Prosthetic ankle and foot combination

#This invention relates to a prosthetic ankle and foot combination arranged to allow damped ankle flexion. The invention also includes a lower limb prosthesis incorporating such an ankle and foot combination.

#Current prosthetic ankle-foot systems are generally aligned for operation as fixed mechanical structures comprising elastic and deformable elements designed to provide stability during standing and walking and to return energy for propulsion into the swing phase of the walking cycle. However, such a device is often uncomfortable for the user whilst standing and walking on ramps and stairs and walking at different speeds. Users have also experienced knee instability and difficulty in maintaining forward motion during roll-over of the foot while standing and walking on ramps and stairs, with consequent impairment of efficiency. These difficulties are particularly important for transfemoral amputees whose stance phase action is normally compromised by significantly reduced knee flexion and extension which would otherwise assist shock absorption and forwards propulsion during the stance phase.

#Another aspect of ankle-foot function and transfemoral amputee locomotion relates to the way in which a typical known prosthesis hinders the amputee, resulting in poor body posture for certain locomotion activities such as ascending and descending stairs and ramps, which diminishes the potential for application of voluntary control, particularly user-generated hip extension torque. The poor posture described is largely caused by inappropriate stiffness and range of motion at the ankle which does not allow the body centre of mass to pass easily over the ankle. Consequently, amputees sometimes have to adopt unnatural compensating actions.

#FIG. 1 is a sectional view of a lower limb prosthesis incorporating a first ankle and foot combination in accordance with the invention;

#FIG. 2 is a block diagram of a control system forming part of the prosthesis of FIG. 1, shown connected to a pair of servo valves for controlling damping resistance;

#FIG. 3 is a flow diagram for dynamic damping control;

#Referring to FIG. 1, a lower limb prosthesis in accordance with the invention has a foot component 10 with a foot keel 12 comprising a rigid carrier 12A. Independently coupled to the rigid carrier 12A are a toe spring 12B and a heel spring 12C. The keel 12 is largely formed from carbon fibre composite material and can be surrounded by a foam cosmetic covering (not shown).

#Coupled to the foot keel 12 is a shin component 14 having, at its distal end, an ankle joint mechanism 16 which is housed largely within the shin component 14 and connects the shin component 14 to the foot keel 12. The shin component 14 defines a shin axis 18. The mounting of the shin component 14 to the foot keel 12 is by way of an ankle flexion pivot 20 defining a flexion axis 20A running in a medial-lateral direction to the anterior of the shin axis 18. The ankle joint mechanism is in the form of a piston and cylinder assembly, the cylinder 22 of which forms an extension of a shin tube centred on the shin axis 18. The cylinder 22 slidably houses two coaxial piston elements 24, 26, the axes of which, in this case, coincide with the shin axis 18. These two piston elements comprise a first piston element 24 for providing a dynamic damping action during locomotion, and a second piston element 26 for independent adjustment of heel-height. As will be understood from the description which follows, these two piston elements are able to move translationally in the cylinder 22 independently of each other so that a heel-height setting can be established without affecting the function of dynamic damping action provided by the first piston element.

#To describe the piston and cylinder assembly in more detail, the dynamic piston element 24 has two pistons 24P, 24D interconnected by an axial piston rod 24R. Located in the space between the two pistons 24P, 24D, the second piston element 26, for heel-height adjustment, is also reciprocable within the cylinder in the space between the pistons 24P, 24D of the first piston element and, itself, has two spaced, apart pistons, 26P, 26D which are interconnected by a respective piston rod 26R. The heel-height adjustment piston 26 has an axial bore running its entire axial length to house the piston rod 24R of the dynamic damping piston element 24. Between the pistons 26P, 26D of the heel-height adjustment piston element 26 is a transversely extending dividing wall 28 which is fixed to the inside of, or is integral with, the cylinder 22, thereby dividing the space between the two pistons of the second piston element 26 into two annular variable-volume chambers 30A, 30B.