Endoscopic Sinonasal Dissection Guide

3    The Use of Anatomic Landmarks: A Stepwise Approach to the Paranasal Sinuses

Roy R. Casiano

In 1994, May and colleagues1 cited six user-friendly anatomic landmarks that are almost always present despite previous surgery:

1.The arch (or convexity) formed by the posterior edge of the lacrimal bone, marking the lacrimal duct at the anterior margin of the middle meatus

2.The anterior superior attachment of the middle turbinate (vertical lamella)

3.The middle meatal antrostomy and its bony "ridge," along its superior border, formed by the junction of the floor of the orbit with the lamina papyracea and resected margin of the posterior fontanelle

4.The lamina papyracea

5.The nasal septum

6.The arch of the posterior choana

Using these landmarks, revision endoscopic sinus surgery (ESS) for recurrent or persistent disease in the maxillary, ethmoid, sphenoid, or frontal sinuses can be safely performed. May was one of the first to acknowledge that in advanced sinus disease, anatomic landmarks, such as the uncinate process, basal lamella, and superior or middle turbinates, are not always readily identifiable. He was also one of the first to point out that the floor of the orbit, as seen through an antrostomy, serves as a consistent landmark from which other structures may be found. The bony "ridge" along the superior border of the antrostomy corresponds to the medial orbital floor, which facilitates identification of the inferior lamina papyracea prior to proceeding with an ethmoidectomy. As will be seen later in this manual, this ridge is also useful in locating the posterior ethmoid and sphenoid sinuses.

Despite prior reports that showed great intersubject variability, May et al2 and Stankiewicz3 suggested that it was clinically efficacious to use standard measurements from the columella to orient the surgeon during ESS.2,3 They based this suggestion on anecdotal experience and prior anatomic studies by others, noting that the distance from the area of the anterior nasal spine to the sphenoid ostium is ~60 mm (range, 47-70 mm).4-6 If 1 cm is added for the length of the columellar base, the mean distance to the sphenoid ostium would be approximately 70 mm. For this reason, May advocated labeling instruments with colored tape to warn the surgeon when the anterior face of the sphenoid is reached (~7 cm).

Today, many instruments come premarked in centimeters from the tip to allow for such measurements. However, there will likely be variability among surgeons' measurements of these distances. In isolation, these measurements have not been shown to be clinically reliable.

Schaefer7 was the first to described a "hybrid or combined technique" that blended the conservation goals of the anteroposterior (AP) approach with the anatomic virtues of the posteroanterior (PA) approach. Surgery begins with identification and complete removal of the uncinate process. If further surgery of the ethmoid sinus is warranted, the maxillary natural ostium is enlarged posteriorly or inferiorly, rather than anteriorly, to avoid injury to the lacrimal canal. Schaefer noted that this immediately exposes the level of the orbital floor. Like May, Schaefer recognized the importance of the medial orbital floor as a very important landmark to facilitate identification of the inferior lamina papyracea prior to proceeding with an ethmoidectomy. He also advocated removal of the inferior two thirds of the ethmoid cells in an AP direction using a 0-degree telescope.

Often this involves removal of most, if not all, of the basal lamella of the middle turbinate to address the drainage area of the posterior sinuses and to facilitate entry into the sphenoid sinus. If the ostium cannot be visualized or palpated, the sphenoid is entered in the inferomedial quadrant of the anterior wall of the sinus. This approach ensures that the surgeon will maintain a safe distance from the skull base. It is only after the sphenoid roof has been identified that a superior dissection of the sphenoid face or ethmoid cavity (if indicated) is performed, as with the PA approach.

Schaefer's approach, like May's, recognizes the importance of performing an antrostomy prior to an ethmoidectomy to identify the orbital floor and medial orbital wall. Schaefer was the first to note the importance of performing an inferior ethmoidectomy before proceeding posteriorly using the medial orbital floor as a reference point. As the surgeon proceeds posteriorly, it is the orbital wall that dictates the trajectory and not some ill-defined and often distorted lamella or turbinate structure, as advocated by proponents of the AP approach.

