Implantable middle ear hearing devices: current state of technology and market challenges

Evaluations on the stability and bio-compatibility of a new piezoelectric microphone for the implantable middle ear microphone

BACKGROUND

A new floating piezoelectric microphone (NFPM), driven by the acoustic vibration of the ossicles, has been manufactured in the lab.

OBJECTIVES

This study aimed at exploring the stability and bio-compatibility of this NFPM.

MATERIALS AND METHODS

The NFPM was implanted into the cat tympanic cavity by clamping it to the handle of the malleus, and then a temporal bone computed tomography (CT) plain scan and three dimensional reconstruction were performed to observe its coupling with the handle of the malleus. After six months of implantation, cats' blood samples were taken for pro-inflammatory factor analysis. Then, the operative cavity was opened to check the NFPM, the auditory ossicular chain and some granulation tissue surrounding the NFPM.

RESULTS

The NFPM was firmly clamped to the handle of the malleus of the cats during the six months of implantation. Besides, there was no obvious systemic inflammatory response in the experimental animals. In addition, local proliferation of granulation tissue occurred in the tympanic cavity without hampering the movement of the auditory ossicle,or causing ischemia of the auditory ossicle.

CONCLUSION

The NFPM could be implanted in our experimental cats for a long period of time and had good bio-compatibility.

Implementation of a fully implantable middle-ear hearing device chip

BACKGROUND AND OBJECTIVE

Recently, with the increase in the population of hearing impaired people, various types of hearing aids have been rapidly developed. In particular, a fully implantable middle ear hearing device (F-IMEHD) is developed for people with sensorineural hearing loss. The F-IMEHD system comprises an implantable microphone, a transducer, and a signal processor. The signal processor should have a small size and consume less power for implantation in a human body.

METHODS

In this study, we designed and fabricated a signal-processing chip using the modified FFT algorithm. This algorithm was developed focusing on eliminating time delay and system complexity in the transform process. The designed signal-processing chip comprises a 4-channel WDRC, a fitting memory, a communication 1control part, and a pulse density modulator. Each channel is separated using a 64-point fast Fourier transform (FFT) method and the gain value is matched using the fitting table in the fitting memory.

RESULTS AND CONCLUSION

The chip was designed by Verilog-HDL and the designed HDL codes were verified by Modelsim-PE 10.3 (Mentor graphics, USA). The chip was fabricated using a 0.18 μm CMOS process (SMIC, China). Experiments were performed on a cadaver to verify the performance of the fabricated chip.

Piezoelectric Actuator with Frequency Characteristics for a Middle-Ear Implant

The design and implementation of a novel piezoelectric-based actuator for an implantable middle-ear hearing aid is described in this paper. The proposed actuator has excellent low-frequency output characteristics, and can generate high output in a specific frequency band by adjusting the mechanical resonance. The actuator consists of a piezoelectric element, a miniature bellows, a cantilever membrane, a metal ring support, a ceramic tip, and titanium housing. The optimal structure of the cantilever-membrane design, which determines the frequency characteristics of the piezoelectric actuator, was derived through finite element analysis. Based on the results, the piezoelectric actuator was implemented, and its performance was verified through a cadaveric experiment. It was confirmed that the proposed actuator provides better performance than currently used actuators, in terms of frequency characteristics.

Difference of auditory brainstem responses by stimulating to round and oval window in animal experiments

ABSTACT To ensure the safety and efficacy of implantable hearing aids, animal experiments are an essential developmental procedure, in particular, auditory brainstem responses (ABRs) can be used to verify the objective effectiveness of implantable hearing aids. This study measured and compared the ABRs generated when applying the same vibration stimuli to an oval window and round window. The ABRs were measured using a TDT system 3 (TDT, USA), while the vibration stimuli were applied to a round window and oval window in 4 guinea pigs using a piezo-electric transducer with a proper contact tip. A paired t-test was used to determine any differences between the ABR amplitudes when applying the stimulation to an oval window and round window. The paired t-test revealed a significant difference between the ABR amplitudes generated by the round and oval window stimulation (t = 10.079, α < .0001). Therefore, the results confirmed that the biological response to round window stimulation was not the same as that to oval window stimulation.

