The Codacs? Direct Acoustic Cochlear Implant Actuator: Exploring Alternative Stimulation Sites and Their Stimulation Efficiency

This work assesses the efficiency of the Codacs system actuator (Cochlear Ltd., Sydney Australia) in different inner ear stimulation modalities. Originally the actuator was intended for direct perilymph stimulation after stapedotomy using a piston prosthesis. A possible alternative application is the stimulation of middle ear structures or the round window (RW). Here the perilymph stimulation with a K-piston through a stapes footplate (SFP) fenestration (N = 10) as well as stimulation of the stapes head (SH) with a Bell prosthesis (N = 9), SFP stimulation with an Omega/Aerial prosthesis (N = 8) and reverse RW stimulation (N = 10) were performed in cadaveric human temporal bones (TBs). Codacs actuator output is expressed as equivalent sound pressure level (eq. SPL) using RW and SFP displacement responses, measured by Laser Doppler velocimetry as reference. The axial actuator coupling force in stimulation of stapes and RW was adjusted to ~ 5 mN. The Bell prosthesis and Omega/Aerial prosthesis stimulation generated similar mean eq. SPLs (Bell: 127.5–141.8 eq. dB SPL; Omega/Aerial: 123.6–143.9 eq. dB SPL), being significantly more efficient than K-piston perilymph stimulation (108.6–131.6 eq. dB SPL) and RW stimulation (108.3–128.2 eq. dB SPL). Our results demonstrate that SH, SFP and RW are adequate alternative stimulation sites for the Codacs actuator using coupling prostheses and an axial coupling force of ~ 5 mN. Based on the eq. SPLs, all investigated methods were adequate for in vivo hearing aid applications, provided that experimental conditions including constant coupling force will be implemented.     

Design of a resilient ring for middle ear’s chamber stapes prosthesis

Abstract

This paper presents the process of designing a new elastic element replacing a membrane in the chamber stapes prosthesis (ChSP). The results of the study are volume displacement characteristics obtained for the prosthesis and physiological stapes. Simulation tests on a 3D CAD model have confirmed that a properly designed ring can stimulate perilymph with the same or greater efficacy as the physiological stapes footplate placed on the elastic annular ligament. The ChSP with a new elastic element creates a good chance of improving hearing in patients suffering from otosclerosis.     

Human cochlear hydrodynamics: A high-resolution μCT-based finite element study

Measurements of perilymph hydrodynamics in the human cochlea are scarce, being mostly limited to the fluid pressure at the basal or apical turn of the scalae vestibuli and tympani. Indeed, measurements of fluid pressure or volumetric flow rate have only been reported in animal models. In this study we imaged the human ear at 6.7 and 3-µm resolution using µCT scanning to produce highly accurate 3D models of the entire ear and particularly the cochlea scalae. We used a contrast agent to better distinguish soft from hard tissues, including the auditory canal, tympanic membrane, malleus, incus, stapes, ligaments, oval and round window, scalae vestibule and tympani. Using a Computational Fluid Dynamics (CFD) approach and this anatomically correct 3D model of the human cochlea, we examined the pressure and perilymph flow velocity as a function of location, time and frequency within the auditory range. Perimeter, surface, hydraulic diameter, Womersley and Reynolds numbers were computed every 45° of rotation around the central axis of the cochlear spiral. CFD results showed both spatial and temporal pressure gradients along the cochlea. Small Reynolds number and large Womersley values indicate that the perilymph fluid flow at auditory frequencies is laminar and its velocity profile is plug-like. The pressure was found 102–106° out of phase with the fluid flow velocity at the scalae vestibule and tympani, respectively. The average flow velocity was found in the sub-µm/s to nm/s range at 20–100 Hz, and below the nm/s range at 1–20 kHz.     

The group delay and suppression pattern of the cochlear microphonic potential recorded at the round window

Background

It is commonly assumed that the cochlear microphonic potential (CM) recorded from the round window (RW) is generated at the cochlear base. Based on this assumption, the low-frequency RW CM has been measured for evaluating the integrity of mechanoelectrical transduction of outer hair cells at the cochlear base and for studying sound propagation inside the cochlea. However, the group delay and the origin of the low-frequency RW CM have not been demonstrated experimentally.

