WO2000009046A1

WO2000009046A1 - Prosthetic ankle element and prosthesis

Info

Publication number: WO2000009046A1
Application number: PCT/NZ1999/000127
Authority: WO
Inventors: Wayne Stuart Alexander
Original assignee: Dashfoot Limited
Priority date: 1998-08-11
Filing date: 1999-08-11
Publication date: 2000-02-24
Also published as: AU5453299A; EP1105072A1

Abstract

A range of embodiments for an ankle element (6, 26, 46, 66, 86, 106, 126) for a prosthesis (2, 22, 42, 62) are provided. The prosthesis has two elements and the ankle element. The ankle element has at least a first portion (6b, 66b) connectable with a lower leg (4, 23, 44, 64) and a second portion (6a, 66a) connectable to the foot (6, 25, 45, 65). The ankle element provides a degree of rotational movement which is limited by the torsional energy absorbable by the material and shaping of the element. The element includes thrust and bearing surfaces, engage with shaped housings on the foot and leg element. Variations in the ease of rotation of the two elements relative to one another is obtained by a method selected from: the use of a centre piece (141) within the ankle element, which can be placed under compression; exchange of ankle elements of different density; and a combination of these.

Description

PROSTHETIC ANKLE ELEMENT AND PROSTHESIS

TECHNICAL FIELD

The present invention relates to a prosthetic ankle element for connection to a prosthetic foot and also to a prosthetic foot incorporating this ankle element. More particularly it relates to a prosthetic ankle element adapted to cope with different types and intensities of physical activity by being readily de-mountable and including allowance for varying its resilience and also its normal angular attitude with respect to an attached artificial leg.

BACKGROUND ART

Many of the developments in the field of prosthetic ankles have been aimed at helping the user duplicate the motions of the natural ankle as much as possible. In combination with the associated artificial foot the ankle prosthesis should provide a stable support permitting the user to walk or run with a normal gait and to participate in a range of activities. The approach taken in the present invention has been to incorporate a resilient torsional member in the ankle in order to more closely replicate the resilient action of a natural ankle as the foot continually rolls through the heel-strike, foot-flat and toe-off cycle.

Resilient materials, such as elastomers, in prosthetic feet and ankle devices are used to advantage for cushioning impact or shock loads necessarily encountered in walking, running and other vigorous physical activities. Resilient or elastomeric material is also well suited to provide stops to help limit the motion of pivoted devices, for example in the ankle of an artificial foot.

Exemplary of the latest developments in the field incorporating these uses of elastomeric elements is U.S. Pat. No. 4,229,839 (Schwemmer). This teaches a joint prosthesis pivot member which includes at least one elastomeric element. One side of the pivot member is connected to an artificial leg; the other to a foot prosthesis.

Different embodiments of the pivot member itself vary with respect to the number of components and mechanical complexity. In all these variants, however, lubrication is not required because the bushing separates the adjacent metal surfaces. A disadvantage of this type of ankle joint is the complexity of moulding required for the different elements of the device.

Schwemmer also includes a discussion of the state of the art, describing other similar designs making use of resilient material in torsion, such as U.S. Pat. No. 3,480,972 (Prahl). Here a tubular bushing of elastomeric material is secured between an outer rigid sleeve and an inner pin. With the outer sleeve connected to an artificial foot, and the pin connected to an artificial lower leg, articulation is permitted by torsional deflection of the bushing. Because of the torsional resilience of the bushing alone, and being without compression bumpers, the ankle joint prosthesis returns to a quiescent position after it has been deflected.

However, all the ankle elements of the prior art require a mechanical stop to limit relative motion between foot and leg elements. Further, there is no ability to alter the ease or difficulty of the relative motion, thus permitting use of the same prosthesis in differing activities. Further, none of the prior art provides a means for adjustment of the angle of ankle roll between the foot and leg sections. This feature enables varying ankle positions to be easily set, catering for the requirements of different activities such a cycling, the operation of pedals in driving a motor vehicle or allowing the user to readily wear high heels or other shoes with varying heel heights. Another feature of the invention is the method of fixing the prosthetic ankle pivot element which allows different elements to be readily exchanged. This demountable attachment permits the ready adjustment of spring rates, with almost infinite variability, to suit the stride and weight of the user, allowing a natural stride and resilience of gait which has not been obtainable by prior art prosthetic devices.

