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

Abstract

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.