Abstract:
The invention relates to an orthopaedic aid, in particular to a prosthesis ( 58 ) or orthosis comprising an orthopaedic fluid damper ( 10 ) with a displacement chamber ( 14 ) formed in a housing ( 12 ), with a piston ( 16 ) mounted in the displacement chamber ( 14 ), with a fluid reservoir for a fluid ( 20 ), with a return flow conduit ( 22 ) connecting the displacement chamber ( 14 ) to the fluid reservoir, with a valve ( 24 ) that can adopt an open position and a closed position, in which it at least partially closes the return flow conduit ( 22 ), and with a joint ( 72 ) that has a first branch ( 60 ) and a second branch ( 66 ), wherein the first branch ( 60 ) is connected to the housing ( 12 ) and the second branch ( 66 ) is connected to the piston ( 16 ). A device ( 84 ) is also provided for detecting a joint force (FB) acting on the joint, which device ( 84 ) is designed to bring the valve ( 24 ) to the closed position when the joint force (FB) exceeds a predefined threshold value. The invention also relates to a method for control of the aid and a fluid damper ( 10 ) fitted therein.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an orthopaedic fluid damper and to an orthopaedic aid with such a damper which is designed for use in a prosthesis or orthesis, with a displacement chamber formed in a cylinder housing, with a piston mounted in the displacement chamber, with a fluid reservoir for a fluid, with a backflow line which connects the displacement chamber to the fluid reservoir, and with a valve which can assume an opening position and a closing position, in which it at least partially closes the backflow line. According to a further aspect, the invention relates to a method for controlling the orthopaedic aid. 
     2. Description of Related Art 
     Orthopaedic fluid dampers are employed in orthopaedic aids, such as, for example, prostheses or ortheses. They serve as a resistance element between two limbs. For example, the orthopaedic fluid damper is arranged between a thigh and a lower leg of a leg prosthesis and increases a bending resistance of a knee joint of the leg prosthesis. 
     Leg prostheses with orthopaedic fluid dampers of this type are rarely found in geriatric patients. For patients of great age who are severely restricted in their muscular and motor capability, blocking knee joints are often employed. 
     Known orthopaedic blocking knee joints for geriatric leg prostheses provide geriatric patients with only two states, blocked or unblocked, for cost reasons. The geriatric patient prefers the blocking knee joint which is blocked by means of the lock. Known orthopaedic blocking joints have the disadvantage that they cause the patient&#39;s gait to have a hobbling appearance, since the patient brings the leg forward by rotating it outwardly (circumduction) or lengthens the healthy leg via a pointed-foot position or slanted-hip position, so that the leg prosthesis can swing through freely. So that he can sit down, the geriatric patient releases the lock manually. In this state, the leg prosthesis offers no bending resistance and therefore, disadvantageously, also no longer any safety. The patient has to support himself with both hands in this situation. Moreover, should the geriatric patient stumble when the leg prosthesis is released, he involuntarily falls down. For this reason, known orthopaedic aids which are equipped with conventional orthopaedic locks are unpopular among geriatric patients. 
     EP 0 309 441 discloses a double-acting hydraulic piston/cylinder unit, by means of which the outer dimensions of a concertina preserve an approximately predetermined form, the concertina spanning large parts of the piston/cylinder unit. The disadvantage of this piston/cylinder unit is that it is complicated to produce. 
     DE 102 14 357 A1 discloses a prosthetic knee joint with a hydraulic damping cylinder. In the prosthetic knee joint, a hydraulic fluid is present, the viscosity of which can be varied via an external force field. The disadvantage of this embodiment is that, although it makes increased walking comfort possible, it nevertheless requires an electric control, which, however, is not justifiable for low-priced prostheses. 
     DE 198 59 931 A1 discloses a leg prosthesis which is likewise damped to a greater or lesser extent via a hydraulic fluid, the viscosity of which can be varied by means of external electrical fields. The disadvantage here, too, is that a control is necessary. 
     BRIEF SUMMARY OF THE INVENTION 
     The object on which the invention is based is to provide an orthopaedic fluid damper, by means of which orthopaedic aids can be produced which are more readily acceptable to geriatric patients. 
     The invention solves the problem by means of an orthopaedic aid having the features of the main claim, which comprises an orthopaedic fluid damper according to the invention and a joint which has a first limb and a second limb, the first limb being connected to the cylinder housing and the second limb being connected to the piston of the fluid damper and having a device which detects the force acting on the joint, in order to bring the valve into a closing position when a threshold value is overshot. According to a second aspect, the invention solves the problem by means of a method according to the invention for controlling an orthopaedic aid according to the invention, having the steps (a) detection of a force (joint force) acting on the joint or the prosthesis and (b) activation of the valve in such a way that it at least partially closes the backflow line. According to a third aspect, the invention solves the problem by means of a generic orthopaedic fluid damper, in which the piston is mounted in the displacement chamber at at least one push-in depth, so as to form an annular gap, in particular a throughflow-determining annular gap. 
     An orthopaedic aid preferably comprises a device for detecting the load acting on the prosthesis, which is designed in order to bring, the valve into the closing position when the joint force overshoots a preset threshold value. This device may, for example, be a sensor which detects a force or a torque. However, the device may also be formed by a purely mechanical connection. 
     The orthopaedic aid is preferably a leg prosthesis with a knee joint and with a device for detecting the load acting on the prosthesis. This device comprises a leg-axis force sensor which detects the leg-axis force acting in the longitudinal direction of the extended leg prosthesis. 
