In Vitro Model for Intraoperative Adjustments in an Implantable Hearing Aid (MET)

Optimum Coupling of an Active Middle Ear Actuator: Effect of Loading Forces on Actuator Output and Conductive Losses

Introduction:

The desired outcome of the implantation of active middle ear implants is maximum coupling efficiency and a minimum of conductive loss. It has not been investigated yet, which loading forces are applied during the process of coupling, which forces lead to an optimum actuator performance and which forces occur when manufacturer guidelines for coupling are followed.

Methods:

Actuator output was measured by laser Doppler vibrometry of stapes motion while the actuator was advanced in 20 μm steps against the incus body while monitoring static contact force. The occurrence of conductive losses was investigated by measuring changes in stapes motion in response to acoustic stimulation for each step of actuator displacement. Additionally, the electrical impedance of the actuator was measured over the whole frequency range at each actuator position.

Results:

Highest coupling efficiency was achieved at forces above 10 mN. Below 1 mN no efficient coupling could be achieved. At 30 mN loading force, which is typical when coupling according to manufacturer guidelines, conductive losses of more than 5 dB were observed in one out of nine TBs. The electrical impedance of the actuator showed a prominent resonance peak which vanished after coupling.

Conclusion:

A minimum coupling force of 10 mN is required for efficient coupling of the actuator to the incus. In most cases, coupling forces up to 100 mN will not result in clinically relevant conductive losses. The electrical impedance is a simple and reliable metric to indicate contact.

Vibroplasty combined with tympanic membrane reconstruction in middle ear ventilation disorders

Although the Vibrant Soundbridge is one of the most frequently used active middle ear implants, data regarding how middle ear ventilation disorders may affect the transmission behavior of its floating mass transducer are still insufficient. Studies involving coupling the floating mass transducer to the stapes head are particularly lacking. This temporal bone study evaluated the influence of simulated middle ear ventilation disorders on the middle ear transfer function in the reconstructed middle ear. The middle ear transfer function was measured using Laser Doppler Vibrometry after vibroplasty onto the stapes head, with or without tympanic membrane reconstruction. Middle ear ventilation disorders were simulated through changes in static pressure via the external ear channel with a maximum pressure of +3 kPa. Slice thickness of tympanic membrane reconstruction material was measured using micro-CT. When the reconstructed ossicular chain and the reconstructed tympanic membrane were mechanically excited by the floating mass transducer under conditions of ambient static pressure, the transmission behavior was found to be independent of the type of tissue used. Increase in static pressure up to +3 kPa caused maximum low frequency transmission loss of 15 dB when elastic grafts were used and 5 dB when stiff tissue was inserted. At high frequencies, measured loss of up to 5 dB was relatively independent of the tissue stiffness. Increase in static pressure led to displacement of the tissues towards the vestibulum and caused stiffening, especially of the annular ligament. Stiffening-induced transmission losses were mainly found at low frequencies and could not be compensated by the floating mass transducer in this range. Above 1300 Hz, the continuous force spectrum of the actuator sufficiently protected against loss of amplitude. To minimize postoperative transmission loss due to persisting ventilation disorders, choosing a very stiff tympanic membrane reconstruction material seems to be appropriate.

Implantable hearing devices

Combined hearing loss is an essential indication for implantable hearing systems. Depending on the bone conduction threshold, various options are available. Patients with mild sensorineural deafness usually benefit from transcutaneous bone conduction implants (BCI), while percutaneous BCI systems are recommended also for moderate hearing loss. For combined hearing losses with moderate and high-grade cochlear hearing loss, active middle ear implants are recommended. For patients with incompatibilities or middle ear surgery, implants are a valuable and proven addition to the therapeutic options.

In situ Probe Microphone Measurement for Testing the Direct Acoustical Cochlear Stimulator

Hypothesis: Acoustical measurements can be used for functional control of a direct acoustic cochlear stimulator (DACS).

Background: The DACS is a recently released active hearing implant that works on the principle of a conventional piston prosthesis driven by the rod of an electromagnetic actuator. An inherent part of the DACS actuator is a thin titanium diaphragm that allows for movement of the stimulation rod while hermetically sealing the housing. In addition to mechanical stimulation, the actuator emits sound into the mastoid cavity because of the motion of the diaphragm.

Methods: We investigated the use of the sound emission of a DACS for intra-operative testing. We measured sound emission in the external auditory canal (P_EAC) and velocity of the actuators stimulation rod (V_act) in five implanted ears of whole-head specimens. We tested the influence various positions of the loudspeaker and a probe microphone on P_EAC and simulated implant malfunction in one example.

