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.
Key points
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Larger contact area between transducer and ossicular chain improves coupling.
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Ensure a tight and stable contact between the actuator and the ossicle (cartilage, coupling devices with bell or clip mechanisms).
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Floating mass transducer movement axis has to be in an orthogonal (perpendicular) projection to the round window membrane plane.
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Vibroplasty with direct coupling to the inner ear fluid should only be used in cases of noncontaminated middle ear.
| AMEI | Active middle ear implant |
| DACS | Direct acoustic cochlear stimulator |
| eq. | Equivalent |
| FEM | Finite element model |
| FMT | Floating mass transducer |
| GME | Middle-ear pressure gain |
| LDV | Laser Doppler vibrometry |
| METF | Middle ear transfer function |
| PORP | Partial ossicular replacement prosthesis |
| RW | Round window |
| RWM | Round window membrane |
| SPL | Sound pressure level |
| STF | Sound transfer function |
| TF | Transfer function |
| THD | Total harmonic distortion |
| TORP | Total ossicular replacement prosthesis |
| VSB | Vibrant Soundbridge |
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. Colletti 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. 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:
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Are there any differences in the mechanical characteristics of the currently available devices, and if so, how do they affect reconstruction?
- •
What is the best coupling site for an AMEI in the different middle ear situations?
- •
How can the device be coupled to the ossicular chain and what is the best direction of coupling?
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.
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. Colletti 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. 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:
- •
Are there any differences in the mechanical characteristics of the currently available devices, and if so, how do they affect reconstruction?
- •
What is the best coupling site for an AMEI in the different middle ear situations?
- •
How can the device be coupled to the ossicular chain and what is the best direction of coupling?
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 mm 2 ) of the tympanic membrane compared with the relatively small area of the stapes footplate (3 mm 2 ) creates the pressure amplification of about 22 dB around the resonant frequency of the middle ear (1 kHz). 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.
Vibration Mode of the Tympanic Membrane
In the intact middle ear, the tympanic membrane is the driving force for the ossicular chain. The bridge between the membrane and the ossicular chain is the malleus handle. Calculations and experiments have shown that the vibration pattern is frequency-dependent. Although at frequencies less than 1 kHz all points of the tympanic membrane move in-phase (inward-outward motion of the entire membrane), the vibration pattern greater than 1 kHz becomes more and more complex. Between 1 and 4 kHz, lower-order modes predominate, with an example being a butterfly vibration pattern and other modes similar to those of microphone membranes. Greater than 4 kHz, the vibration pattern demonstrates multiple obviously independently moving spots of the tympanic membrane ( Fig. 1 ). From new stroboscopic measurements, Rosowski and colleagues concluded that the vibration pattern in the highest frequency range results from an interaction of the modal motion with traveling waves at the surface of the membrane.
Vibration Mode of the Ossicular Chain
The vibration pattern of the ossicular chain is dominated by the tympanic membrane movement. Because of the translational motion of the entire membrane, up to 1 kHz, the malleus and incus rotate around an axis between the short process of incus and the anterior mallear ligament ( Fig. 2 ). The resulting movement of the stapes is pistonlike. Above the first resonant frequency, the different modes of the tympanic membrane lead to a 3-dimensional vibration pattern of the chain. The rotational axis of the malleus and incus is shifted in a manner that is frequency-dependent to their upper part, the incudomalleolar joint and the incudostapedial joint become more mobile, and the movement of the stapes becomes a combination of pistonlike and tilting movements as well as rotational components ( Fig. 3 ).
In the high-frequency range (greater than 4 kHz), the tilting movements can be in the same order of magnitude as the pistonlike movements. In animal experiments, it has been demonstrated that tilting movements also generate hearing sensations. After direct mechanical stimulation of the stapes with a transducer, compound action potentials were measured for tilting movements as well as for pistonlike movements, although the tilting movements were smaller ( Fig. 4 ).
Middle Ear Transfer Function
The middle ear transfer function (METF) has been calculated and measured in several models. It describes, in general, the relation between the energy transmitted to the inner ear to the energy input in the external ear canal. Common definitions of the METF are (1) displacement of the stapes footplate related to the sound pressure at the tympanic membrane, (2) velocity of the stapes footplate related to the sound pressure at the tympanic membrane, or (3) sound pressure in the vestibulum related to the sound pressure at the tympanic membrane also named middle-ear pressure gain (GME). In this article, the first definition is usually meant when METF is used. Fig. 5 shows the displacement of the stapes footplate related to the sound pressure at the tympanic membrane over frequency. The measurements were performed with LDV in temporal bones. The GME can be obtained from measurements of sound pressure in the vestibulum with hydrophones ( Fig. 6 ). In the low-frequency range, GME increases with 6 dB per octave to a maximum of 23.5 dB at 1.2 kHz. Above this frequency, the gain decreases with a slope of 6 dB per octave. The measurements demonstrated that the amplification of the middle ear is only about 22 dB in the frequency range between 0.5 and 1.5 kHz.
