Abstract:
A prosthetic foot having a spring assembly with an anterior and posterior component, a snubber for limiting the movement of the spring assembly during assembly, and anterior and posterior shock absorbers provided between the components of the spring assembly and snubber. The arrangement of the posterior shock absorber, the spring assembly, and the snubber is such that ambulation results in the generation of a compressive force in the posterior shock absorber during a heel-strike phase and tensile force in the posterior shock absorber during a toe-off phase.

Description:
FIELD OF THE INVENTION 
     This invention relates to prosthetic feet, and to prosthetic limbs employing those feet. 
     BACKGROUND OF THE INVENTION 
     As is well known in the art, an individual&#39;s or an amputee&#39;s gait during ambulation or other movement has several discrete phases, of which three are particularly significant. The first phase, known as heel strike, occurs when the heel contacts the surface along which the individual is moving. The second phase, known as flat foot, occurs when the foot is roughly parallel to the surface; and the third phase, known as toe-off, occurs when the heel of the foot is lifted from the surface and only the toe portion of the foot bears upon the surface to propel the individual forward. 
     A variety of different types of prosthetic feet have previously been proposed. One of the earlier examples of a previously proposed foot is shown in FIG.  1  and described in detail in United Kingdom Patent No. 2187102. As shown in FIG. 1, the foot  10  comprises a keel  12  that is fixedly attached to a plate  14  joined to the cosmesis  16  of the foot. A spring member  18 ,  20  is sandwiched between the keel  12  and the mounting plate  14 . The keel  12  is provided with cantilevered snubber arms  22 ,  24  that serve to limit deflection of the spring member during ambulation. The spring arms serve to absorb energy on heel strike, and to release that energy (or at least a proportion thereof) on toe-off to facilitate ambulation of the amputee. 
     Whilst the arrangement shown in FIG. 1 provides many advantages over older systems, it has subsequently been noted that the device can cause discomfort to some amputees in certain circumstances. For example, it has been noted that the co-operating spring member  18  and snubber arm  22  can cause jarring on heel strike. It has also been noted that a significant amount of the energy stored during heel strike is lost, and thus that less stored energy is available to aid the amputee during toe-off. 
     United Kingdom Patent Application No. 2311466 discloses one system that purports to alleviate these and other problems. The device disclosed in the publication employs a compressible energy storage device which can take the form of an elastomeric or rubber buffer or a gas-filled strut, for example. The device employs a heel spring and a toe spring that are each pivotally mounted to a frame within which the energy storage means is provided. A complicated locking arrangement is provided to lock the energy storage means in a compressed state when it has been compressed during heel strike, and to release the energy storage means from that locked compressed state during toe-off. This arrangement, whilst going some way to providing better energy recovery during ambulation, suffers from the disadvantages that it is unduly complicated, potentially noisy and, by virtue of its complicated construction, possibly prone to mechanical failure. 
     Another system that purports to aid energy recovery is disclosed in International Patent Publication No. WO98/53769. This publication discloses a device comprising an ankle member to which a heel member is pivotally attached, and a toe member pivotally attached to the heel member. The heel member includes an arm that extends beyond the pivot point between the ankle member and the toe member. Rubber buffers are provided between the heel member and the ankle member, between the heel member arm and the ankle member, and between the heel member arm and the toe member. Ambulation of the amputee causes compression of the various buffers and toe-off is assisted as these buffers decompress. The publication discloses a variety of different buffers that can serve to reduce the operating noise of the prosthesis. 
