GB2282414A - Artificial limb component - Google Patents

Abstract

A control unit for controlling operation of the knee joint of a lower limb prosthesis for an above-knee amputee has a piston (10) within a cylinder (12). The piston divides the cylinder into two chambers (14, 16) and fluid flow between the chambers as the knee joint of the prosthesis is flexed and extended is controlled by valve arrangements (26, 28) in respective passageways (18, 20) extending from one end of the cylinder to the other. The valve arrangement (26) controlling downward movement of the piston (10) (corresponding to knee joint flexion) has two flow restrictors (30, 32) operating in series. Flow restrictor (30) is pressure-sensitive and comprises a foam element with an internal passage closed by compression of the element by fluid in chamber 36. Restrictor (32) is a needle valve externally adjustable by virtue of the needle valve body (32B) being screw-threaded in the end cap (12C) of the cylinder (12). <IMAGE>

Description

ARTIFICIAL LIMB COMPONENT This invention relates to a pneumatic swing phase control piston and cylinder assembly for an artificial limb, particularly for controlling movement about the knee joint in an above-knee lower limb prosthesis.

Pneumatic swing phase control devices are well known for controlling knee flexion and extension movements in aboveknee lower limb prostheses. Such a device is normally pivotally connected to, firstly, the posterior part of a knee chassis which constitutes a portion of a thigh component and, secondly, a shin component at a distance below the knee axis of rotation. Movement of air to and from the cylinder spaces on either side of the piston is normally controlled by means of adjustable orifices in conjunction with non-return valves so that the resistance to flexion and extension respectively of the knee can be independently controlled. The settings are generally made by a prosthetist to achieve the best gait for the patient during normal walking. While this arrangement has proved satisfactory in most circumstances, it has been found that the patient's gait can be improved at faster and slower walking speeds if the resistance to flexion and/or extension is altered. With this in mind, electronically controlled resistance-varying systems have been devised which operate by sensing the patient's speed of walking and, using a stepper motor controlled by a microprocessor circuit, alter the orifice dynamically to vary the resistance according to the speed. This has proved very successful for many patients, but the system is significantly more expensive than a conventional swing phase control device and relies on battery power, so that battery life can be quite significantly limited by the power requirements of the stepper motor.

It is an object of this invention to provide a piston and cylinder assembly which automatically varies the resistance to flexion and/or extension according to walking speed at a comparatively low cost.

According to a first aspect of this invention, a swing phase control unit for controlling operation of the knee joint of a lower limb prosthesis for an above-knee amputee comprises a cylinder, a piston inside the cylinder dividing the cylinder into closed chambers on opposite sides of the piston, and a valve arrangement for controlling the flow of fluid out of one of the chambers through a passageway in communication with the said one chamber, wherein the valve arrangement comprises a first flow restrictor having an orifice area which is dynamically variable in response to the pressure in the said one chamber, the area reducing as the pressure increases and, in series with the first restrictor, a second fluid restrictor having an orifice area which is pre-settable. Accordingly, it is possible to arrange for the resistance to piston movement to be increased according to the speed of movement. In the context of a lower limb prosthesis for an above-knee amputee, this means that the resistance to flexion of the knee at the end of the stance phase and during the first part of the swing phase is greater at higher speeds of operation due to the reduced orifice area of the first restrictor. The second flow restrictor can be used to set a basic flow resistance independently of the characteristics of the first restrictor.

Preferably, the control unit has a single piston rod extending through the other of the said chambers. The second restrictor may be a needle valve having a screwthreaded needle body which is a externally accessible for adjustment by rotation.

The first restrictor, the pressure-responsive restrictor, may comprise a resiliently mounted pressure-sensing surface and, associated with that surface, means defining a variable orifice, the area of which depends on the position of the surface. This can be achieved using a foam element housed in a cavity forming part of the passageway. The passageway may also include a bore which opens out in to the cavity, the cavity having an annular abutment surface around the bore for receiving an end face of the foam element. The opposite end face defines the pressure-sensing surface, which may be a plate mounted on the body of the foam element, or a substantially impervious skin or film formed on the foam body. This surface is on the upstream side of the foam element, pressure upstream of the foam element thereby causing the element to be compressed against the abutment surface so as to reduce the orifice area of the restrictor.

