Prosthetic pylon having an enclosed compressible volume of fluid to support a patient's weight

This application is a continuation of U.S. patent application Ser. No. 08/939,134, filed Sep. 29, 1997, now U.S. Pat. No. 6,086,615, which is a continuation of U.S. patent application Ser. No. 08/527,514, filed Sep. 12, 1995, now U.S. Pat. No. 5,702,488.

The present invention relates in general to a prosthetic pylon, and in particular to a prosthetic pylon supporting the weight of a patient on an enclosed compressible volume of fluid.

Individuals who lose all or part of a leg have a residual leg stump to which a prosthetic foot is often attached through an elongated pylon. Such a pylon is typically a rigid member made with a material, like a carbon-fiber composite or aluminum, which provides the rigidity necessary to support an individual's weight.

This rigidity can be problematic, because it often makes conventional pylons too stiff for comfortable movement. Each step on such a pylon can be awkward and painful, rather than cushioned and resilient like a natural leg. Therefore, there is a need in the art for a pylon which adequately supports an individual's weight and is more cushioned and resilient than conventional pylons.

An inventive pylon includes a first end portion connected to a patient's leg stump and a second end portion connected to a prosthetic foot. The second end portion is rotatable and axially movable relative to the first end portion about and along a longitudinal axis of the pylon, respectively. Preferably, the second end portion includes a piston, and the first end portion includes a piston chamber receiving the piston. The piston chamber encloses a compressible volume of fluid through which substantially all of the patient's weight applied through the patient's leg stump is supported. The compressible volume of fluid is preferably sealed so that it can be pressurized. Also, a torsional spring couples the piston and piston chamber and resists relative rotation between the piston and piston chamber without supporting any substantial portion of the patient's weight applied through the patient's leg stump. The present invention thus advantageously provides a pylon which is more cushioned and resilient than conventional pylons.

FIG. 1 is an isometric view of a prosthetic system using the inventive pylon.

FIG. 2 is a side cross-sectional view of the inventive pylon of FIG. 1 .

FIG. 3 is a plan cross-sectional view of a torsional spring of the inventive pylon of FIG. 1 .

FIG. 4 is a side cross-sectional view of a pylon having a membrane in accordance with an embodiment of the invention.

FIG. 5 is a side cross-sectional view of a pylon having a bladder in accordance with another embodiment of the invention.

With reference to FIG. 1, a preferred embodiment of the present invention provides a prosthetic system 10 comprising a prosthetic foot 12 and a pylon 14 . The pylon 14 has a first end portion 16 releasably connected to a conventional socket 18 into which a patients leg stump fits, and a second end portion 20 releably connected to the prosthetic foot 12 . The second end portion 20 is rotatable relative to the first end portion 16 about a longitudinal axis 22 of the pylon 14 , and is also axially movable relative to the first end portion 16 .

The pylon 14 of FIG. 1 is shown in more detail in FIG. 2 . The first end portion 16 includes a conventional attachment plate 24 which attaches to the conventional socket (not shown in FIG. 2) in a well known manner. The angle of the attachment plate 24 may be adjusted with adjustment bolts 26 .

The first end portion 16 also includes a piston chamber 28 receiving a piston 30 . The piston chamber 28 is preferably made of aluminum or titanium, and the piston 30 is preferably made of titanium.

The piston 30 has an O-ring seal 32 mounted in a groove about its circumference so that the piston chamber 28 contains an enclosed, sealed area 34 . Alternatively, the piston chamber 28 can be enclosed and sealed by a membrane 32 a (FIG. 4) attached to the inner walls of the piston chamber 28 and positioned orthogonally to the longitudinal axis 22 . As another alternative, the piston chamber 28 can include a fluid bladder 32 b (FIG. 5) which is enclosed and sealed.

A retainer block 36 retains the piston 30 in the piston chamber 28 , and O-ring bumpers 40 cushion the impact of the piston 30 against the retainer block 36 . A bearing, such as a wear ring 44 , is mounted about the circumference of the piston 30 in contact with the inner walls of the piston chamber 28 , and a bearing, such as a wear ring 48 , is mounted in the retainer block 36 in contact with the piston 30 . The wear rings 44 and 48 are preferably made of TURCITE-X®, and will be discussed in more detail below.

