Round Window Membrane Motion Induced by Bone Conduction Stimulation at Different Excitation Sites: Methodology of Measurement and Data Analysis in Cadaver Study

Abstract

Objectives: The aim of this study was to investigate the following: (1) the vibration pattern of the round window (RW) membrane in human cadavers during air (AC) and bone conduction (BC) stimulation at different excitation sites; (2) the effect of the stimulation on the fluid volume displacement (VD) at the RW and compare the VD between BC and AC stimulation procedures; (3) the effectiveness of cochlear stimulation by the bone implant at different excitation sites. Design: The RW membrane vibrations were measured by using a commercial scanning laser Doppler vibrometer. The RW vibration amplitude was recorded at 69 measurement points evenly distributed in the measurement field covering the entire surface of the RW membrane and a part of the surrounding bony surface. RW vibration was induced first with AC and then with BC stimulation through an implant positioned at two sites. The first site was on the skull surface at the squamous part of the temporal bone (implant no. 1), a place typical for bone-anchored hearing aids. The second site was close to the cochlea at the bone forming the ampulla of the lateral semicircular canal (implant no. 2). The displacement amplitude (dP) of the point P on the promontory was determined and used to calculate the relative displacement (drRW) of points on the RW membrane, drRW = dRW − dP. VD parameter was used to analyze the effectiveness of cochlear stimulation by the bone implant screwed at different excitation sites. Results: RW membrane displacement amplitude of the central part of the RW was similar for AC and BC implant no. 1 stimulation, and for BC implant no. 2 much larger for frequency range >1 kHz. BC implant no. 2 causes a larger displacement amplitude of peripheral parts of the RW and the promontory than AC and BC implant no. 1, and BC implant no. 1 causes larger than AC stimulation. The effect of BC stimulation exceeds that of AC with identical intensity, and that the closer BC stimulation to the otic capsule, the more effective this stimulation is. A significant decrease in the value of VD at the RW is observed for frequencies >2 kHz for both AC and BC stimulation with BC at both locations of the titanium implant placement. For frequencies >1 kHz, BC implant no. 2 leads to a significantly larger VD at the RW compared to BC implant no. 1. Thus, the closer to the otic capsule the BC stimulation is located, the more effective it is. Conclusions: Experimental conditions allow for an effective acoustic stimulation of the inner ear by an implant screwed to the osseous otic capsule. The mechanical effect of BC stimulation with a titanium implant placed in the bone of the ampulla of the lateral semicircular canal significantly exceeds the effect of an identical stimulation with an implant placed in the temporal squama at a conventional site for an implant anchored in the bone. The developed research method requires the implementation on a larger number of temporal bones in order to obtain data concerning interindividual variability of the observed mechanical phenomena.

Detailed Analysis of the Space between the Semicircular Canals for the Purpose of Direct Bone Conduction Stimulation of the Inner Ear

“…The results of the mentioned studies [Kwacz et al, 2019;Lachowska et al, 2019;Niemczyk et al, 2020] present a potential of new stimulation sites that could be introduced in the field of BC hearing rehabilitation. However, both new BC stimulations sites involved drilling the bone near the SC, which might be questionable in terms of the safety of this procedure.…”

“…The distance plays an important role, and the shorter the distance is the more efficient the stimulation is [Stenfelt et al, 2000;Stenfelt and Goode, 2005;Eeg-Olofsson et al, 2008, 2013Kwacz et al, 2019;Lachowska et al, 2019;Niemczyk et al, 2020]. Kwacz et al [2019], Niemczyk et al [2020], and Lachowska et al [2019] demonstrated that BC vibrations applied directly to the otic capsule seem to be more efficient than if applied outside the petrous part of the temporal bone. The authors sought to investigate the potential benefits of the proposed alternative stimulation sites, comparing them to the standard BC implants with stimulation applied to the squamosal portion of the temporal bone.…”

“…The first one is in the direct proximity of the ampulla of the lateral SC and the second one is between the superior and lateral SC, which is, to be more precise, the area located more medially, slightly above the lateral SC in the space between the crura of the superior SC. Overall, the second location has been proven to be the most efficient in terms of the transmission of vibratory energy both in cadavers [Kwacz et al, 2019;Niemczyk et al, 2020] and in patients [Lachowska et al, 2019].…”

“…However, until recently, all of these locations described in the literature were situated outside of the real otic capsule with no direct contact with its bone. In recent studies by Kwacz et al [2019] and Niemczyk et al [2020] performed in cadavers and one by Lachowska et al [2019] performed in living humans, 2 new sites for the BC titanium implant were proposed to directly stimulate the cochlea from the closest possible distance, which was the bone forming the ampulla of the lateral semicircular canal (SC) and the bone between the superior and lateral SC. The 2 locations can be approached by the otosurgeon, which was shown in these studies.…”