Namkeun Kim

Inertial Bone Conduction: Symmetric and Anti-Symmetric Components

Of the two pathways through which we hear, air conduction (AC) and bone conduction (BC), the fundamental mechanisms of the BC pathway remain poorly understood, despite their clinical significance. A finite element model of a human middle ear and cochlea was developed to gain insight into the mechanisms of BC hearing. The characteristics of various cochlear response quantities, including the basilar membrane (BM) vibration, oval-window (OW) and round-window (RW) volume velocities, and cochlear fluid pressures were examined for BC as well as AC excitations. These responses were tuned and validated against available experimental data from the literature. BC excitations were simulated in the form of rigid body vibrations of the surrounding bony structures in the x, y, and z orthogonal directions. The results show that the BM vibration characteristics are essentially invariant regardless of whether the excitation is via BC, independent of excitation direction, or via AC. This at first appeared surprising because the cochlear fluid pressures differ considerably depending on the excitation mode. Analysis reveals that the BM vibration responds only to the lower-magnitude anti-symmetric slow-wave cochlear fluid pressure component and not to the symmetric fast-wave pressure component, which dominates the magnitude of the total pressure field. This anti-symmetric fluid pressure is produced by the anti-symmetric component of the window volume velocities. As a result, the BM is effectively driven by the anti-symmetric component of the OW and RW volume velocities, irrespective of the type of excitation. Middle ear modifications that alter the anti-symmetric component of the OW and RW volume velocities corroborate this assertion. The current results provide further clarification of the mechanisms underlying Békésy’s “paradoxical motion” concept.

Effects of ear-canal pressurization on middle-ear bone- and air-conduction responses

In extremely loud noise environments, it is important to not only protect ones hearing against noise transmitted through the air-conduction (AC) pathway, but also through the bone-conduction (BC) pathways. Much of the energy transmitted through the BC pathways is concentrated in the mid-frequency range around 1.5-2 kHz, which is likely due to the structural resonance of the middle ear. One potential approach for mitigating this mid-frequency BC noise transmission is to introduce a positive or negative static pressure in the ear canal, which is known to reduce BC as well as AC hearing sensitivity. In the present study, middle-ear ossicular velocities at the umbo and stapes were measured using human cadaver temporal bones in response to both BC and AC excitations, while static air pressures of +/-400 mm H(2)O were applied in the ear canal. For the maximum negative pressure of -400 mm H(2)O, mean BC stapes-velocity reductions of about 5-8 dB were observed in the frequency range from 0.8 to 2.5 kHz, with a peak reduction of 8.6(+/-4.7)dB at 1.6 kHz. Finite-element analysis indicates that the peak BC-response reduction tends to be in the mid-frequency range because the middle-ear BC resonance, which is typically around 1.5-2 kHz, is suppressed by the pressure-induced stiffening of the middle-ear structure. The measured data also show that the BC responses are reduced more for negative static pressures than for positive static pressures. This may be attributable to a difference in the distribution of the stiffening among the middle-ear components depending on the polarity of the static pressure. The characteristics of the BC-response reductions are found to be largely consistent with the available psychoacoustic data, and are therefore indicative of the relative importance of the middle-ear mechanism in BC hearing.

Reconstruction and exploration of virtual middle-ear models derived from micro-CT datasets

BACKGROUNDnMiddle-ear anatomy is integrally linked to both its normal function and its response to disease processes. Micro-CT imaging provides an opportunity to capture high-resolution anatomical data in a relatively quick and non-destructive manner. However, to optimally extract functionally relevant details, an intuitive means of reconstructing and interacting with these data is needed.nnnMATERIALS AND METHODSnA micro-CT scanner was used to obtain high-resolution scans of freshly explanted human temporal bones. An advanced volume renderer was adapted to enable real-time reconstruction, display, and manipulation of these volumetric datasets. A custom-designed user interface provided for semi-automated threshold segmentation. A 6-degrees-of-freedom navigation device was designed and fabricated to enable exploration of the 3D space in a manner intuitive to those comfortable with the use of a surgical microscope. Standard haptic devices were also incorporated to assist in navigation and exploration.nnnRESULTSnOur visualization workstation could be adapted to allow for the effective exploration of middle-ear micro-CT datasets. Functionally significant anatomical details could be recognized and objective data could be extracted.nnnCONCLUSIONSnWe have developed an intuitive, rapid, and effective means of exploring otological micro-CT datasets. This system may provide a foundation for additional work based on middle-ear anatomical data.

