*Posterior ethmoid cells can become pneumatized far laterally and to some degree superiorly to the sphenoid sinus, in which case they are called sphenoethmoid cells(cellulae sphenoethmoidales) or Onodi cells. Pneumatization of the clinoid process in those cases may originate from the posterior ethmoid cell, also.
*The optic nerve and carotid artery may be exposed in a sphenoethmoid(Onodi) cell. This is clinically significant because the sphenoid sinus is located medially and inferiorly to the most posterior cell of the posteriorethmoid complex. Consequently, attempts to use instrumentation to locate the sphenoid sinus directly behind the last cell of the posterior ethmoid complex may result in serious damage to the optic nerve or carotid artery.
*The most posterior ethmoid cell may becalled a sphenoethmoid cell(Onodi cell) when it pneumatizes laterally and superiorly to the sphenoid sinus and is intimately associated with the optic nerve. Prominence of the optic nerve tubercle or the internal carotid artery is not prerequisite, however. Moreover, the optic nerve tubercle may be prominent in other posterior ethmoid cells as well. Whether ethmoid complex components grow posteriorly alongside the sphenoid sinus or sphenoethmoid cells pneumatize directly into the sphenoid bone has not been resolved, but the answer does not bear on practical issues in diagnosis and sugery: the air space in question is clearly ethmoid.