In the preferred embodiment of the invention, the passageway forms a bypass passage between the chambers on opposite sides of the piston, and includes a non-return valve restricting or preventing fluid flow through the passageway into the said one chamber.

A further bypass passage may be provided between the said chambers, including a further non-return valve preventing flow therethrough from the said one chamber. This further bypass passage preferably includes an adjustable flow restrictor.

Accordingly, the preferred embodiment is a closed system having separate passageways between opposite sides of the piston, one for controlling flexion of the knee joint, and one for controlling extension. In the preferred embodiment, the pressure-sensitive flow restrictor is used to control movement of the piston during joint flexion.

The valve arrangement may include a third flow restrictor located in series with the first and second restrictors, the second and third restrictors being on opposite sides of the first flow restrictor.

According to an another aspect of this invention a pneumatic swing phase control piston and cylinder assembly for an artificial limb includes a gas flow regulator arranged to control the flow of gas through a passageway in communication with a cylinder space bounded by the piston so as to regulate the resistance to movement of the piston, wherein the regulator comprises a resiliently mounted pressure-sensing surface such as a plate, skin, or film and, associated with the surface, means defining a variable orifice, the area of which orifice depends on the position of the surface. In this way, the surface and the orifice means can be arranged to respond to an increased gas pressure differential across the regulator caused by an increased speed of operation of the prosthesis to bring about an automatic reduction in the orifice area so as to increase the resistance to movement of the piston.

The pressure-sensing surface may be a plate movable in a direction normal to its face, the orifice means comprising an element coupled to the plate and having at least one opening facing in a direction transverse to the direction of movement of the plate for transmitting gas travelling through the passageway, whereby movement of the plate in response to an increased pressure differential causes a restriction of the orifice area associated with the opening or openings.

Typically, the passageway is shaped to define, firstly, a cavity and, secondly, a bore which opens out into the cavity, the cavity having an annular abutment surface around the bore, with the orifice means comprising a resilient foam element having opposite end faces and a side face or faces, the element being located in the cavity with one end face seated on the abutment surface so as to cover the bore and the side faces spaced from the sides of the cavity. The plate is preferably located on, and is substantially coextensive with, the other end face of the foam element whereby the foam element is compressible between the plate and the abutment surface by a pressure-responsive movement of the plate towards the bore. The foam element has an internal passage which communicates with the above-mentioned bore and the sides of the cavity so that compression of the elements, leading to reduction of the size of the passage, reduces the orifice area defined by the foam element as the element is deformed. An impermeable layer or membrane, preferably forming an integral part of the foam element, may be substituted for the plate.

Whether the pressure-sensing surface is formed by a plate, a skin, or simply an air impervious surface, the combination of this surface and a closed cell foam block with an internal passage or passages is preferred for its simplicity and its ability to perform both pressure-sensing and orifice area variation functions. The foam element is typically cylindrically shaped, having a diameter which is similar to or greater than its depth. The foam should be elastic with the ability to be compressed repeatedly with comparatively little compression set.

The preferred dimensions of the foam element are a width or diameter in the range of from 4 to 10 mm and a depth within the same range. The compression strength is preferably between 100 and 250 kPa for 50% compression.

Alternatively an open-cell foam block can be used with a fluid-impervious outer layer acting as the pressure-sensing surface. In this case, a compression strength of between 8 and 25 kPa is preferred for 50% compression.

The passageway may be part of a bypass passage forming part of a closed air circuit between cylinder spaces on opposite sides of the piston. In order to provide for variable resistance to both flexion and extension movements, two such passageways may be provided, each including a pressuresensitive flow restrictor as described above, one restrictor being operable to vary orifice area for flexion and the other to vary the orifice area for extension. The or each passageway is preferably formed in a cylinder end cap forming part of the cylinder of the assembly with a tube or tubes extending to the other end of the cylinder.

Alternatively, one or more passageways may be formed in the piston.

Since the assembly is entirely mechanical and can be constructed from relatively few parts, it provides the ability to vary resistance to knee movements at low cost.

