Sound transfer of active middle ear implants

Influence of the Coupling on the Hearing Outcome After Implantation of an Active Middle Ear Implant: Comparison of the Transmission Behavior in Temporal Bone Experiments With Clinical Data

OBJECTIVES

The active middle ear implant, Vibrant Soundbridge (VSB), can be implanted with a variety of couplers. Hearing outcome after implantation has been investigated in both temporal bone (TB) experiments and patient studies, but the relationship between experimental and clinical data is still weak in the literature. Therefore, experimental data from TB experiments should be compared with patient data in a retrospective study, in which the floating mass transducer is used with couplers of the third generation. Actuator coupling structures included the long (LP coupler) and short (SP coupler) incus process, the stapes head (Clip coupler), and the round window membrane (RW soft coupler).

METHODS

In the TB experiments, the sound transmission after vibroplasty on the above-mentioned actuator coupling structures was determined in 32 specimens by means of laser Doppler vibrometry on the stapes footplate. Data of 69 patients were analyzed. The main target audiometric parameters were the postoperative aided word recognition score (WRS) in the free field at 65 dB SPL (WRS 65 dB in %), the preoperative and postoperative pure-tone average (PTA4, including the frequencies 0.5, 1, 2, and 4 kHz) of the bone conduction hearing threshold (PTA4BC), the aided postoperative air conduction hearing threshold in the free field (PTA4FF) and the direct threshold (Vibrogram) at least 6 months postoperatively. The coupling efficiency of the actuator (Vibrogram-PTA4BC) as well as the effective hearing gain (PTA4FF-PTA4BC) was compared between the couplers.

RESULTS

The analysis in the main speech range (0.5-4 kHz) indicated that in the TB experiments, the LP coupler tends to have the best coupling quality at low frequencies (500-1000 Hz). This was up to 15 dB above the worst actuator (RW soft coupler). However, the results missed the significance level ( p > 0.05). In the high frequencies (2000-4000 Hz), the Clip coupler showed the best coupling quality. This was 15 dB above the worst actuator (SP coupler). However, the results missed the significance level ( p > 0.05), too. The postoperative WRS at 65 dB SPL and the postoperative PTA4FF were independent of the actuator coupling structure. The PTA4BC was stable at 6 months postoperatively. For the PTA4 of the coupling efficiency, there were no significant differences between the actuator coupling structures (LP 8.9 dB ± 12.9; SP 9.5 ± 6.5 dB; Clip 5.2 ± 10.5 dB; RW 12.7 ± 11.0 dB). However, the tendential inferiority of the RW soft coupler with regard to transmission in the low-frequency range and the tendential superiority of the Clip coupler in the high-frequency range that have already been displayed experimentally could be confirmed in the clinical results. However, the clinical results missed the significance level, too ( p > 0.05).

CONCLUSIONS

In vivo, there are no significant differences in the postoperative outcome stratified according to coupling the target structure. The differences known from the experimental setting were repressed by individual biasing factors. However, to ensure sufficient postoperative speech intelligibility, the frequency-specific transmission behavior of the couplers should be taken into account when setting the indication for VSB implantation.

Results and Quality of Life after Implantation of Active Middle Ear Implants

The provision of implantable hearing aids represents an area with high development and innovation potential. On the one hand, this review article provides an overview of current indication criteria for the treatment with active middle ear implants. On the other hand, outcome parameters as well as functional results after implantation of active middle ear implants are demonstrated and discussed. The focus is mainly placed on audiological results as well as the subjective health status. "Patient Reported Outcome Measures" (PROMs) have become an integral part of the evaluation of hearing implant treatment. Due to low evidence level criteria, the study situation regarding audiological as well as subjective outcome parameters is not satisfactory. The lack of an international consensus on accepted outcome parameters makes a meta-analytical analysis of results immensely difficult. In the studies published to date, patients with sensorineural hearing loss and patients with conductive or mixed hearing loss offered better speech recognition after implantation of an active middle ear implant compared to conventional hearing aids. Current analyses show a significant improvement in general as well as hearing-specific quality of life after implantation of an active middle ear implant. To date, no validated, hearing-specific quality-of-life measurement instruments exist for assessing the success of fitting in children. Especially in children with complex malformations of the outer ear and the middle ear, excellent audiological results were shown. However, these results need to be substantiated by quality-of-life measurements in future.

[Active hearing implants in chronic otitis media]

In patients with inadequate hearing improvement after tympanoplasty and failure of conventional hearing aid fitting, active hearing implants provide an alternative treatment option. Active middle ear implants function as a vibromechanical bypass of the stiffness and damping effect of a poorly oscillating tympanic membrane and the (reconstructed) ossicular chain. The selection of the hearing system depends on the maximum output levels of the hearing system and the anatomical conditions in mostly multiply operated ears. The development of variable coupling elements for active middle ear implants led to an extension of the indications to include not only purely sensorineural hearing loss but also mixed and conductive hearing loss in patients, as the transducer can now be coupled to the (mobile) stapes or the round window membrane. The article provides an overview of current clinical study results and recommendations on the indications for active hearing implants in patients with chronic otitis media.

