Learning Objective: Describe the anatomy of the bony orbit, including its borders, contents, major foramina, and clinical correlations such as orbital fractures.

The bony orbit is a paired, pyramidal cavity in the skull that houses the eyeball, extraocular muscles, neurovascular structures, and orbital fat. It protects the eyeball, facilitates ocular movement, and provides pathways for nerves and vessels entering and leaving the orbit.

Borders and Anatomical Relations

The orbit has a pyramidal shape with its apex directed posteriorly and base opening anteriorly (orbital rim). It is formed by seven bones:

• Roof (superior wall): Frontal bone + lesser wing of sphenoid; separates orbit from anterior cranial fossa.
• Floor (inferior wall): Maxilla + palatine + zygomatic bones; separates orbit from maxillary sinus.
• Medial wall: Ethmoid + maxilla + lacrimal + sphenoid; ethmoid separates orbit from ethmoid sinus.
• Lateral wall: Zygomatic + greater wing of sphenoid.
• Apex: Optic canal (optic foramen).
• Base: Orbital rim; anterior opening of the orbit.

Contents

• Eyeball
• Extraocular muscles: Move the eyeball and the superior eyelid.
• Eyelids: Cover the anterior orbit.
• Nerves: CN II (optic), III (oculomotor), IV (trochlear), V1 (ophthalmic branch of trigeminal), VI (abducens).
• Blood vessels: Ophthalmic artery; venous drainage via superior and inferior ophthalmic veins.
• Orbital fat: Cushions and stabilizes ocular structures.

Pathways into the Orbit

1. Optic canal: Optic nerve (CN II) + ophthalmic artery.
2. Superior orbital fissure: Lacrimal, frontal, trochlear (CN IV), oculomotor (CN III), nasociliary, abducens (CN VI) + superior ophthalmic vein.
3. Inferior orbital fissure: Zygomatic branch of CN V2, inferior ophthalmic vein, sympathetic fibers.
4. Nasolacrimal canal: Drains tears to the nasal cavity.
5. Supraorbital foramen & infraorbital canal: Carry small neurovascular structures.

Clinical Relevance: Orbital Fractures

Types of orbital fractures:

• Orbital rim fracture: Fracture at sutures of the maxilla, zygomatic, or frontal bones forming the rim.
• Blowout fracture: Herniation of orbital contents through a thin wall (usually medial or inferior) into the ethmoid or maxillary sinus.

Clinical consequences:

• Increased intraorbital pressure → exophthalmos (protruding eye).
• Possible hemorrhage into sinuses.
• Diplopia or ocular motility restrictions occur if the extraocular muscles are trapped.

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