GB2582733A

GB2582733A - Biasing element

Info

Publication number: GB2582733A
Application number: GB1900999.2A
Authority: GB
Inventors: Blakeley Philip
Original assignee: Cambridge Prosthetics Ltd
Current assignee: Cambridge Prosthetics Ltd
Priority date: 2019-01-24
Filing date: 2019-01-24
Publication date: 2020-10-07
Also published as: GB201900999D0; WO2020152469A1

Abstract

There is provided a rate responsive biasing element 10 comprising a longitudinal connection member 20, a first location member 30 coupled to the longitudinal connection member 20, and a biasing means 41,42 located between the first location member 30 and a second location member 35 to provide a biasing force between the first location member 30 and the second location member 35. The biasing means comprises a rate-independent component and 41 a rate-dependent component 42 arranged to act in parallel. The rate independent component may comprise a spring. The rate dependent component may comprise a non-Newtonian material preferably a shear thickening fluid. The rate components may form annular members concentric relative to one another or may comprises a resilient material with a cavity containing the rate dependent component. The rate responsive biasing element 10 may form part of a prosthetic knee joint.

Description

Biasing Element

Field of the invention

The invention relates to a rate-responsive biasing element and to a prosthetic knee 5 joint comprising such a biasing element.

Background

Prosthetic knee joints are integral to trans-femoral (above knee) prostheses. The function of a prosthetic knee joint is to mimic the flexion and extension (bending and straightening) of an anatomical knee joint. A well-designed prosthetic knee joint can support an amputee and provide a natural gait. Many variations of prosthetic knee joints have been designed and these designs range from relatively basic, hinge-like joints to complicated, computer-controlled, multi-axial joints.

Currently, many prosthetic knee joints use microprocessors to electronically control complicated networks of hydraulic pistons in order to mimic the flexion and extension of an anatomical knee joint. Such prosthetic knee joints can be both difficult and expensive to manufacture, and, therefore, are expensive to buy. For example, such prosthetic knee joints can cost in excess of £2,000 to buy. Complete, top of the range prostheses can cost over £100,000 to buy.

Less expensive prosthetic knee joints are often designed to provide resistance against flexion of the joint, for at least a portion of the flexion-extension cycle. Such resistance may be provided by a biasing element such as a spring, which may also assist in the subsequent extension of the knee joint. This arrangement can help to provide a more natural gait for an amputee. However, if an amputee stumbles and applies all, or most, of their weight through their prosthetic knee joint while the knee joint is flexing, this resistance is not sufficient to prevent the prosthetic knee joint from collapsing.

It is an object of the invention to provide an improved prosthetic knee joint. It is another object of the invention to provide an improved prosthetic knee joint which is mechanically simple, reliable, and does not have a high cost of manufacture.

It would be desirable to provide a biasing element that could be used to assist flexion-extension of a knee joint and resist collapse of the knee joint. It would also be desirable to provide a low cost prosthetic knee joint which can reduce the likelihood of an amputee falling due to collapse of the prosthetic knee joint.

Summary of invention

The invention provides a rate responsive biasing element and a prosthetic knee joint as defined by the appended independent claims, to which reference should now be made. Optional or advantageous features are set out in various dependent sub-claims.

According to a first aspect, a rate-responsive biasing element comprises a longitudinal connection member, a first location member coupled to the longitudinal connection member, and a biasing means arranged to provide a biasing force between the first location member and a second location member when the biasing means is elastically deformed by a biasing load. The biasing means comprises a rate-independent component and a rate-dependent component. The rate-independent component and the rate-dependent component arranged in parallel, such that an applied load is distributed between both components. The rate responsive biasing element may comprise the second location member.

In preferred embodiments, the rate responsive biasing element may be used as a biasing element to assist flexion and/or extension of a prosthetic joint, such as a prosthetic knee joint. A prosthetic joint may be part of a prosthetic limb, for example a prosthetic leg. The rate responsive biasing element may have other applications that are not related to prosthetic devices.

Advantageously, the rate responsive biasing element according to the first aspect provides a biasing means comprising a rate-independent component and a rate-dependent component arranged to act in parallel. This means that the response of the biasing means depends on both the rate-independent component and the rate-dependent component. The rate-independent component may be described as an rate-independent component and may have a conventional elastic response.

As used herein, "rate independent" is intended to refer to a component having a response that is substantially independent of load rate or strain rate applied to that component.

As used herein, "rate dependent" is intended to refer to a component having a response that varies or is otherwise affected by the load rate or strain rate applied to that component. For example, a rate-dependent component may offer little resistance to extension or compression when a load is applied at a low load rate, but a high resistance to extension or compression when a load is applied at a high load rate.

A biasing load may be applied to the biasing means via longitudinal movement of the longitudinal connection member in a first longitudinal direction. In such circumstance, the biasing force acts to urge the longitudinal connection member in a second longitudinal direction opposite to the first longitudinal direction.

Elastic deformation of the rate-independent component may be proportional to the applied biasing load and independent of load rate. Deformation of the rate-dependent 5 component may be variable depending on the load rate of the applied biasing load.

The rate dependent component may have a stiffness which varies depending on a load-rate at which a load is applied to the biasing means. The rate dependent component may have a stiffness which is much higher than a stiffness of the rate-independent component at a first, high load-rate, and the rate dependent component may have a stiffness which is much lower than a stiffness of the rate-independent component at a second, lower load-rate. In such an embodiment, at the first, high load-rate, the stiffness of the rate dependent component may dominate the response of the biasing means. In contrast, at the second, lower load-rate, the stiffness of the rate-independent component may dominate the response of the biasing means. Thus, the biasing means may exhibit, at least temporarily, a lesser extension per unit force when that force is applied at the higher load-rate than at the lower load-rate.

The rate dependent component may have a response, such as a stiffness, which varies with applied load-rate, for example a greater stiffness at higher load-rates. The rate dependent component may develop a greater stiffness when loaded at a load-rate of 10 kN/s than when loaded at a load-rate 1 kN/s. The stiffness of the rate dependent component may be greater at higher load-rates, and the rate of change of stiffness of the rate dependent component may increase at higher load-rates.

As used herein, reference to load-rates is intended to indicate a rate of change of load on a component. This may include increasing and decreasing loads.

As used herein, reference to strain-rates is intended to indicate a rate of change of strain of a component. This may include increasing and decreasing strains.

Alternatively, the biasing element according to the first aspect may not necessarily comprise a rate-dependent component. For the avoidance of doubt, optional features described below in relation to the first aspect may also be applicable to a biasing element which may not necessarily comprise a rate-dependent component. It would be clear to the skilled person where descriptions and explanations below would also be applicable to a biasing element which does not necessarily comprise a rate-dependent component.