Being responsive to pressure differential, a nearinstantaneous response to gait variations is obtained. In addition, the device can be made to be substantially maintenance-free and has the potential to be easily adjustable to suit the requirements of each individual patient.

The invention will now be described by way of example with reference to the drawings in which: Figure 1 is a diagrammatic cross-section of a control unit in accordance with the invention; Figures 2A and 2B are respectively an underside view and a longitudinal cross-section of a foam restrictor element.

Referring to Figure 1, a pneumatic swing phase control piston and cylinder assembly for an artificial limb has, as its main elements, a piston 10 coupled to a piston rod 10A which is normally connected to a thigh component of an above-knee lower limb prosthesis, and a cylinder 12 normally for connection to a shin component of the prosthesis. The piston and cylinder together define two enclosed air spaces 14, 16. These air spaces 14, 16 are interconnected by two passageways 18, 20 arranged in parallel, each containing a non-return valve 22, 24. Each passageway 18, 20 also includes a respective valve arrangement 26, 28 housed in a cylinder cap 12C through which the piston rod 10A passes.

The orientation of the non-return valves 22, 24 is such that the valve arrangement 26 controls downward movement of the piston, corresponding to flexion of the knee joint, while valve arrangement 28 controls upward movement of the piston, corresponding to extension of the knee joint.

In accordance with the invention the valve arrangement 26 has a first flow restrictor 30 which is pressure-sensitive and, in series with the first restrictor, a second flow restrictor 32 having an orifice area which is manually adjustable. In this embodiment, the second flow restrictor is a needle valve having a needle body 32B which is mounted in the cylinder cap 12C by means of a screw-thread allowing manual adjustment of the position of the needle body 32B in an opening 34 in the cylinder cap 12 to set the basic resistance to fluid flow through the passageway 18. Once set, the orifice area of the second flow restrictor remains fixed during operation of the assembly.

The first flow restrictor 30 takes the form of a small resilient cylindrical foam block housed in a cylindrical cavity 36 located between an inlet bore 38 in the cylinder cap 12C and an outlet bore 40 in the cylinder cap 12C, both bores forming part of the passageway 18 between opposite sides of the piston 10. A bottom wall of the cavity forms an annular abutment surface 42 around the bore 40 for receiving a lower end face of the foam block 30. The sizes of the foam block 30 and the cavity 36 are such that the side face of the foam block is spaced from the side wall of the cavity.

The foam block 30 is shown in more detail in Figures 2A and 2B. In this embodiment, the foam is a closed cell type, and a blind bore 30B is formed axially in the block 30, opening out on the lower end face 30L. A transverse bore 30T connects the inner end of the blind bore 30B with the side of the block 30. Consequently, air entering the cavity 36 from the bore 38 is normally able to pass into the transverse bore 30T via the space between the side of the block 30 and the side wall of the cavity 36, and thence into the blind bore 30B before passing into bore 40 in the body 32B of the needle valve 32. The air is then exhausted into the chamber 14 containing the piston rod 10A via the orifice formed by the lower end portion of the needle valve body 32B where it enters the opening 34 formed in the cylinder cap 12C.

When flexion of the knee joint occurs sufficiently quickly to cause the pressure in the chamber 16 of the cylinder 12 to rise above a certain level, the foam block 30 is compressed due to the pressure in the cavity 36 with the result that the transverse bore 30T in the block 30 is reduced in cross-sectional area, thereby reducing the orifice area represented by the block 30. This means that the overall resistance to flexion provided by the control unit is increased as the amputee's walking speed increases.

The sensitivity of the arrangement to walking speed can be adjusted by a third flow restrictor formed by a plug 44 housed by means of a screw-thread in the needle valve body 32B, in combination with side openings 46 in the needle body 32B which restrict the flow of air from bore 38 in cap 12C to the cavity 36. In an alternative embodiment of the invention the orifice area of the third restrictor may be fixed. All three restrictors of the valve arrangement 26 are in series with each other, the pressure-sensitive restrictor 32 being between the other two. Since it is housed in the end cap 12C of the cylinder 12, the arrangement is easily accessible for adjustment when the unit is fitted beneath the knee mechanism of a lower limb prosthesis.