A tri-coil bellows-type round window transducer with improved frequency characteristics for middle-ear implants

A number of methods to drive the round window (RW) using a floating mass transducer (FMT) have been reported. This method has attracted attention because the FMT is relatively easy to implant in the RW niche. However, the use of an FMT to drive the RW has been proven to produce low outputs at frequencies below approximately 1 kHz. In this study, a new tri-coil bellows-type transducer (TCBT), which has excellent low frequency output and is easy to implant, is proposed. To design the frequency characteristics of the TCBT, mechanical and electrical simulations were performed, and then a comparative analysis was conducted between a floating mass type transducer (like the FMT) and a fixed type transducer (like the TCBT). The features of the proposed TCBT are as follows. First, the TCBT's housing is fixed to the RW niche so that it does not vibrate. Second, the internal end of a tiny bellows is connected to a vibrating three-pole permanent magnet located within three field coils. Finally, the rim of the bellows bottom is attached to the end of the housing that hermetically encloses the three field coils. In this design, the only vibrating element is the bellows itself, which efficiently drives the RW membrane. To evaluate the characteristics of this newly developed TCBT, the transducer was installed in the RW niche of temporal bones and the velocity of the stapes was measured using a laser Doppler vibrometer. The experimental results indicate that the TCBT can produce 100, 111, and 129 dB SPL equivalent pressure outputs at below 1 kHz, 1-3 kHz, and above 3 kHz, respectively. Thus, the TCBT with one side coupled to the RW via a bellows will be easy to implant and offer better performance than an FMT.

Long-Term Outcome Data in Patients following One Year's Use of a Fully Implantable Active Middle Ear Implant

This study examined the safety and efficacy of a fully implantable active middle ear (AMEI) system. Outcome measures assessed AMEI performance compared with an optimally fitted conventional hearing aid (CHA). Fifty adults with stable, symmetric moderate-to-severe sensorineural hearing loss were implanted at 9 ambulatory settings. Consonant-Nucleus-Consonant (CNC) words, Bamford-Kowel-Bench Speech in Noise test (BKB-SIN), Abbreviated Profile of Hearing Aid Benefit (APHAB), and unaided hearing thresholds in the implanted ear were compared to baseline measures obtained using a personal CHA. Changes in thresholds were observed from pre- to 12-month postoperative assessments. CNC word scores decreased (within 10%), and the BKB-SIN showed no change from pre- to 12-month postoperative time points. The APHAB revealed improvement. Findings suggest no difference in performance between an appropriately fit CHA and the AMEI at 12 months. This study indicates AMEIs have the potential to help individuals who choose not to use CHAs.

Comparison of auditory responses determined by acoustic stimulation and by mechanical round window stimulation at equivalent stapes velocities

Active middle ear implants (AMEIs) have been studied to overcome the limitations of conventional hearing aids such as howling, occlusion, and social discrimination. AMEIs usually drive the oval window (OW) by means of transmitting vibrational force through the ossicles and the vibrational force corresponding to sound is generated from a mechanical actuator. Recently, round window (RW) stimulation using an AMEI such as a floating mass transducer (FMT) to deliver sound to the cochlea has been introduced and hearing improvement in clinical use has been reported. Although previous studies demonstrated that the auditory response to RW stimulation was comparable to a sound-evoked auditory response, few studies have investigated the quantification of the physiologic performance of an AMEI through RW stimulation on the inner ear in vivo. There is no established relationship between the cochlear responses and mechanical stimulation to RW. The aim of this study is to assess the physiologic response in RW stimulation by an AMEI. The transferred energy through the RW to the inner ear could estimate the response corresponding to acoustic stimulation in order to quantify the AMEI output in the ossicular chain or OW stimulation. The parameters of the auditory brainstem responses (ABRs) were measured and compared based on stapes velocities similar enough to be regarded as the same for acoustic stimulation to the external auditory canal (EAC) and mechanical stimulation to the RW in an in vivo system. In conclusion, this study showed that the amplitudes and latencies of the ABRs of acoustic and RW stimulation showed significant differences at comparable stapes velocities in an in vivo system. These differences in the ABR amplitudes and latencies reflect different output functions of the cochlea in response to different stimulation pathways. Therefore, it is necessary to develop a new method for quantifying the output of the cochlea in the case of RW stimulation.

