Figure 1: The MCF approach to the right IAC is shown. The subtemporal dura has been dissected from the middle fossa floor to expose the middle meningeal artery, the greater superficial petrosal nerve, and the arcuate eminence (top image). Part of the bone over the roof of the IAC has been removed. The bone over the arcuate eminence has been drilled to identify the exact location of the superior semicircular canal (left middle image). An enlarged version of the left middle image with additional bone removal exposes the cochlea, located below the middle fossa floor in the angle between the facial and greater petrosal nerves. The IAC is unroofed (right middle image). The semicircular canals are posterolateral and the cochlea is anteromedial to the IAC (left lower image). An enlarged view of the left lower image in included (right lower image). Note that the vertical crest (Bill’s bar) divides the facial and superior vestibular nerves at the IAC. The superior and inferior vestibular nerves are located caudally and the facial and cochlear nerves rostrally within the IAC. The labyrinthine segment of the facial nerve travels superolaterally to the cochlea (images courtesy of AL Rhoton, Jr).

Figure 2: Additional anatomy of the right middle fossa floor is shown. The greater superficial petrosal nerve and the middle meningeal artery in relation to the cavernous sinus are shown (left upper image). The middle fossa floor contents are identified, including the cochlea, semicircular canals, petrous carotid artery, and the facial, cochlear, and superior vestibular nerves in the IAC. The superior semicircular canal is prominent under the arcuate eminence. The cochlear nerve courses underneath the facial nerve to join the cochlea, which is situated above the lateral genu of the petrous carotid in the angle between the pregeniculate facial and greater petrosal nerves (right upper image). This anatomy is further elaborated upon to identify the IAC, cochlea, vestibule, semicircular canals, tympanic cavity, and external meatus. The vestibule is posterolateral and the cochlea is anteromedial to the fundus of the IAC (left lower image). The configuration of the nerves within the IAC from a superior view is demonstrated (right lower image)(images courtesy of AL Rhoton, Jr).

Figure 3: The patient’s position with the zygoma as the highest point in the operative field is illustrated. The single pin of the skull clamp often must be placed on the patient’s forehead to avoid interference of the pins with the incision. The vertex of the patient's head is tilted slightly toward the floor. This maneuver maximizes the effect of gravity retraction on the temporal lobe.

Figure 4: The linear incision, as outlined here in red (upper illustration), is adequate and heals most effectively. This incision starts near the inferior edge of the zygomatic arch, just in front of the superficial temporal artery that may be palpated during planning the incision. The incision extends cranially toward the vertex, just above the superior temporal line. The linear incision mobilizes enough scalp and temporal muscle flaps to provide adequate skull exposure and obviates the need for more extensive incisions. Upon completion of the incision, the scalp is undermined and the attachments of the temporalis muscle to the superior temporal line are disconnected beyond the incision. This maneuver significantly expands the bone exposure through the linear incision.

Figure 5: Craniotomy through a linear incision is illustrated. In this case, the fibers of the temporalis muscle are split longitudinally and the muscle attachments along the superior temporal line are disconnected. When compared with the horseshoe incision, the posterior extent of exposure may be slightly compromised. Regardless which incision is used, the width of the craniotomy is about 4 cm and 2/3 anterior and 1/3 posterior to the external auditory canal. The root of the zygoma is exposed. Intraoperative navigation can help with creation of the craniotomy relative to the the IAC.

Figure 6: After the craniotomy flap is elevated, the remaining base of the temporal squama is reduced flush with the floor of the middle fossa. This goal can be accomplished using rongeurs or a drill with a side-cutting burr. Air cells are often exposed at this step of the operation and must be thoroughly obliterated with bone wax to prevent creation of a CSF fistula. The superior edge of the zygomatic arch may also be drilled to provide an unobstructed view of the middle fossa floor, but this is rarely necessary.

Figure 7: Additional removal of CSF through the lumbar drain relaxes the brain in preparation for its mobilization. Once the floor of the middle fossa is visible, I use a #1 Penfield dissector to detach the dura from the lateral aspect of the petrous bone. A self-retaining retractor may be placed on the dura to protect it during drilling. The remaining bone of the squama and the bony gyrations along the lateral middle fossa floor are drilled completely flush with the lowest level of the floor to improve the operative working angles while minimizing temporal lobe retraction.

