SummaryThe combination of osmium tetroxide staining and highresolution tomographic imaging using monochromatic X rays allows visualizing cellular structures of the human inner ear, that is, the organ of Corti, the stria vascularis and further soft tissues of the membranous labyrinth, in three-dimensional space with isotropic micrometre resolution. This approach permits to follow the course of nerve fibre bundles in a major part of the specimen and reveals the detailed threedimensional arrangement of individual ganglion cells with distinct nuclei by means of X-ray tomography for the first time. The non-destructive neuron cell counting in a selected volume of 125 μm × 800 μm × 600 μm = 0.06 mm 3 gives rise to the estimate that 2000 ganglion cells are present along 1 mm organ of Corti.

The accuracy of optical tracking was near the resolution of the camera system, whereas the accuracy of electromagnetic tracking was lower. Only optical tracking allows for an application accuracy of considerably less than 1 mm in high-resolution datasets.

We performed approximately 250 operations with different systems and introduced navigation at the lateral skull base and the petrous bone with mechanical, optic, and magnetic digitizers. In these anatomical areas, navigation was used successfully; the technical challenge is greatest at the lateral skull base, however.

Intraoperative three-dimensional (3D) "navigation" has significantly improved patient safety during operative procedures on the paranasal sinuses and the frontal skull base. The ISG Viewing Wand (ISG Technologies, Mississauga, Ontario, Canada) is now used routinely for such procedures in our hospital at the present time. Current use with our radiological and intraoperative protocols has demonstrated a clinical accuracy of 1-2 mm. Initial experience with the ARTMA Virtual Patient (ARTMA Biomedical, Vienna, Austria) has allowed endoscopic 3D navigation by using augmented reality techniques and has been found to be very promising. We present our experience with these systems and discuss the impact of these unique techniques on computer-assisted surgery (CAS) in otorhinolaryngology and head and neck surgery.

Frameless stereotactic procedures crucially depend on the firmness of immobilization. Once registered, shifting of the patient leads to inaccuracy, and the patient registration has to be realigned. To overcome the drawbacks of conventional invasive fixation for neurosurgery and the widely accepted fixation with surgical tape in ENT, the Vogele-Bale-Hohner (VBH) head holder has been developed. It permits rigid, noninvasive fixation of the head by using an individualized dental cast attached to the upper jaw by vacuum. Oral intubation is uncomplicated. In addition, a special registration device providing well defined reference points can be mounted to the mouthpiece. We report the first promising clinical applications of this device.