Estimating vibration artifacts in preclinical experimental assessment of actuator efficiency in bone-conduction hearing devices.

OBJECTIVES: Test feasibility of a means to distinguish artifact from relevant signal in an experimental method for pre-clinical assessment of bone conduction (BC) stimulation efficiency based on measurement of intracochlear pressure (ICP). METHODS: Experiments were performed on fresh-frozen human temporal bones and cadaver heads. In a first step, fiber optic pressure sensors inserted into the cochlea through cochleostomies were intentionally vibrated to generate relative motion versus the stationary specimen, and the resulting ICP artifact recorded, before and after attaching the sensor fiber to the bone with glue. In a second step, BC stimulation was applied in the conventional location for a commercial bone anchored implant, as well as two alternative locations closer to the otic capsule. Again, ICP was recorded and compared with an estimated artifact, calculated from the previous measurements with intentional vibration of the fiber. RESULTS: Intentional vibration of the sensor fiber creates relative motion between fiber and bone, as intended, and causes an ICP signal. The stimulus does not create substantial promontory vibration, indicating that the measured ICP is all artifact, i.e. would not occur if the sensor were not in place. Fixating the sensor fiber to the bone with glue reduces the ICP artifact by at least 20 dB. BC stimulation also creates relative motion between sensor fiber and bone, as expected, from which an estimated ICP artifact level can be calculated. The ICP signal measured during BC stimulation is well above the estimated artifact, at least in some specimens and at some frequencies, indicating "real" cochlear stimulation, which would result in an auditory percept in a live subject. Stimulation at the alternative locations closer to the otic capsule appear to result in higher ICP (no statistical analysis performed), indicating a trend towards more efficient stimulation than at the conventional location. CONCLUSIONS: Intentional vibration of the fiber optic sensor for measurement of ICP can be used to derive an estimate of the artifact to be expected when measuring ICP during BC stimulation, and to characterize the effectiveness of glues or other means of reducing the artifact caused by relative motion of fiber and bone.

In-vivo characterisation of an implanted microphone and totally implantable active middle ear implant.

OBJECTIVE: To objectively evaluate acoustic sensitivity of the implanted microphone, and maximum stable gain of a totally implantable active middle ear implant. DESIGN: Prospective, single centre evaluation. STUDY SAMPLE: Fourteen adult patients. RESULTS: Microphone sensitivity is approx. 10 dB lower than an externally worn conventional hearing aid, at frequencies up to 4000 Hz, and substantially lower at higher frequencies. The masking level due to microphone noise, which determines the softest test tones that can be detected, is estimated at <20 to <30 dB HL up to 1000 Hz, and <40 dB HL at higher frequencies. Maximum stable effective gain is the maximum amplification achievable without causing feedback whistling. In sensorineural hearing loss (SNHL) cases, it is 30-40 dB at frequencies up to 2000 Hz, allowing to compensate for even the maximum recommended hearing loss (60-70 dB HL). In both SNHL and mixed hearing loss (MHL) cases, maximum stable effective gain is lower (+20 to -30 dB) around 3000-6000 Hz. CONCLUSIONS: Microphone sensitivity is high enough to achieve aided thresholds of 20-40 dB HL. A strong correlation between actuator coupling efficiency and maximum stable effective gain implies that any effort to improve actuator efficiency should also increase the available gain.

Hearing Aid Treatment in Patients with Mixed Hearing Loss. Part I: Expected Benefit and Limitations after Stapes Surgery.

OBJECTIVE: The purpose of the present study was to determine the fraction of patients with mixed hearing loss who can or cannot expect benefit from power hearing aids (HAs) after stapes surgery. DESIGN: The audiological outcome of 374 stapes surgeries was used to calculate the patients' individual postoperative requirements in terms of gain and output of HAs. These requirements were compared to the available gain and output provided by state-of-the-art power HAs at 0.5, 1.0, 2.0, and 4.0 kHz. According to these comparisons, ears were divided into three groups. For G0, required gain and output lay within the corresponding technical limits of the HAs at all frequencies. In G1, one or both requirements could not be fulfilled at 1 frequency. G2 combined all ears where the requirements lay beyond the HA's technical limitations at 2 or more frequencies. RESULTS: Stapes surgery resulted in an improvement of air-bone gap (ABG) in 84.5% of the cases by 15.7 dB on average. Based on pure-tone average (0.5, 1.0, 2.0, 4.0 kHz), 40.6% of all cases showed an ABG ≤10 dB. 44.9% of all cases did no longer need a HA after stapes surgery. A power HA would fulfill both audiological criteria at all 4 frequencies in 81.6% of cases that needed a HA postoperatively. However, 18.4% would not be sufficiently treatable at 1 or more frequencies (15.0% in G1, 3.4% in G2). CONCLUSIONS: The present study identified a subset of patients with mixed hearing loss after stapes surgery that cannot be treated sufficiently with available power HAs. As the residual ABG is an important reason for this lack of treatment success, the advancement of alternative hearing devices that circumvent the middle ear, such as powerful active middle ear implants, is indicated.

