Thickness of the gerbil tympanic membrane measured with confocal microscopy

Finite-element modelling of interactions of needle with tympanic membrane and middle ear

The tympanic membrane (TM) is one of the most common routes to access the middle ear and inner ear for the treatment of hearing and balance pathologies. Since the TM is a soft thin biological tissue with small dimensions, using needles seems to be among the most practical interventional approaches. In this study, we proposed a finite-element (FE) analysis of needle-TM interactions that combines a 3D model of the TM and other main middle-ear structures in gerbil, and a 2D model of needle insertion into the TM based on the cohesive zone method (CZM). The TM was modelled using a 1st-order Ogden hyperelastic material and its properties were obtained by fitting to the experimental force-displacement plots of large deformation in the TM under needle indentation. The cohesive parameters were also acquired by calibrating the puncture force against the experimental data of needle insertion into the TM. These FE models were then used to obtain the deformation behaviour of the TM and other middle-ear structures due to the insertion force applied at different locations on the TM. Moreover, we investigated the effect of the TM thickness, the geometry of the needle (i.e., diameter and tip angle), and needle material on the insertion of needles into the TM. We also studied the penetration success of deformable needles.

Experimental Study of Needle Insertion into Gerbil Tympanic Membrane

The perforation characteristics and fracture-related mechanical properties of the tympanic membrane (TM) greatly affect surgical procedures like myringotomy and tympanostomy performed on the middle ear. We analyzed the most important features of the gerbil TM perforation using an experimental approach that was based on force measurement during a 2-cycle needle insertion/extraction process. Fracture energy, friction energy, strain energy, and hysteresis loss were taken into consideration for the analysis of the different stages of needle insertion and extraction. The results demonstrated that (1) although the TM shows viscoelastic behavior, the contribution of hysteresis loss was negligible compared to other irreversible dissipated energy components (i.e., fracture energy and friction energy). (2) The TM puncture force did not substantially change during the first hours after animal death, but interestingly, it increased after 1 week due to the drying effects of soft tissue. (3) The needle geometry affected the crack length and the most important features of the force-displacement plot for the needle insertion process (puncture force, puncture displacement, and jump-in force) increased with increasing needle diameter, whereas the insertion velocity only changed the puncture and jump-in forces (both increased with increasing insertion velocity) and did not have a noticeable effect on the puncture displacement. (4) The fracture toughness of the gerbil TM was almost independent of the needle geometry and was found to be around 0.33 ± 0.10 kJ/m2.

Possible clinical implications of the structural variations between the tympanic membrane quadrants

Introduction

Retraction pockets and marginal perforations of the pars tensa of the tympanic membrane (TM) are most commonly found at superior posterior quadrant (SPQ). The patulous Eustachian tube tends to manifest in the same quadrant. Variation in the structure of the TM may explain these observations.

Material and Methods

A line defined by the manubrium was used to divide the pars tensa into anterior and posterior portions. A transverse line centered on the umbo divides the pars tensa into superior and inferior parts, resulting in four quadrants. Surface areas of each of the TM quadrants were measured in a sample of 23 human adult formalin-fixed temporal bones. The TMs were completely excised, faced medially, and placed against graph paper to maintain scale measurements, photoed, and measured.TM thickness was measured on a different set of 20 human temporal bones (TB) preparations with normal external and middle ears. Four random loci were chosen from each pars tensa's TM quadrant. The thickness was measured using high-magnification power microscopy.

Results

The SPQ was the largest and thinnest of the four quadrants. It occupies 31% of the pars tensa area. It is 69 μm as compared to approximately 85 μm in the other quadrants. The radial lines between the umbo and the annulus are in descending order from superior posterior toward the anterior-superior radials.

Conclusion

The SPQ has the largest vibratory area and is the thinnest of the four TM quadrants. Variation in the thickness of the middle, fibrous layer accounts for the variation in the thickness of the TM. These findings may explain the tendency of pathologies related to Eustachian tube dysfunction to preferentially manifest in or originate from the SPQ.

Level of evidence 5

Recovery from tympanic membrane perforation: Effects on membrane thickness, auditory thresholds, and middle ear transmission

Sounds delivered to the ear move the tympanic membrane (TM), which drives the middle-ear (ME) ossicles and transfers the acoustic energy to the cochlea. Perforations of the TM result in hearing loss because of less efficient sound conduction through the ME. The patterns of TM motions, and thus ME sound transmission, vary with frequency and depend on many factors, including the TM thickness. In this study, we measured TM thickness, auditory brainstem responses (ABR), and ME transmission immediately following a controlled pars tensa perforation and after 4 weeks of spontaneous recovery in a gerbil model. It is found that after recovery, the hearing thresholds showed a sloping pattern across frequencies: almost back to normal levels at frequencies between 2 and 8 kHz, sloping loss in the low (<2 kHz) and mid-frequency (8-30 kHz) range, and little restoration at frequencies above 30 kHz. This pattern was confirmed by the measured ME pressure gains. The thickness of the healed TM did not return to normal but was 2-3 times thicker over a significant portion of the membrane. The increased thickness was not limited to the perforated area but expanded into intact regions adjacent to the perforation, which led to an increased thickness in general. Combined, these results suggest that TM thickness is an important factor in determining its vibration patterns and efficiency to transfer sounds to the ossicles and thus influencing ME sound transmission, especially for high-frequency sounds. The results provided both structural and functional observations to explain the conductive hearing loss seen in patients with abnormal TMs, e.g., caused by otitis media, spontaneously healed post-perforation, or repaired via tympanoplasty in the clinic.

