Patent Application
20080228288
The present invention relates to a prosthetic foot and specifically to a prosthetic foot formed of composite tubing.
Prostheses for amputated feet have been used since the times of ancient civilization. However, over time there have been numerous refinements in medicine, surgery, technology and prosthetic science resulting in the development of many designs of prosthetic feet. These feet can generally be divided into five categories:
The basic functions designed to be accomplished by a prosthetic foot are to provide a stable weight-bearing surface, to absorb shock, to replace lost motor function, to replicate the anatomic joint, and to restore cosmetic appearance. The SACH foot mimics ankle plantar flexion with a compliant heel pad which allows for a smooth gait. The single axis foot adds passive plantar flexion and dorsal flexion which increases stability during stance phase. Both the multi-axis foot and the dynamic response foot are energy storing designs. The multi-axis foot adds inversion/eversion to the plantar flexion and dorsal flexion provided by the single axis design with a mechanism that dissipates a substantial amount of the energy input during gait. The multi-axial foot also handles uneven terrain by allowing the foot to conform to the surface while continuing to provide a stable platform for weight bearing. Dynamic response feet utilize a basic metal, nylon or composite leaf spring to store and release energy during gait and are particularly useful for amputees with a very active lifestyle. As the amputee's cadence or activity level increases, more spring comes into play resulting in a greater push off. Some of the most widely recognized commercial embodiments of dynamic response feet include Flexfoot by Ossur, Springlite by Otto Bock, Seattle feet by Seattle Systems and Carbon Copy by Ohio Willow Wood.
The functions of the foot and ankle during gait are numerous and subtle, so that during the initial phase of heel strike the foot absorbs impact through controlled plantar flexion allowing the foot to be flat on the ground shortly after heel strike. Following this there is controlled dorsal flexion coupled with inversion and eversion to cope with irregular terrain. Then the gait proceeds to the rollover phase with the foot deforming during the single-limb stance, transitioning from a flexible shock absorber to a rigid platform for pushing. During late stance phase immediately preceding toe-off there is plantar flexion and power generation. Finally, during swing phase, dorsal flexors are active lifting the toes to prevent toe stubbing and possible stumbling.
Many dynamic response foot prostheses have been created and introduced for use by amputees. However, the performance characteristics desired by each amputee vary substantially and a tunable dynamic response prosthetic foot design providing high and low dynamic response capabilities is desired.
In particular, most dynamic response prosthetic feet, typified by those disclosed in the Van Phillips patents, utilize a J-shaped spring and rely upon the length of the spring element for energy storage. The result is that most dynamic response prostheses are of a high profile design and not really suitable for foot replacement at the ankle. Therefore, a need exists for a lightweight, low profile foot prosthesis providing energy storing dynamic response, plantar/dorsal flexion and inversion/eversion.
An autoclave manufacturing process is utilized on most current composite construction dynamic response prosthetic feet. This process uses a single sided tool to produce components which are generally planar in nature. The shapes are usually gently curved in only one primary direction. The autoclave process is expensive and slow and is unsuited for the manufacture of hollow shapes with a complex geometry. The material near the mid-plane of this planar structure are relatively inefficient, contributing weight but not capable of storing significant flexural energy. Most dynamic response prosthetic feet today are of relatively simple construction, being essentially planar in direction. Such feet are generally loaded almost exclusively in flexure. Delamination failures occasionally occur in current dynamic response prosthetic foot designs when the structure is loaded in a way to incur interlaminar tensile stresses or when interlaminar shear stresses exceed the strength of the relatively weak matrix material, usually epoxy resin, such as when a curved section in the foot is loaded so as to open or flatten or flatten the curve. This delamination occurs because there are no fibers oriented in the direction of the tensile or shear load. Current autoclave construction processes are not conducive to the construction of structures which can place fibers in the direction where these tensile or shear delamination type loads are transmitted.
It is therefore an object of the invention to produce an improved prosthetic foot of hollow composite tubing at reasonable cost, with high strength, great reliability, high level of compliance and terrain conformance.
It is an additional object of the invention to produce a prosthetic foot of hollow composite tubing in a fashion allowing a wide range of geometries to be utilized effectively in foot structure, while providing a relatively light-weight foot capable of supporting and storing high torsional and radial tensile loads with fibers oriented in a way to avoid large interlaminar tensile or shear stresses.
In order to provide low profile dynamic response foot prostheses, the present invention comprises a mounting element such as an ankle plate adapted for attachment to a lower leg pylon and composite fiber tubing forming a rear spring element. Preferably the tubing may form a helical spring, and the foot may also have a forward flexible keel portion fabricated of composite tubing. The tubing might also form a forward frame or posterior heel structure, the heel typically is a reverse design from the forward frame.