Results: Sound emission of the DACS with a signal-to-noise ratio >10 dB was observed between 0.5 and 5 kHz. Simulated implant misplacement or malfunction could be detected by the absence or shift in the characteristic resonance frequency of the actuator. P_EAC changed by <6 dB for variations of the microphone and loudspeaker position.

Conclusion: Our data support the feasibility of acoustical measurements for in situ testing of the DACS implant in the mastoid cavity as well as for post-operative monitoring of actuator function.

[Differential indication of active middle ear implants]

INTRODUCTION

Hearing aids (HA) provide adequate support for many patients with hearing loss, but not all. Around one third of 10.000 patients provided with hearing aids in the Abbreviated Profile of Hearing Aid Benefit felt no actual benefit when using the hearing aid, although they demonstrated the necessary hearing improvement on speech audiometry. Epidemiological data show bad compliance, especially in older people. Only one in three hearing aid owners wears their device regularly. For this subpopulation of patients active middle ear implants (AMEIs) have been used since 1998. In the present review, the current indications for AMEIs are presented.

MATERIAL AND METHODS

A selective literature search in PubMed, as well as a guideline search at the Arbeitsgemeinschaft der Wissenschaftlichen Fachgesellschaften e. V. (German Association of Scientific Societies), was carried out.

RESULTS

The present review shows that when there is an adequate indication the hearing capacity of patients can be thoroughly rehabilitated and thus their quality of life improved with the help of AMEIs. Although most commercially available systems have a satisfactory risk profile, increased extrusion rates, malfunctioning and facial paresis have been reported in older implant series. The advantages of AMEIs include increased hearing gain, reduced feedback, increased hearing quality, increased speech discrimination in the presence of background noise, and an absence of occlusion.

CONCLUSIONS

The audiological indication for AMEIs in primary care is usually controversial, since the functional hearing gain and increase in speech discrimination may be small compared with modern conventional hearing aids. AMEIs thus play a main role in the secondary care of patients who do not have sufficient benefit or who have side effects after having a conventional hearing aid fitted.

Controlled round-window stimulation in human temporal bones yielding reproducible and functionally relevant stapedial responses

Stimulation of the round window (RW) has gained increasing clinical importance. Clinical, as well as human temporal bone and in-vivo animal studies show considerable variability. The influence of RW stimulation on the cochlea remains unclear. We designed a human temporal-bone study with controlled direct mechanical stimulation of the RW membrane to identify conditions for successful RW stimulation. Eight human temporal bones were stimulated on the RW by piezoelectric stack actuators with cylindrical aluminium rods of diameter 0.5 mm and with either flat or 30° inclined top surface. Using a dedicated two-stage positioning protocol for the actuator, we achieved highly reproducible measurements of the stimulus vibration at the RW and of the resultant vibration of the stapes footplate. The reverse transmission, characterized by the displacement ratio of the stapes-footplate relative to the actuator tip on the RW membrane, yielded an average displacement ratio of 0.089 up to 1.2 kHz when the actuator was coupled without angular misalignment to the RW membrane. The results suggest that 90-μm pretension of the RW membrane is essential for optimum and reproducible RW stimulation. The displacements are shown to be roughly consistent with the equal-volume displacement hypothesis under specific assumptions about the displacement mode of the RW membrane. It is further suggested that the large inter-patient variability in the effectiveness of RW stimulation might be due primarily to the success of coupling, rather than to the variability of functionally relevant anatomical parameters.

Anatomic limitations in implantation of middle ear transducer and carina middle ear implants

OBJECTIVES/HYPOTHESIS

The objective of this study was to examine any anatomic limitations in implantation of the semi-implantable middle ear transducer (MET) and fully implantable Carina middle ear implants (Otologics, Boulder, CO).

STUDY DESIGN

Retrospective case series.

METHODS

This study involved high-resolution computed tomography (HRCT) of the temporal bone and surgical findings in 22 middle ear implantations (17 MET, five Carina). The distance between the dura and the superior-posterior wall of the external auditory canal (dura-meatal distance) on the incus projection level was measured in coronal high-resolution computed tomography (HRCT) sections. Extensive bone removal from the tegmen for the fitting of the implant was intraoperatively documented, using as criteria the dura exposure. The correlation between HRCT measurements and dura exposure was examined.

RESULTS

In 10 implantations (45.5%) the dura was exposed. In nine of 10 cases (90%) the dura-meatal distance was less than 8 mm. In 11 out of 12 implantations that were performed without exposing the dura (91.7%), the dura-meatal distance was greater than 8 mm. In two cases with dura-meatal distance less than 5 mm, extensive dura exposure and surgical time were needed. In one of these cases, opening of the dura occurred during later explantation.