Superior-semicircular-canal dehiscence: Effects of location, shape, and size on sound conduction

The effects of a superior-semicircular-canal (SSC) dehiscence (SSCD) on hearing sensitivity via the air-conduction (AC) and bone-conduction (BC) pathways were investigated using a three-dimensional finite-element (FE) model of a human middle ear coupled to the inner ear. Dehiscences were modeled by removing a section of the outer bony wall of the SSC and applying a zero-pressure condition to the fluid surface thus exposed. At each frequency, the basilar-membrane velocity, vBM, was separately calculated for AC and BC stimulation, under both pre- and post-dehiscence conditions. Hearing loss was calculated as the difference in the maximum magnitudes of vBM between the pre- and post-dehiscence conditions representing a change in hearing threshold. In this study, BC excitations were simulated by applying rigid-body vibrations to the model along the directions of the (arbitrarily defined) x, y, and z axes of the model. Simulation results are consistent with previous clinical measurements on patients with an SSCD and with results from earlier lumped-element electrical-circuit modeling studies, with the dehiscence decreasing the hearing threshold (i.e., increasing vBM) by about 35 dB for BC excitation at low frequencies, while for AC excitation the dehiscence increases the hearing threshold (i.e., decreases vBM) by about 15 dB. A new finding from this study is that the initial width (defined as the width of the edge of the dehiscence where the flow of the fluid-motion wave from the oval window meets it for the first time) on the vestibular side of the dehiscence has more of an effect on vBM than the area of the dehiscence. Analyses of dehiscence effects using the FE model further predict that changing the direction of the BC excitation should have an effect on vBM, with vBM being about 20 dB lower due to BC excitation parallel to the longitudinal direction of the BM in the hook region (the x direction) as compared to excitations in other directions (y and z). BC excitation in the x direction and with a center dehiscence located midway along the length of the SSC causes a reduction in the anti-symmetric component of the fluid pressure across the BM, as compared to the other directions of BC excitation, which results in a decrease in vBM at high frequencies. This article is part of a special issue entitled MEMRO 2012.

The importance of the hook region of the cochlea for bone-conduction hearing.

For the most part, the coiled shape of the cochlea has been shown to have only minor importance for air-conductedxa0hearing. It is hypothesized, however, that this coiled shape may play a more significant role for the bone-conducted (BC) route of hearing, through inertial forces exerted by the middle ear and cochlear fluid, and that this can be tested by comparing the results of applying BC stimuli in a variety of different directions. A three-dimensional finite element model of a human middle ear coupled to the inner ear was formulated. BC excitations were simulated by applying rigid-body vibrations normal to the surface of the basilar membrane (BM) at 0.8 (d(1)), 5.8 (d(2)), 15.6 (d(3)), and 33.1 (d(4)) mm from the base of the cochlea, such that relative motions of the fluid within the cochlea produced excitations of the BM. The vibrational direction normal to the BM surface at the base of the cochlea (d(1)) produced the highest BM velocity response across all tested frequencies-higher than an excitation direction normal to the BM surface at the nonbasal locations (d(2)-d(4)), even when the stimulus frequency matched the best frequency for each location. The basal part of the human cochlea features a well-developed hook region, colocated with the cochlear vestibule, that features the largest difference in fluid volume between the scala vestibuli (SV) and scala tympani (ST) found in the cochlea. The proximity of the hook region to the oval and round windows, combined with it having the biggest fluid-volume difference between the SV and ST, is thought to result in a maximization of the pressure difference between the SV and ST for BC stimuli normal to the BM in this region, and consequently a maximization of the resulting BM velocity.

