Impact of Insertion Speed, Depth, and Robotic Assistance on Cochlear Implant Insertion Forces and Intracochlear Pressure: A Scoping Review.

Cochlear implants are crucial for addressing severe-to-profound hearing loss, with the success of the procedure requiring careful electrode placement. This scoping review synthesizes the findings from 125 studies examining the factors influencing insertion forces (IFs) and intracochlear pressure (IP), which are crucial for optimizing implantation techniques and enhancing patient outcomes. The review highlights the impact of variables, including insertion depth, speed, and the use of robotic assistance on IFs and IP. Results indicate that higher insertion speeds generally increase IFs and IP in artificial models, a pattern not consistently observed in cadaveric studies due to variations in methodology and sample size. The study also explores the observed minimal impact of robotic assistance on reducing IFs compared to manual methods. Importantly, this review underscores the need for a standardized approach in cochlear implant research to address inconsistencies and improve clinical practices aimed at preserving hearing during implantation.

Force and pressure measurements in temporal bones.

PURPOSE: Some cochlear implant (CI) patients lose their residual hearing during surgery. Two factors that might play a role in residual hearing loss are the change in intracochlear hydraulic pressure and force on the cochlear wall during electrode insertion. The aim of this study is to investigate whether a difference in peak hydraulic pressure and peak force on the cochlear wall exists during a CI electrode insertion with different insertion techniques. MATERIALS AND METHODS: Twenty fresh frozen temporal bones were used. Hydraulic pressure and force on the cochlear wall were recorded during straight electrode insertions with 1) slow versus fast insertion speed, 2) manual versus automatic insertion method and 3) round window approach (RWA) versus extended RWA (ERWA). RESULTS: When inserting with a slow compared to a fast insertion speed, the peak hydraulic pressure is 239% (95% CI: 130-399%) higher with a RWA and 58% (95% CI: 6-137%) higher with an ERWA. However, the peak force on the cochlear wall is a factor 29% less (95% CI: 13-43%) with a slow insertion speed. No effect was found of opening and insertion method. CONCLUSIONS: As contradictory findings were found for hydraulic pressure and force on the cochlear wall on insertion speed, it remains unclear which insertion speed (slow versus fast) is less traumatic to inner ear structure.

Optimisation of the round window opening in cochlear implant surgery in wet and dry conditions: impact on intracochlear pressure changes.

To preserve residual hearing in cochlear implant candidates, the atraumatic insertion of the cochlea electrode has become a focus of cochlea implant research. In a previous study, intracochlear pressure changes during the opening of the round window membrane were investigated. In the current study, intracochlear pressure changes during opening of the round window membrane under dry and transfluid conditions were investigated. Round window openings were performed in an artificial cochlear model. Intracochlear pressure changes were measured using a micro-optical pressure sensor, which was placed in the apex. Openings of the round window membrane were performed under dry and wet conditions using a cannula and a diode laser. Statistically significant differences in the intracochlear pressure changes were seen between the different methods used for opening of the round window membrane. Lower pressure changes were seen by opening the round window membrane with the diode laser than with the cannula. A significant difference was seen between the dry and wet conditions. The atraumatic approach to the cochlea is assumed to be essential for the preservation of residual hearing. Opening of the round window under wet conditions produce a significant advantage on intracochlear pressure changes in comparison to dry conditions by limiting negative outward pressure.

Objective preclinical measures for bone conduction implants.

The study evaluates the accuracy of predicting intracochlear pressure during bone conduction stimulation using promontory velocity and ear canal pressure, as less invasive alternatives to intracochlear pressure. Stimulating with a percutaneous bone conduction device implanted in six human cadaveric ears, measurements were taken across various intensities, frequencies, and stimulation positions. Results indicate that intracochlear pressure linearly correlates with ear canal pressure (R2 = 0.43, RMSE = 6.85 dB), and promontory velocity (R2 = 0.47, RMSE = 6.60 dB). Normalizing data to mitigate the influence of stimulation position leads to a substantial improvement in these correlations. R2 values increased substantially to 0.93 for both the ear canal pressure and the promontory velocity, with RMSE reduced considerably to 2.02 (for ear canal pressure) and 1.94 dB (for promontory velocity). Conclusively, both ear canal pressure and promontory velocity showed potential in predicting intracochlear pressure and the prediction accuracy notably enhanced when accounting for stimulation position. Ultimately, these findings advocate for the continued use of intracochlear pressure measurements to evaluate future bone conduction devices and illuminate the role of stimulation position in influencing the dynamics of bone conduction pathways.

