Lower limb prosthesis

Details Lower limb prosthesis Lower limb prosthesis

2000-05-03

2003-02-11

Snow, Bruce (Department: 3738)

Prosthesis (i.e., artificial body members), parts thereof, or ai

Having electrical actuator

C623S043000, C623S045000

Reexamination Certificate

active

06517585

ABSTRACT:

This invention relates to a lower limb prosthesis incorporating an adaptive prosthesis control system in the form of a processor-controlled knee flexion control unit which is arranged to resist the flexion at the knee joint in a variable way according to signals picked up from a sensing means on the prosthesis and processed in accordance with a stored program.
British Patent Application No. GB 2216426A discloses such a lower limb prosthesis. A pneumatic cylinder assembly is used as the knee flexion control unit to provide resistance to knee flexion and extension during the swing phase of the walking cycle. This unit includes a valve which is adjustable using a stepper motor to alter the degree of resistance according to signals received from a processor which senses walking speed, so that resistance to movement of the shin component about the knee axis is varied as the walking speed varies.
British Patent Application No. GB 2280609A teaches the programming of an adaptive prosthesis control system using a remote operator control unit allowing convenient setting up of the system by a prosthetist using a handheld remote control unit, whereby walking tests can be conducted at different speeds and a complete set of control data in which flexion resistance settings are mapped onto walking speed ranges can be quickly programmed. In the prosthesis described in Application No. GB 2280609A knee stability is achieved during the stance phase by means of a mechanical load-activated knee braking device consisting of an expandable brake shoe with a friction lining, housed in a brake drum.
One of the difficulties associated with the above-mentioned mechanical friction brake for knee stabilisation during the stance phase is its comparatively sharp transition from a condition in which the knee is free to flex to one in which it is locked. An alternative stance control device found in other lower limb prostheses makes use of a hydraulic piston and cylinder assembly which provides yielding resistance to flexion during the stance phase. This allows the amputee in many cases to achieve a more natural gait due to the ability to roll over the foot in mid stance while achieving smooth initial flexion at the knee. Extension of the knee prior to heel contact is assisted by a coiled compression spring inside the cylinder. However, the need to compress the spring during the latter part of the preceding stance phase leads to many amputees finding such an hydraulic swing and stance control unit tiring to use. The unit also tends to be difficult to adjust in many cases and the mechanism for controlling the onset and release of stance control can produce an unnatural gait.
It is an object of the present invention to provide improved control of knee flexion for a wide variety of amputees and in a wide variety of circumstances.
According to one aspect of this invention, a lower limb prosthesis for an above-knee amputee has an adaptive control system which comprises a knee flexion control device arranged to resist flexion at the knee joint both hydraulically and pneumatically. Sensor means may be provided for generating electrical sensor signals in response to loading of the prosthesis, together with an electronic processing circuit electrically coupled to the sensor means and the control device for automatically adjusting the hydraulic and pneumatic resistance to knee flexion according to the actions of the amputee, the system being arranged such that the resistance to knee flexion is controlled at least predominantly hydraulically during the stance phase of the walking cycle of the prosthesis and at least predominantly pneumatically during the swing phase of the walking cycle. Preferably, the system is configured such that knee flexion is hydraulically resisted substantially only within an initial part of the angular range of flexion of the knee from full extension, typically from zero to between 30° and 35° flexion. Conveniently, extension assistance can be provided pneumatically by the same control element which resists flexion during the swing phase, and terminal extension damping may be achieved hydraulically.
This advantageous combination may be provided in the form of a posteriorly mounted piston and cylinder assembly having two pistons mounted coaxially on a common piston rod and reciprocable together within respective chambers, one containing hydraulic fluid and the other containing a gas, e.g. air. In each case the flow of liquid or gas to or from the respective chambers may be restricted by valves automatically controlled in response to setting signals from the processing circuit whereby the resistance to flexion provided by the hydraulic part of the assembly is varied according to the mode of ambulation of the amputee (such as level walking, walking up or down an incline, walking up or down stairs, and so on), while resistance of the pneumatic part of the assembly is controlled according to the sensed speed of walking, as described in the above-mentioned prior published patent specifications. Arranging for the hydraulic part of the piston and cylinder assembly to resist flexion only during the first 30° to 35° of flexion limits the effect of the hydraulic part mainly to resisting flexion during the stance phase and, with appropriate directing of the hydraulic fluid, damping forward movement of the prosthesis shin part towards the end of the swing phase so as to cushion extension of the knee joint prior to terminal impact at the end of the swing phase.
Adjustment of a pneumatic piston and cylinder assembly according to walking speed by means of a needle valve driven by a stepper motor is described in detail in GB 2280609A. Compression of air against the needle valve during swing phase flexion, together with the comparatively small resistance provided by the hydraulic part of the assembly at large angles of flexion brings the advantage of efficiently generated extension assistance due to the compressed air cushion which exists at maximum flexion, obviating the need for a stiff extension assisting spring, as in prior hydraulic units. The preferred technique for controlling resistance to knee flexion during the stance phase using a piston in a chamber containing incompressible hydraulic fluid makes use of a variable orifice in a first bypass passage between ports in opposite end portions of the chamber and on opposite sides of the piston, the orifice area being controlled electromechanically using, for example, a stepper motor to move a valve member to different positions according to detected modes of amputee activity.
Provision of hydraulic and pneumatic resistance to flexion, both programmable according to activity mode, allows a wide range of adjustment which can be achieved precisely. For instance, coarse adjustment can be performed hydraulically and fine adjustment pneumatically. In this way, it is possible for the prosthetist to program the system to match the level of stance control to the residual muscular control available from the amputee.
The applicants have found that a particular improvement of the ease with which an above-knee amputee can descend a ramp can generally be obtained by increasing stance phase flexion resistance in comparison with the optimum setting for level walking. Consequently, in the preferred control system in accordance with this invention the sensor means and the processing circuit are operable together to generate a descent control signal indicative of the amputee walking down a slope or ramp, this descent control signal causing the stepper motor to move the valve member in the hydraulic bypass passage so as to restrict fluid flow to a greater degree than when the amputee is walking on the level.
Sensing of the ramp descent mode is advantageously performed by means of the processing circuit in combination with a sensor located at the knee level to provide a sensor output signal indicative of kinetic and kinematic parameters around the knee such as knee bending moment, i.e. the moment tending to cause knee flexion during the stance phase. In particular, the processin

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