Middle Ear Mechanics in Research and Otology

PREFACE

CONTENTS

During the mid 18th century it was found that sound could be transmitted through solids and in the 19th century it was generally accepted that a person can perceive sound by bone conduction (BC). Since then, the research community has tried to understand its fundamental mechanisms. This report provides an overview of the present state in BC physiology. Five factors contributing to BC hearing are identified: 1) sound radiated into the ear canal, 2) middle ear ossicle inertia, 3) inertia of the cochlear fluids, 4) alteration of the cochlear space, and 5) pressure transmission from the cerebrospinal fluid. Of these, inertia of the cochlear fluid seems most important. The vibration modes in the skull can be divided into three frequency ranges. At the lowest frequencies, approximately below 400 Hz, the skull moves as a whole with rigid body motion. At higher frequencies, up to 1 kHz, the skull motion can be modelled as a mass-spring system whereas at frequencies above 1 kHz, wave propagation dominates the skull vibration response. The wave propagation differs between the thin-boned cranial vault and the thick and dense bone of the skull base. Both vibration measurement of the cochlea and ear canal sound pressure can estimate BC perception changes caused by stimulation alterations for frequencies above 0.8 kHz; below 0.8 kHz measured hearing thresholds differ from skull vibration and ear canal sound pressure data.

The middle ear is a semi-rigid biological gas pocket which is closed most of the time. It is believed that pressure in the middle ear (ME) is regulated by a complex combined action of the Eustachian tube muscles, eardrum deformation and gas exchange with the blood circulating in the mucosal lining. Sudden pressure variations of thousands of kPa occur in everyday situations and can be slow as well as fast. Both external pressure changes and ME gas exchange processes generate differences between the ME and ambient pressure. To understand the mechanisms and effects involved with quasi-static high amplitude pressure variations, we need to know how middle ear pressure varies in normal circumstances, investigate both fast and slow regulation mechanisms, and determine the effect of such pressures on middle ear mechanics. To measure the normal variations in middle ear pressure, we developed a monitoring system which is used in ambulant patients. We will show some results from direct measurements in intact ears. To investigate a possible fast regulation process, we performed measurements of eardrum deformation in gerbils. We found that volume displacement of the pars flaccida is small compared to middle ear volume, so that its fast pressure regulating function is limited to a very small pressure range. Finally, we will discuss some recent results, obtained in rabbits, of the effect of static pressure variations on middle ear ossicle motions. With heterodyne interferometry, we measured motions of umbo and stapes at pressure change rates between 200 Pa/s and 1.5 kPa/s. We will show that hysteresis in these motions increases as pressure change rate decreases, quite in opposite to the common notion that such hysteresis is mainly caused by visco-elasticity. We conclude that static and dynamic friction are important aspects of ossicle motion at slow pressure variations, new aspects which should be taken into account when trying to model quasi-static ossicle mechanics.

The motion of the ossicle of the guillemot (Uria aalge) has been studied during changes in static pressure using digital video clips. The magnitude of the displacements in the medial to lateral plane have been measure by capturing digital still images and measuring the change in the distance between a fixed point on the oval window margin and a second point on the columella portion of the ossicle at different pressures, using the dimensioning tool of Corel Draw. The flexion of the ossicle at the junction of the columella and extracolumella which has been observed in other bird species is not so obvious in the guillemot, nor is the rocking motion of the stapes which occurs during variations in static pressure. The maximum medial movement of the ossicle when the pressure in the external auditory meatus was increased to 200 dPa was 0.1 mm. This species dives and swims under water and is therefore more likely to be exposed to significant changes in static pressure than other birds.

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Laser Doppler vibrometry has proved to be very useful in diagnosing ossicular chain disorders by measuring umbo velocity. Although it has been supposed that these measurements contain information about both the middle ear and the inner ear, it has not been possible to separate the information, nor to provide information about the inner ear.