Advantageously, in the context of a prosthetic joint, such a prosthetic knee joint, the rate-independent component of the biasing means may resist flexion of the joint to a greater degree when the joint is in a more flexed position. That is, the rate-independent component of the biasing means may exhibit an elastic response and exert a greater force acting to straighten the joint when the rate-independent component is loaded to a greater degree. This may provide a more natural gait because a faster walk naturally results in greater flexion of the knee joint. Thus, the rate-independent component may provide a greater force acting to straighten the leg during a faster walk. For example, the rate-independent component may exert a force which increases with its compression, and may be compressed more during a faster walk.

The term "longitudinal connection member' refers to a connection member having a longitudinal dimension. A longitudinal axis of the longitudinal connection member may extend between the first and second location members. In the context of a prosthetic knee joint, the longitudinal axis of the longitudinal connection member may be arranged to lie substantially parallel to a shin portion of a patient's prosthetic leg, and may be arranged concentrically with a shin portion of a patient's prosthetic leg.

The rate responsive biasing element may be configured such that, if it is loaded at a rate of loading that is lower than a first predetermined rate of loading, the biasing means extends or compresses, but if it is loaded at a rate of loading that is higher than a second predetermined rate of loading, the rate-dependent component of the biasing means resists the extension or compression of the biasing means.

The rate responsive biasing element may be configured such that relative movement between the first location member and the second location member at a rate of movement that is lower than a predetermined rate of movement results in extension of the biasing means, and relative movement between the first location member and the second location member at a rate of movement that is greater than the predetermined rate of movement causes the rate dependent component of the biasing means to stiffen and, at least temporarily, resist the extension of the biasing means.

The rate responsive biasing element may be configured such that relative movement between the first location member and the second location member at a rate of movement that is lower than a predetermined rate of movement results in compression of the biasing means, and relative movement between the first location member and the second location member at a rate of movement that is greater than the predetermined rate of movement causes the rate dependent component of the biasing means to stiffen and, at least temporarily, resist the compression of the biasing means.

The rate responsive biasing element may allow relative movement between the first location member and the second location member if the rate of this relative movement, 35 which may be dependent on the rate of loading of the biasing element, is lower than a predetermined threshold. The rate responsive biasing element may resist relative movement between the first location member and the second location member if the rate of this movement is greater than a predetermined threshold. Such resistance may be a substantially instantaneous resistance. Advantageously, this may allow for relatively slow movement between the first location member and the second location member, but may prevent relatively quick movement between the first location member and the second location member. In the context of a prosthetic knee joint, flexion of the joint may result in relative movement between the first location member and the second location member. In this case, the biasing element allows relatively slow flexion of the joint, such as during 10 walking, but resists relatively quick flexion of the joint, such as if the joint is collapsing under the weight of the wearer of the joint.

Relative movement between the first location member and the second location member may be resisted if the second predetermined rate of loading is equal to or greater than 10 kN/s, for example greater than 50 kN/s, or 80 kN/s, or 100 kN/s, or 150 kN/s, or 15 200 kN/s.

In the context of a prosthetic knee joint worn by a user, a rate of loading higher than the second predetermined rate of loading may indicate that a large portion of the user's weight has been used to flex the joint in a small amount of time. This may suggest that the user has stumbled and, if the stiffness of the rate-dependent component is not high enough, the joint could collapse and the user may fall.

The rate responsive biasing element may be configured such that, if it is loaded at a rate of loading that is higher than a predetermined rate of loading, then the rate dependent component of the biasing means is stiffer than the rate-independent component of the biasing means. For example, the rate responsive biasing element may be configured such that, if it is loaded at a rate of loading that is higher than a predetermined rate of loading, for example higher than 200, 150, 100, 80, 50, or 10 kN/s, then the rate dependent component of the biasing means is at least 10, 20, 50, 100, 200, or 500% stiffer than the rate-independent component of the biasing means.

The rate responsive biasing element may be configured such that, if it is loaded at a rate of loading that is higher than 200, 150, 100, 80, 50, or 10 kN/s, then the biasing means has a stiffness greater than 500, 1000, 2000, 5000, 10000, or 20000 kN/m. For example, if the rate responsive biasing element is loaded at a rate of loading that is higher than 200 kN/s, then the biasing means may have a stiffness greater than 500 kN/m.

The rate responsive biasing element may be configured such that, if it is loaded at a 35 rate of loading that is higher than 200, 150, 100, 80, 50, or 10 kN/s, then the rate dependent component of the biasing means has a stiffness greater than 500, 1000, 2000, 5000, 10000, or 20000 kN/m. For example, if the rate responsive biasing element is loaded at a rate of loading that is higher than 200 kN/s, then the rate dependent component of the biasing means may have a stiffness greater than 500 kN/m.

If the rate dependent component is loaded at a rate of loading that is higher than 200, 150, 100, 80, 50, or 10 kN/s, then the rate dependent component may have a stiffness greater than 500, 1000, 2000, 5000, 10000, or 20000 kN/m. For example, if the rate dependent component is loaded at a rate of loading that is higher than 200 kN/s, then the rate dependent component may have a stiffness greater than 500 kN/m.The longitudinal connection member may be fixedly-coupled to the first location member and movably-coupled to the second location member, such that longitudinal movement of the longitudinal connection member results in relative movement between the first location member and the second location member.

Advantageously, fixedly coupling the longitudinal connection member to the first location member and movably coupling the longitudinal connection member to the second location member may be a simple, and straightforward way to provide relative movement between the first location member and the second location member when the longitudinal connection member moves.

Alternatively, the longitudinal connection member may be fixedly coupled to the 20 second location member and movably coupled to the first location member such that longitudinal movement of the longitudinal connection member causes relative movement between the first location member and the second location member.

The first location member may comprise a flange or a plate extending transversely from the longitudinal connection member. The first location member may comprise an adjustable nut and/or a washer. The rate-independent component and/or the rate dependent component may attach to, connect to, couple to, or abut the first location member. The longitudinal position of the first location member relative to the longitudinal connection member may be adjustable. Adjusting the position of the first location member may change the permissible extension or compression of the rate-independent component and/or the rate dependent component, thereby altering the biasing response of the rate responsive biasing element.

In the context of a prosthetic knee, an adjustable first location member may allow adjustment of the maximum permissible extension or compression of the rate-independent component and/or the rate dependent component when the knee joint is fully extended, or 35 fully straightened, to account, for example, for a user's weight.

Adjusting the adjustable nut or washer may adjust the stiffness of the rate-independent component. For example, where the rate-independent component is a rate-independent spring, adjusting the adjustable nut or washer may compress or extend the rate-independent spring. This may have the effect of increasing the stiffness of the rate-independent spring at any given degree of flexion in the prosthetic knee.

