3 Corridor to the Anterior Skull Base and Orbit

10.1055/b-0039-172565

3 Corridor to the Anterior Skull Base and Orbit

Davide Mattavelli, Giacomo Bertazzoni, Alberto Schreiber, Marco Ravanelli, Luca Pianta

The corridor toward midline anterior skull base and orbit is mostly formed by the ethmoid and frontal sinus. In addition, the sphenoid sinus serves as a natural corridor toward the most posterior portion of the anterior skull base (i.e., planum sphenoidale and tuberculum sellae), but this pathway will be discussed in Chapter 4. The dissection of the ethmoid complex is usually divided into three parts: dissection of the frontal recess, removal of the bullar complex, and dissection of the posterior ethmoid. As in functional endoscopic sinus surgery, these different types of ethmoidectomy can be combined to expose the adjacent skull base based on the specific needs.

The frontal recess consists of the space between the anterior bullar wall posteriorly, frontal process of the maxillary bone anteriorly, middle turbinate medially, and medial orbital wall laterally (mostly by the pars orbitalis ossis frontalis; see below). ¹ This space is compartmentalized by insertions of the uncinate process. The air chambers formed by these insertions, which are overall referred to as the agger complex, ² are intimately adjacent to the drainage pathway of the frontal sinus. The anatomy of the frontal recess is exceedingly variable, including a number of variants that were thoroughly studied and described during the last decades. Since the first attempt of categorizing these air spaces by Kuhn in 1996, ³ it took 20 years to reach a unanimous classification, namely the International Frontal Sinus Anatomy Classification. ⁴ The different air spaces that can be found result from the variable anatomy of the uncinate process. The agger nasi cell is the most anterior air space of the anterior ethmoid; more precisely, it should be defined as the agger nasi cell only when the air space reaches the agger nasi, which is a lateral-to-medial prominence of the frontal process of the maxillary bone where the head of the middle turbinate is inserted. Otherwise, this cell is called the lacrimal cell when it does not extend anteriorly and remains confined nearby the lacrimal fossa. In the same area, a funnel-shaped space with a dead end, called the terminal recess, can be identified instead of agger nasi or lacrimal cell. The supra-agger cell is an air space lying above the agger nasi cell, lacrimal cell, and/or terminal recess. When extended cranially so far that it exceeds the axial plane passing through the floor of the frontal sinus, this cell is defined as the supra-agger frontal cell. An air space extending cranially and pneumatizing the interfrontal sinus septum is called the frontal septal cell. Of note, all these air spaces are formed according to the variable and multiple insertions of the uncinate process, which has been consequently described as a palmlike structure. ⁵ While getting closer to the frontal ostium (the narrowest area between the frontal recess and frontal sinus), the uncinate process inserts posteriorly onto the anterior bullar wall forming the suprainfundibular plate, which in turn can be used as a landmark to define the drainage pathway of the frontal sinus as medial or lateral to the uncinate process. ⁶ Supra-agger (frontal) and frontal septal cells are usually associated with a medial and lateral drainage pathway, respectively; these associations can be considered a hallmark of different developmental mechanisms of the frontal recess. ⁷ – ⁹ In addition to these variants, the middle turbinate can be variably pneumatized: when the pneumatization is limited to the laminar portion, the air space is called interlamellar Grunwald cell; when the air extends to the bulbous portion, the turbinate is defined as concha bullosa.

From a surgical perspective, different degrees of clearance of the frontal recess were described by Draf ¹⁰ : Draf type I frontal sinusotomy consists of a simple dissection of the frontal recess without enlargement of the frontal ostium. In Draf type II frontal sinusotomy, the frontal ostium is widened by simply removing frontal cells and/or mildly enlarging the frontal ostium with a punch (type IIa) or by entirely removing the medial portion of the floor of the targeted frontal sinus (type IIb), which is called frontal beak due to its peaked shape. Draf type III frontal sinusotomy consists in merging the frontal sinuses through an anterosuperi- or septal window, with bilateral removal of the frontal beak and interfrontal sinus septum.

**Fig. 3.1** Schematic architecture of the nasal cavity and paranasal sinuses.

**Fig. 3.2 Diagonal view of the transnasal corridor toward anterior skull base and orbit**. This drawing shows the position of the transnasal corridor toward anterior skull base and orbit with respect to other sinonasal and skull base structures.

