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

Relative importance and interactions of parameters of finite-element models of human middle ear

In the last decades, finite-element models of the middle ear have been widely used to predict the middle-ear vibration outputs. Even with the simplest linear assumption for material properties of the structures in the middle ear, these models need tens of parameters. Due to the complexities of measurements of material properties of these structures, accurate estimations of the values of most of these parameters are not possible. In this study, we benefited from the stochastic finite-element model of the middle ear we had developed in the past, to perform global sensitivity analysis. For this aim, we implemented Sobol' sensitivity analysis which ranks the importance of all uncertain parameters and interactions among them at different frequencies. To decrease the computational costs, we found Sobol' indices from surrogate models that we created using stochastic finite-element results and the polynomial chaos expansion method. Based on the results, the Young's modulus and thickness of the tympanic membrane, Young's modulus and damping of the stapedial annular ligaments, and the Young's modulus of ossicles are among the parameters with the greatest impacts on vibrations of the umbo and stapes footplate. Furthermore, the most significant interactions happen between the Young's modulus and thickness of the tympanic membrane.

Inaccuracies of deterministic finite-element models of human middle ear revealed by stochastic modelling

For over 40 years, finite-element models of the mechanics of the middle ear have been mostly deterministic in nature. Deterministic models do not take into account the effects of inter-individual variabilities on middle-ear parameters. We present a stochastic finite-element model of the human middle ear that uses variability in the model parameters to investigate the uncertainty in the model outputs (umbo, stapes, and tympanic-membrane displacements). We demonstrate: (1) uncertainties in the model parameters can be magnified by more than three times in the umbo and stapes footplate responses at frequencies above 2 kHz; (2) middle-ear models are biased and they distort the output distributions; and (3) with increased frequency, the highly-uncertain regions spatially spread out on the tympanic membrane surface. Our results assert that we should be mindful when using deterministic finite-element middle-ear models for critical tasks such as novel device developments and diagnosis.

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

Tympanometry provides an objective measurement of the status of the middle ear. During tympanometry, the ear-canal pressure is varied, while the response of the ear to sound pressure is measured. The effects of the pressure on the mechanics of the middle ear are not well understood. This study is a continuation of our previous work in which the vibration response of the gerbil eardrum was measured in vivo under quasi-static pressure steps. In this study, we delivered a continuous pressure sweep to the middle ear and measured the vibration response at four locations for six gerbils. Vibrations were recorded using a single-point laser Doppler vibrometer and glass-coated reflective beads (diameter ~ 40 µm) at the umbo and on the mid-manubrium, posterior pars tensa and anterior pars tensa.The vibration magnitudes were similar to those in the previous step-wise pressurization experiments. Most gerbils showed repeatability within less than 10 dB for consecutive cycles. As described in the previous study, as the frequency was increased at ambient pressure, the vibration magnitude on the manubrium increased slightly to a broad peak (referred to as R1) and then decreased until a small peak appeared (referred to as R2), followed by multiple peaks and troughs as the magnitude decreased further. The low-frequency vibration magnitude (at 1 kHz) decreased monotonically as the pressure became more negative except for a dip (about 500 Pa wide) that occurred between - 700 and - 1800 Pa. The lowest overall magnitude was recorded in the dip at mid-manubrium. The vibration magnitudes also decreased as the middle-ear pressure was made more positive and were larger than those at negative pressures. R1 was only visible at negative and small positive middle-ear pressures, while R2 was visible for both positive and negative pressures. R2 split into multiple branches after the middle-ear pressure became slightly positive. No magnitude dip was visible for positive middle-ear pressures.The low-frequency vibration magnitudes at negative middle-ear pressures on the pars tensa were higher than those on the manubrium. R1 was not visible for large negative middle-ear pressures on the pars tensa. R2 appeared as a multi-peak feature on the pars tensa as well, and a higher-frequency branch on the posterior pars tensa appeared as a trough on the anterior pars tensa. The magnitude dip was not present on the pars tensa. The largest overall magnitude was recorded at the R2 peak on the posterior pars tensa.The results of this study expand on the findings of the step-wise pressurization experiments and provide further insight into the evolution of the vibration response of the eardrum under quasi-static pressures.

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.

