The ethmoid bulla, also known as bulla ethmoidalis, is the largest and most consistent air cell of the anterior ethmoid air cells.
It can be classified into three subtypes 1:
- simple bulla (50%): a single cavity that usually communicates in the hiatus semilunaris
- compound bulla (25%): 2-3 separate compartments that communicate with the hiatus semilunaris
- complex bulla (25%): 2-3 separate compartments that communicate with the hiatus semilunaris, ethmoid infundibulum or superior meatus.
It is formed by pneumatization of the bulla lamella and its degree of pneumatization is variable. In its most extreme form (giant ethmoid bulla), it may fill the entire middle meatus. Failure of pneumatization of the ethmoid bulla is termed torus ethmoidalis 3.