     Whilst this arrangement is advantageous over that disclosed in United Kingdom Patent Application No. 2311466, it still demonstrates a significant energy loss during ambulation. Furthermore, it has also been noted that problems can arise with rapid degradation of the buffers particularly when employed in prostheses fitted to amputees of over 90 kg in weight. These problems are exacerbated by the fact that the prosthesis can become significantly warmer during use, the increase in temperature causing a corresponding increase in the rate of buffer decay. 
     SUMMARY OF THE INVENTION 
     Aspects of the present invention variously seek to further improve energy recovery during ambulation, and to avoid problems associated with the use of the prosthesis by heavier amputees (i.e. by amputees of over 90 kg in weight). It will be understood, of course, that the prosthesis of the present invention can also provide significant advantages to amputees of less that 90 kg in weight. 
     In accordance with an aspect of the invention, there is provided a prosthetic foot comprising: a spring assembly having an anterior component and a posterior component; a snubber pivotally connected to said spring assembly for limiting movement of said spring assembly during ambulation; and anterior and posterior shock absorbers provided between respective components of said spring assembly and said snubber; wherein the arrangement of said posterior shock absorber, said spring assembly and said snubber is such that ambulation results in the generation of a compressive force in said posterior shock absorber during a heel strike phase and a tensile force in said posterior shock absorber during a toe-off phase. It should be noted that provision of an anterior shock absorber is optional. 
     This arrangement is advantageous as it allows energy recovery from the posterior shock absorber to occur both in the heel strike and toe-off phases thereby increasing the quantity of energy that can be recovered during toe-off. 
     Preferably, the anterior shock absorber is replaceable, or removable from said prosthesis for replacement. Preferably, the posterior shock absorber is replaceable, or removable from said prosthesis fore replacement. Preferably, the posterior shock absorber is releasably attached to said spring assembly and said snubber. 
     Preferably, the spring assembly comprises a pair of resiliently flexible members each of which have a pair of cantilevered arms. One of the pair of flexible members may be shorter than the other. The flexible members are preferably releasably attached to a keel that forms the remainder of said spring assembly. 
     Alternatively, the spring assembly may comprise a secondary resiliently flexible member fixedly attached to a keel, and one or more resiliently flexible sole members attached to said secondary flexible member, said keel being pivotally attached to said snubber. 
     A further aspect of the invention provides a prosthetic foot wherein at least one of said anterior and posterior shock absorbers are of a multi-layer construction. 
     Preferably, at least one of said anterior and posterior shock absorbers are of a multi-layer construction comprising a sandwich of compressible and substantially non-compressible or rigid layers. 
     Preferably, said posterior shock absorber is less compressible than said anterior shock absorber. 
     A further aspect of the invention provides a shock absorber for a prosthetic foot, said shock absorber having a multi-layer construction comprising a sandwich of compressible or resiliently deformable layers and substantially non-compressible or rigid layers. 
     Preferably the compressible or resiliently deformable layers have a hardness in the region of 65 IHRD or 75 IHRD. 
     A further aspect of the invention provides, in combination, a prosthetic foot as described herein, a shock absorber as described herein as said anterior shock absorber, and a shock absorber as described herein as said posterior shock absorber. A further aspect of the invention provides a prosthetic leg comprising an exoskeleton and a prosthetic foot as described herein. A further aspect of the invention provides a prosthetic leg comprising an endoskeleton and a prosthetic foot as described herein. A further aspect of the invention provides a cosmesis for a prosthetic foot as described herein, said cosmesis comprising strengthening means in the vicinity of a toe region thereof. Preferably, the strengthening means comprises a member embedded in the sole of the cosmesis. The strengthening means may comprise a strip of material of greater stiffness than the remainder of the cosmesis. 
    