Fully Implantable Active Middle Ear Implants After Subtotal Petrosectomy With Fat Obliteration: Preliminary Clinical Results

OBJECTIVES

In patients with chronic middle ear disease, especially after revision surgery for ventilation problems and mixed hearing loss, active middle ear implants may provide an alternative treatment option. The fully implantable active middle-ear implant (FI-AMEI) is designed for implantation in a ventilated mastoid with an intact posterior canal wall. Until now, there have been no reports on audiometric results after implantation of a FI-AMEI in a fat-obliterated cavity after subtotal petrosectomy (SPE).

STUDY DESIGN

Retrospective case review.

SETTING

Tertiary referral center.

PATIENTS

Twelve patients were included after numerous previous tympanoplasty surgeries for severe mixed hearing loss and FI-AMEI implantation.

INTERVENTION

In five patients, the FI-AMEI was implanted in a cavity, with fat obliteration, after SPE. Seven patients received FI-AMEI implantation after intact canal wall (ICW) surgery.

MAIN OUTCOME MEASURE(S)

Audiometric results (pure-tone audiometry, Freiburger monosyllables) are demonstrated for 12 patients after an observation period of 3 months.

RESULTS

The improvement in monosyllable score was 40 to 85% for the 12 patients. Free-field-aided thresholds showed high heterogeneity.

CONCLUSION

FI-AMEI implantation combined with SPE provides an alternative approach to hearing rehabilitation to non-FI-AMEI implantation. Studies with a high number of patients and long-term observation periods are necessary to statistically verify these results.

[Laser Doppler vibrometric measurements on human temporal bones]

Laser Doppler vibrometric (LDV) measurements on human temporal bones represent the standard method for predicting the performance of active middle ear implants (AMEI) and are used as preclinical tests in the development, approval process, and indication expansion of AMEI. The quality of the coupling of the floating mass transducer to the mobile structures of the middle ear is decisive for the performance of the implant and patients' hearing perception. The cochlea can be stimulated via the oval window (forward stimulation) or the round window (reverse stimulation). For forward stimulation, the ASTM standard F2504-05 defines a method to ensure physiologically normal properties of the temporal bones used in the experiments. For reverse stimulation, which depends even more critically on the quality of the temporal bone, a comparable standard method is lacking. Appropriate preparation and storage of the human petrous bone as well as suitable LDV test setups with respect to calibration and reproducibility of measuring positions and angles provide results that allow a comparison of different types of coupling and also correlate well with clinical data.

[Coupling of active middle ear implants-biomechanical aspects]

Active middle ear implants or implantable hearing aids are used to treat sensorineural or combined hearing loss. Their coupling to the middle ear structures has a large impact on the success of rehabilitation. Practical issues such as the coupling site, influence of middle ear status, and forward and backward excitation of the inner ear are discussed in the context of biomechanics. For this purpose, experimental studies, model simulations, and current literature data are evaluated. The explanations are intended to contribute to a better understanding of certain procedures in hearing rehabilitation with active implants.

Feasibility Study of a Mechanical Real-Time Feedback System for Optimizing the Sound Transfer in the Reconstructed Middle Ear

OBJECTIVE

To evaluate electromechanical excitation as an alternative excitation mode for middle ear transfer function (METF) measurements as well as real-time feedback in prosthetic ossicular reconstruction.

METHOD

In eight human cadaveric temporal bones, the ossicular chain was excited using acoustic and mechanical (floating mass transducer, FMT) stimulation to determine the METF. After disconnecting the ossicular chain and reconstruction with partial or total prosthesis the METFs were measured again. Continuous FMT stimulation was then applied to improve the prosthesis' position using real-time feedback of the METF.

RESULTS

Mechanical stimulation of ossicular vibration showed characteristic differences to acoustic excitation resulting from the force characteristics of the FMT. Furthermore, the interspecimen METF variability was greater with electromechanical than acoustic stimulation because of interspecimen variability in the FMT coupling conditions. When the METF with FMT excitation was used as a real-time feedback tool, a measurable improvement in the quality of ossicular reconstruction could be achieved.

CONCLUSIONS

Mechanical excitation is an effective and suitable alternative stimulation method in experimental METF measurements. The system provides real-time feedback for ossicular reconstruction in the experimental setting. Some influencing factors still need to be distinguished for reliable measurements. However, the method does not yet meet the requirements for clinical application as an intraoperative, real-time monitoring tool. However, the system could be an excellent model for high-end cadaveric temporal bone training in ossiculoplasty.