Here, we propose a method to provide information about the inner ear, separating it from middle-ear components, in records of vibration measurements on the human umbo. For this purpose, distortion product otoacoustic emissions (DPOAEs) were measured on the human eardrum using a very sensitive laser Doppler vibrometer. Besides providing valuable information about cochlear status, especially at low level stimulation, velocity-DPOAEs allowed us to objectively estimate the hearing threshold. Thresholds were predicted with a standard deviation of only 8 dB. Moreover, the DPOAEs can provide information about the input and output impedances of the eardrum, opening perspectives for characterising and diagnosing middle- and inner-ear function.

We propose that vibration measurements of DPOAEs on the human eardrum could become a new tool for separating middle-ear from inner-ear disorders and for objective assessment of hearing threshold.

Otoreflectance (OR) is a newly developed hearing-test instrument utilizing acoustic signals, which are presented and recorded in the ear canal. It provides an objective measurement of the middle ear transfer function for a frequency range of 0.25 to 8 kHz. In the present study, reflectance measurements were performed in 25 subjects (50 ears) using an experimental OR system. The energy reflectance (ER) was recorded and plotted against each 1/6^th octave frequencies from 0.25 to 8.0 kHz. Three types of the ER-plot configurations were found in subject with normal middle ear function using the OR test, which may represent different stiffness conditions in the normal middle ear. Notches found at around 1.1 kHz are close to the middle ear resonant frequency region, whereas notches around 3.4 kHz indicate another frequency region of the middle ear that functions efficiently. This may be associated with a number of factors to be found in the ear transmission system. The central frequency of ER-plot notches in both low to mid and the high frequency bands correlated significantly with the frequencies in the corresponding sound pressure variations obtained in the SPL-plots.

To evaluate ossicular mobility in surgery, an apparatus that quasi-statically measures the load and displacement of the ossicles has been developed. This apparatus was used to measure the ossicular mobility in human temporal bones before and after the ossicles were artificially fixed. These results were compared with simultaneous estimates of ossicular mobility obtained with a laser Doppler velocimeter (LDV). In addition, the ossicular mobility of three patients with otosclerosis or chronic otitis media was also measured to evaluate the usability of the apparatus. The apparatus could make estimating the change of mobility between pre and post-treatments for ossicular fixation possible, and positive correlation was seen between ossicular mobility and hearing level. These results suggest that our apparatus has the ability to detect the difference in ossicular mobility quantitatively between fixed and normal ears, and shows that our apparatus could be a useful tool in the surgical estimate of ossicular disorders.

The development of acoustic reflectance measurements to augment middle-ear diagnostic testing may lead to noninvasive tests that provide information currently unavailable from standard audiometric testing. Reflectance measurements were made on both live-human and human-cadaver ears to quantify inter-subject variability relative to intra-subject variability due to temporal changes in either an individual ear or the measurement system, methodological issues including estimates of ear-canal area, and measurement location within the ear canal. To compare intra-subject versus inter-subject variability, multiple reflectance measurements were made on five live human subjects (10 ears). The variability among ears from different subjects was greater than the variability within repeated measurements on a single ear at all frequencies except from 2000 to 4000 Hz, where inter-subject variability was smaller. Reflectance measurements in human-cadaver ears demonstrate that reflectance depends on both the ear-canal location and variations in the ear-canal area. However, these variations are also small relative to inter-subject variations.

No abstract received.

In most experiments, access for direct measurement of stapes motion in line with the piston axis is not available and piston motion is estimated from single component interferometric measurements done under observation directions that make angles up to 60° with the piston axis. We measured the vibration velocity of the stapes in human and gerbil from different observation angles and calculated the complete set of 3-D motion components. We expressed the components in an intrinsic reference system and could foretell the motion component to be recorded with a single axis interferometer at an angle with the piston axis. A cosine factor provides a good correction for the axis offset only for low frequencies (f<1.5kHz) in human; at higher frequencies and for gerbil at all frequencies the piston component cannot be accurately estimated from a single off-axis observation and a cosine correction.

In the normal middle ear, stapes displacement during atmospheric pressure changes is physiologically reduced by the gliding ossicular joints. In fixed ossicular joints, static pressure is assumingly transfered directly towards the inner ear. Therefore unphysiological movements could eventually initiate exceeding displacements and consecutively lead to a risk for inner ear function.