    
     DESCRIPTION OF THE DRAWINGS 
     An embodiment of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: 
     FIG. 1 is a cross-sectional view of a previously proposed prosthetic foot; 
     FIG. 2 is an elevation of a prosthetic foot according to an embodiment of the invention; 
     FIG. 3 is an elevation of an anterior shock absorber suitable for use with the foot of FIG. 2; 
     FIG. 4 is an elevation of a posterior shock absorber suitable for use with the foot of FIG. 2; 
     FIG. 5 is a schematic representation of a mechanism for forming the shock absorber of FIG. 3; 
     FIG. 6 is a schematic representation of a mechanism for forming the shock absorber of FIG. 4; 
     FIG. 7 is a cross-sectional view of a cosmesis; 
     FIG. 8 is a schematic view of an endoskeletal artificial leg; and 
     FIG. 9 is a schematic view of an exoskeletal artificial leg. 
    
    
     DESCRIPTION OF PREFERRED EMBODIMENTS 
     As mentioned above, FIG. 2 shows an elevation of a prosthetic foot  40  according to an embodiment of the invention. For ease of illustration, the foot  40  is shown without a cosmesis, but it will be understood that a cosmesis will be fitted over the foot before use. FIG. 7 shows one such cosmesis, the construction of which will be later described. 
     As shown, the foot  40  comprises a snubber  42  and a spring assembly  44 . The snubber  42  is preferably generally triangular in shape (to reduce the weight of the snubber) and is provided with connecting means  46  by means of which the foot may be connected to a prosthetic limb. Many different connectors are commonly available and the snubber  42  may be modified in shape to accommodate whichever connector is appropriate. 
     The snubber  42  is provided with a bore that is alignable with a similar bore  48  in a component of the spring assembly  44 , and through which an axle pin  50  is inserted. The axle  50  allows the spring assembly  44  to pivot through a predetermined extent controlled by the snubber in a manner to be later described. 
     The spring assembly  44  comprises a keel  52  and, in this embodiment, a sole spring arm  54  and a secondary spring arm  56 . As shown in FIG. 1, it is known to provide a single sole spring arm but problems have been experienced with delamination of the composite arm material, as a result of which it has been proposed to provide a pair of sole spring arms each of which may be of a composite material or a metallic material. Whilst the preferred embodiment to be described employs a single sole spring arm  54  it should be noted that a pair of sole spring arms (or more than a pair of sole spring arms) could be provided instead should one wish to. 
     The spring arms  54 ,  56  are securely sandwiched between a mounting plate  58  and the underside of the keel  52  by fixing means  60  (which are screws in this particular case). The mounting plate  58  is preferably of metal and preferably the cosmesis (not shown) is formed around the mounting plate at the instep region thereof. 
     The sole spring arm  54  forms the sole of the prosthetic foot and is provided with cantilevered toe and heel arms  62 ,  64 . The secondary spring arm  56  is also formed with cantilevered toe and heel arms  66 ,  68  of lesser extent than those of the sole spring arm  54 . Toe and heel buffers  70 ,  72  are fixed to the underside of the toe and heel arms  66 ,  68  to cushion abutment of corresponding portions of the secondary  56  and sole  54  spring arms during ambulation. 
     The keel  52  is generally U-shaped in lateral cross-section and has an anterior portion  74  and a posterior portion  76 . The snubber  42  is generally triangular and also comprises an anterior portion  78  and a posterior portion  80 . An anterior shock absorber  82  is provided between the anterior portion  78  of the snubber  42  and the anterior portion  74  of the keel  52 . The anterior shock absorber  82  is retained in place by a grub screw  84  in the snubber  42  and a retaining pin  86 . A posterior shock absorber  85  is retained in place between the snubber  42  and keel  52  by attaching a portion of the shock absorber to the snubber  42 , and by attaching an opposite portion to the keel  52  by a pair of retaining pins  88 ,  90 . 
     Advantageously, both of the shock absorbers may easily be removed and replaced simply by removing the retaining pins and by disconnecting the posterior shock absorber  85  from the snubber  42 . Easy removal and replacement of the shock absorbers allows for quick and inexpensive maintenance of the prosthesis. 
     Operation of the prosthesis during ambulation or other physical exercise will now be described. Upon heel strike the heel arm  64  of the sole spring arm  54  abuts against the ground and is deflected upwards towards the keel  52 . Deflection of the heel arm  64  causes the keel  52  to pivot about the axle  50  so that the posterior portion  76  of the keel  52  is driven towards the snubber  42  to compress the posterior shock absorber  85 . If the amputee is undertaking more vigorous exercise (such as running for example) then the deflection of the sole spring arm heel portion  52  may cause it to abut the secondary spring arm heel portion  68  and to also deflect the secondary spring arm heel portion towards the keel  52 . 
     As the amputee moves forward the prosthesis rolls over from the heel strike phase to the flat foot phase. Rolling over from the heel strike phase allows the sole spring arm heel portion  64  (and possibly also the secondary spring arm heel portion  68 ) to return to its undeflected state and the posterior shock absorber  85  to return to its non-compressed state. Relaxation of the sole spring arm heel portion  64  and the posterior shock absorber  85  returns energy that facilitates transition from the heel strike phase to the flat foot phase shown in FIG. 2, and during roll over to the toe-off phase. 
