Insertion Depth Angle Prediction Accuracy for Cochlear Implant Electrode Array Selection

Anschütz, Lukas Peter; Weder, Stefan; Mantokoudis, Georgios; Kompis, Martin; Caversaccio, Marco; Wimmer, Wilhelm (27 July 2017). Insertion Depth Angle Prediction Accuracy for Cochlear Implant Electrode Array Selection (Unpublished). In: 15th Symposium on Cochlear Implants in Children. San Francisco, USA. 26. - 29. Juli 2017.

Introduction
It is hypothesized that preoperative planning of a specific angular insertion depth for CI electrode arrays
can be achieved by utilizing the correlation between the cochlear diameter and the cochlear duct length.
In practice, however, the result can be distorted mainly for two reasons: wrongly measured cochlear
diameters (i.e., incorrectly aligned image slices to visualize the cochlea) and an uncontrolled insertion
depth of electrode arrays (i.e., difficulty to reproduce the insertion point and depth). The aim of this ex
vivo study was to evaluate the accuracy of insertion angle prediction with a logarithmic equation under
optimal conditions, i.e. optimally aligned imaging slices to measure the cochlear diameter and a
controlled insertion of electrode arrays in human specimen.
Methods
Eight human temporal bones (Thiel conservation) were imaged with a cone-beam computedtomography
scanner (0.33 mm3 voxel size). An oblique slice was placed according to anatomical
landmarks to ensure a reproducible visualization of the basal turn. In the basal turn plane, the maximum
diameter of the cochlea through the round window was measured. Using a logarithmic equation the
linear insertion depth required for an insertion angle of 540° degrees was computed. Lateral wall
electrode arrays were marked at the specific insertion depth and inserted through the round window.
The insertion was stopped when the marks reached the round window membrane. The electrode arrays
were fixed using bone wax to avoid array migration. After the insertion, CBCT images were taken to
assess the insertion outcome, i.e., the angular insertion depth, the implanted scala, and the array
course.
Results
The mean cochlear diameter was 8.9 mm (range, 8.3 mm to 9.4 mm). On average, the estimated linear
insertion depth for a 540° insertion was 24.9 mm (range, 23.2 mm to 26.3 mm). An average angular
insertion depth of 529° (range, 510° to 555°) was achieved. This corresponds to a mean positioning
error of 11° (standard deviation, ±15°). The differences were not statistically significant (p = 0.11, twotailed
Wilcoxon signed rank test). All electrode arrays had a smooth course along the lateral wall and
were fully implanted into the scala tympani.
Conclusion
The presented results demonstrate the possibilities and limitations of insertion angle prediction using a
logarithmic equation. It was possible to place the electrode arrays closely to the aimed target, however,
clinical settings will introduce additional error sources such as bending of the array within the cochlea
and the difficulty to reproducibly stop at the planned position. Regarding the latter, computer-assisted
planning and controlled insertion devices could help to increase the positioning accuracy of CI electrode
arrays.

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