Carhart's notch: A window into mechanisms of bone-conducted hearing

Otosclerosis is a disease process of the ear that stiffens the stapes annular ligament and results in footplate immobilization. This produces a characteristic loss in bone-conducted (BC) hearing of about 20 dB between 1 and 2 kHz, known as Carharts notch, for which the specific mechanisms responsible have not yet been well understood. In this study, it is hypothesized that this observed pattern of hearing loss results from interactions between compressional and inertial mechanisms of BC hearing. Differences in the basilar-membrane velocity between a normal and otosclerotic human ear were calculated in response to compressional vibration of the cochlear capsule, translational vibration of the skull bone in various directions, and combinations of the two, using an anatomically accurate 3-D finite element model of the middle ear, cochlea, and semicircular canals. Compressional and inertial BC stimuli were found to both be necessary to capture the full behavior of clinical data, with the compressional component dominating below 0.75 kHz, the inertial component dominating above 3 kHz, and the notch between 1 and 2 kHz resulting from the suppression of an ossicular resonance due to stapes fixation. [Work supported by grant R01-DC07910 and R01-DC05960 from the NIDCD of NIH.]

Towards Creation of a Human‐Head Auditory Model for Simulating Bone‐Conduction Pathways

The air‐to‐bone sound transmission difference (i.e., BC limit) of a human auditory system was investigated by a finite‐element (FE) simulation. For the simulation, we developed FE models of a human skull and a human auditory periphery, and then combined them. The prelimiary results show that: (1) several mechanical responses of the FE model were consistent with published data, and (2) the BC limit also showed consistency with published data.

Bone Conduction Hearing: Three-Dimensional Finite Element Model of the Human Middle and Inner Ear

A finite‐element (FE) simulation model of a human auditory periphery was developed to gain insight into the fundamental mechanisms of bone conduction (BC) hearing. Three dimensional geometry of middle ear and cochlea including semi‐circular canal was obtained by μCT images. The simulation effectively focused on the middle ear and then the cochlea fluid‐inertial BC component. The FE model was first tuned and validated against various frequency responses available from the literature. The characteristics of various cochlear response quantities such as the basilar membrane (BM) displacement, window volume velocities, and cochlear fluid pressure were examined for both BC and air conduction (AC) excitations. Especially, the decomposition analysis was applied to window volume velocities and cochlear fluid pressures to separate them into anti‐symmetric and symmetric components. The preliminary result shows that the BM vibration is driven by the part of the fluid pressure that is anti‐symmetric (i.e. differential...

Cochlear anatomy using micro computed tomography (μCT) imaging

A novel micro computed tomography (μCT) image processing method was implemented to measure anatomical features of the gerbil and chinchilla cochleas, taking into account the bent modailosis axis. Measurements were made of the scala vestibule (SV) area, the scala tympani (SV) area, and the basilar membrane (BM) width using prepared cadaveric temporal bones. 3-D cochlear structures were obtained from the scanned images using a process described in this study. It was necessary to consider the sharp curvature of mododailosis axis near the basal region. The SV and ST areas were calculated from the μCT reconstructions and compared with existing data obtained by Magnetic Resonance Microscopy (MRM), showing both qualitative and quantitative agreement. In addition to this, the width of the BM, which is the distance between the primary and secondary osseous spiral laminae, is calculated for the two animals and compared with previous data from the MRM method. For the gerbil cochlea, which does not have much cartilage in the osseous spiral lamina, the μCT-based BM width measurements show good agreement with previous data. The chinchilla BM, which contains more cartilage in the osseous spiral lamina than the gerbil, shows a large difference in the BM widths between the μCT and MRM methods. The SV area, ST area, and BM width measurements from this study can be used in building an anatomically based mathematical cochlear model.