Abstract:
A prosthetic knee joint mechanism has an hydraulic load-activated knee-stabilizing device for resisting joint flexion. A rotary piston ( 34 ) connected to one part of the mechanism is rotatable within a fluid-filled fluid displacement chamber ( 18 ) associated with another part of the mechanism to drive fluid through a fluid passage which contains a main valve ( 38 ) which restricts or allows joint flexion according to the position of a valve member within the valve. The valve member is movable towards an open position in response to fluid pressure in the fluid passage upstream of the valve member caused by application of a flexion torque to the knee joint mechanism, movement of the valve member in the direction of the open position being at least resisted by a valve control arrangement actuated by application of the wearer&#39;s weight. The weight-responsive valve control arrangement is preferably a weight-responsive pilot valve ( 32 ) for hydraulically resisting movement of the valve member of the main valve.

Description:
BACKGROUND OF THE INVENTION 
     This invention relates to a prosthetic knee joint mechanism which includes a load-activated knee-stabilising device for restricting joint flexion. 
     It has long been known to include a stabilised knee as part of a prosthetic leg to achieve a natural looking gait, that is for the knee joint to resist flexion when under the load of part or all of the amputee&#39;s weight. Mechanical friction devices and hydraulic devices have been developed. In a known hydraulic stabilised knee joint mechanism, disclosed in Canadian Patent No. 2,134,999, resistance to flexion during the stance phase of the walking cycle and to extension during the swing phase is provided by restricting the movement of fluid between opposite sides of a rotary piston in a chamber filled with hydraulic fluid. In this mechanism the knee is locked by closing a fluid line by a valve interconnecting chamber parts on opposite sides of the piston using a valve member which moves when the application of the amputee&#39;s weight causes two resiliently connected parts of the mechanism to move relative to each other. The movement required to close the valve so to lock the knee is significant and creates a period of instability until the knee is locked. Also, overloading may damage the valve arrangement as it is directly operated. 
     BRIEF SUMMARY OF THE INVENTION 
     According to the first aspect of this invention, a prosthetic knee joint mechanism comprises first and second knee parts which are rotatable relative to each other in joint flexion and joint extension, and a load-activated knee-stabilising device for resisting joint flexion, the stabilising device comprising means defining a fluid-filled displacement chamber associated with the first knee part and a piston which is connected to the second knee part so as to be driven by rotation of the second knee part relative to the first knee part and which is so arranged within the chamber that it divides the chamber into first and second variable volume chamber parts which are interconnected by a fluid passage, the stabilising device further comprising a valve associated with the fluid passage and including a valve member which is movable between an open position in which fluid can flow through the passage to allow joint flexion and a stabilising position in which such fluid flow is at least restricted, wherein the valve member is movable towards its open position in response to fluid pressure in the interconnecting passage upstream of the valve member caused by application of a flexion torque to the knee joint mechanism, and wherein the stabilising device includes a weight-responsive valve control arrangement to at least resist movement of the valve member in the direction of its open position. 
     A feature of a preferred embodiment of the invention is its compactness, having the displacement chamber housed in the first knee part and being centred on an axis of relative rotation of the knee parts, the piston being in the form of a rotary piston which rotates with the second knee part. In the preferred embodiment, the valve comprises a main valve in which the valve member is movable in a fluid-filled valve cavity. The control arrangement for this valve comprises a weight-responsive pilot valve located in a secondary fluid passage which communicates with the above-mentioned valve cavity for hydraulically resisting or preventing movement of the valve member of the main valve in the direction of its open position. In fact, the main valve may be constructed as a shuttle valve having at least three ports which include an upstream port communicating with one of the variable volume chamber parts, a downstream port communicating with the other variable volume chamber part, and a control port. The upstream port opens into the valve cavity on one side of the valve member. The downstream port is located in the wall of the cavity so that it is fully or partially covered by the valve member in the stabilising position and is in communication with the upstream port when the valve member is in its open position. The control port opens into the valve cavity on the other side of the valve member from the upstream port and also forms part of the secondary passage. 