     So that the leg-axis force can be determined especially accurately and sensitively, a front-foot force sensor arranged in a front foot of a foot of the leg prosthesis and/or a back-foot force sensor arranged in a back foot are preferably provided. These are connected electrically to an electric control which activates the valve as a function of electrical signals which said control receives from the sensors. 
     The leg-axis force sensor may be designed as a carrying element of the prosthesis which, for example, changes its longitudinal extent as a function of the load upon the leg prosthesis. Alternatively, the leg force sensor may comprise strain gages. 
     Alternatively to an electronic measurement, evaluation and actuation, there is provision for the device for detecting the joint force to be set up mechanically and, in the event of load, to displace the valve into the closing position. The mechanical control may take place, for example, via relative displacement of the limbs with respect to one another, in that a pivoting or axial displacement of the limbs with respect to one another is utilized and, for the displacement of the valve, the forces occurring are at least partially transmitted to this. For this purpose, the device for detecting the joint force may be designed as a buckling spring, spring shackle, cam guide or slotted guide or as a lever system, via which the displacement forces are transmitted to the valve. Preferably, the device is coupled to a solenoid valve which is prestressed in the direction of the closing position, so that, when a switching magnet is removed, the valve is closed automatically. This ensures that, in the event of a failure of the mechanism, damping is increased abruptly, thus ensuring the stability of the joint. 
     The switching movement may take place in that the limbs are displaced toward one another in the case of a sufficiently high axial force component. For this purpose, the two limbs are mounted one on the other so as to be displaceable to a limited extent in relation to one another, as a result of which, in the event of mechanical switching, the valve is actuated directly, while, in the event of sensor switching, the displacement travel is detected as a variable to be sensed, via which an actuator signal is triggered. The displacement indicates an axial forte load, for example when a prosthesis user is standing, so that safety during a standing phase can be provided via an increase in damping. To counteract the displacement movement, a spring acting in the axial direction may be provided, which compensates tolerances and causes a return movement into the initial state. 
     In a development of the invention, the damper and the joint are mounted together on a frame or form said frame which is displaceable, in particular shiftable, in relation to one of the limbs. Between the frame and one of the limbs, a spring may be arranged which acts counter to a displacement of the limb towards the frame. The hydraulic damper may be designed as a single-acting or double-acting damper, as a cylinder damper or as rotary hydraulics. 
     In addition to a control dependent on axial force, it is additionally possible that, and there is provision whereby, the device is designed such that a displacement, dependent on the pivot angle φ, of a switching element in relation to the valve takes place, but preferably not solely by mechanical coupling. 
     Within the scope of a method according to the invention, the valve is preferably activated such that it at least partially closes the backflow line when the leg-axis force overshoots a preset closing threshold value. For this purpose, it is possible, but not necessary, that the closing threshold value is explicitly stored, for example, in the electric control. It is also possible that a degree of closing of the valve is dependent monotonically or continuously on the leg-axis force, that is to say the valve is closed the further, the higher the leg-axis force is. 
     Furthermore, there is provision whereby, in a variant of the invention, the valve is switched independently of the position of the limbs, that is to say it is unimportant how the limbs stand in relation to one another or in space, but, instead, the aim is merely to ensure that a sufficiently high axial force component is present. Whether bending moments, torsional moments or horizontal forces also occur in the orthopaedic aid is then insignificant. 
     In a further variant, the valve is switched as a function of the angle φ of the limbs with respect to one another, for example in the case of a bending angle of between 20° and 50° with respect to one another. If a certain fixed bending angle is reached in the case of use in a leg prosthesis, damping is automatically increased, in order to ensure that the joint is secured. Damping may in this case be set such that a slow lowering is still possible. The switching of the valve on account of a displacement or pivoting of the limbs with respect to one another takes place, for example, mechanically, in that a relative movement of the limbs with respect to one another is transmitted to the valve. 
     A fluid damper according to the invention has the advantage that it makes it possible to produce an orthopaedic aid which satisfies the needs of geriatric patients. When the prosthesis is loaded, high damping is available to the patient, which avoids sudden buckling and allows safe walking. Under load, the piston displaces the fluid slowly through the annular gap out of the displacement chamber into the pressureless fluid reservoir. The patient subsides slowly in the knee joint and always has the feeling of being supported by the leg prosthesis. Moreover, an unlocking of the fluid damper is dispensed with. 
     If, by contrast, no load or bending force is applied to the knee joint of the leg prosthesis, the valve can be brought into the opening position and the leg swings through without resistance. There is therefore no need to bring the leg prosthesis forward by rotating it outward or to lengthen the healthy leg via the pointed-foot position. The gait therefore appears more natural. 
     A further advantage is that the orthopaedic fluid damper according to the invention can be produced by simple technical means. In conventional fluid dampers, a good sealing action between the piston and displacement chamber must be ensured. Such a seal is complicated and cost-intensive. Since, according to the invention, an annular gap is provided, markedly lower manufacturing tolerances can be provided in the manufacture of the piston and cylinder housing, thus making manufacture easier and less costly. 
     Moreover, on account of the acceptable lower manufacturing tolerances, it is possible to use cost-effective materials, such as, for example, plastic, which have a low modulus of elasticity, in order to produce the cylinder housing and/or the piston. In conventional fluid dampers, leaktightness between the piston and displacement chamber also has to be ensured under mechanical load. In order under mechanical load to prevent a play which puts leaktightness at risk, materials with a high modulus of elasticity, such as, for example, metals, have to be used. This necessity is avoided in the fluid damper according to the invention. 
