gms | German Medical Science

To determine the feasibility of using pre-operative high resolution CT images as an aid in limiting depth of insertion of cochlear implant electrodes in order to better preserve low-frequency residual hearing. Recent results for hearing conservation with the Nucleus 24 Contour Advance electrode array show that where insertion depths exceed 400 degrees there is a rapid decrease in hearing conservation despite soft-surgery procedures being strictly adhered to. In addition, insertion depth angles vary considerably from 300° to 430° degrees.

Pre-operative CT scans were analysed using rotational tomography software. A two dimensional view of the basal turn of the cochlea was developed by displaying the darkest voxels in a 1.0 mm layer. A characteristic measure of cochlear size was obtained by finding the largest distance between the round window and the lateral wall. Post-operative X-rays were obtained from patients implanted with the Nucleus 24 Contour Advance electrode array using a linear insertion depth of 17mm in order to better preserve residual hearing for electro-acoustic stimulation. These X-rays were analysed using the method of Xu to obtain the insertion depth angles for individual electrodes.

The largest distance between the round window and the lateral wall varied between 8.6 and 10.0 mm (e.g a range of 1.4 mm). The mean variation of insertion depth angle with linear distance for apical electrodes was computed as 48.2°/mm.

Variations in cochlear size may thus account for significant variations in insertion depth angle, approximately 50°/mm. Thus, in small cochleae, further reducing the length of electrode introduced by 1.0 mm may better conserve low-frequency hearing without compromising access to spiral ganglia with characteristic frequencies at and above 500Hz.