     In the preferred embodiment, the main valve has a bleed passage which interconnects the portions of the main valve cavity on opposite sides of the valve member; i.e. it effectively interconnects the upstream port and the control port. This bleed passage is conveniently located in the valve member of the main valve. The mechanism can be so arranged that the secondary fluid passage provides communication between the valve cavity of the main valve and the part of the fluid displacement chamber which increases in volume with joint flexion. 
     The main valve member is preferably resiliently biased towards its stabilising position. The yield of the knee joint may be adjusted by including a yield adjuster which forms an adjustable stop defining the stabilising position of the valve member. This may take the form of a needle stop which is arranged partially to close the bleed passage when the valve member is in its stabilising position. The variable yield under load allows a suitable setting to be found to enable the amputee to descend stairs leg-over-leg. 
     In a preferred embodiment of the invention the first knee part is configured as either a shin or thigh associated component and is divided into a pair of resiliently interconnected portions. One of these portions contains the fluid displacement chamber and the interconnected elements are arranged to execute a weight-responsive relative movement. The pilot valve is arranged in the first knee part so that it opens and closes in response to the relative movement of the resiliently connected portions of the knee mechanism, in particular closing to prevent movement of the shuttle valve member towards its open position during flexion, preferably by removing the force causing such movement during flexion. When weight is applied to the knee joint, the two elements of the first part of the knee mechanism move relative to each other, and the pilot valve responds. As a result, pressure in the secondary fluid passage changes so as to restrain the main valve member from moving to its open position when a flexion moment is applied. The primary passage between the two parts of the chamber containing the piston is restricted by the main valve member, restricting the movement of the piston. Hence, the movement of the two parts of the knee relative to each other is also restricted and the knee is stabilised. 
     According to another aspect of the invention, a prosthetic knee joint mechanism comprises a rotary piston in a fluid-filled displacement chamber, the mechanism being arranged such that the piston and the chamber resist joint flexion in response to weight activation, wherein the mechanism includes: a valve associated with a fluid passage interconnecting parts of the chamber on opposite sides of the piston, the valve having a valve member movable in response to upstream fluid pressure in the passage from a stabilising position, in which fluid flow in the passage is restricted, to an open position, in which fluid is allowed to flow in the passage more freely; and a weight-responsive valve control arrangement to resist movement of the valve member in the direction of its open position thereby to cause the mechanism to resist flexion. 
     The invention also includes a prosthetic knee joint mechanism comprising first and second knee parts which are rotatable relative to each other about an axis of rotation in joint flexion and joint extension, and a load-activated knee-stabilising device for resisting joint flexion, the stabilising device comprising means defining a fluid-filled displacement chamber associated with the first knee part and a piston which is centred on the knee axis of rotation and connected to the second knee part so as to be driven by relative rotation between the knee parts, and which is so arranged within the chamber that it divides the chamber into first and second variable volume chamber parts which are interconnected by a fluid passage, the stabilising device further comprising a valve associated with the fluid passage and including a valve member which is movable between an open position, in which fluid can flow through the passage to allow joint flexion, and a stabilising position, in which such fluid flow is at least restricted, wherein the valve member is movable towards its open position in response to a differential fluid pressure on opposite sides of the valve member caused by application of a flexion torque to the knee joint mechanism, and wherein the stabilising device includes a valve control arrangement substantially to eliminate the said differential pressure when the joint mechanism is loaded. 
     According to a further aspect of the invention, there is provided a lower limb prosthesis including a knee joint mechanism as set out above. One of the knee parts of the mechanism is preferably associated with or constituted by a shin component of the prosthesis and the other of the knee parts is associated with or constituted by the thigh component of the prosthesis. In particular, the said first and second knee parts may be associated with or constituted by the shin component and the thigh component respectively. Typically, but not necessarily, the axis of relative rotation of the first and second knee parts is the knee axis of rotation of the prosthesis. 
