Effect of middle ear components on eardrum quasi-static deformation

Prestrain in the eardrum investigated using laser-ablation perforation: A proof of principle study on the New Zealand white rabbit

While the presence of residual stress (also called prestress) in the tympanic membrane (TM) was hypothesized more than 150 years ago by von Helmholtz (1869), little experimental data exists to date. In this paper, a novel approach to study residual stress is presented. Using a pulsed laser, the New Zealand white rabbit TM is perforated at seven predefined locations. The subsequent retraction of the membrane around the holes is computed using digital image correlation (DIC). The amount of retraction is the so-called prestrain, which is caused by the release of prestress due to the perforation. By measuring the prestrain using DIC, we show that residual stress is clearly present over the entire rabbit TM surface. In total, fourteen TMs have been measured in this work. An automated approach allows tracking the holes' deformation during the measurement process and enables a more robust analysis than was previously possible. We find similar strains (around 5%) as reported in previous work, in which slits were created manually using flattened surgical needles. However, the new approach greatly reduces measurement time, which minimizes dehydration artifacts. To investigate the effect of perforation location on the TM, the spatial decrease of the prestrain (α) around the perforation was quantified. Perforations inferior to the umbo showed the least negative α values, i.e., the most gradual decrease around the hole, and were the most consistent. Perforations on other locations showed more negative α values, i.e., steeper decrease in strain, but were less consistent across samples. We also investigated the effect of the holes' creation sequence but did not observe a significant change in the results. Overall, the presented method allows for consistent residual stress measurements over the TM surface. The findings contribute to our fundamental knowledge of the mechanics of the rabbit TM and provide a basis for future work on human TMs.

Characterization of middle ear soft tissue damping and its role in sound transmission

Damping plays an important role in the middle ear (ME) sound transmission system. However, how to mechanically characterize the damping of ME soft tissues and the role of damping in ME sound transmission have not yet reached a consensus. In this paper, a finite element (FE) model of the partial external and ME of the human ear, considering both Rayleigh damping and viscoelastic damping for different soft tissues, is developed to quantitatively investigate the damping in soft tissues effects on the wide-frequency response of the ME sound transmission system. The model-derived results can capture the high-frequency (above 2 kHz) fluctuations and obtain the 0.9 kHz resonant frequency (RF) of the stapes velocity transfer function (SVTF) response. The results show that the damping of pars tensa (PT), stapedial annular ligament (SAL) and incudostapedial joints (ISJ) can help smooth the broadband response of the umbo and stapes footplate (SFP). It is found that, between 1 and 8 kHz, the damping of the PT increases the magnitude and phase delay of the SVTF above 2 kHz while the damping of the ISJ can avoid excessive phase delay of the SVTF, which is important in maintaining the synchronization in high-frequency vibration but has not been revealed before. Below 1 kHz, the damping of the SAL plays a more important role, and it can decrease the magnitude but increases the phase delay of the SVTF. This study has implications for a better understanding of the mechanism of ME sound transmission.

Rabbit tympanic membrane thickness distribution obtained via optical coherence tomography

Knowing the precise tympanic membrane (TM) thickness variation is crucial in understanding the functional properties of the TM and has a significant effect on the accuracy of computational models. Using optical coherence tomography, we imaged five left and five right TMs of domestic New Zealand rabbits. From these data, ten thickness distribution maps were computed. Although inter-specimen variability is present, similar features could be observed in all samples: The rabbit TM is thickest around the umbo, with values of 150 ± 32 µm. From the umbo towards the TM annulus, the thickness gradually decreases down to 38 ± 7 µm around the midway location, but increases up to 54 ± 19 µm at the TM annulus. The thickness values at the umbo are comparable to literature data for humans, but the rabbit TM is thinner at the TM annulus and in-between the umbo and annulus. Moreover, the rabbit TM thickness distribution is highly symmetrical, which is not the case for the human TM. The results improve our general understanding of TM structure in rabbits and may improve numerical models of TM dynamical behavior.

