Anatomical variations and relations of the bony portion of the eustachian tube.

The bony portion of the Eustachian tube was examined, using anatomical and histological methods, on 200 temporal bones of adults. This part of the tube averages 11.38 mm in length, its tympanic opening has an average diameter of 5.20 X 3.95 mm, and its lumen can be of an irregular (45%), rectangular (35%) or triangular (20%) shape. The dimensions of the bony portion of the tube diminish gradually toward the isthmus, so that in the middle the average diameter is 4.02 X 3 mm and at the level of the isthmus, 2.5 X 1.5 mm. The outer wall forms the tympanic plate of the tympanic bone, which in 65% of the cases is compact and in 35% finely pneumatized. On this wall bony trabeculae often appear, which more or less block the lumen of the tube. The medial part of the upper wall forms the bony septum toward the m. tensor tympani and the lateral represents the tegmen. The average distance from the lumen of the tube to the dura at the level of the tegmen is 3.62 mm. The tegmen above the tube may be pneumatized (45%) or compact (55%). The lower wall in 20% of the cases is represented by a shallow groove, and if formed is usually convex into the tube's lumen, but is less often concave. The jutting out of the lower wall is usually caused by its pneumatization.