Efficacy of the active middle-ear implant in patients with sensorineural hearing loss

Introduction:

This systematic review aims to advise on the effectiveness of the active middle-ear implant in patients with sensorineural hearing loss, compared with external hearing aids.

Methods:

A systematic search of several electronic databases, including PubMed and Embase, was used to identify relevant studies for inclusion.

Results:

Fourteen comparative studies were included. Nine studies reported on the primary outcome of functional gain: one found that the middle-ear implant was significantly better than external hearing aids (p < 0.001), while another found that external hearing aids were generally significantly better than middle-ear implants (p < 0.05). Six of the seven remaining studies found that middle-ear implants were better than external hearing aids, although generally no clinically significant difference (i.e. ≥10 dB) was seen.

Conclusion:

Generally, the active middle-ear implant appears to be as effective as the external hearing aid in improving hearing outcomes in patients with sensorineural hearing loss.

Influence of the middle ear anatomy on the performance of a membrane sensor in the incudostapedial joint gap

There is a great demand for implantable microphones for future generations of implantable hearing aids, especially Cochlea Implants. An implantable middle ear microphone based on a piezoelectric membrane sensor for insertion into the incudostapedial gap is investigated. The sensor is designed to measure the sound-induced forces acting on the center of the membrane. The sensor mechanically couples to the adjacent ossicles via two contact areas, the sensor membrane and the sensor housing. The sensing element is a piezoelectric single crystal bonded on a titanium membrane. The sensor allows a minimally invasive and reversible implantation without removal of ossicles and without additional sensor fixation in the tympanic cavity. This study investigates the implantable microphone sensor and its implantation concept. It intends to quantify the influence of the sensor's insertion position on the achievable microphone sensitivity. The investigation considers anatomical and pathological variations of the middle ear geometry and its space limitations. Temporal bone experiments on a laboratory model show that anatomical and pathological variations of the middle ear geometry can prevent the sensor from being placed optimally within the incudostapedial joint. Beyond scattering of transfer functions due to anatomic variations of individual middle ears there is the impact of variations in the sensor position within the ossicular chain that has a considerable effect on the transfer characteristics of the middle ear microphone. The centering of the sensor between incus and stapes, the direction of insertion (membrane to stapes or to incus) and the effect of additional contact points with surrounding anatomic structures affect the signal yield of the implanted sensor. The presence of additional contact points has a considerably impact on the sensitivity, yet the microphone sensitivity is quite robust against small changes in the positioning of the incus on the sensor. Signal losses can be avoided by adjusting the position of the sensor within the joint. The findings allow the development of an improved surgical insertion technique to ensure maximally achievable signal yield of the membrane sensor in the ISJ and provides valuable knowledge for a future design considerations including sensor miniaturization and geometry. Measurements of the implanted sensor in temporal bone specimens showed a microphone sensitivity in the order of 1 mV/Pa. This article is part of a special issue entitled "MEMRO 2012".

[Implantable hearing aids]

Strictly speaking, implantable hearing aids are technical systems that process audiological signals and convey these by direct mechanical stimulation of the ossicular chain or cochlea. They have certain benefits over conventional hearing aids in terms of wearing comfort and general acceptance. As current studies lack convincing audiological results, the indications for implantable hearing aids are primarily of medical or cosmetic nature. To date, three systems are available in Germany: Vibrant Soundbridge®, Carina®, and Esteem®. Because the performance of the different implantable and nonimplantable hearing systems together with various surgical procedures are currently undergoing major changes, audiological indications may also develop in the future.