     An advantage of the mechanism described in this specification is that it is pilot operated in that a pilot or control arrangement controls the movement of a main valve member, hence the movement required to lock and release the knee is reduced. A near instant response is observed, providing a more stable knee. Another advantage is that it is adjustable in both the point at which stabilisation occurs and the extent of the lock stabilisation, i.e. in terms of locking the knee or allowing degrees of yield. The degree of yield may be altered to allow the amputee to descend stairs leg-over-leg or to descend steep slopes. Yet another advantage is that when unloaded the pilot arrangement, specifically the pilot valve in the preferred embodiment, is in contact with its controlling element, and that in weight activation such contact no longer exists, or the contact force decreases, so that overloading will not damage the pilot arrangement. In the unlocked position, free swing of the shin component is allowed in the flexion direction. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will now be described below by way of example with reference to the drawings, in to which: 
         FIG. 1  is a cross-section of a knee-mechanism in accordance with the invention, the cross-section being in a central anterior-posterior plane; 
         FIG. 2  is a cross-section of the mechanism in a generally vertical medial-lateral plane through the knee axis of rotation, the plane being indicated by the line  2 - 2  in  FIG. 1 ; 
         FIG. 3  is a cross-section of the mechanism taken in the inclined medial-lateral plane  3 - 3  in  FIG. 1 ; 
         FIG. 4  is a second anterior-posterior cross-section of the mechanism in the plane indicated by  4 - 4  in  FIG. 3 ; 
         FIG. 5  is a schematic diagram of an hydraulic system of the mechanism showing primary and secondary fluid passages; 
         FIGS. 6A and 6B  are hydraulic circuit diagrams of the mechanism; 
         FIG. 7  is a cross-section of an alternative knee mechanism in accordance with the invention, the cross-section being in a central anterior-posterior plane; and 
         FIG. 8  is a diagrammatic cross-sectional detail of part of the mechanism of  FIG. 7 , the cross-section being in a medial lateral plane. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring to  FIGS. 1 to 4 , a prosthetic knee joint mechanism in accordance with the invention has an upper part  10  associated with a thigh component (not shown) of a limb prosthesis and a lower part  12  including the upper section of a shin component  14  of the prosthesis. The two joint mechanism parts, upper and lower, are pivotally interconnected, relative rotation occurring about a knee axis  16 . The upper joint part  10  has a chassis  10 A for receiving an alignment coupling (not shown), and associated medial and lateral flanges  10 B which carry a axle  10 C defining the knee axis (see  FIG. 2 ). The axle is non-rotatably secured to the flanges  10 B. The chassis  10 A has posterior bushes  10 AA for pivotal connection of a swing phase control unit (not shown). This unit typically takes the form of a pneumatic piston and cylinder assembly connected at one end to a pin housed in the bushes  10 AA and at its other end to the shin component  14   
     The lower part  12  of the joint mechanism is in two main portions. One of the portions comprises a housing  12 A containing an hydraulic chamber  18 . Housing  12 A has side plates  12 AA ( FIG. 2 ) and rotates on bearings  13  on the axle  10 C. Housing  12 A is also resiliently and pivotally connected to the upper section of the shin component  14  by a spindle  20  housed in medial and lateral side walls  14 A of the shin component  14  (see  FIG. 3 ). Spindle  20  defines a weight-sensing pivot axis  22  spaced from the knee axis of rotation  16  in the anterior-posterior direction. In this embodiment, the weight-sensing axis  22  is on the anterior side of the knee axis  16 . Limited relative rotation of the housing  12 A in the shin component  14  is governed by a resilient interconnection between the housing  12 A and the shin component  14  in the form of an anteriorly extending plunger  24  pivotally mounted on a downwardly depending flange  12 AB of the housing  12 A, and slidably received in a bush  26  which is threaded in an anterior wall  14 B of the shin component  14  (see  FIG. 1 ) so as to be adjustable in position. Located between a posterior flange  24 A on the plunger  24  and the posterior face of the adjustable bush  26  is a weight-sensing spring  28 , here in the form of a stack of conical spring washers encircling plunger  24 . 
     Referring to  FIG. 4 , clockwise rotation of the housing  12 A relative to the shin component  14  about the weight-sensing axis  22  causes abutment of a sensitivity adjusting element in the form of a grub screw  30  threaded in the anterior wall  14 B of the shin component  14  against a control element in the housing  12 A. This control element takes the form of a button  32 B constituting an exposed anterior end of a pilot valve member  32 A slidably housed in a pilot valve cavity to form a pilot valve  32 , which will be described in more detail below. Pilot valve member  32 A is resiliently outwardly biased by an internal valve closure spring  32 C towards a position in which the valve is closed. The maximum extent of rotation of the shin component  14  about axis  22  in the direction of knee extension is limited by a final stop (not shown) on the housing  12 A. 