Newly breeding an inbred strain of ischemia-prone Mongolian gerbils and its reproduction and genetic characteristics

The Mongolian gerbil has been a useful laboratory animal in many research fields, especially in ischemia studies. However, due to the variation of the circle of Willis (COW), the ischemic model is unstable and various. To solve this problem, we newly established an inbred strain of gerbils, restricting breeding and keeping to F₂₃. The data on the breeding and growth of the animals are described in the present study. The genetic characteristics of F₄ to F₂₀ detected by microsatellite DNA and biochemical markers are also shown here. The results demonstrated that the frequency of ischemic model by unilateral carotid occlusion and the frequency of incomplete COW increased, increasing from 50% and 75% in F₁ to 88.89% and 100% in F₂₀, respectively. The ratios of consistent patterns of COW in parents were positively related with the number of inbred generations. A reproductive performance analysis indicated that the average size of litters in the inbred gerbils was less than that of outbred gerbils and that adult body weight was also lower in inbred gerbils; also, the pups in the 2nd litter were the best ones chosen to reproduce. The genetic detection results indicated that 26 out of 28 microsatellite loci and all 26 biochemical markers were homozygous in F₂₀, showing comparably identical genetic composition in inbred gerbils. All the data demonstrated that an inbred strain of ischemia-prone gerbil has been established successfully. This strain can be used in stroke research and can largely reduce the number of animals needed in experiments.

Panel 2: Anatomy (Eustachian Tube, Middle Ear, and Mastoid-Anatomy, Physiology, Pathophysiology, and Pathogenesis)

Objective In this report, we review the recent literature (ie, past 4 years) to identify advances in our understanding of the middle ear-mastoid-eustachian tube system. We use this review to determine whether the short-term goals elaborated in the last report were achieved, and we propose updated goals to guide future otitis media research. Data Sources PubMed, Web of Science, Medline. Review Methods The panel topic was subdivided, and each contributor performed a literature search within the given time frame. The keywords searched included middle ear, eustachian tube, and mastoid for their intersection with anatomy, physiology, pathophysiology, and pathology. Preliminary reports from each panel member were consolidated and discussed when the panel met on June 11, 2015. At that meeting, the progress was evaluated and new short-term goals proposed. Conclusions Progress was made on 13 of the 20 short-term goals proposed in 2011. Significant advances were made in the characterization of middle ear gas exchange pathways, modeling eustachian tube function, and preliminary testing of treatments for eustachian tube dysfunction. Implications for Practice In the future, imaging technologies should be developed to noninvasively assess middle ear/eustachian tube structure and physiology with respect to their role in otitis media pathogenesis. The new data derived from these structure/function experiments should be integrated into computational models that can then be used to develop specific hypotheses concerning otitis media pathogenesis and persistence. Finally, rigorous studies on medical or surgical treatments for eustachian tube dysfunction should be undertaken.

Functional Outcomes of Heparin-Binding Epidermal Growth Factor-Like Growth Factor for Regeneration of Chronic Tympanic Membrane Perforations in Mice

We aim to demonstrate that regeneration of chronic tympanic perforations with heparin-binding epidermal growth factor-like growth factor (HB-EGF) delivered by an injectable hydrogel restored hearing to levels similar to that of nonperforated tympanic membranes. Chronic tympanic membrane perforation is currently managed as an outpatient surgery with tympanoplasty. Due to the costs of this procedure in the developed world and a lack of accessibility and resources in developing countries, there is a great need for a new treatment that does not require surgery. In this study, we show in a mouse model through measurement of auditory brainstem response and distortion product otoacoustic emissions that tympanic perforations lead to hearing loss and this can be predominantly recovered with HB-EGF treatment (5 μg/mL). Our animal model suggests a return to function between 2 and 6 months after treatment. Auditory brainstem response thresholds had returned to the control levels at 2 months, but the distortion product otoacoustic emissions returned between 2 and 6 months. We also show how the vibration characteristics of the regenerated tympanic membrane, as measured by laser Doppler vibrometry, can be similar to that of an unperforated tympanic membrane. Using the best available methods for preclinical evaluation in animal models, it is likely that HB-EGF-like growth factor treatment leads to regeneration of chronic tympanic membrane perforations and restoration of the tympanic membrane to normal function, suggesting a potential route for nonsurgical treatment.

