A Biological Gear in the Human Middle Ear

A three-dimensional (3D) finite element (FE) model was developed to simulate the motion modes of the human middle ear structures. The model was based entirely on 3D reconstruction obtained from micro Computed Tomography (microCT) imaging. We solve the acoustics-structure interaction problem using COMSOL Multiphysics. Our results show that at low frequencies, the classical hinging motion is dominant, as expected. However, at high frequencies, we observe multi-resonance vibration modes at the eardrum and a biological bevel-gear like response at the malleus-incus complex. This suggests that the middle ear motion modes and dynamics are determined by the special anatomical features (saddle shape of the MIJ, asymmetry of the eardrum, and cylindrical malleus shape in larger mammals) and the three-dimensional inertial properties of the ossicles, and the twisting mode of the malleus and incus are required in larger mammals, like humans, in order to reduce the inertia due to ossicle mass.