Orbital surgery can be indicated for a variety of traumatic or pathologic conditions and for esthetic concerns in the contemporary practice of oral and maxillofacial surgery (OMS). OMS surgeons must be familiar with the complex anatomy of the orbital region, both for proper diagnosis and for subsequent treatment of these conditions. This chapter reviews the pertinent orbital anatomy for surgeons who perform orbital surgery. Unfamiliarity with orbital anatomy can have devastating consequences for the patient and the surgeon. Blindness, the most feared iatrogenic complication after internal orbital reconstruction, is fortunately rare. Frequently, deep orbital exploration is required to properly treat the patient’s condition. Deep orbital exploration is safe provided that the anatomy and physiology of the orbit are considered before treatment. Clinically, this includes good visualization with proper lighting, gentle retraction of the globe/muscle cone, and careful subperiosteal dissection.
The Hard Tissue Anatomy
The bony orbit is not a straight, four-walled pyramid as depicted in many textbooks ( Figure 2-1, A ). This simplistic view of the anatomy leads directly to inadequate orbital fracture repair and secondary deformities. Three of the four orbital walls have both concave and/or convex portions that should be reproduced when reconstruction is performed. Only the thick lateral wall of the orbit should be considered straight from anterior to posterior. More conical in shape, the orbit consists of a proximal apex and a distal base, both of which have thicker bone than any of the walls. The base of the cone is rotated laterally such that the visual axis diverges from the orbital axis by 23 degrees.
The orbital entrance measures approximately 4 cm wide by 3.5 cm high. The orbital rim is an important landmark for the structural dimension of the orbit: the orbit’s maximum area is approximately 1 cm behind the rim; the apex is approximately 44 to 55 mm from the medial rim. The lateral walls are approximately 90 degrees to each other; the medial walls are roughly parallel to each other and have a slight convexity proximal to distal. The total volume of the orbit is approximately 30 mL, with the globe comprising about 7 mL of the total ( Figure 2-1, B ). Seven bones form the internal orbit: the frontal, ethmoid, zygomatic, maxillary, lacrimal, palatine, and sphenoid bones ( Figure 2-1, A ). Some of the bony walls are thick and resist fracture, whereas others are quite thin and fracture with regularity. Thin walls also allow easy transmission of infection and invasion by tumors from the paranasal sinuses.
Three bones form the floor of the orbit: the orbital process of the maxilla, the zygomatic bone, and the orbital plate of the palatine bone. The orbital plate of the palatine bone is a crucial consideration in the treatment of deep fractures of the orbital floor. In most low-energy injuries of the orbital floor, this bone does not fracture and can be used to provide a sound ledge for orbital plates or mesh. It should be identified as a small, triangular-shaped bone posterior to the orbital plate of the maxilla and medial to the infraorbital/maxillary nerve. Immediately behind the inferior rim, a concavity in the floor of about 15 mm extends past the inferior orbital fissure. This concavity becomes convex proximally as it approaches the orbital apex. Knowledge of this post-bulbar convexity aids in the reconstruction of the normal floor anatomy and helps to prevent late secondary enophthalmos. Some surgeons routinely obliterate the inferior orbital fissure during fracture repair to prevent extraconal fat from herniating into the infratemporal fossa and thus contributing to secondary enophthalmos ( Figure 2-2 ).
The floor is also the roof of the maxillary sinus. The maxillary division of the trigeminal nerve (cranial nerve [CN] V-2) leaves the foramen rotundum in the middle cranial fossa and enters the orbit in a confluence between the superior and inferior orbital fissures (see Figure 2-1, A ). It continues anteriorly to enter the infraorbital canal in the orbital plate of the maxilla. The canal contains the infraorbital nerve, infraorbital branch of the maxillary artery, infraorbital veins, and postganglionic autonomic fibers from the pterygopalatine ganglion.
The orbital floor and the medial wall are the most commonly fractured areas in orbital trauma. This high incidence can be attributed to the thinness of the orbital floor, which may measure only 0.5 mm thick. Inadequately treated posterior floor fractures can play a significant role in the etiology of post-traumatic enophthalmos.
The orbital roof is composed of three bones. The frontal bone forms the major portion, and a small anterolateral portion of the zygoma and part of the lesser wing of the sphenoid bone posteriorly constitute the remainder. The orbital roof has a concavity immediately behind the superior rim. Once past the concavity, the roof is mainly straight back to the orbital apex. Two important landmarks are found within the anterior roof: the lacrimal fossa anterolaterally and the trochlear fossa medially. Other important landmarks, situated at the junction with the lateral wall, are the superior orbital fissure and the frontosphenoidal suture. The superior rim contains the supraorbital notch/foramen, found at the junction of the medial one third and the lateral two thirds. Injury through trauma or iatrogenic damage to the supraorbital neurovascular bundle may produce altered sensation of the forehead and brow. The supratrochlear vessels are located medial to the supraorbital bundle. The supratrochlear artery is a branch of the ophthalmic artery, and the supratrochlear nerve is a terminal branch of the frontal nerve (CN V-1).
The lateral wall of the orbit is fairly straight from rim to apex and owes its strength to the two bones that form it: the greater wing of the sphenoid (GWS) and the zygomatic bone. Posteriorly, the lateral wall begins at the superior orbital fissure and is composed primarily of the straight, thick greater wing of the sphenoid. Anteriorly, the GWS articulates with the orbital surface of the zygoma at the zygomaticosphenoidal suture. The zygoma contains two foramina and accompanying neurovascular bundles. The zygomaticofacial nerve is a purely sensory supply to the skin over the body of the zygoma. The zygomaticotemporal nerve carries sensory axons to the temporal fossa and postganglionic parasympathetic fibers of the pterygopalatine ganglion to the lacrimal gland. This is by way of an anastomotic branch that connects V-2 with the lacrimal nerve (a branch of CN V-1). Whitnall’s tubercle is a small, bony promontory inside the lateral orbital rim of the zygoma that serves as an attachment for several soft tissue structures.
