Microsurgical anatomy of the protection of the labyrinth with the transpetrosal approach

Objective To obtain more detailed anatomic information about the protection of the labyrinth with the transpetrosal approach. Methods Twenty sides of 10 adult cadaver heads fixed in formalin were used, the anatomy of the bony labyrinth and the relationship of the correlative structures were observed and measured, and meanwhile the actual surgical procedure was mimicked under the surgical microscope. Results The bony labyrinth was a compact and lustrous bone, which encircled the membranous labyrinth. The bony semicircular canal was drilled until it was so thin that the membranous canal was visible as blue color, called blue line. But this manipulation was more difficult in the bad pneumatized mastoid. There was significant variability in the morphometry of the adult temporal bone. The average overlying thickness of the bone overlying the posterior semicircular canal (PSC) was 3.1 mm± 1.3 mm, and that of the bone overlying the superior semicircular canal (SSC) was 1.5 mm ± 0.7 mm. The distance from the PSC to the sigmoid sinus was 8.6 mm± 2.2 mm. Conclusion In order to keep the labyrinth intact, two criteria are important and should be taken into account . The first one is the deviation of the labyrinth location, which is so great that the labyrinth can not be precisely located during operation. The second one is the pneumatization of the mastoid; the more dense the boneis, the more difficult it is to expose the blue line. Recognizing the bony semicircular and never penetrating the blue line are important principles.