3D Finite Element Model of Human Ear with 3-Chamber Spiral Cochlea for Blast Wave Transmission from the Ear Canal to Cochlea

Blast-induced auditory trauma is a common injury in military service members and veterans that leads to hearing loss. While the inner ear response to blast exposure is difficult to characterize experimentally, computational models have advanced to predict blast wave transmission from the ear canal to the cochlea; however, published models have either straight or spiral cochlea with fluid-filled two chambers. In this paper, we report the recently developed 3D finite element (FE) model of the human ear mimicking the anatomical structure of the 3-chambered cochlea. The model consists of the ear canal, middle ear, and two and a half turns of the cochlea with three chambers separated by the Reissner's membrane (RM) and the basilar membrane (BM). The blast overpressure measured from human temporal bone experiments was applied at the ear canal entrance and the Fluent/Mechanical coupled fluid-structure interaction analysis was conducted in ANSYS software. The FE model-derived results include the pressure in the canal near the tympanic membrane (TM) and the intracochlear pressure at scala vestibuli, the TM displacement, and the stapes footplate (SFP) displacement, which were compared with experimentally measured data in human temporal bones. The validated model was used to predict the biomechanical response of the ear to blast overpressure: distributions of the maximum strain and stress within the TM, the BM displacement variation from the base to apex, and the energy flux or total energy entering the cochlea. The comparison of intracochlear pressure and BM displacement with those from the FE model of 2-chambered cochlea indicated that the 3-chamber cochlea model with the RM and scala media chamber improved our understanding of cochlea mechanics. This most comprehensive FE model of the human ear has shown its capability to predict the middle ear and cochlea responses to blast overpressure which will advance our understanding of auditory blast injury.

Real-time measurement of stapes motion and intracochlear pressure during blast exposure

Blast-induced auditory injury is primarily caused by exposure to an overwhelming amount of energy transmitted into the external auditory canal, the middle ear, and then the cochlea. Quantification of this energy requires real-time measurement of stapes footplate (SFP) motion and intracochlear pressure in the scala vestibuli (Psv). To date, SFP and Psv have not been measured simultaneously during blast exposure, but a dual-laser experimental approach for detecting the movement of the SFP was reported by Jiang et al. (2021). In this study, we have incorporated the measurement of Psv with SFP motion and developed a novel approach to quantitatively measure the energy flux entering the cochlea during blast exposure. Five fresh human cadaveric temporal bones (TBs) were used in this study. A mastoidectomy and facial recess approach were performed to identify the SFP, followed by a cochleostomy into the scala vestibuli (SV). The TB was mounted to the "head block", a fixture to simulate a real human skull, with two pressure sensors - one inserted into the SV (Psv) and another in the ear canal near the tympanic membrane (P1). The TB was exposed to the blast overpressure (P0) around 4 psi or 28 kPa. Two laser Doppler vibrometers (LDVs) were used to measure the movements of the SFP and TB (as a reference). The LDVs, P1, and Psv signals were triggered by P0 and recorded simultaneously. The results include peak values for Psv of 100.8 ± 51.6 kPa (mean ± SD) and for SFP displacement of 72.6 ± 56.4 μm, which are consistent with published experimental results and finite element modeling data. Most of the P0 input energy flux into the cochlea occurred within 2 ms and resulted in 10-70 μJ total energy entering the cochlea. Although the middle ear pressure gain was close to that measured under acoustic stimulus conditions, the nonlinear behavior of the middle ear was observed from the elevated cochlear input impedance. For the first time, SFP movement and intracochlear pressure Psv have been successfully measured simultaneously during blast exposure. This study provides a new methodology and experimental data for determining the energy flux entering the cochlea during a blast, which serves as an injury index for quantifying blast-induced auditory damage.

Dual-laser measurement of human stapes footplate motion under blast exposure

Hearing damage is one of the most frequently observed injuries in Service members and Veterans even though hearing protection devices (HPDs, e.g. earplugs) have been implemented to prevent blast-induced hearing loss. However, the formation and prevention mechanism of the blast-induced hearing damage remains unclear due to the difficulty for conducting biomechanical measurements in ears during blast exposure. Recently, an approach reported by Jiang et al. (2019) used two laser Doppler vibrometers (LDVs) to measure the motion of the tympanic membrane (TM) in human temporal bones during blast exposure. Using the dual laser setup, we further developed the technology to detect the movement of the stapes footplate (SFP) in ears with and without HPDs while under blast exposure. Eight fresh human cadaveric temporal bones (TBs) were involved in this study. The TB was mounted in a "head block" after performing a facial recess surgery to access the SFP, and a pressure sensor was inserted near the TM in the ear canal to measure the pressure reaching the TM (P1). The TB was exposed to a blast overpressure measuring around 7 psi or 48 kPa at the entrance of the ear canal (P0). Two LDVs were used to measure the vibrations of the SFP and TB (as a reference). The exact motion of the SFP was determined by subtracting the TB motion from the SFP data. Results included a measured peak-to-peak SFP displacement of 68.7 ± 31.6 μm (mean ± SD) from all eight TBs without HPDs. In five of the TBs, the insertion of a foam earplug reduced the SFP displacement from 48.3 ± 6.3 μm to 21.8 ± 10.4 μm. The time-frequency analysis of the SFP velocity signals indicated that most of the energy spectrum was concentrated at frequencies below 4 kHz within the first 2 ms after blast and the energy was reduced after the insertion of HPDs. This study describes a new methodology to quantitatively characterize the response of the middle ear and the energy entering the cochlea during blast exposure. The experimental data are critical for determining the injury of the peripheral auditory system and elucidating the damage formation and prevention mechanism in an ear exposed to blast.

3D Finite Element Modeling of Blast Wave Transmission from the External Ear to Cochlea

As an organ that is sensitive to pressure changes, the ear is often damaged when a person is subjected to blast exposures resulting in hearing loss due to tissue damage in the middle ear and cochlea. While observation of middle ear damage is non-invasive, examining the damage to the cochlea is difficult to quantify. Previous works have modeled the cochlear response often when subjected to an acoustic pressure input, but the inner ear mechanics have rarely been studied when the ear is exposed to a blast wave. In this study we aim to develop a finite element (FE) model of the entire ear, particularly the cochlea, for predicting the blast wave transmission from the ear canal to cochlea. We utilized a FE model of the ear, which includes the ear canal, middle ear, and uncoiled two-chambered cochlea, to simulate the cochlear response to blast overpressure (BOP) at the entrance of the ear canal with ANSYS Mechanical and Fluent in a fluid-structure interface coupled analysis in the time domain. This model was developed based on previous middle and inner ear models, and the cochlea was remeshed to improve BOP simulation performance. The FE model was validated using experimentally measured blast pressure transduction from the ear canal to the middle ear and cochlea in human cadaveric temporal bones. Results from the FE model showed significant displacements of the tympanic membrane, middle ear ossicles, and basilar membrane (BM). The stapes footplate displacement was observed to be as high as 60 µm, far exceeding the displacement during normal acoustic stimulation, when the 30 kPa (4.35 psi, 183 dB (SPL), Sound Pressure Level) of BOP was applied at the ear canal entrance. The large stapes movement caused pressures in the cochlea to exceed the physiological pressure level [< 10 Pa, 120 dB (SPL)] at a peak of 49.9 kPa, and the BM displacement was on the order of microns with a maximum displacement of 26.4 µm. The FE model of the entire human ear developed in this study provides a computational tool for prediction of blast wave transmission from the ear canal to cochlea and the future applications for assisting the prevention, diagnosis, and treatment of blast-induced hearing loss.