Ethmoidal Sinus

The ethmoidal sinus is by far the most complex paranasal sinus and has the greatest variability of all the sinuses. This sinus, which also is called the ethmoidal labyrinth, forms a pyramid on either side of the nose with the base located posteriorly. The sinus measures approximately 4 to 5 cm in the anteroposterior direction, 2.5 cm in height, 0.5 cm in width anteriorly, and 1.5 cm in width posteriorly (10). The lateral wall of the ethmoidal sinus is the lamina papyracea, which also serves as the "paper thin” medial wall of the orbit. The medial wall of the sinus functions as the lateral wall of the nose. In the midline of the nasal cavity lies the vertical plate of the ethmoid bone. This plate has a superior portion in the anterior cranial fossa called the crista galli and an inferior portion in the nasal cavity called the perpendicular plate of the ethmoid bone (Fig. 28.2). The anterior cranial fossa is separated from the ethmoidal cells superiorly by the horizontal plate of the ethmoid bone. The horizontal plate is composed of a thin medial portion called the cribriform plate and a thicker, more lateral portion called the fovea ethmoidalis, which forms the roof of the ethmoid bone (Fig. 28.2).

FIGURE 28.2. Coronal view through maxillary sinuses.

The horizontal plate of the ethmoid bone varies depending on the position of the cribriform plate with respect to the fovea ethmoidalis. The length of the lateral lamella, the extremely thin lateral part of the cribriform plate (Fig. 28.2), differentiates into three types. In type 1, the cribriform plate is located 1 to 3 mm below the fovea ethmoidalis, making the lateral lamella short or nonexistent. In type 2, the distance is 4 to 7 mm. In type 3, it is 8 to 16 mm. Thus the lateral lamella is lengthened with each configuration. The anterior vertical attachment of the middle turbinate to the horizontal plate is depicted in Fig. 28.2. It separates the horizontal part of the cribriform plate from the lateral lamella and the fovea ethmoidalis. The attachment of the middle turbinate is vital to remember during intranasal dissection (11). Dissecting lateral to the turbinate prevents entrance into the horizontal part of the cribriform plate, and the presence of a Keros type 2 or 3 ethmoid roof should prompt extreme caution to avoid entering the anterior cranial fossa through the bone of the lateral lamella.

The horizontal plate of the ethmoid bone has another weak area called the dome of the ethmoid bone. This is the area where the anterior ethmoidal artery traverses the ethmoid roof. It can be found by means of following the second ethmoturbinal remnant (anterior face of the bulla ethmoidalis) to the ethmoid roof and examining a few millimeters posterior to that point (11). Dissection in this area should be scrupulous to avoid entering the anterior cranial fossa and to prevent rupture of the artery, which can cause orbital hematoma.

The ethmoidal cells are divided into an anterior group that drains into the ethmoid infundibulum of the middle meatus (Fig. 28.3) and a posterior group that drains into the superior meatus (5). The sinus usually consists of 4 to 17 cells per side, with an average of 9 cells (12). The configuration of the cells is not uniform from person to person, and several classification schemes have been proposed. The Ritter classification, which is most commonly used, is based on both the origin and drainage of the ethmoidal cells (12). The most anterior cells, which originate from the anterosuperior growth of ethmoidal cells into the frontal bone, are the frontal recess cells (0 to 4 cells). The cells form the frontal sinus, supraorbital air cells, and other various anterior and superior ethmoidal cells.

FIGURE 28.3. Sagittal view through ethmoidal sinuses with middle turbinate removed.

The next most anterior ethmoidal cells are the infundibular cells (1 to 7 cells). These cells are variable in location, the most consistent being the agger nasi cells, which extend outside the ethmoid capsule. The agger nasi (Fig. 28.3) is a mound of tissue on the lateral nasal wall that represents a superior remnant of the first ethmoturbinal, which is just superior, lateral, and anterior to the attachment of the middle turbinate. Pneumatization of this area produces agger nasi cells that drain into the most superior end of the ethmoid infundibulum, which is otherwise known as the frontal recess.

The bullar cells (1 to 6 cells) constitute the next group of anterior ethmoidal cells. These cells, which are consistent in location, exist in the middle meatus and form the bulla ethmoidalis, which is a partial sphere (Fig. 28.2). The anterosuperior wall of the bulla ethmoidalis faces the frontal recess, and if the wall does not reach the ethmoid roof, a suprabullar recess exists (Fig. 28.2). The bullar cells drain into this area and into the rest of the ethmoid infundibulum along its course. Therefore the ethmoid infundibulum represents a three-dimensional cleft running anterosuperiorly to posteroinferiorly, and the two-dimensional opening to this cleft is the hiatus semilunaris. The bulla ethmoidalis borders the ethmoid infundibulum posteriorly and superiorly; the lateral wall of the nose is lateral, and the uncinate process is anteromedial. The uncinate process (Fig. 28.3) is a thin semilunar piece of bone that is a remnant of the first ethmoturbinal. The superior edge usually is free (Fig.

28.2), but it can insert onto the lamina papyracea or the fovea ethmoidalis and cause variation in drainage patterns (2). The uncinate process can become pneumatized and obstruct drainage in the ethmoid infundibulum.

The structure of the middle turbinate must be clearly understood to comprehend the anatomic relations of the ethmoidal sinus. At the anterior end, the turbinate attaches superiorly to the horizontal plate of the ethmoid bone (Fig. 28.2). This anterior segment becomes pneumatized in about 12% of the population to form a concha bullosa, which can block drainage from the ethmoid infundibulum. Behind the bullar cells, the insertion turns laterally to insert on the lamina papyracea (Fig. 28.4). This attachment, which lies in the frontal plane, represents the third ethmoturbinal (also called the third basal lamella) and is the dividing line between the anterior and posterior ethmoid cells. Posterior to this lateral attachment, the insertion of the middle turbinate turns inferiorly to become horizontal. A sinus lateralis or retrobullar recess exists if the posterior wall of the bulla ethmoidalis does not contact the third basal lamella (Fig. 28.3, Fig. 28.4) (2).

FIGURE 28.4. Axial view shows the three-dimensional structure of the middle turbinate.

The posterior ethmoid cells (1 to 7 cells) are posterior and superior to the third basal lamella. Aside from pneumatizing the posterior ethmoid capsule, these cells can invade the palatine, maxillary, middle turbinate, and sphenoid bones. An Onodi cell is a posterior ethmoid cell that can pneumatize an area of the sphenoid bone superior and lateral to the sphenoidal sinus in 9% to 12% of the population (12). This entity is clinically important because the optic nerve and carotid artery can be exposed and injured during dissection in this area. The posterior ethmoidal cells drain into the superior meatus, the supreme meatus if one exists, and the sphenoethmoidal recess, which is the area between the superior turbinate, the roof of the nose, and the nasal septum.

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