The orbit is composed of seven bones: the zygoma, sphenoid, frontal, ethmoid, maxilla, lacrimal and palatine bones. Residing inside the bony orbit are two fissures, the superior and inferior orbital fissures.
Through the superior orbital fissure (SOF) pass the occulomotor (CN III), trochlear (CN IV), ophthalmic division of the trigeminal (CN V1) and the abducens (CN VI) nerves. It is located in a superolateral position relative to the optic foramen and divides the greater and lesser wings of the sphenoid bone. The inferior orbital fissure (IOF) resides between the lesser wing of the sphenoid and the maxilla and is oriented in an inferolateral direction.
Traversing the IOF are the second division of the trigeminal nerve (CN V2), branches off of the sphenopalatine ganglion and the inferior ophthalmic vein. The optic foramen which transmits the optic nerve is located 42-45 mm posterior to the infraorbital rim.
A tendinous ring, the annulus of Zinn, is located just anterior to the foramen and serves as the common origin of the four rectus muscles, superior oblique and the levator muscles. Whitnall's tubercle is the bony attachment of the lateral canthal tendon, and a check ligament for the lateral rectus, the suspensory ligament of Lockwood, and the lateral extension of the levator aponeurosis.
The tubercle is located 1 cm inferior and 3 mm posterior to the frontozygomatic suture. Lockwood's ligament is actually a hammock-like system that suspends the globe. It has contributions from muscular septae, Tenon's capsule and the lower eyelid retractors. It arises from the fibrous attachments of the inferior rectus posteriorly and continues as the capsulopalpebral fascia anteriorly. It attaches to the lacrimal crest medially and Whitnall's tubercle laterally.
A thorough understanding of the four orbital walls (superior, inferior, medial and lateral) is important for describing fracture patterns and predicting associated injuries. The superior wall, or orbital roof, is moderately resistant to trauma and infrequently fractured, comprising only 1-5% of orbital fractures.
The lateral wall is also moderately strong and made up of the zygomatic and frontal bones. This is the least common area for an isolated orbital fracture, although it is commonly fractured through the frontozygomatic suture line in ZMC fractures.
The medial wall is partially formed from the thin lamina papyracea of the ethmoid and lacrimal bones. As such it is more prone to fracture; however, these medial wall fractures are often observed nonoperatively. Problematic sequelae occur when there is a medial wall fracture in conjunction with injury to other nearby structures.
The lacrimal sac is located in the lacrimal crest and may be lacerated or obstructed by bony fragments requiring immediate or late dacrocystorhinostomy. The medial canthal tendon which inserts on the anterior and posterior lacrimal crests may be torn or avulsed creating traumatic telecanthus. The medial rectus muscle can become entrapped or attenuated resulting in late medial gaze abnormalities.
The inferior orbital rim, composed of the maxillary and zygomatic bones, is the structure most often involved in orbital fractures. A direct force applied to the inferior orbital rim is transmitted to the orbital floor.
Since the orbital floor is a relatively weak structure, it is prone to fracture, resulting in a blow-out fracture of the floor in which the orbital contents (mostly fat) herniated downwards. As the medial floor is thinnest portion, floor fractures typically occur medial to the infraorbital groove.
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