Experimental investigations were performed in order to evaluate the dynamic and quasi-static effect of ossicular joint fixation on the middle ear. Acoustic transfer characteristics and stapes displacement were determined in 12 fresh temporal bone specimens. Stapes vibration amplitude was measured using Laser-Doppler-Vibrometry (LDV) before and afer stepwise artificial fixation of the ossicular joints; the same applied to measurements of stapes displacement during standardized pressure changes in the external ear canal.

In comparison to a regular middle ear, fixed ossicular joints demonstrated a minimal decrease of footplate vibration amplitude in the low frequency range whereas an improvement of the transfer characteristics of less than 10 dB was demonstrable in the middle and high frequency region. It furthermore shows that atmospheric pressure changes in the external ear canal lead to an unphysiological footplate displacement.

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In recent papers we have shown that the mode of vibration of the ossicular chain in cat and human temporal bones is more complex than a simple rotation about a fixed axis and that it also changes with frequency. For the stapes e.g. it was shown that rotations of the footplate about its long and short axis are present besides the predominant pistonlike velocity component. As the footplate motion gives rise to the pressure wave set up in the cochlea, we investigated whether the non-piston components contribute to the pressure produced in the gerbil cochlea: 3-D vibration velocity of the stapes along with the scala vestibuli pressure were measured in the same animals. We found no correlation between non-piston components and scala vestibuli pressure.

As observed in many studies the natural vibration pattern of the stapes on acoustic stimulation reveals a complex motion pattern dependent on the frequency of excitation. Whereas for low frequencies the motion is predominantly piston-like, significant rocking can be found for higher frequencies. The cochlea fluid is mechanically exited by the piston-like motion and two rotational movements along the short and long axis of the footplate. The rotational components produce no net volume flux of the cochlea fluid and, therefore, their influence on the hearing sensation is still an open question. To investigate the response of the cochlea on complex motion of the stapes footplate, different vibration pattern on the stapes have been mechanically applied in anesthetized guinea pigs.

A test rig was built to position the subject, an actuator and a Laser Doppler Vibrometer adjustable by micro manipulators. A three-axis piezoelectric actuator has been designed and coupled to the stapes head of the surgically prepared guinea pig by a coupling rod. For capturing the effective motion of the stapes three-dimensional laser Doppler vibrometry was applied. The excitation procedure for arbitrary stapes motion consists of both an identification and a measurement phase to determine the transducer behavior and the electrophysiological measurements of the cochlea response on the applied motion. Spatial velocity of stapes and cochlea potentials on repeated transducer activation are captured simultaneously by the data acquisition system for appropriate post-processing.

The task of driving arbitrary motion patterns yields a much higher complexity than classical one-dimensional consideration. The setup enables investigations in electrocochleography with different amounts of piston and rocking-like motions of the stapes.

Studies into the vibration modes of the stapes in response to acoustic stimulation of the normal ear have revealed a complex movement pattern of its footplate. These complex vibrations can be expressed as one translational displacement and two rotational movements around the long and short axes of the stapes, known as the three elementary motions. According to the classical theory of hearing, the rotational motions induce no volume displacement of cochlear fluid and, therefore, no cochlear activity (i.e. hearing sensation). It is the goal of this study to verify this hypothesis.

A custom-built, three-axis piezoelectric actuator, capable of eliciting any desired vibration mode, was coupled to the surgically prepared stapes superstructure of anesthetized guinea pigs. When producing different movement patterns, electro-physiological measurements of the cochlear potentials were simultaneously recorded.

Mechanical stimulation of the stapes according to the three elementary motions delivered three cochlear potentials of different amplitude. The greatest potential resulted from translational motions.

The results of the present study show a cochlear excitation in all performed movement patterns of the stapes. Hence, the prevailing hypothesis could not be verified.

