Microprocessor-controlled prosthetic knees, which rely on magnetorheological (MR) technology, have the potential to increase the comfort and quality of life of amputees. The focus of this study is on a prosthetic knee which is currently on the market and manufactured by the company Ossur Inc. The knee uses magnetic fields to vary the viscosity of the MR fluid, and thereby its flexion resistance. The torque transmissibility of the knee greatly depends on the magnetic field intensity in the MR fluid. The objective of this study is to investigate the strength of the magnetic field and the braking torque in the knee, for a few selected design parameters, and to determine which changes can be made to the existing design in order to maximize the torque output. The magnetic field in the fluid is evaluated by finite element analysis and the torque is calculated by using a Bingham visco-plastic model. A parametric study is carried out for several design parameters where the effect of variation in each parameter on the braking torque is observed. The results of this study give a valuable insight into which parameters should be prioritized for future changes of the knee, with regard to strength and comfortability.

Different tasks and conditions in gait call for different stiffness of prosthetic foot devices. The following work presents a case study on design modifications of a prosthetic foot, aimed at variable stiffness of the device. The objective is a proof-of-concept, achieved by simulating the modifications using finite element modeling. Design changes include the addition of a controlled damping element, connected both in parallel and series to a system of springs. The aim is to change the stiffness of the device under dynamic loading, by applying a high damping constant, approaching force coupling for the given boundary conditions. The dynamic modelling simulates mechanical test methods used to measure load response in full roll-over of prosthetic feet. Activation of the element during loading of the foot justifies the damped effect. As damping is in contrast to the main design objectives of energy return in prosthetic feet, it is considered important to quantify the dissipated energy in such an element. Our design case shows that the introduction of a damping element, with a high damping constant, can increase the overall rotational stiffness of the device by 50%. Given a large enough damping coefficient, the energy dissipation in the active element is about 20% of maximum strain energy.