An object of the present invention is to provide an ankle element which permits the resilience of a prosthetic foot relative to the leg and to provide transverse stability to assist balancing when walking on uneven surfaces.

A further object of this invention is to provide a means for adjustment of the angle of ankle roll between the foot and leg sections.

It is a further of the present invention to provide the public with a useful choice. Further aspects and advantages of the present invention will become apparent from the ensuing description which is given by way of example only.

DISCLOSURE OF INVENTION

According to one aspect of the present invention there is provided an ankle prosthesis which includes: a first element connectable with a prosthetic leg; a second element, being an element connectable with a prosthetic foot, which includes a heel and a toe; a pivot element disposed between and coupling said first and second elements, said element comprising: at least one first portion connectable with said first element ; at least one second portion connectable with said second element ; wherein each said portion includes thrust and bearing faces for the absorption and release of energy during heel strike and toe lift during use; and wherein said first element includes a housing for receiving therein said at least one second portion; said second element is adapted for attachment to said at least one first portion; and wherein the first element, the second element and the pivot element are provided with means for varying the resilience of the prosthetic ankle; and wherein the first element, the second element and the pivot element are provided with means for varying the angular attitude of the prosthetic foot with respect to an attached prosthetic leg; and wherein said pivot element is made from an elastomeric material selected from the group: urethane, hardened polymers of high elastic properties; hardened natural and synthetic rubbers, and a combination thereof, whereby said element is capable of torsional movement approximately about a rotation axis of the said pivot element, enabling load transfer in both directions between the prosthetic foot and said pivot element.

Preferably, said pivot element is capable of limited rotation about the two axes perpendicular to the said rotation axis.

Preferably, said second element is integrally formed with the prosthetic foot

Optionally, said ankle element is manufactured integrally. Further, the pivot element is optionally of a uniform density. However, the density may vary from portion to portion (and within both the first or second portions), as is desired. Preferably also, said second element is shaped to engage with said bearing and thrust faces on the said at least one second portion.

Preferably, said first element is shaped to engage with bearing an thrust faces of said at least one first portion.

Preferably, the shape of the pivot element in cross section can be selected from the following: square; rectangular; circular; a mixture of rectangular and circular. The shape of the pivot element in cross section may also be two circles, with the element being formed in two tubular parts.

Optionally, there are two first portions, each first portion being connected separately to the first element. The said first element is formed in the shape of a yoke, with a housing on each side for one of the first portions.

Alternatively, the pivot element includes one first portion and two second portions, the second element being in the form of a yoke.

The present invention further provides a prosthesis which includes: an ankle prosthesis as described above; and a prosthetic foot adapted to engage with said ankle prosthesis.

Preferably said prosthesis is adapted so that said ankle prosthesis is removable and interchangeable with a second ankle prosthesis.

The advantage of the above described ankle element and prosthesis is that the prosthesis can include a foot element in which the toe is 'down' and/or movable relative to the lower leg element. The degree of movement is limited; however, the operation of the torsional energy release ensures that lift is provided to the heel as the foot and leg prosthetic elements interact as the user conducts any activity. Such activities may include normal day to day walking, sporting or other activities.

BRIEF DESCRIPTION OF DRAWINGS

Further aspects of the present invention will become apparent from the following description which is given by way of example only and with reference to the accompanying drawings, in which:

Figure 1 is a first perspective view of a first preferred embodiment of the prosthetic ankle and foot of the present invention;

Figure 2 is perspective view of the second preferred embodiment of the prosthetic foot of the present invention; Figure 3 is a diagrammatic side representation of three positions (a, b, c) of the first preferred embodiment of the prosthesis of the present invention;

Figure 4 is a sectional side view of the ankle element of the first preferred embodiment of the present invention from the heel;

Figure 5 is a sectional side view of the ankle element of the first preferred embodiment of the present invention from the toe;

Figure 6 is a side view of a second preferred embodiment of the prosthesis of the present invention;

Figure 7 is a back elevation of the third preferred embodiment of the prosthesis of the present invention;

