Effectiveness of Bone Conduction Stimulation Applied Directly to the Otic Capsule Measured at Promontory: Assessment in Cadavers

Directional sensitivity of bone conduction stimulation on the otic capsule in a finite element model of the human temporal bone

Sound transmission to the human inner ear by bone conduction pathway with an implant attached to the otic capsule is a specific case where the cochlear response depends on the direction of the stimulating force. A finite element model of the temporal bone with the inner ear, no middle and outer ear structures, and an immobilized stapes footplate was used to assess the directional sensitivity of the cochlea. A concentrated mass represented the bone conduction implant. The harmonic analysis included seventeen frequencies within the hearing range and a full range of excitation directions. Two assessment criteria included: (1) bone vibrations of the round window edge in the direction perpendicular to its surface and (2) the fluid volume displacement of the round window membrane. The direction of maximum bone vibration at the round window edge was perpendicular to the round window. The maximum fluid volume displacement direction was nearly perpendicular to the modiolus axis, almost tangent to the stapes footplate, and inclined slightly to the round window. The direction perpendicular to the stapes footplate resulted in small cochlear responses for both criteria. A key factor responsible for directional sensitivity was the small distance of the excitation point from the cochlea.

Estimating vibration artifacts in preclinical experimental assessment of actuator efficiency in bone-conduction hearing devices

OBJECTIVES

Test feasibility of a means to distinguish artifact from relevant signal in an experimental method for pre-clinical assessment of bone conduction (BC) stimulation efficiency based on measurement of intracochlear pressure (ICP).

METHODS

Experiments were performed on fresh-frozen human temporal bones and cadaver heads. In a first step, fiber optic pressure sensors inserted into the cochlea through cochleostomies were intentionally vibrated to generate relative motion versus the stationary specimen, and the resulting ICP artifact recorded, before and after attaching the sensor fiber to the bone with glue. In a second step, BC stimulation was applied in the conventional location for a commercial bone anchored implant, as well as two alternative locations closer to the otic capsule. Again, ICP was recorded and compared with an estimated artifact, calculated from the previous measurements with intentional vibration of the fiber.

RESULTS

Intentional vibration of the sensor fiber creates relative motion between fiber and bone, as intended, and causes an ICP signal. The stimulus does not create substantial promontory vibration, indicating that the measured ICP is all artifact, i.e. would not occur if the sensor were not in place. Fixating the sensor fiber to the bone with glue reduces the ICP artifact by at least 20 dB. BC stimulation also creates relative motion between sensor fiber and bone, as expected, from which an estimated ICP artifact level can be calculated. The ICP signal measured during BC stimulation is well above the estimated artifact, at least in some specimens and at some frequencies, indicating "real" cochlear stimulation, which would result in an auditory percept in a live subject. Stimulation at the alternative locations closer to the otic capsule appear to result in higher ICP (no statistical analysis performed), indicating a trend towards more efficient stimulation than at the conventional location.

CONCLUSIONS

Intentional vibration of the fiber optic sensor for measurement of ICP can be used to derive an estimate of the artifact to be expected when measuring ICP during BC stimulation, and to characterize the effectiveness of glues or other means of reducing the artifact caused by relative motion of fiber and bone.

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

INTRODUCTION

Until recently, all locations for bone conduction (BC) stimulator described in the literature were situated outside of the real otic capsule. In recent studies 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. They proved to be the most efficient in terms of transmission of vibratory energy into the inner ear and could be introduced in the field of BC hearing rehabilitation. To the best of our knowledge the anatomy of the space between SC has not been studied so far. However, screwing the BC implant into the proposed new locations directly at the otic capsule and drilling the bone near the SC cast doubt on the safety of this procedure. In this study we aimed to present a detailed analysis of the anatomy of the otic capsule, especially as regards the space between the SC that seems to be safer.

METHODS

Sixteen fresh frozen cadaveric temporal bones scanned with micro-computed tomography and analyzed using the multiplanar reconstruction option. The anatomy of the space between the SC was analyzed in detail for the purpose of direct BC stimulation of the inner ear.

RESULTS

At least 3 mm of bony tissue is available above the bony space between the crura of the superior SC above the lateral SC, where the new location for the titanium BC implant is proposed. As regards the limitations of the length of screw the BC implant to be screwed, the smallest distance is at least 4 mm of bone thickness.

CONCLUSIONS

The bone between the crura of the superior SC is the best placement to screw the BC implant directly to the otic capsule. The implant direction should be parallel to the plane of the lateral SC. This location, the direction, and the limitation of the screw length of the BC implant to a maximum of 7 mm present the lowest potential risk of damage to the inner ear.