Elastic characterization of the gerbil pars flaccida from in situ inflation experiments

Combined analysis of finite element model and audiometry provides insights into the pathogenesis of conductive hearing loss

The middle ear transmits sound to the inner ear via vibrations in the eardrum and ossicles, and damage to the middle ear results in conductive hearing loss. Although conductive hearing loss can be corrected by surgery, the currently available clinical investigations cannot always diagnose the ossicular chain pathology underlying the conductive hearing loss, and even intraoperative findings can be equivocal. Acoustic analysis using finite element models (FEMs) can simulate the sound pressure change at an arbitrary site for each frequency. FEMs are used in acoustic engineering to simulate the frequency-dependent sound pressure distribution at discrete cells in a simulated model and analyze the effects of specific parameters on the audiogram. However, few reports have compared the numerical results obtained using FEMs with data from clinical cases. We used FEMs to simulate audiograms of the air-bone gap (ABG) for various ossicular chain defects and compared these with preoperative audiograms obtained from 44 patients with a normal tympanic membrane who had otosclerosis, middle ear malformations or traumatic ossicular disruption. The simulated audiograms for otosclerosis and attic fixation of the malleus-incus complex both exhibited an up-slope but could be distinguished from each other based on the ABG at 1000 Hz. The simulated audiogram for incudostapedial joint discontinuity exhibited a peak at around 750 Hz and a down-slope above 1000 Hz. In general, the simulated audiograms for otosclerosis, attic fixation and incudostapedial joint discontinuity were consistent with those obtained from clinical cases. Additional simulations indicated that changes in ossicular mass had relatively small effects on ABG. Furthermore, analyses of combination pathologies suggested that the effects of one defect on ABG were added to those of the other defect. These FEM-based findings provide insights into the pathogenesis of conductive hearing loss due to otosclerosis, middle ear malformations and traumatic injury.

Necessities, opportunities, and challenges for tympanic membrane perforation scaffolding-based bioengineering

Tympanic membrane (TM) perforation is a global clinical dilemma. It occurs as a consequence of object penetration, blast trauma, barotrauma, and middle ear diseases. TM perforation may lead to otitis media, retraction pockets, cholesteatoma, and conductive deafness. Molecular therapies may not be suitable to treat perforation because there is no underlying tissue matrix to support epithelium bridging. Chronic perforations are usually reconstructed with autologous grafts via surgical myringoplasty. Surgical treatment is uncomfortable for the patients. The grafting materials are not perfect because they produce an opaque membrane, fail in up to 20% of cases, and are suboptimal to restore acoustic function. Millions of patients from developing parts of the world have not got access to surgical grafting due to operational complexities, lack of surgical resources, and high cost. These shortcomings emphasize bioengineering to improve placement options, healing rate, hearing outcomes, and minimize surgical procedures. This review highlights cellular, structural, pathophysiological, and perforation specific determinants that affect healing, acoustic and surgical outcomes; and integrates necessities relevant to bioengineered scaffolds. This study further summarizes scaffolding components, progress in scaffolding strategies and design, and engenders limitations and challenges for optimal bioengineering of chronic perforation.

Analysis of the Mechanical Properties of the Human Tympanic Membrane and Its Influence on the Dynamic Behaviour of the Human Hearing System

The difficulty to estimate the mechanical properties of the tympanic membrane (TM) is a limitation to understand the sound transmission mechanism. In this paper, based on finite element calculations, the sensitivity of the human hearing system to these properties is evaluated. The parameters that define the bending stiffness properties of the membrane have been studied, specifically two key parameters: Young's modulus of the tympanic membrane and the thickness of the eardrum. Additionally, it has been completed with the evaluation of the presence of an initial prestrain inside the TM. Modal analysis is used to study the qualitative characteristics of the TM comparing with vibration patterns obtained by holography. Higher-order modes are shown as a tool to identify these properties. The results show that different combinations of elastic properties and prestrain provide similar responses. The presence of prestrain at the membrane adds more uncertainty, and it is pointed out as a source for the lack of agreement of some previous TM elastic modulus estimations.

Tympanic membrane pressure buffering function at quasi-static and low-frequency pressure variations

Deformation of the tympanic membrane is known to contribute to the pressure regulation processes in the middle ear cleft. In this paper we investigated pressure variations in the rabbit middle ear in response to sinusoidal varying pressures applied to the ear canal, with frequencies ranging from 0.5 Hz to 50 Hz and pressure amplitudes ranging between 0.25 kPa and 1 kPa. The transtympanic pressure difference was found to be smallest in the quasi-static range, and quickly increased as a function of frequency. The response curves showed asymmetry, with larger transtympanic pressures when positive pressures were applied in the ear canal. Normalized transtympanic pressure amplitudes remained fairly constant as a function of input pressure, with values in the range of 60%-70% relative to the applied pressure. The total harmonic distortion of the middle ear pressure signal was calculated and was found to be very small (≤2%) for low-pressure amplitudes and low frequencies. For pressure amplitudes in the order of 0.25 kPa-0.5 kPa, it increased to about 10% at 50 Hz. When a 1 kPa pressure amplitude was applied, variation between animals became large and distortion values up to 30% at 50 Hz were observed. The results showed that pressure buffering due to tympanic membrane displacement was most effective for compensating small transtympanic pressure loads at low frequencies.

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.

Estimation of the quasi-static Young's modulus of the eardrum using a pressurization technique

The quasi-static Young's modulus of the eardrum's pars tensa is an important modeling parameter in computer simulations. Recent developments in indentation testing and inverse modeling allow estimation of this parameter with the eardrum in situ. These approaches are challenging because of the curved shape of the pars tensa which requires special care during experimentation to keep the indenter perpendicular to the local surface at the point of contact. Moreover, they involve complicated contact modeling. An alternative computer-based method is presented here in which pressurization is used instead of indentation. The Young's modulus of a thin-shell model of the eardrum with subject-specific geometry is numerically optimized such that simulated pressurized shapes match measured counterparts. The technique was evaluated on six healthy rat eardrums, resulting in a Young's modulus estimate of 22.8±1.5MPa. This is comparable to values estimated using indentation testing. The new pressurization-based approach is simpler to use than the indentation-based method for the two reasons noted above.

Panel 2

Objective

This report reviews the literature to identify the advances in our understanding of the middle ear (ME)–Eustachian tube (ET) system during the past 4 years and, on that basis, to determine whether the short-term goals elaborated in the last report were achieved and propose updated goals to guide future otitis media (OM) research.

Data Sources

Databases searched included PubMed, Web of Science (1945-present), Medline (1950 to present), Biosis Previews (1969-present), and the Zoological Record (1978 to present). The initial literature search covered the time interval from January 2007 to June 2011, with a supplementary search completed in February 2012.

Review Methods

The panel topic was subdivided; each contributor performed a literature search and provided a preliminary report. Those reports were consolidated and discussed when the panel met on June 9, 2011. At that meeting, the progress was evaluated and new short-term goals proposed.

Conclusions

Progress was made on 16 of the 19 short-term goals proposed in 2007. Significant advances were made in the characterization of ME gas exchange pathways, modeling ET function, and preliminary testing of treatments for ET dysfunction.

Implications for Practice

In the future, imaging technologies should be developed to noninvasively assess ME/ET structure and physiology with respect to their role in OM pathogenesis. The new data derived from form/function experiments should be integrated into the finite element models and used to develop specific hypotheses concerning OM pathogenesis and persistence. Finally, rigorous studies of treatments, medical or surgical, of ET dysfunction should be undertaken.