Effects of Varying Laser Parameters During Laser Stapedotomy on Intracochlear Pressures.

OBJECTIVE: Sensorineural hearing loss is a known complication of stapes surgery. We previously showed that laser stapedotomy can result in intracochlear pressures that are comparable to high sound pressure levels. Optimizing laser settings to those that correspond with the lowest pressure changes may mitigate risk for postoperative hearing loss. Here we quantify the effects of various laser parameters on intracochlear pressures and test the hypothesis that intracochlear pressure changes are proportional to the laser energy delivered. STUDY DESIGN: Basic and translational science. SETTING: Cadaveric dissection and basic science laboratory. METHODS: Cadaveric human heads underwent mastoidectomies. Intracochlear pressures were measured via fiber-optic pressure probes placed in scala vestibuli and tympani. Pulses of varied stimulus power and duration from a 980-nm diode laser were applied to the stapes footplate. RESULTS: Sustained high-intensity pressures were observed in the cochlea during all laser applications. Observed pressure magnitudes increased monotonically with laser energy and rose linearly for lower stimulus durations and powers, but there was increased variability for laser applications of longer duration (200-300 ms) and/or higher power (8 W). CONCLUSIONS: Results confirm that significant pressure changes occur during laser stapedotomy, which we hypothesize may cause injury. Overall energy delivered depends predictably on duration and power, but surgeons should use caution at the highest stimulus levels and longest pulse durations due to the increasing variability in intracochlear pressure under these stimulus conditions. While the risk to hearing from increased intracochlear pressures from laser stapedotomy remains unclear, these results affirm the need to optimize laser settings to avoid unintended injury.

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.

Real-time measurement of stapes motion and intracochlear pressure during blast exposure.

Blast-induced auditory injury is primarily caused by exposure to an overwhelming amount of energy transmitted into the external auditory canal, the middle ear, and then the cochlea. Quantification of this energy requires real-time measurement of stapes footplate (SFP) motion and intracochlear pressure in the scala vestibuli (Psv). To date, SFP and Psv have not been measured simultaneously during blast exposure, but a dual-laser experimental approach for detecting the movement of the SFP was reported by Jiang et al. (2021). In this study, we have incorporated the measurement of Psv with SFP motion and developed a novel approach to quantitatively measure the energy flux entering the cochlea during blast exposure. Five fresh human cadaveric temporal bones (TBs) were used in this study. A mastoidectomy and facial recess approach were performed to identify the SFP, followed by a cochleostomy into the scala vestibuli (SV). The TB was mounted to the "head block", a fixture to simulate a real human skull, with two pressure sensors - one inserted into the SV (Psv) and another in the ear canal near the tympanic membrane (P1). The TB was exposed to the blast overpressure (P0) around 4 psi or 28 kPa. Two laser Doppler vibrometers (LDVs) were used to measure the movements of the SFP and TB (as a reference). The LDVs, P1, and Psv signals were triggered by P0 and recorded simultaneously. The results include peak values for Psv of 100.8 ± 51.6 kPa (mean ± SD) and for SFP displacement of 72.6 ± 56.4 µm, which are consistent with published experimental results and finite element modeling data. Most of the P0 input energy flux into the cochlea occurred within 2 ms and resulted in 10-70 µJ total energy entering the cochlea. Although the middle ear pressure gain was close to that measured under acoustic stimulus conditions, the nonlinear behavior of the middle ear was observed from the elevated cochlear input impedance. For the first time, SFP movement and intracochlear pressure Psv have been successfully measured simultaneously during blast exposure. This study provides a new methodology and experimental data for determining the energy flux entering the cochlea during a blast, which serves as an injury index for quantifying blast-induced auditory damage.

Occluded insertion loss from intracochlear pressure measurements during acoustic shock wave exposure.

