ON THE OPTIMAL COUPLING OF AN IMPLANTABLE HEARING AID – MEASUREMENTS AND SIMULATIONS

The sound transfer is strongly dependent on the coupling of the prosthesis to the ossicles. This holds for both passive but in particular for active implants. Various principles of coupling and different designs of attaching the implant to ossicle are in use showing a different mechanical behavior concerning stiffness and damping. Pressing a driving rod against the ossicles means a unilateral coupling which needs a particular prestress. Due to the nonlinear transfer behavior of the coupling region and ossicular chain, the actual motions, e.g. of actuator, stapes and umbo are dependent on that prestress. An optimal coupling should allow a stapes motion sufficient to compensate a hearing loss, but should avoid a distorted sound transfer or feedback due to sound radiation from ear drum. The static adjustment influences the sound transfer of the dynamical system essentially. To find an optimal adjustment of the actuator, the dynamical behavior of the actuator itself, of the coupling region and the individual ossicular chain has to be regarded. During surgery the stiffness of the individual chain has to be estimated and the configuration when the actuator touches the ossicles has to be detected very carefully.