Prestrain in the rabbit eardrum measured by digital image correlation and micro-incisions

Prestrain in the absence of external loads can have an important effect on the vibrational behavior of mechanical systems such as the middle ear. Studies that measure tympanic membrane (TM) prestrain are scarce, however, and provide no conclusive answer on the existence and nature of the prestrain. In this study, prestrain is measured in the TM of cadaveric rabbit ears by stereo digital image correlation. To release the prestrain, straight incisions of 0.33 mm are made on different locations in the TM with a direction parallel to either the radial or circular fibers in the membrane. The effect of sample dehydration during different stages in the experimental procedure is assessed and eliminated by rehydrating the samples directly before each measurement. The measurements demonstrate average prestrain values around the incisions between 3.52±2.34% and 13.62±7.92% for the different locations, with a noise floor of 0.07%. No clear differences were found between the prestrain values obtained for radial and circular incisions. Observed local variations in TM prestrain could not be clearly related to specific locations on the TM. The results suggest that TM prestrain may need to be considered in future studies of middle-ear function if the findings can be confirmed in human ears.

Are suspensory ligaments important for middle ear reconstruction?

As the resolution of 3D printing techniques improves, the possibility of individualized, 3-ossicle constructions adds a new dimension to middle ear prostheses. In order to optimize these designs, it is essential to understand how the ossicles and ligaments work together to transmit sound, and thus how ligaments should be replicated in a middle ear reconstruction. The middle ear ligaments are thought to play a significant role in maintaining the position of the ossicles and constraining axis of rotation. Paradoxically, investigations of the role of ligaments to date have shown very little impact on middle ear sound transmission. We explored the role of the two attachments in the gerbil middle ear analogous to human ligaments, the posterior incudal ligament and the anterior mallear process, severing both attachments and measuring change in hearing sensitivity. The impact of severing the attachments on the position of the ossicular chain was visualized using synchrotron microtomography imaging of the middle ear. In contrast to previous studies, a threshold change on the order of 20 dB across a wide range of frequencies was found when both ligaments were severed. Concomitantly, a shift in position of the ossicles was observed from the x-ray imaging and 3D renderings of the ossicular chain. These findings contrast with previous studies, demonstrating that these ligaments play a significant role in the transmission of sound through the middle ear. It appears that both mallear and incudal ligaments must be severed in order to impair sound transmission. The results of this study have significance for middle ear reconstructive surgery and the design of 3D-printed three-ossicle biocompatible prostheses.

Structural stiffening in the human middle ear due to static pressure: Finite-element analysis of combined static and dynamic middle-ear behavior

The vibration response of the middle ear (ME) to sound changes when static pressure gradients are applied across the tympanic membrane (TM). To date, it has not been well understood which mechanisms lead to these changes in ME vibration response. In this study, a 3D finite-element model of the human ME was developed that simulates the sound-induced ME vibration response when positive and negative static pressures of up to 4 kPa are applied to the TM. Hyperelasticity of the soft-tissue components was considered to simulate large deformations under static pressure. Some ME components were treated as viscoelastic materials to capture the difference between their static and dynamic stiffness, which was needed to replicate both static and dynamic ME behavior. The change in dynamic stiffness with static preload was simulated by linearization of the hyperelastic constitutive model around the predeformed state. For the preloaded harmonic response, we found that the statically deformed ME geometry introduced asymmetry in the vibration loss between positive and negative pressure, which was due to the TM cone shape. As opposed to previous assumptions, the prestress in the ME due to static pressure had a substantial impact on the vibration response. We also found that material nonlinearity led to a higher stiffening at the umbo but a less pronounced stiffening at the footplate compared to the linear elastic condition. The results suggest that flexibility of the incudomalleolar joint (IMJ) enhances the decoupling of static umbo and footplate displacements, and that viscosity and viscoelasticity of the IMJ could play a role in the transfer of sound-induced vibrations from the umbo to the footplate. The components of the incudostapedial joint had minimal effect on ME mechanical behavior.

