Lower-limb prosthesis : design of the inversion-eversion degree of freedom for active ankle foot prosthesis

In Belgium, approximately a thousand people suffer from major lower-limb amputation every year. These people are affected by the loss of mobility, self-esteem and quality of life caused by amputation. To enable them to regain mobility and quality of life they are fitted with lower-limb prostheses. The majority of prostheses focus on the main degree-of-freedom of walking. The inversion-eversion degree-of-freedom is the movement that enables the ankle to tilt in the frontal plane. This movement allows the foot to remain flat on sloping or unstable ground and therefore ensures stability. It is also involved in everyday movements such as sidestep cutting, which are steps taken to avoid an obstacle without rotating the body. This degree of freedom is still only very rarely included in current ankle-foot prostheses. In the present work, the design of a module that can be fixed to ankle-foot prostheses and add the inversion-eversion degree-of-freedom is performed. The design methodology led to a weight activated passive solution. The aim is to allow the ankle to adjust to the slope of the ground when the weight of the amputee does not apply on it, therefore changing the neutral position of the mechanism. Once the weight acts on the prosthesis, a mechanical impedance ensures stability through a nylon rope via a locking system. The deformation of the nylon rope allows the inversion-eversion of the ankle. Finite elements methods with a CAD software were used to quantify the deformation and the structural strength of the different parts of the mechanism.