Experimental and Numerical Analysis of Tympanic Membrane Biomechanics for Minimally-Invasive Surgical Innovations

Abstract

Otitis media is a group of inflammatory ear diseases of the middle ear that can cause earaches and hearing loss, and even lead to incurable damage of the middle ear structure in extreme cases. The current treatment for different types of otitis media (i.e., acute otitis media and otitis media with effusion) is tympanostomy surgery, which has a risk of general anesthesia in children, the economic burden of surgical costs, and the probability of long-term defects in the tympanic membrane (TM) structure. This thesis provides the scientific foundation for minimally-invasive therapeutic methods and medical devices that rely on tearing and fracture of the TM. In this thesis, we investigated the biomechanics of the TM and its mechanical interactions with needles during the needle-insertion process to establish a fundamental understanding of the fracture-related mechanical properties of the TM. To this end, we exploited numerical modelling, combined with experimental analysis to address these objectives.

To begin with, we utilized finite-element (FE) analysis to model needle insertion into skin, as the most similar biological tissue to the TM with existing relevant experimental data, and to investigate how the various mechanical and geometrical parameters impact the relationship between force and displacement in needle-skin interactions, the required force to puncture the skin, and the mechanical stress generated during this process. Our FE analysis was established based on a 2D bilinear cohesive zone model for which the cohesive parameters including initial stiffness, failure traction, and separation length for the skin tissue were obtained using a three-stage algorithm. Our results showed that the coefficient of friction, between the needle and skin, substantially changed the needle reaction force. Also, the needle's cutting angle primarily influenced stress in the skin tissue, whereas the needle diameter played a more critical role in the needle's reaction force. We also presented an energy analysis on frictional dissipation, damage dissipation, and recoverable strain energy stored in the skin tissue during the insertion process.

In order to develop and establish an experimental methodology to be later applied to the TM, we also conducted an experimental analysis of needle insertion into phantom membranes made of polyethylene and nitrile rubber using an innovative custom-designed experimental apparatus and studied the effect of different insertion and membrane parameters on the most important features of the needle reaction force plots. Our findings revealed that, in most cases, the increase in the needle diameter, membrane thickness, and needle inclination led to increased crack length, maximum insertion force, puncture displacement, kinetic frictional force, and static frictional force. However, the effect of velocity on these factors was generally negligible, except for an increase in the kinetic frictional force with a higher insertion velocity. Our results also highlighted that lubrication had a notable impact on reducing puncture displacement and frictional forces but did not significantly alter the maximum needle insertion force into the membranes. Moreover, we proposed a combined experimental-numerical approach using FE modeling to calculate fracture toughness, resulting in values of 6.66±1.16 kJ/m^2 for polyethylene and 3.98±0.56 kJ/m^2 for nitrile rubber. Additionally, the developed FE model was employed to predict membrane deformation and mechanical stress.

In the next part of this thesis, we investigated the cutting behaviour and fracture-related mechanical properties of the TM using force measurement during a 2-cycle needle insertion/extraction process on the gerbil TM. Energy components, including the fracture energy, friction energy, strain energy, and hysteresis loss, were considered throughout the analysis to understand various stages of the procedure. Our results showed the viscoelastic behaviour of the TM, with hysteresis loss having a minimal effect on the energy dissipation compared to the fracture energy and friction energy. Interestingly, the puncture force of the TM remained stable shortly after the animal's death while increased after a week due to the drying effects of soft tissues. Also, the needle geometry significantly influenced needle insertion features such as the crack length, insertion forces, and puncture displacement. On the other hand, the insertion velocity only affected insertion forces, while did not have a remarkable impact on the puncture displacement. Furthermore, we computed the fracture toughness of the gerbil TM, which was obtained to be 0.33±0.10 kJ/m^2.

We also carried out numerical analysis using a combination of a 3D FE model of the TM and other middle-ear structures and a 2D cohesive zone FE model of needle insertion into the TM to investigate the TM and middle-ear response during the needle insertion process. Using 3D modelling, we proposed a nonlinear Ogden model for the TM and acquired the whole-domain deformational behaviour of the TM and other middle-ear structures. Moreover, using the 2D cohesive zone modelling, we obtained cohesive parameters of the TM, and studied its mechanical behaviour during needle insertion.

This thesis initiated obtaining an understanding of the TM behaviour during needle insertion and cutting, as well as its fracture-related properties. The proposed methodologies and presented results in this thesis serve as a robust starting point for a more fundamental knowledge of TM mechanical damage. Ultimately, this knowledge can contribute to the development of innovative minimally-invasive surgical devices and treatment strategies for hearing and balance diseases.