Differentiation between Posterior Sinuses Using the Medial Orbital Floor as a Landmark.

 This study aims to evaluate the difference in height between the floors of the posterior ethmoid and sphenoid sinuses with respect to the medial orbital floor (MOF) to confirm this difference as a landmark for identification of the posterior sinuses. It also aims to describe this difference regarding the type of pneumatization of the sphenoid sinus (conchal, presellar, and sellar).  A cross-sectional study was conducted.  The study was conducted at the Department of Radiology of Hospital de Clínicas de Porto Alegre, a tertiary care university hospital in Southern Brazil.  A standardized and computerized analysis of 100 tomography scans of sinuses (200 sides), in patients older than 18 years, was carried out.  Mean vertical distance (height) from the MOF to the floor of the posterior ethmoid and sphenoid sinuses was 0.72 ± 1.85 mm and 9.48 ± 3.81 mm, respectively. There was no statistically significant difference as compared with sex and side. We found conchal-type sphenoid sinus pneumatization in 1.5% ( = 3), presellar in 13.5% ( = 27), and sellar in 85% ( = 170), whereas the vertical distance between the MOF and the floor of the sphenoid sinus was 2.04 ± 0.81 in the conchal-type sinuses, 5.71 ± 2.49 in the presellar sinuses, and 10.21 ± 3.52 in the sellar sinuses. No sphenoid sinus showed its floor above the MOF, regardless of the type of pneumatization.  The present study demonstrates that there is a difference between the floor of the posterior sphenoid and ethmoid sinuses in adults, which is more evident when the sphenoid sinus is well pneumatized. These data suggest that the difference in height between the floors of the sinus investigated in our study may be considered during endoscopic sinus surgery to guide adequate localization, but the surgeon should be aware of the type of pneumatization of the sphenoid sinus to use this landmark.