The rate-independent component and the rate dependent component of the biasing means may be separate or separable components.

The rate-independent component may substantially obey Hooke's law up to an applied load of at least 5, 10, 20, or 50 kN. That is, the force required to compress or extend the rate-independent component may be substantially proportional to the compression or extension of the rate-independent component up to an applied load of at least 5, 10, 20, or 50 kN.

The force required to further compress or extend the rate-independent component may be proportional to the compression or extension of the rate-independent component 15 (from its natural length) up to an applied load of at least 5, 10, 20, or 50 kN, plus or minus 20% of the force. As an equation, this may be expressed as: 0.8 F k x 1.2 F In the above equation, F is the tensile (or compressive) force applied to the rate-independent component, x is the extension (or compression) of the rate-independent 20 component, and k is the constant of proportionality between F and x, also known as the spring constant'.

The rate-independent component may comprise a spring such as a helical tension spring or a helical compression spring, a gas strut, a domed washer, or a substantially elastic material. The rate-independent component may comprise an elastomeric material, such as an elastomeric polymer. The rate-independent component may comprise polyurethane, or microcellular polyurethane.

The rate dependent component may comprise a non-Newtonian material, for example a non-Newtonian fluid or a non-Newtonian polymer. The rate dependent component may comprise a shear thickening, or dilatant, material or fluid. The rate dependent component may comprise a material in which viscosity increases with an increasing applied rate of strain, and/or an increasing applied rate of shear strain. The viscosity or shear viscosity may increase with increased applied shear stress or increased rate of applied load. The rate dependent component may comprise a rate sensitive material, for example as supplied by D30® technologies, or a polyurethane foam matrix with a fluid and polyborodimethylsiloxane (PBDMS) as a dilatant dispersed through the foam matrix. By volume, appropriate proportions may be 10-40%, preferably 15-35%, of PBDMS and 30-80%, preferably 40-70%, fluid (the fluid resulting from the foaming process, generally carbon dioxide), the remainder being polyurethane. The rate dependent component may comprise a dilatant fluid such as a suspension of corn starch in water, commonly called "Oobleck". The skilled person would be able to select a suitable material, or combination of materials, for the rate dependent component.

The rate-independent component and the rate dependent component may coexist in the same unitary component. For example, the biasing means may comprise a material which exhibits a partially elastic, rate-independent, response, and a partially rate dependent response. For example, the biasing means may comprise a foam, gel, suspension, polymer, co-polymer, or elastomer which exhibits such a response. The biasing means may comprise a gel, suspension, polymer, co-polymer, suspension, elastomer, or foam, such as a closed cell foam, which encapsulates, or otherwise includes, a non-Newtonian material, for example a non-Newtonian fluid such as a shear thickening fluid. For example, the biasing means may comprise a closed cell foam which encapsulates sections, or droplets, of a shear thickening fluid. The biasing means may comprise a D30® material, such as a shear thickening D3O® material. D30® materials are commercially available materials designed to resist impacts. A D30® material may exhibit a load-rate dependent response. A D30® material may exhibit a combination of a substantially elastic load-rate independent response and a load-rate dependent response. The D30® material may exhibit this combination of response up to a given applied load, for example up to an applied load of at least 5, 10, 20, or 50 kN.

The rate dependent biasing element or the biasing means of the rate dependent biasing means may define a channel, lumen, or bore. The biasing means may be located at least partially within a channel, lumen, or bore. As used from here onwards, the term "channel' is used to mean "channel, lumen or bore". The longitudinal connection member may extend through the channel. That is, at least part of the longitudinal connection member may extend at least partly through the channel.

The rate-independent component of the biasing means may be partially or entirely 30 contained within a channel. The rate dependent component of the biasing means may be partially or entirely contained within a channel. The biasing means may be partially or entirely contained within a channel.

Advantageously, locating at least part of the biasing means within a channel may provide a smaller, or more compact, rate responsive biasing element. Further, providing at least part of the biasing means within the channel may protect at least part of the biasing means from contact with other objects external to the channel.

The rate-independent component of the biasing means and/or the rate-dependent component of the biasing means may be disposed adjacent to, or around, the longitudinal 5 connection member. The rate-independent component of the biasing means and/or the rate-dependent component of the biasing means may surround the longitudinal connection member in at least one plane. For example, the rate-independent component and/or rate-dependent component may be in the shape of a ring, toroid, or tube defining a central opening or lumen, and the longitudinal connection member may extend through this 10 opening or lumen.

The biasing means may comprise a first annular member forming at least a portion of the rate-independent component. The biasing means may comprise a second annular member forming at least a portion of the rate dependent component.

As used herein, the term "annular member" may refer to an incomplete annular member. That is, the annular member may be a partial annular member. The partial annular member may not form a full 360 degree annulus. For example, the annular member may be substantially "C" shaped, or may form less than 350, or 300, degree annulus. The annular member may form at least 180, or 270, degrees of an annulus. Alternatively, the term "annular member" may refer to a complete annular member. For example, the annular member may be substantially "0" shaped.

The first annular member and the second annular member may be disposed concentrically relative to each other. The first annular member and/or the second annular member may be disposed around the longitudinal connection member. The longitudinal connection member may extend through a central opening of the first annular member and/or the second annular member. Advantageously, such an arrangement may facilitate a reduction in the length of the biasing element in a longitudinal direction.

Part or all of the rate-independent component of the biasing means may encapsulate part or all of the rate dependent component. Part or all of the rate-independent component may be in the shape of a tube, or toroid. The rate-independent component may be disposed around the longitudinal connection member. The rate-independent component may be made of an elastic or elastomeric material, thereby providing a biasing force when loaded.

Part or all of the rate dependent component may be in the shape of a tube, or toroid. The rate dependent component may be disposed around the longitudinal connection member. The rate dependent component may comprise a non-Newtonian fluid. The rate dependent component may comprise a non-Newtonian fluid contained within a container.

The biasing means may comprise a longitudinally extending stack of separate biasing members, each separate biasing member comprising a rate-independent 5 component and a rate dependent component arranged to act in parallel.

The biasing means may comprise a longitudinally extending stack of 3, 4, 5, or more separate biasing members. The biasing means may comprise a longitudinally extending stack of up to 8, 9, or 10 separate biasing members.

Each separate biasing member may be separated by a sheet, plate, or slab, of 10 material, for example a polymeric, elastic, or metallic material.

Each biasing member may be in the form of an annular component having a first annular portion forming the rate-independent component and a second annular portion forming the rate-dependent component. The first annular portion and the second annular portion may be disposed concentrically relative to each other.