     An annular gap is understood, within the scope of the present description, to mean, in particular, an interspace between an inside of the displacement chamber and an outside of the piston. This annular gap may, but must not necessarily, have an annular cross section. It is possible that the piston bears in regions against the inner wall and the cross section of the annular gap is therefore crescent-shaped. A further possibility is that the piston bears in wide regions against the inner wall and possesses longitudinal grooves, through which the fluid can pass from the displacement chamber into the fluid reservoir. Contrary to known fluid dampers, in which annular gaps can occur only due to manufacturing tolerances, in the fluid damper according to the invention, the piston is manufactured such that it forms an annular gap of preset cross-sectional area with the cylinder housing or such that the fluid experiences a preset fluid resistance when it flows through the annular gap. In other words, in known fluid dampers, the piston and cylinder housing form a sealing fit, whereas, in the fluid damper according to the invention, a loose clearance fit may be formed. In known fluid dampers, therefore, this results at most in leakage streams, but not, as in the fluid damper according to the invention, defined damping fluid streams. 
     It is possible, but not necessary, that the piston and cylinder housing form an annular gap of identical cross-sectional area in all cross sections perpendicular to the piston longitudinal axis. 
     In a preferred embodiment, the valve leads to a damping of the movement of the piston in the displacement chamber, the damping acting in only one direction of movement of the piston, for example in the push-in direction of the piston into the cylinder housing. In the case of use in an orthopaedic aid, it is usually necessary, but also sufficient, to damp a movement of two limbs of a joint of the aid in the push-in direction. If the orthopaedic fluid damper is installed in a leg prosthesis, this is the bending direction (flexion direction). In order to implement the feature, the valve may, for example, be a nonreturn valve, this being simple and cost-effective. 
     The fluid, when it flows along a fluid path out of the displacement chamber through the annular gap into the fluid reservoir, experiences an annular-gap fluid resistance and, when it flows along a fluid path out of the displacement chamber through the backflow line into the fluid reservoir, experiences a backflow fluid resistance. There is preferably provision for the annular gap to possess a form and/or cross-sectional area which is dimensioned such that, when the valve is in the closing position, the annular-gap fluid resistance is lower than the backflow fluid resistance. That is to say, with the valve closed, when the piston is pushed into the displacement chamber, more fluid passes through the annular gap into the fluid reservoir than through the backflow line. 
     Particularly preferably, the valve and the annular gap are designed such that, when the valve is in the closing position and the piston is pushed into the displacement chamber, essentially the entire fluid flows out of the displacement chamber through the annular gap into the fluid reservoir. The feature that essentially the entire fluid flows in the way described is to be understood as meaning that it is not necessary for the entire fluid to flow through the annular gap in the strict sense. On the contrary, it is possible that a small part stream continues to flow through the valve. This part stream is, for example, lower than 15% of the overall stream. 
     Particularly preferably, the fluid is a hydraulic fluid. This hydraulic fluid may be an oil, for example, mineral oil, but, for example, also water. The use of air is also possible in principle. 
     It is beneficial if the viscosity of the hydraulic fluid is independent of magnetic fields and/or electrical fields. This system may be understood as meaning that the viscosity changes, in particular, by less than 50% when a magnetic field of 0.1 Tesla is applied. In particular, the hydraulic fluid is free of magnetic particles. Such hydraulic fluids are especially cost-effective. 
     A fluid damper which is especially simple and cost-effective to manufacture is obtained when the cylinder housing and/or the piston are/is manufactured, in particular injection-molded, from plastic. A further advantageous configuration comprises a cylinder housing with a metal sleeve injected or embedded into the plastic. The metal sleeve in this case forms the cylinder. This combination reduces the deformation of the cylinder under load. In order to avoid the alternative deformation of the cylinder, the cylinder housing may be surrounded from outside by composite fiber materials. This advantageously allows a cost-effective mass production of simple orthopaedic aids, which also makes them available to patients in countries where the income is low. 
     During use, the piston of the fluid damper is regularly pushed into the displacement chamber and pulled out of this. In other words, a push-in depth of the piston, that is to say the length of the distance over which the piston is pushed into the displacement chamber, changes constantly. Advantageously, the damping by which the fluid damper opposes a further pushing of the piston into the displacement chamber can be varied in that the piston has a contoured configuration. For example, the piston may taper conically, a cross-sectional area of the piston becoming larger, for example, with an increasing push-in depth. In this case, the resistance with which the fluid damper opposes a further pushing of the piston into the displacement chamber rises with the push-in depth. Installed in a leg prosthesis, this has the effect that a bending of the knee joint is damped especially highly toward the end of the sitting down of the patient. 
     Moreover, it is possible that the diameter of the piston decreases with an increasing push-in depth. It is also possible that the piston has a convex or concave design. All the forms mentioned may also be present in portions, and therefore the piston may, for example, possess a conical portion and a cylindrical portion. 
     Alternatively, or additionally, moreover, the displacement chamber has a contoured configuration. This is to be understood as meaning that an inner wall of the displacement chamber may, as outlined above, be designed conically, convexly, concavely and/or, in portions, cylindrically. 
     A fluid damper which is especially simple to manufacture is obtained when the fluid reservoir is designed in order to store the fluid so that it is always essentially pressure-free. Moreover, a loss of fluid is thereby largely avoided. Particularly preferably, the fluid reservoir is a concertina. Such a concertina can be manufactured very easily. The piston has a penetration portion, which can penetrate into the cylinder housing, and a fine portion, which cannot penetrate into the cylinder housing. There is preferably provision for the concertina to be fastened to the cylinder housing and to the free portion. Thus, the concertina is fastened securely and can easily be exchanged for maintenance purposes. However, it is basically also possible to store the fluid under pressure. 