The path of a click stimulus from ear canal to umbo

The tympanic membrane (TM) has a key role in transmitting sounds to the inner ear, but a concise description of how the TM performs this function remains elusive. This paper probes TM operation by applying a free field click stimulus to the gerbil ear and exploring the consequent motions of the TM and umbo. Motions of the TM were measured both on radial tracks starting close to the umbo and on a grid distal and adjacent to the umbo. The experimental results confirmed the high fidelity of sound transmission from the ear canal to the umbo. A delay of 5-15 μs was seen in the onset of TM motion between points just adjacent to the umbo and mid-radial points. The TM responded with a ringing motion, with different locations possessing different primary ringing frequencies. A simple analytic model from the literature, treating the TM as a string, was used to explore the experimental results. The click-based experiments and analysis led to the following description of TM operation: A transient sound pressure on the TM causes a transient initial TM motion that is maximal ∼ at the TM's radial midpoints. Mechanical forces generated by this initial prominent TM distortion then pull the umbo inward, leading to a delayed umbo response. The initial TM deformation also gives rise to prolonged mechanical ringing on the TM that does not result in significant umbo motion, likely due to destructive interference from the range of ringing frequencies. Thus, the umbo's response is a high-fidelity representation of the transient stimulus. Because any sound can be considered as a consecutive series of clicks, this description is applicable to any sound stimulus.

Morphological changes in the tympanic membrane associated with Haemophilus influenzae-induced acute otitis media in the chinchilla

OBJECTIVE

The tympanic membrane (TM) couples sound waves entering the outer ear canal to mechanical vibrations of the ossicular chain in the middle ear. During acute otitis media (AOM), dynamic structural changes in the TM can occur, which potentially affect sound transmission. It has remained unclear whether TM changes contribute significantly to the conductive hearing loss associated with human AOM. Studies that systematically and quantitatively assess the impact of morphological and mechanical characteristics of the TM on hearing in animal models of AOM have been few in number and lack detail. Our current study focused on the identification of quantitative morphological changes in the TM of the adult chinchilla.

METHOD

AOM was produced by transbullar injection of the nontypeable (acapsular) Haemophilus influenzae strain 86-028NP into two treatment groups of chinchillas: one 4 days (4D) post bacterial challenge, and a second treatment group after 8 days (8D) post challenge. Structure and thickness were examined histologically at nine locations over the TM in untreated controls and in animals from both AOM treatment groups.

RESULTS

TM thickness was found to have increased significantly (110-150%) at all measured locations of H. influenzae-infected ears when compared with uninfected (normal) TMs at 4D post bacterial challenge. Cellular proliferation and infiltration in the outer epithelial layer were primary contributors to this thickening. In ears infected for 8D, the TM was substantially thicker, a 200-300% increase from uninfected control values, due to edema and cell proliferation in both the outer and inner epithelial layers. In both 4D and 8D ears, thickening of the TM was more prominent in the superior-anterior quadrant.

CONCLUSION

This study provides unequivocal structural evidence that significant TM thickness increases are associated with AOM induced by a well characterized H. influenzae human clinical isolate of low passage number. These and additional thickness data from early and later stages in middle ear infection will be used to derive the mechanical properties of the TM in a future study from our laboratory.

Finite-Element Modelling of the Response of the Gerbil Middle Ear to Sound

We present a finite-element model of the gerbil middle ear that, using a set of baseline parameters based primarily on a priori estimates from the literature, generates responses that are comparable with responses we measured in vivo using multi-point vibrometry and with those measured by other groups. We investigated the similarity of numerous features (umbo, pars-flaccida and pars-tensa displacement magnitudes, the resonance frequency and break-up frequency, etc.) in the experimental responses with corresponding ones in the model responses, as opposed to simply computing frequency-by-frequency differences between experimental and model responses. The umbo response of the model is within the range of variability seen in the experimental data in terms of the low-frequency (i.e., well below the middle-ear resonance) magnitude and phase, the main resonance frequency and magnitude, and the roll-off slope and irregularities in the response above the resonance frequency, but is somewhat high for frequencies above the resonance frequency. At low frequencies, the ossicular axis of rotation of the model appears to correspond to the anatomical axis but the behaviour is more complex at high frequencies (i.e., above the pars-tensa break-up). The behaviour of the pars tensa in the model is similar to what is observed experimentally in terms of magnitudes, phases, the break-up frequency of the spatial vibration pattern, and the bandwidths of the high-frequency response features. A sensitivity analysis showed that the parameters that have the strongest effects on the model results are the Young's modulus, thickness and density of the pars tensa; the Young's modulus of the stapedial annular ligament; and the Young's modulus and density of the malleus. Displacements of the tympanic membrane and manubrium and the low-frequency displacement of the stapes did not show large changes when the material properties of the incus, stapes, incudomallear joint, incudostapedial joint, and posterior incudal ligament were changed by ±10 % from their values in the baseline parameter set.

