Superior mesotympanic sinus.
This study of the posterior tympanum was performed to better define the anatomy of this complex area of the middle ear. A sinus previously not described was examined. The posterior tympanum of 49 temporal bones was studied by microscopic techniques. The sinus depth, the opening into the sinus, and the relationship of the facial nerve to the sinus were documented. A sinus lying in the superior part of the posterior tympanum was discovered: the superior mesotympanic sinus. The depth of the sinus was from 0.5 to 2 mm, and it was present in 42 of the 49 specimens (86%). The superior border of the sinus is the facial nerve, which was dehiscent in 9 cases. The surgeon can remove disease with more confidence from the tympanum if he or she knows that disease may harbor in this sinus, and that injury to the facial nerve must be avoided.