Vibration Measurements of the Gerbil Eardrum Under Quasi-static Pressure Steps

Tympanometry is a relatively simple non-invasive test of the status of the middle ear. An important step towards understanding the mechanics of the middle ear during tympanometry is to make vibration measurements on the eardrum under tympanometric pressures. In this study, we measured in vivo vibration responses in 11 gerbils while varying the middle-ear pressure quasi-statically, with the ear canal at ambient pressure. Vibrations were recorded using a single-point laser Doppler vibrometer with five glass-coated reflective beads (diameter ~ 40 μm) as targets. The locations were the umbo, mid-manubrium, posterior pars tensa, anterior pars tensa and pars flaccida. As described in earlier studies, the unpressurized vibration magnitude was flat at low frequencies, increased until a resonance frequency at around 1.8-2.5 kHz, and became complex at higher frequencies. At both the umbo and mid-manubrium points, when the static pressure was decreased to the most negative middle-ear pressure (- 2500 Pa), the low-frequency vibration magnitude (measured at 1.0 kHz) showed a monotonic decrease, except for an unexpected dip at around - 500 to - 1000 Pa. This dip was not present for the pars-tensa and pars-flaccida points. The resonance frequency shifted to higher frequencies, to around 7-8 kHz at - 2500 Pa. For positive middle-ear pressures, the low-frequency vibration magnitude decreased monotonically, with no dip, and the resonance frequency shifted to around 5-6 kHz at + 2500 Pa. There was more inter-specimen variability on the positive-pressure side than on the negative-pressure side. The low-frequency vibration magnitudes on the negative-pressure side were higher for the pars-tensa points than for the umbo and mid-manubrium points, while the magnitudes were similar at all four locations on the positive-pressure side. Most gerbils showed repeatability within less than 10 dB for consecutive cycles. The results of this study provide insight into the mechanics of the gerbil middle ear under tympanometric pressures.

Eardrum displacement and strain in the Tokay gecko (Gekko gecko) under quasi-static pressure loads

The eardrum is the primary component of the middle ear and has been extensively investigated in humans. Measuring the displacement and deformation of the eardrum under different quasi-static loading conditions gives insight in its mechanical behavior and is fundamental in determining the material properties of the eardrum. Currently, little is known about the behavior and material properties of eardrums in non-mammals. To explore the mechanical properties of the eardrum in non-mammalian ears, we investigated the quasi-static response of the eardrum of a common lizard: the Tokay gecko (Gekko gecko). The middle ear cavity was pressurized using repetitive linear pressure cycles ranging from -1.5 to 1.5 kPa with pressure change rates of 0.05, 0.1 and 0.2 kPa/s. The resulting shape, displacement and in-plane strain of the eardrum surface were measured using 3D digital image correlation. When middle-ear pressure is negative, the medial displacement of the eardrum is much larger than the displacement observed in mammals; when middle-ear pressure is positive, the lateral displacement is much larger than in mammals, which is not observed in bird single-ossicle ears. Peak-to-peak displacements are about 2.8 mm, which is larger than in any other species measured up to date. The peak-to-peak displacements are at least five times larger than observed in mammals. The pressure-displacement curves show hysteresis, and the energy loss within one pressure cycle increases with increasing pressure rate, contrary to what is observed in rabbit eardrums. The energy lost during a pressure cycle is not constant over the eardrum. Most energy is lost at the region where the eardrum connects to the hearing ossicle. Around this eardrum-ossicle region, a 5% increase in energy loss was observed when pressure change rate was increased from 0.05 kPa/s to 0.2 kPa/s. Other parts of the eardrum showed little increase in the energy loss. The orientation of the in-plane strain on the eardrum was mainly circumferential with strain amplitudes of about +1.5%. The periphery of the measured eardrum surface showed compression instead of stretching and had a different strain orientation. The TM of Gekko gecko shows the highest displacements of all species measured up till now. Our data show the first shape, displacement and deformation measurements on the surface of the eardrum of the gecko and indicate that there could exist a different hysteresis behavior in different species.