Methodology/Principal Findings

This study quantified the intra-cochlear group delay of the RW CM by measuring RW CM and vibrations at the stapes and basilar membrane in gerbils. At low sound levels, the RW CM showed a significant group delay and a nonlinear growth at frequencies below 2 kHz. However, at high sound levels or at frequencies above 2 kHz, the RW CM magnitude increased proportionally with sound pressure, and the CM phase in respect to the stapes showed no significant group delay. After the local application of tetrodotoxin the RW CM below 2 kHz became linear and showed a negligible group delay. In contrast to RW CM phase, the BM vibration measured at location ∼2.5 mm from the base showed high sensitivity, sharp tuning, and nonlinearity with a frequency-dependent group delay. At low or intermediate sound levels, low-frequency RW CMs were suppressed by an additional tone near the probe-tone frequency while, at high sound levels, they were partially suppressed only at high frequencies.

Conclusions/Significance

We conclude that the group delay of the RW CM provides no temporal information on the wave propagation inside the cochlea, and that significant group delay of low-frequency CMs results from the auditory nerve neurophonic potential. Suppression data demonstrate that the generation site of the low-frequency RW CM shifts from apex to base as the probe-tone level increases.     

Middle-ear dynamics before and after ossicular replacement

The mechanism of hearing involves conduction of mechanical vibrations along the ossicular chain to the inner ear. An acoustic wave is collected and transformed as it passes down the ear canal and impacts on the tympanic membrane (ear drum). The drum is connected to the inner-ear by three ossicle bones (malleus, incus, and stapes) in a complex arrangement, which serves to further transform the mechanical vibration before it reaches the cochlea of the inner ear. What is the mechanical function of the ossicular chain, and what are the biomechanical consequences of surgical reconstruction with prostheses? To answer these questions, a three-dimensional finite element model of the outer ear canal and middle ear was generated. The dynamical behaviour was predicted for the normal ear, and an ear reconstructed with partial and total ossicular replacement prostheses. For the normal ear, stapes amplitudes of 1x10(-8) m at low frequencies decrease to 4x10(-10)m at approximately 3kHz with several resonance peeks in between, most significantly at approximately 1kHz. Thereafter a further resonance is predicted at 4kHz associated with the ear canal. The behaviour is changed fundamentally by adding a prosthesis; the partial replacement increases the vibratory coupling of the drum and the stapes compared to the normal ear whereas the total replacement does the opposite, and is predicted to have the disadvantage of bringing several new resonances of the ossicular chain into the hearing range. It is hypothesised that the function of the malleus-incus-stapes arrangement is to link the drum to the oval window with the flexibility required for impedance matching but the rigidity to prevent unconstrainable resonances from occurring in the hearing range. If this is true, then the structural stiffness of ossicular chain is the critical design variable for middle-ear replacement prostheses.     

Study of age-related changes in Middle ear transfer function

Abstract

Osteoporosis (OP) is common with advancing age. Several studies have shown a strong correlation between OP and otosclerosis. However, no studies have investigated OP of the malleus, incus or stapes in the human middle ear, its effect on middle ear transfer function. Here, we investigate whether these three ossicles develop OP, and how this affects middle ear transfer function. The effect of OP on middle ear transfer function was investigated in simulations based on a finite element (FE) method. First, the FE model used in our previous study was refined, and optimized by introducing viscoelastic properties to selected soft tissues of the middle ear. Then, the FE model was used to simulate OP of the three ossicles and assess its influence on middle ear transfer function. Other possible age-related changes, such as stiffness of the joints or ligaments in the middle ear, were also investigated. The results indicated that OP of the ossicles could increase the high frequency displacement of both the umbo and stapes footplate (FP). However, the stiffness of the middle ear soft tissue can lead to the decrease of middle ear gain at lower frequencies. Furthermore, loosening of these joints or ligaments could increase displacement of the umbo and stapes FP. In conclusion, although age-related hearing loss is most commonly conceived of as sensorineural hearing loss (SNHL), we found that age-related changes may also include OP and changes in joint stiffness, but these will have little effect on middle ear transfer function in elderly people.     