     The amputee&#39;s momentum and the energy returned from the heel strike phase causes the foot to roll over from the flat foot phase to the toe-off phase. As the foot rolls over to the toe-off phase, the toe arm  62  of the sole spring arm  54  abuts against the ground and is deflected upwards towards the keel  52 . Deflection of the toe arm  62  causes the keel  52  to pivot about the axle  50  so that the anterior portion of the keel is driven towards the anterior portion of the snubber to compress the anterior shock absorber  82 . Deflection of the toe arm  62  in the manner described also causes the posterior portion  76  of the keel  52  to be pivoted away from the snubber  42 . Pivoting the keel  52  away from the snubber  42  causes the posterior shock absorber  85 , which is connected to both the posterior portion  80  of the snubber  42  and the posterior portion  76  of the keel  52 , to be placed under tension. If the amputee is undertaking more vigorous exercise (such as running for example) then the deflection of the sole spring arm toe portion  62  may cause it to abut the secondary spring arm toe portion  68  and to also deflect the secondary spring arm toe portion towards the keel  52 . 
     When the amputee moves from the toe-off phase relaxation of the sole spring arm toe portion  62  (and possibly also the secondary spring arm toe portion  68 ) returns energy that facilitates movement from the toe-off phase of the amputee&#39;s gait. This energy return is further complemented by the relaxation of the anterior shock absorber  82  from its compressed state to its uncompressed state, and by the recovery of the energy stored therein. In addition, energy recovery is further improved by the relaxation of the posterior shock absorber  85  from its tensioned state to its untensioned state. It can be seen therefore that the prosthesis shown in FIG. 2 greatly increases the amount of energy that may be recovered during ambulation (and thus facilitates ambulation) when compared with previously proposed devices. 
     FIG. 3 is an elevation of an anterior shock absorber suitable for use with the foot of FIG.  2 . As mentioned above, previously proposed shock absorbers have been prone to accelerated wear particularly when used by amputees of over 90 kg in weight. The shock absorbers shown in FIGS. 3 and 4 have been conceived to address these problems, and to further assist in energy recovery. 
     As shown in FIG. 3, the anterior shock absorber  82  is of a sandwich-type construction and comprises alternate layers of compressible material  92  (i.e. resiliently deformable material) and rigid or substantially non-compressible material  94  (such as a metallic or composite material). The compressible material  92  is employed as an energy storage device that aids energy recovery during ambulation. The rigid material  94  helps reduce the degree of deformation of the shock absorber  82  and thus reduce the wear of the shock absorber during ambulation. In the preferred embodiment the anterior shock absorber comprises a sandwich of differing layers arranged thus: rigid material (or substantially non-compressible material)/compressible material/rigid material/compressible material/rigid material/compressible material/rigid material. 
     Many different types of compressible material may be employed for the compressible layers  92 . A presently preferred composition for the anterior shock absorber  82  compressible material provides a hardness of 65 IRHD and is available from Rapra Technology Limited, Shawbury, Shrewsbury, United Kingdom. The 65 IHRD composition comprises the ingredients set out below in Table 1. 
     FIG. 4 is an elevation of a posterior shock absorber suitable for use with the foot of FIG.  2 . The posterior shock absorber  85  is of similar construction to that of the anterior shock absorber and is preferably of a greater hardness in the order of 75 IRHD. As shown, the posterior shock absorber  85  comprises a sandwich of compressible layers  96  and rigid or substantially non-compressible layers  98 . As with the anterior shock absorber  82 , the compressible material  96  is employed as an energy storage device that aids energy recovery during ambulation. The rigid material  98  helps reduce the degree of deformation of the shock absorber  85  and thus reduce the wear of the shock absorber during ambulation. In the preferred embodiment the posterior shock absorber comprises a sandwich of differing layers arranged thus: rigid material (or substantially non-compressible material)/compressible material/rigid material/compressible material/rigid material/compressible material/rigid material. The secondarymost rigid material layer  100  is formed with a pair of roughly triangular sidearms that each have a bore  102  therethrough to allow the posterior shock absorber  85  to be connected to the snubber  42  as described above. 
     Once again, many different types of compressible material may be employed for the compressible layers  96 . A presently preferred composition for the posterior shock absorber  85  compressible material provides a hardness of 75 IRHD and is available from Rapra Technology Limited, Shawbury, Shrewsbury, United Kingdom. The 75 IHRD composition comprises the ingredients set out below in Table 1. 
     The rigid or non-compressible layers  94 ,  98  are preferably of a metallic material and a variety of suitable metals, and alternative sufficiently rigid materials, will be apparent to those persons skilled in the art. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 65 IHRD Composition 
                 75 IHRD 
               
