WO2013064832A1

WO2013064832A1 - Prosthesis

Info

Publication number: WO2013064832A1
Application number: PCT/GB2012/052729
Authority: WO
Inventors: Andrew Clive Taylor
Original assignee: Finsbury (Development) Limited
Priority date: 2011-11-02
Filing date: 2012-11-01
Publication date: 2013-05-10
Also published as: GB201118937D0

Abstract

An acetabular prosthesis comprising a plastics cup-shaped body, a plurality of barbed stakes extending from at least a portion of the bone-facing surface of the body, said stakes being sized and shaped to provide fixation on impaction and a single alignment rod, said alignment rod being free of components which provide fixation, said alignment rod being located on the bone-facing surface at an angle of 15 to 25 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body,

Description

PROSTHESIS

The present invention relates to a prosthesis. More particularly, it relates to an acetabular prosthesis.

The efficient functioning of the hip joint is extremely important to the well-being and mobility of the human body. Each hip joint is comprised by the upper portion of the femur which terminates in an offset bony neck surmounted by a ball-headed portion which rotates within the acetabulum in the pelvis. Diseases such as rheumatoid- and osteo-arthritis can cause erosion of the cartilage lining of the acetabulum so that the ball of the femur and the hip bone rub together causing pain and further erosion. Bone erosion may cause the bones themselves to attempt to compensate for the erosion which may result in the bone becoming misshapen.

Operations to replace the hip joint with an artificial implant are well-known and widely practiced. Generally, the hip prosthesis will be formed of two components, namely: an acetabular component which lines the acetabulum; and a femoral component which replaces the femoral head. The femoral component may be total femoral head replacement in which case the component includes a head, neck and a stem which in use is inserted into the end of a prepared femur. Alternatively, where appropriate, the femoral head component may he a resurfacing prosthesis which is attached to the head of the femur once it has been suitably machined .

In an operation to insert a prosthetic acetabulum in a patient's pelvis the surgeon first uses a reamer to cut a cavity of appropriate size in the patient's pelvis. An acetabular cup is then inserted into the cavity. By "appropriate size" is meant a size which is selected by the surgeon as being the most appropriate for that particular patient. Normally, it is desirable to retain as much of the original healthy bone surface as possible.

Commercially available acetabular cups are sold in a range of sizes to suit the needs of individual patients. Generally, acetabular cups are available in sizes of from 42 mm to 62 mm diameter with 2 mm increments between neighboring sizes.

I There axe a number of different types of prosthetic acetabular cups. One type of cup is that made from polyethylene. They are generally cemented into the acetabulum and require only light pressure to seat them in the cement.

One alternative cup type has a polyethylene liner unit for articulation with the femur and a metal shell for insertion into the pelvic cavity. These cups with metal shells maybe implanted without cement such that they rely on a jam fit between the metal shell and the patient's acetabulum. However, in some arrangements, screws may be used to secure the cup shell in position in the pelvis before the liner is applied into position. The insertion of the metal shell into the pelvis requires considerable force. As the surgeon applies this force, there is a risk that the metal shell can become damaged or deformed. There is also a possibility that during the application of the force, the shell may be moved so that it is not in the optimum alignment in the acetabulum. Often the metal shells have outer surfaces or coatings which encourage bone to grow into them over time.

W th this type of prosthesis, the polyethylene liner unit is snapped or screwed into the metal shell after the metal shell has been seated in the acetabulum. Thus the inner surface of the liner forms the socket part of the joint.

More recently, ceramic liners have been used as an alternative to the plastics liner. In this arrangement, the metal shell, which is generally formed from titanium, is first inserted into the acetabulum. The ceramic liner is then inserted into the shell

Whilst these various prior art arrangements offer relief to patients from the pain of the worn joint, there is a continuing desire to provide prostheses which provide improved results to the patient, particularly in terms of wear. This is particularly important for younger patients where revision operations may be required if the prosthesis itself becomes worn or becomes loosened in the acetabulum. There is therefore an ongoing need for new materials and or new structures for prostheses which more closely mimic the natural bone while minimizing the wear problems thereof. Polyetheretherketone (PEEK) polymers have been suggested as being suitable materials for use in orthopaedic implants. This material is discussed in "Taking a PEEK at Material Options for Orthopedics", Kinbum A., Medical Design Technology Magazine; 1 Jan 2009 page 26ff, It is suggested that particular advantages can be obtained if the PEEK is reinforced with carbon fibres. The use of PEEK in a horseshoe-shaped cup is discussed in "Biomechanics of a PEEK Horseshoe-Shaped Cup: Comparisons with a Predicate Deformable Cup" Manley, M. T., et a! Poster No 1717 at 53^rd Annual Meeting of the Orthopaedic Research Society.

