Patent Publication Number: US-2007100466-A1

Title: Prosthetic foot devices

Description:
This application claims priority from U.S. Provisional App. No. 60/720,433 filed Sep. 24, 2005, which is hereby incorporated by reference. 
    
    
     TECHNICAL FIELD OF TEH INVENTION  
      The present invention relates to a prosthetic foot. More particularly, it relates to an improved prosthetic foot with characteristics of a dynamic response device.  
     BACKGROUND AND SUMMARY OF THE INVENTION  
      Prosthetic feet have undergone major developments in the past several decades, largely spurred by patients demanding full functionality in their prosthesis. Bioengineering research has begun to consider the presence of many complex inter-functionalities in the human form and to address these with a more sophisticated prosthetic design.  
      There are two general types of current, high-end prosthetic feet: dynamic response and articulating. Dynamic response feet are feet that may be semi-rigid or have a flexible keel, while articulating feet attempt to recreate foot and ankle function.  
      Popular articulating type prosthetic designs include the Navy ankle, the Greissinger foot, the SACH (Solid Ankle Cushioned Heel) foot, and the Tru-Step™ foot, all of which employ rubber spacers to allow flexure and impact absorption. The benefits of these feet are many, including the fact that they generally have good re-creation of the foot&#39;s intact functioning. Unfortunately, their extremely high maintenance requirements and material fatigue make them less than optimal. Additional drawbacks include relatively high weight, complexity of construction, noise resulting from pivoting at bushings, and threat of catastrophic failure.  
      On the other hand, dynamic type devices are typically lightweight and relatively highly stable. A popular, exemplary conventional dynamic response type foot is known as the Flex-Foot™. It incorporates a flexible carbon fiber shank and heel spring that allows the entire length of the prosthesis (rather than just the foot) to flex, absorb and return energy. Other dynamic response prosthetic feet are currently available with a range of different approaches. Generally using some type of composite (laminated or injection molded) in conjunction with metallic hardware, they are conjoined to an endo-skeletal assembly, which joins the prosthetic foot to the stump socket of the wearer. The carbon beam types are quite popular with users because of their robust and lightweight nature. In some models, the laminated beams may be split down the centerline, allowing for either side of the foot to move relatively independently of the other, providing increased response and stability. Unfortunately, however, the presently available dynamic response type devices lack the flexibility and accurate ankle replication response of the articulating devices. In addition, many of the dynamic devices require a dedicated type of leg shaft.  
      An improved prosthetic foot is described by the present Applicant in U.S. patent application Ser. No. 10/832,610, entitled “Prosthetic Foot Devices,” which is hereby incorporated by reference. Although the prosthetic foot described by that application is an improvement over the prior art, there is still a need for a more robust prosthetic foot design that weighs less while still providing correct anatomical behavioral characteristics.  
     SUMMARY OF THE INVENTION  
      An object of the invention, therefore, is to provide an improved prosthetic foot. Embodiments of the invention can achieve minimal weight, robust structure, and anatomically correct behaviors by means of structural arrangement and maximized material application. A preferred embodiment uses a minimum of components while allowing for maximum user adjustment through a modular approach to design.  
      The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter. It should be appreciated by those skilled in the art that the conception and specific embodiments disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      For a more thorough understanding of the present invention, and advantages thereof, reference is now made to the following descriptions taken in conjunction with the accompanying drawings, in which:  
       FIG. 1A  shows an exploded view of a preferred embodiment of a prosthetic foot according to the present invention.  
       FIG. 1B  shows an assembled view of the preferred embodiment of  FIG. 1A .  
       FIG. 1C  shows a forefoot component from the embodiment of  FIG. 1B .  
       FIG. 2A  is a top view of a natural foot showing the talocrural (T c ) and talocruronavicular (T cn ) axes.  
       FIG. 2B  is a side view of a natural foot showing the talocrural (T c ) and talocruronavicular (T cn ) axes.  
