Mechanical Aspects of the Round Window Stimulation

Abstract

During the last years, the round window (RW) has become a well established application position for active middle ear implants¹. The coupling condition between actuator and round window membrane (RWM) is the critical point regarding the lasting function of the reconstruction. A preload force is needed to maintain sound transmission dealing with an unilateral contact. In this work, the RW stimulation is examined from the mechanical point of view. Based on laboratory experiments and computational simulations, the effects of parameter variations on the dynamical behaviour of the reconstructed ear are revealed in sensitivity analyses and case studies.

Force-displacement measurements are carried out in order to capture the nonlinear stiffening and the relaxation behaviour of the RWM. Its characteristics are quantifed by mechanical parameters based on visco-elastic models². A simplified mechanical multibody model is used to simulate the transfer of sound in the natural ear. Hereby, the transmission ratio between oval window and RW has to be taken into account. In case of otosclerosis, stiffening of the annular ring leads to alterations in the transfer behaviour. A hearing loss is observed mainly in the low frequency range, whereas increased hearing sensation may occur in the higher frequency range.

With free floating actuators, vibration is transmitted via the actuator housing. The working principle is based on the inertia effect of an internal seismic mass. As an example, the Floating Mass Transducer (FMT) is presented, which is acting as a force transducer. In the not implanted case, the actuator exhibits freqency dependent spatial motions. For assessing the dynamics of the reconstructed ear, the natural structure and the actuator have to be considered as a whole. Based on a multibody model, various influences are investigated by means of virtual experiments, e.g. the preload force, the actuator suspension and the intermediate layer in the contact area. The preload force leads to stiffening of the natural system due to its nonlinear behaviour. As a consequence, resonances are shifted to higher frequencies and low frequency amplitudes are reduced. With increasing preload force, the dynamic force amplitude transmittable without lift-off in the contact area is increased. The corresponding maximum level of equivalent sound pressure is determined by the mechanical properties of the natural structure, but not by not the actuator. The suspension of the actuator housing should be designed to be as compliant as possible in order to maintain the mobility of the actuator during stimulation and to preserve the preload force during large quasistatic deformations of the RWM. Increased motion transfer is observed in the higher frequency range in case of additional damping in the intermediate layer.