Normal sinonasal endoscopic surgical anatomy

The goal of endoscopic sinus surgery (ESS) is, when possible, to restore the normal mucociliary clearance function of the paranasal sinuses. This entails opening the natural sinus drainage pathway with preservation of normal mucosa, unless it is irreversibly damaged. Understanding and being facile with the normal sinonasal anatomical landmarks is critical to performing safe and efficacious ESS. In the primary surgical case, the landmarks are less variable and generally more familiar to the surgeon. In revision ESS, some of the more common landmarks relied upon may not be present or may be distorted, making their utility unclear. It is for this reason that a thorough understanding of all the normal landmarks be described and understood, but particular attention must be directed to those constant reference points used in revision surgery. Clearly, there is an overlap in the landmarks that are used in primary and revision surgery.

It is important to remember that the use of triplanar computer-aided image guidance is endorsed by the American Academy of Otolaryngology—Head and Neck Surgery and the American Rhinologic Society in primary ESS for pathology of the posterior ethmoid, frontal, and sphenoid sinuses. It is also indicated in revision ESS, skull base dehiscences, pathology abutting critical neurovascular structures, extensive sinonasal polyposis, and benign or malignant neoplasms. However, there is no substitute for a thorough and clear understanding of the anatomical landmarks, as this instrumentation can harbor error and cannot be solely relied upon.

Obtaining a triplanar computed tomography of the paranasal sinuses is critical to the preoperative evaluation and surgical planning of any ESS. The imaging must be carefully studied to assess for anatomical variants including Haller cells, Onodi cells, frontal sinus outflow variations, bony dehiscences, aplastic or hypoplastic sinuses, and Keros classification of the lateral lamella. Careful examination of the computed tomography is particularly important in revision surgery.

Upon entering the nose with the endoscope, one should immediately notice the septum, inferior turbinate, and head of the middle turbinate (Figure 1A). Advancing the endoscope along the septum posteriorly, the eustachian tube orifice, arch of the posterior choana, and nasopharynx are easily seen (Figure 1B). It is important to keep the orientation of the endoscope, whether angled or straight, in line with the choanal arch and with vertical orientation of the septum to avoid superior angled penetration of the skull base or lateral penetration through the lamina papyracea (Figure 1C). Using cadavers, Casiano determined standardized measurements originating from the columella of 5 cm to the face of the posterior ethmoid, 7 cm to the face of the sphenoid, and 9 cm to the posterior sphenoid (Figure 1D). These measurements are important to guiding depth and avoiding skull base injuries.

The maxillary sinus drains via the natural ostium into the middle meatus. If the middle turbinate is medialized then the nasolacrimal convexity, uncinate process, and ethmoid bulla are seen above the level of the inferior turbinate (Figure 2A). Once the natural ostium of the maxillary sinus is identified posterolateral to the uncinate process and connected to the accessory ostium (by removing the posterior fontanel), the posterior maxillary sinus wall is easily visualized (Figure 2B). Most importantly, the medial orbital floor (MOF) defines the horizontal maxillary antrostomy ridge and perpendicularly meets the vertically oriented lamina papyracea (medial orbital wall) (Figure 2C). The vertical antrostomy ridge is formed by the ethmoid crest of the palatine bone in close proximity to the greater palatine canal and sphenopalatine foramen.

The drainage of the anterior ethmoid cells, which include the agger nasi, the bulla ethmoidalis, and the suprabullar or supraorbital cells, is through the ethmoid infundibulum into the middle meatus. The convexity of the bulla ethmoidalis is seen just posterior to the uncinate process (Figure 2A). This cell can be opened to its posterior extent that can be confluent with the horizontal ground lamella of the middle turbinate; there can be a potential space between the back of the bulla ethmoidalis and the ground lamella named the sinus lateralis. The horizontal ground lamella of the middle turbinate separates the bulla ethmoidalis from the inferior-posterior ethmoid cell. The vertical ground lamella of the middle turbinate separates the suprabullar or supraorbital cells from the superior-posterior ethmoid air cells (Figure 2D).