       FIG. 3  is a top down view of a forefoot according to the present invention showing the forefoot centerline, center of mass path, and T cn  flexure line.  
       FIG. 4A  shows an exploded view of another preferred embodiment of a prosthetic foot according to the present invention.  
       FIG. 4B  shows an assembled view of the preferred embodiment of  FIG. 4A .  
       FIG. 5A  shows an exploded view of another preferred embodiment of a prosthetic foot according to the present invention.  
       FIG. 5B  shows an assembled view of the preferred embodiment of  FIG. 5A .  
       FIG. 6A  is a graph showing the nonlinear moment of resistance in an intact ankle.  
       FIG. 6B  shows a graph of the moment of resistance versus its angle of deflection for a model of an intact ankle. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS  
       FIG. 1A  shows a preferred embodiment of a right side prosthetic foot  100  according to the present invention. (A left side foot is a mirror of the foot  100  and thus will not be discussed.) The foot  100  generally comprises a forefoot  120 ; a heel assembly  160  with a heel left spring  130  and a heel spring plate  140 ; and a pyramid adapter mount  150 . In the following sections, the forefoot  120 , heel leaf spring  130 , and heel spring plate  140  will separately be addressed with regard to their structures and operations.  
      Pyramid adapter mount  150  is preferably composed of titanium metal that may also be embodied in a fiber reinforced, injection molded plastic or a number of other metal or plastic materials. The pyramid adapter mount  150  serves as the intermediary mounting surface between the forefoot leaf spring and a typical industry standard pyramid adapter  170 . In the embodiment shown in  FIG. 1B , pyramid adapter mount  150  is located on top of the forefoot  120  directly over heel assembly  160 . The pyramid adapter mount is preferably completely modular in that a number of iterations could be provided with the prosthetic at different heights, allowing multiple configurations of adapter build height. This allows the user to modify behavioral characteristics of the prosthetic to optimize the foot under foreseeable individual requirements and environmental situations.  
      Forefoot  
      The preferred forefoot  120  comprises a leaf spring  121  preferably made of a laminated carbon composite material. Alternatively, the forefoot leaf spring  121  may be composed of a titanium metal or a solid, three-dimensional weave carbon composite. Forefoot  120  is preferably concavely curved toward the limb of the wearer of the prosthetic foot and extends distally (away from the wearer&#39;s body) in the same manner as a natural foot. The forefoot  120  is located between the pyramid adapter mount  150  and the heel assembly  160 . In the preferred embodiment shown in  FIG. 1A  to  FIG. 1C , raised channel architecture is used to give additional longitudinal support to the forefoot leaf spring without significantly increasing the weight of forefoot  120 .  
      In a preferred embodiment, the raised channel  122  forms an open box structure over much of the length of the forefoot device. In the embodiment shown in  FIG. 1A  to  FIG. 1C , the open side of the open box structure is facing down toward the bottom of the prosthetic foot. As shown in  FIG. 1A  to  FIG. 1C , the channel is located generally in the center of forefoot leaf spring and is raised above the surface of the forefoot leaf spring. Skilled persons will recognize that the center channel need not be located in the exact center of the forefoot. For example, the center channel could be offset toward the outside edge of the forefoot leaf spring so that the inner uninterrupted surface of the forefoot leaf spring is wider than the outer portion. The location of the center channel may be manipulated in order to more closely approximate an individual&#39;s particular gait and anatomical variations.  
      When viewed from the bottom, forefoot  120  would give the appearance of an interrupted platform. As discussed below, an inverted channel could also be used, with the inverted channel below the upper surface of the forefoot leaf spring  121  and the open side facing up, away from the bottom of the prosthetic foot.  
      In addition to providing structural support, the open channel also serves as a transverse “hinge,” allowing for the expression and isolation of rotational forces at the front of foot  100  (inversion and eversion) separate from the rear of the foot. This in turn isolates the anatomical stump/socket interface from torque and friction forces magnified along the prosthetic foot&#39;s artificial lever. The hinging action also helps advantageously distribute the pressure of the body weight on the foot in a manner akin to a natural foot. As shown in  FIG. 3 , the channel also forms a path for the progression of the center of mass of the body (shown by line  340 ) as it progresses through the gait process.  
