Analysis of Intersubject Variations in Intracochlear and Middle Ear Surface Anatomy for Cochlear Implantation

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 &mgr;CT 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.