We described the experience of a microscope-based navigational system for opening of the posterior wall of the internal auditory canal (IAC) via the retrosigmoid route. Computed tomographic findings for 47 acoustic neuroma cases were divided into three groups, on the basis of the relationship between the labyrinth and the sigmoid-fundus line (medial,on the line,or lateral). The shortest distances between the most medial labyrinthine extension (MMLE) and the resection line were measured. In 20 acoustic neuroma operations, the different features and the practicality of the microscope-based navigational system for opening of the IAC were evaluated.

The mean anatomic localization errors were (0,67 ± 0,2) mm for navigation to the IAC and (0,71 ± 0,37) mm for navigation to the posterior semicircular canal. The average distances between the MMLE and the resection line were 3,65; 3,36, and 2,0 mm for the lateral, on-the-line, and medial groups, respectively. Direct contouring of structures at risk does not take into account the localization error, nor does it provide reliable navigational information. A novel indirect contouring concept that takes into account the localization error (the safety corridor method) was therefore introduced.