      Forefoot  120  uses engineered pivot points in the architecture of the leaf spring to control the location of flexure. As shown in  FIG. 1C , the raised channel  122  runs from the proximal end or rear of the forefoot device toward the distal or front of the forefoot device. Because the raised channel serves to give additional longitudinal support, upward force exerted on the forefoot device will not cause the forefoot device to flex along the length where the raised channel is present. The location of termination points  310  and  320 , each located toward an outer edge of the forefoot device, determine the location of a flexure line, as indicated by line  330 . Because no additional structural support is present in this portion of the forefoot device, upward pressure applied to the forefoot device will cause the device to flex along this mean line of flexure.  
      When the mean line of flexure  330  is compared to the T c  axis of the natural foot shown in  FIG. 2A  and  FIG. 2B , it is evident that the line  330  is substantially parallel to the natural T c  axis but forwardly displaced from it. Shifting the mean line of flexure  330  forward of the natural T c  axis in this manner is advantageous as it increases the user&#39;s ankle stability while allowing a normal range of motion.  
      The channel&#39;s terminations thus form a flexural location across the forefoot that closely approximate the intact anatomy&#39;s talo-crural-navicular (T cn ) axis as shown in  FIG. 2A  and  FIG. 2B . This flexural location means that the intact anatomical behavior of the ankle is facilitated without compromising the stability of the rear-foot of the prosthetic. Without the allowance for this biomechanical behavior, other prosthetics experience high stresses upon more traditional mechanical pivots, which lead to component degradation and/or failure, as well as creating unnecessary torquing moments on the stump socket.  
      In a preferred embodiment of the present invention, the distal termination points for the forefoot channel (whether raised or inverted as discussed below) can be custom tuned for an individual wearer. Depending on an individual&#39;s anatomy and preferences, it may be desirable to locate the T cn  axis flexure at a particular location. By changing the location of the channel termination points, including changing the location of the termination points relative to each other, the exact location of the T cn  axis flexure can be customized for a particular wearer.  
      The channel  122  may be solid or hollow. While a raised channel is described above, other structures may be employed to provide the same characteristics. Relative to currently available designs, this allows a minimum of material usage and weight while still maximizing robustness.  
      Heel Assembly  
       FIG. 1A  and  FIG. 1B  also show a preferred embodiment of a heel leaf spring  130  and a heel spring plate  140  according to the present invention. Together, heel leaf spring  130  and heel spring plate  140  form heel assembly  160 .  
      Heel leaf spring  130  is preferably a resilient leaf spring composed of a laminated carbon composite material that may also be embodied in spring titanium metal or a solid, three-dimensional weave carbon composite. The heel leaf spring  130  is a concavely curved body located in the heel assembly between the forefoot component  120  and the heel spring plate component  140 . The heel leaf spring serves as the initial loading structure as the foot enters the heel strike portion of the gait process, absorbing impact forces and storing them for energy return later in the gait process. The heel leaf spring works in combination with the heel spring plate  140  as a progressively loading spring structure.  
      Heel spring plate  140  is preferably a leaf spring composed of a laminated carbon composite material that may also be embodied in spring titanium metal or a solid, three-dimensional weave carbon composite. The heel spring plate  140  is a concavely curved body located in the heel assembly below the heel leaf spring. The heel spring plate serves as the secondary loading structure as the foot enters the heel strike portion of the gait process, absorbing impact forces and storing them for energy return later in the gait process.  
      In combination, heel leaf spring  130  and heel spring plate  140  provide a progressive loading response when the lower surface of heel leaf spring  130  comes into contact with the ground. This progressive loading response is due to the shortening of the heel leaf spring  130  as its contact point progresses down the curve of the face of the heel spring plate  140 , the changes in effective spring length achieving a desired non-linear load response to the load placed upon it in order to more closely mimic intact anatomical musculo-skeletal arrangements. This causes initial low resistance with increasing resistance as heel strike progresses.  