Figure 8 is a side elevation of a fourth preferred embodiment of the prosthesis of the present invention;

Figure 9 is a section elevation from the front of the fourth preferred embodiment of the prosthetic ankle and foot of the present invention;

Figure 10 is a section elevation from the front of the fourth preferred embodiment of the ankle element of the present invention; and

Figure 11 is an exploded perspective of the first and second elements of the prosthetic ankle from one side, for use with a fifth preferred embodiment of the pivot element of the present invention;

Figure 12 is a perspective view of the fifth preferred embodiment of the pivot element of the present invention;

Figures 13 & 14 are perspective views of, respectively, the sixth, and seventh preferred embodiments of the external shape of the pivot element of the present invention;

Figure 15 is a diagrammatic cross-sectional representation of three positions (a, b, c) of the fifth preferred embodiment of the prosthesis of the present invention; Figure 16 is a side view of the fifth preferred embodiment of the foot element of the present invention; and

Figure 17 is a plan view from above of the fifth preferred embodiment of the lower leg element of the present invention; and in which

Figure 14 also shows a perspective section of the eighth preferred embodiment of the ankle element of the present invention.

BEST MODES FOR CARRYING OUT THE INVENTION

As can be seen from the description of the drawings and when viewing the drawings, there are eight preferred embodiments of the prosthesis (which includes the leg, foot and ankle elements as described below). However, there are in fact only seven preferred embodiments of the ankle element. For readability, every time a new embodiment of the foot and leg elements are presented, the same numeral is used to refer to the embodiment of the ankle element, whether or not there is in fact a new embodiment of the ankle element. It will be appreciated that the third embodiment of the ankle element (shown in Fig. 6) is identical with the first embodiment shown in Fig. 1.

Referring to Figures 1 to 3, a first and a second preferred embodiment of the ankle elements and prosthesis are thereshown. In the first preferred embodiment, the prosthesis 2 includes a lower leg prosthetic element 3 (which includes two spars 4), a foot 5 and an ankle element 6 (Fig 3). The foot 5 includes a heel pad 7, central portion 8 and toe 9. The heel pad 7 and toe 9 are optionally of known type and are made of urethane or other flexible material.

The central portion 8 includes an ankle element housing 10 (Fig. 2) which has a shaped hole 11 therethrough. The housing 10 with hole 11 is of a shape complementary to the profile of the ankle element 6. The top and bottom of the housing 10 about the hole 11 includes two holes 12 adapted to receive therethrough a vertical pin (not shown). The vertical pin, which is of known type, is adapted to take up or cater for any slack in the tolerance of the housing 10 and the ankle element 6, so that there is no or substantially no relative movement between the housing 10 and the ankle element 6.

The lower leg element 3 includes a hole 13 through the lower portion of each spar 4. The position of the holes 13 is similar to that of the ankle on a human foot. About each hole 13 is a rectangular housing 14 extending to each side of the respective spar 4. The rectangular housing 14 extends completely around the ankle element 6 (when in position). The rectangular housing 14 includes two pin holes 15 along each side. Pins (of known type, not shown) for securely locating the ankle element 6, relative to the spars 4, are secured through the ankle element 6 and holes 15, in known manner.

Referring to Figures 4 and 5, the arrangement of the spars 4 and central portion 8 of the foot 5 and the respective housings (14, 10) is such that a short gap g is left between the housings (14, 10 and 10, 14) when viewed in a front elevation. Thus the ankle element 6 is in three portions, two outer portions 6a and a central portion 6b. The gap g aids in the translation of the torsion energy in adjacent portions 6a and 6b of the ankle element 6. The angling shown within the gap g is a diagrammatic representation of the torsional rotation of which the ankle element 6 is capable.

Referring to Fig. 2, the second preferred embodiment thereshown differs from the first preferred embodiment only in the angle of the hole 10 relative to the base of the foot 5. The hole 10 is backwardly inclined, allowing the toe to be permanently in a down position.