To characterize the interaction of AMEIs with the middle ear, observation of the vibration mode and data from STF are required. STF provides information on the overall energy transfer from the device to the middle ear. Vibration modes are useful for optimizing coupling and stimulation characteristics. Of course, the mechanical behavior of the implantable hearing aid itself plays an important role.
Mechanical characteristics of implantable hearing devices
A common characteristic of all implantable hearing aids is that the transducer is coupled to the ossicular chain or directly to the inner ear fluid. Consequently, implantable transducers have to fulfill specific mechanical, biological, and surgical requirements.
Mechanical requirements are mainly determined by the middle ear mechanics. Based on the knowledge of middle ear mechanics from experimental data and simulation models, the input impedance and motion of the ossicular chain are known. Thus, displacement and force characteristics for different points of the ossicular chain to which transducers may be coupled are provided. Fig. 7 B shows the displacement of the stapes footplate (center point in direction of the footplate normal) at 1 Pa (equal to 94 dB sound pressure level (SPL)) sound pressure excitation at the tympanic membrane (green curve and scale on the left). The blue curve in Fig. 7 A shows the required force acting on the stapes to produce the same displacement. That means that a transducer coupled to the stapes has to reach these values of force and displacement to produce a hearing sensation equivalent (eq.) to 94 dB SPL. Rosowski and colleagues defined the eq. ear canal sound pressure as a measure to compare the performance of AMEI: the quotient of the electrovibrational transfer function (TF) (ie, the AMEI-aided METF) and the normal METF. It yields the eq. SPL that the transducer can generate in the middle ear for a certain driving voltage or current. The actuator performance should be constant at the frequency range of speech (about 90–6000 Hz) or even increase with frequency, to account for high-frequency hearing loss.
Further requirements and limitations for AMEIs are low-energy consumption, distortion-free transfer characteristics, safety regulations concerning voltage and current, insensitivity to electromagnetic fields of electronic devices, risk-free CT and MRI examination, limited space, surgical approach, biocompatibility, biostability, implantation with hearing preservation, and residue-free explantation, to name just the most important ones. Because it is not possible to meet all of these requirements at the same time, developing implantable transducers is a matter of finding a reasonable compromise and this has led to different systems of implantable transducers. An overview of AMEIs is presented in Table 1 . The table also includes systems used in the past and systems not yet in clinical use but at an advanced development stage. Devices may be distinguished by indication (sensorineural or combined hearing loss), the kind of device fixation (rigidly supported at the mastoid vs free floating), or the type of actuator (piezoelectric vs electrodynamic) ( Table 3 ). The boundaries between the groups are mostly fluid.
| System | Company, Institution | Implantation | Actuator | Coupling | Published Studies | |||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
| Semi | Total | Electrodynamic | Piezoelectric | Umbo, TM | Malleus, Incus Body | Incus Long Process | Stapes | OW | RW | Cochlea | Experimental | Clinical | ||
| Soundbridge | Med-El | x | x | x | x | Head, footplate | x | x | x | x | x | |||
| Esteem | Envoy Medical Corp. | x | x | Head | x | x | ||||||||
| (MET), Carina | Otologics | (x) a | x | x | x | x | Head, footplate | x | x | x | x | |||
| C-DACS b | Cochlear | x | x | x | x | x | x | x | ||||||
| DACS-PI b | Sonova | x | x | x | x | x | x | |||||||
| Rion Device E-type | Japan | x | x | Head | x | |||||||||
| TICA | Implex, Universität Tübingen, Germany | x | x | x | x | x | x | |||||||
| DDHS | Soundtec | x | x | x | Head | |||||||||
| SIMEHD | Cleveland, OH, USA | x | x | x | x (Animal) | |||||||||
| AMEI Tübingen | Universität Tübingen, Germany | x | x | x | x | |||||||||
| Ear Lens | EarLens Corp., USA | x | x | x | x | |||||||||
a MET is semi-implantable, successor to Carina total implantable.
