Mechanics of the Middle Ear

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Background: The middle ear consists of a chain of three bones (malleus, incus, and stapes) that are suspended by ligaments in an air-filled cavity in the head and are acted upon by two muscles, the stapedius and tensor tympani. The eardrum separates the middle ear from the ear canal and is connected to the malleus. Our understanding of the middle ear's biomechanics has been enhanced by computer models. However, current models cannot be used to simulate high intensity sounds and reflexive contraction of the stapedius muscle. Extension of computer models is important for the engineering and design of diagnostic, assistive and protective devices.***Hypothesis: The inclusion of appropriate material properties in computer models will allow the extension to high intensity sounds.***Objectives: (1) Measure vibrations of the middle-ear bones and eardrum with and without muscle contractions; (2) measure changes in middle-ear geometry caused by muscle contraction; (3) develop software for constructing computer models from images; (4) estimate appropriate material properties for use in models and and test the models by comparing simulations to measured data; and (5) demonstrate the application of the models.***Approach: We will measure vibrations of the eardrum and middle-ear bones in cadaveric ears using a state-of-the-art laser Doppler vibrometer. Muscle contractions will be applied using custom-built hardware. We will image changes in middle-ear geometry with and without muscle contractions using micro-imaging techniques. To automatically generate the geometry of computer models from medical images, we will use modern digital image processing methods. Parameters specifying the material properties of the modeled tissues will be estimated by optimizing the model so simulation results match a select portion of the measurements, and the refined models will be tested by comparing simulation results to data not used for parameter estimation. The refined models will then be using to simulate the acoustic reflex test in which an intense sound is applied to contract the stapedius muscle. By varying the parameters and geometry of the model systematically, we will develop an understanding of how middle ear geometry and pathology affect the acoustic reflex test.***Impact: We will provide rigorous training to biomedical engineering students that will make them highly employable. We will be the first to (1) measure vibrations in response to high intensity sounds with and without muscle contractions, (2) use micro-imaging to measure geometric changes caused by muscle contractions, (3) develop and validate computer models that are valid for intense sounds, and (4) simulate the acoustic reflex test. Additionally, we will develop software for automatic model construction. Our software, models and measured validation data will be made available to the hearing biomechanics research community.*********