Evaluation of Vibrant® Soundbridge™ positioning and results with laser doppler vibrometry and the finite element model

Abstract

The etiology of hearing loss originates from genetic factors and includes several other events including infections, working or living environment, as well as several endocrine and metabolic disorders. The Vibrant^® Soundbridge^™ (VSB) is an implantable hearing aid whose floating mass transducer (FMT) is attached to the long process of the incus. The device is used for pure sensorineural hearing loss with an intact middle ear. Variations in the manner of attachment may occur. Knowledge of the impact of such variations on the overall device performance may guide towards optimal transducer attachment during surgery. A mechanical modelling of the ear was first reported by von Békésy and indicated that the tympanic membrane (TM) moves as a stiff plate, and that the mallear and incudal ligaments act as a rotation axis for the ossicular chain at low frequencies. Experimental investigations and simulations with the model yield the same main results. The first fitting situation, where the FMT floats freely in the middle ear, provides by far the worst possible results. Contact to the stapes supra-structure of the FMT is necessary for optimal performance of the FMT. The mastoid specimen preserves its acoustic properties that have been shown to be similar to those in the vital human ear, under these conditions. Properly coupling the electromagnetic transducer to the ossicles can be difficult and it requires a certain degree of experience. A finite-element model (FEM) is useful for functional evaluation of the VSB since it enables easy modelling of the complicated middle ear structures and simulation of their dynamic behavior which makes it easy to understand it in detail without experiments.

Keywords: Vibrant Soundbridge; doppler effect; ear; implants; incus; middle.

Figure 1

Schematic representation of the FEM. FEM, finite element model.

Figure 2

Schematic representation of the FEM. FEM, finite element model.

Figure 3

Schematic representation of the FEM. FEM, finite element model.

Figure 4

Positioning of the FMT (3 different positions) in regard to the stapes structure during the temporal bone experiments. FMT, floating mass transducer.

Figure 5

Change in transfer function when direction of excitation changes from no contact to the stapes to 0° and 45°-60° (on temporal bone). FMT, floating mass transducer.

Figure 6

Scheme of the FMT attached to the stapes head with direction of excitation of 0° (black line), 15° (blue line) and 45° (red line). (modified from the original image, courtesy of Med-El GmbH).

Figure 7

Change in transfer function when direction of excitation changes to 15° and 45° (on FEM). FEM, finite element model.