WO2009/097412 suggests that PEEK canbe useful in a multi-layered hemispherical prosthesis. The multi-layered system comprises an outer layer formed from a porous metal and an inner layer formed from a polyaryletherketone in which the polyaryletherketone at least partially permeates the pores of the first material . The Applicants suggest that this arrangement allows the two layers to be securely bonded together. They also suggest that the prosthesis has a lower stiffness than that achieved for prior art arrangements.

WO 2011/095813 describes an improved prosthesis comprising:

an inner layer formed from a polyaryletherketone;

a first outer layer adjacent to said inner layer formed from a porous polyaryletherketone at least some of said pores having located therein material to promote osteointegration; and

a second outer layer adjacent to said first outer layer formed from a porous polyaryletherketone, a portion of said pores being free of material to promote osteointegration.

A second embodiment described in WO 201 1 /095813 relates to a prosthesis which comprises: an inner layer formed from a polyaryletherketone;

a first outer layer adjacent to said inner layer formed from a porous polyaryletherketone, at least some of said pores having located therein material to promote osteointegration having a crystallinity of from about 60% to about 90%; and

a second outer layer adjacent to said first outer layer farmed from a porous polyaryletherketone, at least a portion of said pores having located therein material to promote osteointegration having a ciysta!linity of less than about 50%. These prostheses offer various advantages over prior art arrangements. The prosthesis is lightweight, while having the required strength. In addition, it has improved wear resistance over prior art arrangements.

In the arrangement of WO 2011/095813 osteointegration is provided for by means of an outer layer having pores at least some of which are provided with material which promotes osteointegration.

Other approaches to enhance osteointegration have been suggested. For example in some arrangements, the bone-facing surface of the prosthesis is supplied with upstanding projections which allow for improved osteointegration. For example, in EP 1527757, an element is described which comprises a rear face, a front face, and a body extending between the rear face and the front face, wherein the front face comprises a basal surface and at least one upstanding portion projecting above the basal surface and having a transverse hole extending therethrough from one side of the upstanding portion to another. These elements are incorporated into the bone-facing surface of the prosthesis. Examples of other osteointegration projections located on the surface of prosthesis are given in WO2009/034429, EP0668062, FR2548889 and EP0552950. In an alternative approach, it has been suggested that the surface of the prosthesis should be roughened or include grooves or channels. Examples of these arrangements can be found in GB2268408 and EP1854430.

Whilst these arrangements all assist osteointegration to occur, it does take some time for the bone to integrate with the prosthesis and hold it firmly in place. Thus when the prosthesis is first impacted into position, the hold later provided by osteointegration does not apply. There is therefore a need to provide a means for holding the prosthesis in position during the initial stages of use.

Whilst conventionally screws or similar mechanical fixation methods can be used to hold the prosthesis in position, these are generally undesirable. For example, in an acetabular cup prosthesis, the presence of screw head(s) is disadvantageous as it will interrupt the articulating surface or will require the use of a liner within the cup such that the screw head does not impinge on the articulating surface. There is therefore a need for an improved prosthesis which addresses the problem of initial fixation and which does not have the problems noted with conventional mechanical fixation means.

In co-pending application filed by Finsbury (Development) Limited on 28^tn April 201 an accorded application number PCT/GB2011/050853 there is described a prosthesis comprising a plastics body and a plurality of barbed stakes extending from a bone-facing surface of said body; said stakes being sized and located to provide fixation on impaction.

In one embodiment of the arrangement described in PCT/GB2011/050853 at least one locating rod may extend from the bone-facing surface. Bi use this is located in a well that has been drilled in the bone by the surgeon and assist to ensure that the prosthesis is located in the correct position before impaction occurs, hi one arrangement three rods are suggested located in a triangular configuration.

Whilst this anrangement goes some way to address the problems of the prior art, there is a requirement for an optimized prosthesis.

