Effects of model definitions and parameter values in finite element modeling of human middle ear mechanics.

BACKGROUND: Despite continuing advances in finite element software, the realistic simulation of middle ear response under acoustic stimulation continues to be challenging. One reason for this is the wide range of possible choices that can be made during the definition of a model. Therefore, an explorative study of the relative influences of some of these choices is potentially very helpful. METHOD: Three finite element models of the human middle ear were constructed, based on high-resolution micro-computed tomography scans from three different human temporal bones. Interesting variations in modeling definitions and parameter values were selected and their influences on middle ear transmission were evaluated. The models were compared against different experimental validation criteria, both from the literature and from our own measurements. Simulation conditions were restricted to the frequency range 0.1-10 kHz. RESULTS: Modeling the three geometries with the same modeling definitions and parameters produces stapes footplate response curves that exhibit similar shapes, but quantitative differences of 4 dB in the lower frequencies and up to 6 dB around the resonance peaks. The model properties with the largest influences on our model outcomes are the tympanic membrane (TM) damping and stiffness and the cochlear load. Model changes with a small to negligible influence include the isotropy or orthotropy of the TM, the geometry of the connection between the TM and the malleus, the microstructure of the incudostapedial joint, and the length of the tensor tympani tendon. CONCLUSION: The presented results provide insights into the importance of different features in middle ear finite element modeling. The application of three different individual middle ear geometries in a single study reduces the possibility that the conclusions are strongly affected by geometrical abnormalities. Some modeling variations that were hypothesized to be influential turned out to be of minor importance. Furthermore, it could be confirmed that different geometries, simulated using the same parameters and definitions, can produce significantly different responses.

A single-ossicle ear: Acoustic response and mechanical properties measured in duck.

To date, the single-ossicle avian middle ear (ME) is poorly understood, despite its striking resemblance to the design of many currently used ossicular replacement prostheses. This study aims to improve comprehension of this system. The acoustic response and the mechanical properties of the mallard middle ear were studied by means of optical interferometry experiments and finite element (FE) simulations. A finite element model was constructed based on µCT data and validated using the experimental results. Stroboscopic holography was used to measure the full-field displacement of the tympanic membrane (TM) under acoustic stimulation, and the transfer function was obtained with laser Doppler vibrometry. A sensitivity analysis concluded that the most influential parameters for ME mechanics are the elasticity of the TM, the extracolumella (the cartilaginous part of the columella) and the annular ligament of the columellar footplate. Estimates for the Young's modulus of the TM were obtained by iteratively updating the FE model to match experimental data. A considerable inter-individual variability was found for the TM's elasticity. Comparison of the experimental results and the optimized FE model shows that, similar to the human middle ear, damping needs to be present in the TM to describe the specific spatial and frequency dependent vibrations of the TM. In summary, our results indicate which mechanical parameters are essential to the good functioning of the avian ME and provide a first estimation of their values.

Pressure buffering by the tympanic membrane. In vivo measurements of middle ear pressure fluctuations during elevator motion.

OBJECTIVES: The tympanic membrane (TM) represents a pressure buffer, which contributes to the overall pressure regulation of the middle ear (ME). This buffer capacity is based on its viscoelastic properties combined with those of the attached ossicular chain, muscles and ligaments. The current work presents a set of in vivo recordings of the ME pressure variations normally occurring in common life: elevator motion. This is defined as a situation of smooth ambient pressure increase or decrease on a limited range and at a low rate of pressure change. Based on these recordings, the purpose was a quantitative analysis of the TM buffer capacity including the TM compliance. METHODS: The pressure changes in seven normal adult ME's with intact TM's were continuously recorded directly inside the ME cavity during four different elevator trips using a high precision instrument. The TM buffer capacity was determined by the ratio between the changes in ME and the ambient pressure. Further, the ME volumes were calculated by Boyle's Law from pressure recordings during inflation-deflation tests; subsequently the TM compliance could also be calculated. Finally, the correlation between the ME volume and buffer function was determined. RESULTS: Twenty-one elevator trips could be used for the analysis. The overall mean TM pressure buffering capacity was 23.3% (SEM = 3.4), whereas the mean overall compliance was 28.9 × 10-3 µL/Pa (SEM = 4.8). A strong negative linear correlation was found between the TM buffer capacity and the ME volumes (R2 = 0.92). CONCLUSIONS: These results were in fair agreement with the literature obtained in clinical as well as temporal bone experiments, and they provide an in vivo reference for the normal ME function as well as for ME modeling. The TM buffer capacity was found more efficient in smaller mastoids. Possible clinical implications are discussed.

On the connection between the tympanic membrane and the malleus.

BACKGROUND: The tympano-mallear connection (TMC) is the soft-tissue connection between the tympanic membrane (TM) and the manubrium of the malleus. Some studies suggest that its mechanical properties may have a substantial influence on the mechanics and transfer function of the middle ear. However, relatively little is known about the dimensions of the TMC and its variability among individuals. METHOD: Thirteen samples were collected from human temporal bones, consisting of only the malleus and the TM. They were imaged using µCT without contrast enhancing agent. From the µCT images, the TMC dimensions were measured in both anterior-posterior direction (TMC width) and medial-lateral direction (TMC thickness). Three selected samples were examined using histological microscopy. RESULTS: Both TMC width and thickness featured a large variability among individuals. The minimal TMC width along the manubrium for different individuals covered a range between 83 and 840 µm. The minimal thickness ranged from 48 to 249 µm and the maximal thickness from 236 to 691 µm. Histological sections showed that the TMC consists of a narrow core of dense regular connective tissue, surrounded by loose connective tissue. In some samples, either of these two components was absent in the TMC at some manubrium locations. The configuration of these components varied among the samples as well. CONCLUSION: Our data confirm that a large inter-individual variability exists in the properties of the TM-malleus connection in humans in terms of its dimensions, tissue composition and configuration. Average data and their variability margins will be useful input for testing the importance of the TMC in finite element models.