Scaling of the marsupial middle ear and its functional significance

The marsupial middle ear performs an anatomical impedance matching for acoustic energy travelling in air to reach the cochlea. The size of the middle ear sets constraints for the frequencies transmitted. For generalized placental mammals, it has been shown that the limit for high-frequency hearing can be predicted on the basis of middle ear ossicle mass, provided that the ears fulfil requirements of isometry. We studied the interspecific size variation of the middle ear in 23 marsupial species, with the following measurable parameters: skull mass, condylobasal length, ossicular masses for malleus, incus and stapes, tympanic membrane area, oval window area, and lever arm lengths for malleus and incus. Our results show that the middle ear size grows with negative allometry in relation to body size and that the internal proportions of the marsupial middle ear are largely isometric. This resembles the situation in placental mammals and allows us to use their isometric middle ear model to predict the high-frequency hearing limit for marsupials. We found that the isometry model predicts the high-frequency hearing limit for different marsupials well, indicating that marsupials can be used as auditory models for general therian mammalian hearing. At very high frequencies, other factors, such as the inner ear, seem to constrain mammalian hearing.     

The effects of cetrimide and potassium bromate on the potassium ion concentration in the inner ear fluid of the guinea-pig

The mammalian inner ear is located deep within the temporal bone. The organ of Corti, the delicate sensory system for sound, is surrounded by two fluid systems; the potassium-rich endolymph and the sodium-rich perilymph. The pathogenesis of inner ear deafness is thought to be largely due to an imbalance of potassium and sodium ions in the inner ear fluids. Dynamic changes in K+ in the endolymph and perilymph were studied in the guinea-pig following cetrimide (cetrimonium bromide, a powerful cationic detergent which shows ototoxicity) applications on the round window membrane, intramuscular injection of potassium bromate (bread whitener, known to cause renal damage and permanent deafness in animals and man). Maximum fall in K+ concentration in the endolymoh (mM/min) and maximum K+ conductance (mM/min/mV) were 3.54 +/- 1.65 and 0.036 +/- 0.02 in cetrimide, and 1.85 +/- 0.35 and 0.021 +/- 0.009 in potassium bromate, respectively. In view of these findings, the influence of the active transport mechanism to K+ concentrations are discussed in comparison with dynamic changes in endolymph K+ induced by asphyxia and ethacrynic acid.     

A chronic implant for recording of cochlear potentials in primates

A new technique for the continuous recording of peripheral bioelectrical activity in the auditory system of primates is described. Because of basic differences in the anatomy of the temporal bone, the approach to the round window of the cochlea is more difficult in most primates than in lower animals. A relatively simple surgical approach, which made possible the placement of an electrode into the perilymph of the inner ear via the well-demarcated horizontal semicircular canal was therefore developed and is described in detail. The bared tip of a Teflon-coated wire was cemented into the canal opening with carboxylate cement, and the wire attached to a permanent electrical connector on the skull. Cochlear microphonic and action potentials of 50 to 100 μV amplitude were thus recorded on a continuing basis at the same time that behavioral studies of primate auditory acuity were conducted.     

3D finite element model of the chinchilla ear for characterizing middle ear functions

Chinchilla is a commonly used animal model for research of sound transmission through the ear. Experimental measurements of the middle ear transfer function in chinchillas have shown that the middle ear cavity greatly affects the tympanic membrane (TM) and stapes footplate (FP) displacements. However, there is no finite element (FE) model of the chinchilla ear available in the literature to characterize the middle ear functions with the anatomical features of the chinchilla ear. This paper reports a recently completed 3D FE model of the chinchilla ear based on X-ray micro-computed tomography images of a chinchilla bulla. The model consisted of the ear canal, TM, middle ear ossicles and suspensory ligaments, and the middle ear cavity. Two boundary conditions of the middle ear cavity wall were simulated in the model as the rigid structure and the partially flexible surface, and the acoustic-mechanical coupled analysis was conducted with these two conditions to characterize the middle ear function. The model results were compared with experimental measurements reported in the literature including the TM and FP displacements and the middle ear input admittance in chinchilla ear. An application of this model was presented to identify the acoustic role of the middle ear septa—a unique feature of chinchilla middle ear cavity. This study provides the first 3D FE model of the chinchilla ear for characterizing the middle ear functions through the acoustic-mechanical coupled FE analysis.     

Isolation of mumps virus from the inner ear after sudden deafness.

A 26-year-old woman with bilateral otosclerosis underwent right stapedectomy with an excellent result. One year later, however, she developed symptoms of mumps and within two days was completely deaf in the right ear. Prompt surgical exploration excluded a complication of the otosclerosis and a perilymph fistula, but culture of a sample of perilymph grew mumps virus. The case provides direct evidence of a relation between mumps virus infection and inner-ear damage.     