The above described ankle element 6 and prosthesis 2 works as follows: referring to Figs 1 , and 3 to 5, the prosthesis is assembled by aligning the hole 10 with that of the housings 14 in the spars 4. The ankle element 6 is inserted through the holes (13, 10). The pins (not shown) are inserted through their respective holes (12, 15) to ensure the ankle element 6 stays securely in place whatever the motion of the prosthesis 2.

The principal motion that the prosthesis 2 must provide for is the flexion of the foot 5 with respect to the artificial leg 4 as it rolls through the heel-strike, foot-flat and toe-off cycle of normal walking. This flexion occurs about an axis more or less perpendicular to the longitudinal axis of the leg 4 and transverse to the longitudinal axis of the foot 5. Thus the flexion is approximately about the longitudinal axis A (Fig. 3) of the ankle element 6.

The operation of the ankle joint prosthesis 2 is described during walking or running action, as illustrated in Fig. 3. The user extends the prosthetic leg 4 and foot 5 to take a step. As the heel of the artificial foot 5 strikes the ground (Fig. 3c), the heel 7 of the foot 5 rotates toward the prosthetic leg 4 approximately about the longitudinal axis A of the ankle element 6. The elastomeric ankle element 6 that secures the artificial foot 5 to the prosthetic leg 4 deflects in torsional shear to permit the relative rotation that occurs between the heel 7 and the prosthetic leg 4. The resilience of the ankle element 6 limits the rotational movement and assists in the absorption of impact load imposed on the foot 5. As the weight of the user comes forward on the foot 5, the heel 7 begins to rotate away from the prosthetic leg 4. The rotation relieves the torsional deflection of the ankle element 6, imparting a thrust to the foot 5. The foot 5 passes through the natural quiescent position for the ankle element 6 (Fig. 3a), and as the weight of the user moves forward, the ankle element 6 is again deflected in torsional shear. The resilience of the ankle element 6 limits relative movement of the heel 7 away from the prosthetic leg 4. When the user lifts the artificial foot 5 from the ground, as he prepares to take another step (Fig. 3b), the loads on ankle element 6 are relieved, imparting a lift to the foot 5.

The resilient ankle element 6 is also able to deflect in bending about the two axes perpendicular to its longitudinal axis. This bending deflection permits the prosthesis 2 to accommodate a limited degree of rotation about an axis generally parallel to the longitudinal axis of the foot 5. This provides the user with transverse stability when walking on uneven surfaces. This bending deflection also permits the prosthesis 2 to accommodate a limited degree of rotation generally about the longitudinal axis of the leg 4.

Whilst the ankle element 6 is shown in the first and second preferred embodiments as rectangular in cross-section, it will be appreciated that this may be altered, to make more efficient use of thrust and bearing faces on the ankle element 6. For example, the element 6 may be circular, or of a rounded shape that is non-uniform is profile.

Referring to Fig.s 6 and 7, a third preferred embodiment of the present invention is thereshown. The prosthesis 22 includes a foot 25 with a yoked central portion 28. The lower leg prosthetic element 23 is a single pylon. The ankle element 6 is unchanged from that of the first preferred embodiment, except for the arrangement of the housing 34. The holes and pin holes (of the housing 10 for the spars 4 in the first preferred embodiment, not here shown) are here shaped as a part of each flange (34a and 34b) of the yoke 34 of the central foot portion 28. Similarly the housing for the central portion 28 of the first embodiment is not at the lower end of the pylon 23. The prosthesis 22 otherwise is assembled and operates in the manner described for the first preferred embodiment of the prosthesis 2.

Referring to Figs. 8 to 10, the fourth embodiment of the prosthesis 42 is thereshown. In this embodiment the ankle element 46 is in two parts (46u and 46I). Each part (46u and 46I) is cylindrical and of circular cross-section. The housing (not detailed) comprises an upper housing and a lower housing in each of the feet 45 and leg spars 44. The manner of securement and retention of the ankle element 46 is as described above in first two preferred embodiments. The point of rotation of the foot element 46 relative to the spars 44 of the lower leg element will be positioned between the two parts of the element 46.