In the preferred embodiment, the valve arrangement 28 controlling upward movement of the piston 10, corresponding to extension of the knee joint, is a simple needle valve 48 having a needle portion located in an opening 50 in the cylinder end cap 12C. This presents a fixed resistance to flow from chamber 14 to chamber 16 via passageway 20 as the piston 10 rises. Alternatively, it is possible to incorporate a pressure-sensitive foam restrictor similar to that of valve arrangement 26 in order to provide varying resistance to extension, according to walking speed.

The preferred foam element is cylindrical, having a diameter and depth of about 6 and 5 mm respectively. The foam material is preferably an closed-cell chemically crosslinked polyethylene foam with a density of approximately 50kg/m3. The compressive strength of the element is about 160 kPa for 50% compression. The blind bore 30B and transverse bore 30T are typically 2mm and lmm in diameter respectively, with values in the range of from 0.5mm to 3mm being possible. In an alternative embodiment an open-cell foam block may be used having a similar diameter. In this case a distinct pressure sensing layer, film or plate is provided on the end face of the block which is opposite to the end facing the bore 40 in the cylinder end cap 12C.

Bores 30B and 30T are unnecessary since air passes through the cells of the block from the side to the bottom end face and the bore 40. Typically the compressive strength for the open-cell block is between 15 and 20 kPa for 50% compression.

Claims (13)

1. A control unit for controlling operation of a knee joint of a lower limb prosthesis for an above-knee amputee, the unit comprising a cylinder, a piston inside the cylinder dividing the cylinder into closed chambers on opposite sides of the piston, and a valve arrangement for controlling the flow of fluid out of one of the chambers through a passageway in communication with the said one chamber, wherein the valve arrangement comprises a first flow restrictor having an orifice area which is dynamically variable in response to the pressure in the said one chamber, the area reducing as the pressure increases, and, in series with the first restrictor, a second flow restrictor having an orifice area which is pre-settable.

2. A control unit according to claim 1, having a single piston rod extending through the other of the said chambers.

3. A control unit according to claim 1 or claim 2, wherein the second restrictor is a needle valve having a screw threaded needle body which is externally accessible for adjustment by rotation, the second restrictor being downstream of the first restrictor.

4. A control unit according to any preceding claim, wherein the pressure-responsive restrictor comprises a resiliently mounted pressure-sensing surface and, associated with the surface, means defining a variable orifice, the area of the orifice depending on the position of the surface.

5. A control unit according to claim 4, wherein the orifice means comprises a foam element.

6. A control unit according to claim 5, wherein the passageway includes a cavity, and a bore which opens out into the cavity, the cavity having an annular abutment surface around the bore, and wherein the foam element is located in the cavity, the foam element having one end face seated on the abutment surface and an opposite end face defining the pressure-sensing surface, the foam element being compressible against the abutment surface by a pressure responsive movement of the pressure-sensing surface in the direction of the bore.

7. A control unit according to any preceding claim, wherein the passageway forms a bypass between the chambers on opposite sides of the piston, and wherein the passageway includes a non-return valve restricting or preventing fluid flow through the passageway into the said one chamber.

8. A control unit according to claim 7, including a further bypass passage between the said chambers, which further passage includes a non-return valve preventing flow therethrough from the said one chamber.

9. A control unit according to claim 8, wherein the further bypass passage includes an adjustable flow restrictor.

10. A control unit according to any preceding claim, wherein the valve arrangement includes a third flow restrictor located in series with the first and second restrictors, the second and third restrictors being on opposite sides of the first flow restrictor.

11. A swing phase control unit for an artificial limb, comprising a piston and cylinder assembly including a fluid flow restrictor arranged to control the flow of fluid through a passageway in communication with a cylinder space bounded by the piston so as to regulate the resistance to movement of the piston, wherein the restrictor comprises a resiliently mounted pressure sensing surface such as a plate, skin or film and, associated with the surface, means defining a variable orifice, the area of which orifice depends on the position of the surface.

12. A lower limb prosthesis for an above-knee amputee, the prosthesis including a control unit according to any preceding claim.

13. A control unit for an artificial limb, the unit being substantially as herein described and shown in the drawings.