     A fluid reservoir formed by a concertina is put at risk by mechanical actions, and therefore the fluid reservoir is preferably surrounded by a sleeve, in particular a plastic sleeve, which advantageously has at least one pressure compensation orifice, so that the concertina can expand or a diaphragm can variably form a reception volume. 
     To actuate the valve, the orthopaedic fluid damper preferably possesses a magnet arranged outside the cylinder housing, the valve being capable of being brought from the opening position to the closing position by means of the magnet. Switching via a magnet may also be used in other types of fluid damper, in which there is no annular gap present or which have a double-acting set-up or are set up as rotary hydraulics. This may be implemented, for example, in that the magnet is an electromagnet which cooperates with a ferromagnetic valve ball of the valve. This takes place, for example, in that an application of current to the electromagnet moves the valve ball such that the valve is brought into the opening position or alternatively into the closing position. Alternatively, there is provision for the magnet to be a permanent magnet which is mounted displaceably and in an automatically actuable way via an actuator arranged outside the cylinder housing or via mechanical coupling and which cooperates with the valve ball. 
     According to the invention, moreover, a generic orthopaedic fluid damper, in which the cylinder housing and/or the piston are/is manufactured, in particular injection-molded, from plastic, is provided. Such a fluid damper preferably has the feature of the characterizing part of claim  23  and/or one or more of the abovementioned features. There may then be provision for the piston to be mounted in the displacement chamber via a seal, for example an O-ring seal. In this case, in addition to the backflow line, a further line may be provided which connects the fluid reservoir to the displacement chamber. 
     Embodiments of the invention are explained in more detail below with reference to the accompanying drawings. The same reference symbols designate identical or identically acting components or elements in the figures. In these: 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1   a  shows an orthopaedic fluid damper according to the invention in a cross-sectional view, in which a piston is pushed into the displacement chamber to a low push-in depth, 
         FIG. 1   b  shows the fluid damper according to  FIG. 1   a , in which the piston is pushed essentially completely into the displacement chamber, 
         FIG. 2   a  shows, in the form of a detail, a diagrammatic sectional view of the valve of the fluid damper from  FIGS. 1   a  and  1   b  in a closing position, 
         FIG. 2   b  shows the valve according to  FIG. 2   a  in an opening position, 
         FIG. 3   a  shows the sectional view of a valve of a further embodiment of an orthopaedic fluid damper according to the invention with a bypass line in which is arranged a bypass valve which is closed, 
         FIG. 3   b  shows the valve according to  FIG. 3   a  in an opening position, 
         FIG. 4   a  shows a sectional view of a valve of a further embodiment of an orthopaedic fluid damper according to the invention with an electromagnet in the dead state, 
         FIG. 4   b  shows the valve according to  FIG. 4   a  in the live state, 
         FIG. 5  shows an orthopaedic aid according to the invention in the form of a leg prosthesis in an extended position, 
         FIG. 6  shows the leg prosthesis according to  FIG. 5  in a bent position, 
         FIG. 7   a  shows a first embodiment of a leg force sensor, 
         FIG. 7   b  shows an alternative embodiment of a leg force sensor, 
         FIG. 7   c  shows a further alternative embodiment of a leg force sensor, 
         FIGS. 8   a , 8   b  show a variant of the fluid damper in two positions, 
         FIGS. 9   a , 9   b  show a further variant of the fluid damper in various positions, 
         FIGS. 10   a ,  10   b  show a leg prosthesis in the non-loaded and the loaded state, and 
         FIG. 11  shows a variant of the leg prosthesis with angle-dependent hydraulic switching. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       FIG. 1   a  shows an orthopaedic fluid damper  10  which comprises a cylinder housing  12  made from a plastic with a displacement chamber  14  formed therein, a piston  16  mounted in the displacement chamber  14  and consisting of plastic, a fluid reservoir in the form of a concertina  18 , manufactured from elastomer, for a fluid in the form of a hydraulic fluid  20 , a backflow line  22  formed in the cylinder housing  12 , and a valve  24 . The piston  16  is designed cylindrically and is mounted in the cylinder housing  12  so as to form an annular gap  26 . The valve  24 , which is shown diagrammatically in  FIG. 1   a  in a closing position, comprises a valve seat  32 , a valve ball  34  and a helical spring  36  which prestresses the valve ball  34  onto the valve seat  32 . 
     The cylinder housing  12  and the piston  16  may alternatively also consist of aluminum or high-grade steel or comprise, as a liner, a metal sleeve which is embedded in plastic or around which plastic is injection-molded. 
     The distance which the piston  16  has covered in the cylinder housing  12  constitutes a push-in depth T. When the piston  16  is pushed further into the cylinder housing  12  so that the push-in depth T increases, hydraulic fluid  20  present in the displacement chamber  14  is displaced through the annular gap  26  along an annular-gap fluid path  28  into the concertina  18 , since a backflow-line fluid path  30  through the backflow line is shut off by the valve  24 . 
     The hydraulic fluid  20  experiences along the annular-gap fluid path  28  an annular-gap fluid resistance which is dependent on a speed at which the piston  16  is pushed into the cylinder housing  12 . This speed depends, in turn, on a push-in force F E  with which the piston  16  is pushed in. The movement of the piston  16  into the cylinder housing  12  is thus damped by the annular-gap fluid resistance. 
       FIG. 1   b  shows a piston  16  which is pushed essentially completely into the displacement chamber  14  and which is pulled out of the displacement chamber  14  by means of a pull-out force F A . Hydraulic fluid  20  flows out of the concertina  18  along a backflow path  38  through the valve  24  into the displacement chamber  14 . The valve  24  is designed as a nonreturn valve and allows hydraulic fluid  20  free passage in this direction. 