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%.

Measurement of thickness and profile of a transparent material using fluorescent stereo microscopy

Full-field thickness measurement for a thin transparent film is of interest for biological, medical, electronic, and packaging materials. It is a challenging task when the film is curvy, delicate and its thickness varies with location. We report herein a method to measure the thickness of a transparent (flat or curved) material and its topography using a stereo fluorescent profilometry technique. In this technique, two different types of fluorescent particles are deposited to both sides of the transparent film. Selected fluorescent excitation and emission are used to allow the observation of each one surface of the film at a time to determine the surface profile using stereo-based digital image correlation techniques. After the surface profiles for both surfaces are determined, subtraction of one surface profile from the other provides accurate thickness distribution of the film. Validation experiments were conducted using transparent films with known thickness. As an application, a measurement on a contact lens was conducted. The technique is appropriate for measurement of the full-field thickness of objects at other scales, such as soft transparent or translucent biofilms, with which thickness can hardly be measured accurately with other techniques.

A study of sound transmission in an abstract middle ear using physical and finite element models

The classical picture of middle ear (ME) transmission has the tympanic membrane (TM) as a piston and the ME cavity as a vacuum. In reality, the TM moves in a complex multiphasic pattern and substantial pressure is radiated into the ME cavity by the motion of the TM. This study explores ME transmission with a simple model, using a tube terminated with a plastic membrane. Membrane motion was measured with a laser interferometer and pressure on both sides of the membrane with micro-sensors that could be positioned close to the membrane without disturbance. A finite element model of the system explored the experimental results. Both experimental and theoretical results show resonances that are in some cases primarily acoustical or mechanical and sometimes produced by coupled acousto-mechanics. The largest membrane motions were a result of the membrane's mechanical resonances. At these resonant frequencies, sound transmission through the system was larger with the membrane in place than it was when the membrane was absent.

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.

Feedback characteristics between implantable microphone and transducer in middle ear cavity

With the advent of implantable hearing aids, implementation and acoustic sensing strategy of the implantable microphone becomes an important issue; among the many types of implantable microphone, placing the microphone in middle ear cavity (MEC) has advantages including simple operation and insensitive to skin touching or chewing motion. In this paper, an implantable microphone was implemented and researched feedback characteristic when both the implantable microphone and the transducer were placed in the MEC. Analytical and finite element analysis were conducted to design the microphone to have a natural frequency of 7 kHz and showed good characteristics of SNR and sensitivity. For the feedback test, simple analytical and finite element analysis were calculated and compared with in vitro experiments (n = 4). From the experiments, the open-loop gain and feedback factor were measured and the minimum gain margin measured as 14.3 dB.

Energy localization and frequency analysis in the locust ear

Animal ears are exquisitely adapted to capture sound energy and perform signal analysis. Studying the ear of the locust, we show how frequency signal analysis can be performed solely by using the structural features of the tympanum. Incident sound waves generate mechanical vibrational waves that travel across the tympanum. These waves shoal in a tsunami-like fashion, resulting in energy localization that focuses vibrations onto the mechanosensory neurons in a frequency-dependent manner. Using finite element analysis, we demonstrate that two mechanical properties of the locust tympanum, distributed thickness and tension, are necessary and sufficient to generate frequency-dependent energy localization.

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.

Realistic 3D computer model of the gerbil middle ear, featuring accurate morphology of bone and soft tissue structures

In order to improve realism in middle ear (ME) finite-element modeling (FEM), comprehensive and precise morphological data are needed. To date, micro-scale X-ray computed tomography (μCT) recordings have been used as geometric input data for FEM models of the ME ossicles. Previously, attempts were made to obtain these data on ME soft tissue structures as well. However, due to low X-ray absorption of soft tissue, quality of these images is limited. Another popular approach is using histological sections as data for 3D models, delivering high in-plane resolution for the sections, but the technique is destructive in nature and registration of the sections is difficult. We combine data from high-resolution μCT recordings with data from high-resolution orthogonal-plane fluorescence optical-sectioning microscopy (OPFOS), both obtained on the same gerbil specimen. State-of-the-art μCT delivers high-resolution data on the 3D shape of ossicles and other ME bony structures, while the OPFOS setup generates data of unprecedented quality both on bone and soft tissue ME structures. Each of these techniques is tomographic and non-destructive and delivers sets of automatically aligned virtual sections. The datasets coming from different techniques need to be registered with respect to each other. By combining both datasets, we obtain a complete high-resolution morphological model of all functional components in the gerbil ME. The resulting 3D model can be readily imported in FEM software and is made freely available to the research community. In this paper, we discuss the methods used, present the resulting merged model, and discuss the morphological properties of the soft tissue structures, such as muscles and ligaments.