     The hydraulic chamber  18  has a cross-section in the form of a sector of a circle centred on the knee axis  16 . Housed sealingly within the chamber  18  is a rotary piston in the form of a vane  34  which is rotationally fixed with respect to axle  10 C. Indeed, in this embodiment vane  34  is integral with axle  10 C. When the knee mechanism is in the fully extended state, i.e. corresponding to full extension of the knee, the vane  34  is near its clockwise limit of rotation in the chamber  18 , as it appears in  FIGS. 1 and 4 . As the knee is flexed, the vane sweeps around the chamber  18 , displacing hydraulic fluid in a manner to be described below. 
     Weight-sensing occurs as a result of relative movement of the housing  12 A and the shin component  14  about weight-sensing axis  22 . Depending on the position of the ground reaction vector from the prosthetic foot (not shown) relative to the weight-sensing axis  22 , application of the amputee&#39;s weight to the prosthesis gives rise to an anticlockwise moment on the housing  12 A as viewed in  FIGS. 1 and 4 , tending to compress the spring  28 , the plunger  24  moving anteriorly in bush  26 . As a result, the housing  12 A in the region of the pilot valve button  32 B moves away from the grub screw element  30  in the anterior wall  14 B of the shin component  14 , and the pilot valve member  32 A slides in its cavity in the housing  12 A under the influence of the pilot valve closure spring  32 C, thereby tending to close the pilot valve  32 . Removal of the weight-responsive moment on the housing  12 A allows the housing  12 A to return to its unloaded position owing to the biasing force applied by spring  28 . 
     In this embodiment of the invention, the pilot valve  32  has a central axis lying in a normally vertical anterior-posterior plane. Also housed in housing  12 A is a main valve  38  which forms part of a primary passage interconnecting the two variable volume parts of the hydraulic chamber  18  which are separated from each other by the vane  34 . This main valve  38  has an axis running in the medial-lateral direction. The two variable volume parts of the chamber  18  are also interconnected by a non-return valve  40  in the housing  12 A (see  FIG. 1 ). 
     The manner in which the pilot valve is activated by a knee flexion moment is adjustable. On the one hand, the bush  26 , which forms a stop for the spring  28 , is threaded in anterior wall  14 B of the shin component  14  so that preloading of the spring  28  when no load is applied can be adjusted. This means that the stiffness of the weight-responsive resilient movement of the housing  12 A relative to the shin component  14  can be adjusted depending on the weight of the amputee and his or her gait characteristics. This manifests itself as resilience tending to bias the joint towards full extension. On the other hand, sensitivity of operation of the pilot valve  32  is adjusted by screwing grub screw element  30  in or out. In effect, the adjustment of the grub screw  30  alters the amount of weight-responsive deflection of the housing  12 A relative to the shin component  14  required to close the pilot valve  32 , and also the point at which the pilot valve opens as weight is removed. Accordingly, the screw element  30  adjusts the timing of the locking and release of the knee mechanism, as will be appreciated from the description which follows. 
     Details of the main valve  38 , the pilot valve  32  and their interaction will now be described with reference to  FIGS. 5 ,  6 A and  6 B. 
     Referring to  FIG. 5 , the main valve  38  is a shuttle valve having an upstream port  38 A opening into one end of a valve cavity  38 B, and a downstream port  38 C in a sidewall formed, in this case, by a honed sleeve  38 D. Cavity  38 B extends axially of the valve to a control port  38 E at the opposite end of the cavity from the upstream port  38 A. Housed in the cavity  38 B is a cylindrical shuttle valve member  38 F which slides in a sealed manner in the sleeve  38 D and is biased by an internal spring  38 G against a shoulder in the cavity  38 B. It will be seen that the shuttle member divides the cavity  38 B into two portions, one communicating with the upstream port  38 A and the other communicating with the control port  38 E. These two cavity parts are interconnected by a bleed passage  38 H which, in this case, is an orifice formed by a narrow axial bore  38 H in the shuttle valve member  38 F. A threaded plug  38 I closes off the valve cavity  38 B. 