Strain distribution in rabbit eardrums under static pressure

Full-field strain maps of intact rabbit eardrums subjected to static pressures are presented. A stochastic intensity pattern was applied to 12 eardrums, and strain maps were measured at the medial site using a stereoscopic digital image correlation setup for pressures between -2 and 2 kPa. Ear canal overpressures induced circumferential orientated positive strains between manubrium and the eardrum border that increased almost linearly with pressure. Radially orientated negative strains were found at the border and manubrium. Ear canal underpressures caused negative circumferential strains between manubrium and the tympanic annulus but radially orientated positive strains at the borders. The magnitudes of these negative strains at underpressures were larger than those of positive strains at overpressures and were nonlinearly proportional to pressure. In three ears, strains were calculated with intact and removed cochlea. The effect of cochlea removal on the peak-to-peak strain was found to be no more than 3%.

Nonlinear Vibration Response Measured at Umbo and Stapes in the Rabbit Middle ear

Using laser vibrometry and a stimulation and signal analysis method based on multisines, we have measured the response and the nonlinearities in the vibration of the rabbit middle ear at the level of the umbo and the stapes. With our method, we were able to detect and quantify nonlinearities starting at sound pressure levels of 93-dB SPL. The current results show that no significant additional nonlinearity is generated as the vibration signal is passed through the middle ear chain. Nonlinearities are most prominent in the lower frequencies (125 Hz to 1 kHz), where their level is about 40 dB below the vibration response. The level of nonlinearities rises with a factor of nearly 2 as a function of sound pressure level, indicating that they may become important at very high sound pressure levels such as those used in high-power hearing aids.

Experimental study of vibrations of gerbil tympanic membrane with closed middle ear cavity

The purpose of the present work is to investigate the spatial vibration pattern of the gerbil tympanic membrane (TM) as a function of frequency. In vivo vibration measurements were done at several locations on the pars flaccida and pars tensa, and along the manubrium, on surgically exposed gerbil TMs with closed middle ear cavities. A laser Doppler vibrometer was used to measure motions in response to audio frequency sine sweeps in the ear canal. Data are presented for two different pars flaccida conditions: naturally flat and retracted into the middle ear cavity. Resonance of the flat pars flaccida causes a minimum and a shallow maximum in the displacement magnitude of the manubrium and pars tensa at low frequencies. Compared with a flat pars flaccida, a retracted pars flaccida has much lower displacement magnitudes at low frequencies and does not affect the responses of the other points. All manubrial and pars tensa points show a broad resonance in the range of 1.6 to 2 kHz. Above this resonance, the displacement magnitudes of manubrial points, including the umbo, roll off with substantial irregularities. The manubrial points show an increasing displacement magnitude from the lateral process toward the umbo. Above 5 kHz, phase differences between points along the manubrium start to become more evident, which may indicate flexing of the tip of the manubrium or a change in the vibration mode of the malleus. At low frequencies, points on the posterior side of the pars tensa tend to show larger displacements than those on the anterior side. The simple low-frequency vibration pattern of the pars tensa becomes more complex at higher frequencies, with the breakup occurring at between 1.8 and 2.8 kHz. These observations will be important for the development and validation of middle ear finite-element models for the gerbil.

Measurements of three-dimensional shape and sound-induced motion of the chinchilla tympanic membrane

Opto-electronic computer holographic measurements were made of the tympanic membrane (TM) in cadaveric chinchillas. Measurements with two laser wavelengths were used to compute the 3D-shape of the TM. Single laser wavelength measurements locked to eight distinct phases of a tonal stimulus were used to determine the magnitude and the relative phase of the surface displacements. These measurements were made at over 250,000 points on the TM surface. The measured motions contained spatial phase variations consistent with relatively low-order (large spatial frequency) modal motions and smaller magnitude higher-order (smaller spatial frequency) motions that appear to travel, but may also be explained by losses within the membrane. The measurement of shape and thin shell theory allowed us to separate the measured motions into those components orthogonal to the plane of the tympanic ring, and those components within the plane of the tympanic ring based on the 3D-shape. The predicted in-plane motion components are generally smaller than the out-of-plane perpendicular component of motion. Since the derivation of in-plane and out-of plane depended primarily on the membrane shape, the relative sizes of the predicted motion components did not vary with frequency.