INTRODUCTION: Injuries to the peripheral auditory system are among the most common results of high intensity impulsive noise exposure. Hearing protection can mitigate this injury, but careful assessment of the insertion loss they provide is necessary. Insertion loss is typically measured using microphone-based acoustic manikins to measure the decrease in sound pressure level transmitted into the ear canal, which precisely measure the change in air conducted sound, but neglect alternate pathways to the inner ear such as bone conduction. In a previous study we reported intracochlear pressures in cadaveric human specimens to acoustic shock waves, which revealed a substantial bone conducted component (Greene, et al., 2018). Here we evaluate insertion loss to several hearing protection devices (HPDs) in those same specimens using intracochlear pressure measurements. METHODS: Human cadaver heads were exposed to impulsive acoustic pressure waves with peak overpressures of 7 and 28 kPa (171 & 183 dB SPL). Ear canal (EAC), middle ear, and intracochlear sound pressure levels were measured bilaterally with fiber-optic pressure sensors. Surface-mounted sensors measured SPL and skull strain near the opening of each EAC and at the forehead. Responses were measured with specimen ears unoccluded, as reported previously, as well as fitted with four types of HPDs. Impulse peak insertion loss (IPIL) and impulse spectrum insertion loss (ISIL) were calculated for each HPD. RESULTS: For all HPDs, IPIL generally increases with exposure level, though ISIL tended to be more consistent, and the spectral characteristics across frequency appear to be highly dependent on exposure level. ISIL measured in the ear canal tended to overestimate insertion loss measured in the cochlea, particularly at frequencies > 1 kHz; however, low signal-to-noise in intracochlear pressures limited comparisons. As a proof of concept, 36 low-level unoccluded exposures, were averaged together, and the resulting signal-to-noise ratio was improved by up to 15 dB. CONCLUSIONS: Insertion loss measured in the cochlea was lower than in the ear canal, suggesting substantial contributions from transmission pathways in parallel with air conduction (e.g., bone conduction) were present, which will require novel strategies to mitigate. However, high variance was observed, and noise reduction strategies should be utilized in future studies to facilitate more precise insertion loss estimates.

Intracochlear pressure and temporal bone motion interaction under bone conduction stimulation.

BACKGROUND: Under bone conduction (BC) stimulation, the otic capsule, and surrounding temporal bone, undergoes a complex 3-dimentional (3D) motion that depends on the frequency, location and coupling of the stimulation. The correlation between the resultant intracochlear pressure difference across the cochlear partition and the 3D motion of the otic capsule is not yet known and is to be investigated. METHODS: Experiments were conducted in 3 fresh frozen cadaver heads, individually on each temporal bone, resulting in a total of 6 samples. The skull bone was stimulated, via the actuator of a BC hearing aid (BCHA), in the frequency range of 0.1-20 kHz. Stimulation was applied at the ipsilateral mastoid and the classical BAHA location via a conventional transcutaneous (5-N steel headband) and percutaneous coupling, sequentially. Three-dimensional motions were measured across the lateral and medial (intracranial) surfaces of the skull, the ipsilateral temporal bone, the skull base, as well as the promontory and stapes. Each measurement consisted of 130-200 measurement points (∼5-10 mm pitch) across the measured skull surface. Additionally, intracochlear pressure in the scala tympani and scala vestibuli was measured via a custom-made intracochlear acoustic receiver. RESULTS: While there were limited differences in the magnitude of the motion across the skull base, there were major differences in the deformation of different sections of the skull. Specifically, the bone near the otic capsule remained primarily rigid across all test frequency (above 10 kHz), in contrast to the skull base, which deformed above 1-2 kHz. Above 1 kHz, the ratio, between the differential intracochlear pressure and the promontory motion, was relatively independent of coupling and stimulation location. Similarly, the stimulation direction appears to have no influence on the cochlear response, above 1 kHz. CONCLUSIONS: The area around the otic capsule appears rigid up to significantly higher frequencies than the rest of the skull surface, resulting in primarily inertial loading of the cochlear fluid. Further work should be focused at the investigation of the solid-fluid interaction between the bony walls of the otic capsule and the cochlear contents.

Intracochlear pressure as an objective measure for perceived loudness with bone conduction implants.