Advantageously, such a concentric arrangement may provide a shorter biasing element. In other words, the length of the biasing element in a longitudinal direction may be reduced.

In some embodiments the biasing means may comprise a first, load-rate dependent, portion comprising the rate-independent component and the rate-dependent component which are arranged to act in parallel, and a second, elastic, portion comprising a resilient means. The second portion may be arranged in series with the first dependent portion such that there is always a degree of biasing response generated by the second portion irrespective of the rate at which the biasing element is loaded. Advantageously, this configuration allows the second portion to act as a 'cushion' in the event of a high applied load rate or strain rate.

The resilient means may comprise a helical spring, for example a helical tension spring or a helical compression spring, a gas strut, a domed washer, or a substantially elastic material.

The second portion, or the resilient means of the second portion, may be stiffer than 30 the rate-independent component, for example at least 10, 25, 50, 100, or 200% stiffer.

According to an aspect of the invention, there is provided a prosthetic joint. In preferable embodiments a prosthetic knee joint is provided.

A prosthetic knee joint may comprise an upper leg portion pivotably connected to a lower leg portion. Flexion of the prosthetic knee joint is assisted or resisted by a biasing element. The biasing element comprises a biasing means having a rate-independent component. The biasing means may also have a rate dependent component. The rate-independent component and the rate dependent component of the biasing means may be arranged to act in parallel. The biasing element may preferably be a rate responsive biasing element as described above in relation to the first aspect of the invention.

The rate-independent component of the biasing means may resist flexion of the joint to a greater degree when the knee is flexed to a greater extent. That is, the rate-independent component of the biasing means may exert a greater force acting to straighten the joint when the joint is more flexed. This may provide a more natural gait because a faster walk naturally results in greater flexion of the knee joint. Thus, the rate-independent component may provide a greater force acting to straighten the leg during a faster walk.

The rate-independent component of the biasing means may resist flexion of the joint more when the joint is flexed more, up to a given point. For example, the rate-independent component of the biasing means may resist flexion of the joint more when the knee is flexed more, between a first, fully extended position of the joint and a second position of the joint. This second position of the joint may occur when the angle between the upper leg portion and the lower leg portion is between 15 and 75 degrees, for example between 20 and 70 degrees, or between 30 and 60 degrees. The rate-independent component of the biasing means may resist flexion of the joint less when the joint is flexed beyond the second position of the joint, or may assist flexion of the joint when the joint is flexed beyond the second position of the joint. Thus, the rate-independent component of the biasing means may resist flexion of the joint the most at the second position of the joint. The second position of the joint is determined by the pivoting mechanism. The second position may be the point at which the rate-independent component, such as a spring, is compressed the most. This may be achieved by designing the knee joint such that flexion from a straight joint initially results in motion of the longitudinal connection member in a first direction, and further flexion beyond the second position of the joint results in motion of the longitudinal connection member in a second direction, different to the first direction, for example opposite to the first direction.

In other words, starting from a fully extended prosthetic knee joint, the rate-independent component of the biasing means may resist flexion more as the joint is flexed until this resistance reaches a maximum. Beyond this maximum, the rate-independent component of the biasing means may resist flexion less as the joint is flexed more, or may assist flexion as the joint is flexed more. Thus, when a user is sitting down with the prosthetic knee joint flexed to, say, 90 degrees, the force acting to straighten the joint is less than if this force were to monotonically increase as the joint is flexed further.

Advantageously, this may provide a more natural gait by providing a greater restoration force (a force acting to straighten the joint) at greater flexion during walking, without the restoration force continuing to increase as the knee is flexed beyond a certain point which is not expected during walking, for example whilst sitting.

The rate responsive biasing element may be located in the lower leg portion. The longitudinal connection member may be arranged to move longitudinally when the joint is flexed or extended. Flexion of the joint may cause the longitudinal connection member to move in a first direction and extension of the joint may cause the longitudinal connection member to move in a second direction, opposite to the first direction.

Preferably, flexion of the knee joint at a rate of movement that is greater than a predetermined rate of movement causes the load-rate dependent component of the biasing element to stiffen, thereby resisting flexion of the knee joint.

Flexion of the knee joint may apply a load to the biasing element. Flexion of the knee joint may cause relative movement between the first location member and the second 15 location member.

The prosthetic knee joint may comprise a first stop configured to provide a maximum degree of flexion of the knee joint. For example, the prosthetic knee joint may comprise a first stop configured to provide a maximum degree of flexion of the knee joint of greater than 100, 110, or 120 degrees and/or less than 160, 150, or 140 degrees.

The prosthetic knee joint may comprise a second stop configured to prevent the knee joint from extending (from a flexed position) past a given point such as a point when the upper leg portion and the lower leg portion lie along the same axis (a fully extended, or straight position).

The invention may provide a prosthetic limb comprising a rate responsive biasing element as described herein. For example, a prosthetic leg may be provided comprising a prosthetic knee joint as described above. Such a prosthetic leg may provide a natural gait at low cost compared to existing prosthetic legs while providing some protection against collapse of the knee joint when suddenly loaded.

Features described in relation to the first aspect may be applicable to the second 30 aspect and vice versa.

List of figures The invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figures 1 and 2 are schematic illustrations showing the components and operation of a rate responsive biasing element according to the invention.

Figure 3 is a cross-sectional view of a first biasing element according to the invention, the first biasing element being disposed in a first position.

Figure 4 is a cross-sectional view of the first biasing element according to the invention, the first biasing element being disposed in a second position.

Figure 5 is a cross-sectional view of a first polycentric, prosthetic knee joint when the joint is fully extended.

Figure 6 is a cross-sectional view of the first polycentric, prosthetic knee joint shown 10 in Figure 5 when the joint is flexed by around 30 degrees.

Figure 7 is a cross-sectional view of the first polycentric, prosthetic knee joint shown in Figure 5 when the joint is flexed by around 135 degrees.

Figure 8 is a cross-sectional view of a second polycentric, prosthetic knee joint when the joint is fully extended.

Figure 9 is a cross-sectional view of the second polycentric, prosthetic knee joint when the joint is flexed by around 30 degrees.

Figure 10 is a cross-sectional view of the second polycentric, prosthetic knee joint when the joint is flexed by around 135 degrees.

Figure 11 is a front view and a cross-sectional view of a third polycentric, prosthetic 20 knee joint when the joint is fully extended.

Figure 12 is a front view and a cross-sectional view of a fourth polycentric, prosthetic knee joint when the joint is fully extended.

Specific description

Specific embodiments of the invention will now be described by way of example only.