A more complete appreciation of the invention and the advantages is obtained with reference to the following detailed description considered in connection with the accompanying drawings wherein:
Referring now to the drawings wherein like numbered reference numerals designate corresponding parts throughout the several views, according to the embodiments of the invention illustrated in the non-limiting
Turning then to the heel spring 30, this element extends from proximal end 31 through first spring loop 32, second spring loop 33 and third spring loop 34 to distal end 35. The term “wire” is commonly used in the spring industry to refer to solid or hollow finger-like members of a spring. All of the wire sections of heel spring 30 are preferably hollow. In some cases, the hollow area may be quite small on the order of only three-hundredths of an inch, and in other cases the hollow area may be relatively large having a diameter on the order of 0.5 inches, or equal to as much as 90 percent of the outer wire diameter. Similarly, wall thicknesses may be relatively thin on the order of three or four-hundredths of an inch, or much thicker and very nearly equal to the radius of the wire section. The heel spring 30 is preferably manufactured from long composite fibers such as carbon, Kevlar, or fiberglass preimpregnated with curable resin, which are wrapped around an inflatable bladder and placed within a mold. The bladder is then inflated and the mold is heated to a temperature sufficient to melt the resin and activate the curing process. This forms the composite fibers into a circle or other tubular shape and this configuration permits the composite material to handle sheer stresses very effectively. Indeed there is little or no sheer stress between layers of fiber in a hollow composite tube. The result is a stiff tubular frame that is extraordinarily light. The diameter of the composite tube, the cross-sectional shape of the tube, the thickness and number of layers of composite material utilized and the composition of the composite materials utilized may be altered to achieve optimum performance characteristics. The result is a heel spring adequate to support a 1200 pound load, as is representative of the load that may be placed on the foot by a 300 pound amputee. It will be understood that many variations in the fore spring sound rear spring 30 are possible, and tubular “u” shaped spring elements are also useful in their prosthetic foot designs.
The preferred manufacturing technology to create the shaped hollow composite tubes utilizes matched female molds with an internal cavity forming the outer shape of the product. Resin impregnated fiber material is either placed in the tubular cavity or wrapped about an internal pressure bladder which is placed into the cavity. Several examples of this manufacturing technology are disclosed as used in various industries in U.S. Pat. Nos. 5,624,519; 6,340,509; 6,270,104; 6,143,236; 6,361,840; 5,692,970; 5,985,197; 6,248,024; 5,505,492; 5,534,203; and 6,319,346.
Many variations are possible in the manufacturing process of hollow composite tubing. For example, disentegratable core material may be used inside the bladder to rigidize the bladder, making it easier to place fiber materials on the bladder. The entire assembly, consisting of fiber overwrapping the bladder with an internal core may then be placed inside the mold, the mold can then be closed and heated, and air or other gas is used to pressure the bladder internally, compacting and applying pressure to the fiber resin composite structure. In addition, fiber material may also be placed directly on the tool mold cavity surfaces. Some fiber material could be placed in the tool and some material placed on the bladder.
Pre-impregnated fiber material is generally used, which has uncured epoxy resin already impregnated into the fiber. Dry fiber can also be used, such as woven or braided material. If dry materials are used, liquid epoxy resin can be injected during cure using an external pump or a transfer device inside the tool which forces a volume of resin to be moved from a precharged reservoir in the tool into the part during cure. Inflation of the internal pressure bladder can be coordinated with the resin injection in this case.
A preferred construction of composite fiber tubing utilizes unidirectional fiber oriented along the wire sections consisting of roughly 25% to 75% of the total laminate thickness. Additional layers of fiber are oriented at ±45° and at 90° to the wire center line. The fibers may also be oriented at other angles corresponding to the principle directions of stress within the structure. The use of ±45° fiber in the hollow tubing wall allows the springs to efficiently store, release and carry torsional loads. Prior art dynamic response prosthetic feet produced in autoclaves lack this ability and their geometries are significantly restricted.
The use of ±45° and 90° fiber orientation in the composite fiber tubing walls sections also greatly strengthens the resistance to delamination type forces. In sum, the use of hollow composite tubular walled wire sections containing ±45° and 90° fiber in the cross section walls allows the spring to become a torsional spring in some or all areas rather than a pure flexural spring as in prior art dynamic response feet. The ability to carry torsional loads allows a more complex geometry, which in turn allows designs to be developed with longer wire lengths. This allows greater compliance in the foot while minimizing breakage and delamination problems. The use of hollow cross sections also removes inefficient material from the prosthetic foot, reducing the weight of the foot. If a wide flat cross section is desired, multiple hollow cavities extending the length of the section may be utilized in what is referred to as a multi-celled hollow structure.
Prosthetic feet according to this invention will usually be designed to utilize torsional loading to store at least half of the energy load dynamics retained by the biasing elements of the shoe. It will also be understood that the hollow tubing may be filled with non-structural material for damping or to adjust the weight or profile of the foot.