The bullar complex is a group of air spaces enclosed between the anterior bullar wall anteriorly, basal lamella of the middle turbinate posteriorly, middle turbinate medially, and medial orbital wall laterally. ² Frequently, air spaces within the bullar complex are disposed in a cranial–caudal fashion, with the cells that are adjacent to the skull base being called suprabullar cells. The latter can extend within the orbital roof or toward the frontal sinus, thus acquiring the name of supraorbital ethmoid cell or suprabullar frontal cell, respectively. When a suprabullar frontal cell is encountered, the frontal ostium is narrowed and pushed anteriorly. By virtue of its relationship with the medial orbital wall and anterior skull base, the safest area to begin the dissection of the bullar complex is at the inferior–medial corner. In some cases, surgical access to the frontal sinus requires the dissection of this area, especially when the bullar complex, by protruding anteriorly, hampers surgical maneuvers with curved instruments. Otherwise, an intact bulla frontal sinusotomy can be performed. ¹¹

Given the anatomical variability of the frontoethmoidal area, some information must be collected before dissecting the frontal sinus. In particular, keeping in mind the medial or lateral position of the frontal sinus drainage pathway is of utmost importance. Indeed, the dissection is performed by marsupializing the air spaces that surround the drainage pathway in a centrifugal fashion, proceeding from the middle nasal meatus to the frontal sinus. The total number of air spaces to be opened and the presence of cells extending toward the frontal sinus are also important. The agger-bullar classification is a valuable tool to systematically assess these anatomic features before and during surgery. ² This classification was aimed at providing a systematic, anatomic approach to perform endoscopic frontal sinusotomy.

The boundaries of the posterior ethmoid are the basal lamella of the middle turbinate anteriorly, basal lamella of the superior turbinate posteriorly, superior turbinate medially, and medial orbital wall laterally. The sphenoethmoidal recess is a narrow space lying between the basal lamella of the superior turbinate and the anterior sphenoidal wall. The widest anatomical variability in the posterior ethmoid is found at its posterior limit. In well-pneumatized posterior ethmoids, the superior turbinate ends posteriorly onto the anterior sphenoidal wall, just lateral to the sphenoidal ostium, and the sphenoethmoidal recess is absent. When the pneumatization of the posterior ethmoid exceeds the anteri- or wall of the sphenoid sinus in an anterior-to-posterior direction, it forms a sphenoethmoidal cell called the Onodi cell. The latter is formally defined only if the air space is in contact with the optic canal. In poorly pneumatized posterior ethmoids, the basal lamella of the superior turbinate turns laterally toward the medial orbital wall and the sphenoethmoidal recess takes shape. In these cases, an additional turbinate, called supreme turbinate, can be found within the sphenoethmoidal recess.

Particular attention should be taken when dissecting the lateral, superior, and medial boundaries of the ethmoid compartments.

The lateral boundary is the medial orbital wall, which is made up of bones with variable thickness. The anterior portion of the medial orbital wall is formed by the lacrimal bone inferiorly and pars orbitalis ossis frontalis superiorly, the former being very thin and the latter thick similar to the ethmoidal and orbital roofs. The posterior portion of the medial orbital wall is the lamina papyracea, which is usually thin as suggested by the name deriving from the Latin term papyrus.

The superior boundary of anterior and posterior ethmoids is formed by the ethmoidal roof (also defined fovea ethmoidalis), which is a thick lamina of the frontal bone. The ethmoidal arteries run from the ethmoidal foramina to the olfactory groove parallel to the ethmoidal roof. They are usually two, one per ethmoidal compartment; however, a middle ethmoidal artery can be found in 29 to 38% of cases. ¹² – ¹⁴ The cranial–caudal position with respect to the ethmoidal roof and anteroposterior position with respect to ethmoidal lamellae is variable: the anterior ethmoidal artery runs most frequently below the skull base and along the basal lamella of the middle turbinate; consequently, the anterior ethmoidal artery cannot be exposed when the bullar complex is left completely intact; the posterior and middle ethmoidal arteries lie mostly within the skull base and in a variable location between the basal lamella of the middle turbinate and anterior sphenoidal wall. ¹²

The medial boundary of the ethmoidal box is formed by the middle turbinate, superior turbinate, their common lamella (also called conchal plate), ¹⁵ and the vertical portion of the cribriform plate. The latter structure is so delicate that it is considered the locus minoris resistentiae of the entire skull base. ¹⁶ Moreover, its cranial–caudal length and grade of tilting are remarkably variable and considerably affect the risk of skull base injury during sinus surgery.