A, The insert shows the site of the exposure directed through the mastoid. The spine of Henley at the posterosuperior margin of the external meatus is a superficial landmark that approximates the deep site of the lateral semicircular canal and the tympanic segment of the facial nerve. The mastoidectomy has been completed. The superior petrosal and sigmoid sinuses, the jugular bulb, and the facial nerve are usually skeletonized in the approach, leaving a thin layer of bone over them. The semicircular canals, which are located in the cortical bone medial to the cancellous mastoid and the mastoid antrum, have been exposed. The dura between the sigmoid and superior petrosal sinuses, the jugular bulb, and the labyrinth, which faces the cerebellopontine angle, is referred to as Trautman’s triangle. B, The mastoid antrum opens through the aditus into the epitympanic part of the tympanic cavity, which contains the upper part of the malleus and incus. The tympanic segment of the facial nerve passes between the lateral canal and the stapes in the oval window and then turns downward as the mastoid segment. The chorda tympani arises from the mastoid segment of the facial nerve and passes upward and forward along the deep surface of the tympanic membrane crossing the neck of the malleus. The incus, the head of which is located in the epitympanic area, has a long process that attaches to the stapes. C, The semicircular canals and vestibule have been removed and the dura lining the internal acoustic meatus has been opened to expose the vestibulocochlear nerve. D, The dura has been opened to expose the petrosal cerebellar surface and the structures in the cerebellopontine angle. Anatomic variants that limit the exposure include an anterior position of the sigmoid sinus, a high jugular bulb, or a low middle fossa plate. The jugular bulb may extend upward into the posterior wall of the internal acoustic meatus and be encountered as the posterior meatal wall is being removed by either the translabyrinthine or retrosigmoid approaches. (Images courtesy of AL Rhoton, Jr.)