Possible factors that affect measurement of the middle ear transfer function were investigated using human cadaveric temporal bones. As done by many others in the past, a laser Doppler vibrometer measured the stapes velocity (V_st) and a probe-tube microphone measured pressure (P_ec) in the external ear canal (EAC). The ratio of the two variables V_st/P_ec is the stapes velocity transfer function (SVTF). Three different types of experiments were conducted: (1) artificial vs. natural ear canal, (2) different locations of probe tube microphone relative to the eardrum, and (3) varying levels of humidity. The mean SVTF with artificial EAC (A-EAC) was similar as previously reported data, and the differences between SVTF with the natural EAC and that with A-EAC were less than 5.0 dB in magnitude up to 15 kHz and less than 45 degrees in phase angle. Significant differences between SVTF magnitude slopes with N-EAC and A-EAC were not detectable. The position of the probe-tube microphone (PT mic) significantly changed the SVTF at high frequencies. At 8 kHz 10 mm difference with the PT mic position caused 20 dB elevations in SVTF magnitude. The most influential factor that affects on SVTF at high frequencies was the position of the probe-tube microphone which is well known to be due ear canal acoustics. Dryness due to a decrease in humidity decreased the SVTF mainly at low frequencies.

No abstract received.

We re-examined ossicular vibrations in the widely opened middle ear of the chinchilla, a species with a relatively narrow hearing bandwidth, similar to that of humans. The magnitude of vibration velocity at the head of the stapes is relatively constant (− 0.3 mm/s/Pa) up to 24 kHz, thus exceeding the bandwidth of hearing in chinchilla (<20 kHz). Phase lag relative to pressure in the external ear canal increases approximately linearly between 0 and 20 kHz, with a slope equivalent to a pure delay of 76 μs. The present findings support the contention that, in general, the middle ears of amniotic vertebrates do not limit the bandwidth of hearing. The magnitude of vibration of the incus lenticular plate is similar to that of the head of the stapes but substantially larger than the vibration of the incus long process in the range 15–24 kHz. Thus, the flexibility of the incus pedicle seems to boost the bandwidth of middle ear transmission.

For the past years, surgeons performing lateral skull base surgery have been paying more attention to the development and advancement of mechatronically aided surgical procedures. Especially the interconnection between electronic hearing implants (i.e. MET Otologics Transducer) and the ossicular chain requires an accurate milling of skull cavities to ensure a proper angle of coupling and a long term stable fixation. We developed a PC-based computer and robot assisted system (CAS/RAS) for the precise and efficient planning and execution of surgical processes at the lateral skull. Employing multimodal sensory feedback, the geometry of the surgical area can be analyzed with a resolution of several μm.

The incus once removed at the first stage can be used in ossicular reconstruction at the second stage, when it is preserved appropriately. The present study was conducted to investigate if the mastoid bowl is a suitable site for storage. The study group included 24 ears of 23 patients who underwent staged tympanoplasty for the treatment of middle ear cholesteatoma. The average interval between the two stages was 8.3 months (range 6–12 months). The incus was identified in all cases at the second stage: ten incudes were found to be covered with a thin mucosa layer, 12 were buried in fibrous or granulation tissue, and two were joined to the surrounding bone. In 19 cases, the incus was available as a short columella for ossicular reconstruction. The remaining five cases were reconstructed using a hydroxyapatite ossicle as a long columella, since the stapes superstructure was missing at the second stage. Preservation of the incus in the mastoid bowl is an effective option in staged tympanoplasty, when the incus is considered useful for ossicular reconstruction at the second stage.

No abstract received.

Anterior tympanic membrane blunting results in persistent conductive hearing loss and patient dissatisfaction despite successful closure of a perforation. Tympanic membrane compliance and efficiency in sound transmission are impaired. In extreme cases the entire tympanic membrane may lateralize giving a false fundus with maximal conductive loss. The causes of blunting may be both surgical technique and poor wound healing. We describe our techniques for prevention of blunting which include adequate support and anchoring of the fascia graf in an underlay technique and the use of thin split thickness skin grafs. Post operative follow up in clinic to monitor healing is important. Examples of the techniques and results are presented.

A prospective parallel group study was performed of hearing outcomes in ears undergoing primary cholesteatoma surgery using a laser-assisted intact canal wall technique.