Thus according to the present invention there is provided an acetabular prosthesis comprising a plastics cup-shaped body, a plurality of barbed stakes extending from at least a portion of the bone-facing surface of the body, said stakes being sized and shaped to provide fixation on impaction and a single alignment rod, said alignment rod being free of components which provide fixation, said alignment rod being located on the bone-facing surface at an angle of 15 to 25 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body.

By components which "provide fixation" is meant barbs and the like which in use would prevent the prosthesis being removed from the bone.

With this arrangement, the location of the prosthesis in the acetahulam can be optimised. Generally, the prosthesis will be provided with a plurality of barbed stakes. By "barbed" stake, we mean that the stake has at least one barb which whilst allowing the stake to be impacted into the bone make its extraction difficult. The barb may be a backward facing protrusion such as one would find on, for example, a fish-hook or arrow or may be a protrusion extending substantially perpendicularly from the stake. In one arrangement, the barb may extend around the stake. Whilst the, or each, barb may be located at any point on the stake, in a preferred arrangement it will generally be located on the distal end of the stake, i.e. the end remote from the surface of the prosthesis. In one arrangement however, further barbs may be provided on the stake closer to the surface of the prosthesis. Whilst in one arrangement, the distal end of the stake may be pointed to facilitate impaction into the bone, the surface maybe of any suitable shape. In one arrangement it is curved.

The present invention enables the stakes to be driven into the bone as the prosthesis is impacted into position. The, or each, barb prevents the prosthesis from being withdrawn and therefore serves as an initial fixing means. The stakes will generally be angled such that they align with the impaction angle of the prosthesis. Where the prosthesis is an acetabiilar cup prosthesis, this angle does not align with the axis passing through the pole of the cup. The alignment will depend on the design of the particular cup but will generally be at about 10° to about 30°, preferably about 15° to about 25°, or about 20° from the axis passing through the pole of the cup. In addition to being in the impaction direction this will also be in the loading direction in use therefore improving stability.

For strength, the stakes and barbs will generally be formed as a single unit. Since the stakes are integral with the body of the prosthesis, they will generally be formed with the prosthesis. In a particularly preferred arrangement, the prosthesis is formed from a polymer and the stakes are formed integrally with the prosthesis as part of the moulding process. The barbs may be formed as part of the mould, or in one arrangement the stakes, once formed, may be contacted with a heated tool to form a barb arrangement at the distal end of the pre-formed stake. It will be understood that where the stake is formed from the thermoplastic polymer, the tool will be heated to a temperature above the melting point of the polymer. Heating of the tool maybe achieved by any suitable means. In one arrangement this may be achieved by induction heating. The stakes may be of any suitable size. Suitable sizes are generally of from about 0.3mm to about 3mm in length. The length of the stakes may be the same or different. The width of the shaft of the stake may be from about 0.2mm to about 3mm. The width of the stake at the widest point of the barb will be wider than that of the shaft of the stake and may be twice the width thereof. The shaft of the stake may be straight or in one arrangement it may taper away from the body of the prosthesis to the underside of the barb.

They stakes maybe selected to be the same length from the surface of the bone-facing surface of the body of the prosthesis and/or they may be selected so that their distal ends align in a particular orientation as desired which may mean that the stakes are of differing length to achieve the desired configuration.

The stakes will generally be configured to have the required strength to withstand the impaction forces and to allow the stakes to be driven into the bone. Where the prosthesis is to be inserted into the acetabulum stakes must be able to be driven into the acetabulum post machining, The stiffness of the acetabulum post machining would be approximately that of metphysis or lower, and may approach cancellous. As indicated in Taylor, Tanner & Freeman, J. Biomechanics 31 (1998) 303 - 319, the Young's Modulus of diaphysis cortical bone is 17GPa, metaphysis cortical bone is 5Gpa and that of cancellous bone is 0.3 - 0.4 GPa,

Stakes may be provided over the whole of the bone-facing region of the prosthesis or over a portion thereof. Whilst stakes may be provided extending over the whole bone-facing region, since the stakes are aligned on the direction of impaction, in one arrangement, at least the portion of the cup in the region of the opening will not have stakes. Generally, from about 10% to about 30% of the area of the bone-facing region of the prosthesis will have stakes extending therefrom. However, amounts of from about 20% to about 25% of the area of the bone- facing region of the prosthesis are preferred. in a preferred arrangement the stakes will generally be spaced from about 0.5mm to about 15mm, with spaeings of I mm or 2 mm to about 12 mm., and 3mm to about 10mm being useful, in one arrangement the stakes will be located at the main articulation area of the prosthesis i.e. in the region where most loading is applied to the prosthesis. Where the surface of the prosthesis is to be coated with a material to promote osteointegration such as hydroxyapatite, it is important that the spacing of the stakes is not so close that the gap between the stakes fills up with the bone ingrowth material, The osteointegration material will generally be applied at a thickness of about 0.3 mm but it can build up against external projections.

