball and its associated structures. This article explores the borders, contents, and clinical correlations of the bony orbit.

Borders and Anatomical Relations

The orbit is a pyramidal structure with the apex pointing posteriorly and the base situated anteriorly. The boundaries of the orbit are formed by seven bones. Understanding these borders and their anatomical relations is crucial for clinical considerations, particularly in the spread of infections and trauma.

Borders of the Bony Orbit

• Roof (Superior Wall): Formed by the frontal bone and the lesser wing of the sphenoid. The frontal bone separates the orbit from the anterior cranial fossa.
• Floor (Inferior Wall): Formed by the maxilla, palatine, and zygomatic bones. The maxilla separates the orbit from the underlying maxillary sinus.
• Medial Wall: Formed by the ethmoid, maxilla, lacrimal, and sphenoid bones. The ethmoid bone separates the orbit from the ethmoid sinus.
• Lateral Wall: Formed by the zygomatic bone and the greater wing of the sphenoid.
• Apex: Located at the opening to the optic canal, the optic foramen.
• Base: Opens out into the face, bounded by the eyelids, and is also known as the orbital rim.

Contents of the Bony Orbit

The bony orbit houses the eyeballs and their associated structures, including muscles, nerves, and blood vessels.

Extra-Ocular Muscles

These muscles are responsible for the movement of the eyeball and the superior eyelid.

Eyelids

The eyelids cover the orbits anteriorly, protecting the eye and aiding in its function.

Nerves

Several cranial nerves supply the eye and its structures:

• Optic Nerve (CN II)
• Oculomotor Nerve (CN III)
• Trochlear Nerve (CN IV)
• Trigeminal Nerve (CN V)
• Abducens Nerve (CN VI)

Blood Vessels

The primary blood supply to the eye comes from the ophthalmic artery, with venous drainage via the superior and inferior ophthalmic veins.

Orbital Fat

Any space within the orbit not occupied by other structures is filled with orbital fat, which cushions the eye and stabilizes the extraocular muscles.

Pathways into the Orbit

There are three main pathways by which structures can enter and leave the orbit:

Optic Canal

Transmits the optic nerve and ophthalmic artery.

Superior Orbital Fissure

Transmits the lacrimal, frontal, trochlear (CN IV), oculomotor (CN III), nasociliary, and abducens (CN VI) nerves, as well as the superior ophthalmic vein.

Inferior Orbital Fissure

Transmits the zygomatic branch of the maxillary nerve, the inferior ophthalmic vein, and sympathetic nerves.

Other Openings

• Nasolacrimal Canal: Drains tears from the eye to the nasal cavity, located on the medial wall of the orbit.
• Supraorbital Foramen and Infraorbital Canal: Carry small neurovascular structures.

Clinical Relevance

Fractures of the Bony Orbit

Fractures of the orbit can significantly affect the eye and surrounding structures. There are two major types of orbital fractures:

Orbital Rim Fracture

A fracture of the bones forms the outer rim of the bony orbit, typically occurring at the sutures joining the maxilla, zygomatic, and frontal bones.

‘Blowout’ Fracture

This fracture involves the partial herniation of the orbital contents through one of its walls, usually due to blunt force trauma. The medial and inferior walls are the weakest, leading to herniation into the ethmoid and maxillary sinuses, respectively.

Any fracture of the orbit increases intraorbital pressure, causing exophthalmos (protrusion of the eye). Hemorrhage into one of the neighboring sinuses may also occur, further complicating the injury.

Points to Remember

• The bony orbit is a pyramidal structure housing the eye and its associated structures.
• It has seven borders formed by different cranial bones.
• The orbit contains extra-ocular muscles, eyelids, nerves, blood vessels, and orbital fat.
• Major pathways into the orbit include the optic canal, superior orbital fissure, and inferior orbital fissure.
• Orbital fractures can lead to significant complications, including exophthalmos and hemorrhage.

Bibliography

• TeachMeSeries Ltd (2024). Anatomy of the Bony Orbit. TeachMeSeries.