Is There a “Biological Gear” in the Human Middle Ear?

Objective A mammal’s ability to hear high‐frequency sound is due to unique structures such as 3 distinct middle‐ear (ME) ossicles. In larger mammals such as humans, the ME features a cylindrical malleus cross section, differing eardrum areas on each side of the malleus handle, and a mobile saddle‐shaped malleus‐incus joint (MIJ). Method Based entirely on 3D reconstructions of micro‐CT images, a finite element biocomputation model was constructed, which includes the ear canal, eardrum, ossicles, suspensory soft tissue attachments, and ME joints modeled as mobile and fluid‐filled. Frequency responses of acoustics‐structure interactions were calculated using COMSOL Multiphysics solvers. Results Results indicate that at low frequencies, hinge‐like motion is dominant as expected, and that the orthotropy of the eardrum boosts the ME gain due to increased peak displacement. However, at high frequencies we observe multi‐resonance vibration modes at the eardrum and a bevel‐gear‐like motion at the MIJ, and the orthotropy of the eardrum makes the rotation axis of the malleus more coincident with its long axis. This supports the hypothesis that a “twisting” motion of the malleus and incus is required in larger mammals at high frequencies in order to compensate for larger moments‐of‐inertia of the larger ossicular masses. Conclusion We argue that a new “twisting” gear‐like motion is necessary for efficient high‐frequency sound transmission in larger mammals, due to higher moments of inertia for hinge‐like motion in these species. These features favor the existence of the twisting motion of the malleus‐incus complex in addition to the classical hinge‐like motion.