Schaefer's study, however, did not define the vertical extent of the initial "inferior ethmoidectomy" from the level of the medial floor of the orbit. Mosher8 has shown that the height of the ethmoid labyrinth ranges from 2.5 to 3 cm; however, this height may vary even more depending on whether it is measured anteriorly or posteriorly.

Similarly, the distance of two thirds of the ethmoid cells, as described by Schaefer, can be quite variable. The maximum vertical distance permitted for an "inferior ethmoidectomy" as the surgeon proceeds posteriorly before critical skull base structures are at risk remains unclear. Similarly, the distances to the critical structures in the posterior and lateral walls of the sphenoid sinus remain undefined.

In 2001, Casiano9 confirmed May's and Schaefer's observations on a series of human cadavers. In this study, two examiners, with varying experience in endoscopic sinus surgery, performed endoscopic and direct measurements from the columella and medial orbital floor to critical orbital and skull-base structures. The distances to four critical skull-base or orbital structures (the carotid artery, optic nerve, mid-ethmoid roof, and anterior ethmoid artery) and to the anterior and posterior walls of the sphenoid sinus were measured (Fig. 3.1). The mean, ranges, and standard deviations for all measurements (endoscopic and direct) were calculated. In addition, the variability in measurements between examiners and between the endoscopic and direct measurements was also determined. The mean and range of values for each of the variables correlated well both between examiners and between endoscopic and direct measurements. The columellar measurements appeared to be very consistent between examiners and between endoscopic and direct measurements (Fig. 3.2).

Fig. 3.1(a) Distances from the columella and antrostomy ridge to critical structures: 9 cm to the posterior sphenoid; 7 cm to the anterior sphenoid or posterior wall of posterior ethmoid; 5 cm to the anterior wall of the posterior ethmoids. (b) Shaded area denotes the "safe zone" of inferior orbital dissection within 1 cm of the antrostomy ridge. Arrows denote the key measurement points illustrated in Fig. 3.2b.

Fig. 3.2(a) Mean, minimum, maximum, and standard deviation (SD) for measurements from the columellar base (in millimeters). (b) Mean, minimum, maximum, and SD for measurements from the antrostomy ridge (in millimeters). PM, posterior maxillary sinus; AS, anterior sphenoid sinus; PS, posterior sphenoid sinus; ON, optic nerve at canalicular portion; CA, cavernous carotid artery; AA, anterior ethmoidal artery; E, ethmoid roof at the junction of the orbital wall. Columellar measurements greater than 9 cm represent "extra-sinus" extension into the orbit or skull base or posterior sphenoid wall (black dashed line in a). Staying within 1 cm of the antrostomy ridge along the inferior orbital wall keeps the surgeon away from most critical neurovascular structures (white solid line in b).

When the antrostomy ridge and adjacent medial orbital floor was used, there was some slight variability between the individual measurements of the examiners and between endoscopic and direct measurements. However, the differences in measurements were no more than a few millimeters and did not appear to affect the overall clinical utility of these values. Casiano concluded that the bony ridge of the antrostomy and adjacent medial orbital floor, when combined with the use of columellar measurements, are easily identifiable and consistent anatomic landmarks that provide even the most inexperienced surgeon with very reliable information to navigate through even the most distorted paranasal sinus cavities. For example, staying within 1 cm of the antrostomy ridge, along the medial orbital wall and anterolateral sphenoid sinus, keeps the surgeon well away from critical skull base structures (Figs. 3.1b and 3.2b). This is particularly important for advanced cases with distorted anatomy of the paranasal sinuses, due to prior surgery or significant inflammatory disease (e.g., polyps). This critical anatomic landmark as well as others are reviewed throughout the course of this dissection manual, and their practical use in endoscopic sinus surgery and in maintaining the surgeon's orientation within this complex anatomic area is illustrated.

References

1 May M, Schaitkin B, Kay SL. Revision endoscopic sinus surgery: six friendly surgical landmarks. Laryngoscope 1994;104(6 Pt 1):766-767.

2 May M, Sobol SM, Korzec K. The location of the maxillary os and its importance to the endoscopic sinus surgeon. Laryngoscope...