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Since Portmann suggested surgical drainage of the endolymphatic sac in 1927, several similar procedures have been described. These procedures all share a common problem: the endolymphatic sac must be identified without injuring the labyrinthine structures or the facial nerve.

Anson and Donaldson have provided the otologist with a detailed and accurate description of the anatomy of the endolymphatic sac but did not provide the surgeon a convenient and reliable way of safely locating this structure through Trautmann's triangle via a mastoidectomy approach.

Forty normal temporal bones were examined and measurements obtained. In 20 temporal bones, the vestibular portion of the endolymphatic duct was injected with dye, and a complete mastoidectomy was performed. Accurate measurements were obtained. In addition, 20 normal temporal bones were sectioned down to the inferior-most extent of the endolymphatic sac, and measurements from local landmarks were obtained.

The distance between the short process of the incus and the bottom of the otic capsule of the posterior semicircular canal is 10.4 mm (range 9.5 to 11.5 mm), and the distance from the tip of the short process of the incus to the inferior-most point of the endolymphatic sac is 16.9 mm (range 13.0 to 19.0 mm). Hence, the distance from the otic capsule of the posterior canal to the bottom of the sac is 4.3 mm (range 1.9 to 6.6 mm). Therefore, the conservative surgeon can approach the endolymphatic sac below the level of the posterior semicircular canal and minimize risk of injury to that structure.

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