Hearing Aid Treatment for Patients with Mixed Hearing Loss. Part II: Speech Recognition in Comparison to Direct Acoustic Cochlear Stimulation.

BACKGROUND: The conventional therapy for severe mixed hearing loss is middle ear surgery combined with a power hearing aid. However, a substantial group of patients with severe mixed hearing loss cannot be treated adequately with today's state-of-the-art (SOTA) power hearing aids, as predicted by the accompanying part I of this publication, where we compared the available maximum power output (MPO) and gain from technical specifications to requirements for optimum benefit using a common fitting rule. Here, we intended to validate the theoretical assumptions from part I experimentally in a mixed hearing loss cohort fitted with SOTA power hearing aids. Additionally, we compared the results with an implantable hearing device that circumvents the impaired middle ear, directly stimulating the cochlea, as this might be a better option. OBJECTIVES: Speech recognition outcomes obtained from patients with severe mixed hearing loss supplied acutely with a SOTA hearing aid were studied to validate the outcome predictions as described in part I. Further, the results obtained with hearing aids were compared to those in direct acoustic cochlear implant (DACI) users. MATERIALS AND METHODS: Twenty patients (37 ears with mixed hearing loss) were provided and fitted with a SOTA power hearing aid. Before and after an acclimatization period of at least 4 weeks, word recognition scores (WRS) in quiet and in noise were studied, as well as the speech reception threshold in noise (SRT). The outcomes were compared retrospectively to a second group of 45 patients (47 ears) using the DACI device. Based on the severity of the mixed hearing loss and the available gain and MPO of the SOTA hearing aid, the hearing aid and DACI users were subdivided into groups with prediction of sufficient, partially insufficient, or very insufficient hearing aid performance. RESULTS: The patients with predicted adequate SOTA hearing aid performance indeed showed the best WRS in quiet and in noise when compared to patients with predicted inferior outcomes. Insufficient hearing aid performance at one or more frequencies led to a gradual decrease in hearing aid benefit, validating the criteria used here and in the accompanying paper. All DACI patients showed outcomes at the same level as the adequate hearing aid performance group, being significantly better than those of the groups with inadequate hearing aid performance. Whereas WRS in quiet and noise were sensitive to insufficient gain or output, showing significant differences between the SOTA hearing aid and DACI groups, the SRT in noise was less sensitive. CONCLUSIONS: Limitations of outcomes in mixed hearing loss individuals due to insufficient hearing aid performance can be accurately predicted by applying a commonly used fitting rule and the 35-dB dynamic range rule on the hearing aid specifications. Evidently, when outcomes in patients with mixed hearing loss using the most powerful hearing aids are insufficient, bypassing the middle ear with a powerful active middle ear implant or direct acoustic implant can be a promising alternative treatment.

Validation of methods for prediction of clinical output levels of active middle ear implants from measurements in human cadaveric ears.

Today, the standard method to predict output levels of active middle ear implants (AMEIs) before clinical data are available is stapes vibration measurement in human cadaveric ears, according to ASTM standard F2504-05. Although this procedure is well established, the validity of the predicted output levels has never been demonstrated clinically. Furthermore, this procedure requires a mobile and visually accessible stapes and an AMEI stimulating the ossicular chain. Thus, an alternative method is needed to quantify the output level of AMEIs in all other stimulation modes, e.g. reverse stimulation of the round window. Intracochlear pressure difference (ICPD) is a good candidate for such a method as it correlates with evoked potentials in animals and it is measurable in cadaveric ears. To validate this method we correlated AMEI output levels calculated from ICPD and from stapes vibration in cadaveric ears with outputs levels determined from clinical data. Output levels calculated from ICPD were similar to output levels calculated from stapes vibration and almost identical to clinical data. Our results demonstrate that both ICPD and stapes vibration can be used as a measure to predict AMEI clinical output levels in cadaveric ears and that ICPD as reference provided even more accurate results.