Figure 1 is a schematic diagram representing a rate responsive biasing element 10 according to the invention. The rate responsive biasing element 10 comprises a longitudinal connection member 20 in the form of a longitudinally extending rod. A first location member 30 is fixedly coupled to an end of the longitudinal connection member 20. The longitudinal connection member 20 is slideably engaged with a second location member 35 which is longitudinally spaced from the first location member 30. A biasing means 40 is arranged to provide a biasing force acting to bias the first location member 30 away from the second location member 35 when the biasing means 40 is deformed by a compressive biasing load.

The biasing means 40 comprises two components, a first, rate-independent component 41 and a second, rate-dependent component 42. The rate-independent component 41 and the rate-dependent component 42 are arranged in parallel such that a portion of any biasing load is transmitted through the rate-independent component 41 and a further portion of any biasing load is transmitted through the rate-dependent component 42. In figure 1 the rate-independent component 41 and the rate-dependent component 42 are illustrated as two separate elements. In specific embodiments these two components may be separate elements arranged to act in parallel, but may also be a single unitary component that has a combination of a rate-dependent and a rate-independent response under a load.

In figure 1 the rate-responsive biasing element 10 is illustrated in a relaxed condition in which no load is applied to the biasing means 40. Figure 2 is a schematic diagram illustrating the rate-responsive biasing element in a condition in which a compressive biasing load has been applied to the biasing means 40. The compressive biasing load is applied by a longitudinal movement of the longitudinal connection member 20 relative to the second location member 35 in the direction indicated by the arrow. As the first location member 30 is fixedly coupled to the longitudinal connection member 20, the first location member 30 moves towards the second location member 35 thereby compressing the rate-independent component 41 and the rate-dependent component 42 of the biasing means 40. Compression of the biasing means 40 results in a biasing force which acts to urge the first location member 30 away from the second location member 35 and thereby return the rate responsive biasing element 10 to its relaxed condition as illustrated in figure 1.

The rate-independent component 41 of the biasing means 40 is a component that generates a biasing force when it is loaded. The deformation of the rate-independent component, and therefore the biasing force generated, is related to the load applied and is independent of the rate at which the load is applied. The deformation of the rate-dependent component 42, by contrast, varies depending on the rate at which the load is applied. At a low load rate, which generates a low strain rate in the rate-dependent component, the rate-dependent component deforms with low resistance to deformation. In this circumstance, an applied load deforms the biasing means and the biasing means swiftly generates a biasing force that is related to the applied load. At a high load rate, however, the rate-dependent component develops a high resistance to deformation. Thus, the biasing means is prevented from substantial deformation, at least initially, and the rate-responsive biasing element effectively locks, preventing longitudinal movement of the longitudinal connection member 20.

If a rate-responsive biasing element as schematically illustrated in figures 1 and 2 was used as a biasing element in a joint, for example a knee joint, then the rate responsive 5 nature of the biasing element may be used to good effect. For example, under normal walking conditions, flexion of the knee joint may result in loading of the biasing means at a low load rate. This loading results in a biasing force which acts to assist in extension of the knee joint. Under severe loading rates, however, such as may occur if a user of the knee joint stumbles, the rate-responsive element 42 of the rate responsive biasing means may 10 lock, thereby preventing collapse of the knee joint.

Specific embodiments will now be discussed. Figure 3 is a cross-sectional view of a rate responsive biasing element 100 according to an embodiment the invention when disposed in a first, relaxed, position. The biasing element 100 comprises a longitudinal connection member in the form of a steel rod 112. The steel rod is threaded at each of its two ends. Threaded onto an upper threaded end of the steel rod 112 is a top nut 113, and threaded onto a lower threaded end of the steel rod 112 is a bottom nut 114. The bottom nut 114 is fixedly couple to the steel rod 112 and acts as a first location member.

A biasing means 105 is located between the first location member (bottom nut 114) and a second location member 106. The second location member may be a component part of the rate responsive biasing element or may be a part of a component comprising the rate responsive biasing element. The biasing means 105 comprises a stack of three annular steel washers, first washer 116, second washer 120, and third washer 124. A first annular spring element 118 is disposed between the first washer 116 and the second washer 120, and a second annular spring element 122 is disposed between the second washer 120 and the third washer 124. The steel rod 112 extends through central openings in each of the washers and the annular springs. The biasing means 105 is located between the first location member 114 and the second location member 106 such that movement of the steel rod 112 relative to the second location member 106 can cause a compressive load to be applied to the biasing means.

In the specific embodiment shown in Figure 3, a first annular spring element 118 is formed of a first inner annular component 117 and a first outer annular component 119. The inner annular component 117 exhibits a load-rate dependent response and the outer annular component 119 exhibits an elastic, load-rate independent response. As such, the first annular spring element 118 as a whole exhibits a combination of an elastic, load-rate-independent response, and a load-rate dependent response. Similarly, a second annular spring element 122 is formed of a second inner annular component 121 and a second outer annular component 123. The inner annular component 121 exhibits a load-rate dependent response and the outer annular component 123 exhibits an elastic, load-rate independent response. As such, the second annular spring element 122 as a whole exhibits a combination of an elastic, load-rate-independent response, and a load-rate dependent response. In this embodiment, each of the first and second annular spring elements 118, 122 comprise an elastic, load-rate-independent inner annular component concentric with a load-rate dependent outer annular component. However, in an alternative embodiment, the inner annular component may exhibit an elastic, load-rate independent response and the outer annular component may exhibit load-rate dependent response. In yet another embodiment, each of the first and second annular spring components 118, 122 could be unitary components which exhibit a combination of an elastic, load-rateindependent response and a load-rate dependent response, for example a unitary component formed from a suitable polymeric material or a suitable D30® material.

The first and second annular spring elements 118, 122 in Figure 3 have an outer diameter of about 20 mm and a thickness of about 10 mm when the biasing means is relaxed.

The inner annular components 117, 121 of the annular spring elements 118, 122 are made of a material such as microcellular polyurethane that exhibits a substantially elastic, load-rate-independent response. The outer annular components 119, 121 of the annular spring elements 118, 122 are made of a material such as a material produced by D3O®, and exhibit a load-rate dependent response. In this embodiment, the outer annular components 119, 121 are less stiff than the inner annular components 117, 121 at low loading rates and are more stiff than the inner annular components 117, 121 at high loading rates. A number of suitable materials for the inner annular components 117, 121 are produced by D30®. At low loading rates, the annular springs 118, 122 exhibit a substantially elastic response and at higher loading rates, the annular springs 118, 122 exhibit a load-rate dependent response. Specifically, the annular springs 118, 122 have a higher stiffness at higher loading rates because the outer annular components 119, 123 have a higher stiffness at higher loading rates. The annular springs 118, 122 exhibit a substantially elastic response up to a certain, predetermined, load-rate and, at load-rates above this predetermined load-rate, the stiffness of the annular springs 118, 122 could vary. For example, above this given load-rate, the stiffness could increase approximately linearly, or exponentially, with load-rate. Alternatively, the stiffness of the annular springs 118, 122 could vary at low load-rates but by insignificant, or negligible, amounts.

