Ethmoid Bulla

Schematic construction of the anterior ethmoid sinuses starts with a sphere. This sphere is then sectioned by a plane, and the smaller portion of the sphere is discarded. The thin sectioning plane or layer represents the lamina papyracea. The partial sphere attached to the lamina papyracea represents the ethmoid bulla ( Fig. 1.6A ). As implied from our basic construction of the ethmoid complex, the ethmoid bulla is typically a rather large anterior ethmoid air cell that takes origin from the lamina papyracea along the medial wall of the orbit. Although ethmoid anatomy is often quite complicated, the ethmoid bulla is the most consistent and recognizable of the ethmoid cells. As the remainder of the ethmoid anatomy is added to the diagram, the relationship of the ethmoid bulla to surrounding structures will become evident: medial to the lamina papyracea, posterior to the uncinate process, anterior to the vertical basal lamella of the middle turbinate, and postero-inferior to the frontal recess. Stammberger describes minimal ethmoid bulla pneumatization or absence of ethmoid bulla occurring in ~8% of patients, referred to as a torus lateralis.4

Uncinate Process, Ethmoid Infundibulum, and Hiatus Semilunaris

A crescent-shaped line is added anterior to the ethmoid bulla and parallel to it. From this line, an outgrowth is then extended posteriorly and slightly away from the lamina papyracea and lateral nasal wall. This hook-shaped outgrowth thus forms a trough with a vertical plane that parallels the anterior surface of the sphere (or ethmoid bulla). The term for this hook-shaped outgrowth is the uncinate process. As seen in Fig. 1.6B , the uncinate process conforms to the anterior and inferior aspects of the ethmoid bulla, and has a free edge without bony attachments posteriorly. At its anterior superior aspect, the uncinate process attaches to the ethmoidal crest of the maxilla and the posterior portion of the lacrimal bone, near the agger nasi region.5 At its posterior inferior aspect, the uncinate process fuses to the ethmoidal process of the inferior turbinate bone.

The superior and posterior attachment of the uncinate process deserves special attention, as this attachment can have implications for the configuration of the frontal recess and frontal sinus outflow tract. Publications in the early 20th century, based on frontal recess anatomic dissections by Schaeffer6 and Kasper,16 indicate that the developmental origins of the frontal sinus and surrounding frontal recess structures are varied. Due to such variations, the frontal sinus outflow tract may drain directly into the superior aspect of the ethmoid infundibulum (less common), or into the middle meatus without a direct connection to the superior aspect of the ethmoid infundibulum (more common).6,16 The uncinate process most commonly inserts laterally onto the lamina papyracea, resulting in frontal sinus drainage that occurs medial to the uncinate process insertion and directly into the middle meatus between the superior aspect of the uncinate process and the middle turbinate attachment to the skull base.5,17 Less frequently, the uncinate process may insert onto the superior aspect of the middle turbinate or directly onto the skull base. In both of these cases, the frontal sinus drainage pathway will be lateral to the uncinate process and into the ethmoid infundibulum, which is described later. Finally, the posterior superior aspect of the uncinate process may have multiple attachments to the lamina papyracea, skull base, and middle turbinate.17 It is important to recognize that, due to the extreme variability of pneumatization of the frontal recess, these descriptions of the frontal sinus outflow tract are quite simplified and serve only as a framework for a basic understanding of this region.

The trough that is formed lateral to the uncinate process, within the anterior ethmoid complex, is called the ethmoid infundibulum. The ethmoid infundibulum is bounded medially by the lateral aspect of the uncinate process, laterally by the lamina papyracea, and posterosuperiorly by the ethmoid bulla.5,17 In addition, the frontal process of the maxilla and the lacrimal bone may also contribute to the anterior and superior boundaries of the ethmoid infundibulum.5,17 The importance of the superior attachment of the uncinate process and its relationship to the frontal sinus outflow tract have already been described. Of note, in the most common scenario of uncinate attachment to the lamina papyracea, the superior aspect of the infundibulum that is formed by the uncinate process attaching laterally to the lamina papyracea is a blind-ended configuration called the recessus terminalis.17

Access to the three-dimensional ethmoid infundibulum is through a two-dimensional semilunar-shaped gap or opening between the free margin of the uncinate process and the ethmoid bulla, called the hiatus semilunaris. Although commonly referred to simply as the hiatus semilunaris, Grunwald further classified this entrance to the ethmoid infundibulum as the hiatus semilunaris inferior.18 The hiatus semilunaris superior is then defined as the two- dimensional entrance to the sinus lateralis (described later) that is formed by the space between the posterior aspect of the ethmoid bulla and the anterior aspect of the middle turbinate basal lamella.

