Abstract:
An artificial foot has a connecting part for a lower leg part, and a base foot part rotatably connected to the connecting part via an ankle joint. The base foot part extends rearwards of the ankle joint in a heel section and forwards of the ankle joint in a midfoot section. A forefoot part is rotatably connected to the midfoot section. In order to allow an automatic adaptation of the foot to varying heel heights while ensuring a good stability, an articulated connection transmits the angular position of the connecting part in relation to the base foot part in a proportional manner to the angular position of the forefoot part in relation to the base foot part.

Description:
FIELD OF THE INVENTION 
     The invention relates to an artificial foot with a connecting part for a lower leg part, with a base foot part which is pivotably connected to the connecting part via an ankle joint and extends rearward of the ankle joint into a heel section and forward of the ankle joint into a midfoot section, and with a forefoot part connected pivotably to the midfoot section. 
     BACKGROUND 
     An artificial foot of this kind is known from U.S. Pat. No. 5,913,902. The ankle joint is located in the axis of a tubular lower leg part of a below-knee prosthesis received by an adapter piece of the connecting part. The ankle joint and the joint between forefoot part and base foot part are pivotable independently of one another, the pivotability being elastically damped by compression springs fitted between the respective joint parts. This foot construction permits the use of the artificial foot with several heel heights, since the foot can be adapted to different angles of the forefoot and of the connecting part relative to the base foot part. However, the static arrangement in this case alters as a function of the heel height, so that widely different walking dynamics arise also as a function of the heel height. 
     DE 100 10 302 A1 discloses a prosthetic foot with a movable ankle joint and with a similarly movable forefoot part. The object of the disclosed foot construction is to substantially prevent dropping of the knee joint, and thus of the hip joint, during heel-to-toe movement across the toe area. To achieve this, a plantar flexion is sought between foot and lower limb, that is to say between base foot part and connecting part. For this purpose, a coupling rod is secured between the forefoot part and the base foot part, the securing in the base foot part being effected by an oblong hole. One end of the coupling rod protrudes into the area of the connecting part and, because of the bending of the forefoot part, limits the angle between connecting part and base foot part. By limiting the angle, further dropping of the knee, and thus of the hip, is avoided. Adaptation of the foot to different heel heights is neither provided for nor indeed possible in the described construction. 
     Prostheses are known in which the sagittal adaptation of the prosthetic foot can be done by the patient, in order to permit adaptation to different heel heights. A wrong adjustment cannot be ruled out. To ensure that the forefoot can adapt to the shape of the shoe, it is made soft. As a result, however, the load uptake in the forefoot area at the end of the stance phase is insufficient. 
     SUMMARY 
     The object of the present invention is to design an artificial foot of the aforementioned type in such a way that it can automatically adapt to different heights of shoe heel and, in so doing, guarantees good stability in a standing position. 
     In an artificial foot of the type mentioned at the outset, this object is achieved, according to the invention, by an articulated connection which transmits the angular position of the connecting part relative to the base foot part in a proportional manner to the angular position of the forefoot part relative to the base foot part. 
     The foot according to the invention thus provides for a forced coupling between the connecting part and the forefoot part in respect of the angular position relative to the base foot part. According to this forced coupling, a change in the angular position of the connecting part in a rearward direction leads to a proportional lifting of the forefoot part. If the artificial foot, starting from a position without heel (barefoot position), is inserted into a shoe with a high heel, the position of the lower leg (and thus of the connecting part) relative to the base foot part changes in the sense of a rearward shifting of the angle. Accordingly, the forefoot part is lifted until it again lies parallel to the contact surface (front sole of the shoe). It is therefore not necessary for the forefoot part to be made soft so that the forefoot part lies firmly on the supporting surface and thus takes up the required load for a secure stance, even with a high heel. 
     The articulated arrangement is in this case a multiple-joint arrangement and can in particular be a four-joint arrangement. 