BACKGROUND: The generally accepted method to assess the functionality of novel bone conduction implants in a preclinical stage is to experimentally measure the vibratory response of the cochlear promontory. Yet, bone conduction of sound is a complex propagation phenomenon, depending on both frequency and amplitude, involving different conduction pathways. OBJECTIVES: The aim of this study is to validate the use of intracochlear sound pressure (ICP) as an objective indicator for perceived loudness for bone conduction stimulation. It is investigated whether a correlation exists between intracochlear sound pressure measurements in cadaveric temporal bones and clinically obtained results using the outcome of a loudness balancing experiment. METHODS: Ten normal hearing subjects were asked to balance the perceived loudness between air conducted (AC) sound and bone conducted (BC) sound by changing the AC stimulus. Mean balanced thresholds were calculated and used as stimulation levels in a cadaver trial (N = 4) where intracochlear sound pressure was measured during AC and BC stimulation to assess the correlation with the measured clinical data. The intracochlear pressure was measured at the relatively low stimulation amplitude of 80 dBHL using a lock-in amplification technique. RESULTS: Applying AC and BC stimulation at equal perceived loudness on cadaveric heads yield a similar differential intracochlear pressure, with differences between AC and BC falling within the range of variability of normal hearing test subjects. CONCLUSION: Comparing the perceived loudness at 80 dB HL for both AC and BC validates intracochlear pressure as an objective indicator of the cochlear drive. The measurement setup is more time-intensive than measuring the vibratory response of the cochlear promontory, yet it provides direct information on the level of the cochlear scalae.

Intracochlear pressure in cadaver heads under bone conduction and intracranial fluid stimulation.

BACKGROUND: The frequency dependent contributions of the various bone conduction pathways are poorly understood, especially the fluid pathway. The aim of this work is to measure and investigate sound pressure propagation from the intracranial space to the cochlear fluid. METHODS: Stimulation was provided sequentially to the bone (BC) or directly to the intracranial contents (hydrodynamic conduction, or HC) in four cadaver heads, where each ear was tested individually, for a total of 8 samples. Intracranial pressure was generated and monitored via commercial hydrophones, while the intracochlear sound pressure (ICSP) levels were monitored via custom-made intracochlear acoustic receivers (ICAR). In parallel, measurements of the 3D motion of the cochlear promontory and stapes were made via 3D Laser Doppler Vibrometer (3D LDV). RESULTS: Reliability of the intracochlear sound pressure measurements depends on the immobilization of the ICAR relative to the otic capsule. Regardless of the significant differences in absolute stapes and promontory motion, the ratios between the otic capsule velocity, the stapes volume velocity (relative to the cochlea), and the intracochlear pressure were very similar under BC and HC stimulus. Under HC, the cochlear fluid appears be activated by an osseous pathway, rather than a direct non-osseous pathway from the cerebrospinal fluid (CSF), however, the osseous pathway itself is activated by the CSF pressure. CONCLUSIONS: Data suggests that the skull bone surrounding the brain and CSF could play a role in the interaction between the two CSF and the cochlea, under both stimulation conditions, at high frequencies, while inertia is dominant factor at low frequencies. Further work should be focused on the investigation of the solid-fluid interaction between the skull bone walls and the intracranial content.

Effect of Underwater Insertion on Intracochlear Pressure.

Background: The importance of intracochlear pressure during cochlear electrode insertion for the preservation of residual hearing has been widely discussed. Various aspects of pre-insertional, intra-insertional, and post-insertional relevant conditions affect intracochlear pressure. The fluid situation at the round window during electrode insertion has been shown to be an influential factor. Aims/Objectives: The aim of the study was to compare various insertion techniques in terms of the fluid situation at the round window. Material and Methods: We performed insertion of cochlear implant electrodes in a curled artificial cochlear model. We placed and fixed the pressure sensor at the tip of the cochlea. In parallel to the insertions, we evaluated the maximum amplitude of intracochlear pressure under four different fluid conditions at the round window: (1) hyaluronic acid; (2) moisturized electrode, dry middle ear; (3) middle ear filled with fluid (underwater); and (4) moisturized electrode, wet middle ear, indirectly inserted. Results: We observed that the insertional intracochlear pressure is dependent on the fluid situation in front of the round window. The lowest amplitude changes were observed for the moisturized electrode indirectly inserted in a wet middle ear (0.13 mmHg ± 0.07), and the highest values were observed for insertion through hyaluronic acid in front of the round window (0.64 mmHg ± 0.31). Conclusions: The fluid state in front of the round window influences the intracochlear pressure value during cochlear implant electrode insertion in our model. Indirect insertion of a moisturized electrode through a wet middle ear experimentally generated the lowest pressure values. Hyaluronic acid in front of the round window leads to high intracochlear pressure in our non-validated artificial model.