The spacing of the stakes may be the same throughout the surface carrying the stakes or in one other arrangement, they may be more closely spaced. For example, they may be more densely packed in an area carrying most load.

The stakes may be provided as discrete protrusions. They may be of any cross-section with circular or square being preferred.

However, in an alternative arrangement, a plurality of stakes may be conjoined to provide a network arrangement. Where a network is present, more than one network may be present. The network may be of any suitable configuration, in one arrangement, the stake may be elongated such that it becomes a wall with a barb extending from each side thereof. The barb may be located at or near the top of said wall. The walls may be arranged in any suitable arrangement, hi one arrangement, the walls may be arranged to in a triangular or hexagonal arrangement. Where walls meet, the region of intersection may be free of stake/wall or may have a lower height region which may or may not have a barb. The benefit of using walls is that the stakes are stronger and less likely to break during impaction.

The prosthesis may be formed of any suitable material. In one arrangement it is formed from a thermoplastic polymer. Any thermoplastic polymer may be used provided that it is biocompatible or is coated with biocompatible material Suitable thermoplstics include a member of the family of polyaryletherketones with polyetheretherketone and polyetherketoneketone being particularly useful, in one arrangement, the tliermopiastics polymer may be reinforced. The stakes will generally be formed with the prosthesis and will therefore generally be formed from the same material. Where the prosthesis is formed from a thermoplastic polymer, it maybe reinforced. Any suitable material may be used to reinforce the polymer such as the polyaryletherketone although carbon fibre is particularly preferred as the reinforcing material. Where the thermoplastic polymer is reinforced, it may be reinforced throughout the prosthesis such that reinforcing continues into the, or each, stake. Alternatively, reinforcement may only be provided in a portion of the prosthesis, for example in a region remote from the bone-facing surface,

In one arrangement, the reinforcement maybe aligned to provide specific desired properties. For example, they may be aligned within each stake to maximise their strength.

The prosthesis may be formed of more than one layer. Where more than one layer is present a layer which will form the articulating surface may be reinforced and a layer forming the bone- facing surface may not be reinforced, in one arrangement, the prosthesis may comprise the dual layer configuration described in 02011/095813.

Material to promote osieointegration may be coated on the bone-facing surface of the prosthesis and/or on the surface of the stakes and/or barbs. Any suitable material may be used including hydroxyapetite. in use the locating rod is located in a well drilled in the bone by the surgeon and ensures that the prosthesis is located in the correct position before impaction occurs. The location of the well can be obtained using any suitable locating device or jig. The presence of the alignment rod enables the surgeon to have confidence that the cup is correctly aligned before it is impacted and the barbs on the stakes interact with the bone.

The alignment rod will generally be longer than the stakes and/or may be of a larger cross- section. In one arrangement the rod will be from about 15 mm to about 30 mm, preferably from about 20 mm to about 25 mm in length.

The alignment rod is located such that in use it is located into the ileopubic bone, in one preferred arrangement it will be located into the thickest part of the ileopubic bone. The alignment rod is located at an angle of about 15 to about 25 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body. The exact position will be a function of the size of the cup but in one arrangement it will be located at an angle of about 20 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body, The rod will generally be perpendicular to the surface of the cup such that an axis extending through the rod would pass through the centre of the notional sphere from which the cup can be considered to be taken, in one arrangement, the rod is also located on plane taken through the cup at a position which is notionally defined by the position of the acetabula fossa. The alignment rod may be of any suitable configuration. Although it may be of consistent cross- section along its length, in one arrangement, it may taper to the end of the alignment rod located remote from the body.

Whilst the rod may be circular in cross-section, in a preferred arrangement, it may be configured to resist rotation of the rod in the well drilled in the acetabulam. Thus in one arrangement it may have two or more legs extending from a central point. Three, four, five, and six legs are particularly preferred. In the arrangement where the stakes form walls located in a triangular configuration, the alignment rod will be located at the point where three or more walls meet and the legs of the alignment rod will then generally align with the wal ls. In one arrangement flanges may be located between the legs and extend at least part way up the walls. Generally they will extend from about 50% to about 75% of the height of the walls.