     The articulated connection is preferably formed by the midfoot section and by a linking rod that is connected in an articulated manner to forefoot part and connecting part, these elements then being disposed in a four-joint or multiple-joint arrangement, wherein the midfoot section and linking rod can lie substantially parallel to one another. 
     In a preferred embodiment of the invention, the connecting part is provided with an extension piece which extends downward across the ankle joint and on which the linking rod is secured pivotably via one of its free ends. 
     The articulated connection according to the invention can be formed by pivot joints, but also by material hinges, said material hinges being formed by a flexible section of a coupling piece of the articulated connection. 
     The function of the artificial foot according to the invention entails the base foot part being designed as a rocker which is mounted more or less centrally between the heel and the area of the ball of the foot. The prosthetic foot thus generates a structure that allows the loading line to extend at all times through the bearing point of the rocker. This corresponds to the circumstances of the natural foot. By virtue of the coupling between the connecting part and the forefoot part, the forefoot always extends approximately parallel to the ground. This corresponds to the design of shoes with a constant sole thickness in this area, which is necessary, since otherwise the foot would be pressed into the point of the shoe. 
     If the foot according to the invention is constructed only with rigid elements, it very quickly generates, in the middle of the standing phase, a forefoot resistance that does not correspond to the feeling when standing on a natural foot. To permit adaptation to the natural feeling when standing, it is advantageous for at least one coupling piece of the articulated arrangement to be elastically deformable in the longitudinal direction. This is particularly expedient for a coupling piece that is subjected predominantly to tensile loading in the articulated arrangement. In this case, the coupling piece can be designed as a curved leaf spring which initially responds gently to tension and then gradually increases the stiffness. 
     Further possible improvements are obtained in respect of the dynamics of walking. When the heel touches down during walking, the base foot part designed according to the invention as a rocker tilts forward, as a result of which the connecting part is inclined relatively rearward and, consequently, the forefoot part is drawn upward. If one wishes to avoid this effect, a distinction has to be made between a momentary, substantial loading of the heel section of the base foot part when the heel touches down during walking, and the longer-term loading for adaptation of the height of the heel. This can be achieved by providing an adjustment element for the angle between base foot part and connecting part, which adjustment element becomes effective only when there is a load that continues for several seconds. An adjustment element of this kind can be a hydraulic cylinder. The latter is expediently combined with an elastic member that is deformable under momentary loading (when the heel touches down), in order to achieve a cushioning action when the heel touches down during walking. 
     The distinction between momentary loading when the heel touches down and long-term adaptation of the height of the heel can preferably be achieved by a valve arrangement being inserted into a circuit path of the hydraulic cylinder, which valve arrangement interrupts the flow of the hydraulic fluid in the circuit path when the flow velocity suddenly increases. By contrast, in the case of a slow adaptation, the valve arrangement does not shut the circuit, so that the hydraulic fluid is able to flow through for changing the adjustment element formed by the hydraulic cylinder. 
     During the walking process, the foot normally rolls forward across the forefoot area. In doing so, the lower leg (the connecting part) is tilted forward relative to the base foot part, so that the coupling according to the invention leads to a pressing down of the forefoot part, that is to say the forefoot “digs in”. This can be alleviated by designing the articulated connection so as to have a reduced transmission factor in respect of the angular positions. However, this also impairs the adaptation to the different heel heights. 
     It is therefore preferred that at least one coupling piece of the articulated arrangement is designed to be adjustable in length, the length adjustment being able to be controlled by at least one sensor that detects parameters of the walking or standing situation. The length adjustment can be performed by a hydraulic cylinder. 
     It is in each case expedient to arrange an elastic member between forefoot part and base foot part, which elastic member permits in particular the length adjustment of a coupling piece, in particular of the linking rod. 
     The elastic design of the linking rod can be supplemented or replaced by an elastic design of the extension piece of the connecting part on which the linking rod is articulated. The extension piece itself can be made of an elastic material or can be connected pivotably and in an elastic manner to the rest of the body of the connecting part. 
