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Orbital Bony Anatomy
Overview
The bony orbit is built from seven bones: the frontal, zygomatic, maxillary, ethmoid, sphenoid, lacrimal, and palatine. Together they form a pear-shaped cavity of roughly 30 mL, narrowing at its apex into the optic canal, the stem through which the optic nerve passes. Every wall is a composite of two or more of these bones. That composite structure is the reason fracture patterns, the spread of sinus disease, and surgical access routes all come back to the same question: which bone is thin, which is thick, and what sinus sits directly behind it.
The Four Walls
Roof
The orbital roof is formed mainly by the orbital plate of the frontal bone, with a small posterior contribution from the lesser wing of the sphenoid. It separates the orbit from the anterior cranial fossa above and the frontal sinus in front of that.
Floor
The floor is formed mostly by the maxillary bone, with contributions from the zygomatic and palatine bones. It slopes downward roughly 20 degrees from posterior to anterior and sits directly above the maxillary sinus, thin enough to be the wall most often involved in a blowout fracture, where orbital fat and the inferior rectus muscle herniate down into the sinus below.
Medial Wall
The medial wall is built from the ethmoid bone, specifically its paper-thin lamina papyracea, with contributions from the lacrimal, maxillary, and sphenoid bones. It is the thinnest wall in the orbit.
Lateral Wall
The lateral wall is the thickest and strongest of the four. The zygomatic bone forms its anterior portion; the greater wing of the sphenoid forms the posterior portion.
Foramina, Fissures, and Surgical Landmarks
Lamina Papyracea
This is the thin ethmoid segment of the medial wall; its name literally means "paper-thin layer." It separates the orbit from the ethmoid sinus air cells with almost no bony resistance, which makes it the usual route for sinus infection to spread into the orbit as orbital cellulitis. The same thinness makes it the most frequent site of medial wall blowout fractures.
Fovea Ethmoidalis
The fovea ethmoidalis forms the roof of the ethmoid sinus complex, which also sits as part of the floor of the anterior cranial fossa immediately above the medial orbit and the lacrimal fossa. During both lacrimal surgery (dacryocystorhinostomy) and endoscopic sinus or orbital procedures, it is the landmark that keeps dissection out of the cranial cavity: going too far superiorly risks breaching it, with a real risk of cerebrospinal fluid leak or intracranial injury.
Whitnall's (Lateral Orbital) Tubercle
Whitnall's tubercle is a small bony elevation on the zygomatic bone, just inside the lateral orbital rim. Several soft-tissue structures attach here at once: the lateral horn of the levator aponeurosis, the lateral canthal tendon, the check ligament of the lateral rectus, and the Lockwood suspensory ligament. Any procedure that needs to locate or reattach one of them, such as a lateral canthal tendon repair, uses this tubercle as a reference point.
Superior Orbital Fissure
The superior orbital fissure lies between the greater and lesser wings of the sphenoid bone. It transmits cranial nerves III, IV, and VI, the ophthalmic division of V (V1), and the ophthalmic veins. A lesion compressing it produces superior orbital fissure syndrome: ophthalmoplegia, ptosis, and V1 sensory loss together.
Inferior Orbital Fissure
Lower down, between the orbital floor and the lateral wall, the inferior orbital fissure connects the orbit to the pterygopalatine and infratemporal fossae. The maxillary nerve (V2) passes through it and continues forward along the orbital floor as the infraorbital nerve, which is why a floor fracture so often produces cheek and upper lip numbness. The zygomatic nerve, a separate V2 branch, also runs through this fissure before dividing along the lateral orbital wall.
Optic Canal
The optic canal runs through the lesser wing of the sphenoid at the orbital apex, carrying the optic nerve, the ophthalmic artery, and sympathetic nerve fibers. Its location at the apex explains why apex pathology, whether compressive, traumatic, or inflammatory, threatens vision directly, and why the medial wall offers the shortest surgical path to this region.
Fibrous Dysplasia of the Orbit
Fibrous dysplasia is a disease of bone, not a soft-tissue orbital mass: normal bone is gradually replaced by fibro-osseous tissue, most often in the sphenoid and frontal bones of the orbit. It usually presents in the first two decades of life with slowly progressive bony expansion and facial asymmetry. On CT, it has a characteristic ground-glass appearance that sets it apart from the more discrete lytic or sclerotic patterns of other bone lesions.
- Floor = maxillary (mostly), thin, slopes ~20°: the classic blowout fracture site.
- Medial wall = ethmoid's lamina papyracea, the thinnest wall in the orbit and the classic route for sinus infection to spread into it.
- Optic canal runs through the sphenoid's lesser wing; the superior orbital fissure sits between the sphenoid's greater and lesser wings.
- Fibrous dysplasia of the orbit shows a ground-glass appearance on CT.
Don't mix up the two sphenoid wings. The optic canal is carried in the lesser wing; the superior orbital fissure sits between the lesser and greater wings, with the greater wing also forming the posterior lateral orbital wall alongside the zygomatic bone. Confusing the two changes which structures you'd expect to be affected together in apex versus fissure pathology.
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