Referring to Fig.s 11 , 12 and 17, the fifth embodiment of the prosthesis 62 is thereshown. In this embodiment, the prosthesis 62 includes an ankle element 66, a foot 65 and lower leg element 63. The lower leg element 63 is in the form of a yoke 74, which a standard male pyramid adapter 73, between the two sides of the yoke 74 (74a, 74b). The adapter 73 is of known type and includes means for attaching the yoke 74 to the lower end of lower leg element or to a pylon 23 of the type shown in Fig. 8. The foot 65 includes heel pad 7 and toe 9. These elements are as described above for the first preferred embodiment. The central portion 68 includes an ankle element housing 70 which has a shaped hole 71 therethrough.

The ankle element 66 has three portions (66a, 66b, 66a). The two outer portions 66a are circular in cross-section. The central portion 66b is octagonal in cross- section. The shape of the hole 71 is complementary to the cross-section profile of the central portion 66b. The central portion 66b includes one threaded hole centrally on the face thereof. The central portion 68 includes three threaded holes positioned about the top portion, above the hole 71 , and within the housing 70, such that grub screws 72 can secure the ankle element 66 within the housing 70 so that the ankle element 66 bears against the front location screw 72 (depicted in Fig. 16 as 72b).

Each side 74a and 74b of the yoke 74 has a circular hole therethrough. There are four holes spaced about each side 74a or 74b through which grub screws 72 can be threaded. In each outer portion 66a of the ankle element 66 there is a slotted inset 61 which can receive the end of a threaded pin 64. The ankle element 66 is inserted through the holes in the sides 74a and 74b and the hole 71 in the housing 70. With appropriate rotation and settling of the ankle element 66, the pins 64 can be used to locate the ankle element 66 with reference to the lower leg element 63, and with the use of the grub screw 72, can be inserted through and secured to the housing 70.

It will be appreciated that with a threaded hole on each of the faces of the octagon of the central portion 66b, the angle of the foot 65 with respect to the lower leg element 63 can be varied without the need to modify the shape of any of the parts; ankle element 66, foot 65, and lower leg element 63. Optionally, each outer portion 66a may include a sleeved bush 67 with slots arranged to overlay the insets 61. The outer diameter of the bush is equal to the outer diameter of the octagon of the central portion 66b. Whilst this embodiment has been described with reference to the cross-section of the central portion 66b as being an octagon, it will be appreciated that other symmetrical multi-faceted shapes may be used, for example a hexagon or a 12 sided shape. Also, it will be appreciated that the cross-section of the outer portions 66a may be other than circular. For example, Fig.s 13 & 14, showing embodiments six and seven of the ankle element of the present invention (86, 106), show other shapes that may replace the shape of the fifth embodiment. The sixth embodiment of the ankle element 86 is octagonal in cross-section. For optimal engagement of the outer portions of the element 86 (86 a) a flanged sleeve 87 is used. The sleeve 87 permits a circular bearing face 89 to be presented on the outer surface of the element 86 for engagement with a circular housing (as shown, for example, on the fifth preferred embodiment in Figs. 11 , 12 & 17). The sleeve 87 includes a locking shoulder 88 to prevent rotation of the sleeve 86, when the element 86 is in use.

The above described fifth preferred embodiment of the ankle element 66 and prosthesis 62 works as follows: referring to Fig.s 11 , 12, 15 and 17, the prosthesis 62 is assembled as described above. The pins 64 and screws 72 are inserted and tightened in their respective holes. As the user walks, the motion of the prosthesis 62 is as shown in Fig. 15, from a heel strike position (Fig. 15a), through a flat position (Fig. 15b), to a toe off (heel lift) position (Fig. 15c). The centre line of the yoke 74 is represented by the numeral 75. The centre line of the foot 65 is represented by numeral 76. The position of the inserts 61 with the respective pin 64 is the same for each inset, although only one is shown in detail in Fig. 15.

As the heel strikes, the pin 64 is sitting against one side of the respective inset 61. As the foot progresses to the flat position, the movement of the pin 64 relative to the respective inset 61 is to the opposite face. The pin 64 bears against this opposite face of the inset 61 , forcing the prosthesis 62 onto the toe 9, thus ensuring heel lift. The pin 64 returns to the position as shown in Fig. 15a as the heel strikes the ground again.

