In-plane motions of the stapes in human ears

The piston-like (translation normal to the footplate) and rocking-like (rotation along the long and short axes of the footplate) are generally accepted as motion components of the human stapes. It has been of issue whether in-plane motions, i.e., transversal movements of the footplate in the oval window, are comparable to these motion components. In order to quantify the in-plane motions the motion at nine points on the medial footplate was measured in five temporal bones with the cochlea drained using a three-dimensional (3D) laser Doppler vibrometer. It was found that the stapes shows in-plane movements up to 19.1 ± 8.7% of the piston-like motion. By considering possible methodological errors, i.e., the effects of the applied reflective glass beads and of alignment of the 3D laser Doppler system, such value was reduced to be about 7.4 ± 3.1%. Further, the in-plane motions became minimal (≈ 4.2 ± 1.4% of the piston-like motion) in another plane, which was anatomically within the footplate. That plane was shifted to the lateral direction by 118 μm, which was near the middle of the footplate, and rotated by 4.7° with respect to the medial footplate plane.