The pylon 14 also has a torsional spring including cushions (not shown in FIG. 2) and a torsion key 52 rotatably mounted at one end in a bearing 54 seated in an interior end of the piston chamber 28 . The cushions and the torsion key 52 are retained by a stop plate 56 . A slot in the head of the piston 30 receives the other end of the torsion key 52 in a torsion slide 58 preferably made of TURCITE-X®. As a result, the torsion key 52 slides into and out of the head of the piston 30 with little resistance, but it is forced to rotate in its bearing 54 against the cushions (not shown in FIG. 2) about the longitudinal axis 22 when the piston 30 rotates relative to the piston chamber 28 . The torsion key 52 and cushions will be described in more detail below.

In operation, the area 34 contains a compressible volume of fluid, preferably air. However, the fluid can also be, for example, another compressible gas or an incompressible liquid containing compressible objects 25 . Those having skill in the field of this invention will understand that a wide variety of compressible volumes of fluid fall within the scope of this invention. Also, it is preferable that the pylon 14 include a fitting 60 so that the compressible volume of fluid can be pressurized. This allows a patient to customize the response of the pylon 14 .

As the patient moves forward, the heel of the prosthetic foot (not shown in FIG. 2) strikes the ground. At this time, a force F W due to the patient's weight is impressed on the piston 30 , and the piston 30 slides into the piston chamber 28 resisted by a pressure force F P as the compressible volume of fluid compresses. As a result, substantially all of the patient's weight applied to the piston 30 is resiliently cushioned by the compressible volume of fluid. At the same time, the torsion key 52 slides into the slot in the head of the piston 30 .

As the patient continues forward, the patient's weight is transferred from the heel to a point forward on the prosthetic foot (not shown in FIG. 2) impressing a moment on the piston 30 relative to the piston chamber 28 which may, for example, be resolved into a couple including a moment M W centered on the wear ring 48 and a force F a , imposed on the wear ring 48 . This moment M W impresses a force F b on the wear ring 44 , for example, as follows:

F b =( M W /d b )  (1)

where d b is the length of the moment arm at the wear ring 44 . Because of the coefficients of friction μ a , μ b between the wear rings 48 and 44 and the piston 30 and the inner wall of the piston chamber 28 , respectively, the forces F a and F b cause the wear rings 44 and 48 to resist motion by the piston 30 with a friction force F F equal to:

F F =(μ a *F a +μ b *F b )  (2)

At a point during the patient's movement forward when the toes of the prosthetic foot (not shown in FIG. 2) are on the ground and the heel is beginning to rise, the moment M W and the force F a reach their maximum. At this point in the patient's gait, the pressure force F P tries to urge the piston 30 out of the piston chamber 28 , but the friction force F F (which is at its maximum because of the maximized moment M W and force F a ) advantageously restricts motion by the piston 30 .

When the moment M W and the force F a decrease from their maximum values as the patient continues forward, and the force F W due to the patient's weight lessens as the patient's weight shifts to the other leg, the friction force F F decreases sufficiently to allow the pressure force F P to rapidly urge the piston 30 out of the piston chamber 28 . As a result the patient receives an advantageous boost forward. The timing and amount of the boost can, of course, be controlled by varying the pressurization of the compressible volume of fluid through the fitting 60 , by varying the length of the moment arm d b , and by varying the coefficients of friction μ a and μ b .

The torsion key 52 is shown in more detail in FIG. 3 . It includes projections 52 a and 52 b which compress cushions 64 a , 64 b , 64 c , 64 d when the torsion key 52 is rotated by the piston (not shown in FIG. 3) about the longitudinal axis 22 of the pylon (not shown in FIG. 3 ). In this manner, the torsion key 52 resists rotation by the piston, and hence by the patient's leg stump (not shown in FIG. 3 ), about the longitudinal axis 22 . The cushions 64 a , 64 b , 64 c , 64 d are preferably made of polyurethane, and they allow a range of rotation by the torsion key 52 of approximately ±15°.

In an alternative embodiment, the shapes, positions and spring rates of the cushions 64 a , 64 b , 64 c , 64 d may be selected so that the cushions 64 a , 64 b , 64 c , 64 d resist clockwise rotation by the torsion key 52 with a different force than they resist counter-clockwise rotation. It may, for example, be advantageous to have a different resistance to clockwise and counter-clockwise rotation according to whether the pylon (not shown in FIG. 3) is attached to a right or a left leg stump. Also, it will be understood that a wide variety of alternative configurations for the cushions are possible—including more or less than four cushions—and that these alternative configurations fall within the scope of the present invention.

The present invention thus advantageously provides a pylon which is cushioned and resilient. It also provides a patient a boost forward at an advantageous moment in their stride.

Although the present invention has been described with reference to a preferred embodiment, the invention is not limited to this preferred embodiment. Rather, the invention is limited only by the appended claims, which include within their scope all equivalent devices or methods which operate according to the principles of the invention as described.