Group A had a continuous ossicular chain. Group B had a disrupted chain with an intact stapes superstructure onto which an ossiculoplasty had been performed.

The four frequency air-bone gap was calculated for each operated ear.

The Belfast Rules of Thumb were also used to calculate a dichotomous measure of whether the operated ear would provide useful hearing.

61 patients underwent surgery resulting in an intact chain. The median air-bone gap in this group was 15dB HL. 82% satisfied the requirements of the Belfast Rules.

84 patients underwent surgery requiring an ossiculoplasty onto an intact stapes. The median air-bone gap in this group was 20 dB HL. 50% satisfied the Belfast Rules.

The two groups are statistically discernible by both outcome measures (P < 0.001, Mann-Whitney ; P< 0.0002, χ², ν = 1 respectively).

Preservation of the ossicular chain provides better hearing than any reconstructed chain afer cholesteatoma surgery. Moreover, the requirements of hearing preservation influence the entire surgical technique for the treatment of cholesteatoma.

No abstract received.

If the functionality of the middle ear is considered, mostly the ability of effectively transmitting sound energy from the air in the ear canal to the fluids in the inner ear is examined. However, to assess the “quality” of the middle ear it is necessary to consider also other aspects such as the reaction to external forces and to general changes in the mechanic system. The human middle ear turns out to provide a larger auditory frequency range, lower general parameter sensitivity and better protection of the inner ear than a columella ear found in birds and reptiles.

The favourable properties are best understood by regarding the vibrations of the ossicular chain under normal and impaired conditions and for different kinds of excitation. Vibrations were computed by means of a generalised circuit model. All the resulting patterns of vibration taken together reveal a remarkable conceptual design of the human middle ear: The incudomalleal joint acts as an element protecting the inner ear against external forces. Such a robust overload protector necessarily introduces a rather heavy mass centre which tends to worsen the transmission at high frequencies. It turns out that the design of the human middle ear does not only circumvent a decrease in transmission at too low frequencies, but even takes advantage from the mass centre. The surprising features are achieved by the position of the mass centre outside the direct path of transmission in combination with a favourable design of the elastic elements involved.

A 3-D finite element (FE) model of human ear with accurate geometry of the ear canal and middle ear has been recently published by our group. In this paper, the acoustic-structure-acoustic coupled FE analysis was conducted to the model for normal and pathological ears such as the tympanic membrane (TM) perforations. Two laser vibrometers were used to measure simultaneously the TM and stapes footplate vibrations. The transfer function of the middle ear under normal and perforation conditions from the model show frequency-dependent behavior similar to that measured from temporal bones. The FE model provides good prediction on effect of perforation location and size on middle ear transfer function.

Two laser vibrometers were used to measure simultaneously the movement of the tympanic membrane and stapes footplate in human temporal bones. After control study of the normal ear, the stapedial tendon, posterior incudal ligament, tensor tympani tendon, and superior malleus/incus ligament were sectioned sequentially. The displacements of the umbo and footplate were measured repeatedly for each section. A 3-D finite element model of human ear which has accurate anatomic structure was used to mimic the middle ear structure changes and to derive the umbo and footplate vibrations in response to those alterations. The results show that the effects of ligaments on transfer function of the middle ear are frequency sensitive and vary with individual ligament.

In this study we used a Finite Element model of the middle ear to evaluate LDV (Laser-Doppler-Vibrometer) measurements of sound induced umbo vibrations. Simulations were performed for the intact ossicular chain, otosclerosis, incus luxation and malleus head fixation. Simulation results are comparable to results from clinical investigations. Incus luxation and malleus head fixation can be clearly distinguished. Results for otosclerosis range from almost unchanged to moderate changes in umbo vibrations, depending on particular middle ear morphology.

Hearing sensations are caused by air and bone guided sound. Of course other biological materials like tendons, muscles and tissue are also involved during conduction of sound. To study the influence of bone conduction (BC) a formerly developed finite element model was excited by harmonic pressure signals at the cochlea wall. Clinical findings during middle ear surgery, namely the increase of bone conduction sensitivity with removed footplate was confirmed.

