Dynamic insertion study on cochlear implant electrodes with x-ray microscopy visualization.

Background: The movements of the electrode cable of a cochlear implant inside the cochlea during the insertion procedure generally are invisible, even in temporal bone experiments. Yet, the development of new designs of electrodes, their positioning near to the modiolus etc, requires an exact knowledge on the dynamic behaviour of the electrodes inside the cochlea. The exclusive method for looking through the undamaged cochlea walls is by x-ray technique. Methods: A specific x-ray tube with a focal spot size of barely 5 mum allows useful direct magnifications of more than 20-30 times, We performed temporal bone experiments with this tube and studied the behaviour of conventional and perimodiolar electrode cables of a MedEl-CI during the insertion procedure under fluoroscopic viewing. Results: The high resolution imaging revealed the influence of stiffness of the electrode cable on the insertion depth of a conventional electrode. if the angle of the direction of insertion was too steep, the resulting kinking of the cable at the medial wail of the basal turn was instantly visible. The direct visualisation of the movements of the cable was useful in optimizing the design and stiffness of various prototypes of perimodiolar electrodes for a deep insertion. Conclusions: The dynamic examination technique disclosed that the stiffness of actual electrode cables is not optimally adapted to the form of the cochlear cavity for deep insertion. The non-destructive visualisation technique will facilitate the development of advanced electrode design, especially for various concepts of perimodiolar electrodes.