Dynamic intracochlear pressure measurement during cochlear implant electrode insertion.

Background: Electrode insertion into the cochlea can cause significant pressure changes inside the cochlea with assumed effects on the cochlea's functionality regarding residual hearing. Model-based intracochlear pressure (ICP) changes were performed statically at the cochlear helix. Aims/objectives: The aim of this study was to observe dynamic pressure measurements during electrode insertion directly at the cochlear implant electrode. Material and methods: The experiments were performed in an uncurled cochlear model that contained a volume value equivalent to a full cochlea. A microfibre pressure sensor was attached at one of two positions on a cochlear implant electrode and inserted under different insertional conditions. Results: We observed the ICP increase depending on the insertional depth. A sensor-position-specific pressure change is insertional-depth dependent. Interval insertion did not lead to a lower peak insertional ICP. Conclusions and significance: In contrast to the static pressure-sensor measurement in the artificial model's helix, a dynamic measurement directly at the electrode shows the pressure profile to increase based on the insertional depth. A mechanical traumatic relevance of the observed pressure values cannot be fully excluded.

Impedances of the inner and middle ear estimated from intracochlear sound pressures in normal human temporal bones.

For almost a decade, we have measured intracochlear sound pressures evoked by air conducted (AC) sound presented to the ear canal in many fresh human cadaveric specimens. Similar measurements were also obtained during round window (RW) mechanical stimulation in multiple specimens. In the present study, we use our accumulated data of intracochlear pressures and simultaneous velocity measurements of the stapes or RW to determine acoustic impedances of the cochlear partition, RW, and the leakage paths from scala vestibuli and scala tympani, as well as the reverse middle ear impedance. With these impedances, we develop a computational lumped-element model of the normal ear that illuminates fundamental mechanisms of sound transmission. To calculate the impedances for our model, we use data that passes strict inclusion criteria of: (a) normal middle-ear transfer function defined as the ratio of stapes velocity to ear-canal sound pressure, (b) no evidence of air within the inner ear, and (c) tight control of the pressure sensor sensitivity. After this strict screening, updated normal means, as well as individual representative data, of ossicular velocities and intracochlear pressures for AC and RW stimulation are used to calculate impedances. This work demonstrates the existence and the value of physiological acoustic leak impedances that can sometimes contribute significantly to sound transmission for some stimulation modalities. This model allows understanding of human sound transmission mechanisms for various sound stimulation methods such as AC, RW, and bone conduction, as well as sound transmission related to otoacoustic emissions.

Intracochlear pressure in response to high intensity, low frequency sounds in chinchilla.

Exposure to high intensity (blast) sounds can result in both conductive and sensorineural damage to hearing. This includes rupture of the tympanic membrane and dislocation of the middle ear ossicles, as well as damage to the inner and outer hair cells in the cochlea. A clearer understanding of how the hearing system responds to blast could help us better prevent auditory trauma, and support those who have been exposed to such sounds. Chinchillas are often used in studies of hearing due to the similarity between the chinchilla and human audiograms. The suitability of their use in research on auditory trauma from blast noise will depend on the extent to which cochlear pressures generated in chinchillas compare to those in humans. In order to gain a more detailed understanding of the response of the ear to high intensity sounds, a custom built sound concentrating horn was used to expose chinchilla cadaveric ears to a series of single frequency tones between 10 and 1280 Hz, with varying intensities from 90 to 194 dB SPL while intracochlear pressures were measured simultaneously in the scala vestibuli and scala tympani. These results were then compared to similar, previously published data from human cadavers. In both human and chinchillas, intracochlear pressures increased with applied sound pressure up to about 120 dB SPL, but began to saturate at higher intensities. The exact saturation point and the saturation pressures showed a strong frequency dependence. Intracochlear pressure magnitudes in chinchillas show some similarities with those measured in humans, but also significant differences, particularly at very high intensity levels such as those found in a blast. These differences should be taken into account when conducting blast studies in chinchillas.

Intracochlear pressure measurements during acoustic shock wave exposure.