The cup prosthesis of the present invention maybe a conventional cup shape, However, since in the natural pelvis the acetabulum is not a hemisphere and is more akin to a horseshoe shape, the prosthesis may be of a similar horseshoe shape, it is known that even when a cup shaped prosthesis is used, the wear on the cup by the femoral head prosthesis subscribes a horseshoe.

In one alternative arrangement the prosthesis may be of a combination structure whereby the cup is overall of conventional hemispherical cup but is thicker in the bearing region, i.e. in the horseshoe, and thinner in the remaining area. This thinner area may be the location of fixing means to the bone. Thus where bone screws are to be used, they may be inserted through the prosthesis at this position. Where a combination structure is used, the outer surface may generally be a conventional hemispherical cup and the increased thickness in the bearing region will lead to a raised bearing surface in the inner surface of the cup,

The prosthesis of the present invention when of a conventional hemispherical structure will be of any suitable size and the walls of any suitable thickness. The thickness will generally be the minimum required to conserve bone and maintain as much of the natural femur head diameter as possible. Ei one arrangement, the wall thickness will be from about 2 to about 5 mm with about 3 mm being generally useful. The ratio of the thickness of the inner layer to the outer layer may be adjusted as required. However, it is believe that a ratio in the region of 1 : 1 may be desirable. However a variation in the thickness ratio allows the properties of the prosthesis to be tailored such that the natural distribution of loads into the pelvis is replicated.

Where the prosthesis is of the arrangement where there is an increased thickness in the load bearing area and a thinner area in non-load bearing positions, the thickness of the load bearing area will be similar to that discussed above for conventional prosthesis. In one arrangement, an aperture may be located in the non-load bearing position. This can be useful to assist with introduction of the prosthesis and allows lubrication to flow through the prosthesis.

In one arrangement of the present invention, a portion of the bone-facing surface of the cup to which stakes are not applied, maybe provided with surface dimples. These dimples maybe of any configuration but will generally be oval. For the purposes of the present invention oval includes circular. These dimples serve to improve the surface area available for coating of the bone ingrowth material such as hydioxvapatite. In addition, their presence increases the shear force required to remove the bone ingrowth material and hence increases the strength of the coating. The dimples may be of any suitable size. Where they are of circular configuration they may have a diameter of from about 2 mm to about 6 mm.

According to the second aspect of the present invention there is provided a method of fonning the prosthesis of the above first aspect comprising the steps of:

moulding the prosthesis and said integral stakes; and locating said stakes against a heated tool to form said barbs. The tool is preferably heated by induction heating.

The present invention will now be described, by way of example, with reference to the accompanying drawings in which:

Figure 1 is a perspective view from one side of an acetabular cup prosthesis according to the present invention;

Figure 2 is a view from above of the acetabular cup prost hesis of Figure 1;

Figure 3 is a view from one side of the acetabular cup prosthesis of Figure 1 ;

Figure 4 is a similar view to that illustrated in Figure 3 viewed with the opening of the cup horizontal;

Figure 5 is a view of the cup of Figure 1 viewed from above with the opening horizontal and viewed from the position of the acetabula fossa in use;

Figure 6 is a perspective view of the cup of Figure 1 from an alternative angle;

Figure 7 is an enlarged view from above of the alignment rod of Figure 2;

Figure 8 is a view from above of the femoral facing side of the prosthesis of Figure

Figure 9 is a perspective view of Section A~A of Figure 8;

Figure 10 is a perspective view of an alternative prosthesis in accordance with the present invention; Figure 11 is a schematic illustration of the acetabular cup prosthesis of Figure 1 in position in the pelvis from one angle; and

Figure 12 is a schematic illustration of acetabular cup prosthesis of Figure 1 in position in the pelvis viewed from a different position.

In all of the Figures filed herewith, the size of the stakes has been exaggerated to improve the clarity of the illustration. In addition, the barbs have been omitted for clarity. Details of the barbs and their formation can be found in co-pending application no PCT/GB20141/050853.