Sensor-actuator component for a Floating Mass Transducer-based fully implantable hearing aid

We propose a novel system based on the Floating Mass Transducer (FMT) to be used as the active component of a fully implantable, Vibrant Soundbridge-like middle ear implant. The new system replaces the external microphone used in the currently available design with an implantable piezoelectric sensor that is inserted into the incudostapedial joint and picks up the vibrations transmitted to the long process of the incus. The FMT is coupled to the round window of the cochlea. We characterize the system by measuring the gain in intracochlear sound pressure using laser Doppler vibrometry at a surgically installed "third window" into the cochlea of six temporal bones. Closed-loop feedback oscillations limit the system's available output. We show that using an adaptive control algorithm, a mean functional gain of up to 40 dB is achieved, which is similar to Soundbridge functional gain. The concept matches the FMT's one-point fixation philosophy and offers several advantages over other designs, namely an easy and time-efficient surgery, reversibility of implantation, and natural hearing for the prospective patient.

Air- and Bone-Conducted Sources of Feedback With an Active Middle Ear Implant

OBJECTIVES

Active middle ear implants (AMEI) have been used to treat hearing loss in patients for whom conventional hearing aids are unsuccessful for varied biologic or personal reasons. Several studies have discussed feedback as a potential complication of AMEI usage, though the feedback pathway is not well understood. While reverse propagation of an acoustic signal through the ossicular chain and tympanic membrane constitutes an air-conducted source of feedback, the implanted nature of the device microphone near the mastoid cortex suggests that bone conduction pathways may potentially be another significant factor. This study examines the relative contributions of potential sources of feedback during stimulation with an AMEI.

DESIGN

Four fresh-frozen, hemi-sectioned, human cadaver specimens were prepared with a mastoid antrostomy and atticotomy to visualize the posterior incus body. A Carina active middle ear implant actuator (Cochlear Ltd., Boulder, CO) was coupled to the incus by two means: (1) a stereotactic arm mounted independently of the specimen and (2) a fixation bracket anchored directly to the mastoid cortical bone. The actuator was driven with pure-tone frequencies in 1/4 octave steps from 500 to 6000 Hz. Acoustic sound intensity in the ear canal was measured with a probe tube microphone (Bruel & Kjær, Nærum, Denmark). Bone-conducted vibration was quantified with a single-axis laser Doppler vibrometer (Polytec Inc., Irvine, CA) from both a piece of reflective tape placed on the skin overlying the mastoid and a bone-anchored titanium screw and pedestal (Cochlear Ltd., Centennial, CO) implanted in the cortical mastoid bone.

RESULTS

Microphone measurements revealed ear-canal pressures of 60-89 dB SPL, peaking in the frequency range below 2 kHz. Peak LDV measurements were greatest on the mastoid bone (0.32-0.79 mm/s with mounting bracket and 0.21-0.36 mm/s with the stereotactic suspension); peak measurements on the skin ranged from 0.05 to 0.15 mm/s with the bracket and 0.03 to 0.13 mm/s with stereotactic suspension.

CONCLUSION

AMEI produce both air- and bone-conducted signals of adequate strength to be detected by the implanted device microphone, potentially resulting in reamplification. Understanding the relative contribution of these sources may play an important role in the development of targeted mitigation algorithms, as well as surgical techniques emphasizing acoustic isolation.

An Intracochlear Pressure Sensor as a Microphone for a Fully Implantable Cochlear Implant

OBJECTIVE

To validate an intracochlear piezoelectric sensor for its ability to detect intracochlear pressure and function as a microphone for a fully implantable cochlear implant.

METHODS

A polyvinylidene fluoride (PVDF) piezoelectric pressure sensor was inserted into a human fresh cadaveric round window at varying depths. An external sound pressure stimulus was applied to the external auditory canal (EAC). EAC pressure, stapes velocity, and piezoelectric sensor voltage output were recorded.

RESULTS

The PVDF sensor was able to detect the intracochlear sound pressure response to an acoustic input to the EAC. The frequency response of the pressure measured with the intracochlear sensor was similar to that of the pressure at the EAC, with the expected phase delay of the middle ear transmission. The magnitude of the response increased and smoothened with respect to frequency as the sensor was inserted more deeply into the scala tympani. Artifact measurements, made with the sensor in air near the round window, showed flat frequency response in both magnitude and phase, which were distinct from those measured when the sensor was inserted in the round window.

CONCLUSION

This study describes a novel method of measuring intracochlear pressure for an otologic microphone composed of a piezoelectric polymer, and demonstrates feasibility. Our next goal is to improve device sensitivity and bandwidth. Our long-term objective is to imbed the piezoelectric sensor within a conventional cochlear implant electrode, to enable a device to both measure intracochlear sound pressure and deliver electrical stimulus to the cochlea, for a fully implantable cochlear implant.

[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.