               
                 Ingredient 
                 Part Hardness Rubber (phr) 
                 Composition 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                 SMR CV 60 
                 100.0 
                 100.0 
               
               
                 HAF N330 
                 60.0 
                 80.0 
               
               
                 Circo Light Rubber Oil 
                 10.0 
                 10.0 
               
               
                 Zinc Oxide 
                 10.0 
                 10.0 
               
               
                 Stearic Acid 
                 2.0 
                 2.0 
               
               
                 Flectol H 
                 2.0 
                 2.0 
               
               
                 Santoflex 13 
                 2.0 
                 2.0 
               
               
                 Strucktol WB16 
                 — 
                 2.0 
               
               
                 Sulphur 
                 1.5 
                 1.5 
               
               
                 CBS 
                 1.2 
                 1.2 
               
               
                 IMTD 
                 0.2 
                 0.2 
               
               
                   
               
             
          
         
       
     
     Some or all of the materials listed in Table 1 may be TradeMarks (registered or otherwise) of Rapra Technology Limited or others and all rights are hereby acknowledged as belonging to their respective owners. 
     FIGS. 5 and 6 are schematic representations of mechanisms for forming the shock absorbers of FIGS. 3 and 4, respectively. 
     As shown, the shock absorbers  82 ,  85  are preferably formed by a moulding process. The moulds  110 ,  120  are each three-piece moulds comprising backing plates  112 ,  122 , shaping plates  114 ,  124 , top plates  115 ,  125  and plungers  116 ,  126 . To fabricate the shock absorbers, the shaping plates  114 ,  124  are each secured to a corresponding backing plate  112 ,  122  and then layers of substantially non-compressible or rigid material are arranged at appropriate positions in the shaping plates  114 ,  124 . The top plates  115 ,  125  are secured to the shapingplates and suitable molten compressible material is then driven into the shaping plates  114 ,  124  by means of respective plungers  116 ,  126 . Once the compressible material has solidified the mould pieces can be removed to leave the finished shock absorber. 
     FIG. 7 is a cross-sectional view of a cosmesis that is suitable for use with the prosthesis of FIG. 2 or any other suitable prosthesis. The cosmesis  130  is of a suitable elastomeric material known to persons skilled in the art and is shaped to look like a foot. The cosmesis is hollow to accommodate the prosthesis and, in this embodiment, is provided with strengthening means to help alleviate problems caused by unwanted flexing of the cosmesis  130 . 
     In addition to the various constructional problems outlined above, it is also important for the cosmesis  130 , and indeed the foot as a whole, to behave as similarly to a normal human foot as possible in order to make the prosthesis less conspicuous for the amputee. For example, it has been noted that the cosmesis can flex across the front of the foot in the region of the toes during ambulation. This flexing provides an immediate indication to other persons that the foot is part of a prosthesis as it is wholly unnatural. 
     To help alleviate this problem it is proposed to provide strengthening means  132  in the sole of the cosmesis  130 . The strengthening means provides extra resilience to flexing at the very point of the cosmesis where it is most needed, namely in the region of the toes. In this embodiment the strengthening means is formed within the sole, but it will of course be apparent that it could be formed in an upper portion of the cosmesis forming the front of the foot. 
     It is important that the strengthening means does not prevent flexing as this too would be unnatural, and thus the choice of material from which the strengthening means is made is important. Investigations have shown that a material known as Ferravel LX 36 1  is particularly suitable, but other suitable materials will be apparent to persons skilled in the art. By incorporating a strengthening means of a suitable material into the cosmesis in the manner described it is possible to reduce unnatural flexing of the cosmesis and thus to further improve the appearance of the prosthesis. 
       1  Ferravel LX 36 is a TradeMark of Chamberlain Phipps Materials Limited of Wanlip Road, Syston, Leicester, United Kingdom.  
     As mentioned above, FIG. 8 is a schematic view of an endoskeletal artificial leg  200  which includes a prosthetic foot  40  such as that shown in FIG.  2 . The leg comprises an internal skeleton which in this embodiment comprises a shin tube  210 , a hinge  220  to simulate a knee and a thigh tube  240 . The tubes may be of carbon fibre, titanium or of a variety of other materials. The lover end of the shin tube is provided with a socket  250  to which the connecting means  46  of the foot  40  may be connected, and the upper end of the thigh tube  240  is provided with a socket  260  by means of which the leg may be fixed to a patient. Packing  270  can be provided about the endoskeleton to simulate the bulk of a normal human leg. 
     FIG. 9 is a schematic view of an exoskeletal artificial leg  300 . The leg of this figure comprises a shin portion  310  hingedly attached by a hinge  320  to a thigh portion  330 . A prosthetic foot  40  may be affixed to the shin portion  340  by engaging the connecting means  46  of the foot  40  with a socket  350  attached or comprising a part of the shin portion  310 . The upper end of the thigh portion  330  is provided with a socket  360  by means of which the leg may be fixed to a patient. 
     It will be understood that illustrative embodiments of the invention have been described herein, and that modifications may be made thereto without departing from the scope of the invention. 
     For example, whilst the shock absorbers of the present embodiment are of a multi-layer construction, it will be appreciated that conventional shock absorbers may be provided instead. It is also possible for only one of the posterior or anterior shock absorbers to be of a multi-layer construction. 
     Similarly, whilst the legs shown in FIGS. 8 and 9 are designed to replace lost legs substantially in their entirety, it will be apparent that the leg may be adapted if the patient has only lost their leg below the knee, for example.