How does prestrain in the tympanic membrane affect middle-ear function? A finite-element model study in rabbit

Experiments have shown that prestrain exists in the rabbit tympanic membrane (TM), also in the absence of external loads. To date, it is unclear how prestrain influences the vibration response of the middle ear (ME). In this study, a detailed 3D finite-element model of the rabbit ME was constructed based on experimentally validated material properties. The model incorporates different degrees of prestrain in the TM and simulates the ME vibration response to sound as a linear harmonic perturbation around the prestressed reference state. To account for finite deformations associated with large prestrains, a framework was developed that iteratively updates the initial unstrained geometry until the prestrained geometry is in agreement with the given reference geometry. After validating the model using quasi-static and acoustic measurement data, it was shown that small levels of prestrain already have a substantial impact on the normal umbo and footplate response due to a phenomenon known as prestress stiffening. Although the approach is not preferable, it was possible to replicate the effect of prestrain in the normal ME by appropriately scaling the elastic moduli and damping factors in the base model. To evaluate the effect of possible changes in TM prestrain when the normal state of the ear is altered due to pathological modifications in the ME structure, we created a model with a perforation in the TM. It was shown that the change in vibration response after perforation is affected at low frequencies by a release of TM prestrain. In future studies, it may be necessary to incorporate prestrain in ME models to better understand the function of the diseased or reconstructed ME, which may be relevant for the development of reconstructive tissue grafts in the middle ear.