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Figure 2 Graph illustrating minimum, maximum, and mean distances (in mm) and standard deviations (bars) from the antrostomy ridge and adjacent MOF to the optic nerve (ON), carotid artery (CA), roof of the ethmoid (E), and anterior ethmoid artery (AA). The solid black line (10 mm) represents the initial safe distance from the medial orbital floor, as seen through a middle meatal autrostomy, when performing an ethmoidectomy.

Figure 2 Graph illustrating minimum, maximum, and mean distances (in mm) and standard deviations (bars) from the antrostomy ridge and adjacent MOF to the optic nerve (ON), carotid artery (CA), roof of the ethmoid (E), and anterior ethmoid artery (AA). The solid black line (10 mm) represents the initial safe distance from the medial orbital floor, as seen through a middle meatal autrostomy, when performing an ethmoidectomy.

by proponents of the AP approach. It is only after the sphenoid roof has been identified that a superior dissection of the ethmoid cavity (if indicated) is performed, as with the PA approach.

Schaefer's study does not, however, define the vertical extent of the initial "inferior ethmoidec-tomy" from the level of the medial floor of the orbit. Mosher has shown that the height of the ethmoid labyrinth ranges from 2.5 to 3 cm (3); 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 skullbase structures are at risk remains unclear. Similarly, the level of the initial penetration (superiorly or inferiorly) into the sphenoid if the superior or middle turbinates are absent or distorted is open to interpretation. As with the AP approach, entering blindly through the "inferior and medial quadrant" of the anterior sphenoid wall does not clearly define the appropriate point of entry.

Despite their practical clinical utility, May's and Schaefer's observations are limited by the lack of studies to confirm the consistency or reliability of their measures through endoscopic means. Also, it is not clear how consistent the distance from the medial orbital floor is from the critical orbital or skull-base structures and how this landmark can be better used to facilitate endoscopic surgical orientation.

In 2001, Casiano confirmed May's and Schaefer's observations on a series of human cadavers (37). In this study, two examiners, with varying experience in endoscopic sinus surgery, performed endoscopic and direct measurements from the columnella 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 wall of the sphenoid sinus, were measured. The mean, ranges, and standard deviations for all measurements (endoscopic and direct) were calculated. In addition, the variability in mea surements between examiners and between the endoscopic and direct measurements was also determined.

The author found that the mean and range of values for each of the variables correlated well both between examiners and between endoscopic and direct measurements. The columnellar measurements (Figure 1) appeared to be very consistent between examiners and between endoscopic and direct measurements. When the antrostomy ridge and adjacent medial orbital floor was used (Figure 2), 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 columnellar measurements, are easily identifiable and consistent anatomical landmarks that provide even the most inexperienced surgeon with reliable information to navigate through even the most distorted paranasal sinus cavities.

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