     Shuttle valve  38  lies in a primary passage  42  in the housing  12 A (see  FIG. 1 ) interconnecting the variable volume parts of hydraulic chamber  18 . The upstream port is “upstream” in the sense that it is upstream when the knee joint flexes. 
     The pilot valve  32  has already been briefly described with reference to  FIG. 4 . As will be seen from  FIG. 5 , the pilot valve member has four main parts as follows. Firstly the valve member  32 A has a wide cylindrical body portion  32 AA housed in a first cylindrical bore in the housing  12 A. Adjacent body portion  32 AA is a flange portion  32 AB having an outwardly directed sealing wall with an annular sealing ring  32 AC lying in a plane perpendicular to the pilot valve axis. Projecting axially from this axially directed sealing face is a narrow stem  32 AD carrying at its end the pilot valve button  32 B referred to above. The material of the housing  12 A surrounding the flange  32 AB and the stem  32 AD is bored to provide an annular pilot valve cavity spaced laterally of both such portions  32 AB and  32 AD, and an annular shoulder against which the sealing ring  32 AC abuts when the valve member  32 A is allowed to move outwardly under the influence of closure spring  32 C (when weight is allied to the knee mechanism as described above with reference to  FIGS. 1 and 4 ). 
     The pilot valve forms part of a secondary passage between the control port  38 E of the main valve  38  and the downstream port  38 C of the main valve  38 , a bore  44  being formed in the body of the housing  12 A to interconnect the main valve control port  38 E with the pilot valve cavity  32 D on one side of the sealing member  32 AC, and another bore  46  opening into the pilot valve cavity  32 D adjacent the stem  32 AD, i.e. on the opposite side of the sealing member  32 AC. 
     The disposition of the main valve  38  and pilot valve  32  in the hydraulic circuit of the knee joint mechanism is more clearly shown in  FIGS. 6A and 6B . Bleed passage  38 H appears as a restricted flow passage between the upstream side of the main valve  38  and the secondary passage  44 ,  46  on the control port side of the main valve  38 . The disposition of the non-return valve  40  is also shown. 
     Operation of the valve is generally as follows. Without weight-activation, flexion of the knee joint causes hydraulic fluid to be driven by the vane  34  from one variable volume part of the chamber  18  through the upstream and downstream ports  38 A and  38 C to the other variable volume part of the chamber  18 . If the unit is weight-activated either during or prior to flexion of the knee joint, the shuttle valve interrupts the flow of fluid between the two chambers causing the knee to lock (i.e. to be stabilised). During extension of the knee joint, the vane  34  forces the fluid in the opposite direction via the non-return valve  40  ( FIGS. 6A and 6B ) to ensure free swing in the extension direction at all times subject, of course, to any resistance imposed by a swing phase control unit attached between the first and second parts of the mechanism. 
     In the swing phase, prior to rotation at the knee joint, the housing  12 A is held in such a position by the spring  28  (see  FIG. 1 ) as to maintain the pilot valve  32  open, providing communication between the control port  38 E of the main valve  38  and the downstream part of the primary passage between the two variable volume parts of the chamber  18 . At this time, the shuttle valve member  38 F is urged against the shoulder in the main valve cavity  38 B by the spring force of spring  38 G, covering the downstream port  38 C, as shown in  FIG. 5 . 
     Flexion of the knee joint causes an increase in pressure upstream of the shuttle valve  38 . Bearing in mind that the control port is open to the downstream side so long as pilot valve  32  is open, the differential pressure across the shuttle valve  38  causes the valve member to slide away from the shoulder stop in cavity  38 B, compressing the spring  38 G and, thereby, uncovering the downstream port  38 C. As a result, fluid may flow from the upstream port  38 A to the downstream port  38 C, and the shin component  14  may freely swing. 