The lateral walls should form a 45-degree angle at the orbital apex with the medial orbital walls and a 90-degree angle with each other in the axial plane (see Figure 2-1, C ). The recurrent meningeal branch of the ophthalmic artery exits the orbit near the posterior aspect of the frontosphenoidal suture at the meningeal foramen, which lies between the roof and lateral wall within the GWS. This artery then anastomoses with the middle meningeal artery, a branch off the first part of the maxillary artery, to supply the dura inside the skull.
The medial wall is bounded from anterior to posterior by four bones: the maxillary, lacrimal, and ethmoid bones and the lesser wing of the sphenoid. The lamina papyracea of the ethmoid bone is the thinnest bone in the medial wall, often fracturing in blow-out fractures. The medial wall contains two foramina: the anterior ethmoidal foramen and the posterior ethmoidal foramen. The posterior ethmoidal neurovascular bundle is an important landmark for deep orbital dissection. As the surgeon dissects posteriorly, the posterior ethmoidal foramen is located 5 to 10 mm anterior to the optic canal.
These foramina are located at the junction between the medial wall and the orbital roof at the frontoethmoidal suture, which denotes the level of the cribriform plate. Both ethmoidal neurovascular bundles leave the orbit at the level of this suture to enter the roof of the nasal cavity.
The lacrimal sac fossa lies anteriorly between the anterior and posterior lacrimal crests (see Figure 2-1, D ). The anterior lacrimal crest is within the frontal process of the maxilla, blending with the inferior orbital rim. The posterior lacrimal crest lies within the lacrimal bone. The medial wall displays a slight convexity in the axial plane from front to back, and this should be reproduced in a medial wall plate or mesh during fracture treatment.
The orbital apex is a complex anatomic region that the surgeon must completely understand. All important nerves and blood vessels traverse this area. A complete review of the sphenoid bone anatomy is required to understand the orbital apex. The two most important features of the orbital apex are the superior orbital fissure and the optic canal (see Figure 2-1, A ). The optic canal lies between the roof and the end of the medial wall at the orbital apex in the vertical dimension. The optic canal is entirely within the lesser wing of the sphenoid and is oriented laterally in the axial plane. The superior orbital fissure (SOF) is between the lesser and greater wings of the sphenoid and houses several important cranial nerves: the oculomotor nerve (CN III), the trochlear nerve (CN IV), the ophthalmic division of the trigeminal nerve (CN V-1), and the abducens nerve (CN VI). Cranial nerves III and VI enter the orbit inside the annulus of Zinn to run within the muscle cone ( Figure 2-3 ). The tendinous origin of the lateral rectus muscle divides the SOF into two compartments, one superior and one inferior. The area of the SOF encircled by the annulus of Zinn is called the oculomotor foramen. The ophthalmic nerve (CN V-1) enters the orbit outside the annulus/muscle cone in the superior compartment to proceed forward as the frontal and lacrimal nerves. The trochlear nerve (CN IV) lies in the superior compartment, just outside the annulus of Zinn, in close proximity to the superior ophthalmic veins. *
* References .
The ophthalmic artery is the first branch off the internal carotid artery (ICA) after it enters the skull ( Figure 2-4 ). The artery lies below the optic nerve and runs forward in the dural-arachnoid sheath, eventually piercing and then emerging outside the sheath as it exits the optic canal lateral and inferior to the optic nerve. The central retinal artery (CRA) branches off the ophthalmic artery near the lateral rectus muscle origin and reenters the optic nerve sheath on the way to the retina. Because the CRA is an “end artery,” the retina has no collateral arterial supply. The ophthalmic artery supplies the muscle cone, globe, and all superior orbital structures. The external carotid artery (ECA) contributes to the lower orbit through the maxillary, infraorbital, zygomaticofacial, and zygomaticotemporal arteries. Thus, the orbit has a dual blood supply from the ICA superiorly and the ECA inferiorly.
The superior ophthalmic veins pass through the SOF outside the muscle cone in the superior compartment and drain to the cavernous sinus. The inferior ophthalmic veins pass through the inferior orbital fissure to communicate with the pterygoid plexus and anastomose with the superior ophthalmic veins posteriorly to drain to the cavernous sinus. Anteriorly, the superior and inferior ophthalmic veins can communicate with the angular vein on the face, creating the so-called danger triangle. The supposed lack of valves within the angular and ophthalmic veins would allow easy passage of bacterial emboli to the cavernous sinus. In 2010, Zhang and Stringer reported finding valves in cadavers using stereomicroscopy. The angular vein was found to drain to the facial vein or to the superior ophthalmic vein in this study.
The Soft Tissue Anatomy
The eyelids represent composite structures that cover the anterior globe and form the palpebral fissure ( Figure 2-5, A ). The three layers of the lids are called lamellae (a single layer is a lamella ) . There are three lamellae of the upper lids and three lamellae of the lower lids. In the upper lid, the anterior lamella consists of the skin and orbicularis oculi muscle; the middle lamella is composed of the orbital septum and levator aponeurosis; and the posterior lamella consists of the tarsal plate, Müller’s muscle, and palpebral conjunctivae. In the lower lid, the anterior lamella is composed of the skin and orbicularis oculi muscle; the middle layer consists of the orbital septum; and the posterior lamella consists of the palpebral conjunctiva, capsulopalpebral fascia (i.e., lower lid retractor), and tarsal plate. The medial and lateral locations where the two eyelids meet are referred to as the canthi (singular, canthus ).