A thorough understanding of sinonasal anatomy is crucial to performing safe and successful sinus surgery. It cannot be understated; however, that knowledge of the regional anatomy is critical to avoiding the complications discussed in this chapter. Orbital complications resulting from sinus surgery are fortunately rare, and this stems from understanding the relationship of the orbit and its internal structures with the sinuses themselves. The orbit comprises seven bones: maxilla, zygoma, frontal, lacrimal, sphenoid, palatine, and ethmoid. The seven bones together form a conical-shaped structure that is approximately 4 cm wide by 5 cm long. The contents of the orbit are divided into the anterior and posterior compartments, which are divided anatomically by the orbital septum and the globe. The orbital septum is the reflection of the periorbita into the tarsal plates. Separating the anterior and posterior eye chambers, the septum is tough and holds the orbital contents in place. To its detriment, the orbital septum also holds orbital effusions such as hemorrhage and infection within the orbit, preventing the fluid from passing directly into the preseptal space and instead allowing potentially dangerous increases in orbital pressure. The anterior compartment of the orbit contains the lids, orbital soft tissue, and the lacrimal apparatus. The posterior orbital compartment, or retrobulbar space, is subdivided into the intraconal and extraconal spaces, which are separated by a fascial sling that envelops the extraocular muscles.

The intraconal space contains the vital orbital structures, including the optic nerve, portions of the ophthalmic artery, branches of the ophthalmic artery (central retinal artery, long and short ciliary arteries, lacrimal artery, portions of the anterior ethmoidal artery, and the posterior ethmoidal artery), the ophthalmic veins, and orbital fat. The blood supply to the retina, chiefly the choroid choriocapillaris (outer half) and the central retinal artery and branches, lies within the intraconal space as well. The blood supply is protected by various compensatory mechanisms so that retinal blood flow is maintained even if the entire system is under stress.

The extraconal space can be subdivided into superior, inferior, medial, and lateral compartments. The superior extraconal space contains the levator muscle, supratrochlear and supraorbital nerves, the trochlear nerve, and the supraorbital artery. The lateral extraconal space is not as relevant in sinus surgery; however, it contains the lacrimal gland, the lacrimal neurovascular structures, and fat. The inferior extraconal
space contains fat and the infraorbital nerve and vessels. The medial extraconal space is the most relevant space in sinus surgery and contains a plethora of vascular structures: distal portions of the posterior ethmoidal artery, the distal portion of the ophthalmic artery, and the anterior ethmoidal artery as well as fat. Of extreme importance is the relationship of the ophthalmic artery and the anterior ethmoidal artery. The ophthalmic artery courses anteriorly in the extraconal space just inferior to the superior oblique muscle before giving rise to the anterior ethmoidal artery near the anterior ethmoidal foramen along the medial orbital wall. Radiographically, this is best identified in the coronal plane along the convergence of the medial rectus muscle and the superior oblique muscle (Fig. 44.1). In approximately 10% of cases, this neurovascular bundle can be suspended from the anterior ethmoid skull base in a bony mesentery, often associated with a supraorbital ethmoid air cell (1). Considering the limen nasi as a fixed landmark, the artery is located approximately 55 degrees from the nasal floor and at 50 mm.

Superficial to the orbital fat in the extraconal space is the orbital periosteum, or periorbita. The periorbita is tough and fibrous but can be elevated easily except at the suture lines, where it passes through to fuse with the periosteum on the opposite side. Medial to the periorbita is the lamina papyracea of the ethmoid bone, which is paper thin and is easily susceptible to injury during sinus surgery. The lamina papyracea is the lateral-most extent of the ethmoid sinuses. Finding the lamina papyracea is essential to preventing orbital complications. The orbit and the paranasal sinuses are intimately related on three sides.

Posteriorly, the orbit converges and permits passage of neurovascular structures through the superior orbital fissure, the inferior orbital fissure, and the optic canal. Through the optic canal traverses the optic nerve and ophthalmic artery, as well as various vasa nervosa that supply the optic nerve itself. Posteriorly, this optic canal narrows and can form an indentation in the sphenoid sinus at the junction of the cavernous carotid artery known as the opticocarotid recess (Fig. 44.2), which anatomically relates to the anterior clinoid process of the sphenoid bone. This region is notable for dehiscence of the carotid artery and optic nerve, which can occur in 4% to 22% (2,3,4,5) and 4% to 8% (3,5) of cases, respectively.