Agger Nasi Region

Within the anterior portion of the lateral nasal wall, a mound of bone that is often pneumatized is present. The term for this nasal mound is the agger nasi. When this area is pneumatized, it is called an agger nasi cell ( Fig. 1.7A ). The agger nasi region or cell is found anterior and inferior to the frontal sinus and frequently forms a portion of the anteromedial floor of the frontal sinus. The boundaries of the agger nasi cell are the frontal sinus superiorly and frontal recess superiorly and posteriorly, the frontal process of the maxilla anterolaterally, the nasal bones anteriorly, the lacrimal bones inferolaterally, and the uncinate process inferomedially.5 In the surgical treatment of frontal sinus disease, the agger nasi cell is important to recognize and remove, as it may contribute significantly to narrowing of the frontal recess and frontal sinus outflow tract. In cases of revision frontal sinus surgery, retained remnants of unopened agger nasi cells may also be identified narrowing outflow from the frontal sinus.19

Middle Turbinate

The addition of the middle turbinate completes the construction of the anterior ethmoid complex ( Fig. 1.7A ). The middle turbinate provides a medial and posterior boundary to the anterior ethmoid complex. The middle turbinate is a complex, three-dimensional structure, whose shape may not be intuitive initially. The portion of the middle turbinate that is initially visible on anterior rhinoscopy or nasal endoscopy is oriented in the parasagittal plane, with a free mucosal edge anteriorly and inferiorly. This portion of the middle turbinate may be pneumatized, forming a concha bullosa air cell. Bony attachment of the parasagittal portion of the middle turbinate occurs anterosuperiorly at the crista ethmoidalis of the maxilla, in the region of the agger nasi cell.4,17 The middle turbinate attachment to the skull base in the parasagittal plane occurs adjacent to the lateral lamella of the cribriform plate of the ethmoid bone.

The portion of the middle turbinate that runs in the coronal plane and attaches to the skull base superiorly and the lamina papyracea laterally is called the vertical portion of the middle turbinate basal lamella ( Fig. 1.7A–D ). The basal lamella of the middle turbinate separates the anterior and posterior ethmoid complexes. Although the concept of the vertical portion of the middle turbinate basal lamella may now be easy to grasp via simplified diagrams ( Fig. 1.7A ), in reality the middle turbinate basal lamella is rarely a smooth structure. The middle turbinate basal lamella is frequently indented from both anterior and posterior aspects by the ethmoid complexes on either side, adding to the intricacy of its shape.4,17 The vertical portion of the middle turbinate basal lamella may also be pneumatized, forming an interlamellar cell.4,5,18 Pneumatization of an interlamellar cell has been described as originating from the superior meatus.4

Posteriorly, the middle turbinate basal lamella is oriented in a more axial plane, forming the horizontal portion of the middle turbinate basal lamella. The posterior bony attachment of the middle turbinate to the lateral nasal wall occurs at the crista ethmoidalis of the perpendicular process of the palatine bone, which is often used as an anatomic marker anterior to the sphenopalatine foramen.5,17

With the middle turbinate now visualized, the term ostiomeatal complex or ostiomeatal unit may be addressed. A functional rather than truly anatomic term, the ostiomeatal unit refers to the conglomerate of structures and sinuses that surround or drain into the middle meatus.20 Included are the anterior ethmoid, maxillary, and frontal sinuses; the uncinate process; and the ethmoid infundibulum. Due to the confluent anatomy of this region, and potentially narrow middle meatus drainage pathway, a relatively minor blockage in this important area may lead to obstruction of the frontal, anterior ethmoid, and maxillary sinuses.

Retrobullar and Suprabullar Recesses (Sinus Lateralis)

Situated between the bulla ethmoidalis and the middle turbinate basal lamella are the retrobullar and suprabullar recesses, which lie posterior and superior to the bulla ethmoidalis, respectively ( Fig. 1.8 ). Sometimes referred to collectively as the sinus lateralis, these spaces are not actually sinuses but instead are potential spaces, or recesses, that are bounded by the ethmoid complex structures. The two-dimensional entrance to the sinus lateralis from the middle meatus is the hiatus semilunaris superior, situated between the posterior aspect of the ethmoid bulla and the anterior aspect of the middle turbinate basal lamella, as described previously.

The sinus lateralis may be pneumatized to varying degrees, and at times, bony partitions may divide the suprabullar and retrobullar recesses. The sinus lateralis is bounded by the lamina papyracea laterally, the ethmoid bulla anteriorly, the middle turbinate basal lamella posteriorly, and the skull base superiorly.

Frontal Recess

Typically there is a superior projection from the bulla toward the roof of the ethmoid cavity which attaches to the skull base. This attachment is called the bulla lamella ( Fig. 1.7A ). The space anterior to the bulla lamella, which is bounded by the agger nasi cell anteriorly, the lamina papyracea laterally, and the middle turbinate medially, leads toward the frontal sinus ostium. This space is the frontal recess ( Fig. 1.8 ). If a bulla lamella is not present connecting the ethmoid bulla to the skull base, the frontal recess may communicate with the suprabullar recess posterior to the ethmoid bulla.4,17 The frontal recess is described in more detail in the Frontal Sinus section of this chapter and in other chapters in this text specifically devoted to the anatomy and surgery of the frontal sinus.