      The changes in effective spring length make it easier to achieve a desired non-linear load response to the load that is placed upon it in order to more closely mimic intact anatomical muscular-skeletal arrangements. This is of course desirable. Put another way, the nonlinear loading response of the varying effective spring length of the heel leaf spring  130  causes initial low resistance with increasing resistance as heel strike progresses. The load response, of course, is also a function of the designed spring characteristics of the heel leaf spring itself. For example, if the heel leaf spring is made from a carbon fiber laminate composite, factors such as material type, layer density, and ply orientation can be selected, as known to persons of skill in the art, to provide a heel member with desired spring load characteristics. To a certain extent, the heel leaf spring can be designed to have a non-linear response, but it has been found that a desired response can be more readily attained by controllably shortening its effective length as it is being depressed (as discussed above) in cooperation with the use of a suitable heel spring plate.  
      With reference to  FIGS. 6A and 6B , in one embodiment, both heel members are designed so that the load response of heel assembly corresponds to the nonlinear moment of resistance in the ankle for an intact person, which is depicted in  FIG. 6A . One aid to achieving this is through the use of the graph of  FIG. 6B , which shows the moment of resistance versus its angle of deflection for a model of an intact ankle, derived by Dr. Mark Pitkin and described in his article entitled, “Mechanical Outcomes of a Rolling Joint Prosthetic,” American Academy of Orthotists and Prosthetists, Journal of Prosthetics and Orthotics, Vol. 7, No. 4, pp. 114-123 (1995) and its enumerations of the non-linear moment of resistance in the ankle, and the moment of resistance in the ankle versus the angle of deflection. In a preferred embodiment, the non-linear, progressive loading of heel assembly  160  substantially matches the anatomical curve as described by Pitkin.  
      In a preferred embodiment, heel spring plate  140  also serves to stiffen the overall flexibility of the heel assembly  160  and thus acts as a stabilizing feature when an amputee is standing in place.  
      Another preferred embodiment of a prosthetic foot according to the present invention is shown in  FIG. 4A  and  FIG. 4B . In this preferred embodiment, an inverted channel  422  is used instead of the raised channel discussed above. Like the embodiment shown in  FIG. 1A  to  FIG. 1C , channel  422  forms an open box structure over much of the length of the forefoot device. In this embodiment, however, the open area of the open box structure is facing up, away from the bottom of the prosthetic foot, and is bounded by channel sidewalls  424 .  
      The inverted channel of  FIG. 4A  and  FIG. 4B  will be more robust than the embodiment shown  FIG. 1A  to  FIG. 1C , while retaining many of the same features and advantages, such as the hinging action discussed above. When the raised channel leaf spring  120  is flexed in a downward direction, the channel walls (not shown) will tend to spread apart. This spreading tends to lessen the longitudinal support provided by the channel and also contributes to laminate shear when the leaf is composed of a laminated carbon composite material. The inverted channel of  FIG. 4A  and  FIG. 4B , however, will not spread when the leaf  422  is bent downward. Instead, the walls of the inverted channel  424  will bow inward, increasing the stiffness and helping to prevent laminate shear. As shown by  FIG. 4A  and  FIG. 4B , the profiles of the heel assembly components  460  (including heel leaf spring  430  and heel spring plate  440 ) can be adapted to “cup” into the open channel on the bottom of forefoot  420 , providing for increased stability and easier assembly.  FIG. 4A  shows a heel leaf spring  430  that also has an inverted channel  432  formed so that it will mate with the inverted channel of the forefoot.  