INTRODUCTION: Injuries to the peripheral auditory system are among the most common results of high intensity impulsive acoustic exposure. Prior studies of high intensity sound transmission by the ossicular chain have relied upon measurements in animal models, measurements at more moderate sound levels (i.e. < 130 dB SPL), and/or measured responses to steady-state noise. Here, we directly measure intracochlear pressure in human cadaveric temporal bones, with fiber optic pressure sensors placed in scala vestibuli (SV) and tympani (ST), during exposure to shock waves with peak positive pressures between ∼7 and 83 kPa. METHODS: Eight full-cephalic human cadaver heads were exposed, face-on, to acoustic shock waves in a 45 cm diameter shock tube. Specimens were exposed to impulses with nominal peak overpressures of 7, 28, 55, & 83 kPa (171, 183, 189, & 192 dB pSPL), measured in the free field adjacent to the forehead. Specimens were prepared bilaterally by mastoidectomy and extended facial recess to expose the ossicular chain. Ear canal (EAC), middle ear, and intracochlear sound pressure levels were measured with fiber-optic pressure sensors. Surface-mounted sensors measured SPL and skull strain near the opening of each EAC and at the forehead. RESULTS: Measurements on the forehead showed incident peak pressures approximately twice that measured by adjacent free-field and EAC entrance sensors, as expected based on the sensor orientation (normal vs tangential to the shock wave propagation). At 7 kPa, EAC pressure showed gain, calculated from the frequency spectra, consistent with the ear canal resonance, and gain in the intracochlear pressures (normalized to the EAC pressure) were consistent with (though somewhat lower than) previously reported middle ear transfer functions. Responses to higher intensity impulses tended to show lower intracochlear gain relative to EAC, suggesting sound transmission efficiency along the ossicular chain is reduced at high intensities. Tympanic membrane (TM) rupture was observed following nearly every exposure 55 kPa or higher. CONCLUSIONS: Intracochlear pressures reveal lower middle-ear transfer function magnitudes (i.e. reduced gain relative to the ear canal) for high sound pressure levels, thus revealing lower than expected cochlear exposure based on extrapolation from cochlear pressures measured at more moderate sound levels. These results are consistent with lowered transmissivity of the ossicular chain at high intensities, and are consistent with our prior report measuring middle ear transfer functions in human cadaveric temporal bones with high intensity tone pips.

Intracochlear Sound Pressure Measurements in Normal Human Temporal Bones During Bone Conduction Stimulation.

Bone conduction (BC) is heavily relied upon in the diagnosis and treatment of hearing loss, but is poorly understood. For example, the relative importance and frequency dependence of various identified BC sound transmission mechanisms that contribute to activate the cochlear partition remain unknown. Recently, we have developed techniques in fresh human cadaveric specimens to directly measure scalae pressures with micro-fiberoptic sensors, enabling us to monitor the input pressure drive across the cochlear partition that triggers the cochlear traveling wave during air conduction (AC) and round-window stimulation. However, BC stimulation poses challenges that can result in inaccurate intracochlear pressure measurements. Therefore, we have developed a new technique described here that allows for precise measurements during BC. Using this new technique, we found that BC stimulation resulted in pressure in scala vestibuli that was significantly higher in magnitude than in scala tympani for most frequencies, such that the differential pressure across the partition-the input pressure drive-was similar to scala vestibuli pressure. BC (stimulated by a Bone Anchored Hearing Aid [Baha]) showed that the mechanisms of sound transmission in BC differ from AC, and also showed the limitations of the Baha bandwidth. Certain kinematic measurements were generally proportional to the cochlear pressure input drive: for AC, velocity of the stapes, and for BC, low-frequency acceleration and high-frequency velocity of the cochlear promontory. Therefore, our data show that to estimate cochlear input drive in normal ears during AC, stapes velocity is a good measure. During BC, cochlear input drive can be estimated for low frequencies by promontory acceleration (though variable across ears), and for high frequencies by promontory velocity.

Cochlear implant electrode sealing techniques and related intracochlear pressure changes.

