3D finite element modeling of blast wave transmission in human ear from external ear to cochlear hair cells

Abstract

Auditory system dysfunction caused by exposure to blast waves is one of the leading causes of disability among military servicemembers and veterans. While the external and middle ear response to blast overpressures (BOPs) have been characterized experimentally, the inner ear behavior is much more difficult to measure, especially the micro-level cochlear hair cells in the organ of Corti (OC) responsible for converting pressure waves into electrical signals. Recently, computational finite element (FE) models have advanced to predict blast wave transmission from the external ear into the cochlea. However, in published FE models the anatomy of the inner ear is still insufficient. The objective of this study was to develop a 3D FE model of the human ear that included a 3-chambered cochlea to improve inner ear anatomy, validate the model’s results, and simulate the behavior of the OC during blast wave transmission. The human ear FE model consists of the ear canal, middle ear, and spiral cochlea with 3 chambers (scala vestibuli, scala media, and scala tympani) separated by Reissner’s membrane (RM) and the basilar membrane (BM). The model was run as a coupled fluid-structural analysis in ANSYS. An experimentally recorded blast waveform was applied as input to the entrance of the ear canal, and the model outputs included the ear canal (P1) and cochlear pressures, and the displacements of the tympanic membrane (TM), stapes footplate (SFP), and BM. The results of the model were compared to experimental measurements from blast tests in order to validate the FE model’s results. In addition, a microscale structural model of the OC was developed that used the FE model-derived BM displacement at 16.75 mm from the BM base end as input. This model reported some preliminary results describing the motion of the outer hair cells (OHCs) and hair bundles (HBs). The FE model of the human ear successfully predicted the middle and inner ear tissue displacements and fluid pressures. The P1 pressure, cochlear pressures, TM displacement, and SFP displacements were validated against blast test results. The incorporation of the 3-chambered cochlea improved the model’s accuracy compared to previous cochlea models used for blast transmission and demonstrated the influence of the RM and scala media chamber on cochlear biomechanics. These results were used to predict the likelihood of auditory injury. In addition, the preliminary results of the OC model showed radial variation in the OHC and HB behavior and indicated some potential mechanisms of sensory hair cell injury. The FE model reported in this thesis successfully improved the simulation of human cochlear anatomy and was validated against experimental blast measurements. A microscale model of the OC was connected with the full human ear model, giving some preliminary insight into blast-induced OC behavior. Future work with this model will improve the connection between the cochlea model and OC model and apply the FE model of the human ear to hearing loss prediction and the evaluation of earplug effectiveness.