Figure 8: The dura over the floor of the anterior aspect of the middle fossa is now elevated. The middle meningeal artery (red arrow, lower image), entering through the foramen spinosum, is identified. The artery may need to be coagulated on its dural side and transected. Next, I pack the foramen with bone wax and oxidized cellulose. Stripping of the dura anteriorly must stop at this point to avoid direct injury to the greater (GSPN)(yellow arrow) and lesser (LSPN) superficial petrosal nerves, potentially leading to dry eyes. Inadvertent traction on the GSPN also places the facial nerve at risk. The surgeon can minimize the risk of this type of injury by lifting the dura in a posterior-to-anterior direction.

Figure 9: The next step is to identify the meatal plane—the area of bone covering the IAC. Numerous methods of locating the meatal plane have been described. The Garcia-Ibanez technique relies on the relationship between the GSPN and the arcuate eminence, usually located at an angle of 120 degrees. Bisection of this angle provides the general location of the IAC and a starting point for drilling the petrous ridge. Similarly, the Fisch technique uses an approximately 60-degree angle between the long axis of the arcuate eminence and the meatal plane to estimate the location of the IAC. The relevant anatomy of the middle fossa floor is illustrated. A House-Urban retractor is used for elevation of the temporal lobe.

Figure 10: The anatomy of the middle fossa floor is widely variable, and the arcuate eminence is not always a reliable locator of the superior semicircular canal. As a general rule, the IAC courses at a 45-degree angle from a line that runs from the arcuate eminence perpendicular to the petrous ridge; this is my preferred method to locate the IAC. Intraoperative navigation confirms this anatomic localization. Once the presumed location of the IAC is identified, its roof is removed using a 3-mm coarse diamond burr along with continuous irrigation. The generous use of irrigation removes bone dust and decreases the likelihood of thermal injury to the underlying nerves. The bone removal begins at the petrous ridge, exposing the proximal dura over the IAC.

Figure 11: These are other methods of localizing the IAC. The right-sided superior semicircular canal is identified either by image guidance or, more accurately, via blue-lining of the canal (upper image, blue arrows). The IAC (black lines) bisects the angle between the course of the GSPN (yellow arrows) and the route of the superior semicircular canal. These anatomic relationships vary, and more than one method of localization should be employed. The anatomy of the left middle fossa floor is demonstrated in the lower image (courtesy of AL Rhoton, Jr). All of the intraoperative photos and illustrations refer to a right-sided approach, whereas the above anatomic photo refers to a left temporal bone.

Figure 12: As the meatal bone is removed in layers, the curved shape of the dura as it enters the IAC verifies the location of this canal. Bone removal proceeds from a medial-to-lateral direction, skeletonizing the medial half of the IAC 270 degrees in its circumference. Skeletonization is less complete in the lateral half of the IAC to avoid injury to the cochlea or superior semicircular canal ampulla. The surgeon must avoid entry into the cochlea while drilling lateral to the IAC; the cochlea is located under the apex of the angle formed by the GSPN and the superior semicircular canal.

Figure 13: A T-shaped dural incision is made, with the first incision parallel and just inferior to the superior petrosal sinus. The location of the facial nerve in the IAC is then identified transdurally via electrical stimulation. The second limb of the incision is made parallel to the IAC away from the location of the facial nerve that commonly lies within the anterior aspect of the IAC, so the dura is typically incised posteriorly. The two dural flaps are reflected and secured, exposing the contents of the IAC.

Figure 14: The facial nerve and its edges are precisely identified with electrical stimulation and visually under high magnification of the microscope. The schwannoma capsule is opened away from the facial nerve and the tumor is internally debulked. A fine dissector is used to develop a plane between the tumor and the facial/cochlear nerves. Dissection is performed in a medial-to-lateral direction to minimize injury to the tiny fascicles of the cochlear nerve found at the lateral extent of the IAC, entering the modiolus.

Figure 15: The final result of tumor resection and the anatomically intact facial nerve are shown.