The lamina is curved and gives way superiorly to the supraorbital ethmoidal cells and inferiorly to the maxillary sinus. It is important to define the lamina surgically when working in the ethmoid and maxillary sinuses to avoid entrance into the orbit via compromise of the lamina papyracea and periorbita and injuring the vasculature or musculature. It is appropriate to stay near the superior part of the inferior turbinate when making a middle meatal antrostomy and to not open the true ostium any more anteriorly than the anterior end of the middle turbinate. Specifically, the true ostium of the maxillary sinus is situated along the inferior half of the maxillary line in 94% of specimens and just inferior to the mid-point of the maxillary line in 60% of cases (Fig. 44.3). The os is obliquely oriented and less than 5 mm posterior to the lacrimal duct. Dissecting anterior to the true ostium only encounters the blind bony pouch of the infundibulum and does not provide any additional functional improvement in maxillary sinus mucociliary clearance. Moreover, dissection more anteriorly than this places the lacrimal duct at risk of injury. After the antrostomy is made, the inferolateral aspect of the lamina papyracea can be identified superior to the ostia and if the opening is large enough, the floor of the orbit can be identified as well. Proceeding through the maxillary os in a lateral or superolateral direction will likely result in injury to the orbit; therefore, once the os is identified, entry in the maxillary sinus is performed in an inferolateral direction to safely avoid the lamina papyracea.

The structures of the orbit viewed through an endoscope appear different on the right and left sides (6). The actual right nasal and meatal structures lie visually straight back. On the left, the ethmoid sinuses are actually more medial in location, especially anteriorly and superiorly. Because of altered perception, a right-handed surgeon operating on the left side who takes the same straight-back approach as on the right, especially during an endoscopic procedure, contacts lamina papyracea and enters the orbit, particularly in the superior and lateral orbit area. It is important to stay in the medial aspect against the middle turbinate. See Table 44.1 for a tabulation of important orbital anatomy.

The surgeon should be aware that the skull base slopes downward beginning at the frontal recess and terminating at the planum sphenoidale (Fig. 44.4). It is a mistake to assume that this slope is both smooth and two-dimensional. Anteriorly, the skull base is variable in structure as described by Keros (7), as the ethmoid roof (a.k.a. fovea ethmoidalis) is generally higher laterally than it is medially where the cribriform plate is identified. The anterior ethmoidal artery can pierce the cribriform plate and even be intracranial as it travels medially toward the septum. The bone at this point is 10 times thinner than in other areas of the cribriform plate and thus can be easily violated. Surgeons need to beware of a low skull base or cribriform plate, as found at computed tomography (CT).
Two excellent articles relating to anatomic variations on CT scanning important to the sinus surgeon describe and enhance this anatomic discussion and are highly recommended (8,9). As the skull base transitions posteriorly to meet the planum sphenoidale, the ethmoid roof can be identified at the same level as the cribriform plate.

The sphenoid sinus is the key to identifying the skull base. The sphenoid sinus is bounded anteriorly and superiorly by the sphenoid crest, which articulates with the perpendicular plate of the ethmoid bone. Inferiorly and anteriorly, the sphenoid bone converges into the rostrum, which articulates with the vomer of the septum. Lateral to the sphenoid crest are the two sphenoid ostia, both of which measure approximately 2mm in dimension. Each ostium is located approximately 15-20mm above the choanal arch. The ostia are the anatomic keys to the skull base. These ostia can be obstructed by posterior ethmoidal disease making them difficult to directly visualize. However, a beaded probe or image-guidance probe placed onto the sphenoid ostium safely identifies the appropriate area for opening into the sphenoid sinus, which is anatomically inferior and medial. The sphenoid ostium can be approached in a position medial to the middle turbinate (transethmoid) or approached laterally in a plane between the superior turbinate and the septum, just superior to the choanal bridge (transnasal). The superior turbinate is a well-established landmark for the ostium of the sphenoid sinus since it is typically medial to the lower one-third to one-half of the superior turbinate. Viewed in an axial plane, the anterior wall of the sphenoid sinus approximates the posterior wall of the maxillary sinus. The only intervening anatomic structure is the pterygopalatine fossa, which is thin and permits the sphenopalatine artery and it branches to enter the sinonasal cavity. The anterior aspect of the sphenoid bone usually is extremely thin, but thickens both medially near the rostrum and sphenoid crest and laterally near the orbital apex. If resistance is felt, the bone should not be penetrated blindly, rather entered cautiously with a drill or other instrument after the anatomy is confirmed radiographically. The posterior ethmoidal cells can be opened in a medial direction through the posterior middle turbinate to allow visualization of the sphenoid ostium, which safely identifies the lateral aspect of the sphenoid and posterior ethmoidal cells that can be penetrated.