The lower nut 114 may be tightened so that the biasing means 105 is under compression. The more the lower nut 114 is tightened at this stage, the stiffer the biasing element 100 will be in its first position, as shown in Figure 3. In this context, tightening the lower nut 114 means threading the lower nut 114 further up the lower threaded end of the steel rod 112.

Figure 4 is a cross-sectional view of a biasing element according to the invention in a second, loaded, position.

In order to reach the second position of the biasing element, the top nut 113, steel rod 112, and lower nut 114 have travelled upwards and compressed the biasing means 105 against the second location member 106. This movement is illustrated by the position of the dashed line 126 marking the position of the lowermost point of the steel rod in the first position of the biasing element 100. The location of the first steel washer 116 has not changed. Thus, the distance travelled by the lower nut 114 has been accommodated by further compression of the biasing means 105. That is, the annular spring elements, or annular springs, 118, 122 are more compressed in the second position of the biasing element 100 than the first position of the biasing element 100.

At lower loading rates, the annular springs 118, 122 exhibit a substantially elastic response, but at higher loading rates, the annular springs 118, 122 stiffen significantly. Thus, the greater the loading rate on the washers (or the quicker the steel rod 112 attempts to move upwards), the greater the effective stiffness of the annular springs 118, 122 (and the more the movement is resisted).

Whilst Figures 3 and 4 illustrate how an upwards movement of the steel rod 112 may be resisted by compressive forces in the biasing means, it is clear that the biasing element 100 could function in a number of other ways. For example, downwards motion of the steel rod 112 from the first position of the biasing element 100 could be resisted by tensile forces in the biasing means.

Figure 5 is a cross-sectional view of a first polycentric, prosthetic knee joint when the joint is fully extended. This knee joint includes a non-rate-responsive biasing element and is described to illustrate the function of a biasing element in such a knee joint.

The joint 310 comprises an upper leg portion 312 connected to a lower leg portion 314. The upper leg portion 312 comprises a pyramid fitting 313. This pyramid fitting 313 is a standard fitting which is connectable to a standard socket attachable to an amputee's residual limb. The lower leg portion 314 of the joint 310 comprises a steel shin tube 315 defining an internal channel 320. A steel rod 318 extends longitudinally within the channel 320, and a helical compression spring 316 is positioned around the rod 318 and within the channel 320. An upper end of the spring 316 abuts a surface at the upper end of the tube 315. A lower end of the spring 316 is coupled to the steel rod 318 via a washer 319 and a nut 321 threaded onto the steel rod 318. As such, longitudinal movement of the steel rod 318 downwards relative to the tube 315 causes the spring 316 to extend, and longitudinal movement of the steel rod 318 upwards relative to the tube 315 causes the spring 316 to compress. The spring 316, as shown in Figure 5, is put under compression using the nut 321 so that the spring 316 exerts a force on the steel rod 318 which is acting to move the steel rod 318 downwards. The steel rod 318 is restricted to movement in the longitudinal direction -the direction of the steel shin tube 315. Effectively, the steel rod 318, the washer 319, the nut 321, and the helical spring 316 form a non-rate-responsive biasing element that can act to resist flexion and assist extension of the knee joint.

A first connector 322 of the joint 310 is coupled to a first pivot point 324 and a second pivot point 325. The upper leg portion 312 is coupled to the first pivot point 324 and the lower leg portion 314 is coupled to the second pivot point 325. A second connector 327 of the joint 310 is coupled to a third pivot point 329 and a fourth pivot point 331. The upper leg portion 312 is coupled to the third pivot point 329 and the lower leg portion 314 is coupled to the fourth pivot point 331. The first connector 322 and second connector 327 are steel plates. The four pivot points are part of a four-bar linkage system. The skilled person would be aware of the dynamics of four-bar linkage systems. From the fully extended position of the joint in figure 5, the first connector 322 is able to pivot anticlockwise about the first pivot point 324 as the lower leg portion 314 simultaneously pivots clockwise about the second pivot point 325. At the same time, the second connector 327 is able to pivot anticlockwise about the third pivot point 329 as the lower leg portion 314 simultaneously pivots clockwise about the fourth pivot point 331.

A section 326 of the connector 322 engages the steel rod 318 via a further pivot point and is shaped such that, from the fully extend position of the joint shown in Figure 5, as the lower leg portion 314 pivots clockwise about the second pivot point 325, the connector 322 causes the steel rod 318 to move longitudinally upwards in the channel 320. This further compresses the helical compression spring 316. So, flexion of the knee joint, from a fully extended position, resulting in upwards movement of the steel rod 318, is resisted by the compression spring 316.

When the knee joint 310 is fully extended, as shown in Figure 5, the first pivot point 324 is positioned closer to the back of the knee (the right-hand-side of the page) than the second pivot point 325. As such, the amputee's weight, acting through the first pivot point 35 324 and the second pivot point 325, is acting to further extend the knee joint 310. This is prevented by a steel stop 328. As such, the fully extended position of the joint 310 is a relatively stable position.

However, if the knee joint 310 is flexed such that the first pivot point 324 is positioned further from the back of the knee than the second pivot point 325, then a force 5 acting upwards through the second pivot point 325, such as the weight of an amputee, will act to flex the knee joint 310. In practice, this means that, if an amputee puts their weight through the prosthetic knee joint when the joint is bent, the only force provided by the prosthetic knee joint 310 to prevent flexion is a resistance force from the spring 316. However, any force provided by the spring 316 is likely to be significantly smaller than an amputee's weight. As such, the knee joint 310 is likely to quickly flex, or collapse, under the amputee's weight.

Figure 6 is a cross-sectional view of the first polycentric, prosthetic knee joint shown in Figure 5 when the joint is flexed by around 30 degrees.

It can be seen in Figure 6 that the steel rod 318 has moved towards the upper end of the steel shin tube 315. Flexion from the fully extended position of Figure 3 to the position of Figure 4 has been resisted by the spring 316, and the resistance provided by the spring 316 has increased as the joint has flexed more. The position shown in Figure 6 shows the position of the joint in which the spring 316 is compressed the most. The position of the joint at which the spring is compressed the most is determined by the design prosthetic knee joint, specifically the design of the cam action of the mechanism for flexing the joint. Beyond this point at around 30 degrees of flexion, the shape of the section 326 is such that the steel rod 318 travels back towards the lower end of the steel shin tube 315 as the knee joint flexes even further.