Virtual biopsy of rat tympanic membrane using higher harmonic generation microscopy

Multiharmonic optical microscopy has been widely applied in biomedical research due to its unique capability to perform noninvasive studies of biomaterials. In this study, virtual biopsy based on back-propagating multiple optical harmonics, combining second and third harmonics, is applied in unfixed rat tympanic membrane. We show that third harmonic generation can provide morphologic information on the epithelial layers of rat tympanic membrane as well as radial collagen fibers in middle fibrous layers, and that second harmonic generation can provide information on both radial and circular collagen fibers in middle fibrous layers. Through third harmonic generation, the capillary and red blood cells in the middle fibrous layer are also noted. Additionally, the 3-D relationship to adjacent bony structures and spatial variations in thickness and curvature are obtained. Our study demonstrates the feasibility of using a noninvasive optical imaging system for comprehensive evaluation of the tympanic membrane.

Morphology of the human tympanic membrane annulus

OBJECTIVE

To study the full panoramic view with figuring of the morphology and topography of the human tympanic annulus.

STUDY DESIGN

Postmortem material analysis.

SETTING

University-affiliated hospital.

SUBJECTS AND METHODS

Twenty-three single, normal human adult tympanic membranes were completely extracted from formalin-fixed temporal bones. They were faced medially and placed at the same level of a graph paper mounted on a board. High-quality images of the tissue preparations were taken, and computer-aided measurements of the annular caliber were calculated at nine reference points. The 6 o'clock direction served as a midpoint, and another four reference points were set anteriorly and posteriorly in clockwise and counterclockwise directions.

RESULTS

The annulus has a horseshoe-like shape with a small part absent above the neck of the malleus. The maximal mean caliber at the manubrial axis (6 o'clock direction) was 748 +/- 201 mum. The annulus gradually thins out almost symmetrically anteriorly and posteriorly, until it reaches about 15 percent of the maximal caliber at its end points (152 +/- 87 and 113 +/- 42 mum, respectively). Significant differences were found between adjacent reference points on both anterior and posterior sides.

CONCLUSIONS

The annulus has a horseshoe-like shape and gradually thins out almost symmetrically, reaching anteriorly and posteriorly about 15 percent of the maximal caliber at the manubrial axis. These new data may provide guidance in transcanal middle ear exploration and suggest the possibility of varied functions attributable to the annulus regarding middle ear sound transmission and TM vibratory properties. The data may contribute to understanding the development of marginal perforations and posterior superior retraction pockets.

Measuring the quasi-static Young's modulus of the eardrum using an indentation technique

Accurate estimation of the quasi-static Young's modulus of the eardrum is important for finite-element modeling. In this study, we adapted a tissue indentation technique and inverse finite-element analysis to estimate the Young's modulus of the eardrum. A custom-built indentation apparatus was used to perform indentation testing on seven rat eardrums in situ after immobilizing the malleus. Testing was done in most cases on the posterior pars tensa. The unloaded shape of each eardrum was measured and used to construct finite-element models with subject-specific geometries to simulate the indentation experiment. The Young's modulus of each specimen was then estimated by numerically optimizing the Young's modulus of each model so that simulation results matched corresponding experimental data. Using an estimated value of 12 microm for the thickness of each model eardrum, the estimated average Young's modulus for the pars tensa was found to be 21.7+/-1.2 MPa. The estimated average Young's modulus is within the range reported in some of the literature. The estimation technique is sensitive to the thickness of the pars tensa used in the model but is not sensitive to relatively large variations in the stiffness of the pars flaccida and manubrium or to the pars tensa/pars flaccida separation conditions.

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.

Structural and Functional Properties of the Healed Tympanic Membrane

HYPOTHESIS

The short-term healing scar that forms after experimental laser myringotomy will revert to a normal lamina propria in the long run. The mechanical stiffness will stay normal.

BACKGROUND

Recent studies have shown severe structural changes in the fibrous layer in the early course after experimental laser myringotomy, whereas the scar quickly restored the strength of the tympanic membrane (TM). A reorganization of the fiber layer is expected to occur.

METHODS

Potassium titanyl phosphate laser myringotomy was made on one side of the TM in Sprague-Dawley rats. The ear of the other side was untouched and used for control. After half a year of observation, the stiffness and strength of the healed TMs were measured with moiré interferometry and examined with otomicroscopy and light and electron microscopy.

RESULTS

The interferometry readings showed a slightly reduced strength in the myringotomized and healed TMs. After half a year, still there were immense structural changes including increased thickness over a wide area of the pars tensa with increased amounts of fibers. An obvious reorganization of the fiber layer was lacking.

CONCLUSION

Laser myringotomy causes profound, long-standing, or permanent structural changes in the lamina propria of the pars tensa, whereas the strength of the TM may become slightly reduced.

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.