Analysis of intersubject variations in intracochlear and middle ear surface anatomy for cochlear implantation

Abstract

Hypothesis: We hypothesize that surface landmarks surrounding the round window typically used to guide electrode placement during cochlear implantation (CI) exhibit substantial variability with respect to intracochlear anatomy.

Background: Recent publications suggest that both atraumatic electrode insertion and electrode location within the scala tympani can affect auditory performance after CI. However, current techniques for electrode insertion rely on surface landmarks alone for navigation, without actual visualization of intracochlear structures other than what can be seen through a surgically created cochleostomy. In this study, we quantify how well the position of intracochlear anatomy is predicted by surface landmarks surrounding the round window.

Methods: Structures representing middle ear surface and intracochlear anatomy were reconstructed in μCT scans of 10 temporal bone specimens. These structures were then reoriented into a normalized coordinate system to facilitate measurement of inter-subject anatomical shape variations.

Results: Only minor intersubject variations were detected for intracochlear anatomy (maximum deviation, 0.71 mm; standard deviation, 0.21 mm), with greatest differences existing near the hook and apex. Larger intersubject variations in intracochlear structures were detected when considered relative to surface landmarks surrounding the round window (maximum deviation, 0.83 mm; standard deviation, 0.54 mm).

Conclusion: The cochlea and its scala exhibit considerable variability in relation to middle ear surface landmarks. While support for more precise, atraumatic CI electrode insertion techniques is growing in the otologic community, landmark guided insertion techniques have limited precision. Refining the CI insertion process may require the development of image-guidance systems for use in otologic surgery.

Figure 1

Shown in the figure are a sagittal (left) and axial (right) cut of a microCT volume of one of the specimens used in this study. Also shown are contours around the manually delineated scala tympani (red) and scala vestibuli (blue).

Figure 2

This image depicts the middle ear surface landmarks (green) surrounding the round window overhang that were used to optimally align individual temporal bone specimen images.

Figure 3

Shown in the figure are 3D renderings of the average shape of the scala tympani and scala vestibuli. The color at each point on the surfaces corresponds to the mean deviation (in mm) of the position of that point on the structure when measured with respect to surgical landmarks, i.e., surface structures surrounding the round window (right column), and when measured with respect to the cochlea (left column).

Figure 4

Shown in the figure are 3D renderings of middle ear surface structures surrounding the round window (green), scala tympani (red) and scala vestibuli (blue) from all 10 volumes registered with respect to the middle ear surface structures. The bottom row shows the average shape from volumes 1-10. Also shown is a 1 mm diameter yellow tube that represents the optimal electrode insertion trajectory in average anatomy.