Additionally the effects of an immobile footplate and round window aplasia and bone conduction are studied. The maximum basilar membrane displacement with the closure of the oval and the round window and both with BC is examined numerically.

275 inserted Clip-Pistons type “ÀWengen” within three years revealed difficulties in 14.5% of the cases. In those cases it was necessary to make adjustments to the clip shape (plastic deformation) before insertion due to the individual dimension of the long incudal process. During 100 middle ear surgeries the cross sections of the long incudal processes where the clip is attached was measured. This resulted in data hitherto unknown. By virtue of a Finite Element Model (FEM) these data were used for optimizing the clip shape. Design criteria were a minimal variation of the contact force for different cross-sections and to minimize the force necessary to slide the clip over the incudal process. The new clip has a lower stiffness and can therefore be applied onto different incus diameters. The lower contact force reduces the risk of arrosion. Due to its optimized shape, the maximal stress in the clip is lowered preventing plastic deformation during the application procedure. The application force was decreased by up to 45% depending on the application points. This leads to easy and safe application reducing the risk of damaging the ossicular chain.

The sound transfer is strongly dependent on the coupling of the prosthesis to the ossicles. This holds for both passive but in particular for active implants. Various principles of coupling and different designs of attaching the implant to ossicle are in use showing a different mechanical behavior concerning stiffness and damping. Pressing a driving rod against the ossicles means a unilateral coupling which needs a particular prestress. Due to the nonlinear transfer behavior of the coupling region and ossicular chain, the actual motions, e.g. of actuator, stapes and umbo are dependent on that prestress. An optimal coupling should allow a stapes motion sufficient to compensate a hearing loss, but should avoid a distorted sound transfer or feedback due to sound radiation from ear drum. The static adjustment influences the sound transfer of the dynamical system essentially. To find an optimal adjustment of the actuator, the dynamical behavior of the actuator itself, of the coupling region and the individual ossicular chain has to be regarded. During surgery the stiffness of the individual chain has to be estimated and the configuration when the actuator touches the ossicles has to be detected very carefully.

No abstract received.

An anatomically based three-dimensional (3D) compuational model requires detailed morphometry of middle ear structures. We report methods and results using the microCT imaging modality for the middle ear. Stacks of images were obtained from cadaveric temporal bone ears of human, cat, chinchilla and guinea pig. The images were segemented and 3D structures reconstructed. The structures include the eardrum (thickness map and 3D shape), volumes of each of the three middle ear bones, and suspensory ligaments and tensor tympani tendon. The volume reconstruction of the malleus indicates that it has a solid rod-like shape in human, is a hollow cylinder in cat, and has an I-beam like shape in guinea pig and chinchilla. From the ossicle volumes, principal axis of rotation and principal moments of inertia, both around the center of gravity, were calculated. The results indicate that for the guinea pig and chinchilla, where the malleus-incus joint is fused, the clasical axis of rotation through the malleus and incus heads holds. However, the data also indicate a new mode of operation at high frequencies in animals where the malleus-incus joint is mobile (human and cat). We hypothesize that the malleus shape, along with a mobile joint, provides optimal transfer of energy to the cochlea at high frequencies through a rotation motion (torsion) around the long axis (superior-inferior) of the malleus.

No abstract received.

The transcranial transmission is important in bone conduction (BC) audiometry and for fitting BC hearing aids. In BC audiometry, the BC hearing threshold depends on the stimulation position, and the transcranial transmission determines the relative amount of the sound that reaches the contralateral cochlea when fitting BC hearing aids. Previously reported transcranial transmission results seem to depend on the method used. Here, a comparison between the transcranial transmission, measured with BC hearing thresholds and ear canal sound pressure (ECSP), is performed for both open and occluded ear canals. The transcranial transmission was similar for BC hearing thresholds and ECSP above 800 Hz; this indicates that the ECSP can be used to estimate changes of the BC hearing perception produced by alterations of the stimulation. The transcranial transmission results are also similar to vibration measurements of the cochleae made in earlier studies. Hence, vibration measurements of the cochleae can also estimate relative BC hearing.