Referring to Fig. 16, the amount of torsional stress that the ankle element 66 absorbs for a given deflection can be adjusted, if so desired. On the back of the foot 65, a rebound spring 67 may be positioned in co-operation with a screw 69 which is threaded through an additional hole (not shown) in back of the housing 70 and bearing against the central portion 66b of the ankle element 66. This spring and screw 69 can combine to apply tensile forces to the rear of the housing 70.

Referring to Fig. 14, an eighth preferred embodiment of the ankle element 126 is also thereshown. In this embodiment, the ankle element 126 has a cylindrical hollow along the length of the axis 140. About the centre point of the axis 140 is positioned a shaped centre piece 141 which is embedded in the material of the ankle element 126.

In Fig. 14 the centre piece 141 is shown as cylindrical, with a radius greater than the radius of the central hollow. However, it will be appreciated that the shape of the centre piece 141 may be other than cylindrical, as is desired.

On each side of the centre piece 141 and inserted in the cylindrical hollow is a metal half-axle or stub axle 142. The axle 142 is hollow and threaded internally. A threaded screw (not shown) inserted in the axle 142 can place the centre piece 141 under a variable amount of compression.

It will be appreciated by those skilled in the art that this arrangement can be used to apply a pre-tension of the centre piece 141. By adjustment of the or each screw bearing on the centre piece 141 , the amount of pre-tension on the ankle element 126 can be varied. Thus the ability of the ankle element 126 to absorb torsional stress may be varied on one or both sides of the centre piece 141.

Thus the user of the prosthesis in which the ankle element 126 is inserted may vary the rigidity or ease of movement of the prosthesis, depending on the activity being pursued by adjustment of the or each screw within the half-axle 142.

It will be appreciated by those skilled in the art that with appropriate and known modifications of any embodiment of the lower leg and foot of the eight embodiments described above, any of the seven embodiments of the ankle element may be used with any of the embodiments described for the foot and lower leg element described above.

The ankle element (any embodiment) is made from a material selected from the group: urethane, hardened polymers of high elastic and viscous properties, other dense elastomeric materials; hardened natural and synthetic rubbers, and a combination thereof. The central portion (8, 28, 68) of the foot element, the spars 4 and housings (10, 14, 34, 74) may be made from the group: steel, aluminium, aluminium alloy, carbon fibre mouldings, reinforced carbon fibre mouldings, a hard plastic; and a combination thereof. The heel and toe (7, 9) may be made from any of these materials or from a plastic, elastomer, a resilient polymer, or a combination of these materials.

If so desired, a range of ankle elements (of any particular embodiment) made from material of varying resilience and elastic hardness may be interchanged when the user is undertaking various activities. For example, a very much harder ankle element could be used for sports than for normal daily walking.

Aspects of the present invention have been described by way of example only and it should be appreciated that modifications and additions may be made thereto without departing from the scope thereof.

Claims

CLAIMS:

1. An ankle prosthesis which includes: a first element connectable with a prosthetic leg; a second element, being an element connectable with a prosthetic foot, which includes a heel and a toe; a pivot element disposed between and coupling said first and second elements, said element comprising: at least one first portion connectable with said first element ; at least one second portion connectable with said second element ; wherein each said portion includes thrust and bearing faces for the absorption and release of energy during heel strike and toe lift during use; and wherein said first element includes a housing for receiving therein said at least one second portion; said second element is adapted for attachment to said at least one first portion; and wherein the first element, the second element and the pivot element are provided with means for varying the resilience of the prosthetic ankle; and wherein the first element, the second element and the pivot element are provided with means for varying the angular attitude of the prosthetic foot with respect to an attached prosthetic leg; and wherein said pivot element is made from an elastomeric material selected from the group: urethane, hardened polymers of high elastic properties; hardened natural and synthetic rubbers, and a combination thereof, whereby said element is capable of torsional movement approximately about a rotation axis of the said pivot element, enabling load transfer in both directions between the prosthetic foot and said pivot element.

2. An ankle prosthesis as claimed in claim 1 wherein said pivot element is capable of limited rotation about the two axes perpendicular to the said rotation axis.

3. An ankle prosthesis as claimed in either claim 1 or claim 2 wherein, said second element is formed integrally with the prosthetic foot.