The role of active middle-ear implants in the rehabilitation of hearing loss

The surgical implantation of auditory devices to improve or restore the sensation of hearing in affected individuals is a rapidly growing area of modern ear, nose and throat, and audiological practice. Following the enormous success of cochlear implantation and set to take an increasing role in the rehabilitation of deafness is the active middle-ear implant. They should be viewed as an alternative to conventional hearing aids for individuals who are either unable to wear hearing aids or reject them for a variety of reasons. This article discusses the different types of middle-ear implant that are currently in use and examines the significant challenges that remain to be overcome to further advance this field.

Passive and active middle ear implants

Besides eradication of chronic middle ear disease, the reconstruction of the sound conduction apparatus is a major goal of modern ear microsurgery. The material of choice in cases of partial ossicular replacement prosthesis is the autogenous ossicle. In the event of more extensive destruction of the ossicular chain diverse alloplastic materials, e.g. metals, ceramics, plastics or composits are used for total reconstruction. Their specialised role in conducting sound energy within a half-open implant bed sets high demands on the biocompatibility as well as the acoustic-mechanic properties of the prosthesis. Recently, sophisticated titanium middle ear implants allowing individual adaptation to anatomical variations are widely used for this procedure. However, despite modern developments, hearing restoration with passive implants often faces its limitations due to tubal-middle-ear dysfunction. Here, implantable hearing aids, successfully used in cases of sensorineural hearing loss, offer a promising alternative. This article reviews the actual state of affairs of passive and active middle ear implants.

Systematic review of middle ear implants: do they improve hearing as much as conventional hearing AIDS?

OBJECTIVE

A systematic review to determine whether middle ear implants (MEIs) improve hearing as much as hearing aids.

DATA SOURCES

Databases included MEDLINE, EMBASE, DARE, and Cochrane searched with no language restrictions from 1950 or the start date of each database.

STUDY SELECTION

Initial search found 644 articles, of which 17 met the inclusion criteria of MEI in adults with a sensorineural hearing loss, where hearing outcomes and patient-reported outcome measures (PROMs) compared MEI with conventional hearing aids (CHAs).

DATA EXTRACTION

Study quality assessment included whether ethical approval was gained, the study was prospective, eligibility criteria specified, a power calculation made and appropriate controls, outcome measures, and analysis performed. Middle ear implant outcome analysis included residual hearing, complications, and comparison to CHA in terms of functional gain, speech perception in quiet and in noise, and validated PROM questionnaires.

DATA SYNTHESIS

Because of heterogeneity of outcome measures, comparisons were made by structured review.

CONCLUSION

The quality of studies was moderate to poor with short follow-up. The evidence supports the use of MEI because, overall, they do not decrease residual hearing, result in a functional gain in hearing comparable to CHA, and may improve perception of speech in noise and sound quality. We recommend the publication of long-term results comparing MEI with CHA, reporting a minimum of functional gain, speech perception in quiet and in noise, complications, and a validated PROM to guide the engineering of the new generation of MEI in the future.

Comparisons of electromagnetic and piezoelectric floating-mass transducers in human cadaveric temporal bones

Electromagnetic floating-mass transducers for implantable middle-ear hearing devices (IMEHDs) afford the advantages of a simple surgical implantation procedure and easy attachment to the ossicles. However, their shortcomings include susceptibility to interference from environmental electromagnetic fields, relatively high current consumption, and a limited ability to output high-frequency vibrations. To address these limitations, a piezoelectric floating-mass transducer (PFMT) has recently been developed. This paper presents the results of a comparative study of these two types of vibration transducer developed for IMEHDs. The differential electromagnetic floating-mass transducer (DFMT) and the PFMT were implanted in two different sets of three cadaveric human temporal bones. The resulting stapes displacements were measured and compared on the basis of the ASTM standard for describing the output characteristics of IMEHDs. The experimental results show that the PFMT can produce significantly higher equivalent sound pressure levels above 3 kHz, due to the flat response of the PFMT, than can the DFMT. Thus, it is expected that the PFMT can be utilized to compensate for high-frequency sensorineural hearing loss.