Figure 7 is a cross-sectional view of the first polycentric, prosthetic knee joint shown 25 in Figure 5 when the joint is flexed by around 135 degrees.

It can be seen in Figure 7 that, from the position shown in Figure 6, the steel rod 318 has moved back towards the lower end of the steel shin tube 315. Since the helical compression spring 316 is always under compression, this further flexion from the position shown in Figure 6 to the position shown in Figure 7 is assisted by the spring 316.

It is clear that the prosthetic knee joint 310 could function with a tension spring instead of a compression spring. For example, a tension spring could be located below the washer 319. In such an embodiment, an upper end of the tension spring would be coupled to the steel rod 318 via the washer 319 and nut 321, and a lower end of the tension spring would be fixed relative to the steel shin tube 315, for example fixed at a lower end of the steel shin tube 315. In this embodiment, motion of the steel rod 318 upwards would cause the tension spring to extend further so would be resisted by the spring.

The prosthetic knee joint 310 shown in Figures 5, 6, and 7 is mechanically simple and relatively cheap to manufacture. However, one potential problem with this prosthetic 5 knee joint is that, if a user, for example an amputee wearing a prosthetic leg incorporating the knee joint, were to stumble and put their weight through the prosthetic joint when the joint is bent, the only force provided by the prosthetic knee joint 310 to prevent flexion would be from the spring 316. However, the force provided by the spring 316 is likely to be significantly smaller than an amputee's weight. As such, the knee joint 310 is likely to 10 quickly flex, or collapse, under the amputee's weight.

The second prosthetic knee joint 610 is similar to the first prosthetic knee joint 310. The spring 316 of the first prosthetic knee joint 310 has been replaced by a rate responsive biasing element, substantially as described above in relation to figures 3 and 4. The same reference numbers have been given for elements of the knee joint in figure 8 that are the same as those described in figures 3 and 4.

Thus, the second knee joint 610 comprises a steel shin tube 615, a steel rod (or longitudinal connection member) 112, and a nut (or first location member) 114 threaded onto the lower end of the steel rod 112. The joint 610 further comprises a biasing means located around the steel rod 112. The biasing means has a first steel washer 116, a first annular spring 118, a second steel washer 120, a annular spring 122, and a third steel washer 124. In some embodiments, the biasing member may include further washers and annular springs. The annular springs 118, 122 have an outer diameter of about 20 mm and a thickness of about 10 mm when the biasing means is relaxed. The annular springs 118, 122 exhibit a combination of an elastic, load-rate-independent response and a load-rate dependent response.

An upper surface of the first steel washer 116 abuts a surface at the upper end of the steel shin tube 615, which acts as a second location member 106. Thus, the first steel 30 washer 116 cannot move upwards relative to the steel shin tube 615.

As the joint 610 flexes from a fully extended, or straight, position like that shown in Figure 8, to the position shown in Figure 9, the steel rod 112 moves upwards relative to the 35 steel shin tube 615. The mechanism causing this movement is identical to that described in relation to the first prosthetic knee joint shown in Figures 5, 6 and 7. Movement of the steel rod 112 upwards brings the nut 114 closer to the first steel washer 116, thus compressing the biasing means. The position shown in Figure 9 is the position in which the steel rod 112 is at its highest point, and the stack of washers is at its point of maximum compression, during flexion of the joint.

As the joint 610 flexes from the position shown in Figure 9, where the biasing means is at its point of maximum compression, to the position shown in Figure 10, the steel rod 112 moves downwards relative to the steel shin tube 615. The mechanism causing this movement is identical to that described in relation to the first prosthetic knee joint shown in Figures 5, 6 and 7. Movement of the steel rod 112 downwards moves the nut 114 further from the first steel washer 116, thus extending the stack of washers. It is worth noting that the stack of washers are always under compression due to the tightening (i.e. threading further upwards) of the nut 114 before use of the joint 610.

Thus, if an amputee applies all, or most, of their weight through the joint 610 in short period of time, for example if the amputee stumbles, then the annular springs 118, 122 are subjected to a temporary high rate of loading. Thus, the annular springs temporarily stiffen. This temporary increased stiffness reduces the movement of the knee during the stumble by resisting motion of the steel rod 618. This may allow the amputee time to regain their balance. This stumble could occur before or after a point of maximum compression of the stack of washers. If the amputee stumbles during flexion from the fully extended or straight position shown in Figure 8 to the 30 degree flexion shown in Figure 9, then the stumble (acting to further flex the knee) will act to quickly compress the stack of washers. On the other hand, if such a stumble occurred past the point of maximum compression of the stack of washers (in other words, during flexion from the 30 degree flexion shown in Figure 9 to the 135 degree flexion shown in Figure 10), then the stumble (acting to further flex the knee) will act to quickly extend the stack of washers. Both of these motions would result in a high rate of change of load in the stack of washers, causing the annular springs 118, 122 to stiffen and 'break the fall'.

Figure 11 is a front view and cross-sectional view of a third polycentric, prosthetic knee joint when the joint is fully extended.

The third joint 910 is identical to the second joint 610, except the biasing means 901 is no longer comprises a stack of washers and annular springs. The biasing means 901 in 35 this third embodiment of a knee joint comprises a spring component 912 located between a lower washer 914 and an upper washer 916. The spring component 912 comprises an elastomeric toroid 918 encapsulating a non-Newtonian fluid 920.

The elastomeric toroid 918 could be made of any suitable elastomer, such as an elastomeric polymer. The non-Newtonian fluid 920 could comprise, or be, any suitable shear thickening fluid.

Similarly to the first and second joints, before use, a nut (or first location member) 922 may be adjusted to ensure that the component 912 is under compression at all points of flexion of the joint 910.

Operation of the joint is substantially as described above in relation to figures 5, 6, and 7. Compression of the biasing means 901 elastically deforms the elastomeric toroid 918, which exerts a corresponding biasing force. Deformation of the elastomeric toroid 918 causes the non-Newtonian fluid 920 encapsulated within the toroid 918 to move, or flow, and a shear stress is applied to the fluid 920. This applied shear stress causes shear strain in the fluid. In other words, flexion of the joint causes a change in shape of the toroid 918, and the change in shape of the toroid 918 creates shear strain in the fluid 920.

At low load-rates, the shape of the elastomeric toroid changes slowly, thus causing a slow change in shear stress and a slow change in shear strain in the fluid 920. However, at high load-rates, the shape of the elastomeric toroid 918 changes quickly, thus causing a quick change in shear stress and a quick change in shear strain in the fluid 920. In other words, a high load-rate causes a high rate of change of shear strain, or a high shear rate, in the fluid 920.