     However, when weight is applied to the limb in such a way as to produce a flexion moment within the knee mechanism sufficient to rotate the housing  12 A against the spring  28  ( FIG. 1 ), button  32 B of the pilot valve  32  is allowed to move outwardly in the housing  12 A, the pilot valve thereby closing under the force of its closure spring  32 C. The control port  38 E of the shuttle valve  38  is now closed off and the secondary passage interrupted. Pressure on the control port side of the shuttle member  38 F is then equalised with that at the upstream port  38 A owing to fluid flow through the bleed passage  38 H. The previously described differential pressure across the valve member  38 F is thereby removed. Should weight be applied prior to flexion of the knee joint, the shuttle valve member  38 F is prevented from moving away from its closed position, i.e. with downstream port  38 C closed. Should weight be applied during flexion of the knee (for instance when the amputee stumbles), the shuttle valve member  38 F is returned from an open position to its closed position by the spring force of spring  38 G. In the closed position of the shuttle valve member  38 F, the downstream port  38 C is cut off from the upstream port  38 A, thereby interrupting the flow of fluid between the two variable volume parts of the chamber  18 , causing the knee to stabilise. 
     It will be understood, then, that the knee mechanism is locked or stabilised during the stance phase of the walking gait cycle, when the flexion moment caused by the ground reaction vector overcomes the restoring force of the spring  28  ( FIG. 1 ). When that flexion moment is insufficient to overcome the spring force, the housing  12 A rotates about weight-sensing pivot axis  22 , forcing the pilot valve  32  to open. This, in turn, causes the pressure on the control port side of the shuttle valve member  38 F to drop and, therefore, the shuttle valve member  38 F slides to its open position due to upstream pressure in the upstream port  38 A, allowing fluid to flow once again between the two parts of the hydraulic chambers  18 . 
       FIG. 6A  illustrates the state of the hydraulic circuit when the knee joint is being flexed without weight application.  FIG. 6B  shows the hydraulic state when a flexion moment is applied when the mechanism is weight-activated. As described above, when weight-activated, the pilot valve  32  is closed, removing the differential pressure across the shuttle valve  38  so that the latter closes. 
     The arrangement of the shuttle valve shown in  FIG. 5  is simplified. In the preferred shuttle valve  38 , means for adjusting the stabilising action of the mechanism is provided in the form of a yield adjuster  50 , as shown in  FIG. 3 . In this preferred embodiment, the plug  38 I has a threaded axial bore which receives an axial needle member  38 J having a threaded screw head  38 JA. The needle member  38 J extends through the upstream part of the main valve cavity  38 B to a free end which abuts the shuttle valve member  38 F when it is in its closed position. Advantageously, the needle  38 J has a tapered end which is received in the bleed passage  38 H. Depending, therefore, on the position of the needle member  38 J, equalisation of the pressure on opposite sides of the shuttle valve member when the pilot valve  32  is closed does not cause complete covering of the downstream port  38 C. This variable leak has the effect of an adjustable yield instead of full lock. It follows that if the amputee wishes to descend stairs “leg-over-leg”, the shuttle valve closed position can be set to produce the appropriate level of support. This support may also be used by the amputee for descending steep slopes. 
     The tapered end of the needle member  38 J is received in the bleed passage  38 H (see  FIG. 5 ) of the shuttle valve member  38 F when the latter it is its closed position. The action of the tapered end entering the bleed passage hydraulically damps the movement of the valve member  38 F, largely preventing any significant noise associated with valve closure. 
     A particular property of the pilot valve  32  described above with reference to  FIG. 5  is that the sealing area, determined by the annular seal  32 AC and/or the associated parts of the pilot valve member  32 A, is smaller than the sealing area of the wide valve body portion  32 AA, with the result that a very high upstream pressure transmitted through the bleed passage  38 H in the main valve  38  causes the pilot valve to open against the spring  32 C. The pilot valve  32 , therefore, acts as an over-pressure release valve to prevent hydraulic pressure damaging the mechanism. In effect, by using different sealing areas for the pilot valve closed position, the valve can be made to open at a predetermined limiting pressure. The effect felt by the amputee is a yielding of the knee when loaded excessively. 
     Parts of an alternative knee mechanism in accordance with the invention are shown in cross-section in  FIG. 7 . The main differences are the adoption of a leaf spring  128  to bias the vane housing  12 A towards its unloaded position relative to the shin component  14 , and the medial-lateral disposition of the pilot valve  32  in the housing  12 A, the exposed end of the pilot valve stem  32 AD projecting from a side face  12 AS of the housing  12 A to abut an adjustable projection mounted in the adjacent sidewall  14 A of the shin component  14 , as shown in the detail of  FIG. 8 . 
