A new CBCT-based classification of posterior extramural ethmoid cells.

Purpose: The study aimed to evaluate the prevalence of extramural expansion of posterior ethmoid cells using Cone beam computed tomography (CBCT), as well as to propose anatomical criteria and a classification system for Onodi cells based on their spatial relationship with the sphenoid sinus.

Methods: Cone beam computed tomography (CBCT) scans of 513 ENT (Ear, Nose, and Throat) and dental patients of outpatient clinics, 199 men and 314 women, mean age 36 ± 13 years (range: 18-84 years).

Results: Several extramural variants of posterior ethmoid (PE) cells were examined, such as sphenoethmoidal cells (where PE cells extend into the sphenoid sinus) and posterior ethmomaxillary cells (characterized by extension into the maxillary sinus). Among sphenoethmoidal cells, the following subtypes were identified: Onodi cells (PE cell extension toward the optic canal), inferolateral sphenoethmoidal cells (inferolateral expansion of PE cells relative to the sphenoid sinus without reaching the optic canal), and supraseptal posterior ethmoid cells (migration of PE cells into the region between the nasal septum and the cribriform plate). Among 513 patients examined, sphenoethmoidal cells were identified in 66 cases (89 cells), demonstrating considerable anatomical diversity. We observed a spectrum of variants that included predominant Onodi cell types (superior [48.2%], superolateral [19.2%], and lateral [18%]), less frequent Onodi forms (superoposterior [11.3%] and intersphenoidal [1.1%]), as well as other sphenoethmoidal variants such as supraseptal and inferolateral cells.

Conclusion: Based on the data obtained, we introduce an updated classification of Onodi cells that accounts for their spatial relationship with surrounding structures. Posterior extramural ethmoid cells may pneumatize the sphenoid bone to such an extent that they can be radiologically mistaken for the sphenoid sinus. Accurate differentiation between these structures is crucial, as inflammatory processes may develop independently within either the sphenoid sinus or the sphenoethmoidal cells. Misidentification may lead to inappropriate treatment strategies. Therefore, precise radiological assessment is essential to determine the true origin of inflammation and ensure targeted, effective clinical management.