Since the fluid 920 is a shear thickening fluid, its viscosity increases with shear rate. So a high-load rate applied through the joint causes the viscosity of the fluid 920 to increase. This increased viscosity of the fluid causes the fluid to have an increased resistance to flow. Since the fluid 920 needs to flow in order for the toroid 918 to change shape, and therefore for the joint 910 to flex or straighten, the increased viscosity effectively increases the resistance to flexing or straightening the joint. Thus, a high load-rate applied to the joint 910 results in a higher resistance to flexion or extension of the joint 910 due to the resulting, temporary, increased viscosity of the fluid 920.

Thus, at low load-rates, such as during walking as the joint 910 flexes from a fully straight position to, say, 15 degrees flexion, the elastomeric toroid 918 compresses slowly. The fluid 920 therefore has a relatively low viscosity and the compression, or change in shape, of the toroid is not significantly resisted by the fluid 920. Thus, the response of the component 912 at low load-rates is dominated by the response of the elastomeric toroid.

The elastomeric toroid 918 exhibits a substantially elastic response at these low load-rates.

At high load-rates, for example when the joint is flexed by 10 degrees and an amputee suddenly applies all of their weight through the joint, the joint cannot flex quickly over a relatively large range of motion. That is, the joint cannot collapse. This is because, as explained above, this high load-rate increases the viscosity of the fluid 920 and thus increases the resistance of the joint 910 to flexion.

Figure 12 is a cross-sectional view of a fourth polycentric, prosthetic knee joint when the joint is fully extended.

The fourth joint 1010 is identical to the second joint 610, except the biasing means comprises a closed cell foam 1012 encapsulating droplets of a shear thickening fluid. The closed cell foam 1012 is in the shape of a truncated cone. The fourth joint 1010 works in an identical manner to the third joint 910. That is, the foam of the fourth joint 1010 compresses or extends and changes shape in an identical manner to the elastomeric toroid 918 of the third joint 910, and the droplets of shear thickening fluid in the fourth joint 1010 behave in an identical manner to the shear thickening fluid 920 of the third joint 910.

Claims

Claims 1. A rate-responsive biasing element comprising: a longitudinal connection member, a first location member coupled to the longitudinal connection member, and a biasing means arranged to provide a biasing force between the first location member and a second location member when the biasing means is elastically deformed by a biasing load, wherein the biasing means comprises a rate-independent component and a rate-dependent component arranged in parallel.
2. A rate-responsive biasing element according to claim 1 in which the biasing means is located between the first location member and the second location member.
3. A rate-responsive biasing element according to claim 1, or 2, in which the biasing load is applied to the biasing means via longitudinal movement of the longitudinal connection member in a first longitudinal direction and the biasing force acts to urge the longitudinal connection member in a second longitudinal direction opposite to the first longitudinal direction.
4. A rate responsive biasing element according to any preceding claim, in which elastic deformation of the rate-independent component is proportional to the applied biasing load and independent of load rate, and deformation of the rate-dependent 20 component is variable depending on the load rate of the applied biasing load.
5. A rate responsive biasing element according to any preceding claim, configured such that relative movement between the first location member and the second location member at a rate of movement that is lower than a predetermined rate of movement results in extension or compression of the biasing means, and relative movement between the first location member and the second location member at a rate of movement that is greater than the predetermined rate of movement causes the rate-dependent component of the biasing means to resist the extension or compression of the biasing means.
6. A rate responsive biasing element according to any preceding claim in which the longitudinal connection member is fixedly coupled to the first location member and movably coupled to the second location member such that longitudinal movement of the longitudinal connection member results in relative movement between the first location member and the second location member.
7. A rate responsive biasing element according to any preceding claim, in which the rate-independent component comprises a spring such as a helical tension spring or a helical compression spring, a gas strut, a domed washer, or a substantially elastic material such as a polymeric spring or a series of polymeric springs.
8. A rate responsive biasing element according to any preceding claim, in which the rate-dependent component comprises a non-Newtonian material, for example a non-5 Newtonian fluid, preferably a shear-thickening fluid.
9. A rate responsive biasing element according to any preceding claim, in which the biasing means defines a through-hole, slot, or channel and the longitudinal connection member extends through the through-hole, slot, or channel.
10. A rate responsive biasing element according to any preceding claim, in which the biasing means comprises a first annular member forming the rate-independent component and a second annular member forming the rate-dependent component, the first annular member and the second annular member being disposed concentrically relative to each other.
11. A rate responsive biasing element according to any preceding claim, in which the 15 rate-independent component of the biasing means partially or entirely encapsulates the rate-dependent component of the biasing means.
12. A rate responsive biasing element according to claim 11 in which the rate-independent component comprises a resilient material and the resilient material defines one or more internal cavities containing at least a portion of the rate-dependent component.
13. A rate responsive biasing element according to claim 12 in which the rate independent component comprises at least one substantially toroidal member formed from the resilient material and the rate-dependent component is a non-Newtonian liquid contained within the at least one substantially toroidal member.
14. A rate responsive biasing element according to any preceding claim, in which the biasing means comprises a longitudinally extending stack of separate biasing members, each separate biasing member comprising rate-independent component and a rate dependent component arranged to act in parallel, preferably in which each separate biasing member is separated by a sheet or plate of polymeric, elastic or metallic material.
15. A rate responsive biasing element according to any preceding claim, in which the biasing means comprises a first, load-rate dependent, portion comprising the rate-independent component and the rate-dependent component which are arranged to act in parallel, and a second, elastic, portion comprising a resilient means, the second portion being arranged in series with the first dependent portion such that there is always a biasing response generated by the second portion irrespective of the rate at which the biasing element is loaded.
16. A rate responsive biasing element according to claim 15, in which the resilient means of the second portion comprises a helical spring.
17. A prosthetic knee joint comprising an upper leg portion pivotably connected to a lower leg portion, flexion of the prosthetic knee joint being assisted or resisted by a biasing element, in which the biasing element comprises a biasing means having an rate-independent component and a rate-dependent component arranged to act in parallel.
18. A prosthetic knee joint according to claim 17 in which the biasing element is a rate 10 responsive biasing element as defined in any of claims 1 to 16.
19. A prosthetic knee joint according to claim 18 in which the rate responsive biasing element is located within a cavity defined by the lower leg portion and the longitudinal connection member is arranged to move longitudinally when the joint is flexed or extended.
20. A prosthetic knee joint according to claim 18 or 19 configured such that flexion of the knee joint results in longitudinal movement of the longitudinal connection member and compression of the biasing means, preferably in which the prosthetic knee joint is configured such that flexion of the knee joint at a rate of movement that is greater than a predetermined rate of movement causes the rate-dependent component of the biasing element to stiffen, thereby resisting flexion of the knee joint.