Introduction: 

Endoscopic access to the skull base is available via the nasal cavity and the ethmoidal air cells. Although the topography of the ethmoid bone is rather complex, its relations to the nasal turbinates show certain consistency. Of special interest is the 3rd basal lamina which anchors the middle turbinate to the external surface of the anterior cranial fossa.

Objective: 

We have examined the landmarks (e.g. uncinate process, ethmoid bulla, 3rd basal lamina) visible during the transethmoidal approach step-by-step to consider the possible surgical pitfalls due to the anatomical relations.

Materials and methods: 

3 formaline-fixed cadaver heads and 2 dry skull specimens were used to describe the anatomical boundaries. 5 fresh cadavers, injected with red gelatine, were subjected to endoscopic phantom operation. Cone Beam CT images were taken of 30 patients, together with a fixed cadaver and a dry skull specimen. The gender or the age of the patients or cadaver specimen were not recorded.

Results: 

In the majority (95,31%) of the cases, the lamina papyracea has formed the lateral wall of the ethmoid bulla. A retrobullar recess was identified in 35,94% of the cases. Its longest sagittal length was 3,55 mms (varied between 1,00–6,60 mms). The 3rd basal lamina was poorly identifiable without prior CBCT-examination, or with a small (12,50%), or absent (64,06%) retrobullar recess. The vertical segment of the 3rd basal lamina could not be easily ascribed to one plane due to its intricate relation to the ethmoidal cells. The retrobullar segment of the 3rd basal lamina was found to be connected to anterior ethmoidal cell(s), or the ethmoid bulla in 32,81% or 31,25% of the cases, respectively. A septate sphenoidal sinus may mimick a large ethmoidal air cell.

Conclusion: 

A CBCT screening is imperative prior to a surgical intervention because of the danger of breaking into the skull base or the orbit. Endoscopic investigations of the ethmoid bulla may allow for the identification of the lateral border of the surgical corridor. Out of the five laminae, the 3rd basal lamina bears with the highest clinical significance, although it still cannot be considered as a constant and safe landmark due to its morphological variability.