Walls and margin
The orbit is built from seven bones: frontal, sphenoid, zygomatic, maxilla, ethmoid, lacrimal and palatine.
| Wall | Bones | Features |
|---|---|---|
| Roof | Orbital plate of frontal, lesser wing of sphenoid | Lacrimal gland fossa laterally, trochlear fovea medially; frontal sinus and anterior cranial fossa above |
| Lateral wall | Zygomatic, greater wing of sphenoid | The strongest wall; zygomatic foramina |
| Floor | Maxilla, zygomatic, orbital process of palatine | Infra-orbital groove and canal; maxillary sinus below; thin |
| Medial wall | Frontal process of maxilla, lacrimal, orbital plate of ethmoid, body of sphenoid | The thinnest wall (lamina papyracea); fossa for the lacrimal sac; ethmoidal foramina |
The orbital margin is formed by the frontal bone above, the zygomatic bone and the zygomatic process of the frontal bone laterally, the zygomatic bone and maxilla below, and the frontal process of the maxilla and the frontal bone medially. The frontozygomatic suture is palpable on the lateral margin. The supra-orbital notch or foramen lies at the junction of the medial third and lateral two-thirds of the upper margin, and the infra-orbital foramen lies just below the lower margin on the maxilla, roughly in line with it.
Fissures, canals and foramina
The orbit communicates with the middle cranial fossa, the nose and the fossae behind the maxilla through named openings.
| Opening | Location | Transmits |
|---|---|---|
| Optic canal | Lesser wing of sphenoid, at the apex | Optic nerve with its meninges, ophthalmic artery |
| Superior orbital fissure | Between greater and lesser wings of sphenoid | Lacrimal, frontal and trochlear nerves and superior ophthalmic vein outside the tendinous ring; oculomotor (both divisions), nasociliary and abducens nerves inside it |
| Inferior orbital fissure | Between greater wing of sphenoid and maxilla | Infra-orbital and zygomatic nerves (V2), infra-orbital vessels, veins to the pterygoid plexus |
| Infra-orbital groove, canal and foramen | Floor to face | Infra-orbital nerve and vessels |
| Supra-orbital notch or foramen | Upper margin | Supra-orbital nerve and vessels |
| Anterior and posterior ethmoidal foramina | Frontoethmoidal suture on medial wall | Ethmoidal nerves and arteries |
| Nasolacrimal canal | Front of floor, medially | Nasolacrimal duct to inferior meatus |
The ethmoidal foramina lead the surgeon to the apex. A common teaching rule places the anterior ethmoidal foramen about 24 mm behind the anterior lacrimal crest, the posterior foramen about 12 mm further back, and the optic canal about 6 mm beyond that.
Relations: the lateral skull and pterion
The pterion is the H-shaped junction of the frontal, parietal, squamous temporal and greater wing of sphenoid bones in the temporal fossa, just behind the lateral wall of the orbit. It is a surface landmark about two finger-breadths above the zygomatic arch and a thumb's breadth behind the frontal process of the zygomatic bone.
The bone here is thin, and the anterior (frontal) branch of the middle meningeal artery runs in a groove on its inner surface. A blow to the side of the head can fracture the pterion and tear the artery, producing an extradural haematoma.
Around the orbit, the frontal sinus lies above the medial roof, the ethmoidal air cells behind the medial wall, and the maxillary sinus below the floor. Through the piriform aperture of a dry skull, the bony conchae of the lateral nasal wall are visible medial to the orbit.
Clinical relevance
The thin floor and medial wall give way before the strong orbital margin, which shapes most orbital injuries.
- Blow-out fracture: a blow to the eye raises orbital pressure and fractures the floor into the maxillary sinus. Signs are numbness over the cheek and upper lip (infra-orbital nerve), diplopia on upgaze from inferior rectus entrapment, and enophthalmos.
- Medial wall fracture: through the lamina papyracea into the ethmoid cells, causing orbital emphysema when the patient blows the nose.
- Orbital cellulitis: ethmoid sinusitis spreads through the lamina papyracea, especially in children.
- Superior orbital fissure and orbital apex syndromes: lesions at the fissure affect III, IV, V1 and VI together, causing ophthalmoplegia and forehead numbness; involvement of the optic canal adds visual loss.
- Extradural haematoma at the pterion: classically a lucid interval, then rapid deterioration.
On the specimen
The specimen is a dry skull seen from the front and side, so name each part by the bone it belongs to.
- Superior or inferior orbital fissure: the superior fissure is a slit at the back of the orbit between the two sphenoid wings, running upwards and laterally; the inferior fissure runs along the junction of the lateral wall and floor, opening towards the infratemporal fossa.
- Greater or lesser wing of sphenoid: the lesser wing is above the superior fissure and carries the optic canal; the greater wing forms the back of the lateral wall and reaches the side of the skull at the pterion.
- Lacrimal bone: the small, thin plate between the frontal process of the maxilla in front and the ethmoid behind, with the posterior lacrimal crest on it.
- Infra-orbital groove or foramen: the groove runs forward in the floor; the foramen opens on the face below the margin.
- Posterior ethmoidal foramen: a small hole high on the medial wall near the apex, along the frontoethmoidal suture.