The bony orbit is formed by numerous bones of the cranium and face. It is important to know the various foramina that enter the orbit and their contents.
Structure
The bony orbit is pyramidal in shape, with the base directed externally and the apex facing inwards. The central axis of the orbit is directed at about 30 degrees away from the median plane. It has a roof, a floor, and medial and lateral walls. It also contains numerous foramina. The bones are coated with periosteum (termed the periorbita) which is continuous with the dura of the cranium.
Boundaries
The roof is formed by the orbital plate of the frontal bone.
The floor is formed by the orbital plate of the maxilla. It contains the infraorbital groove for the infraorbital nerve (V2).
The medial wall is formed by the ethmoid posteriorly, the lacrimal bone anteriorly, the frontal bone above and the maxilla below. It is notable for the anterior and posterior ethmoidal foramina.
The lateral wall is formed by the lesser and greater wings of the sphenoid posteriorly and the zygomatic bone anteriorly.
The apex of the orbit contains the optic canal as well as the superior and inferior orbital fissures.
The base of the orbit is closed by soft tissues - the eyelids and eyeball.
Foramina
Optic Canal
The optic canal lies in the apex and is the most medial of the apical foramina. It is circular in shape and passes through the sphenoid.
Contents
The optic canal transmits the optic nerve (II) and the ophthalmic artery from the middle cranial fossa to the orbit.
Superior orbital fissure
The superior orbital fissure is a slit-like opening between the greater and lesser wing of the sphenoid. The frontal bone closes the aperture laterally and the body of the sphenoid closes it medially. The common tendinous ring, the point of attachment for the various extraocular muscles, overlaps the superior orbital fissure and divides it into three parts. The cavernous sinus lies immediately posterior to the superior orbital fissure. The superior orbital fissure is an important communication between the orbit and the middle cranial fossa.
Contents
Above the attachment of the common tendinous ring:
- The recurrent meningeal artery (a branch of the ophthalmic artery)
- The superior ophthalmic vein (draining the orbital contents to the cavernous sinus)
- The trochlear nerve (IV)
- The frontal and lacrimal branches of the ophthalmic nerve (V1)
Within the common tendinous ring:
- The superior and inferior branches of the oculomotor nerve (III)
- The abducens nerve (VI)
- The nasociliary nerve (V1)
Below the common tendinous ring:
- The inferior ophthalmic vein (if present, it may drain directly into the superior ophthalmic vein).
Inferior orbital fissure
The inferior orbital fissure lies between the greater wing of the sphenoid (above) and maxilla/palatine bone (below). The zygomatic bone forms its lateral boundary. It communicates with the pterygopalatine fossa posteriorly.
Contents
The inferior orbital fissure contains:
- The infraorbital and zygomatic nerve (V2)
- The infraorbital artery (branch of the maxillary artery)
- Venous anastamoses between the pterygopalatine fossa and the orbit
Ethmoidal foramina
The anterior and posterior ethmoidal foramen lie on the medial wall of the orbit. They are very small and carry their named vessels (branches / tributaries of the ophthalmic artery/veins) into the ethmoidal air cells, frontal sinus and anterior cranial fossa. The foramina also carry branches of the nasociliary nerve (V1) into the anterior cranial fossa to supply the dura of that region.
Supraorbital foramen / notch
As it departs the orbit, the supraorbital nerve passes through either a notch or a foramen to reach the forehead.
Infraorbital groove / canal
The infraorbital nerve (V2) and vessels initially lie in a groove on the floor of the orbit. About midway along the orbit, they pass into the infraorbital canal. This conveys them to the cheek.
Relations
Superior
The anterior cranial fossa and frontal sinus.
Medial
The ethmodial air cells and nasal cavity.
Lateral
The temporalis muscle, subcutaneous tissue and skin.
Inferior
The maxillary sinus
Posterior
The cavernous sinus, uncus of temporal lobe and trigeminal ganglion.
Surface anatomy
The lateral commissure of the eyelids lies above the lateral wall of the orbit; the same is true for the medial comissure.
Imaging Appearance
To be added
Physiological Variations
Occasionally, union of the maxilla and sphenoid laterally may exclude the zygomatic bone from the inferior orbital fissure.