The drainage of the posterior ethmoid cells, which include the inferior and superior-posterior ethmoid cells, is into the superior meatus, or the supreme meatus, if present (Figure 3A). The MOF and the transitional antrostomy ridge together are important in defining the trajectory of the inferior-posterior ethmoid air cell, at the level of the MOF, and the sphenoid sinus below it. The transitional area of the antrostomy ridge defines the point of “safe” entry into the inferior-posterior ethmoid cell just posterior to the basal lamella of the middle turbinate (Figure 3B). The fovea ethmoidalis defines the roof of the ethmoid air cells and ends at the frontal sinus infundibulum. It is lowest at the face of the sphenoid and gently slopes superiorly toward the frontal sinus ostium. The fovea ethmoidalis is thicker and higher laterally adjacent to the lamina paprycea and thinner and lower medially near the vertical lamella of the middle turbinate. The posterior ethmoid artery and anterior ethmoid artery are found along the fovea ethmoidalis from posterior to anterior, and the latter is located at the posterior septation of the suprabullar cell about 1 cm posterior to the frontal sinus ostium (Figure 3C).

The natural drainage pattern of the sphenoid sinus is through the natural ostium adjacent to the septum into the sphenoethmoid recess. (Figure 2A). If the endoscope is advanced along the septum posteriorly then the eustachian tube orifice, arch of the posterior choana, and nasopharynx are easily seen (Figure 1B). If the middle turbinate is lateralized, its tail becomes visible along with the superior turbinate (and tail) and the medially adjacent natural sphenoid ostium above the posterior septal branches of the sphenopalatine artery that course along the inferior aspect of the posterior choanal arch (Figure 3D). The face of the sphenoid correlates with the posterior wall of the most inferior-posterior ethmoid cell. The natural ostium is located approximately 2 cm above the posterior choanal arch, equivalent to nearly one-third of the way up the tail of the superior turbinate. The sinus should be opened inferiorly and medially to avoid injury to the optic nerve and carotid artery that are located laterally and superiorly. The posterior septal branches may be encountered and require cauterization with electrocautery if injured.

The frontal sinus is often the most difficult sinus to address owing to its anatomical variations. It drains into the frontal recess and either the middle meatus (laterally), if the uncinate attaches to the lamina papyracea, or the ethmoid infundibulum (medially), if the uncinate attaches to the vertical lamella of the middle turbinate or skull base (Figure 4A). The anterior-superior attachment or “axilla” of the middle turbinate attaches to the lateral nasal wall just posterior to the nasolacrimal convexity and defines the location of the agger nasi cell, which forms the anterior boundary of the frontal sinus outflow tract (Figure 4B). In the 2-dimensional sagittal plane, the frontal sinus infundibulum is between the suprabullar ethmoid cells posteriorly and the agger nasi and frontal sinus cells, if present, anteriorly (Figure 4C). In a 3-dimensional endoscopic view, the frontal sinus infundibulum is defined by the lamina papyracea laterally, the suprabullar cells posteriorly, and the vertical lamella of the middle turbinate medially (Figure 4D). The agger nasi cell, suprabullar cells, and frontal sinus cells (types 1, 2, or 3) are then systematically removed until the frontal sinus ostium is clearly visualized and confirmed with transillumination of the forehead or with image guidance. The frontal sinus ostium is superiorly located in a coronal plane at the anterior limit of the maxillary natural ostium and parallel to the nasolacrimal convexity. When palpating for the frontal sinus ostium, a curved probe should be used and guided with 2 fingers in a lateral direction along the medial orbital wall.

The normal landmarks and sinus outflow patterns discussed in this article enables safe and efficient access to the paranasal sinuses. The uncinate process; bulla ethmoidalis; and inferior, middle, and superior turbinates are excellent normal landmarks. When these are not present, the nasolacrimal convexity, MOF, and arch of the choana are constant reference points that allow for confident navigation through all ESS procedures.