Nearly all devices were initially developed for pure sensorineural hearing loss. The indication was then subsequently extended to conductive hearing loss. This trend boosted the usage of the devices. With application of prostheses and coupling elements, the systems now cover all types of tympanoplasty. In conjunction with the floating mass transducer (FMT) and its different types of application, the term “vibroplasty” is currently widely used. At the Middle Ear Mechanics in Research and Otology meeting in 2006, Huber also proposed a classification of the different types of applications of AMEIs. The authors would like to implement this suggestion and propose a surgical classification of vibroplasty ( Table 2 ). This classification is similar to the definition of different types of tympanoplasty and is applicable for all types of AMEIs. The classification is mainly based on medical and surgical criteria. The main types thus represent the surgical approach and the different levels of severity and risks (infection, acoustic trauma). The subgroups take into account the attachment point and the load impedance for the actuator. Both conditions are important for successful vibroplasty. Further subgroups may be added once data are available that necessitate refined differentiation.
| Vibroplasty | Description | Characteristics | Subtype | Coupling Location | AMEI |
|---|---|---|---|---|---|
| Type A | Coupling to the intact ossicular chain | Driving the whole and intact chain | A1 | Tympanic membrane, malleus handle | DDHS, Ear lens |
| A2 | Incus body, malleus head | Soundbridge, Carina, SIMEHD | |||
| A3 | Incus long process | Soundbridge, Carina | |||
| A4 | Stapes footplate | (Carina) a | |||
| Type B | Coupling to interrupted/reconstructed chain | Driving a part of the ossicular chain | B1 | Incus body | TICA, Soundbridge |
| B2 | Incus long process, stapes head | Rion, TICA, Soundbridge, Carina, Esteem | |||
| B3 | Stapes footplate | Soundbridge, Carina | |||
| Type C | Membrane coupling to the inner ear | Driving the inner ear fluid via membrane | C1 | RW | Soundbridge, Esteem, Carina, DACS, AMEI T. |
| C2 | Oval window with membrane | ||||
| C3 | Third window | Soundbridge | |||
| Type D | Inner ear fluid coupling | Driving the inner ear fluid directly | D1 | Via RW | |
| D2 | Via oval niche | Soundbridge, DACS | |||
| D3 | Via third window |
| Actuator type | Ideal displacement driven actuator | Real actuator | Ideal force driven actuator |
| Force characteristics | No limitation (ie, the actuator can generate more force than necessary) | Due to limitations in size, mass and energy usually a resonance characteristics with less force at low or high frequencies | The achievable force of the actuator limits the eq. SPL that the actuator can generate (in the whole frequency range) |
| Load impedance | Insensitive to the load impedance; actuator characteristics is independent from the attached structure | The achievable force and the frequency characteristics depend on the characteristics of the individual middle ear and the coupling point | Sensitive to the load impedance; the attached structure determines the actuator characteristics |
| Displacement characteristics | Usually limited displacement at low frequencies due to size and design limitations and safety considerations concerning excitation voltage | No limitation (ie, the displacement of the actuator in unloaded condition is larger than necessary) | |
| Actuator examples | Stacked piezo actuator | Most devices in Table 1 | Electrodynamic actuator with separate coil and magnet (eg, EarLens ) |
The status of the middle ear undoubtedly affects the performance of the actuator. A reduced load impedance of a partial ossicular chain is usually an advantage that improves the system performance. Studies with the MET showed a 10- to 20-dB increase in the AMEI-aided METF when only stapes and inner ear fluid were present. With a partial ossicular chain, the required force of the actuator decreases, whereas the required displacement remains the same. An exception is direct cochlea fluid excitation, which might require greater displacements depending on the piston diameter (for details, see Mechanical aspects of type D vibroplasty). It will also be more difficult to use the direct acoustic cochlear stimulator (DACS) off-label (ie, to drive the ossicular chain), demanding greater force. The DACS is more attributed to the force-driven (ie, force-limited devices).
Many devices are rigidly supported with one end attached to a bony structure (cranium, tympanic cavity wall, ear canal wall). The other end is attached to the ossicular chain or the inner ear fluid. The sole exception of a free-floating actuator is the Vibrant Soundbridge (VSB)’s FMT that is only coupled to the ossicular chain. Electrodynamic actuators with separated coil and magnet are in between, because the coil is usually fixed in the ear canal and the magnet is attached to the ossicular chain. The rigidly supported transducers require more care for placement and the surgical procedure is usually more difficult. However, there is more space available in the mastoid than in the tympanic cavity, which allows for stronger transducers. The systems with separated coil and magnet can combine the advantages of easy placement and bigger coils and magnets. Their big drawback, that the performance heavily depends on the distance between coil and magnet, however, outweighs these advantages.
The devices can also be differentiated by the type of transducer: piezoelectric versus electrodynamic (electromagnetic), the latter with combined or separated coil and magnet. Depending on the specific design, piezoelectric transducers usually reach higher forces but are more limited in displacement. The opposite usually holds for electrodynamic transducers. If the operating range between the 2 idealized actuators, force-driven and displacement driven, is defined, nearly all current devices are within this range. In Fig. 8 , the characteristics of the FMT can be seen as a typical example. At lower frequencies, the limited force of the FMT restricts the performance, whereas greater than 1 kHz, force and displacement are limiting factors. The force and displacement limits can be obtained from Fig. 7 A, B, where forces and displacements in the middle ear are opposed to forces and displacements of the FMT.