     Moreover,  FIG. 1   b  shows a penetration portion E of the piston  16  and a free portion A which cannot penetrate into the cylinder housing  12 . The concertina  18  is fastened, on the one hand, to an end  40  of the piston  16  which faces away from the cylinder housing  12 , and consequently in the free portion A, and, on the other hand, to an end face  42 , facing the end  40 , of the cylinder housing  12 . For example, the concertina is glued on or welded on or engages in each case in grooves, not depicted, in the piston  16  or the cylinder housing  12 . 
       FIG. 2   a  shows, in the form of a detail, a view of an end, facing away from the piston  16 , of the cylinder housing  12 . It can be seen that the valve ball  34  can be actuated via an actuator which comprises a pin  44 , an actuator diaphragm  46  and an actuator basic body  48 . In the non-actuated state which is shown in  FIG. 2   a , the valve ball  34  lies on the valve seat  32  and prevents a fluid stream out of the displacement chamber  14  into the backflow line  22 . By contrast, hydraulic fluid  20  can flow along the backflow path  38  in that it lifts off the valve ball  34  from the valve seat  32  counter to the force of the helical spring  36 . 
       FIG. 2   b  shows the actuator in an actuation position in which the pin  44  has pressed the valve ball  34  from the valve seat  32 . In this position, hydraulic fluid can pass through the valve  24  both along the backflow path  38  and in the opposite direction. 
       FIG. 3   a  shows an alternative embodiment of a fluid damper  10  which possesses in addition to the backflow line  22  a bypass line  50  which likewise connects the displacement chamber  14  to the concertina  18 , not depicted in  FIG. 3   a . In this embodiment, no annular gap  26  is necessary. Arranged in the bypass line  50  is a bypass valve  52  which can completely or partially close the bypass line  50  so that a defined fluid resistance can be set. When the piston  16  is pulled out of the cylinder housing  12 , hydraulic fluid  20  can flow along the backflow path  38  through the backflow line  22  into the displacement chamber  14 . When the piston  16  is pushed into the displacement chamber  14 , the hydraulic fluid  20  is displaced through the annular gap  26  into the concertina (cf.  FIG. 1   a ) in the situation shown in  FIG. 3   a . By contrast, a bypass-line fluid path  54  is shut off by the closed bypass valve  52 , and the backflow line  22  is closed by the valve  24 . 
       FIG. 3   b  shows the situation in which the bypass valve  52  is open so that, when the piston  16  is pushed into the displacement chamber  14 , hydraulic fluid can flow along the bypass-line fluid path  54 . 
       FIG. 4   a  shows an electromagnet  56  which is mounted outside the cylinder housing  12  and which can act on the ferromagnetic valve ball  34 . In  FIG. 4   a , the electromagnet  56  is unenergized, and therefore the valve ball  34  is pressed onto the valve seat  32  by the helical spring  36 . 
       FIG. 4   b  shows the situation in which the electromagnet  56  is energized and lifts off the valve ball  34  from the valve seat  32 . The electromagnet  56  may partially or completely surround the cylinder housing  12  annularly, so that it can cooperate especially effectively with the valve ball  34 . 
     Alternatively, a permanent magnet is provided, which is coupled displaceably to an actuator. A movement of the permanent magnet by means of the actuator leads to a movement of the valve ball  34 . 
       FIG. 5  shows an orthopaedic aid according to the invention in the form of a leg prosthesis  58  which comprises a fluid damper  10  according to the invention, a thigh  60  with a proximal thigh end  62  and a distal thigh end  64 , and a lower leg  66  with a proximal lower leg end  68  and a distal lower leg end  70 . The thigh  60  and the lower leg  66  are connected to one another in a knee joint  72  and run in a longitudinal direction L in the extended position. The fluid damper  10  is connected by means of its piston  16  to the distal thigh end  64  and by means of its cylinder housing  12  to the proximal lower leg end  68  and causes a damping of a pivoting movement of the thigh  60  in relation to the lower leg  66  over a pivot angle φ. When the thigh  60  pivots in relation to the lower leg  66 , the push-in force F E  on the piston  16  is generated, and the damping action described above occurs. 
     The leg prosthesis  58  possesses a front foot  74  and a back foot  76 . A front-foot force sensor  78  for measuring a front-foot force F v  is arranged on the front foot  74 , and a back-foot force sensor  80  for measuring a heel force F F  is mounted on the back foot. The two sensors are connected via an electrical line, not depicted, to an electric control  82  which is part of the fluid damper  10 . Moreover, the electric control  82  is connected to a leg force sensor  84 . The front-foot force sensor  78 , the back-foot force sensor  80  and the leg force sensor  84  are designed to determine a leg-axis force F B  which runs from the proximal thigh end  62  to the distal lower leg end  70 . The electric control  82  detects the leg-axis force F B , compares this with a closing threshold value or an enabling threshold value, which are stored in an electrical memory of the electric control  82 , and, for example, activates the electromagnet  56  (cf.  FIG. 4   b ) on the basis of this comparison. If, for example, the leg-axis force F B  overshoots the preset closing threshold value, this is a sign that the leg prosthesis  58  is loaded by a patient and a high bending resistance is necessary. The electromagnet  56  is then switched off, current-free, by the electric control  82 , so that the backflow-line fluid path  30  (cf.  FIG. 1   a ) for the hydraulic fluid  20  is shut off and a high push-in force F E  has to by exerted in order to push the piston  16  into the cylinder housing  12 . The leg prosthesis  58  consequently possesses a high bending resistance and affords a high degree of safety to the patient when standing. Moreover, in standing, the leg prosthesis  58  is secured geometrically, since a load line of the applied load runs in front of the knee joint  72  and consequently does not bend the leg prosthesis  58 . 
