The ethmoid bulla is a consistent landmark unless the patient has had previous surgery. Its ostium normally opens posteriorly or medially. It can be attached to the skull base. If it is not attached to the skull base, a suprabullar recess is formed and the anterior ethmoid artery is often found in this area. The space that forms behind the ethmoid bulla is called the retrobullar recess. The posterior wall of the retrobullar recess is made up of the basal lamella of the middle turbinate (Fig. 5.31 a, b).

The anterior cells are separated from the posterior ones by a condensation of bone called the basal lamella. The basal lamella is formed as an almost vertical wall in the coronal plane behind the ethmoid bulla. The posterior root of the middle turbinate merges with the basal lamella.

The number and size of the remaining anterior eth-moidal air cells vary according to the degree of pneumatization of the air cells. In about 8% of individuals the cells extend forward to the extent that they lie medial to the position of the lacrimal sac, and these are called lacrimal cells. If they are large, they can form a prominence of the lateral nasal wall where the anterior end of the middle turbinate is attached (Fig. 5.32a, b). The height and position of the agger nasi cells has been classified by Bent et al. (1994). Agger nasi cells are present in the majority of patients, but their size and position vary. If they are large they "push" the frontal recess medially and posteriorly next to the middle turbinate.

Other anterior ethmoid cells vary and can be attached laterally to the lamina papyracea or to the skull base. A cell that is based on the floor of the orbit or the roof of the maxillary sinus is called a Haller cell (Fig. 5.33).

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