The lateral longitudinal support 40 of the prosthetic foot begins at proximal end 41 formed with or attached to distal end 35 of heel spring 30 and extends through central length to distal end 42 at about corner 43 of front toe 45. From the toe 45, the illustrated composite tube structure rounds corner 44 to arch section 46 which extends to a first loop 52 of forespring 50. The lateral longitudinal support 40 is flexible, that flexibility being variable according to the diameter of the composite tube over the length from proximal end 41 to distal end 42 as well as the thickness, layers and composition of the composite materials are chosen for use and manufacture of this section of the foot 10. A foot plate, not shown, may be added to the lateral longitudinal support 40, or indeed, support 40 may be omitted or replaced by such a foot plate joining the heel and fore sections of the foot. The toe section 45 extending from the outer corner 43 to inner corner 44 helps adapt the front portion of the foot 10 to uneven terrain just as the distal end 35 of heel spring 30 helps conform the rear portion of the prosthetic foot 10 to uneven terrain. The arch 46 and forespring 50 form the keel of the foot and provide energy storage functionality improved over that accomplished by J-shaped leaf springs of higher profile prior art dynamic response foot prostheses and the rear spring 30 and forespring 50 allow for inversion/eversion motion as well as plantar and dorsal flexion. In the illustrated embodiment, the axis of both the heel spring 30 and forespring 50 is substantially vertical, and this dynamically stores the vertical impact of the amputee's weight as a torsional load. If the anticipated impact is more forward, as in the case of the running amputee, the axis of the forespring 50 might be adjusted to descend forward from the ankle plate 20.
The composite frame including heel spring 30, support 40, toe 45, arch 46 and forespring 50 are preferably manufactured in several pieces which are then attached together by polyacrylate or other secure adhesive resin. A frame made from two or three separate pieces would be a typical construction. When made from separate pieces, the individual pieces may be mixed and matched to an individual amputee's weight and mobility. In addition, a foot shell may be added over the composite framework to provide desirable cosmetic appearance and provide additional support for the prosthetic foot 10. The toe 45 of the composite frame is preferably positioned at about the location of the ball of the anatomical foot and it is contemplated that an add on foot shell will have a flexible toe forward of the frame toe 45.
In use, heel spring 30 bottoms out on itself instead of failing, and for this reason, it is desirable that the helical coils or loops 32,33,34 of heel spring 30 be relatively closely spaced. It may be noted that the circumference of loops 32,33,34 increases or telescopes outward as the heel spring 30 proceeds downward from attachment point 23 on ankle plate 20 to the distal end 35 of the heel spring 30. This increasing circumference may also be combined with increasing tubing diameter and the layering, thickness and composition of composite materials so that the compression of the heel spring will not be linear but may instead provide increasing resistance to compression. The presently preferred heel design has a smaller pitch for the first loop 32 and third loop 34 and a medium pitch for the middle loop 33. Alternative pitch modifications are possible for particular performance and characteristics. Ideally, the coils are pitched to close on each other before failure.
The helical structure of the spring 30 allows the efficient storage of torsional loads over a relatively long wire length. The cross section of the wire in the loops of the heel spring 30 may also vary to alter the compression profile of the spring.
Apart from changing composition of composite materials utilized, such as utilizing fiberglass for lower modulus and higher flexibility in portions of the composite frame, the fiber orientation may also be changed to provide additional strength in certain directions. For instance, the fibers are preferably aligned at about a 45 degree angle to the axis of the tubing to manage the torsional load in the helical spring portions 30,50 of the frame. It will also be seen that the forward arch 46 design accommodates the inclusion of an arch in a cosmetic foot shell and within a shoe worn over the foot 10 better than typical prior art dynamic response prosthetic feet. It is to be appreciated that the length of the spring, or the springs' effective wire length, the distance between attachment point 31 and distal end 35 of heel spring 30 and between attachment point 51 and inner corner 44 on the forespring 50 and arch 46 comprising the keel, are proportional to the amount of energy that the spring may store. By utilizing helical spring elements 30,50, additional effective length is added to the springs while providing relatively lower profile for the dynamic responsiveness or energy sharing capacity of the foot.
Turning then to alternative embodiments of the prosthetic foot according to the present invention,
All publications, patents, and patent documents are incorporated by reference herein as though individually incorporated by reference. Numerous alterations of the structure herein disclosed will suggest themselves to those skilled in the art. However, it is to be understood that the present disclosure relates to the preferred embodiment of the invention which is for purposes of illustration only and not to be construed as a limitation of the invention. All such modifications which do not depart from the spirit of the invention are intended to be included within the scope of the appended claims.
The present application claims priority to the Mar. 13, 2007 filing date of U.S. provisional patent application, Ser. No. 60/906,687.
| Number | Date | Country | |
|---|---|---|---|
| 60906687 | Mar 2007 | US |