SUMMARY

A new method for simultaneously measuring the shape and sound-induced motion of the tympanic membrane is utilized to estimate the 3D motion on the membrane surface. This article is part of a special issue entitled "MEMRO 2012".

Mechanical properties of human tympanic membrane in the quasi-static regime from in situ point indentation measurements

The tympanic membrane is a key component of the human auditory apparatus. Good estimates of tympanic membrane mechanical properties are important to obtain realistic models of middle ear mechanics. Current literature values are almost all derived from direct mechanical tests on cut-out strips. For a biomedical specimen like the tympanic membrane, it is not always possible to harvest strips of uniform and manageable geometry and well-defined size suitable for such mechanical tests. In this work, elastic and viscoelastic properties of human tympanic membrane were determined through indentation testing on the tympanic membrane in situ. Indentation experiments were performed on three specimens with a custom-built apparatus that was also used in previously published works. Two types of indentation tests were performed on each specimen: (i) sinusoidal indentation at 0.2 Hz yielding the quasi-static Young's modulus and (ii) step indentation tests yielding viscoelastic properties in the quasi-static regime (0-20 Hz). In the cyclic indentation experiments (type i), the indentation depth and resulting needle force were recorded. The unloaded shape of the tympanic membrane and the membrane thickness were measured and used to create a specimen-specific finite element model of the experiment. The Young's modulus was then found through optimization of the error between model and experimental data; the values that were found for the three different samples are 2.1 MPa, 4.4 MPa and 2.3 MPa. A sensitivity analysis showed that these values are very sensitive to the thickness used in the models. In the step indentation tests (type ii), force relaxation was measured during 120 s and the relaxation curves were fitted with a 5 parameter Maxwell viscoelastic model. The relaxation curves in the time domain were transformed to complex moduli in the frequency domain, yielding viscoelastic properties in the quasi-static regime only.

Nonlinearity in eardrum vibration as a function of frequency and sound pressure

It is generally accepted that the middle ear acts mainly as a linear system for sound pressures up to 130 dB SPL in the auditory frequency range. However, at quasi-static pressure loads a strong nonlinear response has been demonstrated. Consequently, small nonlinear distortions may also be present in the middle ear response in the auditory frequency range. A new measurement method was developed to quickly determine vibration response, nonlinear distortions and noise level of acoustically driven biomechanical systems. Specially designed multisines are used for the excitation of the test system. The method is applied on a gerbil eardrum for sound pressures ranging from 90 to 120 dB SPL and for frequencies ranging from 125 Hz to 16 kHz. The experiments show that nonlinear distortions rise above noise level at a sound pressure of 96 dB SPL, and they grow as sound pressure increases. Post-mortem changes in the middle ear influence the nonlinear distortions rapidly until a stabilization occurs after approximately 3h.

Tympanic membrane boundary deformations derived from static displacements observed with computerized tomography in human and gerbil

The middle ear is too complex a system for its function to be fully understood with simple descriptive models. Realistic mathematical models must be used in which structural elements are represented by geometrically correct three-dimensional (3D) models with correct physical parameters and boundary conditions. In the past, the choice of boundary conditions could not be based on experimental evidence as no clear-cut data were available. We have, therefore, studied the deformation of the tympanic membrane (TM) at its boundaries using X-ray microscopic computed tomography in human and gerbil while static pressure was applied to the ear canal. The 3D models of the TM and its bony attachments were carefully made and used to measure the deformation of the TM with focus on the periphery and the manubrium attachment. For the pars flaccida of the gerbil, the boundary condition can, for the most part, be described as simply supported. For the human pars flaccida, the situation is more complicated: superiorly, the membrane contacts the underlying bone more and more when pushed further inward, and it gradually detaches from the wall when sucked outward. In gerbil, the attachment of the TM to the manubrium can be described as simply supported. In human, the manubrium is attached underneath the TM via the plica mallearis and the contact of the TM with the bone is indirect. For both human and gerbil, a simple boundary condition for the peripheral edge of the pars tensa is not appropriate due to the intricate structure at the edge: the TM thickens rapidly before continuing into the annulus fibrosis which finally makes contact with the bone.

