Basic Knowledge

Importance of the orbital cavity for the eye: The orbital cavity is the protective bony socket for the globe together with the optic nerve, ocular muscles, nerves, blood vessels, and lacrimal gland. These structures are surrounded by orbital fatty tissue. The orbital cavity is shaped like a funnel that opens anteriorly and inferiorly. The six ocular muscles originate at the apex of the funnel around the optic nerve and insert into the globe. The globe moves within the orbital cavity as in a joint socket.

Bony socket: This consists of seven bones (Fig. 15.1):

The bony rim of the orbital cavity forms a strong ring. Its other bony surfaces include very thin plates of bone (see adjacent structures).

Adjacent structures: The close proximity of the orbital cavity to adjacent structures is clinically significant. The maxillary sinus inferior to the orbital cavity is separated from it by a plate of bone 0.5 mm thick. The ethmoidal air cells located medial and posterior to the orbital cavity are separated from it by a plate of bone only 0.3 mm thick or by periosteum alone. The following other structures are also located immediately adjacent to the orbital cavity.

❖ Sphenoidal sinus.

❖ Middle cranial fossa.

❖ Region of the optic chiasm.

❖ Pituitary gland.

❖ Cavernous sinus.

Superior adjacent structures include the anterior cranial fossa and the frontal sinus. Table 15.1 lists the various bony openings into the orbital cavity and

404 15 Orbital Cavity — Anterior aspect of the left orbital cavity. —

Supraorbital notch-, OPtic canal

Anterior and posterior ethmoidal foramina

Orbital plate of the ethmoid

Frontomaxillary suture

Nasal

Anterior lacrimal crest

Posterior lacrimal crest

Infraorbital suture

404 15 Orbital Cavity — Anterior aspect of the left orbital cavity. —

Supraorbital notch-, OPtic canal

Anterior and posterior ethmoidal foramina

Orbital plate of the ethmoid

Frontomaxillary suture

Nasal

Infraorbital suture

Greater wing of the sphenoid

— Orbital plate of the zygomatic

Zygomatic

Inferior orbital fissure

Zygomaticofacial foramen

Zygomaticomaxillary suture

Infraorbital sulcus

Fig. 15.1 Diagram of the seven orbital bones and the openings into the orbital cavity.

Zygomaticomaxillary suture

Superior orbital fissue

Greater wing of the sphenoid

— Orbital plate of the zygomatic

Zygomatic

Inferior orbital fissure

Infraorbital sulcus

Zygomaticofacial foramen

Fig. 15.1 Diagram of the seven orbital bones and the openings into the orbital cavity.

the anatomic structures that pass through them. Because of this anatomic situation, the orbital cavity is frequently affected by disorders of adjacent structures. For example, inflammations of the paranasal sinuses can result in orbital cellulitis.

The walls of the orbital cavity are lined with periosteum, which is also referred to as periorbita. Its anterior boundary is formed by the orbital septa extending from the orbital rim to the superior and inferior tarsal plates, the lateral and medial palpebral ligaments, and the eyelids.

Arterial supply: The orbital cavity is supplied by the ophthalmic artery, a branch of the internal carotid artery. The ophthalmic artery communicates with the angular artery, a branch of the external carotid artery, via the supraorbital and supratrochlear arteries.

Stenosis of the internal carotid artery can result in reversed blood flow through the supraorbital and supratrochlear arteries. This can be demonstrated by Doppler ultrasound studies.

15.2 Examination Methods 405 Table 15.1 Openings into the orbital cavity and the structures that pass through them

Orbital openings Structures

❖ Ophthalmic artery

Superior orbital fissure

❖ Oculomotor nerve

❖ Trochlear nerve

❖ Abducent nerve

❖ Ophthalmic nerve:

- Lacrimal nerve

- Frontal nerve

- Nasociliary nerve

❖ Superior ophthalmic veins

Inferior orbital fissure

❖ Infraorbital nerve

❖ Zygomatic nerve

❖ Inferior ophthalmic vein

Infraorbital canal

❖ Infraorbital nerve

Venous drainage from the orbital cavity: The orbital cavity drains through the inferior ophthalmic vein into the pterygoid plexus, through the superior ophthalmic vein into the cavernous sinus, and through the angular vein into the facial veins.

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