Navigation-guided opening of the internal auditory canal via the retrosigmoid route for acoustic neuroma surgery: Cadaveric, radiological, and preliminary clinical study

OBJECTIVE: We investigated the usefulness of a microscope-based navigationa l system (Multi Koordinaten Manipulator; Zeiss, Oberkochen, Germany) for re moval of the posterior wall of the internal auditory canal (IAC) via the re trosigmoid route. METHODS: A cadaveric study was performed to assess the navigational localiz ation error for the retrosigmoid approach to the IAC. Computed tomographic findings for 47 acoustic neuroma cases were divided into three groups, on t he basis of the relationship between the labyrinth and the sigmoid-fundus l ine (medial, on the line, or lateral). Furthermore, the shortest distances between the most medial labyrinthine extension 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 th e IAC were evaluated. RESULTS: The mean anatomic localization errors were 0.67 +/- 0.2 mm (95th p ercentile, 1.32 mm) for navigation to the IAC and 0.71 +/- 0.37 mm (95th pe rcentile, 1.68 mm) for navigation to the posterior semicircular canal. The average distances between the most medial labyrinthine extension and the re section line were 3.65, 3.36, and 2.0 mm for the lateral, an-the-line, and medial groups, respectively. Direct contouring of structures at risk does n ot take into account the localization error, nor does it provide reliable n avigational information. A novel indirect contouring concept that takes int o account the localization error (the safety corridor method) was therefore introduced. CONCLUSION: The value of navigational assistance for opening of the IAC is promising but still limited. Further development is required before the cli nical effects of this navigational approach can be evaluated.