Implantable hearing aids are gaining importance for the treatment of sensorineural hearing loss and also for mixed hearing loss. The various hearing aid systems, combined with different middle ear situations, give rise to a wide range of different reconstructions. This article attempts to summarize the current knowledge concerning the mechanical interaction between active middle ear implants (AMEIs) and the normal or reconstructed middle ear. Some basic characteristics of the different AMEIs are provided in conjunction with the middle ear mechanics. The interaction of AMEIs and middle ear and the influence of various boundary conditions are discussed in more detail.

Keywords

Active middle ear implant

Transfer function

Vibroplasty

Middle ear

Reconstruction

Ossicular chain

Key points

Introduction

Implantable hearing aids are gaining importance not only for the treatment of sensorineural hearing loss but also for mixed hearing loss. The various hearing aid systems, combined with different middle ear situations, give rise to a wide range of different reconstructions. The actuators of the implantable hearing aids, also referred to as active middle ear implants (AMEIs), and the middle ear form one mechanically interacting system (the term AMEI is used synonymously for the actuator and the complete implantable hearing aid). Understanding the mechanical characteristics of this system and the interactions is a prerequisite for explaining and improving the results of active middle ear reconstructions.

The tympanic membrane and the connected ossicular chain with joints and ligaments are the natural part of this mechanical system. In recent years, knowledge about the normal function of the middle ear has improved thanks to optical laser Doppler vibrometry (LDV) measurements on animal and temporal bone models. The frequency-dependent 3-dimensional vibration patterns of the tympanic membrane and ossicular chain have been demonstrated in many publications and are widely accepted. In addition, computer models have been developed to simulate the sound transfer function (STF) under different conditions. The model simulations provide details of the complex vibration patterns and the data of the frequency specific cochlea input.

The AMEIs form the artificial part of the combined mechanical system. These devices were primarily developed for the healthy middle ear and the intact ossicular chain to replace conventional hearing aids in special cases such as external ear canal problems. Their use in chronic otitis media was not in the range of indications for implantable hearing devices for several medical and mechanical reasons. Colletti1 was the first surgeon to place an AMEI on the stapes head and in the round window (RW) niche. Experiments on coupling implantable hearing aids together with passive middle ear prosthesis were performed during the same time period.2 Consequently, the classical indication for implantable hearing devices was extended to chronic otitis media and reconstructed ossicular chain. The advantages are obvious. Although in cases of passive middle ear prosthesis, the tympanic membrane is the driving force for the reconstructed ossicular chain, after insertion of an AMEI, the power comes from the device itself. The tympanic membrane is no longer necessary for the sound transfer to the ossicular chain because, in many cases, insufficient function of the pathologically changed tympanic membrane seems to explain poor hearing results. AMEIs can also be considered to be a solution for dysfunction of the tympanic membrane in cases of middle ear effusion. However, apart from this advantage, there are many unanswered questions concerning AMEI application from the biomechanical point of view:

This article does not intend to provide answers to all the questions but rather will attempt to summarize the current knowledge concerning the mechanical interaction between AMEIs and the normal or reconstructed middle ear. Some basic characteristics of the different AMEIs are provided in conjunction with the middle ear mechanics. The interaction of AMEIs and middle ear and the influence of various boundary conditions are discussed in more detail and presented according to the different surgical situations.

Basics of sound transfer of the middle ear—present knowledge

The normal middle ear is the reference system whose performance AMEIs have to reach and surpass. The intact middle ear consists of the tympanic membrane, the 3 ossicles (malleus, incus, stapes) with ligaments and joints, and the air-filled tympanic cavity. This mechanical system is designed to transfer the sound waves from the external ear canal into mechanical vibrations of the tympanic membrane and ossicular chain. The vibration of the stapes results at least in the traveling wave of the inner ear. Because of the impedance difference between air and fluid, the function of the middle ear can be considered to couple the sound energy of the air to the inner ear and match the impedance difference as well. This function is mainly performed by the hydraulic factor between the tympanic membrane and the stapes footplate. The large area (90 mm2) of the tympanic membrane compared with the relatively small area of the stapes footplate (3 mm2) creates the pressure amplification of about 22 dB around the resonant frequency of the middle ear (1 kHz).3 Other factors in sound transmission, such as the catenary factor of the tympanic membrane or the lever ratio of the chain, are marginal.

This well-known common knowledge about middle ear function has been supplemented by new information in the past 20 years obtained from LDV measurements on temporal bones and calculations using finite element models (FEMs). Applying both these methods provided new insights into the vibration mode of the tympanic membrane and ossicular chain.4–7