     It is particularly expedient, for the present invention, if the ankle joint is arranged at a distance forward of an adapter attachment on the connecting part receiving the lower leg part. Good stability is thus achieved, and the necessary torques for controlling the foot are applied. 
    
    
     
       DESCRIPTION OF THE DRAWINGS 
       The invention will be explained in more detail below on the basis of illustrative embodiments depicted in the drawing, in which: 
         FIG. 1  shows the basic structure of an artificial foot according to a first embodiment, depicted in the barefoot state; 
         FIG. 2  shows a schematic view of the foot according to  FIG. 1 , in a shoe with a high heel; 
         FIG. 3  shows a view according to  FIG. 1 , illustrating the position of the ankle joint; 
         FIG. 4  shows a view according to  FIG. 3 , for a forwardly shifted load when standing; 
         FIG. 5  shows a view of the foot according to  FIG. 1  during walking, when the heel touches down; 
         FIG. 6  shows a schematic view of a variant of the foot according to  FIG. 1 , corresponding to a second illustrative embodiment; 
         FIG. 7  shows a schematic view of a possible mode of action of a damper; 
         FIG. 8  shows a third embodiment of a foot according to the invention with an elastic linking rod, in the rest state; 
         FIG. 9  shows a view according to  FIG. 8 , for a heel-to-toe movement across the forefoot; 
         FIG. 10  shows a fourth embodiment of the foot according to the invention, with a length-adjustable linking rod; 
         FIG. 11  shows a fifth embodiment as a design variant to the embodiment according to  FIG. 10 ; 
         FIG. 12  shows a sixth embodiment with an articulated arrangement comprising a joint in the manner of a hinge joint. 
     
    
    
     DETAILED DESCRIPTION 
     A first embodiment of a foot according to the invention is shown schematically in  FIGS. 1 to 5 . The basic structure of the foot includes a base foot part  1 , which is connected to a connecting part  3  via an ankle joint  2  designed as a pivot joint. The connecting part  3  has an adapter attachment  4  in the form of an upturned truncated cone which is used to receive, in an adjustable manner, a tubular lower leg part  5  of a below-knee prosthesis. The adapter attachment  4  thus protrudes from an upper delimiting surface  6  of a cosmetic cover  7  of the foot prosthesis which encloses the functional parts of the foot prosthesis up to the end face  6 . 
     The base foot part  1  forms, rearward of the ankle joint  2 , a heel section  8  which slopes rearward and downward from the connecting part  3  and ends in a heel contact surface  10  close to the sole  9  of the cosmetic cover  7 . The base foot part  1  extends forward of the ankle joint  2  in a substantially rectilinear midfoot section  11 , on the front end of which a forefoot part  12  representing the toe area is articulated via a pivot joint  13 . The forefoot part  12  forms an approximately triangular wedge whose underside  14  lies parallel to the sole  9  of the cosmetic cover  7  in the area of the forefoot  12 . The pivot joint  13  is located at the upper tip of the triangular forefoot part  12 . In the lower area of the triangular forefoot part  12  there is a further pivot joint  15  via which the forefoot part  12  is connected by means of a linking rod  16  to the connecting part  3 . For this purpose, the connecting part  3  has a downwardly extending and rigid extension piece  17  on which there is a pivot joint  18  for securing the linking rod  16 . The connecting part  3  is therefore L-shaped, with a horizontal branch lying approximately parallel to the end face  6 , and, arranged approximately at right angles to the latter, a downwardly extending branch in the form of the extension piece  17 . 
     The axes of the pivot joints  2 ,  13 ,  15 ,  18  lie parallel to one another and transverse to the sagittal plane of the foot, that is to say parallel to the frontal plane of the patient. In the first embodiment shown, the spacings between the pivot joints  13 ,  15 , on the one hand, and  2 ,  18 , on the other, are approximately the same, so that the midfoot section  11  and the linking rod  16  are oriented approximately parallel to one another (with respect to the connection line between the pivot joints  2 ,  13 , on the one hand, and  15 ,  18  on the other). In the embodiment shown, the articulated connection between the connecting part  3  and the forefoot part  12  thus corresponds to a parallelogram linkage. 
