School of Mechanical Engineering Seminar
Wednesday 20.10.2023 at 14:00

ZOOM SEMINAR

Numerical biomechanics modeling of a passive device for treatment of heart failure with preserved ejection fraction

Ilan Benoliel

M.Sc. student of Dr Gil Marom

School of Mechanical Engineering, Tel Aviv University, Tel Aviv, Israel

Heart failure, defined as a global pandemic, describes a diseased state of the heart where its pumping ability has been reduced to the extent that it cannot meet the body's demands. The prevalence of heart failure with preserved ejection fraction (HFpEF) appears to be increasing over time with the aging of the population. HFpEF is characterized by the inability of the heart to properly relax and expand during diastole to be filled with enough blood from the left atrium. To date, the diagnosis of HFpEF remains challenging and no treatment has been shown to convincingly reduce mortality and morbidity in patients. Therefore, an effort to develop and evaluate device-based therapies is an important emerging area. This study aims to simulate and optimize the function of passive assist devices for the treatment of HFpEF in a beating pathologic heart. Thus, the current study proposes to focus on and analyze a left ventricle (LV) expander device type that is designed to improve the heart’s relaxation performance. The device, implanted inside the LV, acts like a spring that stores the elastic potential energy from its systolic deformation and helps expand the ventricle during diastole by pushing the relaxed muscle. Therefore, the device should have a direct mechanical effect on the diastolic function and the filling performance. The aims of the present study include two steps. First, to model the hypertrophy of a HFpEF heart by modifying a model of a healthy heart. Second, to optimize the shape, size, and material of the passive assist device. To achieve this aim, device implantation will be modeled inside the diseased heart model from the first aim. The heart model in this study is based on the Living Heart Human Model (Simulia, Dassault Systèmes). The devices considered in this study have a similar design to the Corolla device, but several geometries and materials have been considered and compared. The effect of the various devices on the heart’s function was quantified based on the calculated pressure-volume loops, focusing on the increased ventricular volume during relaxation while not compromising the ejection fraction. Finally, fatigue analyses were performed on the optimal design to assess the long-term impact and durability of the device. The results revealed that cobalt-chromium material seems to be more appropriate than nickel-titanium for this type of application. It was also found that the LV expander consistently demonstrated LV pressure reduction and an increase in the LV volume, while larger devices were able to improve cardiac performance more than the smaller ones. Furthermore, it was demonstrated that the modeled device is incapable of enduring 4∙10⁸ cycles, equivalent to 10 years, but capable to endure 10⁸ cycles. Nonetheless, it is worth considering that severe HFpEF patients typically have a life expectancy of less than 10 years, and therefore further analysis is needed to check the device in the context of clinical needs.

https://tau-ac-il.zoom.us/j/86497933118

SCHOOL OF MECHANICAL ENGINEERING SEMINAR
Wednesday December 20.12.2023 at 14:00

ZOOM SEMINAR

Stress Fractures in the Tibia Bone: A New Approach

Elisheva Berkowicz

M.Sc. student under the supervision of Prof. Dov Sherman

School of Mechanical Engineering, Tel Aviv University, Israel

Abstract - A stress fracture is an injury occurring mostly in the cortical tibia bone, in the third lower part of it. It is characterized by growing damage to the bone and hence increasing pain to the patient.  Stress fractures occur in newly recruited soldiers- men and women, sportsmen and women, and the elderly, primarily due to overuse.

It is commonly accepted that stress fractures are generated by muscles’ off-axis tensile force acting parallel to the bone’s longitudinal axis to reduce the compressive loads acting on the bone during physical activity, but at the same time, it causes bending deformation, hence creating high tensile stresses on the auxiliary part of the bone. It is argued, that these tensile stresses are the direct cause of stress fracture. Microstructural treatment of the damage mechanisms is not considered.

Practically, stress fracture is not detectable by X-Ray diffraction at the early stages of the injury, and appears as a thin line thereafter (no crack opening). It is assumed subjected to load control conditions (muscles’ activity) that is not in line with the first findings above and with the principles of fracture mechanics. Hence, we integrate fracture mechanics principles by conceptualizing the bone material as a composite structure resembling fiber-reinforced plastic. In this analogy, the osteons represent the fibers, and the interstitial lamella is the cohesive matrix binding the osteons together.

 These have led us to suggest different mechanisms for stress fractures. First and foremost, since the stress fracture (the crack) is stably propagated, it is caused by displacement control conditions. Additionally, stress fractures emerge due to repetitive compressive loads that lead to the deterioration of the bone's matrix material, thereby substantially diminishing the constraint force acting on the osteons. This degradation facilitates the failure of osteons through localized collapse under buckling. Crucially, we propose that the accumulation of bone edema, or damage in the matrix, is a prerequisite for stress fractures. This model was studied using Finite Element Analysis of fibrous material under compression and local buckling.  

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https://tau-ac-il.zoom.us/j/86497933118