     Details of this alternative embodiment will be described only where it differs from the embodiment of  FIGS. 1 to 5  but, to aid understanding, the same reference numerals are used in  FIGS. 7 and 8  as in  FIGS. 1 to 5  where the two embodiments have corresponding parts. 
     Referring to  FIG. 7 , the leaf spring  128  is fixed at a proximal end to an anterior face of the housing  12 A, and extends distally behind the anterior wall  14 B of the shin component  14  to abut an adjustable abutnent member  126  threaded in the anterior shin component wall  14 B. Leaf spring  128  biases the shin component  14  about weight-sensing axis  22  in the direction of knee extension. As in the case of the bush  26  of the embodiment of  FIGS. 1 to 5 , the adjustment member  126  in this alternative embodiment, being threaded in the anterior wall  14 B of the shin component  14 , can be adjusted to alter the preloading of the spring  128  so as to define the stiffness of the connection between the housing  12 A and the shin component  14 . 
     Referring to  FIG. 8 , the exposed end portion  32 B of the pilot valve stem  32 AD is generally in registry with a domed inner end  130 A of the adjustable projection  130 , button  32 B sliding over domed end  130 A as the shin component  14  moves with respect to the vane housing  12 A. More specifically, subject to the ground reaction vector from the prosthetic foot (not shown) passing to the posterior of the weight sensing axis  22 , when the amputee applies weight to the prosthesis sufficiently to cause deflection of the vane housing against the biasing force of the leaf spring  128 , the axis  132 X of the pilot valve button  32 B moves away from the axis  130 X of the adjustment member  130  so that the pilot valve closure spring  32 C causes the pilot valve stem  32 AD to move to the position in which the pilot valve  32  is closed. This is the position of the pilot valve stem  32 AD shown in  FIG. 8 . When the amputee&#39;s weight is removed, button  32 B moves to a position more in registry with adjustment member  130 , causing the pilot valve to open. 
     In this embodiment, sealing between the valve stem  32 AD of the pilot valve  32  and the walls of the pilot valve cavity in the housing  12 A occurs by direct contact between the valve stem  32 AD and the cavity wall, in this case by abutment of a conical portion  132  of the valve stem  32 AD with a coaxial annular shoulder in the cavity wall. 
     Overload protection of the hydraulic circuit is achieved as in the embodiment of  FIGS. 1 to 5 , in that the sealing diameter of that portion of the valve stem determining the pressure required to open the valve against the closure spring  32 C is marginally larger than the diameter of the annular seal formed between the conical surface  132  of the valve stem  32 AD and the wall of the pilot valve cavity  32 D. The respective seals, i.e. of the wide valve body portion  32 AA and the sealing surface  132  are on opposite sides of the valve cavity  32  with respect to the port formed by the secondary passage  44  where it opens into the cavity  32 D. 
     The main valve  38  has a construction in this embodiment similar to that described above with reference to  FIG. 3 , in that it has a yield adjuster  50  with a needle member  38 J in registry with the bleed hole  38 H in the shuttle valve member  38 F. 
     Use of a leaf spring  128 , as shown, with its major cross-sectional axis in the medial-lateral direction, as well as the medial-lateral disposition of the pilot valve  32  and pilot valve axis  132 X, results in a lighter and more compact mechanism compared with that of  FIGS. 1 to 5 , particularly in terms of its anterior extent relative to the knee axis  16 . 
     Variations on the structure described above are possible. For instance, the knee part  10  may be integral with a thigh component of a lower limb prosthesis. The weight-responsive parts of the limb may be associated with the prosthetic thigh rather than the shin as shown. A notable feature of the mechanism is that the main valve member is normally in its closed position and is pushed open by the operating pressure resulting from knee joint flexion. Movement of the valve member towards the open position is a result of such pressure. Such movement is not possible when the control arrangement operates to prevent application of a differential pressure on opposite sides of the valve member. It is possible, however, to prevent movement of the main valve member mechanically rather than hydraulically. Allowing the main valve member to be pushed open by operating pressure has the advantage that the secondary control function can be brought about other than as a result of movement of the main valve member with the consequent advantage of minimal take-up movement before the fluid passage between opposite sides of the piston is interrupted and, in addition, precise adjustment.