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M01.02.031 The Bony Orbit
The bony orbits (or eye sockets) are bilateral, symmetrical cavities in the skull that house the eyeballs and associated structures. This article covers the borders, contents, pathways, and clinical relevance of the bony orbit.
Borders and Anatomical Relations
The bony orbit is a pyramidal structure with its apex directed posteriorly and its base anteriorly. Seven bones form its boundaries. These relations are critical in trauma and the spread of infections.
Borders
| Border | Bones Involved | Significance |
|---|---|---|
| Roof (Superior) | Frontal bone, Lesser wing of the sphenoid | Separates the orbit from the anterior cranial fossa. |
| Floor (Inferior) | Maxilla, Palatine, Zygomatic | Separates the orbit from the maxillary sinus. |
| Medial Wall | Ethmoid, Maxilla, Lacrimal, Sphenoid | Separates the orbit from the ethmoid sinus. |
| Lateral Wall | Zygomatic, Greater wing of the sphenoid | Provides lateral support to the orbit. |
| Apex | Located at the optic canal (optic foramen). | Entrance for the optic nerve and ophthalmic artery. |
| Base | Formed by the orbital rim (bounded by the eyelids). | Provides the anterior opening for structures entering or exiting the orbit. |
Contents of the Bony Orbit
The orbit houses essential structures related to vision and eye movement.
Key Components
- Eyeballs – Occupy the central space within the orbit.
- Extraocular Muscles – Enable eye and eyelid movements.
- Eyelids – Provide anterior protection to the orbit.
- Nerves – Include the optic nerve (CN II), oculomotor (CN III), trochlear (CN IV), trigeminal (V1), and abducens (CN VI).
- Blood Vessels
- Arterial supply: Primarily from the ophthalmic artery.
- Venous drainage: Superior and inferior ophthalmic veins.
- Orbital Fat – Fills unoccupied space, cushioning and stabilizing other structures.
Pathways into the Orbit
Structures enter or exit the orbit through specific pathways.
| Pathway | Structures Transmitted |
|---|---|
| Optic Canal | Optic nerve, Ophthalmic artery. |
| Superior Orbital Fissure | Lacrimal nerve, Frontal nerve, Trochlear nerve (CN IV), Oculomotor nerve (CN III), Nasociliary nerve, Abducens nerve (CN VI), Superior ophthalmic vein. |
| Inferior Orbital Fissure | Zygomatic nerve (branch of maxillary nerve), Inferior ophthalmic vein, Sympathetic nerves. |
| Other Minor Openings | Nasolacrimal canal (drains tears to nasal cavity), Supraorbital foramen, Infraorbital canal (carry neurovascular structures). |
Clinical Relevance
Fractures of the Bony Orbit
- Orbital Rim Fracture
- Occurs at the sutures between the maxilla, zygomatic, and frontal bones.
- Often results from blunt trauma.
- Blowout Fracture
- Involves herniation of orbital contents through the medial or inferior wall.
- Commonly affects the ethmoid and maxillary sinuses.
- Clinical features:
- Exophthalmos: Protrusion of the eye due to increased intraorbital pressure.
- Sinus Involvement: Hemorrhage or infection in adjacent sinuses.
Points to Remember
- Roof of Orbit: Protects the brain; susceptible to trauma near the anterior cranial fossa.
- Inferior Wall: Weakest, prone to blowout fractures with herniation into the maxillary sinus.
- Clinical Importance of Pathways: Knowledge of the pathways aids in diagnosing nerve or vascular injuries in orbital trauma.
- Orbital Fat: Essential for cushioning and support; its displacement indicates pathology.
Figures
Key Illustrations
- Borders of the Orbit – Anterior and lateral views.
- Pathways – Highlighting optic canal, superior and inferior orbital fissures.
- Clinical Imaging – CT scan of a blowout fracture showing herniation into the maxillary sinus.
Bibliography
- TeachMeSeries Ltd. (2024). Bony Orbit Anatomy.
- James Heilman, MD. (2024). CT Imaging of Orbital Fractures.