Background: The identification of anatomic landmarks in endoscopic skull base or revision sinus surgery can be challenging. Normal anatomy is significantly altered with many paranasal tumors. Traditional endoscopic surgical landmarks extrapolated from inflammatory disease, such as the superior turbinate, may have been previously removed or involved in pathology. A frequently used rule to enter the sphenoid, "stay below or at the level of the orbital floor as dissection proceeds posteriorly and one will avoid the skull base," is assessed anatomically.
Methods: The maxillary sinus roof height, relative to the nasal floor, was assessed as an operative landmark. Computed tomography (CT) performed on paranasal sinuses was studied. The relative height, ratio, and proportions of the maxillary sinus, ethmoid roof, cribriform fossa, and sphenoid planum were measured using computerized assessments.
Results: Three hundred paranasal sinus systems were evaluated. The roof of the maxillary sinus was below the level of the skull base in 100% relative to the cribriform and 100% relative to the sphenoid planum. The mean distance of the maxillary roof below the skull base was 10.1 +/- 2.7 mm for the cribriform and 11.0 +/- 2.9 mm for the sphenoid.
Conclusion: The maxillary sinus roof can be used as a robust landmark to allow safe dissection and debulking of pathology. Pathology removal can proceed posterior with this landmark to enable a safe entry to the sphenoid sinus, and thus the true skull base, when normal structures such as the superior turbinate and ostium are not available.