     The artificial foot is shown in  FIG. 1  in the barefoot state, i.e. without the heel of a shoe. Compared to this,  FIG. 2  shows the foot according to  FIG. 1  in a shoe  19  that has a high heel  20 . Accordingly, the connecting part  3  or lower leg part  5  is tilted rearward relative to the base foot part  1 , as a result of which the forefoot part  12  is swiveled upward relative to the base foot part  1  via the articulated connection comprising joints  2 ,  13 ,  15 ,  18 . Since the base foot part  1  in the shoe  19  is directed obliquely forward and downward because of the high heel  20 , the upward swiveling of the forefoot part  12  is set such that the underside  14  of the forefoot part  12  extends parallel to the sole  21  of the shoe  19 . The articulated connection shown thus ensures automatic adjustment of the foot to the height of the heel  20  with the aid of the swiveling movement of the forefoot part  12  by means of the articulated connection at the pivot joints  2 ,  13 ,  15 ,  18 . 
       FIGS. 3 and 4  illustrate the forces taken up by the foot according to the invention. The weight force is introduced directly into the ankle joint  2 . The ankle joint  2  is positioned at a distance forward of the adapter attachment  4  and below-knee prosthesis  5 , as a result of which the ankle joint  2  engages approximately centrally on the base foot part  1  which is articulated as a rocker on the ankle joint  2 . The ground reaction forces are introduced in the area of the heel contact surface  10  and in the area of the ball of the foot, that is to say approximately level with the pivot joints  13 ,  15  of the forefoot part  12 . Accordingly, the ground reaction forces are located more or less symmetrically to either side of the introduced weight force when there is a stable stance in equilibrium. 
     If the weight is now shifted forward in accordance with  FIG. 4 , a torque arises on the lower leg part  5  and connecting part  3  in the direction of the curved arrow in  FIG. 4 . In this way, a tensile force is exerted on the linking rod  16  in a rearward direction, as a result of which the front tip of the forefoot part  12  is pressed downward. The tip is therefore in a position to take up additional ground reaction forces, as is indicated in  FIG. 4  by the additional arrow in the area of the tip of the forefoot part  12 . The ground reaction force acting on the heel contact surface  10  has in this case decreased as a result of the forward shift in weight. 
       FIG. 5  shows the foot according to  FIG. 1  in a heel touch-down phase during walking. In this case, the lower leg part  5  or connecting part  3  is tilted rearward relative to the base foot part  1 , so that the forefoot part  12  has been swiveled upward slightly, in an anatomically correct manner. When the foot is set down in the heel area, the rocker forming the base foot part  1  swivels abruptly downward with the midfoot section  11  around the ankle joint  2 , so that the rearward swivel angle of the lower leg part  5  relative to the base foot part  1  is increased further, as a result of which the forefoot part  12  is swiveled upward to an extreme degree. 
     It may therefore be expedient, according to  FIG. 6 , to attenuate the swiveling of the base foot part  1  at heel touch-down by inserting a damper  22 , for example in the form of a hydraulic damper, between the heel section  8  of the base foot part  1  and the connecting part  3 . The elastic damper prevents the midfoot section  11  from dropping down in an undamped manner when the heel touches down. 
       FIG. 7  illustrates a preferred mode of action of the damper  22 . The latter is made up of a cylinder chamber  23  and of a piston  24 . The relative movement between heel section  8  and connecting part  3  is damped via a circuit line  25  through which the hydraulic fluid has to flow from one side to the other side of the piston  24 . A valve arrangement  26 , shown schematically here and consisting of a shut-off ball  28  held by springs  27 , is preferably inserted into the circuit line  25 . At a low flow velocity of the hydraulic fluid in the circuit line  25 , as occurs for example in a quasi-static state for adaptation to the different heights of a heel  20  of a shoe, the required distance compensation takes place in damped form by means of the hydraulic fluid. 