      As in the raised channel embodiment discussed above, forefoot  420  uses engineered pivot points in the architecture of the leaf spring to control the location of flexure. Because the inverted channel also serves to give additional longitudinal support, upward force exerted on the bottom surface of the forefoot device will not cause the forefoot device to flex along the length where the inverted channel is present. Thus, as with the raised channel embodiment discussed above, the location of termination points  311  and  321 , each located toward an outer edge of the forefoot device, determine the location of a flexure line. In a preferred embodiment this mean line of flexure will be substantially parallel to the T c  axis of a natural foot but forwardly displaced from it.  
       FIGS. 5A and 5B  show another preferred embodiment of a prosthetic foot according to the present invention. In the embodiment shown, a raised channel  522  is employed. Instead of a separate pyramid adapter mount located on top of the forefoot, pyramid adapter mount  550  is formed as part of heel spring plate  540 . Mounting holes  552  allow the mount to pass through heel leaf spring  530  and forefoot  520  to reach a typical industry standard pyramid adapter (not shown). Spacer  524  is located between forefoot  520  and heel leaf spring  530  and within the open channel  522  on the bottom of forefoot  520 . Spacer  524  serves both as a locator for easier assembly of the prosthetic foot assembly and as a support to prevent channel  522  from deforming if the connections between the forefoot and heel assemblies are over-tightened or if the foot is exposed to a large degree of force (as when a wearer jumps down from a higher surface onto a lower surface).  
      In a preferred embodiment of the present invention, the forefoot leaf spring outboard edge extends below the horizontal plane with respect to the inboard edge, thereby creating an imbalanced contact surface on the forefoot. This acts to force a more natural supinated foot rollover response as the gait process progresses from heel strike into foot flat, as well as providing increased proprioception for the user, avoiding “foot slap” and other undesirable gait behaviors common among prosthetic users.  
      As shown in  FIGS. 4A and 4B , the forefoot leaf spring  421  may also employ a longitudinal split  442  along a pre-determined path which divides the forefoot into two separate “toes,” a “big toe” (on the inboard edge) and a “little toe” (on the outboard edge). These toes serve to re-create the anatomical functions of the first through second tarsal-metatarsal group and the third through fifth tarsal-metatarsal groups, respectively, as the foot progresses from the foot flat into the toe-off portions of the gait process. Heel leaf spring  430  can employ a similar split  444 . The divided “toes” can also serve to provide compliance of the foot&#39;s contact surface with the ground and any irregularities by providing relatively separate flexure bodies.  
      Preferred embodiments of the present invention allow for provision of adjustment of relative heel height, thereby allowing the prosthetic foot to be used with a range of shoes. Some embodiments include a “clip-on” type forefoot pad. This forefoot pad is preferably manufactured as a co-molded plastic/rubber structure of multiple iterations at different heights serving to allow multiple configurations of build height of the forefoot relative to the heel, effectively raising the heel height and allowing the use of a wide range of shoes.  
      This application describes multiple aspects of an improved prosthetic foot. Several of the aspects are thought to be individually novel and not obvious, and separately patentable. Not every embodiment requires every one of these aspects, and the claims are not limited to such an embodiment.  
      The invention in the preferred embodiments provides correct anatomical behavioral characteristics by progressive loading of the foot components and by relieving torque stresses on the stump and increasing proprioceptive feedback to the user, thus increasing confidence and consequently quality of life through usage. While the structures describe above can provide those advantages, the invention is not limited to the structures described, and alternative structures can also be used.  
      Although the present invention and its advantages have been described in detail, it should be understood that various changes, substitutions and alterations can be made to the embodiments described herein without departing from the spirit and scope of the invention as defined by the appended claims. Moreover, the scope of the present application is not intended to be limited to the particular embodiments of the process, machine, manufacture, composition of matter, means, methods and steps described in the specification. As one of ordinary skill in the art will readily appreciate from the disclosure of the present invention, processes, machines, manufacture, compositions of matter, means, methods, or steps, presently existing or later to be developed that perform substantially the same function or achieve substantially the same result as the corresponding embodiments described herein may be utilized according to the present invention. Accordingly, the appended claims are intended to include within their scope such processes, machines, manufacture, compositions of matter, means, methods, or steps.