Structure and function of the mammalian middle ear. II: Inferring function from structure

Anatomists and zoologists who study middle ear morphology are often interested to know what the structure of an ear can reveal about the auditory acuity and hearing range of the animal in question. This paper represents an introduction to middle ear function targetted towards biological scientists with little experience in the field of auditory acoustics. Simple models of impedance matching are first described, based on the familiar concepts of the area and lever ratios of the middle ear. However, using the Mongolian gerbil Meriones unguiculatus as a test case, it is shown that the predictions made by such 'ideal transformer' models are generally not consistent with measurements derived from recent experimental studies. Electrical analogue models represent a better way to understand some of the complex, frequency-dependent responses of the middle ear: these have been used to model the effects of middle ear subcavities, and the possible function of the auditory ossicles as a transmission line. The concepts behind such models are explained here, again aimed at those with little background knowledge. Functional inferences based on middle ear anatomy are more likely to be valid at low frequencies. Acoustic impedance at low frequencies is dominated by compliance; expanded middle ear cavities, found in small desert mammals including gerbils, jerboas and the sengi Macroscelides, are expected to improve low-frequency sound transmission, as long as the ossicular system is not too stiff.

Experimental and modeling study of human tympanic membrane motion in the presence of middle ear liquid

Vibration of the tympanic membrane (TM) has been measured at the umbo using laser Doppler vibrometry and analyzed with finite element (FE) models of the human ear. Recently, full-field TM surface motion has been reported using scanning laser Doppler vibrometry, holographic interferometry, and optical coherence tomography. Technologies for imaging human TM motion have the potential to lead to using a dedicated clinical diagnosis tool for identification of middle ear diseases. However, the effect of middle ear fluid (liquid) on TM surface motion is still not clear. In this study, a scanning laser Doppler vibrometer was used to measure the full-field surface motion of the TM from four human temporal bones. TM displacements were measured under normal and disease-mimicking conditions with different middle ear liquid levels over frequencies ranging from 0.2 to 8 kHz. An FE model of the human ear, including the ear canal, middle ear, and spiral cochlea was used to simulate the motion of the TM in normal and disease-mimicking conditions. The results from both experiments and FE model show that a simple deflection shape with one or two major displacement peak regions of the TM in normal ear was observed at low frequencies (1 kHz and below) while complicated ring-like pattern of the deflection shapes appeared at higher frequencies (4 kHz and above). The liquid in middle ear mainly affected TM deflection shapes at the frequencies higher than 1 kHz.

External and middle ear sound pressure distribution and acoustic coupling to the tympanic membrane

Sound energy is conveyed to the inner ear by the diaphanous, cone-shaped tympanic membrane (TM). The TM moves in a complex manner and transmits sound signals to the inner ear with high fidelity, pressure gain, and a short delay. Miniaturized sensors allowing high spatial resolution in small spaces and sensitivity to high frequencies were used to explore how pressure drives the TM. Salient findings are: (1) A substantial pressure drop exists across the TM, and varies in frequency from ∼10 to 30 dB. It thus appears reasonable to approximate the drive to the TM as being defined solely by the pressure in the ear canal (EC) close to the TM. (2) Within the middle ear cavity (MEC), spatial variations in sound pressure could vary by more than 20 dB, and the MEC pressure at certain locations/frequencies was as large as in the EC. (3) Spatial variations in pressure along the TM surface on the EC-side were typically less than 5 dB up to 50 kHz. Larger surface variations were observed on the MEC-side.

Effect of opening middle-ear cavity on vibrations of gerbil tympanic membrane

This paper presents in vivo experimental measurements of vibrations on the pars flaccida, along the manubrium and at several points on the pars tensa in the gerbil with open middle-ear cavity. The effects of progressive opening of the middle-ear cavity are presented, with up to five different extents of opening. In all manubrial, pars-tensa and pars-flaccida responses, opening the cavity causes an increase in the low-frequency magnitude and a shift of the main middle-ear resonance to lower frequencies and introduces an antiresonance. However, opening the cavity has little or no effect on either the mode of vibration of the manubrium or the breakup frequency of the pars tensa. When the opening is gradually widened, the antiresonance frequency moves to higher frequencies. When the opening is made as wide as anatomically possible, the antiresonance moves to almost 10 kHz. The main increase in the low-frequency response magnitude happens upon making the smallest hole in the cavity wall, and further progressive enlarging of the opening has little or no effect on the low-frequency magnitude. The antiresonance interferes with the response shapes. An identification method is suggested for eliminating the effect of the antiresonance in order to estimate the ideal open-cavity response. The method is validated and then applied to manubrial and pars-tensa responses. Estimating the ideal open-cavity responses will simplify comparison of the data with numerical models which do not include the air cavity. The data collected at intermediate stages of opening will be useful in validating models that do include the cavity.