Bone Conduction Stimulation Applied Directly to the Otic Capsule: Intraoperative Assessment in Humans

Aim was to investigate the innovative method of direct acoustic bone conduction (BC) stimulation applied directly to the otic capsule and measured intraoperatively by promontory displacement in living humans. The objective was to find the best stimulation site that provides the greatest transmission of vibratory energy in a living human and compare it with the results previously obtained in cadavers.

The measurements were performed in 4 adult patients referred to our department for vestibular schwannoma removal via translabyrinthine approach. The measurements were performed in the operated site. The cadaver data were obtained in our previous study and here they are reanalyzed for comparison. Promontory displacement was measured using a commercial scanning laser Doppler vibrometer. The laser Doppler vibrometer points located on the promontory were used to analyze the promontory displacement amplitude. Cochlear stimulation was induced with BC stimulation through an implant positioned at three sites. The first site was on the skull surface at the squamous part of the temporal bone (BC No. 1), the second at the bone forming the ampulla of the lateral semicircular canal (BC No. 2), and the third between the superior and lateral semicircular canals (BC No. 3). BC No. 2 and BC No. 3 were located directly on the otic capsule. Four frequencies in total were tested (500, 1000, 2000, and 4000 Hz), one at a time.

In patients, the detailed analysis of promontory displacement amplitudes revealed the BC No. 1 magnitude to be the smallest and significantly different from BC No. 2 and No. 3 at all measured frequencies. Transmission of vibratory energy at BC No. 2 and BC No. 3 was the most effective and similar with a small and insignificant difference at 500, 1000, and 4000 Hz, and a significant difference at 2000 Hz. The results observed in cadavers were similar to those in living humans. However, a few differences were observed when comparing patients and cadavers. Small and insignificant differences were found for BC No. 1. Almost the same results were obtained for BC No. 2 and BC No. 3 in cadavers as in living humans, with only BC No. 3 measurements results at 500 Hz at the limit of statistical significance, with no other significant differences observed.

The results of this study indicate that the promontory vibration amplitude increases when the BC stimulation location approaches the cochlea. BC No. 1 stimulation located on the squama caused overall smaller displacement than both BC No. 2 and No. 3 screwed to the ampulla of the lateral semicircular canal and to the midpoint between the semicircular canals, respectively. In our opinion, the results of BC stimulation applied directly to the otic capsule present a potential new stimulation site that could be introduced in the field of BC hearing rehabilitation.