Measurement of Young’s Modulus of Human Tympanic Membrane at High Strain Rates

The mechanical behavior of human tympanic membrane (TM) has been investigated extensively under quasistatic loading conditions in the past. The results, however, are sparse for the mechanical properties (e.g., Young's modulus) of the TM at high strain rates, which are critical input for modeling the mechanical response under blast wave. The property data at high strain rates can also potentially be converted into complex modulus in frequency domain to model acoustic transmission in the human ear. In this study, we developed a new miniature split Hopkinson tension bar to investigate the mechanical behavior of human TM at high strain rates so that a force of up to half of a newton can be measured accurately under dynamic loading conditions. Young’s modulus of a normal human TM is reported as 45.2–58.9 MPa in the radial direction, and 34.1–56.8 MPa in the circumferential direction at strain rates 300–2000 s−1. The results indicate that Young’s modulus has a strong dependence on strain rate at these high strain rates.

Evaluation of a model for studies on sequelae after acute otitis media in the Mongolian gerbil

CONCLUSIONS

The model appears relevant for studies on sequelae after acute otitis media (AOM), and may be the seed of a new, chronic tympanic membrane perforation model in the gerbil.

OBJECTIVES

To evaluate an experimental model for abortive otitis media and to assess the structural and functional changes of the tympanic membrane in the resolving phase.

MATERIALS AND METHODS

The middle ears of 16 Mongolian gerbils were inoculated with type 6a Streptococcus pneumoniae. Half of the animals were treated with antibiotics on days 4-6, when otoscopy was performed as well. After 1, 2, 3 or 4 weeks the animals were sacrificed and their tympanic membranes were examined by otoscopy, dissection microscopy, light microscopy and moire interferometry.

RESULTS

On days 4 and 6 AOM was produced in approximately 80% of the animals and perforations prevailed in approximately 30% at the study end points. Clinical signs of AOM and oedema of the tympanic membrane had already started to reduce after 1 week, and often resolved within 2 weeks. The mechanical stiffness of the tympanic membrane remained relatively unharmed in the non-perforated ears. The antibiotic treatment seemed to reduce the duration of oedema but not the perforation rate.

Functional effects of repeated pressure loads upon the tympanic membrane: mechanical stiffness measurements after simulated habitual sniffing

In experimental studies it was found that otitis media causes stiffness loss in the tympanic membrane, possible precursors to retraction pockets and cholesteatoma. Besides otitis media habitual sniffing behaviour is associated with the development of retractions. The present study aims to test the hypothesis that repeated sniffing manoeuvre may cause not only structural, epithelial tympanic membrane changes presumed to be possible precursors to retractions, but also tympanic membrane stiffness loss, another possible mediator for the development of retractions. An experimental model with a pressure chamber was used to mimic the pressure conditions for the tympanic membrane in habitual sniffers' ears. The stiffness properties of twelve Mongolian gerbil tympanic membranes were measured with moiré interferometry after varying time up to 12 days with repeated pressure loading. Three days later, lower overall displacement were obtained in two ears; after 7-12 days the displacement readings were normal. This study with maximum of 12 days of pressure loading did not verify the hypothesis that habitual "sniffing" impairs the stiffness of the tympanic membrane.

A nonlinear finite-element model of the newborn middle ear

A three-dimensional static nonlinear finite-element model of a 22-day-old newborn middle ear is presented. The model includes the tympanic membrane (TM), malleus, incus, and two ligaments. The effects of the middle-ear cavity are taken into account indirectly. The geometry is based on a computed-tomography scan and on the published literature, supplemented by histology. A nonlinear hyperelastic constitutive law is applied to model large deformations. The middle-ear cavity and the Young's modulus of the TM have significant effects on TM volume displacements. The TM volume displacement and its nonlinearity and asymmetry increase as the middle-ear cavity volume increases. The effects of the Young's moduli of the ligaments and ossicles are found to be small. The simulated TM volume changes do not reach a plateau when the pressure is varied to either -3 kPa or +3 kPa, which is consistent with the nonflat tails often found in tympanograms in newborns. The simulated TM volume displacements, by themselves and also together with previous ear-canal model results, are compared with equivalent-volume differences derived from tympanometric measurements in newborns. The results suggest that the canal-wall volume displacement makes a major contribution to the total canal volume change, and may be larger than the TM volume displacement.