CONCLUSIONS

When dura-meatal distance is greater than 8 mm, implantation of the MET or Carina is a safe procedure. By contrast, in cases with a dura-meatal distance of less than 8 mm, the surgery introduces a high risk of complications. When dura-meatal distance is less than 5 mm, MET or Carina implantation is not recommended.

Intraoperative adjustments to optimize active middle ear implant performance

CONCLUSION

After initial contact of the active middle ear implant (AMEI) on the incus, significant increases in device performance can be achieved intraoperatively without affecting residual hearing by additional static loading of the incus with 62 μm (quarter turn) to 125 μm (half turn) increments via an adjustment screw.

OBJECTIVES

To assess the performance gains of driving the incus with an AMEI under increasing static loads in cadaveric temporal bones.

METHODS

Incus drive efficacy was assessed using laser Doppler velocimetry measurements of stapes velocities over a frequency range of 0.25 to 8 kHz. Results were compared to stapes velocities following acoustic stimulation via insert earphone. Maximum equivalent ear canal sound pressure level (L(Emax)) and residual hearing loss after initial loading of the AMEI (first contact) were compared in each temporal bone. Additional increases in incus load were induced by turning an adjustment screw in quarter turn steps, corresponding to 62 μm increments per step. L(Emax)and residual hearing loss were reassessed after each step. For each temporal bone, experiments were repeated for three different AMEIs.

RESULTS

On average across bones, incus stimulation upon initial contact produced an L(Emax)of 125, 127, and 121 dB SPL and residual hearing losses of -2, -1, and -1 dB with respect to unloaded, unaided conditions for the three AMEIs, respectively. Across bones and transducers, increasing static transducer load by incrementing the AMEI up to 125 μm significantly improved performance without affecting residual hearing loss. Loading beyond 125 μm (half turn) did not improve performance but significantly increased residual hearing loss.

A totally implantable hearing system--design and function characterization in 3D computational model and temporal bones

Implantable middle ear hearing devices are emerging as an effective technology for patients with mild to moderately severe sensorineural hearing loss. Several devices with electromagnetic or piezoelectric transducers have been investigated or developed in the US and Europe since 1990. This paper reports a totally implantable hearing system (TIHS) currently under investigation in Oklahoma. The TIHS consists of implant transducer (magnet), implantable coil and microphone, DSP-audio signal processor, rechargeable battery, and remote control unit. The design of TIHS is based on a 3D finite element model of the human ear and the analysis of electromagnetic coupling of the transducer. Function of the TIHS is characterized over the auditory frequency range in three aspects: (1) mass loading effect on residual hearing with a passive implant, (2) efficiency of electromagnetic coupling between the implanted coil and magnet, and (3) functional gain of whole unit in response to acoustic input across the human skin. This paper focuses on mass loading effect and the efficiency of electromagnetic coupling of TIHS determined from the FE model of the human ear and the cadaver ears or temporal bones. Some preliminary data of whole unit function are also presented in the paper.

Implantable hearing aids

The aim of this article is to give readers a general overview of the concepts involved in the latest generation of implantable hearing aids. A section on ear biomechanics has also been included to familiarize readers with the basic concepts involved. These devices have been developed over the last 20 years, driven by problems with conventional hearing aids and by advances in the understanding of middle-ear mechanics. The use of technology borrowed from cochlear implants has enabled the first generation of fully implantable aids to be trialled. The author examines the theoretical advantages and disadvantages of implantable hearing aids over conventional aids and then reviews the technology and clinical results of a range of devices that have been trialled.

A hydrodynamic model of labyrinth to study the stimulation of perilymph compartments by audioprostheses

CONCLUSIONS

The hydrodynamic model of the labyrinth spaces (LHM) is a useful tool for research on implantable audioprostheses, in particular to develop suitable actuators using MEMS technology (micro-electromechanic machine system). It has other potential applications for auditory research.

OBJECTIVES

The energy reaching the labyrinth fluids is crucial information for developing prostheses to substitute the tympanic-ossicular system because adequate stimulation of the cochlear partition is essential. However, in vivo measurements in human ears are not currently available. Therefore a model of the normal labyrinth resembling its hydrodynamic properties becomes a valuable tool. It could allow comparison of different processing systems, algorithms and transducers, to develop new audioprostheses and improve their effectiveness and efficiency.

MATERIALS AND METHODS

This work presents one LHM that emulates the conduction of the stimuli from the stapes footplate through the labyrinthine fluids, including its dimensions and physical properties, and some examples of measurements of perilymph stimulation by different audioprostheses and algorithms. RESULTS. As shown in the reported examples, this LHM provided effective measurement of acoustic stimulation across the whole human auditory frequency and intensity spectrum. Air-delivered and direct stimulation methods are possible. This provided convenient information for the actuator development and allowed comparison between different prototypes, stimulation patterns and algorithms.