Mechanics of the inner ear of the bullfrog (Rana catesbeiana): the contact membranes and the periotic canal

The frog inner ear consists of a complex of fluid-filled membranous sacs and canals containing eight distinct clusters of sensory hair cells. In this study we attempt to delineate the potential pathways for acoustic energy flow toward two of these clusters located within the amphibian papilla and the basilar papilla. Detailed morphological measurements of the periotic canal based on internal casts of the inner ear in the bullfrog (Rana catesbeiana) revealed that it is divided into a wide, tapered section and a narrower section comprised of two branches – one short and blind projecting into the endolymphatic space and another longer, terminating in the round window. Additionally, we used laser Doppler velocimetry to record the velocity responses of the contact membranes of the amphibian papilla and basilar papilla. We found that the acoustic energy flow through these two structures is frequency dependent such that the amphibian papilla contact membrane displays a peak velocity amplitude at frequencies less than 500 Hz, whereas the basilar papilla contact membrane velocity response exhibits a maximum above 1100 Hz. Our data advocate a mechanical substrate underlying the frequency segregation in the auditory nerve fibers innervating the amphibian papilla and the basilar papilla. Accepted: 9 March 2000     

The human otitis media with effusion: a numerical-based study

Otitis media is a group of inflammatory diseases of the middle ear. Acute otitis media and otitis media with effusion (OME) are its two main types of manifestation. Otitis media is common in children and can result in structural alterations in the middle ear which will lead to hearing losses. This work studies the effects of an OME on the sound transmission from the external auditory meatus to the inner ear. The finite element method was applied on the present biomechanical study. The numerical model used in this work was built based on the geometrical information obtained from The visible ear project. The present work explains the mechanisms by which the presence of fluid in the middle ear affects hearing by calculating the magnitude, phase and reduction of the normalized umbo velocity and also the magnitude and phase of the normalized stapes velocity. A sound pressure level of 90 dB SPL was applied at the tympanic membrane. The harmonic analysis was performed with the auditory frequency varying from 100 Hz to 10 kHz. A decrease in the response of the normalized umbo and stapes velocity as the tympanic cavity was filled with fluid was obtained. The decrease was more accentuated at the umbo.     

Fast Reverse Propagation of Sound in the Living Cochlea

The auditory sensory organ, the cochlea, not only detects but also generates sounds. Such sounds, otoacoustic emissions, are widely used for diagnosis of hearing disorders and to estimate cochlear nonlinearity. However, the fundamental question of how the otoacoustic emission exits the cochlea remains unanswered. In this study, emissions were provoked by two tones with a constant frequency ratio, and measured as vibrations at the basilar membrane and at the stapes, and as sound pressure in the ear canal. The propagation direction and delay of the emission were determined by measuring the phase difference between basilar membrane and stapes vibrations. These measurements show that cochlea-generated sound arrives at the stapes earlier than at the measured basilar membrane location. Data also show that basilar membrane vibration at the emission frequency is similar to that evoked by external tones. These results conflict with the backward-traveling-wave theory and suggest that at low and intermediate sound levels, the emission exits the cochlea predominantly through the cochlear fluids.     

In Vivo Impedance of the Gerbil Cochlear Partition at Auditory Frequencies

The specific acoustic impedance of the cochlear partition was measured from 4 to 20 kHz in the basal turn of the gerbil cochlea, where the best frequency is ∼40 kHz. The acoustic impedance was found as the ratio of driving pressure to velocity response. It is the physical attribute that governs cochlear mechanics and has never before been directly measured, to our knowledge. The basilar membrane velocity was measured through the transparent round window membrane. Simultaneously, the intracochlear pressure was measured close to the stapes and quite close to the cochlear partition. The impedance phase was close to −90° and the magnitude decreased with frequency, consistent with stiffness-dominated impedance. The resistive component of the impedance was relatively small. Usually the resistance was negative at frequencies below 8 kHz; this unexpected finding might be due to other vibration modes within the cochlear partition.     

Biomechanics of the tympanic membrane

The tympanic membrane is a key component of the human auditory apparatus which is a complex biomechanical system, devoted to sound reception and perception. Over the past 30 years, various bioengineering approaches have been applied to the ear modeling and particularly to the middle part. The tympanic membrane, included in the middle ear, transfers sound waves into mechanical vibration from the ear canal into the middle ear. Changes in structure and mechanical properties of the tympanic membrane due to middle ear diseases or damages can deteriorate sound transmission. An accurate model of the tympanic membrane, which simulates the acoustic-mechanical transmission, could improve clinical surgical intervention. In this paper a detailed survey of the biomechanics and the modeling of the tympanic membrane focusing on the finite element method is conduced. Eight selected models are evaluated and compared deducing the main features and most design parameters from published models, mainly focusing on geometric, constraint and material aspects. Non-specified parameters are replaced with the most commonly employed values. Our simulation results (in terms of modal frequencies and umbo displacement), compared with published numerical and experimental results, show a good agreement even if some scattering appears to indicate the need of further investigation and experimental validation.     