BACKGROUND: The inserted cochlear implanted electrode is covered at the site of the round window or cochleostomy to prevent infections and leakage. In a surgically hearing preservational concept, low intracochlear pressure changes are of high importance. The aim of this study was to observe intracochlear pressure changes due to different sealing techniques in a cochlear model. METHODS: Cochlear implant electrode insertions were performed in an artifical cochlear model and the intracochlear pressure changes were recorded in parallel with a micro-pressure sensor positioned in the apical region of the cochlea model to follow the maximum amplitude of intracochlear pressure. Four different sealing conditions were compared: 1) overlay, 2) overlay with fascia pushed in, 3) donut-like fascia ring, 4) donut-like fascia ring pushed in. RESULTS: We found statistically significant differences in the occurrence of maximum amplitude of intracochlear pressure peak changes related to sealing procedure comparing the different techniques. While the lowest amplitude changes could be observed for the overlay technique (0.14 mmHg ± 0.06) the highest values could be observed for the donut-like pushed in technique (1.79 mmHg ± 0.69). CONCLUSION: Sealing the electrode inserted cochlea can lead to significant intracochlear pressure changes. Pushing in of the sealing tissue cannot be recommended.

Insertional depth-dependent intracochlear pressure changes in a model of cochlear implantation.

CONCLUSION: Over time, a homogenous increase in intracochlear pressure was seen in every experiment. Significant reductions in terms of amplitude variation and insertion depth were observed over time, using the one-point-supported insertion method. The frequency of peaks between the thirds was significantly lower when using the two-points-supported insertion method. OBJECTIVES: The preservation of residual hearing and minimization of intracochlear trauma are two of the major goals in modern cochlear implantation (CI) surgery. It is assumed that intracochlear pressure measurements yield information about the intracochlear behavior of the electrode itself in the cochlea. The aim of this study was to investigate temporal intracochlear fluid pressure changes using two different kinds of insertion conditions. METHOD: Cochlear implantations with the Advanced Bionics IJ® electrode were performed in an artificial cochlear model with a constant insertional speed of 0.5 mm/s provided by a linear actor. Amplitude pressure changes and number of pressure peaks were evaluated for every part. RESULT: Intracochlear fluid pressure changes are assumed to affect the preservation of residual hearing and should be minimized. The stability and reduction of movement of a lateral wall IJ® electrode increase at deeper insertion and affect intracochlear fluid pressure amplitude.

Stapes displacement and intracochlear pressure in response to very high level, low frequency sounds.

The stapes is held in the oval window by the stapedial annular ligament (SAL), which restricts total peak-to-peak displacement of the stapes. Previous studies have suggested that for moderate (<130 dB SPL) sound levels intracochlear pressure (PIC), measured at the base of the cochlea far from the basilar membrane, increases directly proportionally with stapes displacement (DStap), thus a current model of impulse noise exposure (the Auditory Hazard Assessment Algorithm for Humans, or AHAAH) predicts that peak PIC will vary linearly with DStap up to some saturation point. However, no direct tests of DStap, or of the relationship with PIC during such motion, have been performed during acoustic stimulation of the human ear. In order to examine the relationship between DStap and PIC to very high level sounds, measurements of DStap and PIC were made in cadaveric human temporal bones. Specimens were prepared by mastoidectomy and extended facial recess to expose the ossicular chain. Measurements of PIC were made in scala vestibuli (PSV) and scala tympani (PST), along with the SPL in the external auditory canal (PEAC), concurrently with laser Doppler vibrometry (LDV) measurements of stapes velocity (VStap). Stimuli were moderate (∼100 dB SPL) to very high level (up to ∼170 dB SPL), low frequency tones (20-2560 Hz). Both DStap and PSV increased proportionally with sound pressure level in the ear canal up to approximately ∼150 dB SPL, above which both DStap and PSV showed a distinct deviation from proportionality with PEAC. Both DStap and PSV approached saturation: DStap at a value exceeding 150 µm, which is substantially higher than has been reported for small mammals, while PSV showed substantial frequency dependence in the saturation point. The relationship between PSV and DStap remained constant, and cochlear input impedance did not vary across the levels tested, consistent with prior measurements at lower sound levels. These results suggest that PSV sound pressure holds constant relationship with DStap, described by the cochlear input impedance, at these, but perhaps not higher, stimulation levels. Additionally, these results indicate that the AHAAH model, which was developed using results from small animals, underestimates the sound pressure levels in the cochlea in response to high level sound stimulation, and must be revised.