The frontal sinus and the ethmoidal labyrinth.

In the "standard" anatomic description, the frontal bone and cribriform plate of the ethmoid bone form the base of the anterior cranial fossa. We studied the development of the ethmoidal bone as well as its relations to the frontal bone in macerated disarticulated skull bones and macerated skull bases of 35 individuals between 9 and 35 years of age. In 19 cases the ethmoidal cells were completely or partly uncovered by the frontal bone. In 6 of 19 cases the frontal bone did not cover any of the ethmoidal cells; in 10 further cases the frontal bone covered only the anterior and in 3 cases the anterior and middle ethmoidal cells. In a 60-year-old subject the ethmoidal cells were incorporated in the base of the anterior cranial fossa, a rare finding. Thus, a depressed lamina cribrosa is not the only danger in ethmoidectomy. Based on the present data ethmoidal cells uncovered by the frontal bone may involve a serious risk during ethmoidectomy even if the surgeon remains lateral to the insertion of the middle concha. The discrepancy between common descriptions of this region and our own findings may be related to imprecise data concerning the life stage of the cases described in the literature.