     If a markedly higher force prevails on the front-foot force sensor  78  than on the back-foot force sensor  80 , this is a sign that the patient would like to sit down, and the electric switch control  82  likewise closes the valve  24 . This situation is shown in  FIG. 6 . 
     If, however, the leg-axis force F B  is low, the leg prosthesis  56  is non-loaded and the electric control  82  applies current to the electromagnet  56 , so that hydraulic fluid can also flow through the backflow-line fluid path  30  (cf.  FIG. 1   a ). The lower leg  66  ( FIG. 5 ) can then swing freely in relation to the thigh  60 . 
       FIG. 7   a  shows an embodiment of the leg force sensor  84  with a sleeve  86  into which a measuring piston  88  can be pushed counter to a resistance exerted by a filling material  90  in an adhesive joint. The leg force sensor  84  comprises a means for determining the relative position of the measuring piston  88  in relation to the sleeve  86  which is proportional to the leg-axis force F B . 
       FIG. 7   b  shows an alternative embodiment of the leg force sensor  84 , in which two L-shaped measuring elements  92   a ,  92   b  are connected via an elastic element  94 . The leg force sensor  94  comprises, once again, a means, not depicted, for determining the relative position of the two measuring elements  92   a ,  92   b  in relation to one another, which constitutes a measure of the acting leg-axis force F B . 
       FIG. 7   c  shows a further alternative embodiment of the leg force sensor  84  which differs from the leg force sensor shown in  FIG. 7   b  in that the two measuring elements  92   a ,  92   b  are connected via two webs  96   a ,  96   b.    
       FIGS. 8   a  and  8   b  illustrate a fluid damper  10  which corresponds essentially to the fluid damper in  FIGS. 1   a  and  1   b . Instead of the annular gap  26  which is open and allows a backflow of the hydraulic fluid out of the displacement chamber  14  into the fluid reservoir of the concertina  18 , in the variant according to  FIGS. 8   a  and  8   b  the annular gap  26  is closed via a seal  29 . Instead of flowing through the annular-gap fluid path  28  according to the embodiment of  FIGS. 1   a  and  1   b , the hydraulic fluid  20  flows out of the displacement chamber  14  through a backflow line  22   a , so that a backflow-line fluid path  28   a  is formed. Within the backflow line  22   a  is arranged a valve  25  which is preferably designed as a solenoid valve and which can variably change the cross section of the backflow line  22   a . The further the valve  25  is opened, the more easily can the hydraulic fluid  20  flow out of the displacement chamber  14  back into the fluid reservoir of the concertina  18 , and the further the valve  25  displaces the flow cross section, the higher the resistance against the penetration of the piston  16  becomes. Due to the rise in the push-in force F E , the bending resistance in the knee joint of the leg prosthesis then rises. In addition to a configuration of the valve  25  as a solenoid valve, other valve configurations may also be provided, in particular actuating valves which allow a rapid and simple variation of the flow cross section in the backflow line  22   a.    
     In the exemplary embodiment illustrated, the fluid reservoir is formed by the volume within the concertina  18 , while the fluid damper  10  is designed as a linearly moving hydraulic damper. The fluid reservoir may also be designed in alternative configurations, in particular the fluid reservoir may also be acted upon by pressure so that the hydraulic fluid is pressed into the reservoir or pressed out of this counter to a pressure which may also be variable. Instead of a configuration of the fluid damper  10  as a linear piston damper, this may also be designed as rotary hydraulics in which a pivoting piston moves pivotally to and fro. The displacement chambers formed on both sides of the piston then form the respective fluid reservoir for the hydraulic fluid flowing through the housing. 
     A variant of the fluid damper  10  is illustrated in  FIGS. 9   a  and  9   b .  FIG. 9   a  shows the fluid damper  10  in the extended position, and  FIG. 9   b  shows it in the retracted position. Instead of an elastic concertina  18 , such as is formed in the exemplary embodiments according to  FIGS. 1 ,  5 ,  6  and  8 , the hydraulic damper according to  FIGS. 9   a  and  9   b  has a dimensionally stable protective cap  13  which is arranged at the piston-side end of the housing  12 . Instead of the concertina  18 , a diaphragm  15  is provided, which is designed elastically and can be displaced within the cap  13  in the direction of the housing  12 . The diaphragm  15  surrounds an annular gap  19  sealingly, so that a fluid reservoir  19  is formed between the diaphragm  15  and the housing  12 .  FIG. 9   a  illustrates the piston  16  in the extended position, so that the hydraulic fluid is collected in the displacement chamber  14 . The fluid reservoir  19  assumes a minimum size, this being achieved in that the diaphragm  15  is displaced in the direction of the housing  12 . This gives rise, on that side of the diaphragm  15  which faces the cap  13 , to a compensating volume  17  which can be filled with air through holes or pressure compensation orifices, not illustrated, in the cap  13 . When the piston is retracted, as illustrated in  FIG. 9   b , a larger volume is required within the fluid reservoir  19 , so that the diaphragm  15  is pressed outward in the direction of the cap  13 . The air present in the compensating volume  17  is pressed out of the cap  13 , with the result that the volume of the fluid reservoir  19  can be increased. In the exemplary embodiment illustrated, the fluid reservoir  19  is connected to the hydraulic fluid circuit via the backflow line  22 , but it is also possible, in principle, to provide a corresponding connection via a backflow line  22   a  provided with a valve  25 . 