     In the event of a momentary load peak, as occurs when the heel touches down during walking, the ball  28  is pressed against a shut-off surface  29  by the high flow velocity and blocks the flow path for the hydraulic fluid. In this case, the heel touch-down does not lead to a change in the spacing adjustment, effected by the hydraulic cylinder  22 , between the heel section  8  and the connecting part  3 . 
     In order to permit an elastic touch-down of the heel, it is expedient for an elastic member, for example a spring, to be coupled in series with the damper  22 . 
     The embodiment shown in  FIGS. 8 and 9  entails the same arrangement of the pivot joints  2 ,  13 ,  15 ,  18 , but with the difference that the linking rod  16 ′ between the pivot joints  15  and  18  is formed by a curved leaf spring.  FIG. 8  shows the state when the person is standing steady. When the weight is shifted forward, the rigid linking rod  16  according to  FIG. 1  would cause an immediate and considerable loading of the forefoot, which would not correspond to the natural feeling when standing. When the weight is shifted forward according to  FIG. 9 , the linking rod  16 ′ can, by contrast, lengthen slightly through elasticity, so that, with a further tensile force on the linking rod  16 ′, it provides a progressively increasing counter-force for the forefoot. A certain “swaying”, when the patient is standing, is thus stabilized by an elastic, gradually increasing counterforce, which corresponds to the natural feeling when standing. 
       FIG. 10  shows an embodiment of the artificial foot in which the linking rod  16 ″ is designed to be adjustable in length by means of a hydraulic cylinder  30 . Like the hydraulic cylinder  22  in  FIG. 7 , the hydraulic cylinder  30  in this case can, in the event of a momentary load, delay a damping of the transmission of the angular position of the connecting part  3  relative to the base foot part  1  to the angular position of the forefoot part  12  relative to the base foot part  1 . By means of the change of length of the hydraulic cylinder  30 , the transmission ratio of the parallelogram linkage between the connecting part  3  and the forefoot part  12  can be changed such that excessive downward pressing of the foot part  12  is avoided at a forwardly inclined angular position of the connecting part  3  relative to the base foot part during heel-to-toe movement. A direct transmission of the angular position would lead to a “digging in” of the toe area of the forefoot part  12  during the heel-to-toe movement across the forefoot part  12 . This digging-in effect can be avoided by virtue of a change in the transmission ratio between connecting part  3  and foot part  12 , brought about by the change in length of the hydraulic cylinder  30 . Arranged between the forefoot part  12  and the midfoot section  11  of the base foot part  1  there is a spring  31  which defines a preferred position of the forefoot element. The spring  31  can advantageously be replaced by a spring/damper combination arranged in series. 
       FIG. 11  shows an embodiment having the same action as the embodiment according to  FIG. 10 , but with structural modifications. Thus, the base foot part  1 ′ extends from the heel area  8  curved only slightly upward into the midfoot section  11 ′. The midfoot section  11 ′ has an extension piece  32  which protrudes past the pivot joint  13  and carries the spring  31  or spring/damper combination acting on the forefoot part  12 . 
     The connecting part  3 ′ is again approximately L-shaped, but the downward extension piece  17 ′ is now arranged at the rear end, as a result of which the pivot joint  18  is shifted rearward in the direction of the heel area. A longer travel is therefore available for the linking rod  16 ″ with the hydraulic damper  30 . 
     The base foot part  1 ′ is articulated on the ankle joint  2  via an upward extension piece  33  of the base foot part  1 ′. 
     The embodiment shown in  FIG. 12  corresponds substantially to the embodiment according to  FIG. 1 , the only difference being that the pivot joint  13  is omitted and is replaced by a flexible section  13 ′ of the midfoot section  11 , which thus merges into the forefoot part  12 ′ via the flexible section  13 ′ formed by tapering of the material. Of course, the embodiment with the joint  13 ′ can be combined with the variants shown in the other figures of the drawing, for example with a flexible linking rod  16 ′ or with a length-adjustable linking rod  16 ″.