A micropower miniature piezoelectric actuator for implantable middle ear hearing device

This paper describes the design and development of a small actuator using a miniature piezoelectric stack and a flextensional mechanical amplification structure for an implantable middle ear hearing device (IMEHD). A finite-element method was used in the actuator design. Actuator vibration displacement was measured using a laser vibrometer. Preliminary evaluation of the actuator for an IMEHD was conducted using a temporal bone model. Initial results from one temporal bone study indicated that the actuator was small enough to be implanted within the middle ear cavity, and sufficient stapes displacement can be generated for patients with mild to moderate hearing losses, especially at higher frequency range, by the actuator suspended onto the stapes. There was an insignificant mass-loading effect on normal sound transmission (<3 dB) when the actuator was attached to the stapes and switched off. Improved vibration performance is predicted by more firm attachment. The actuator power consumption and its generated equivalent sound pressure level are also discussed. In conclusion, the actuator has advantages of small size, lightweight, and micropower consumption for potential use as IMHEDs.

Finite element analysis of the effects of a floating mass transducer on the performance of a middle ear implant

Experimental evidence has shown that floating mass transducers (FMTs) play a key role in the performance of middle ear implants. However, because of the tiny size and complex structure of the middle ear, systematic experimental study of the influences of FMTs is difficult to carry out. In this paper we develop a FMT-attached middle-ear finite element model to investigate some effects of a FMT on the performance of a middle ear implant. This model was constructed based on a complete set of computerized tomography section images of a healthy volunteer's left ear. The validity of the developed model was verified by comparing the model-predicted motion of the tympanic membrane and stapes footplate with published experimental data. The result shows that the FMT produces a mass loading effect prominently at high frequencies, the force required to drive the incus to the equivalent of 100 dB sound pressure level (SPL) is about 89 microN, and setting the attachment position of the FMT close to the incudostapedial joint can enhance the driving effect.

Mastoid cavity dimensions and shape: method of measurement and virtual fitting of implantable devices

Temporal bone implants can be used to electrically stimulate the auditory nerve, to amplify sound, to deliver drugs to the inner ear and potentially for other future applications. The implants require storage space and access to the middle or inner ears. The most acceptable space is the cavity created by a canal wall up mastoidectomy. Detailed knowledge of the available space for implantation and pathways to access the middle and inner ears is necessary for the design of implants and successful implantation. Based on temporal bone CT scans a method for three-dimensional reconstruction of a virtual canal wall up mastoidectomy space is described. Using Amira software the area to be removed during such surgery is marked on axial CT slices, and a three-dimensional model of that space is created. The average volume of 31 reconstructed models is 12.6 cm(3) with standard deviation of 3.69 cm(3), ranging from 7.97 to 23.25 cm(3). Critical distances were measured directly from the model and their averages were calculated: height 3.69 cm, depth 2.43 cm, length above the external auditory canal (EAC) 4.45 cm and length posterior to EAC 3.16 cm. These linear measurements did not correlate well with volume measurements. The shape of the models was variable to a significant extent making the prediction of successful implantation for a given design based on linear and volumetric measurement unreliable. Hence, to assure successful implantation, preoperative assessment should include a virtual fitting of an implant into the intended storage space. The above-mentioned three-dimensional models were exported from Amira to a Solidworks application where virtual fitting was performed. Our results are compared to other temporal bone implant virtual fitting studies. Virtual fitting has been suggested for other human applications.

Recent advances in hearing restoration

This review is based on Pubmed, Medline and Internet literature searches, supplemented by knowledge from textbooks, conference presentations, and personal communications with experts in the field of hearing restoration and patients. We have not specifically selected a time limit for our literature searches; however, the majority are articles from the past 5 years.