Detailed models of middle ear morphology are an important input to improve realism of computer models of middle ear mechanics. Established imaging techniques all have their specific limitations. Orthogonal-plane fluorescence optical sectioning or OPFOS, is an important additional technique which, after adequate specimen preparation, produces high quality, perfectly aligned sectional images in nearly real-time of both bone and soft tissue simultaneously. The technique was introduced by A. Voie with a slicing resolution of 14μm. Using improved optical design and adding motorized scanning along a direction of the image plane, we were able to reduce slicing thickness to 2μm, thus creating a high-resolution OPFOS (HROPFOS) technique. Sections from HROPFOS data show histological detail, f.i. the hollow structure of the stapes head, cavities within the body of the ossicles, etc. From the section data, we reconstructed accurate 3-D models of the middle ear ossicles and its soft tissue structures, such as the stapedial muscle and artery which are nearly invisible with X-ray CT. By imaging bone as well as sof tissue, these models are an important input for finite-element modeling. Measurements were obtained in preparations of intact temporal bones of gerbils, as the technique allows region-of-interest scanning.

In everyday life the human ear is subject not only to sound but also to slow, quasi-static pressure variations with amplitudes in the order of tenths of a mm. In the present study we investigated how the ossicular chain deforms and changes its 3-D configuration when acted upon by large static pressures. To determine with good precision the small deformations of the ossicular chain we use ‘hyperelastic warping’, a deformable image registration method. In this relatively new technique a 3-D finite element model, based on careful segmentation of a template dataset, is deformed until it aligns with a target dataset at a different pressure. 3-D displacement of the ossicles will be shown and expressed mathematically by plotting the displacement of a few landmark points as a function of pressure. We will also describe the 3-D changes in configuration and show how the slippage in the ossicle joints performs as a protecting mechanism of the middle ear.

Computer-aided Opto-Electronic Holography (OEH) was used to measure the sound-induced displacement of the tympanic membrane (TM) of cadaver cats and chinchillas. Real-time time-averaged holograms (gathered at video rates) were used to roughly determine the frequency dependence of TM displacements as tone frequency was swept from 0.4 to 20 kHz. Stroboscopic holography was used at selected frequencies to measure the difference in displacement between condensation and rarefaction phase of the pressure stimulus and enable discrimination of inward and outward displacements of the TM surface with nanometer resolution. The time-averaged holographic data demonstrate standing wave patterns on the cat's TM surface, which move from simple uni-modal or bi-modal patterns at low frequencies, through complicated multi-modal patterns above 3 kHz, to highly ordered arrangements of displacement waves with tone frequencies above 15 kHz. The frequency boundaries of the different wave patterns are lower in chinchilla (simple patterns below 0.6 kHz, ordered patterns above 4 kHz) than cat. The stroboscopic holography measurements indicate wave-like motion patterns on the TM surface, where the number of wavelengths captured along a transect of the TM increased with stimulus frequency with as many as 11 wavelengths visible on the chinchilla TM at 16 kHz. Counts of the visible number of wavelengths on TM transects with different sound stimulus frequency provided estimates of wave velocity along the TM surface that ranged from 5 m/s at frequencies below 8 kHz and increased to 25 m/s by 20 kHz.

Superior canal dehiscence (SCD) syndrome is a non-middle-ear source of conductive hearing loss [3, 5]. The purpose of this study is to develop a better understanding of the physiology and mechanics behind SCD-induced changes in auditory sensitivity to both air-conducted (AC) and bone-conducted (BC) stimuli. To achieve this goal we surgically introduced a hole (dehiscence) into the chinchilla superior canal (SC) and then monitored cochlear potential (CP) in response to both BC and AC stimuli. As a result of SCD, low frequency sensitivity to BC stimuli increased and in response to AC stimuli it decreased, producing an air-bone gap of 12dB. Our results support the hypothesized ‘third window’ mechanism, which suggests that the SCD introduces a pathological shunt pathway that alters inner-ear fluid motion and results in an air-bone gap.

No abstract received.

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No abstract received.