     In addition to a one-sidedly acting piston  16 , as is shown in the exemplary embodiments illustrated, it is likewise possible to provide a double-sidedly acting piston  16  and to use it in a hydraulic damper  10 . The cap  13  serves particularly for ensuring a mechanical protection of the diaphragm  15  which assumes the function of the concertina  18 . 
     Various load states of a leg prosthesis are illustrated in  FIGS. 10   a  and  10   b .  FIG. 10   a  shows a leg prosthesis  58  in a non-loaded state. A thigh part  60  with a knee joint  72  and with a fluid damper  10  mounted thereon is resiliently mounted, via a spring element arranged at the distal end, so as to be displaceable with respect to the lower leg  66 . Between the thigh shank  60  and the lower leg shank  66 , a gap S is provided, which has the maximum extent S a  in  FIG. 10   a . If, then, an axial force is exerted on the lower leg  66 , the spring element  100  is compressed, as illustrated in  FIG. 10   b . In the exemplary embodiment illustrated, the spring element  100  is designed as a cylindrical spring element which is compressed to the maximum extent to its block length in  FIG. 10   b . The gap S is then minimal and is identified in  FIG. 10   b  by reference symbol S b . The difference between S a  and S b  is the displacement travel which corresponds essentially to the spring excursion of the spring  100  or the spring element  100 . Owing to the compression of the spring element  100 , there follows between the fluid damper  10  and the lower leg  66  a relative movement which may be utilized either for generating a sensor signal for switching a valve or for a direct mechanical switching of the valve. Moreover, the spring  100  causes a slight damping of the tread, while the prosthesis wearer has no feeling of uncertainty on account of a relatively short spring excursion. In the event of axial load, that is to say when the patient is treading or standing, switching is caused such that an increased hydraulic resistance is set in the fluid damper  10  so as to give a patient the highest possible feeling of safety, without a complicated mechanical unlocking of the knee joint having to be carried out in the relieved state, for example during sitting. In the relieved state according to  FIG. 10   a , a pivoting about the pivot axis  73  of the knee joint  72  can take place, and, in the loaded state according to  FIG. 10   b , the resistance is increased substantially, ideally locked hydraulically, so that no bending is possible without the destruction of mechanical components. 
     The selected arrangement of the spring element  100  is in this case very low within the lower leg  66 , in order to place the force introduction points as far apart as possible so as to reduce the loads on the mechanical components. The switching of the fluid damper  10  takes place independently of the orientation of the force applied to the lower leg  66 , and, after a threshold value of an axial force fraction has been overshot, switching is triggered either via a sensor or via a mechanical device, such as a solenoid valve  24 ,  25 . 
     If, for example, a fluid damper  10  according to  FIGS. 8   a  and  8   b  is installed in the leg prosthesis  58 , in which the valve  25  is designed as a solenoid valve, direct switching can take place by means of a displacement of a switching magnet with respect to the solenoid valve  25 . The solenoid valve  25  is in this case prestressed in the direction of a closing position, so that, after the lapse of a counterforce by the switching magnet, the solenoid valve  25  closes automatically. If, therefore, a switching magnet is arranged in the lower leg  66  and a compression of the spring element  100  is brought about, the housing  12  of the fluid damper  10  is displaced in relation to the lower leg  66  and consequently the solenoid valve  25  is displaced in relation to the switching magnet. If the lower leg  66  is sufficiently loaded, the switching magnet is moved away from the solenoid valve  25  to an extent such that the switching force does not overshoot the prestress, so that the solenoid valve  25  closes. If the lower leg  66  is relieved, the housing  12  moves into the initial position according to  FIG. 10  due to the return force of the spring  100  and the solenoid valve  25  is opened. The damping is thereby reduced, because the hydraulic fluid  20  can flow, virtually unimpeded, out of the displacement chamber  14  into the fluid reservoir in the concertina  18 . In the exemplary embodiment according to  FIGS. 10   a  and  10   b , the fluid damper  10  has a concertina, but alternatively to this a fluid damper according to  FIGS. 9   a  and  9   b  may also be used, with a cap  13  as mechanical protection either of the concertina  18  or of the diaphragm  15 . Basically, however, the fluid damper  10  according to  FIGS. 1   a  and  1   b  is also possible and provided, and a fluid damper according to  FIGS. 1   a  and  1   b  may likewise be provided with a protective cap  13  against damage to the concertina  18 . Alternatively to a solenoid valve, the actuation of a valve may also take place by means of a buckling spring, a slotted guide or a lever mechanism. 
     Alternatively to the illustrated embodiment of the spring element, other telescopic devices and spring elements may be provided, which allow a relative displacement of the thigh  60  or thigh shank  60  and of the lower leg  66  with respect to one another. In the exemplary embodiment according to  FIGS. 10   a  and  10   b , the knee joint  72 , together with the fluid damper  10  and with a strut, not illustrated in any more detail, is designed as a kind of frame and is mounted on the thigh or thigh shank  60 , so that these components can be displaced together in relation to the lower leg  66 . The arrangement of the spring element  100  in the lower leg  66  is not mandatory, and basically another relative displacement between the lower leg  66  and damper  10  may also be implemented. 