Finite-element analysis of middle-ear pressure effects on static and dynamic behavior of human ear

A finite-element analysis for static behavior of middle ear under variation of the middle-ear pressure was conducted in a 3D model of human ear by combining the hyperelastic Mooney-Rivlin material model and geometry nonlinearity. An empirical formula was then developed to calculate material parameters of the middle-ear soft tissues as the stress-dependent elastic modulus relative to the middle-ear pressure. Dynamic behavior of the middle ear in response to sound pressure in the ear canal was predicted under various positive and negative middle-ear pressures. The results from static analysis indicate that a positive middle ear pressure produces the static displacements of the tympanic membrane (TM) and footplate more than a negative pressure. The dynamic analysis shows that the reductions of the TM and footplate vibration magnitudes under positive middle-ear pressure are mainly determined by stress dependence of elastic modulus. The reduction of the TM and footplate vibrations under negative pressure was caused by both the geometry changes of middle-ear structures and the stress dependence of elastic modulus.

Laser interferometry measurements of middle ear fluid and pressure effects on sound transmission

An otitis media with effusion model in human temporal bones with two laser vibrometers was created in this study. By measuring the displacement of the stapes from the medial side of the footplate, the transfer function of the middle ear, which is defined as the displacement transmission ratio (DTR) of the tympanic membrane to footplate, was derived under different middle ear pressure and fluid in the cavity with a correction factor for cochlear load. The results suggest that the DTR increases with increasing frequency up to 4k Hz when the middle ear pressure was changing from 0 to 20 or -20 cm H20 (e.g., +/-196 daPa) and fluid level was increasing from 0 to a full middle ear cavity. The positive and negative pressures show different effects on the DTR. The effect of fluid on DTR varies between three frequency ranges: f < 1k, between 1k and 4k, and f > 4k Hz. These findings show how the efficiency of the middle ear system for sound transmission changes during the presence of fluid in the cavity and variations of middle ear pressure.

Quasi-static transfer function of the rabbit middle ear' measured with a heterodyne interferometer with high-resolution position decoder

Due to changes in ambient pressure and to the gas-exchange processes in the middle ear (ME) cavity, the ear is subject to ultra-low-frequency pressure variations, which are many orders of magnitude larger than the loudest acoustic pressures. Little quantitative data exist on how ME mechanics deals with these large quasi-static pressure changes and because of this lack of data, only few efforts could be made to incorporate quasi-static behavior into computer models. When designing and modeling ossicle prostheses and implantable ME hearing aids, the effects of large ossicle movements caused by quasi-static pressures should be taken into account. We investigated the response of the ME to slowly varying pressures by measuring the displacement of the umbo and the stapes in rabbit with a heterodyne interferometer with position decoder. Displacement versus pressure curves were obtained at linear pressure change rates between 200 Pa/s and 1.5 kPa/s, with amplitude +/-2.5 kPa. The change in stapes position associated with a pressure change is independent of pressure change rate (34 microm peak-to-peak at +/-2.5 kPa). The stapes displacement versus pressure curves are highly nonlinear and level off for pressures beyond +/-1 kPa. Stapes motion shows no measurable hysteresis at 1.5 kPa/s, which demonstrates that the annular ligament has little viscoelasticity. Hysteresis increases strongly at the lowest pressure change rates. The stapes moves in phase with the umbo and with pressure, but the sense of rotation of the hysteresis loop of stapes is phase inversed. Stapes motion is not a simple lever ratio mimic of umbo motion, but is the consequence of complex changes in ossicle joints and ossicle position. The change in umbo position produced by a +/-2.5 kPa pressure change decreases with increasing rate from 165 microm at 200 Pa/s to 118 microm at 1.5 kPa/s. Umbo motion already shows significant hysteresis at 1.5 kPa/s, but hysteresis increases further as pressure change rate decreases. We conclude that in the quasi-static regime, ossicle movement is not only governed by viscoelasticity, but that other effects become dominant as pressure change rate decreases below 1 kPa/s. The increasing hysteresis can be caused by increasing friction as speed of movement decreases, and incorporating speed-dependent friction coefficients will be essential to generate realistic models of ossicle movements at slow pressure change rates.

