Microsurgical anatomy of the internal acoustic meatus as seen using the retrosigmoid approach

Hypothesis: To show via a retrosigmoid approach the bony labyrinth anatomy and its relationship with the internal acoustic meatus so as to provide guidelines for a safer drilling to the fundus using this approach.Background: Few studies deal with the complex anatomy of petrous bone structures as observed by a retrosigmoid approach.Methods: Ten retrosigmoid approaches were performed bilaterally on five fresh cadaveric heads. Afterward high-resolution computed tomographic scans were obtained. Measurements of landmarks and distances between important topographic structures of the pyramid were obtained on its surface using a navigation system. Semicircular canals, vestibular aqueduct, and internal acoustic meatus were dissected to show their anatomy and relationships.Results: The anatomy of the inner ear structures was shown. Opening of the internal acoustic meatus was accomplished without injury to the labyrinth in 9 out of 10 sides. The distance between the drilled bone of the internal acoustic meatus and the vestibule was calculated on the postoperative computed tomographic scan. The mean value was 1.43 mm (SD, 0.30 mm; range, 1.0-1.8 mm).Conclusion: A better knowledge of the anatomy of the semicircular canals and of the vestibular aqueduct as observed by a retrosigmoid approach, together with their relationships to the fundus and other petrous bone landmarks, can be useful to get a general orientation in acoustic neuroma surgery. Using this information together with the neuronavigation, we were able to successfully open the internal acoustic meatus without entering labyrinthine structures in 90% of the study dissections.