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Middle Fossa Anatomy
Middle Fossa Anatomy
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  Middle Fossa Approach

In the middle fossa approach, the internal auditory canal is approached from above, through a temporal craniotomy located anterior to the ear and above the zygoma. The dura under the temporal lobe is elevated from the floor of the middle cranial fossa until the arcuate eminence and greater petrosal nerve are identified. The distance from the inner table of the skull to the facial hiatus, through which the greater petrosal nerve passes, ranges from 1.3 to 2.3 cm (average 1.7 cm). In separating the dura from the floor of the middle fossa, the surgeon should remember that bone may be absent over all or part of the geniculate ganglion. In a previous study of 100 temporal bones, it was found that all or part of the geniculate ganglion and genu of the facial nerve were exposed in the floor of the middle fossa in 15 bones (15 percent). In 15 other specimens, the geniculate ganglion was completely covered but there was no bone extending over the greater petrosal nerve, The greatest length of greater petrosal nerve covered by bone was 6.0 mm. It is also important to remember that the petrous segment of the internal carotid artery may be exposed without a bony covering in the area below where the greater petrosal nerve passes below the lateral margin of the trigeminal ganglion. The foramen spinosum and middle meningeal artery and the foramen ovale and third trigeminal division are situated at the anterior margin of the extradural exposure. Usually the extradural exposure can be completed without obliterating the middle meningeal artery at the foramen spinosum. The tensor tympani muscle and eustachian tube, although not exposed in this approach, are located beneath the floor of the middle fossa roughly parallel to, and in front of the horizontal portion of the petrous carotid artery.

In completing the middle fossa approach, bone is removed over the greater petrosal nerve to expose the geniculate ganglion and genu of the facial nerve. From here the labyrinthine portion of the facial nerve is followed to the lateral end of the internal auditory canal by removing bone. The lateral part of the bone removal is limited posteriorly by the superior semicircular canal, which is located a few millimetres behind and is oriented parallel to the labyrinthine segment of the facial nerve. The anterior edge of the exposure is limited by the cochlea, which sits only a few millimetres anterior to the site of bone removal in the angle between the labyrinthine portion of the facial nerve and the greater petrosal nerve. It is important that the cochlea and semicircular canals not be damaged in this approach because it is used only for removing small tumors in which there is an opportunity to preserve hearing. The vertical crest, which is identified at the upper edge of the lateral end of the internal acoustic canal provides a valuable land­mark for identifying the facial nerve. In the final stage of bone removal, the upper wall of the internal auditory canal is removed to expose the dura lining the entire superior surface of the internal auditory canal from the vertical crest to the porus acusticus.


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