       FIG. 11  illustrates diagrammatically a variant of the invention. A thigh shank  60  is connected to a lower leg  66  pivotably via the axis of rotation  73  of the knee joint  72 . The lower leg  66  has a housing  166 , within which the hydraulic damper  10  and a bar  150  are mounted. The bar  150  carries the axis of rotation  73  at its proximal end and the spring element  100  at its distal end. The distal end of the housing  12  is likewise mounted on this bar  150 . The proximal end of the piston  16  is arranged on the thigh shank  60  or thigh  60 . Within the housing  166  is arranged a cam  130  which projects in the direction of the damper housing  12 . A switching magnet  110  is mounted resiliently on the housing  12  via a spring tongue  120 . The switching magnet  110  lies opposite the solenoid valve  25 . 
     An axial guide  140  for the bar  150  is likewise provided within the housing  166  and ensures that the bar  150  moves only axially, so that a lateral displacement of the bar  150  and consequently of the knee joint  120  and thigh  60  or thigh shank  60  in relation to the lower leg  66  is avoided. When an axial force which has a sufficiently high component in the direction of the axis of rotation  73  is exerted on the lower leg  66 , the spring element  100  is compressed, so that the spring tab  120  is displaced in relation to the cam  130 . On account of the curved form of the spring tab  120 , the pressure decreases in the event of an increasing axial displacement in the distal direction, so that the switching magnet  110  is moved increasingly away from the solenoid valve  25  as a result of the return force of the spring tab  120 . As soon as the switching magnet  110  has been moved sufficiently far from the solenoid valve  25 , the solenoid valve  25  closes instantaneously and increases the damping abruptly. The same mechanism may also be employed for the angle-dependent control of the damper force. If the thigh  60  or thigh shank  60  is pivoted about the pivot axis  73 , the bearing point of the piston  16  on the thigh shank  60  executes a circular movement which, in addition to a vertical component, also has a horizontal component. Via the horizontal displacement of the piston  16  and, along with this, of the housing  12 , the switching cam  130  is moved away from the spring tab  120 , so that, with an increasing pivot angle φ of the thigh  60  in relation to the lower leg  66 , the solenoid valve  25  is activated in that the switching magnet  110  is displaced away from the housing  12 . The embodiment illustrated has the advantage that, in the event of a failure of the control, for example if the spring tongue  120  breaks or an unwanted axial displacement occurs, the damping is increased instantaneously so that the knee joint  72  is kept stable. This is extremely important for geriatric patients so that they feel safe. 
     In addition to a magnetic activation of the valve  25 , other mechanical couplings may also take place, for example via cam disks, slotted guides or lever devices, which close the valve in the event of a sufficient displacement in the axial direction or sufficient bending. 
     The spring element  100  is designed as a spring block which, in addition to making available a displacement travel between the thigh shank  60  and the lower leg shank  66 , also serves for absorbing transverse forces and for avoiding possible play within the joint  72  or the prosthesis  58 . The block spring  100  can absorb transverse forces and provides a certain tread damping, while at the same time a stop and limit the axial displacement travel that can be set via the spring  100 . 
     The use of a fluid damper in a geriatric knee joint has hitherto been considered too complicated, but it has become apparent that fluid dampers are highly suitable, since they have no “stick/slip” effect which occurs in other mechanical interlocks or braking devices. In addition to the illustrated mechanical switching of the valve  25 , electronic detection of a displacement or axial force and a corresponding actuation of the valve via an actuator are likewise possible. The angle-dependent control applies a relatively high resistance to the joint  70  from a relatively small bending angle, in order to provide high damping and standing phase safety at a relatively early stage. The switching threshold at which the valve  25  closes and provides increased resistance can be set and preferably lies between a 20° and 50° bending angle. Within this angular range of 20° to 50°, there is a switch from a relatively low resistance within the fluid damper to a high resistance. 
     LIST OF REFERENCE SYMBOLS 
     
         
           10  Fluid damper 
           12  Cylinder housing 
           13  Cap 
           14  Displacement chamber 
           15  Diaphragm 
           16  Piston 
           17  Free space 
           18  Concertina 
           19  Fluid reservoir 
           20  Hydraulic fluid 
           22  Backflow line 
           22   a  Backflow line 
           24  Valve 
           25  Valve 
           26  Annular gap 
           28  Annular-gap fluid path 
           28   a  Fluid path backflow 
           29  Seal 
           30  Backflow-line fluid path 
           32  Valve seat 
           34  Valve ball 
           36  Helical spring 
           38  Backflow path 
           40  End 
           42  End face 
           44  Pin 
           46  Actuator diaphragm 
           48  Actuator basic body 
           50  Bypass line 
           52  Bypass valve 
           54  Bypass-line fluid path 
           56  Electromagnet 
           58  Leg prosthesis 
           60  Thigh 
           62  Proximal thigh end 
           64  Distal thigh end 
           66  Lower leg 
           68  Proximal lower leg end 
           70  Distal lower leg end 
           72  Knee joint 
           73  Axis of rotation 
           74  Front foot 
           76  Back foot 
           78  Front-foot force sensor 
           80  Back-foot force sensor 
           82  Electric control 
           84  Leg-axis force sensor 
           86  Sleeve 
           88  Measuring piston 
           90  Filling material 
           92   a,b  Measuring elements 
           94  Elastic element 
           96   a,b  Webs 
           100  Spring element 
           110  Magnet 
           120  Spring 
           130  Cam 
           140  Guide 
           150  Bar 
           166  Housing 
         T Push-in depth 
         F E  Push-in force 
         F A  Pull-out force 
         F B  Leg-axis force 
         F v  Front-foot force 
         F F  Heel force 
         E Penetration portion 
         A Free portion 
         L Longitudinal angle 
         φ Pivot angle 
         S Displacement travel