Abstract
Background: Among the most popular methods employed to classify the depth of the olfactory fossa is Keros classification. This study aims to assess Keros classification of the ethmoid roof, any possible association between Keros types and gender, and the incidence of asymmetry between right and left sides among Iraqi patients. 
 Methods: A retrospective cross-sectional study was conducted at Al-Shaheed Gazi Al-Hariri Teaching Hospital, Medical City, Baghdad, Iraq. The archived reports and the CT scans images (nose and paranasal sinuses) of 126 patients who have undergone functional endoscopic sinus surgery between January 2019 and January 2020 were reviewed. Univariate and bivariate statistical analysis was performed using SPSS version 24. The statistically significant was considered at less than 0.05.
 Results: More than half of patients were females (54.0%) with a mean age of 31.52 ± 11.38 (SD) years (range: 10-57 years). Among the total patients, the mean depth of olfactory fossa (OF) was 3.58 ± 0.02 mm. Results showed that Keros type I was the most common type (71.0%), followed by type II (27.4%) and type III (1.6%) respectively. The difference in the olfactory fossa depth between the right and left sides was ≥ 1 mm in 16 (12.7%) patients and < 1 mm in 110 (87.3%) patients. Moreover, there was no significant relation between symmetry/asymmetry and gender ( p-value > 0.05).
 Conclusion: Keros type I was the most common type, which carries the lowest risk of inadvertent intracranial injury during endoscopic sinus surgery; besides the relatively low percentage of asymmetry in the depth of the two olfactory fossae among patients, surgeons should always be cautious during surgery to avoid iatrogenic injury concerning the thin lateral lamella of the cribriform plate.
Highlights
Among the most popular methods employed to classify the depth of the olfactory fossa is Keros classification
A variable segment of the lateral wall of the olfactory fossa, will be exposed during the endoscopic dissection of the ethmoid region where Keros type III being the most exposed one, besides the fact that the thinnest bone of the entire skull base is the lateral lamella of the cribriform plate [10,11], leading it to be at a higher risk for iatrogenic injury during endoscopic ethmoid procedures [8, 12, 5, 13]
Each computed tomography (CT) scan was counted as two cases for a total of 252 cases
Summary
Among the most popular methods employed to classify the depth of the olfactory fossa is Keros classification. The importance of computed tomography (CT) scan of the paranasal sinuses, both in the diagnosis and preoperative planning, has increased during the last thirty years, especially with the growing popularity of different functional endoscopic sinus surgery (FESS). The term ‘dangerous ethmoid’ was first introduced by Kainz and Stammberger and defined it depending on the depth of the olfactory fossa, as in Keros type III [8]. A variable segment of the lateral wall of the olfactory fossa, (depending on the olfactory fossa depth) will be exposed during the endoscopic dissection of the ethmoid region where Keros type III being the most exposed one, besides the fact that the thinnest bone of the entire skull base is the lateral lamella of the cribriform plate [10,11], leading it to be at a higher risk for iatrogenic injury during endoscopic ethmoid procedures [8, 12, 5, 13]. Badia et al [14] found that the anatomical variations of the paranasal sinuses might differ radiologically in their prevalence between different ethnic groups
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