As illustrated in Figures 1 to 7, the prosthesis of the present invention comprises a cup 1, a portion of the bone-facing surface 2 of which is provided with a plurality of conjoined stakes 3. In the illustrated arrangement, the stakes form walls which together form a network extending over a portion of the surface, The region over which the stakes extend is selected to ensure that in use the prosthesis is firmly anchored and therefore are positioned to resist movement occasioned by the articulation of the femoral head or prosthesis therefor,

In the arrangement illustrated, the stakes 3 have been formed as part of the injection moulding process for the acetabular cup prosthesis which is formed from polyetheretherketone and as such are integral with the prosthesis 1. Once the stakes have been formed, they are pressed into a hot tool such that they splay to form the barbs.

In the illustrated arrangement the stake tapers from the base towards the point thereof but it will be understood that it may be of any suitable arrangement. As the stake is contacted with the heated tool under force, it is deformed such that a portion of the stake splays out to form a barb which extends along both sides of the stake.

An alignment rod 4 is provided to assist in the location of the prosthesis in the pelvis. The prosthesis in position is illustrated in Figures 11 and 12. For clarity only aportion of the network of stakes is illustrated and the pelvis is shown as translucent. The alignment rod 7 as illustrated has three main walls and 5a, 5b and 5c. These are illustrated most clearly in Figure 7. Flanges 6 extend between the walls to increase the strength of the rod and its resistance to rotation. The flanges extend part way up the walls 5. In one arrangement they extend about 75% of the way up the walls.

The preferred location of the rod is illustrated in Figure 9,

The embodiment of the invention where the prosthesis has a thicker region in a horse shoe configuration is illustrated in Figure 8. The thicker region 8 provides the articulating surface. An aperture 9 is provided in the thinner region 10.

As illustrated in Figure 10, dimples 11 maybe provided on an area of the bone- facing surface not provided with stakes. The arrangement of the dimples in the Figure is for illustration only and it will he understood that any suitable configuration may be used, in the illustrated arrangement they are oval.

Whilst the present invention has been discussed in detail in connection with an acetabular- prosthesis, it will be understood that the invention may be applied to other prosthesis such as prosthesis in the shoulder. Further, it will be understood that various modifications, uses or adaptations of the invention may be made within the scope of this invention.

Claims

An acetabular prosthesis comprising a plastics cup-shaped body, a plurality of barbed stakes extending from at least a portion of the bone-facing surface of the body, said stakes being sized and shaped to provide fixation on impaction and a single alignment rod, said alignment rod being free of components which provide fixation, said alignment rod being located on the bone-facing surface at an angle of 15 to 25 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body.

A prosthesis according to Claim 1 or 2 wherein the stake is from about 0.3 mm to about mm in length,

A prosthesis according to Claim 1 or 2 wherein there are a plurality of stakes.

A prosthesis according to Claim 3 wherein the stakes are spaced from about 0.5 mm to about 15 mm,

A prosthesis according to Claim 3 or 4 wherein the stakes are conjoined to form a network.

A prosthesis according to any one of Claims 1 to 5 wherein the prosthesis is formed from a thermoplastic polymer.

A prosthesis according to Claim 6 wherein the thermoplastic material is polyetheretherketone or polyetherketoneketone which may reinforced.

A prosthesis according to any one of Claims 1 to 7 wherein the each stake extends parallel to the locating rod.

9. A prosthesis according to any one of Claims 1 to 8 wherein the alignment rod is from about 15 mm to about 30 mrn in length.

10, A prosthesis according to any one of Claims 1 to 8 wherem the alignment rod is from about 20 mm to about 25 mm in length.

11. A prosthesi s according to any one of Claims 1 to 10 wherein the alignment rod is located at an angle of about 20 degrees to the axis passing through the apex of the body and extending perpendicular from a plane across the opening in said cup-shaped body.

12, A prosthesis according to any one of Claims 1 to 11 wherein the rod tapers to an end of the alignment rod located remote from the body.

13, A prosthesis according to any one of Claims 1 to 12 wherein the alignment rod has two or more legs extending from a central point.

14, A prosihesis according to Claim 13 wherein there are three, four, five and six legs.

15. A prosthesis according to Claim 13 or 14 wherein the legs are located in a triangular configuration and flanges are located within the legs,

16. A prosthesis according to Claim 15 wherem the flanges extend from about 50% to about 75% of the height of the walls.

17, A method of forming the prosthesis of any one of Claims 1 to 16 comprising the steps of:

moulding the prosthesis and said integral stakes; and

locating said stakes against a heated tool to form said barbs.

18. A method according to Claim 17 wherein the tool is heated by induction heating.