Details of human middle ear morphology based on micro-CT imaging of phosphotungstic acid stained samples.

A multitude of morphological aspects of the human middle ear (ME) were studied qualitatively and/or quantitatively through the postprocessing and interpretation of micro-CT (micro X-ray computed tomography) data of six human temporal bones. The samples were scanned after phosphotungstic acid staining to enhance soft-tissue contrast. The influence of this staining on ME ossicle configuration was shown to be insignificant. Through postprocessing, the image data were converted into surface models, after which the approaches diverged depending on the topics of interest. The studied topics were: the ME ligaments; morphometric and mechanical parameters of the ossicles relating to inertia and the ossicular lever arm ratio; the morphology of the distal incus; the contact surface areas of the tympanic membrane (TM) and of the stapes footplate; and the thickness of the TM, round window of the cochlea, ossicle joint spaces, and stapedial annular ligament. Some of the resulting insights are relevant in ongoing discussions concerning ME morphology and mechanical functions, while other results provide quantitative data to add to existing data. All findings are discussed in the light of other published data and many are relevant for the construction of mechanical finite element simulations of the ME.

Volume shrinkage of bone, brain and muscle tissue in sample preparation for micro-CT and light sheet fluorescence microscopy (LSFM).

Two methods are especially suited for tomographic imaging with histological detail of macroscopic samples that consist of multiple tissue types (bone, muscle, nerve or fat): Light sheet (based) fluorescence microscopy (LSFM) and micro-computed tomography (micro-CT). Micro-CT requires staining with heavy chemical elements (and thus fixation and sometimes dehydration) in order to make soft tissue imageable when measured alongside denser structures. LSMF requires fixation, decalcification, dehydration, clearing and staining with a fluorescent dye. The specimen preparation of both imaging methods is prone to shrinkage, which is often not mentioned, let alone quantified. In this paper the presence and degree of shrinkage are quantitatively identified for the selected preparation methods/stains. LSFM delivers a volume shrinkage of 17% for bone, 56% for muscle and 62% for brain tissue. The three most popular micro-CT stains (phosphotungstic acid, iodine with potassium iodide, and iodine in absolute ethanol) deliver a volume shrinkage ranging from 10 to 56% for muscle and 27-66% for brain, while bone does not shrink in micro-CT preparation.

Viscoelastic properties of the human tympanic membrane studied with stroboscopic holography and finite element modeling.

A new anatomically-accurate Finite Element (FE) model of the tympanic membrane (TM) and malleus was combined with measurements of the sound-induced motion of the TM surface and the bony manubrium, in an isolated TM-malleus preparation. Using the results, we were able to address two issues related to how sound is coupled to the ossicular chain: (i) Estimate the viscous damping within the tympanic membrane itself, the presence of which may help smooth the broadband response of a potentially highly resonant TM, and (ii) Investigate the function of a peculiar feature of human middle-ear anatomy, the thin mucosal epithelial fold that couples the mid part of the human manubrium to the TM. Sound induced motions of the surface of ex vivo human eardrums and mallei were measured with stroboscopic holography, which yields maps of the amplitude and phase of the displacement of the entire membrane surface at selected frequencies. The results of these measurements were similar, but not identical to measurements made in intact ears. The holography measurements were complemented by laser-Doppler vibrometer measurements of sound-induced umbo velocity, which were made with fine-frequency resolution. Comparisons of these measurements to predictions from a new anatomically accurate FE model with varied membrane characteristics suggest the TM contains viscous elements, which provide relatively low damping, and that the epithelial fold that connects the central section of the human manubrium to the TM only loosely couples the TM to the manubrium. The laser-Doppler measurements in two preparations also suggested the presence of significant variation in the complex modulus of the TM between specimens. Some animations illustrating the model results are available at our website (www.uantwerp.be/en/rg/bimef/downloads/tympanic-membrane-motion).

Eustachian tube pressure equilibration. Temporal analysis of pressure changes based on direct physiological recordings with an intact tympanic membrane.

Eustachian tube function is important in pressure regulation of the middle ear. The efficacy or magnitude of pressure equilibration by tube openings should be determined by the gradient between middle ear and ambient pressure, but in theory also the duration of the tube opening may play a role. This study employed direct measurements of middle ear pressure in patients, who after parotidectomy had a catheter inserted into the mastoid with a pressure transducer connected. Thus, monitoring of the middle ear pressure in response to experimentally induced pressure changes could be performed under physiological conditions with an intact tympanic membrane. A set of six experiments was performed in four healthy subjects with different pressure deviations, where the counter-regulation was recorded over 10 min's time frames; a total of 75 events of tube openings were recorded. The transducer had a high accuracy of ±0.1 daPa, and data were sampled at 10 Hz, so that detailed parameters for each tube opening event could be obtained: the pressure change, the pressure gradient, and the duration of the opening were determined. The pressure changes in response to Eustachian tube openings showed significant positive correlation to the pressure gradient and ambient pressure (p < 0.001). However, the duration of the opening time was not related to the pressure gradient (p = 0.16), as well as the pressure change was also not related to the duration of the opening time (p = 0.34). This meant that the magnitude of a pressure equilibration during tube openings was only determined by the pressure gradient and not variations in the duration of the opening time. Additional correlations were investigated including the pressure change rate. In conclusion, under physiological conditions the opening of the Eustachian tube behaves similarly to a reflex mechanism with relative constant duration. Therefore, in order to equilibrate higher pressure gradients, series of Eustachian tube openings are needed, rather than the tube will open during a longer period of time. This article is part of a special issue entitled "MEMRO 2012".