Anterior to the planum sphenoidale, the bone transitions into the cribriform plate, the lateral cribriform lamella, and the fovea ethmoidalis (ethmoid roof). This anatomy is best considered in the coronal plane despite its
three-dimensional variance. The cribriform plate represents the lowest midline structure in the medial aspect of the nasal vault. The lateral mass of the ethmoid is suspended from the fovea ethmoidalis or ethmoid roof. Intervening between the ethmoid roof and cribriform plate is the lateral cribriform lamella, or simply lateral lamella. The length of the lateral lamella (and hence the depth of the cribriform plate) was noted by Keros (7) in several anatomic dissections. Keros classified the anterior skull base depth based upon the length of the lateral lamella: 1 to 3 mm (type I), 4 to 7 mm (type II), and 8 to 16 mm (type III). The skull base may also be asymmetric in design (Fig. 44.5). Some (10) have shown that type II anatomy is encountered most frequently and that patients with a lower skull base depth (i.e., type I) have a lower incidence of skull base injury.

Suspended from the articulation of the lateral lamella and the cribriform plate is the sagittal attachment of the middle turbinate; therefore, it is clear that the middle turbinate is an important landmark. In addition to directing the surgeon to the transition between the cribriform and the lateral lamella, the anterior most portion of the middle turbinate marks the anterior limit of maxillary dissection, marks the medial-superior extent of the lacrimal sac, and is important in identifying the frontal recess. The sagittal portion of the middle turbinate transitions posteriorly as the (third) basal lamella and is visualized in coronal and horizontal planes. The basal lamella identifies the transition between the anterior and posterior ethmoidal sinuses, and the lower half of the middle turbinate and its insertion into the choana help to identify the entrance into the sphenoid sinus. These characteristics, not to mention its host of olfactory neurons, make the middle turbinate a structure worth preserving. However, turbinates that are mechanically obstructive, such as concha bullosa or those with significant polypoid degeneration, may need partial or total removal (Table 44.1).

Measuring the anatomic structures is important to avoiding complications. The distance from the nasal spine or nasal opening to the frontal recess is 6 cm, to the base of skull is 7 cm, and to the anterior wall of the sphenoid is 7 cm in adults of normal size. The nasopharyngeal wall approximates the back wall of the sphenoid sinus, usually to within 1 cm. The choanal bridge (just above the choana and below anterior wall sphenoid) is 7 cm. The basal lamella of the middle turbinate is 6 cm. In children, these dimensions are variable, and the surgeon needs to exercise caution. However, one can always find the distance to the sphenoid bone and the base of the skull by measuring the distance to the choanal bridge just above the choana (Fig. 44.6).

The relationship between the maxillary and ethmoid sinuses should be examined in the coronal and sagittal planes on CT imaging prior to the start of surgery. In particular, the ratio of the posterior ethmoid to maxillary sinus height should be assessed. In patients with a vertical height ratio of 1:1 (maxillary sinus height to posterior ethmoid height) are less likely to incur skull base injury. However, in patients with a 2:1 or greater than 2:1 maxillary sinus height to posterior ethmoid height (Fig. 44.7), the posterior ethmoid skull base is encountered rather quickly due to the sloping nature of the skull base in this region. Inadvertent injury to the skull base may occur once
proceeding through the basal lamella if the patient has a 2:1 or greater than 2:1 ratio. Therefore, the surgeon must consider this “angle of attack” when penetrating the basal lamella and entering the posterior ethmoid cavity. Entering the posterior ethmoids via the medial-inferior aspect of the coronal face of the basal lamella, along the roof of the maxillary sinus or floor of the ethmoid bulla, will provide a much safer avenue toward the sphenoid sinus and away from the skull base (Fig. 44.8).