4. An ankle prosthesis as claimed in any one of the preceding claims wherein said second element is shaped to engage with said bearing and thrust faces on the said at least one second portion.

5. An ankle prosthesis as claimed in any one of the preceding claims wherein the shape of a first portion, in cross-section is selected from: square, rectangular, circular, and hexagonal or other multi-faceted symmetrical shape.

6. An ankle prosthesis as claimed in claim 5 wherein the shape of a first portion, in cross-section is octagonal, and said a flanged bush is positioned therearound with an outside face that is of a circular cross section.

7. An ankle prosthesis as claimed in any one of the preceding claims wherein the shape of the at least one second portion, in cross-section is selected from: square, rectangular, circular, and hexagonal or other multi-faceted symmetrical shape.

8. An ankle prosthesis as claimed in any one of the preceding claims wherein said ankle prosthesis includes a pivot element with two first portions, disposed one on each side of a single second portion.

9. An ankle prosthesis as claimed in claim 8 wherein said first element is formed in the shape of a yoke, and further includes a housing for each said first portion; and wherein said second element includes a single housing for said second portion.

10. An ankle prosthesis as claimed in any one of claims 1 to 7 wherein said ankle prosthesis includes a pivot element with two second portions, disposed one on each side of a single first portion.

11. An ankle prosthesis as claimed in claim 10 wherein the second element is formed in the shape of a yoke, and further includes a housing for each said second portion, and wherein said first element includes a single housing for said first portion.

12. An ankle prosthesis as claimed in claim 9 and in claim 11 wherein between adjacent housings there is a fixed spatial gap.

13. An ankle prosthesis as claimed in claim 9 or claim 11 wherein the portion of the yoke between the two housings is positioned above the pivot element.

14. An ankle prosthesis as claimed in claim 5 wherein said pivot portion includes two first portions, that are circular in cross-section, positioned one on each side of a hexagonal second portion; and wherein each said first portion includes a plurality of insets evenly spaced about the circumference thereof; and said second element includes a plurality of threaded holes which are spaced about the top thereof; said ankle prosthesis including a plurality of pins, each engageable with an inset.

15. An ankle prosthesis as claimed in claim 14 wherein said pivot portion further includes: a pivot portion with one central second portion that is octagonal in cross-section; a bushing sleeve about part or all of each first portion, the diameter of the sleeve being equal to the outer diameter of the second portion.

16. An ankle prosthesis as claimed in any one of the preceding claims wherein said ankle prosthesis further includes a rebound spring which is secured to the back of the second element and to the heel.

17. An ankle prosthesis as claimed in any one of the preceding claims wherein said pivot element further includes: a cylindrical hollow along the longitudinal axis thereof; a separate central piece positioned within a space at the centre of the longitudinal axis of the pivot element, said central piece having a large diameter than the cylindrical hollow; and two threaded half axles, positioned one on each side of the central piece, within said hollow; with a screw threaded screw engageable therewith and capable of bearing on said central piece.

18. An ankle prosthesis as claimed in any one of the preceding claims wherein said pivot element is comprised of material of variable density, said material being selected from the group: urethane, hardened polymers of high elastic properties; hardened natural and synthetic rubbers, and a combinations thereof.

19. A prosthesis which includes: an ankle prosthesis as claimed in any one of the preceding claims: and a prosthetic foot adapted to engage with said ankle prosthesis.

20. A prosthesis as claimed in claim 19 wherein said ankle prosthesis is removable and interchangeable with a second ankle prosthesis as claimed in any one of the preceding claims.

21. An ankle prosthesis and as claimed in any one of claims 1 to 17 and substantially as herein before described and with reference to: Fig 1 and 3 to 5; Figs 2; Fig 6 and 7; Figs 8 to 10; Figs 11 ,12, 15, 16 and 17; Figs 13; and Fig 14 of the accompanying drawings.

22. A prosthesis as claimed in either claim 19 or claim 20 and substantially as herein before described and with reference to: Fig 1 and 3 to 5; Figs 2; Fig 6 and 7; Figs 8 to 10; Figs 11 ,12, 15, 16 and 17; Figs 13; and Fig 14 of the accompanying drawings.