Objective assessment of stapedotomy surgery from round window motion measurement

As prostheses and techniques related to stapes surgery develop and improve, there is a need to assess the functional outcomes of the surgery objectively. This study provides a bench test method to assess the functional results of stapes surgery by measuring volume displacement at the round window (RW), which is closely related to pressure propagation of the travelling wave inside the cochlea and thus to hearing.

Motion of the RW membrane in fresh temporal bones was measured using a scanning laser Doppler interferometry system for normal and reconstructed conditions, and the performance of the reconstruction with stapes surgery was quantitatively assessed by comparison of the volume displacements at the RW between the two conditions. To obtain optimal measurements, reflectivity of the laser beam of the scanning laser Doppler interferometry system was improved by retroreflective beads coated onto the surface of the RW, and orientation of the RW membrane relative to the laser beam was obtained using micro-computed tomography imaging.

From measurements in 12 temporal bones, difference in the RW volume displacement between normal ears and ears reconstructed with stapes surgery was approximately 15 dB below 2 kHz and approximately 10 dB above 4 kHz, which was comparable with air-bone gaps in patients after stapes surgery. Two different sizes of the stapes prostheses were also tested (n = 3), and a tendency toward a better outcome with a larger diameter was found.

The method developed in this study can be used to assess various prostheses and surgical conditions objectively in controlled laboratory environments. It may also have potential for providing ways to assess other middle- and inner-ear surgeries, and to study other aspects of hearing science.