Infrasonic hearing in birds: a review of audiometry and hypothesized structure–function relationships

The perception of airborne infrasound (sounds below 20 Hz, inaudible to humans except at very high levels) has been documented in a handful of mammals and birds. While animals that produce vocalizations with infrasonic components (e.g. elephants) present conspicuous examples of potential use of infrasound in the context of communication, the extent to which airborne infrasound perception exists among terrestrial animals is unclear. Given that most infrasound in the environment arises from geophysical sources, many of which could be ecologically relevant, communication might not be the only use of infrasound by animals. Therefore, infrasound perception could be more common than currently realized. At least three bird species, each of which do not communicate using infrasound, are capable of detecting infrasound, but the associated auditory mechanisms are not well understood. Here we combine an evaluation of hearing measurements with anatomical observations to propose and evaluate hypotheses supporting avian infrasound detection. Environmental infrasound is mixed with non‐acoustic pressure fluctuations that also occur at infrasonic frequencies. The ear can detect such non‐acoustic pressure perturbations and therefore, distinguishing responses to infrasound from responses to non‐acoustic perturbations presents a great challenge. Our review shows that infrasound could stimulate the ear through the middle ear (tympanic) route and by extratympanic routes bypassing the middle ear. While vibration velocities of the middle ear decline towards infrasonic frequencies, whole‐body vibrations – which are normally much lower amplitude than that those of the middle ear in the ‘audible’ range (i.e. >20 Hz) – do not exhibit a similar decline and therefore may reach vibration magnitudes comparable to the middle ear at infrasonic frequencies. Low stiffness in the middle and inner ear is expected to aid infrasound transmission. In the middle ear, this could be achieved by large air cavities in the skull connected to the middle ear and low stiffness of middle ear structures; in the inner ear, the stiffness of round windows and cochlear partitions are key factors. Within the inner ear, the sizes of the helicotrema and cochlear aqueduct are expected to play important roles in shunting low‐frequency vibrations away from low‐frequency hair‐cell sensors in the cochlea. The basilar papilla, the auditory organ in birds, responds to infrasound in some species, and in pigeons, infrasonic‐sensitive neurons were traced back to the apical, abneural end of the basilar papilla. Vestibular organs and the paratympanic organ, a hair cell organ outside of the inner ear, are additional untested candidates for infrasound detection in birds. In summary, this review brings together evidence to create a hypothetical framework for infrasonic hearing mechanisms in birds and other animals.     

The permeability of SPION over an artificial three-layer membrane is enhanced by external magnetic field

Background  

Sensorineural hearing loss, a subset of all clinical hearing loss, may be correctable through the use of gene therapy. We are testing a delivery system of therapeutics through a 3 cell-layer round window membrane model (RWM model) that may provide an entry of drugs or genes to the inner ear. We designed an in vitro RWM model similar to the RWM (will be referred to throughout the paper as RWM model) to determine the feasibility of using superparamagnetic iron oxide (Fe₃O₄) nanoparticles (SPION) for targeted delivery of therapeutics to the inner ear.     

Computer-integrated finite element modeling of human middle ear

 The objective of this study was to produce an improved finite element (FE) model of the human middle ear and to compare the model with human data. We began with a systematic and accurate geometric modeling technique for reconstructing the middle ear from serial sections of a freshly frozen temporal bone. A geometric model of a human middle ear was constructed in a computer-aided design (CAD) environment with particular attention to geometry and microanatomy. Using the geometric model, a working FE model of the human middle ear was created using previously published material properties of middle ear components. This working FE model was finalized by a cross-calibration technique, comparing its predicted stapes footplate displacements with laser Doppler interferometry measurements from fresh temporal bones. The final FE model was shown to be reasonable in predicting the ossicular mechanics of the human middle ear. Received: 18 February 2002 / Accepted: 6 June 2002 The preparation of temporal bone histological sections of Robert K. Dyer, Jr., MD is gratefully recognized. The Whitaker Foundation supported this work (Research Grant RG-98-0305).