Characterization of the nonlinear elastic behavior of chinchilla tympanic membrane using micro-fringe projection.

The mechanical properties of an intact, full tympanic membrane (TM) inside the bulla of a fresh chinchilla were measured under quasi-static pressure from -1.0 kPa to 1.0 kPa applied on the TM lateral side. Images of the fringes projected onto the TM were acquired by a digital camera connected to a surgical microscope and analyzed using a phase-shift method to reconstruct the surface topography. The relationship between the applied pressure and the resulting volume displacement was determined and analyzed using a finite element model implementing a hyperelastic 2(nd)-order Ogden model. Through an inverse method, the best-fit model parameters for the TM were determined to allow the simulation results to agree with the experimental data. The nonlinear stress-strain relationship for the TM of a chinchilla was determined up to an equibiaxial tensile strain of 31% experienced by the TM in the experiments. The average Young's modulus of the chinchilla TM from ten bullas was determined as approximately 19 MPa.

Mechanical damage of tympanic membrane in relation to impulse pressure waveform - A study in chinchillas.

Mechanical damage to middle ear components in blast exposure directly causes hearing loss, and the rupture of the tympanic membrane (TM) is the most frequent injury of the ear. However, it is unclear how the severity of injury graded by different patterns of TM rupture is related to the overpressure waveforms induced by blast waves. In the present study, the relationship between the TM rupture threshold and the impulse or overpressure waveform has been investigated in chinchillas. Two groups of animals were exposed to blast overpressure simulated in our lab under two conditions: open field and shielded with a stainless steel cup covering the animal head. Auditory brainstem response (ABR) and wideband tympanometry were measured before and after exposure to check the hearing threshold and middle ear function. Results show that waveforms recorded in the shielded case were different from those in the open field and the TM rupture threshold in the shielded case was lower than that in the open field (3.4 ± 0.7 vs. 9.1 ± 1.7 psi or 181 ± 1.6 vs. 190 ± 1.9 dB SPL). The impulse pressure energy spectra analysis of waveforms demonstrates that the shielded waveforms include greater energy at high frequencies than that of the open field waves. Finally, a 3D finite element (FE) model of the chinchilla ear was used to compute the distributions of stress in the TM and the TM displacement with impulse pressure waves. The FE model-derived change of stress in response to pressure loading in the shielded case was substantially faster than that in the open case. This finding provides the biomechanical mechanisms for blast induced TM damage in relation to overpressure waveforms. The TM rupture threshold difference between the open and shielded cases suggests that an acoustic role of helmets may exist, intensifying ear injury during blast exposure.

Dynamic Properties of Tympanic Membrane in a Chinchilla Otitis Media Model Measured With Acoustic Loading.

Otitis media is the most common infectious disease in young children, which results in changes in the thickness and mechanical properties of the tympanic membrane (TM) and induces hearing loss. However, there are no published data for the dynamic properties of the TM in otitis media ears, and it is unclear how the mechanical property changes are related to TM thickness variation. This paper reports a study of the measurement of the dynamic properties of the TM in a chinchilla acute otitis media (AOM) model using acoustic loading and laser Doppler vibrometry (LDV). AOM was created through transbullar injection of Haemophilus influenzae into the middle ear, and AOM samples were prepared 4 days after inoculation. Vibration of the TM specimen induced by acoustic loading was measured via LDV over a frequency range of 0.1-8 kHz. The experiment was then simulated in a finite element (FE) model, and the inverse-problem solving method was used to determine the complex modulus in the frequency domain. Results from 12 ears (six control and six AOM) show that the storage modulus of the TM from AOM ears was on average 53% higher than that of control ears, while the loss factor was 17.3% higher in control ears than in AOM ears at low-frequency (f < 1 kHz). At high-frequency (e.g., 8000 Hz), there was a mean 40% increase in storage modulus of the TM from AOM compared to control samples. At peak frequency (e.g., 3 kHz), there was a 19.5% increase in loss factor in control samples compared to AOM samples. These findings quantify the changes induced by AOM in the chinchilla TM, namely, a significant increase in both the storage and loss moduli.