Low-frequency finite-element modeling of the gerbil middle ear

The gerbil is a popular species for experimental middle-ear research. The goal of this study is to develop a 3D finite-element model to quantify the mechanics of the gerbil middle ear at low frequencies (up to about 1 kHz). The 3D reconstruction is based on a magnetic resonance imaging dataset with a voxel size of about 45 microm, and an x-ray micro-CT dataset with a voxel size of about 5.5 microm, supplemented by histological images. The eardrum model is based on moiré shape measurements. Each individual structure in the model was assumed to be homogeneous with isotropic, linear, and elastic material properties derived from a priori estimates in the literature. The behavior of the finite-element model in response to a uniform acoustic pressure on the eardrum of 1 Pa is analyzed. Sensitivity tests are done to evaluate the significance of the various parameters in the finite-element model. The Young's modulus and the thickness of the pars tensa have the most significant effect on the load transfer between the eardrum and the ossicles and, along with the Young's modulus of the pedicle and stapedial annular ligament, on the displacements of the stapes. Overall, the model demonstrates good agreement with low-frequency experimental data. For example, (1) the maximum footplate displacement is about 35 nm; (2) the umbo/stapes displacement ratio is found to be about 3.5; (3) the motion of the stapes is predominantly piston-like; and (4) the displacement pattern of the eardrum shows two points of maximum displacement, one in the posterior region and one in the anterior region. The effects of removing or stiffening the ligaments are comparable to those observed experimentally.

Thickness of the gerbil tympanic membrane measured with confocal microscopy

Thickness data for the gerbil tympanic membrane, an extremely thin biological membrane, are presented. Thickness measurements were performed on fresh material using fluorescence images taken perpendicular through the membrane with a commercial confocal microscope. Thickness varies strongly across the membrane. Similar thickness distributions in all samples (pars tensa n = 11; pars flaccida n = 3) were observed. The pars tensa has a rather constant thickness of about 7 microm in the central region curving as a horse shoe upwards around the manubrium. In the most superior parts of the pars tensa thickness becomes gradually twice as large. Thickness increases also steeply from the central region towards the edges (about 35 microm near the annulus and 20 microm near the manubrium). A pronounced, local thickening of about 30 microm is present close to the edge and extends as a ring along the entire annular periphery of the pars tensa. Overall, the pars flaccida is thicker than the pars tensa and has a rugged surface. Its central region has a mean thickness of about 24 microm with a mean variation of about 4 microm. The average thickness in the inferior region is slightly larger than in the superior region. The pars flaccida thickens steeply, up to about 80 microm, near the edges.

Response of the cat eardrum to static pressures: mobile versus immobile malleus

A phase-shift shadow moiré interferometer was used to measure the shape of the cat eardrum with a normal mobile malleus and with an immobile malleus as it was cyclically loaded with static middle-ear pressures up to +/-2.2 kPa. The shape was monitored throughout the loading and unloading phases, and three complete cycles were observed. The mobile-manubrium measurements were made in five ears. In three ears, the malleus was then immobilized with a drop of glue placed on the head of the malleus. Eardrum displacements were calculated by subtracting shape images pixel by pixel. The measurements are presented in the form of gray-level full-field shape and displacement images, of displacement profiles, and of pressure-displacement curves for selected points. Displacement patterns with a mobile malleus show that pars-tensa displacements are larger than manubrial displacements, with the maximum pars-tensa displacement occurring in the posterior region in all cats except one. Displacements vary from cycle to cycle and display hysteresis. For both the mobile-malleus and immobile-malleus cases, the eardrum response is nonlinear. The response is asymmetric, with lateral displacements being larger than medial displacements. With a mobile malleus, manubrial displacements exhibit more pronounced asymmetry than do pars-tensa displacements.